Discovering Hidden Traps

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1515

in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes

LUCA D'AMARIO

ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-554-9911-2 UPPSALA urn:nbn:se:uu:diva-320187 2017 Dissertation presented at Uppsala University to be publicly examined in Häggsalen, Ångströmlab, Uppsala, Wednesday, 7 June 2017 at 21:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor James Durrant (Faculty of Natural Sciences, Department of Chemistry, Imperial College London).

Abstract D'Amario, L. 2017. Discovering Hidden Traps. in Nickel Oxide Nanoparticles for Dye- Sensitised Photocathodes. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1515. 95 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9911-2.

The finite nature of fossil fuels and their effect on the global climate, raised the need to find an alternative source of energy. This source should be environment compatible, cheap and abundant. The light coming from the Sun is a promising alternative. To be fruitful, the solar energy needs to be transformed in storable and transportable energy forms like electricityor fuels. Amongst the most studied techniques dye sensitised devices offer the possibility to be designed for both the scopes: solar-to-electricity and solar-to-fuel conversions. In these applications a photocathode and a photoanode, constructed by mesoporous semisconductor films sensitised with dyes, are placed in series with one another.It follows that the photocurrent generated by one electrode should be sustained by the photocurrent produced by the other electrode. At the moment there is a substantial difference between the conversion efficiencies and the photocurrent produced by photoanodes and photocathodes. In this thesis the reasons for this discrepancy are investigated. The main responsible of the bad performance is identified in the semiconductor normally used in photocathodes, Nickel Oxide (NiO). Electrochemical impedance spectroscopy was used to elucidate the electrical properties of mesoporous NiO films. The study revealed that NiO films are able to carry a large enough current to establish that conductivity is not a limiting factor. The recombination reactions were then accused as the cause of the power losses. A time resolved spectroscopic study revealed that NiO can host two kinds of holes. One of these holes is responsible for a fast dye-NiO recombination (100 ns) and the other one for a slow recombination (10 ms). A cell featuring only the slow dye-NiO recombination would possibly reach high efficiency. The characterisation of the species associated with these two holes was performed by density-of-state assisted spectroelectrochemistry. The holes were found to be trapped by Ni2+ and Ni3+ sites located on the NiO surface forming respectively Ni3+ and Ni4+ states. A study by fs and ns transient absorption spectroscopy revealed that Ni3+ sites can trap a hole in subpicosecond time scale and this hole relaxes into a Ni2+ trap in ns timescale. The control of the Ni2+/Ni3+ratio on the NiO surface was found to be crucial for a high cell photovoltage. In the thesis these results are discussed and used to propose an explanation and some solutions to the poor performance of NiO-based dye sensitised cells.

Luca D'Amario, Department of Chemistry - Ångström, Physical Chemistry, Box 523, Uppsala University, SE-75120 Uppsala, Sweden.

ISSN 1651-6214 ISBN 978-91-554-9911-2 urn:nbn:se:uu:diva-320187 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-320187) Alla mia famiglia.

List of papers

This thesis is based on the following papers, which are referred to in the text by their roman numerals.

I Tuning of Conductivity and Density of States of NiO Meso- porous Films Used in p-Type DSSCs Luca D’Amario, Gerrit Boschloo, Anders Hagfeldt, and Leif Ham- marström J. Phys. Chem. C, 2014, 118 (34), 19556-19564

II Kinetic Evidence of Two Pathways for Charge Recombina- tion in NiO-Based Dye-Sensitized Solar Cells Luca D’Amario, Liisa J. Antila, Belinda Pettersson Rimgard, Gerrit Boschloo, and Leif Hammarström J. Phys. Chem. Lett., 2015, 6 (5), 779-783

III Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application Luca D’Amario, Roger Jiang, Ute Cappel, Elizabeth A. Gibson, Ger- rit Boschloo, Håkan Rensmo, Licheng Sun, Leif Hammarström and Haining Tian ACS Appl. Mater. Interfaces, accepted.

IV Unveiling Hole Trapping and Surface Dynamics of NiO Na- noparticles Luca D’Amario, Jens Föhlinger, Gerrit Boschloo and Leif Hammarström Manuscript ready for submission

Reprints were made with permission from the publishers. Papers not in the thesis

During the Ph.D. studies the author contributed in other scientiﬁc works that are not reported in this thesis.

They are listed in the following: A comprehensive comparison of dye-sensitized NiO photocathodes for solar energy conversion, Wood C. J., Summers G. H., Clark C. A., Kaeﬀer N., Braeutigam M., Carbone L. R., D’Amario L., Fan K., Farre Y., Narbey S., Oswald F., Stevens L. A., Parmenter C. D. J., Fay M. W.,La Torre A., Snap C. E., Dietzek B., Dini D., Hammarström L., Pellegrin Y., Odobel F., Sun L., Artero V., and Gibson, E. A.Phys. Chem. Chem. Phys., 2016, 18, 10727-10738; Supramolecular hemicage Cobalt mediators for dye-sensitized solar cells, M. Freitag, W. Yang, L. A. Fredin, L. D’Amario, K. M. Karlsson, A. Hagfeldt, G. Boschloo, ChemPhysChem 2016, 17, 3845; Ultra long-lived electron-hole separation within water-soluble colloidal ZnO nanocrystals: Prospective applications for solar energy production, Cieslak A.M., Pavliuk M.V., D’Amario L., Abdellah M., Sokolowski K., Rybinska U., Fernandes D.L.A., Leszczynski M.K., Mamedov F., El-Zhory A.M., Fohlinger J., Budinska A., Wolska-Pietkiewicz M., Hammarstrom L., Lewinski J., Nano Energy, 2016, 30, 187-192. Contribution to papers

Paper I: Main responsible for the design of the project, performed all the measurements, the analysis of the data and the interpretation. Wrote the ﬁrst draft of the manuscript.

Paper II: Participated in the design of the project and co-supervised the student that performed the measurements. Analysis of the major part of the data and main responsible for the interpretation. Wrote the ﬁrst draft of the manuscript.

Paper III: Participated in the design of the project, prepared the sample and performed all the spectroscopic measurements and their analysis. Main responsible for the interpretation of the results and wrote the ﬁrst draft of the manuscript.

Paper IV: Main responsible for the design of the project, performed the ns-transient absorption measurements and steady state measurements and their analysis. Main responsible for the interpretation of the results. Wrote the ﬁrst draft of the manuscript. List of abbreviations

BG band gap C343 coumarin c343 dye ued in Paper I CB conduction band CE counter electrode DOS Density of States (cm−3eV−1) DSC dye-sensitised solar cell DSFC dye-sensitised solar fuel cell FTO Fluorine-doped Tin Oxide HEC Hydrogen Evolving Catalyst HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital NP nanoparticle OEC Oxygen Evolving Catalyst P1 dye used in Paper III RE reference electrode Ru-NMI ([Ru(dcb)2(NMI-phen)](PF6)2)dye TA transient absorption TAS transient absorption spectroscopy VB valence band WE working electrode C capacitance (in F) E electrochemical potential (in J/mol) E energy (in J or eV) EF Fermi level (in eV) q EF quasi-Fermi level (in eV) e− electron FF ﬁll-factor fkww stretched exponential function h+ hole, electron vacancy 2 Jsc short circuit current density(in A/cm ) R resistance (in Ω) Voc open circuit voltage (in V) β stretching parameter of the fkww τ time constant of the fkww Contents

1 Introduction ...... 11 1.1 Motivations ...... 11 1.2 Solar energy conversion challenges ...... 13

2 Fundamentals ...... 23 2.1 Fermi level and Density of states (DOS) ...... 23 2.2 quasi-Fermi level ...... 25

3 Materials and methods ...... 26 3.1 Materials ...... 26 3.1.1 Semiconductor: Nickel Oxide ...... 26 3.1.2 Sensitisers ...... 27 3.2 Techniques ...... 29 3.2.1 Electrochemical Impedance Spectroscopy (EIS) ...... 29 3.2.2 ns-Transient Absorption Spectroscopy (ns-TAS) ...... 34

4 Main ﬁndings and Discussion ...... 38 4.1 DSC performance: n-type vs. p-type ...... 38 4.2 NiO electrical conductivity ...... 40 4.3 The lithium eﬀect on the DOS and conductivity ...... 43 4.4 The recombination issue ...... 45 4.5 NiO trap characterisation ...... 52 4.6 NiO surface trap dynamics ...... 55 4.7 The role of the NiO surface in DSCs operation ...... 65 4.8 Future work ...... 67

5 Summary ...... 69

6 Sommario divulgativo ...... 71

7 Populärvetenskaplig sammanfattning ...... 74

8 Acknowledgments ...... 77

Appendix A: Earth’s energy balance ...... 80

Bibliography ...... 82

1. Introduction

This chapter aims to introduce the reader into my ﬁeld of work, motivating the relevance of my science. The introduction has purposely been made so that only little prior knowledge of the basic concepts is required.

1.1 Motivations In thermodynamics a “closed system” is an imaginary box that can exchange energy with the external world, but can not exchange material [1, 2]. The system where we live, the surface1 of Earth together with the atmosphere, can be considered a closed system. I will refer to it as the surface-atmosphere system. In fact this system exchanges energy with the external space but its mass remains rather fixed and confined (at least during a human lifespan) [3, 4]. This last statement is true with some few exceptions: the atmosphere releases light gases (He and H2) into space, the volcanoes inject some minerals and gas from the internal part of Earth and finally humans introduce fossil hydrocarbons into the system [3, 5–8]. The latter has been ascribed, by the scientific community, as the cause of global warming [9–11]. There are several reasons at the basis of global warming, in Appendix A this phenomenon is described in more details. In a few words, the amount of carbon we are introducing in the surface-atmosphere system, turns up directly in the atmosphere in the form of CO2, increasing the green house effect [4, 12]. This leads to a rise of the Earth’s average temperature that is potentially lethal for the ecosystems [12–15]. The most accepted origin of the fossil hydrocarbons is attributed to the decomposition of biological material that around 250 million years ago sed- imented at the bottom of the oceans [16–18]. If this is true, it would mean that the entire mass of carbon stored in the fossil hydrocarbons reservoirs was previously contained in living materials, i.e. participating in the carbon cycle. In terms of flux of material we can see the fossil fuel extraction like the inverse process of petroleum formation. Petroleum formation re- moved material from the surface-atmosphere system transferring it inside the Earth’s crust and 250 millions years later fuel extraction brings it back. In these terms petroleum extraction does not seem so dangerous, but it

1By surface is meant the whole material contained in the ﬁrst few hundreds meters of the Earth’s crust.

11 actually is. It is estimated that the amount of carbon contained in the fossil hydrocarbon resources is about half of the one contained in the oceans and ten times the one in the biosphere [19–21]. The surface-atmosphere system, in particular the biosphere, had 250 million years to adapt to the present amount of carbon. Now humans are resuming the carbon level to the one of 250 millions years ago in only 150 years. It is like deflating a balloon in 2 seconds and blowing it up again in 1 microsecond. The balloon would probably explode. The system we live in is much more complex than a balloon, predicting its behaviour upon such a quick perturbation is not easy, but a reaction is surely expected. Especially, if one considers that the perturbation affects the atmosphere, which is the main responsible for the mean temperature of our planet by the green house effect. In the worst case scenario the thermal shock will be lethal for some ecosystems, and we might observe the sixth big mass extinction in history of our planet, that for the first time would have been caused by a choice [22, 23]. The choice in fact is whether or not to replace our main energy source from the fossil hydrocarbons to a “renewable” kind of source. A renewable energy source is able to re-create itself in a reasonably short amount of time, it is thus not finite [24]. Additionally, this form of energy should keep the system closed to perturb the cycles of matter as little as possible. It follows that this form of energy cannot come from the system itself, and it should not be carried by matter (like fossil hydrocarbons). In fact the majority of renewable energies can be attributed to the main energy fluxes arriv- ing on the surface of Earth [24]: Sun (direct light-energy conversion, wind, hydropower, ocean waves), Earth’s nucleus (geothermal) and gravitational forces (tidal energy). In particularly the Sun delivers about 150 PW (1PW=1015 W) to Earth. The human need for energy is 17 TW (1TW=1012 W) [25]. Its abundance makes this source of energy very profitable. Unfortunately, how to convert solar light into a transportable, storable, energy-dense and cheep form of energy remains unknown. One of the first who wondered why humans use “solar produced” fossil hydrocarbons instead of direct solar energy was G. Ciamician, an Italian chemist, who dreamed of “chimney and smoke free industries” in 1912. [26, 27] Inspired by similar dreams, I gladly dedicated my doctoral research in solar energy conversion.

12 1.2 Solar energy conversion challenges For solar energy conversion it is usually meant the conversion of electromagnetic energy into another form of energy that is either electrical or chemical. The discussion of all the possible ways of converting the solar energy is beyond the scope of this thesis. Thus I will just brieﬂy summarise them (for a better overview see [28–30]). The most common strategies for solar energy conversion into electricity involves the use of a p-n junction as light harvesting site and a charge separation system to generate an electric current. The techniques based on this principle are: single junction cells (Si and Ge as most common semiconductors), multi-junction cells (world record of energy conversion) and thin ﬁlm cells [31–33]. Organic photovoltaics use a similar strategy but the charge separation is carried out with donor and acceptor polymers [34, 35]. Other techniques involve the conversion of the solar energy into heat which is then used to either produce steam for turbines or perform a chemical reaction [32, 36]. The conversion in chemical energy is currently accomplished in the production of “biofuels” [37, 38]. Here, plants using photosynthesis transform

CO2 and O2 into organic material, an oil, that is extracted by squeezing the plant and used as fuel. There is a big effort in the scientific community to genetically modify living organisms, like algae or bacteria, to be able to artificially direct the bio-synthesis for product excretion avoiding the de- struction of the living organism [39–41].

The research I carried out in these years of doctoral studies is focused on the study and characterization of p-type NiO based photocathodes. These electrodes could be used in electrochemical devices to convert solar energy to electricity or chemical energy. These devices are called respectively dye- sensitized solar cells (DSCs) and dye-sensitized solar fuel cells (DSFCs).

Dye-sensitized solar cells As already mentioned, the most common way to convert light into electricity is to use a doped semiconductor that is able to absorb photons and create a charge separation. In these devices the semiconductor material is shaped in the form of a ﬁlm or a foil. One of the two sides of the foil is p- doped, the other one is n-doped, thus a p − n junction is created. The sides are covered with transparent electrodes for charge collection. The light harvesting occurs by fundamental absorption (or band gap excitation) where an electron is promoted from the valence band (VB) to the conduction band (CB) of the semiconductor [32, 42]. The charge separation instead happens in the p-n junction. The depletion layer created by the junction builds a local electric ﬁeld transversal to the semiconductor foil. The hole (h+) and electron (e−) created in the band gap excitation can be separated by

13 the local electric field and flow away from each other and finally reach the electrodes. This mechanism is used in all the photovoltaic devices based on the p-n junction. It is possible to calculate the maximum theoretical conversion efficiency of the p-n junction cell. The conversion efficiency is the ratio between the power obtained from the system after the conversion and the power delivered to the system. The power of a solar cell is proportional to the product of the voltage and current that the cell can deliver at a certain time [42, 43]. This is intuitive, the voltage account for the potential difference of the separated charges, i.e. the energy stored in each h+-e− couple, while the current account for their flux. The potential of the h+-e− couple cannot be higher than the photon that generated it and it is assumed to be the energy of the band gap.2 With this assumption the voltage of the cell becomes strictly related with the band gap of the semiconductor. Now it is easy to see that the power delivered by a p-n junction solar cell has a theoretical maximum. In fact, a material with a large band gap will show a high voltage but will suffer from a small current since the material will not be able to absorb the low energy photons of the solar spectrum. Contrarily a material with low band gap will cover the entire solar spectrum, thus will have a large current but it will have a low potential. The theoretical maximum conversion efficiency3 of a single p-n junction solar cell is called “Shockley-Queisser” limit and is equal to 34% with an optimal band gap of 1.34 eV [44]. Devices that exceed this limit have been built by coupling in series several p-n junctions with different band gaps [45]. Currently a multi-junction devices holds the record of solar energy conversion efficiency of 46% [46].

A diﬀerent approach respect to the p-n junction device was adopted in developing the dye-sensitized solar cell. In this kind of device the light is absorbed by a molecular light harvester, a dye, and the charge separation occurs at the interface between the dye and the mesoporous semiconductor where the dye is linked [43, 47]. A schematic representation of the working principle of a DSC is presented in Figure 1.1. DSCs can be divided in two main kinds, p-type and n-type, based on the kind of semiconductor used in the cell. The vast majority of scientiﬁc work regarding DSC has been made on n-type, mainly using

TiO2 as semiconductor. Most of the concepts and characterization methods used in this thesis have been developed studying TiO2-based DSCs. The working principle of the two kinds of DSCs is similar. p-type DSCs is the

2The absorbed photons with higher energy than the band gap are considered to loose the excess of energy by thermalization. 3This is done assuming that every photon contributes to the generation of a charge pair, i.e. a quantum eﬃciency = 1.

14 main subject of this thesis, therefore, in the following, only the working mechanism of p-type DSCs is presented. A p-type DSC consists of two transparent electrodes, a “photocathode” and a counter electrode, see Figure 1.1 left side. The photocathode is covered by the active material, a mesoporous ﬁlm (∼1μm thick) of a wide band gap p-type semiconductor. Normally the ﬁlm consists of nanoparticles (NP) but DSC based on nano-rods, nano-wires, nano-leafs, etc. have also been reported [48–50]. The nanoparticles are sensitized with a suitable dye that can inject a hole in the semiconductor VB upon photon absorption. The photocathode faces the counter electrode typically by a distance of few tens of micrometer. The electrodes are kept in electric contact by an electrolyte solution of a redox couple that wets both of them.

Figure 1.1. Schematic representation of a p-type DSC. Left scheme: from the left, the photocathode, an FTO (Fluorine-doped Tin Oxide) glass with semiconductor NPs (NiO) deposited on it; the dye linked to the surface of the NPs is in contact with the redox couple, this closes the circuit at the platinized counter electrode (FTO) on the right. In the right scheme: 1, photo-excitation of the dye; 2, hole injection to the valence band of NiO; 3, regeneration of the reduced dye by the redox couple; 4, recombination between the reduced dye and the VB hole; 5, recombination redox couple-VB hole.

The right side of Figure 1.1 describes the processes occurring in the DSC. The absorption of a photon brings the dye in an excited state, reaction 1. The excited state of the dye is oxidative enough to inject a hole in the VB of the semiconductor, reaction 2. This creates a charge separation; the hole, h+, resides in the nanoparticle while the electron, e−, is located in the dye that in this phase is reduced. If the charge separation lives long enough the dye is regenerated by the redox couple that accepts the e−, reaction 3.

15 The redox couple carries the e− to the counter electrode while the semiconductor transports the h+ to the photocathode closing the circuit. It follows that the output voltage of the cell is given by the difference of the q 4 potential of the holes, the quasi-Fermi level (EF ) , and the one of the redox couple. The beauty of this mechanism is comparable only to the frustration of the scientist working on p-type DSC trying to make it work well, since it does not. In fact, each of the steps discussed above can be suppressed by a competitive process that ends up recombining the h+ and the e− created by hole injection. Every time this happens the energy accumulated in the charge separation is lost. Normally just two processes are considered to be the main source of power losses, reaction 4 and 5 in the scheme, respectively dye-hole recombination and electrolyte recombination.Inthedye-hole recombination the h+ recombines with the e− in the reduced dye, while in the electrolyte recombination the h+ reacts with the reduced part of the redox couple. There are also other factors that can cause power losses. Two examples are the resistance to the transport of the charges in the electrolyte or the semiconductor, and the resistance to the charge transfer between two parts of the cell (i.e. redox couple-counterelectrode) [43, 51, 52]. In terms of power losses there is a substantial difference between p-type and n-type DSCs. One of the important points of this thesis is that the main differences between n-type and p-type DSC should be addressed to the particular material, TiO2 vs. NiO.

The concept of DSC has been present in the scientiﬁc literature from 1991 when B. O’Regan and M. Grätzel published their work on a n-type

TiO2 based DSC [47]. In n-type DSCs the working mechanism follows the reaction steps described above with the difference that, instead of a hole, an electron is injected in the conduction band of the semiconductor while the dye oxidises. So far the field of n-type DSC has developed and has become one of the main research fields of chemical science. Moreover, the record of solar conversion efficiency of a TiO2 based DSC reached 14% [53], meaning that many concepts regarding the mechanism introduced above have been understood and well applied. From the first work by Grätzel the field of DSC has evolved and transformed. New branches of research were born and the two most popular ones are the Solid-State-DSC and the perovskite solar cell [54, 55]. In the Solid-State-DSC the liquid contact between the two electrodes has been substituted with a solid conductor. Perovskites on the other hand are generated from the attempt of replacing the molecular sensitizer with an inorganic sensitizer. Perovskite-based solar cells became the new research fashion in the solar energy conversion community replacing DSC in just three years from the first publication. The reason for this is the high con-

4For more information about the quasi-Fermi level see Section 2.1 and 2.2

16 version eﬃciency of ∼20% obtained even in early works.

In contrast to n-type DSC, the field of p-type DSC, mainly based on NiO as semiconductor, is rather new (see Lindquist, 1999 [56]) and it did not have the same success as that of its n-type counterpart. Despite years of research and huge knowledge inherited from the n-type field there are only three cases where the conversion efficiency exceeds 0.5% [57–59]. Many attempts have been made to improve the performances of the p-type cell: different sensitizers were used [60–67], different linker groups [68–72] and many redox couples [56, 58, 59, 73–75]. Instead, the semiconductor was modified very few times [76–88]. It seems that the secret of the poor performances is hidden in some property of the semiconductor material. The interest for p-type DSC is not purely conceptual. A working p- type DSC could be coupled to a n-type DSC to build the so called tandem DSC [56, 89]. In a tandem DSC the counter electrode of an n-type DSC is substituted with a p-type photocathode. The two electrodes have complementary absorptions covering together the entire visible range. Similarly to a multi-junction solar cell the theoretical maximum efficiency of a tandem DSC exceeds the Shockley-Queisser limit.

Solar fuel cells By photosynthesis nature has built the entire biosphere using light, inorganic carbon (CO2) and molecular oxygen to produce carbohydrates and other highly energetic molecules. The branch of science that tries to emulate this process using chemistry principles is called artiﬁcial photosynthesis. At the moment its main scope is restricted to water splitting or CO2 reduction [90–92].

The ﬁrst process would use solar energy to decompose H2O in its two components:

−−→hν 1 H2O(l) 2 O2(g) +H2(g)

This is a highly endoergonic reaction, 141.8 kJ/g(H2), which makes hydrogen a powerful combustible. In comparison, the combustion energy of propane is 50.3 kcal/g(C3H8) (all the combustion heat in the thesis are retrieved by CRC Handbook of Chemistry and Physics [93]). Moreover, burning hydrogen produces mainly water that solves the problem of smog. Unfortunately, hydrogen has a very low energy density which is inconve- nient for transportation. The energy density, dE, of a system is the energy stored per unit of volume [94, 95]. For example, the dE of a fuel is the energy released by burning a litre of fuel. Hydrogen, as a gas, has an energy density of about 0.01 MJ/L at 1 atm (5.6 MJ/L if compressed at 700 atm) while LPG/propane has 25.3 mJ/L. The scientiﬁc community is putting a

17 lot of effort searching for ways to store hydrogen in an efficient and safe way [95–97]. A few examples of recent progresses in this direction are: cryo-compression, intercalation in clathrate or carbon nanotubes, as metal- hydride and adsorbed into metal-organic frameworks. One more handicap in the use of hydrogen as fuel is that today’s transportation infrastructure is based on hydrocarbon thus it would need a complete reconstruction. De- spite the disadvantages, using hydrogen as fuel seems to solve most of the environmental problems. This perspective has attracted the attention of the researchers to find the way to perform the water splitting reaction in the most efficient way.

Along with water splitting, CO2 reduction is considered the main alternative for replacing fossil fuels [98–100]. The research in CO2 reduction aims to eﬃciently reduce CO2 in one of these products: methanol, formic acid, carbon monoxide. These three substances are aimed because each of them can be used in the present industrial infrastructure to produce any other carbon based chemical from fuel to plastic. This would facilitate the conversion to a fossil fuel free society since it would keep most of the present hydrocarbon based technology intact. Burning hydrocarbons derived from artiﬁcial photosynthesis would not increase the net amount of carbon in the surface-atmosphere system, this is thus considered a “zero emission” strategy. Though, it would probably not improve the smog issue.

There are several ways to convert solar light into chemical energy. Usu- ally the conversion into high energy chemicals occurs by a redox reaction at an electrode. A simple way to achieve this transformation is to convert light into electricity with a photovoltaic device and then use an electrochemical cell to perform the chemical reaction. This is not considered here as a solar fuel cell. A solar fuel cell performs the light harvesting and the energy conversion in the same device. The light harvesters and the catalysts, which allow the chemical transformations, are deposited together on the electrodes. In the case of the water splitting cell these two catalysts are the oxygen evolution catalyst (OEC) and the hydrogen evolution catalyst (HEC) [101]. The light harvester can also be of the same material as that of the catalyst. For example one of the first cases of artificial photosynthesis was the discovery that the band gap excitation of TiO2 initiates water oxidation [102]. After promoting an electron in the CB, the hole left in the VB of TiO2 is so oxidising that it can oxidise water in its proximity. The majority of the research in this field is done with inorganic systems, like in this case TiO2, which can function like a light harvester as well as a catalyst. The choice of such a material is then made on the basis of the reaction that needs to be performed. In the case of water splitting, for example, the energetics of the reaction requires at least a photon of 1.23 eV to occur. Moreover the position of the VB of the semiconductor that performs the water oxidation needs to be more positive than +1.23 V vs.

18 NHE in the reduction potential scale. In the same way the reduction potential of the CB of the material that realises the water reduction needs to be more negative than 0 V vs. NHE. Despite the complexity there are already working devices that can execute photosynthesis, one example is the “artificial leaf” which uses a silicon solar cell as a light harvester and a cobalt OEC and a nickel HEC [103]. Another approach is to use molecules as catalysts and light harvesters immobilized on transparent electrodes. These cells are called dye-sensitized solar fuel cells (DSFCs) [104]. As shown in Figure 1.2 the DSFC has many features similar to a tandem DSC. The DSFC is composed by two transparent electrodes: the photocathode where the reduction takes place and a photoanode where the oxidation occurs. As in the tandem DSC, on top of each of the electrodes a mesoporous film of a semiconductor is sintered. At the photocathode a p-type material is used while an n-type one is applied at the photoanode. The two mesoporous films are sensitized with different dyes with complementary absorption. Moreover at the photocathode a catalyst for the reduction reaction is co-sensitised with the dye. Likewise, a catalyst for the oxidation reaction is co-sensitized at the photoanode. In the case of water splitting cell the photocathode is co-sensitised with a HEC, and the photoanode with OEC, see Figure 1.2. The two electrodes are im- merse in the media that contains the substrate for the photosynthesis. In the water splitting case it is buffered water.

Figure 1.2. Schematic representation of a solar fuel cell. On the left the spatial conﬁguration of the various components. On the right the schematic of the mechanism: the arrows indicate the movement of an electron.

19 In right of Figure 1.2 each step of the electron cycle across the cell is represented. Starting from the NiO side, the excitation of the dye causes the hole injection into the VB which reduces the dye. The reduced dye transfers an electron to the HEC which can perform water reduction. In the photoanode, TiO2 side, after the excitation an electron is injected into the CB resulting in the oxidation of the dye. The oxidised dye is regenerated by the OEC which now can perform water oxidation. The electron in the TiO2 CB reaches the electrode and recombines with the hole in the NiO VB by travelling in the external short circuit. The charge neutrality of the cell is assured a ﬂow of ions between the two electrodes.

Similar to DSCs there are several issues that need to be understood and solved before this mechanism can fully work. Here the recombination issue can be considered even worse since the dye is regenerated by a species, the catalyst, that stays on the surface of the nanoparticle. This enhances the probability of recombination in DSFCs compared to DSCs where the regeneration occurs by a species that after the reaction is released in the bulk, far away from the nanoparticle. One of the major issues for DSFCs is the design of the water oxidation catalyst and the related problem of the charge accumulation. In fact, contrarily to DSC, there is a need of accumulation of charges for the redox reaction. For the water oxidation reaction, for example, the catalyst needs to transfer 4 electrons per molecule of oxygen. The catalyst then needs to store those electrons or catalyse a multi-step process.

The importance of the semiconductor In both types of cells, DSC and DSFC, the semiconductor covers a fundamental role. In the first prototypes of DSCs, the dye was deposited on a flat surface [105]. The amount of dye that a flat surface can host might be thousands of times less than that of a mesoporous surface. In DSCs this is essential since only the dyes attached to the surface of the semiconductor are active in electron transfer. Due to the difference in the amount of adsorbed dyes, the mesoporous film enhances the optical density of the electrode by thousands, in some cases resulting in an absorption of more than 99% of the light. In DSCs wide band gap semiconductors are used since they need to be transparent to allow the light harvesting of the dye. Moreover, the bands of the semiconductor need to be properly aligned with the HOMO/LUMO of the dye to allow the photoinduced electron transfer. For example ZnO has the VB potential so positive that most of the p-type dyes cannot inject a hole in it, thus it can not be used in a p-type DSC. One of the most important differences between the DSC and the DSFC is the relative positions of the semiconductor bands. In the tandem DSC the output voltage of the cell is given by the difference between the CB

20 potential of the n-type semiconductor and the VB potential of the p-type semiconductor. Thus it is important, for a tandem DSC, that the bands of the two semiconductors express the largest difference in potential. While for DSFC the difference in potential of the two bands should ideally be zero. In the DSFC, in fact, the converted energy should be as much as possible in the chemical products, while a difference in potential between the two bands would convert energy into electricity. This is why the commonly used

TiO2 and NiO are a good couple of semiconductors for a tandem DSC but not for a DSFC: the potential diﬀerence between the TiO2-CB and NiO-VB is about 1 V. The semiconductor is also very important for the charge dynamics. The semiconductor is the carrier of the charges from the NP surface to the FTO electrode. It is important that the semiconductor has a good electrical conductivity in the working condition of the cell. Moreover, the dynamics of charge transfer and charge stabilization occurring in the interface dye/semiconductor or electrolyte/semiconductor or catalyst/semiconductor is crucial for preventing charge recombination events. An important drawback is that the majority of knowledge about semiconductors, especially NiO, regards bulk properties while most of the mechanisms occurring in these complex systems are happening on the surface. Considering the nanoscopic dimension of these semiconductors (NP ∅=4-50 nm), the surface is probably aﬀecting the properties of the entire material. A better understanding of these concepts is needed to interpret the behaviour of the present materials and for the design of new ones.

The research I carried out in these years was sparked by the mystery surrounding the poor performances of NiO based DSCs. At the beginning of my Ph.D. studies the maximum efficiency obtained from a p-type DSC was 0.5%. The research in this particular field had then already existed for more than 10 years. The first question I wanted to answer was whether the NiO is capable of driving enough current density to allow a high conversion efficiency. At that time, it was a common belief that NiO hole mobility was too poor to sustain a large current density [106]. The conductivity of NiO was studied in Paper I which showed the contrary, NiO can result in sufficiently large current density to allow a high efficiency. The research on NiO conductivity raised new questions on the nature of the NiO-hole and its role in the recombination reaction. The dye-NiO recombination was studied in Paper II where the presence of two kinds of holes was discovered. One of the two kinds of holes was found responsible for a rapid dye-NiO recombination which in turn is believed to cause the power losses in NiO DSCs. The chemical nature of these two holes was revealed in Paper III where the high valence Ni states, Ni3+and Ni4+, were spectroscopically characterized. Finally in Paper IV the surface dynamics of the holes were resolved using transient absorption spectroscopy.

21 It turned out that Ni3+ sites, that are known to be on the NiO surface, can trap the injected hole very rapidly (subpicosecond time scale) in a Ni4+ state. This brings the hole to the surface of the nanoparticle increasing the rate of dye-NiO and electrolyte recombinations. The Chapter 4 of this thesis is intended to be a discussion about the ﬁndings of my papers, presenting the major results, their connections and meaning. Instead Chapter 2 and 3 introduce the reader to some important concepts and methods used in the thesis.

22 2. Fundamentals

In this section I am going to brieﬂy describe some fundamental concepts that are essential in the understanding of the treated matters. The concepts of Fermi level, quasi-Fermi level and density of states (DOS) are going to be used often in this thesis. It is useful to have a reference on the physical meaning of these concepts without exploring all the details of how these can be estimated or calculated by principles. A more extensive introduction can be found in ref. [107] and [108].

2.1 Fermi level and Density of states (DOS) In thermodynamics it is useful to deﬁne a quantity that can measure the contribution of the amount of matter to the balance of an equilibrium. This quantity is called chemical potential [1]. This concept is clear to a chemist for systems like solutions or gasses, but it might be unknown for solid-state materials. Brieﬂy, given an arbitrary number of substances N, they are in equilibrium with each other, at constant pressure and temperature, when the Gibbs free energy (G) of the system reaches the minimum:

dG = 0 where G = f(P, T, n1,n2, ..., nN ),

P is the pressure, T is the temperature, ni is the molar quantity of substance i.WhenP and T are constant dG is only a function of the composition of the system: N dG N dG = dni = μidni (2.1) dni i=1 (P,T,n2,..nN ) i=1 where, dG μ i = dn i (P,T,n2,..nN )

μi is the chemical potential of the species i. Basically, the chemical potential tells about the contribution of a certain quantity of a chemical species to the overall Gibbs free energy. The chemical system tries to reach a minimum of the free energy. Thus, if there is a way to convert a high chemical potential species to a low chemical potential species, this conversion happens following equation 2.1. In this way the concept of chemical potential

23 can explain all the basic chemical phenomena like reactions, diﬀusion, phase changes, etc. The chemical potential can easily be related with the electric potential deﬁning the electrochemical potential [107]:

μ¯i = μi + ziF ΦE where zi is the net charge of the species i, F is the Faraday constant and ΦE is the electrostatic potential in the point where the electrochemical potential is considered. In a sample in equilibrium, the electrochemical potential is constant across the entire sample, this allows to calculate the concentration proﬁle of the species contained in it.

The chemical potential, especially in electron transfer reactions, is associated to the energy level of the species. Thus for a molecule, where the energy levels are discrete, it has a well defined value. In other words it is easy to sum the energy of the system counting every single contribution. For solid-state systems, where the energy levels are a continuum, the defi- nition of a chemical potential is more complicated. Since the levels are not discrete, there is the need to define a quantity that allows for counting the energy levels in the system at a specific energy. This quantity is the density of states (DOS) that counts the number of energy states available for a certain particle per volume of the solid [108]. The DOS is a function of the energy and it is the quantity that, in the case of the electron, shapes the valence band and conduction band of a solid. Intuitively, if the electrons were a fluid, the VB were a tank then the DOS is the shape of the tank. At the thermal equilibrium, the probability of finding an electron in the VB at a certain energy is given by the Fermi-Dirac distribution:

1 p()= (e(−EF )/kT +1)

where is the energy, k the Boltzmann constant, T the temperature, EF the Fermi level. The center of this distribution, i.e. the energy where the probability of ﬁnding an electron is 0.5, is called the Fermi level, EF ,andit is deﬁned to be the chemical potential of the electrons of the material. At room temperature, the Fermi-Dirac distribution is practically 1 at energies below the EF and 0 at higher energies. The region where 0.1

24 2.2 quasi-Fermi level In a system where the semiconductor is an electrode in contact with a solution, the Fermi level of the material is at the potential of the electrode. This means that if we set the potential of the electrode to a specific value vs. a reference, the entire Fermi level will shift. If the system is at equilibrium the electrochemical potential is constant across the entire sample, i.e. the Fermi level of the material matches the chemical potential of the solution. If this is not true, the system is not in equilibrium and a reaction can occur like an electron transfer from the solution to the semiconductor or vice versa. This builds an electrostatic potential that will move the electrochemical potential, eventually bringing the system to the equilibrium. A DSC in the darkness should be in equilibrium thus no voltage should be read at the electrodes. Under illumination the DSC is at working condition, the voltage read-out at the electrodes is the difference in potential between electrons in the semiconductor and the species in contact to the counter-electrode. The potential of the electrons of the material cannot be considered as the chemical potential or the Fermi level because the system is not at equilibrium. The system is instead at a stationary state. This means that the electrons in the semiconductor might occupy the bands with a different distribution than the Fermi-Dirac. In general there might exist several local Fermi levels where the electron occupation probability is q 0.5, they are called quasi-Fermi levels (EF ) [107]. Usually, in n-type DSC research, the term quasi-Fermi level is used for the one created in the conduction band by the injected electrons [43]. This is considered the location of the potential of the electrons.

25 3. Materials and methods

In this chapter the materials and the techniques relevant to my research are discussed, The sensitizer and the preparation methods for the NiO ﬁlms are shortly described. A brief introduction to the two main techniques, electrochemical impedance spectroscopy and ns-transient absorption spectroscopy, is presented.

3.1 Materials 3.1.1 Semiconductor: Nickel Oxide The semiconductor material concerned in this thesis is mesoporous nickel oxide. Nickel oxide (NiO) is a 3.5 eV indirect band-gap semiconductor. Indirect BG differs from direct BG by the mechanism of absorption of light, i.e. the way that an electron can be excited from the VB to the CB by the means of photon absorption [108]. Direct BG semiconductors feature the maximum of the VB aligned with the minimum of the CB in the energy- momentum space, see Figure 3.1. Thus the excitation of the electron occurs with the absorption of a photon with energy equal to the BG. In indirect BG semiconductors the two bands are not aligned respect to the momentum, see Figure 3.1. Thus the excitation from VB to CB needs to occur with a photon together with a particle that carries momentum, like a phonon. For probability reasons, this two-particles-mechanism makes the extinction coefficient of indirect BG much lower than the one of direct BG. For the same reason, indirect BG have a very low fluorescence quantum yield, ΦF , like that found for NiO (ΦF 0). This fact implies that after BG excitation the possible electron-hole recombination dissipates the energy entirely in heat, which was observed and discussed in Paper III. NiO has a p-type character. This is mainly due to the presence of Ni3+ impurities in the crystal structure due to Ni2+ vacancies [109–111]. When NiO contains no Ni3+ defects it is a mixed Mott/charge-transfer insulator [112, 113]. NiO RT bulk conductivity is about 10−13 Ω−1cm−1 [114], but it can be increased up to 10−1 Ω−1cm−1 by introducing Ni2+ vacancies [115, 116]. The concentration of these vacancies is oxygen partial pressure dependent and affects the NiO conductivity [111].

26 Figure 3.1. Direct (left) and indirect (right) band-gap excitations. “Pht.” stands for photon and “phn.” for phonon.

The mesoporous structure is obtained by aggregation of NiO nanoparticles. The nanoparticles can be created by precipitation, spray pyrolysis, and sol-gel sintering [117, 118]. Usually nanoparticles are made from a pre- cursor (NiOH, Ni(NO3)2, NiCl2, Ni(AcO)2) and subsequently sintered to form the oxide. The NiO mesoporous films used in this thesis were prepared by sintering a NiCl2 sol-gel. The sol-gel is prepared mixing1gofNiCl2 anydrous, 1 g of triblock polymer F108, 3 g of H2O and 6 g of ethanol (99.5%). After complete dissolution of the solids (overnight sonication), the sol-gel is spread on the transparent substrate by doctor-blading. The ethanol is allowed to evaporate for 2 min. The films are then sintered in a closed oven at 450◦C for 30 min (with 30 min ramping). In the oven the solvents evaporate and the polymer burns leaving the NiCl2 nanoparticles that are transformed into NiO by calcination. The mesoporous film was prepared in this way on conductive FTO, CaF2 windows or fused silica. The prepared films were used in the construction of the DSC or used in the electrochemical or spectroscopic measurements. The exact preparation of the cells varied depending on their sought purpose and it is reported in the papers.

3.1.2 Sensitisers In Paper I-III NiO was sensitized with a dye to test the photocathode in working condition or to trigger a hole injection to study the recombination process. Three diﬀerent dyes were used: coumarin C343 in Paper I, Ru-NMI in Paper II and P1 in Paper III. The formula of these dyes are reported in Figure 3.2. The summary of the properties of these dyes is reported in Table 3.1.

27 Figure 3.2. Molecular structure of the dyes used in this work. From left to right: coumarin C343, Ru-NMI, and P1.

Table 3.1. Spectroscopic and electrochemical characteristics of the dyes used in this thesis.

Dye λmax HOMO LUMO ref. 1 (nm) ( cm·M ) (V vs.NHE) (V vs.NHE) C343 422 4.4·104 1.4 -1.2 [119] Ru-NMI 470 1.2·104 1.52 -1.12 [60] P1 468 5.8·104 1.38 -0.87 [65]

Coumarin C343. It has for many years been the reference dye for NiO-based DSC [120, 121]. It is a yellow green laser-dye; the performances are very low but are reproducible. Therefore it was taken as standard. The injection and recombination dynamics is clariﬁed by ultrafast spectroscopy [119, 122]. The fast recombination kinetics (ns-timescale) makes it impossible to use it with a slow diﬀusing redox couple like Co-based complexes. – – Hence, it is used with I /I3 redox couple.

Ru-NMI. It is one of the few Ru-based dyes for p-type DSC, it was synthesized in reference [60]. It oﬀers the possibility to be used with slow diﬀusing redox couple since its recombination kinetics is very slow (10−6-0.1 s). The reduced dye spectrum, showed in Figure 3.3, is well characterized [60]. Ru-NMI is then a good choice for recombination studies, it was in fact used in Paper II.

28 Figure 3.3. TAS of Ru-NMI− obtained by exciting Ru-NMI in presence of Ferrocene (FeCp2) as electron donor. Exc. wavelength 480 nm, [Ru-NMI]=5 mM FeCp2=100 mM. Data reprinted from my master thesis [123].

P1. It is another standard dye for NiO-based DSC [117, 120]. Even ∼ – – though it features a fast self recombination, 200 ps, the I /I3 redox couple can regenerate the dye quite eﬃciently, probably thanks to a pre-associated complex formed with the dye [124, 125]. In fact it is used as a reference sensitizer for DSC and DSSFC and was used in Paper III.

3.2 Techniques 3.2.1 Electrochemical Impedance Spectroscopy (EIS) Any electrochemical cell can be rationalized as an electric circuit. An electrochemical cell, as any electric circuit, can be analysed with principles proper of electrodynamics [126].

First of all, the concepts of the electrochemical system and cell need an explanation to avoid misunderstanding. An electrochemical system is anything that can be connected to two electrodes to be able to apply an electric potential between them and measure the current of electrons that ﬂows through them. Thus, it can be almost anything from a water solution of NaCl to a lithium battery. The electrochemical system together with electrodes constitute the electrochemical cell. Two kinds of cells are commonly used in electrochemistry: the three electrode cell and the two electrode cell. In the ﬁrst one, the electrochemical system is connected to three electrodes: a working electrode (WE) which is the probe into the

29 electrochemical system; the reference electrode (RE) which constitutes the reference for the potential applied at the WE; the counter electrode (CE) where the electrons collected (or expelled) from the WE come from (or end up to), see Figure 3.4. In other words the potential of the WE is applied versus the RE while the current flows to the CE. There are many characteristics that RE and CE need to fulfil to be able to work as described. Without entering into details, the most important ones are: the RE needs to have very low polarizability, i.e. its electrochemical potential should not change when electrons are exchanged from the electrode; the CE instead should be able to deliver or store a large quantity of electrons changing its potential as little as possible. In the two electrode cell the RE and the CE are the same electrode. This can be crucial during some electrochemical analysis since the function of one electrode can influence the other.

In general, the scope of the electrochemical investigation is to analyse the response of the system to electrical stimuli (usually a potential variation) to extract thermodynamic and kinetic information. There are several ways to test an electrochemical system. The simplest is to apply a potential at the WE varying its magnitude linearly with time across an interval of potential, the current that ﬂows in the WE is then measured. The analysis of the behaviour of the current respect to the applied potential gives information about the chemical or physical processes occurring at the WE. This technique is called cyclic voltammetry.

Another way to test an electrochemical system is to apply an oscillating potential to the WE. Usually this potential has a sinusoidal shape, i.e. it is a wave, and the operator can control its frequency (f, f= 1/τ), amplitude (A) and potential oﬀset respect to the RE, see Figure 3.4. The current generated from this potential modulation is recorded. This current has a sinusoidal shape too. The analysis of its amplitude and phase shift respect to the potential wave gives information about the system. The physical concept that is used here is the impedance. The easiest way to describe it is to use Ohm’s law. For direct current circuits, DC, the well known Ohm’s law V = R · I describes the relation of the current I across a resistance R caused by a constant potential diﬀerence V . The same concept is applied when the voltage is modulated in an alternate current circuit, AC. V has the form:1

V (t)=|V |·ei(ωt+φV )

1This was given in complex form, it can be transformed in real form by Euler’s formula cos(ωt + φ)=[ei(ωt+φ) + e−i(ωt+φ)]

30 Figure 3.4. Schematic representation a EIS measurement to a three electrode cell. On the left: the schematic graph of the shape of the applied potential with respect to time. On the right: the registered current vs time, where the change in amplitude and phase shift is emphasised.

where t is the time, i is the imaginary number, ω is the frequency of the oscillation and φV is the phase shift of the oscillation (the translation of the entire wave with respect to the time). The current that is created across the circuit assumes an oscillatory form as well:

I(t)=|I|·ei(ωt+φI ) the variables deﬁnitions are analogue to the voltage ones. The Ohm’s law is valid any time in this system and it is written as V = Z · I, where Z is the impedance. Here the form of the impedance is found substituting the expressions for V and I:

|V |·ei(ωt+φV ) Z = = |Z|·ei(φV −φI ) = |Z|·eiθ |I|·ei(ωt+φI ) The impedance of a circuit is then found quite easily knowing the amplitudes of the potential and measuring the current and the phase shift of the current respect to the potential: |V | |Z| and θ φ − φ = |I| = V I

Impedance is a complex number, it is then represented in a two-dimensional space, Re(Z) vs. Im(Z), the complex plane. In an EIS measurement the voltage modulation is applied to the WE in a three electrode cell, the frequency of the modulation is varied and the impedance is measured in function of the frequency. There are two common ways to represent such measurement: the Nyquist plot and the Bode phase plot. The Nyquist plot simply reports the measured impedances in a complex plane. The

31 Bode phase plot reports the phase shift vs. the frequency (in a logarithmic scale). In Figure 3.5 an example of how these two plots are used is given.

Figure 3.5. Schematic representation a EIS measurement to an interface (left) and its representation in Nyquist plot (center) and Bode phase plot (right).

EIS is often used to analyse charge dynamics at interfaces. In the left plot of Figure 3.5 an interface is represented by the contact of light and dark gray areas, e.g. the electrode in contact with an electrolyte. Usually, an interface is rationalised as a capacitor in parallel with a resistor, which resemble the non faradaic and faradaic behaviour of the interface, respectively. At really high frequencies (ω = ∞ limit) the impedance is 0 since the current pass entirely through the capacitor, see Nyquist plot, and the phase shift is π/2, see Bode phase plot. At really low frequencies (ω = 0 limit) the impedance is R since the current pass entirely through the resistor and the phase shift is 0.2 At frequencies in between this range the Nyquist plot assumes a semicircular behaviour. This is due to the impedance of the system becom- ing the sum of the contributions from the capacitor, purely imaginary, and from the resistor, purely real. The frequency top of the Nyquist semicircle is given by the characteristic frequency of the RC circuit, ωRC =1/RC. Usually, the Nyquist plot shows more than one semicircle, this is due to more than one interface present in the system. This can happen only if the two interfaces have diﬀerent enough characteristic frequencies associated with their RC circuits.

Equivalent circuit The way to perform an EIS analysis is based on the formulation of the so called equivalent circuit [126–128]. The equivalent circuit is an electric model of the system that should resemble the behaviour of the electrochemical cell under test. The equivalent circuit can then be simulated and the

2Intuitively it is easy to see that the phase shift of a capacitor is π/2 while the one of a resistor is 0. The alternate current measured across a capacitor will be at the max when the applied voltage is transiting around zero while it will be zero at the potential peaks, i.e. θ = π/2. While for a resistor, the current is at the max when the potential is at its max, i.e. θ =0.

32 Nyquist plot and the Bode phase plot can be ﬁtted, extracting the parameters of the equivalent circuit. It is important to wisely design the equivalent circuit to extract the correct information.

In Paper I EIS was used to analyse the electrical properties of a bare NiO film. EIS was performed in a three electrode cell using as WE a NiO mesoporous film (0.7 μm) sintered on a FTO glass. The supporting electrolyte was LiClO4 0.5 M in acetornitrile. The Nyquist plot produced by such a system is represented in Figure 3.6. The equivalent circuit used to fit the data is also reported on the right.

Figure 3.6. Left: representation of the Nyquist plot obtained in the experiment of Paper I. Right: quivalent circuit associated to the system.

The analysed system produces three different regions in the Nyquist plot. The semicircle is due to the FTO-electrolyte interface created by the space on the surface of FTO left free from the semiconductor. The 45◦ angle straight line is due to the charge transport in the film and it is associated with a so called “Warburg resistance”. The vertical straight line is associated with the capacitance of the double layer formed at the film/electrolyte interface. The equivalent circuit was formulated following the transmission line model (for more information see ref. [127]). The Rs is the infinite frequency resistance of the cell. The Wft indicates the Warburg resistance related to the film charge transport. The Rfto and CPEfto are the resistance and the capacitance related with the FTO-electrolyte interface. The Rdl and CPEdl are the resistance and the capacitance related to the film-electrolyte interface.

33 3.2.2 ns-Transient Absorption Spectroscopy (ns-TAS) The light-matter interaction, besides being one of the most charming aspects of nature, can be used as a powerful tool for scientiﬁc investigation [129]. In the everyday experimental work, spectroscopic techniques like IR, UV-Vis absorption spectroscopy and ﬂuorescence spectroscopy are used to characterise substances and investigate chemical processes. The ones just mentioned are called steady state techniques. Namely a beam of light passes through a sample (the spectrum of the light is measured prior and after the passage) and the absorbed light is then analysed. The light absorbed by the sample can trigger chemical reactions that can modify the absorption of the sample. This is not “seen” from a steady state technique since the signal detected is averaged over a time (0.1s-1s)muchlongerthanmost chemical reaction kinetics. In other words the absorption is detected when the system is in a steady state produced by the light itself. This is usually not important since the intensity of light used in these techniques is not high enough to modify the sample noticeably.

In studies of reaction dynamics or mechanism it is often needed to involve techniques which can time-resolve the property changes. In transient absorption spectroscopy (TAS) the sample is excited, usually with a light pulse, and the following changes in absorption is monitored. Obviously, the sample excitation should be as short as possible in relation to the studied process. For this purpose laser pulses are used since they can be produced in a wide range of durations from attoseconds (10−18 s) to continuous. The time resolved techniques working in the range as-ps are called ultrafast- TAS, they work in a substantially diﬀerent way from ns-TAS and will not be discussed further in this thesis. Usually, in a ns-TAS system a laser pulse of about 5-10 ns long is produced from a Q-switched Nd:YAG laser (in our case a Spectra-Physics Quanta-Ray Pro-230). The fundamental emission of such a laser is in the IR, 1064 nm. The energy of each pulse, after being ampliﬁed, is about 1 J/pulse, which is enough to be used in non-linear optics to undergo double harmonic generation (532 nm) and triple harmonic generation transforming the fundamental to 355 nm light. The 355 nm beam can be directed into an optical parametric oscillator that can convert it to light of any wavelength in the range of 440-780 nm. The light pulse (∅ ∼ 6 mm) is directed in the sample chamber and hits the sample, usually a 1 cm cuvette, with a 90◦ angle respect to the probe beam. The probe beam is generated by a Xe arc lamp that produces a strong and well collimated white light. The probe light hits the sample and is then directed into the detector chamber where it is analysed by a charge-coupled-device (CCD) camera or a photo multiplier tube (PMT).

34 Figure 3.7. Upper schemes: intensity vs. time of the pump and probe beam. The rainbow area indicates that the spectrum is measured as a whole in the TA, instead the turquoise area represents the use of monochromtic light in the transient kinetics. Bottom schemes: the transient absorption sectrum (left) and kinetic trace (right).

The CCD camera can analyse the entire spectrum of the probe beam at a speciﬁc delay after excitation, t0, see Figure 3.7. The PMT instead is coupled with a monochromator so it can monitor one single wavelength of the evolving spectrum in a range of time. The CCD camera measures the spectrum of the probe beam in two times, prior excitation and after the excitation at a given time. The diﬀerence of these two spectra is called transient absorption or delta absorption,TA. A transient absorption spectrum can have negative and positive features, see Figure 3.7. The negative bands are called bleaches and arise from the disappearance of a chemical species after the excitation, such as the ground state molecules that were excited by the pump pulse. The positive bands are due to the spectrum of the population of species that were created upon excitation. The formation and evolution of these bands can be monitored by the PMT, recording the so called transient kinetics. After excitation the molecule can evolve by itself or react with other molecule. These processes can be followed by transient kinetics and basic principles of chemical kinetics can be used to extract information about the occurring mechanisms.

35 Stretched exponential function The kinetic informations are usually extracted performing fitting of the kinetic traces. The fitting functions are normally set by the kinetic law of the process in study. Usually this is enough to discriminate between first order or second order kinetics. Though in processes happening at an interface or on the surface of mesoporous materials, as in the case of DSC, the kinetics do not follow simple laws of bulk chemistry. There is not a very good physical model for describing interface kinetics and so far various strategies have been adopted to compare kinetics of this kind. The simplest way to treat non-ordinary decay is to fit the trace with a sum of exponential functions with different time constants and then compare the weighted times. The sum of exponentials is a concept that is at the base of another function used to fit non-ordinary decays: the stretched exponential or Kolraush-Williams-Watt function (kww) [130, 131]. The stretched exponential was used in Paper IIto describe the dye-NiO recombination kinetics. The kww-function can be written as:

β −(t/τ) fkww(t)=e where t is the time, τ is the observed time constant and β is a parameter that can assume values from 0 to 1. The inﬂuence of β on the function is shown in Figure 3.8. When β is 1, the function is a pure exponential and as β approaches 0 the function behaves faster for t<τ and slower for t>τ, see Figure 3.8 a.

Figure 3.8. The fkww function in: a linear scale and b in logarithmic scale. fkww is represented using the same τ but with diﬀerent β.

Representing the fkww function in the logarithmic scale, the role of β becomes more clear, see Figure 3.8 b. The β sets the slope of the straight line found around τ. If one imagines the fkww function as a sum of exponentials then the more the β value approaches 0 the more exponentials are needed to describe the fkww function. In other words the kinetics becomes more disperse as the β value decreases.

36 The kinetics constant τ does not describe very well the behaviour of the function since much depends on the β value. For this reason the value of τ is normally given together with an “averaged” τ, so called τ which is calculated as: τ = τ · Γ(1 + β−1) . This gives unreliable results when β becomes very small, i.e. at β<0.1 τ assumes values larger than the lifetime of 98% of the species.

Decay Associated Spectra (DAS) In Paper IV fs-TAS has been used and the data are presented in form of decay associated spectra (DAS), which are introduced in the following. In a fs-TAS a series of transient absorption spectra are collected at different delay times. Often the TA spectra can be several tens or hundreds, this allows to analyse the entire set of data with a particular algorithm called global fitting. The global fitting tries to reproduce the TA spectra (ΔAbs) by the following sum: n −t/τ ΔAbs(λ, t)= αi(λ) · e i i where αi(λ) is a spectral component of the TA spectrum evolving with time constant τi. n is the number of components used for the fitting and it should be the smallest possible. Figure 3.9 shows schematically the result of a global fitting. On the left the TA spectra recoded at three times, the two bands on the left decay together while the band on the left decays more slowly. The DAS on the right shows the two DAS associated to these TAs.

Figure 3.9. Left: TA spectra of an hypothetical experiment. Right: DAS related to the TA of the same experiment.

37 4. Main ﬁndings and Discussion

In this chapter the most important ﬁndings of my research are presented. I will discuss their interrelation and the connections with some other results from the DSC community.

Reading note: from now on the Figure n of paper M will be referred as Figure n(M) with M in Roman numbers.

4.1 DSC performance: n-type vs. p-type As already mentioned in the introduction, the performance of the tandem- DSC is limited by the photocathode. The conversion eﬃciency of a solar cell is given by the ratio between the maximum electrical power gained from the cell, Pmax, and the power delivered to it, Pin [43]. In formula:

Pmax Voc · Jsc · FF η = = Pin Pin where Voc is the voltage at open circuit, Jsc is the current density at short circuit and FF is the fill-factor, a factor that accounts for the nonideality of the cell.1 In Figure 4.1 the typical result of a J-V test of an n-type and a p-type DSC are shown. The difference in performance between the photocathode and the photoanode is evident. The p-type DSC shown here shows weaknesses in all the three factors important for high efficiency: Voc, Jsc and FF. The efficiency is around 0.1- 0.5% and for many years this was the normal result that was achieved by dye sensitised photocathodes. The two cases where the efficiency exceeds unity (1.3% and 2% [58, 59]), report a great improvement of the Voc, while the Jsc and the FF remain prohibitive. A photocathode featuring low Jsc and FF is a limiting factor not only for the single cell but also for the tandem DSC and DSFC, where the currents of the photocathode and photoanode must be comparable. Since no improvement was gained with modification of the dye and/or redox couple the attention of the community shifted on the electrical properties of NiO. Investigations on the mobility of the charges in the bulk of a working p-type DSC revealed that the mobility of the holes in

1The FF is the ratio between the product V · J measured at the maximum power point, Pmax, and the product Voc · Jsc, see Figure 4.1.

38 Figure 4.1. Typical J-V curve of an n-type DSC and a p-type DSC. The light gray area indicates the maximum power delivered by the n-type cell. One can compare this area with the teoretical one constructed by Jsc and Voc.

NiO VB is about three order of magnitude less than the electron mobility −8 −7 2 in the TiO2 CB (charge diﬀusion coeﬃcients respectively 10 -10 cm /s vs. 10−5-10−4 cm2/s [106, 132, 133]). This result could explain the poor 2 Jsc and the apparent high cell resistance. The poor Jscwas also attributed to a high rate of recombination between dye and NiO that was proven to be important in most of the cases [132, 134, 135]. Moreover, in the case where the rate of the recombination is a function of the electrode potential the recombination losses can be associated to the low FF, see Paper IIand [136]. With such a rapid recombination it is possible that the holes that were probed for the charge mobility are only a part of the original pool of injected holes. The holes that are lost in the recombination are probably the more reactive ones and most likely the highly reactive hole are also the most mobile. It follows that the holes that are measured at the electrode might be the ones less reactive and less mobile.

The main purpose of Paper I of this thesis was to measure the electric properties of mesoporous NiO in conditions where the recombination issue can be ignored.

2The cell resistance can be easily estimated from the J-V plot. It is the slope of the straight line tangent to the J-V curve in the Voc point. It is easy to see that in the n-type is relatively low (few Ohms) while it is very high in the p-type case (hundreds Ohms).

39 4.2 NiO electrical conductivity In Paper I the NiO film and NiO-electrolyte interface were characterised in absence of a dye. A NiO coated electrode film was prepared with the same procedure used for the DSCs, a 0.7 μm mesoporous film on FTO glass. The NiO electrode was used as WE in a device-like (two electrodes) cell.3 The electric contact between the two electrodes was achieved by an electrolyte, – – I /I3 in acetonitrile, which has been the standard electrolyte for DSCs. The cyclic voltammetry of such a cell is reported in Figure 4.2, black curve.

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Figure 4.2. Cyclic voltammetry plot of Li doped NiO ﬁlm on FTO glass with device-like cell (2 electrode cell, with a platinised counter electrode): orange FTO without NiO ﬁlm, black 0% Li, red 0.1% Li, blue 0.5% Li green 1% Li. The measurement was performed in darkness and with a scan rate of 100 mV/s, the − − redox couple was I /I3 100/100 mM in acetonitrile. –Reprinted from Figure 1 of Paper I

The electrode exhibits oxidation and reduction currents. The inert win- dow between the two currents is remarkably small, ∼200 mV. Considering − − ∼ that the I /I3 redox potential is believed to be 180 mV above the NiO VB, this is not the behaviour expected from this electrochemical system. In Figure 4.3 the scheme of two semiconductor-redox couple conﬁgurations are considered. As depicted in Figure 1.1, the redox couple should have a potential as negative as possible in comparison to the VB of the semiconductor. This will ensure a high Voc. This is the case 1 reported in Figure 4.3. A cyclic voltammetry resulting from this system is also represented in Fig- ure 4.3 right. The voltammogram shows only the oxidation current. This is due to the position of the redox couple with respect to the VB. Since

3The cell was composed by a platinised counter electrode separated by 50 μm Surlyn spacer.

40 Figure 4.3. Schematic representation of two semiconductor-redox cases. On the left the energetic scheme, on the right the cyclic voltammetry expected from these sytems.

the cell in discussion is a two electrode cell the reference potential is set vs. the CE, thus vs. the redox couple. The electron can be transferred from the redox couple to the VB only when the Fermi level has reached the VB edge, i.e. when the applied potential has reached V1. In case 1 the reduction current cannot occur because there are no states available above the redox couple reduction potential. In other words, case 1 behaves like a diode, a gate that allows the current ﬂow in one direction but blocks the current in the opposite one. Instead case 2 features the reduction potential of the redox couple inside the VB of the semiconductor. This allows the occurrence of both the currents, positive and negative, separated by an – – overpotential, if there is any. The system NiO-I /I3 should fall in case 1 but it behaves like case 2. This is due to some defect states on NiO surface whose reduction potential is 300-400 mV above the VB, see Figure 4.4. – – These states are also above the I /I3 reduction potential, this allows the reduction current in Figure 4.2. In terms of DSC this means that the Voc of such a NiO-based cell would derive only from the shift of the quasi-Fermi level due to the hole injection. This also means that the insulation of the semiconductor-electrolyte interface is a critical factor that determines the V – – oc in a NiO-I /I3 cell.

Another important finding of this simple experiment is the large magnitude of the current sustained by the film or in other words the low charge transport resistance of the NiO film. In the paper it is pointed out that the current reached at +1.0 V by this device are comparable with the current observed in high performing n-type DSCs. This statement implies that charge transport of the semiconductor should not be a limitation in p-type DSC. Though the fact that we observe a high current that was electrochem-

41 ically generated does not necessarily mean that it can be observed in a solar energy conversion device. This has to do with the electrical conductivity of the material in the two diﬀerent conditions. The conductivity, σc due to a given charge carrier c is given by:

σc = μc · Cc where μc is the mobility of the carrier and Cc is the concentration of the carrier [107]. The result presented in Paper I shows that the conductivity is sufficient to carry a large current. Assuming a low charge mobility, the carrier concentration Cc might be the key of the high current flow. Cc can be very different in the two cases. In the electrochemically generated current the carrier concentration is determined by the position of the Fermi level vs. the DOS that is set by the applied potential (see Section 2.1) and [107]. The same current density could be obtained in a DSC, if similar Cc would be created. In a working DSC the carrier concentration Cc is determined by the steady state concentration of holes in the NiO nanoparticles. This concentration is due to the balance between the fluxes of injected holes and of the ones lost in the recombination or driven out in the circuit. This is summarised in the following (not formal) scheme:

where the kinj,kJ and krec are the rate of injection, charge transport and recombination4; D∗, C, J and D, the population of the exciton, charge carrier, circuit charges and recombined holes. At the stationary state, the carrier concentration can be estimated5 by:

∗ kinj · D Cc ∝ kJ + krec

The rate of charge transport, kJ , is proportional to the conductivity, thus it is a function of the Cc as well: kJ is higher at higher Cc. Thekeytoa fast charge transport is to reduce the recombination rate to such an extent that a high Cc can be accumulated which increases the conductivity. The concentration of holes in this stationary state is also the one that determines the position of the quasi-Fermi level and consequently the Voc.

4For recombination is intended the sum of the dye-hole and electrolyte recombinations. 5Here ﬁrst order kinetics have been assumed. All these rates should be normalised per particle or per ﬁlm volume.

42 Thus another way to describe the strategy just discussed is that one should increase the quasi-Fermi level shift at a given photon ﬂux. This was par- tially achieved in Paper III. In that study, the electrolyte recombination of the P1-NiO DSC was blocked by two diﬀerent strategies. The better charge accumulation caused an increase of the Voc of about 100 mV, see Figure 4(III). An improvement of the FF was also observed, as expected, but unfortunately no improve of the current density was obtained. This was attribute to the poor dye-loading of the treated cells. On this topic a recent study by Odobel’s group explored the importance of the engineering of the NiO-dye-electrolyte interface for high Voc[137]. In particular they were able to almost double the Voc of a p-type DSC using – – 3/2+ a standard redox couple (I /I3 and Co complexes). The trick was to co-adsorb an inert steroid acid, chenodeoxycholic acid (cheno), together with the dye on the surface of NiO. Cheno helped blocking the electrolyte recombination reaction by physical obstruction of the recombination sites allowing a better charge accumulation.

4.3 The lithium eﬀect on the DOS and conductivity

In Paper I LiCl was added to the NiCl2 sol-gel in various concentrations: 0, 0.1, 0.5, 1 mol%, to study the effect of Li+ on the electrical conductivity of NiO. The addition of small percentage of Li, 0.01-0.1 mol%, to NiO macrocrystals improves the conductivity of the bulk material 3-6 orders of magnitude, mainly increasing the charge carrier concentration [116, 138]. This is obtained by interstitial substitution of a Ni2+ cation with a Li+. This produces the oxidation of a neighbouring Ni2+ to Ni3+ to neutralise the loss of positive charge. In the system studied in Paper I, Li does not have a linear effect on the electrical properties as one would expect from a doping effect, see Figure 4.2 and Figure 4.4. The analysis concluded that there are two main effects due to the Li. One, related with the concentration of Li in the bulk of the particle, affects the charge transport through the film. The electrical resistance of the NiO film, Rw, was measured by EIS. As expected, the film resistance could reach lower values at higher concentration of Li, see Figure 6a(I). This can also easily be seen from the voltammograms of Figure 4.2, where the slopes of the traces in the active region increase at higher Li concentrations. The other effect of the Li doping is related to the surface of the particle. Li-doping was found to be the cause of a dramatic change of the DOS of NiO. The DOS distributions, estimated from capacitance measurements (CPEdl by EIS, see Sec. 3.2.1), are reported in Figure 4.4. At that time we were not able to explain the mechanism behind the changes in the DOS. We understood that the presence of Li removes some

43 h otmga fFgr ..A euigptnil,we i sable is NiO when potentials, I reducing the explain At can reduce This 4.2. to Figure carriers. charge of both voltmmogram for the states trap as working electron an ahohr hseulbimi ayt hf ihrwt hmclo physical or chemical with either shift Paper to In easy treatments. is equilibrium This other. each I 44 ol o dniytentr ftoesae.Tease rie ihthe with arrived answer paper The of Ni states. work those we the but of of surface nature findings the the of activity identify electric not the could for essential are that states surface DOS The NiO shown. the 6b( not Figure of is from capacitance which -Reprint layer EIS. units double by arbitrary of measured values in films by energy extracted the were is distributions DOS the for 4.4. Figure h em ee slweog oices h ocnrto fNi of concentration the increase to enough low is level Fermi the ais ti hslre hnaNi a than larger thus is It radius. h qiiru fNi of equilibrium the htalwteoxidation. the allow that 3 iue4.5. Figure – 2+ h ffc fltimi o locaie.Li clarified. also now is lithium of effect The ssoe ceaial nFgr ..Ised toiiigpotentials oxidising at Instead, 4.5. Figure in schematically showed as n Ni and ffc fteL-oigcnetaino h i O,tex-axis the DOS, NiO the of concentration Li-doping the of Effect 3+ ufc ttscmoiina aiu em ee positions. level Fermi various at composition states Surface

– ttspeeto h ufc fNOaei qiiru with equilibrium in are NiO of surface the on present states /I 3 –

$ ope h em ee fNOi iheog oshift to enough high is NiO of level Fermi the couple, 2+ % &'( )* IV -Ni !"!" !"!" epoe htteeNi these that proved we +, +, +, 3+ III oad Ni towards and 2+ $ nPaper In IV. ain 3p ais ti possible is It radius. pm 83 cation, 2+ (* - +, hsealsterdcinof reduction the enables This . I)ofPaperI 3+ + ## sa o iha9 pm 90 a with ion an is ttscnhs oeor hole a host can states III

edsoee that discovered we 3+ states that in small concentrations and in large NiO crystals, Li+ are uniformly distributed in the bulk. Instead, in small nanoparticles and with relatively high concentration (0.1-1%), the Li+ are most likely restricted to the surface of the nanoparticle where the destabilisation effect of the larger dimension and the different charge is minimised. Close to the Li sites Ni3+ are formed due to charge neutralisation. This Ni3+ is forced to remain in this oxidation state by the proximity of a Li+ and cannot contribute to the equilibrium Ni2+-Ni3+. When the concentration of Li is low, the electrical activity of NiO is preserved since only few Ni3+states are locked, see Figure 4.6 A and B. When the concentration of Li is able to saturate the surface with Ni3+ions, the surface becomes inactive, see Figure 4.6 C. Lithium then works like a passivation agent building a tunneling barrier on the surface of NiO. The effect of the tunneling barrier is clearly shown in the voltammogram of Figure 4.2, where at high concentrations of Li the inert region of the system increases to about 1.5 V.

Figure 4.6. Surface states composition at various Li doping concentratios. The green dots represent the Ni3+ states in equilibrium with Ni2+. The red Ni3+ are the one locked by the Li ions (in blue).

4.4 The recombination issue From the beginning of this thesis is clear that suppressing any kind of recombination reaction is the first step towards a working DSC or DSFC, ensuring a good quantum efficiency. In section 4.2 the relation between the recombination and the Voc was also discussed. The dye-hole recombination, see section 1.2, is considered one of the major issue of p-type DSC since often occurs so quickly to prevent the regeneration reaction. The dye-hole recombination was extensively studied by the community in the field of n-type DSC [139–142]. Two kinds of dye-hole recombination can be identified: the geminate recombination and the non-geminate recombination. In the geminate recombination the electron in the reduced dye

45 and the hole in the nanoparticle recombine before diffusion of one of the two occurs, see Figure 4.7 left. This might occur when the electron-hole pair are still coupled and feel the electrostatic attraction of each other [143–145]. Normally geminate recombination are associated with first order kinetics. The non-geminate recombination occurs after the hole or the electron or both have diffused away from each other [136, 141, 146–148], see Figure 4.7 right. Usually this is the kind of recombination observed in DSC. The hole can diffuse in the core of the nanoparticle or even jump to a neighbouring nanoparticle. The electron instead can diffuse on the surface of the nanoparticle jumping between dyes by a self exchange reaction. Normally, the non-geminate recombination does not exhibit a specific reaction order, in this case the kinetics is complicated by the presence of defect states in the nanoparticle and by inhomogeneity of the surface. The non-geminate recombination kinetics is often fitted with a stretched exponential function, see Section 3.2.2. The usual β values that are measured for recombination decays are between 0.6 and 0.2.

Figure 4.7. Left: geminate recombination. Right: non geminate recombination.

There are many factors that can retard the dye-hole recombination. An important factor is to decouple the hole from the electron in the dye. For example longer recombination rates are observed when the dye features an electron withdrawing moiety or when the electron is located far away from the surface of the nanoparticle, i.e. from the hole [60, 146, 149, 150]. Dyes based on metal complexes (Ir-group complexes) usually show longer recombination kinetics than organic dyes [151, 152]. In fact inorganic dyes benefit from the effect of the metal core that decouples the electron from the hole. There are many studies that investigate the fundamental mechanism of the recombination. One of the most significant studies, in n-type DSC, examines the relation of the recombination rate and the quasi-Fermi level position with respect to the oxidised dye reduction potential [139, 142]. In an open-circuit DSC the quasi-Fermi level position is affected by the rate of injected charges, the rate of recombination and the shape of the DOS of

46 the band in consideration. It was estimated that during normal working condition the conduction band of TiO2 contains about 10 electrons per nanoparticle [43]. The recombination kinetics can be studied with transient absorption techniques which do not work in stationary state conditions. In a pump/probe experiment the number of injected electrons per particle can be estimated by the experimental conditions: number of nanoparticles per electrode area, absorption of the sample, injection eﬃciency and photons per laser ﬂash. Thus, the position of the quasi-Fermi level can be changed varying the intensity of the excitation, see Figure 4.8 left.

Figure 4.8. Eﬀect of the light intensity to the quasi-Fermi level, left. Experi- mental setup for a potential controlled TA measurement.

The quasi-Fermi level can also be controlled by electrochemical methods. Using a three electrode cell and placing the photoactive electrode in the WE position, the Fermi level of the semiconductor is fixed vs. the reference by the applied potential, see Figure 4.8 right. Keeping the light intensity fixed, the quasi-Fermi level position is determined by the applied potential. In the work of ref. [139, 142] the two experiments lead to the same conclusion. When the quasi-Fermi level is on the bottom of the CB, i.e. in low light intensities (≤10 injected electron per particle) or low bias, the recombination is slow. While at high light intensities or high bias the recombination becomes fast. With this condition the recombination rate could vary from picoseconds to milliseconds. The recombination showed the same kind of kinetics, close to a first order whose time constant was smoothly changing trough the potential change. The interpretation that is given to these ob- servations is that the recombination rate is different if the electron resides in a trap state or inside the CB. A trap state is a localised state in the nanoparticle that is able to host an electron whose potential is located in the band gap. Since these states are localised, their wavefunctions do not extend far, meaning that electron transfer (by tunneling) becomes more improbable with distance. Instead, the electron in the CB is more reactive. First of all it is more energetic which might increase the electron transfer

47 rate, secondly the wavefunction of such an electron is very delocalised and might overlap better with the empty orbitals in the oxidised dye. There is a possibility that an electron in a trap state is excited by thermal energy into the CB. In this case the recombination from this trap state is faster than normal. This kind of trap states are called shallow traps while the traps whose electron cannot be excited to the CB by thermal energy are called deep traps. This model is widely used in n-type DSC where it was found to describe experimental behaviour in multiple cases [146, 153–155]. The behaviour of the dye-hole recombination in NiO DSC with respect q to the EF was not examined before the results of Paper II. In Paper II we performed the same experiment done with TiO2 using instead a NiO based photocathode. The two systems were considered the specular image of each other thus a similar result was expected: a faster recombination kinetics at more positive quasi-Fermi level. We used Ru-NMI dye as sensitiser which was previously characterised by our group, see Sec. 3.1.2. The reduced dye spectrum is very characteristic, see Figure 3.3, and does not interfere with the NiO-hole. The recombination kinetics was measured as a function of the excitation light intensity and applied bias potential, see Paper II. The intensity of the excitation was varied to produce from 0.3 to 4.9 injected hole per particle (IHP), this should be enough to shift substancially the quasi-Fermi level and produce an alteration of the kinetics. The applied potential was set using as reference the NiO DOS measured in Paper I. Thus the Fermi level was set from 100 mV above the surface states, where the NiO DOS is practically 0, to 200-300 mV internal to the VB. The measured kinetics showed a clear biphasic behaviour, see Figure 4.9 and Figure 2(II), an initial monoexponential phase followed by a stretched exponential phase. In contrast to expectations the rates did not change

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Figure 4.9. Recombination kinetics of the Ru-NMI-NiO system (ex. 460 nm, q probe 560 nm). Eﬀect of the applied bias potential (i.e. EF ) on the biphasic ricombination kinetics. -Reprint from Figure 3(II)

48 that much upon light intensity variation or applied bias potential. The only variation we could observe was the relative amplitude of the two phases. This was attributed to the presence of two kinds of holes, a fast reacting one (by the monoexponential phase) and a slow reacting one (by the stretched exponential phase). The fast reacting hole can be converted to the slow reacting hole. The conversion kinetics determines the amplitudes of the two kinetic phases. To be able to see the two phases the rate of conversion should be close to the rate of the fast recombination. This is rationalised considering the following reaction system:

where hf is the fast reacting hole, that is assumed to be the hole that is generated by injection; hs is the slow reacting hole; D is the product of the recombination; kf , ks and kc are the rate constant, respectively, of the fast recombination, slow recombination and hole conversion (called relaxation in Paper II). The biphasic behaviour can be analysed by the following kinetic model. The signal measured in the TAS is the signal of the reduced dye that corresponds to the sum of the amount of the two kind of holes:

− Δabs ∝ [dye ]=[hf ]+[hs] the rate associated with hf and hs consumption are given by:

d h d h − [ f ] h · k h · k − [ s] − h · k h · k dt =[ f ] f +[ f ] c dt = [ f ] c +[ s] s hf is consumed in the hole conversion and in the fast recombination while hs is produced by the hole conversion and is consumed in the slow recombination. After few mathematical passages the sum of the concentrations of the two holes as a function of the time is:

[hf ] [hs] k −(kf +kc)·t c −ks·t −(kf +kc)·t [hf ]+[hs]=[hf ]0 · e + e − e kf − ks + kc rearranging k k c −(kf +kc)·t c −ks·t [hf ]+[hs]=[hf ]0 · 1− ·e + e kf − ks + kc kf − ks + kc

49 The first exponential decay in the sum is the fast recombination plus the hole conversion while the second term of the sum is the slow kinetics. It is easy to see now that the magnitude of the hole conversion rate is crucial for the observation of the biphasic kinetics. There are two limiting cases where the biphasic kinetics does not appear. When kc kf the second term of the sum is basically 0 since the fraction nulls, only the first term remains which is just the fast recombination since it is the dominant in the sum. When kc kf the first term of the sum nulls and only the second one remains which is the slow recombination. This is shown graphically in Figure 4.10 where the kinetic constant kc was varied to reproduce the two limit cases and the case where fast recombination and hole conversion compete.

Figure 4.10. Graphic representation of the two limit cases of the competitive kinetics and one non limiting case: dashed-dot line is for the slow limit case, the dashed one for the fast limit case, the full line for the non-limiting case.

Even though the kinetic of the slow recombination observed in Paper II is not a simple exponential we can see that the shape of the decay resembles the competitive non-limiting case.

In Paper II the change in amplitude of the two decays was attributed to a change of the hole conversion rate due to a diﬀerent driving force induced by the diﬀerent position of the Fermi level (bias potential). At the time a more detailed explanation was not possible due to lacking of information about the nature of the observed holes. With the new results found in Paper III it is possible to assign the fast recombining holes to the ones 3+ 4+ trapped by Ni sites (Ni(h+), see below) while the slow holes are the ones 2+ 3+ trapped by Ni sites (Ni(h+)). The bias potential changes the ratio of

50 these two traps on the surface of NiO as it is shown in Paper III.Inthat work is also shown that Ni2+ and Ni3+ states are in equilibrium with each other which might explain the driving force change observed with the bias potential variation. In other words the hole conversion observed in Paper 3+ 4+ 2+ 3+ II is the conversion of a Ni -hole, Ni(h+),toaNi -hole, Ni(h+),which are in equilibrium by the chemical equation:

keq =kc 4+ 2+ −−−−−→ 3+ 3+ , Ni(h+) +Ni ←−−−−− Ni +Ni(h+) (4.1) k−eq

The kinetics of this reaction is governed by the concentration of the respective species. The hole conversion discussed above is fast when the concentration ratio [Ni3+]/[Ni2+] is small, i.e. at negative potentials (high driving force) and it is going to be slow when the concentrations ratio [Ni3+]/[Ni2+] is high (low driving force).

The biphasic kinetics of Paper II could also have had another explanation. Assuming that the trapping rate does not depend on which trap the hole end up in, the ﬁnal concentrations of the two trapped holes are proportional to the initial concentration of the trapping states, i.e. the concentration of Ni2+ and Ni3+, respectively. The above assumption was proven to be not true in Paper IV. In fact it was found that a hole can be trapped by a surface Ni3+ state on a subpicosecond time scale while the Ni2+ traps are active in the nanosecond time scale. This will be discussed in more detail in the following sections.

The rate of the slow recombination phase of Figure 4.9 is remarkably low. The stretched exponential used for fitting it featured a β value of about 0.15 and a τ of ∼100 ms. The prospective to be able to build a p-type DSC with such slow dye-hole recombination was resuscitating the hope in NiO based DSC. Such slow dye-hole recombination coupled with similarly slow electrolyte recombination could give the possibility to increase the Voc and the charge transport to finally make the NiO cell working. Assuming that the competitive reaction model is correct there are two ways to increase the ratio slow-holes/fast-holes. The first is to inhibit the fast recombination kinetics to a point that it is irrelevant in the rate competition. Some results found in literature show a large jump in recombination lifetimes upon a not so important change in molecule design of NiO-dye interface [73, 150]. This could be explained by the competition of the fast recombination and the hole conversion. A small change in the system properties could lower the fast recombination rate by one or two orders of magnitude. This could shift the system to a slow recombination phase suddenly improving the overall recombination kinetics by many orders of magnitude.

51 Another way to increase the population of slow reacting holes is to make the conversion reaction much faster to be more competitive against the fast recombination. This is what directed my research in understanding the basics of the trapping process in NiO nanoparticles and the phenomenon of trap interconversion, as described in the following sections.

4.5 NiO trap characterisation A trap in solid state physics is a state that can host an electron or a hole and does not belong to the band structure, i.e. it is located in the band gap [107, 108].6 A trap state is physically located in a defect of the crystal lattice which can be an ion vacancy, an interstitial or substitutional impurity, misalignments of the crystal, or interruption of the crystal. Defects are mostly found on the surface of the crystal since it is easier for the crystal to stabilise them by lattice modiﬁcation. Often a trap is located to a point defect thus its energy is well deﬁned. Due to the random nature of these states their distribution in energy is considered to be exponentially decreasing from the band edge towards the band gap, see Figure 4.11 left. Sometimes one kind of defect might occur more often than others, for example in the case of impurities. In those cases the distribution of the energy of these states results in a small band in the DOS, see Figure 4.11 right.

Figure 4.11. Illustration of the diﬀerent distribution of defects in the energy scale producing two DOS shapes. On the left the exponential distribution of the defects, on the right the impurities defect with local, discrete populations.

It is clear that the high ratio surface/volume of the nanocrystals allows a high concentration of defects. The presence of trap states becomes an important factor to consider in the study of the physical and chemistry properties of nanomaterials.

6There is the possibility to ﬁnd a trap state at energies internally to a band but these traps are considered less important from a chemical point of view.

52 Deﬁnition box: traps, empty traps and ﬁlled traps.

Talking about trap states for holes and for electrons in the same discussion can be confusing because of the nature of the hole, i.e. an electron vacancy. In Paper IV the discussion can be even more complicated by the fact that Ni3+ states can work as either electron traps or hole traps. The nomenclature of traps in this thesis is: • An electron (or a hole) trap is a state that can host an electron (or a hole). In the electron trapping reaction: Ni3+ +e– −−→ Ni2+ 3+ 2+ Ni is the trap and Ni(e−) is the trapped electron. The corresponding nomenclature is used for hole traps; • An electron trap is said to be “empty” when its energy level, located above the Fermi energy, does not contain an electron. It is said to be “filled” when it does contain one; • A hole trap is empty when its energy level, located below the Fermi energy, does contain an electron, on the contrary it is filled when it does not. The comment “located above/below the Fermi energy” that I used in the description is fundamental for understanding that not any empty level can be considered a trapped hole (or not any filled energy level is a trapped electron).

The distribution of trap states found in TiO2 nanocrystals used in DSC is mostly exponential [156]. In NiO instead the distribution of traps is more aﬀected by impurities; thus it resembles the DOS on the right of Figure 4.11, see Figure 4.4. Moreover, the concentration of traps in NiO is about

10 times more than the one in TiO2, see Paper I. 3+ The chemical nature of the NiO (trapped) hole is believed to be a Ni(h+) state. This hypothesis is supported by years of fundamental and experimental research on NiO bulk properties [109, 112, 157, 158]. Even though the exact determination of the electronic structure of NiO is still under debate, it was proven that the NiO VB edge has mainly Ni 3d character [159, 160]. Furthermore, it was already mentioned that the conductivity of bulk NiO increases with higher concentration of Ni3+ defects (e.g by Lithium doping or oxygen partial pressure variation). The presence of Ni3+ in NiO nanoparticles is clear by the color of the mesoporous ﬁlm. The NiO band gap is ∼3.5 eV thus the mesoporous ﬁlm should appear transparent or white, 3+ it is instead brownish. The two known oxides of Ni ,Ni2O3 and NiOOH, are both black. The brownish color of NiO is then attributed to Ni3+ impurities. A NiO mesoporous electrode is electrochromic, it changes color

53 upon electric stimuli. In particular, the film turns transparent if a negative potential is applied while it changes to black if a positive potential is used. This phenomenon is again attributed to the conversion Ni2+/Ni3+ and, for mesoporous NiO, it was studied for the first time by Boschloo who performed spectroelectrochemistry on a NiO film in water [161]. He found that mesoporous NiO can perform two reversible oxidations one around +0.35 V (vs. SCE) and another at +0.70 V. The two oxidations produced two similar UV-Vis spectra. The assignment of these oxidations to a chemical species was done later by Dini’s group, using XPS. They assigned the first oxidation to Ni2+/Ni3+ and the second to Ni3+/Ni4+ [162]. The potential at which Ni2+ is oxidised is slightly above the NiO VB edge, while the 3+ oxidation potential of Ni is slightly below the VB edge (EVB ∼250 mV 3+ 4+ vs. Ag/AgNO3 [74]). It is then possible that both Ni and Ni can be produced by hole trapping. This could explain the chemical nature of the

Figure 4.12. Assignment of the DOS peaks found in Paper I and the chemical explanation of the fast and slow recombination of Paper II. The “unknown” peak in the DOS is probably due to another Ni2+ oxidation to a different Ni3+ form. phenomena seen in Paper II. In a first place Dini’s result can be used to assign chemical species to the peaks of the NiO DOS found in Paper I, see Figure 4.12 left. Secondly, this assignment is used in Paper II to interpret the fast and slow recombination, see Figure 4.12 right. With this assignment it became clear that the characterisation of the hole relaxation is required for the understanding of the mechanism behind 3+ 4+ the conversion of the trapped Ni(h+) hole to the Ni(h+)hole. This became possible with the spectroelectrochemistry findings of Paper III. The com- bined use of NiO DOS and NiO spectroelectrochemistry gave, for the first time, the characteristic UV-Vis spectra of Ni3+ and Ni4+ states in mesoporous NiO, Figure 4.13. The spectroelectrochemistry (SEC) in Paper III

54 was recorded in acetonitrile with LiClO4 as supporting electrolyte, this closely resembles the condition of a DSC. In fact, in the spectroelectrochemistry performed in buﬀered water by Boschloo, the NiO surface can exchange protons in the redox processes. Instead, the scarce amount of water present in the acetonitrile solution used in Paper III limited the ion exchange to the supporting electrolyte, in this case Li+, which is similar to the electrolyte used in DSC. The assignment of the characteristic spectra to the Ni3+ and Ni4+ chemical species was further supported by optical characterisation of the chemical and physical reduction of the Ni3+ impurities, see Figure 6(III).

Figure 4.13. Diﬀerence of spectroelectrochemistry (Δ-SEC) extracted using DOS as a guideline (voltages vs. Ag/AgNO3). The green Δ-SEC corresponds to the Ni3+ spectrum; the blue spectrum is associated with the Ni4+ species. -Reprint from Figure 9(III)

4.6 NiO surface trap dynamics 3+ The discovery of the characteristic spectra of the two trapped holes, Ni(h+) 4+ and Ni(h+), gave the possibility to study their chemistry, in particular their interconversion dynamics, by transient absorption spectroscopy. As emerged in Section 4.4, the hole conversion should occur in a time scale between 10-100 ns, which is a range that can be studied by ns-TAS. Here, the basic idea is to photoproduce, as rapidly as possible, a population of 4+ 3+ Ni(h+) holes and follow their conversion to Ni(h+) with time. Once the kinetics of the conversion is known, changing a reaction parameter allows for the elucidation of the actual chemical mechanism. For example, changing

55 the temperature or the initial concentration of the reactants, helps in the understanding of the reaction order and energetics. There are mainly two strategies that can be used to photoproduce holes in a semiconductor: photoinduced hole injection and direct excitation by fundamental absorption. The photoinduced hole injection requires the use of a sensitiser which should feature several characteristics: 1) it should have an excited state reduction potential aligned with the 4+ ≥ VB of NiO deep enough to be able to populate the Ni(h+), i.e. 0.7 V vs NHE; 2) the hole injection should occur at least one order in magnitude faster than the hole conversion. Otherwise the hole injection becomes a limiting factor in the reaction system, which makes the hole conversion impossible to be resolved. This excludes, for example, Ru-NMI that injects a hole in NiO in about 5 ns7; 3) the dye regeneration should be faster than the fast recombination, i.e. faster than 100 ns. This is very improbable since most of the regeneration agents used in liquid DSC work in a diffusion controlled regime which occur in 0.1-1μs.8 As evident from this description the dye sensitisation strategy appears to be quite complicated. Therefore, the fast hole dynamics was studied by band gap excitation which offers the advantage to produce an immediate electron-hole pair and can be studied by ns-TAS and ultrafast TAS. This was done in Paper IV. NiO band-gap (BG) is about 3.55 eV corresponding to a photon of wavelength 350 nm, the absorption tail extends to 370 nm allowing our tripled Nd:YAG laser pulse at 355 nm to be used for BG excitation (BG ex.). In Figure 4.14A the scheme of the BG ex. of a semiconductor is represented. After the excitation, reaction 1, an electron-hole couple is formed. In doped semiconductors or in presence of impurities (like the NiO case) the e−-h+ couple might form a bound exciton which means that the created charges are spatially close to each other (this will be considered later in the discussion). The presence of empty levels below the CB might separate the bound exciton and trap the electron, reaction 2. Simultaneously the hole can be trapped in filled levels above the VB, reaction 3. If the Fermi level is in the middle of the band-gap the annihilation between the trapped electron and the trapped hole can occur, reaction 4. TAS performed in system A of Figure 4.14 would show the transient absorptions (TA) of the CB e− and the VB h+ at early times after exci-

7Data measured by Dr. Allison Brown in our lab but not published yet. 8 – – P1 dye and the I /I3 redox couple is a singular case where the regeneration agent, – I3 , forms a pre-associated complex with the dye that speeds the regeneration kinetics to sub-nanosecond time scale. An attempt with this system was done but the absorption of the products of the regeneration and the following electrolyte recombination made the study much more complex than expected with no clear results.

56 Figure 4.14. Scheme of the BG excitation: A) BG ex. with no electron scavenger and no acceptor energy level above the FL; B) a BG ex. with electron scavenger but no acceptor energy level above the FL; and C) a BG ex. with electron scavenger and acceptor energy level above the FL.

tation and at later times the trapped e− together with the trapped h+. With the characteristic spectra of every species, the reactions 2, 3 and 4 can be resolved. The signal from the CB e− and the VB h+ should feature broad bands due to their nature of free particles [163]. The absorption of the trapped e− or h+ might be more structured than the free particle absorption since they correspond to a localised species. An electron scavenger9 might be used to remove the electron from the conduction band and leave only the hole in the semiconductor. Figure 4.14B shows the reaction scheme with this experimental conﬁguration. Here reaction 4 becomes the electron extraction towards the scavenger. This electron transfer can happen very quickly (0.1-10 ns), it can even anticipate the electron trapping. This could simplify the investigation of reaction 3 since the signal of the reduced electron scavenger is known and can be subtracted from the TA. In Figure 4.14C the same experimental conﬁguration is showed but in this system there are also energy levels above the Fermi level that can act as electron deep traps. Since the energy of these traps is below the LUMO of the scavenger, the CB electron might fall into one of these traps before being caught by the e− scavenger. In Paper IVwas found that bare NiO nanoparticles adopt this last behaviour. In fact, in bare NiO the high level of Ni3+ impurities locates the Fermi level in between the oxidation potentials of the Ni2+ and Ni3+ states. This potential is roughly 150 mV

9An electron scavenger is a substance that has the right reduction potential to be used to remove an electron from a system possibly in an irreversible way. Normally in photochemistry a solution of methylviologen is used with this scope.

57 10 vs. Ag/AgNO3 , which is far below the edge of the NiO DOS, see left of Figure 4.12 as reference. This leaves empty electron traps in the form of 3+ Ni at around -100 mV vs. Ag/AgNO3 and empty hole traps at around 3+ +400 mV vs. Ag/AgNO3, both in the form of Ni . After BG excitation these states can operate the following reactions. The Ni3+ can perform 2+ 4+ electron deep trapping toaNi(e−) state and hole trapping toaNi(h+) state, respectively reaction 4 and 3 in scheme C. The electron deep trapping is differentiated from the electron shallow trapping that is considered to be reaction 2 in scheme C. fs-ps hole-electron trapping. In Paper IV the characterisation of the reaction 2, 3 and 4 occurring in bare NiO, was performed by fs-TAS. In Figure 4.15 A the decay associated spectra (DAS, Sec.3.2.2) of the fs-TAS recorded after BG excitation are reported. We were able to use the optical signatures of the trapped species that we identified in Paper III to follow the formation and disappearance of the different trapped species, see Figure 4.15 B.

Figure 4.15. A:Decay associated spectra of the fs-TA of NiO after 330 nm excitation. -Reprint from Figure 4 of Paper IV- B: Comparison of the DAS 126 ps and 2.9 ps with the associated species, Ni3+ and Ni4+, characteristic spectra found in Paper III.

Brieﬂy, a 2.9 ps process was assigned to trapping of the electron in shallow traps, just below the CB, based on the disappearance of the absorption of the free electron (τ1 DAS). Unfortunately we were not able to identify the shallow state where the CB e− is trapped. The hole is trapped into a 4+ τ Ni(h+) state on a subpicosecond time scale, see 3 DAS in Figure 4.15 A and Ni4+ spectrum in Figure 4.15 B. The deep trapping of the electron was

10This was measured electrochemically by me.

58 2+ τ found to occur in 126 ps into a Ni(e−) state, see 2 DAS in Figure 4.15 A and Ni3+ spectrum in Figure 4.15 B. It is not really clear why the Ni3+ trap is reactive more towards the hole than towards the electron. One possible explanation involves the concept of a bound exciton discussed in the beginning of the section. In the following paragraph I describe this hypothesis which needs to be considered only as a possible explanation of the data that does not have an experimental conﬁrmation yet.

An exciton is usually attracted by impurities where it is stabilised by electrostatic forces, it forms then a bound exciton [108]. If a bound exciton is created in a NiO nanoparticle after BG excitation it will probably locate close to a Ni3+ impurity, see Figure 4.16 left.

Figure 4.16. Representation of a bound exciton in a NiO nanoparticle after BG ex. close to a Ni3+ impurity.

There is a clear possibility that an electron transfer occurs either from the e− of the exciton to the Ni3+ or from the Ni3+ to the h+ of the exciton. If the electron transfer occurs in one step, then the reduction of the hole, 4+ to create Ni(h+), seems to be the most likely to occur. In fact the potential difference between the VB h+ and the Ni4+/3+ couple is about 0-200 mV, while the one between the CB e− and the Ni3+/2+ is about 3 V. The electron transfer does not occur with an emission of a photon since no fluorescence was detected upon BG excitation. Therefore the excess energy needs to be dissipated by heat, i.e. by phonon emission. The emission of a phonon with energy greater than 1.5 eV is practically impossible. Thus, this electron transfer needs to be coupled with a multiphonon emission which makes it less probable than the electron transfer involving the VB h+. These concepts are similar to the ones at the basis of the Marcus theory for the kinetics of non-radiative molecular electron transfer [164, 165]. After reduction of the VB h+ the electron is left alone in the CB and is then free to move, see Figure 4.16 right. Moving, the CB e− can fall in a shallow trap and finally in a Ni3+ deep trap.

59 ns trap annihilation. After deep trapping the electron is located in 2+ 4+ aNi(e−) state and the hole in a Ni(h+) state. These two species can react and restore the initial state of the system, see reaction 5 in Figure 4.14C:

4+ 2+ −→ 3+ 3+, Ni(h+) +Ni(e−) Ni +Ni (4.2) This reaction is seen as a trap annihilation since after this process there are no trapped particles. In Paper IV the trap annihilation was studied by ns-TAS since it occurs on the ns-μs time scale. The reaction was observed to follow a second order kinetics with a half-life time of ∼200 ns, see Figure S11(IV). A second order kinetics is in agreement with known models for electron-hole recombination, that assume a random walk of the species in mesoporous semiconductor surface [166]. The reaction of electron and hole trapping followed by annihilation of the traps is well known in the semiconductor ﬁeld. Few examples where it has been previously observed are:

ZnO, SnO2,Fe2O3,Co3O4, and TiO2 [166–173]. Regarding this last case, there are proofs that the oxygen vacancy and the hydroxyl groups present on the surface of TiO2 can act as electron and hole traps [34, 171, 173, 174]. The characterization of NiO electron-hole trapping is then consistent with other results from the community.

The observation of the second order kinetic is a sign that the species 4+ 2+ Ni(h+) and Ni(e−) are uncoupled and need to migrate on the surface of the nanoparticle to meet and react. It was not possible to understand which 4+ 2+ species is the more mobile one, Ni(h+) or Ni(e−). Both of them might hop between Ni3+ sites by a self exchange reaction:

4+ 3+−→ 3+ 4+ Ni(h+) +Ni Ni +Ni(h+) 2+ 3+−→ 3+ 2+ Ni(e−) +Ni Ni +Ni(e−) The migration could occur by random walk, this means that the kinetics of the trap annihilation could be altered by obstruction of the charge hopping. An attempt to decrease the surface available for the diﬀusion of the trapped particles was tried by doping the NiO with inert bivalent cations like Ca2+ or Mg2+. In fact, a substitution of a Ni2+ cation by these two inert ions would remove an active hop-site, this should deviate the path of the random walk. Doping concentration between 0.1 and 1 mol% were used and BG ex. TAS measurement were performed, the normalised traces are shown in Figure 4.17. In the presence of the doping cations slightly faster kinetics is observed. The reduced area available for the random walk could force the trapped charges to meet in shorter times. Even though these are just preliminary data and a complete study still needs to be done, it seems reasonable to think that the trapped charges are free to move on the NiO surface.

60 Figure 4.17. Change of TA kinetic traces of Ca2+ doped (left) and Mg2+ doped NiO (right) with respect to bare NiO. The traces were recorded using the same absorbed energy per pulse.

The second order rate constant of the annihilation reaction could give an idea of how fast this hopping is. Unfortunately the kinetic constant could not be estimated due to the lack of information about the extinction coeﬃcients of the various involved species11.

This mechanism of a surface diﬀusion controlled annihilation is highly aﬀected by the surface concentration of Ni3+ species. In fact,the hopping mechanism can be inhibited if the concentration of Ni3+ sites is low. If 4+ 2+ 3+ the Ni(h+) holes are surrounded by Ni sites they can be oxidised to Ni 3+ producing a Ni(h+):

4+ 2+−→ 3+ 3+ Ni(h+) +Ni Ni +Ni(h+)

In the TAS measurements this reaction would show a monoexponential de- 4+ cay of the Ni(h+) species since it belongs to the class of quasi-first order 2+ 4+ reaction ([Ni ]»[Ni(h+)]). The observed rate of such a reaction is directly proportional to the concentration of Ni3+. Attempts to control the concentration of Ni3+ in the NiO films resulted in limited success. As discussed in Paper III the factors affecting the Ni3+ concentration are temperature, oxygen partial pressure and humidity. This made the Ni3+ concentration study unreliable. Moreover the initial concentration of Ni3+ of the freshly prepared NiO film is unknown and difficult to measure. If the surface concentration of Ni3+ could be determined, a further in- sight into the mechanism of charge hopping could come from a temperature study of the trap annihilation kinetics. The temperature study could clar- ify if the hopping of the trapped states occurs across an energy barrier,

11By knowing the extinction coeﬃcient of the Ni4+ spectrum an estimation of the 4+ 2+ initial concentration of the Ni(h+) and Ni(e−) could be done.

61 moreover such a study could measure the activation energy of the trap annihilation process.

Trap annihilation vs. hole relaxation. In Paper IV the annihilation reaction, equation 4.2, is assumed to have the same kinetics as that of the 4+ 3+ Ni(h+)-Ni(h+) hole relaxation, see equation 4.1 and Paper II. This assumption is based on the similarity of the species involved in the two processes. For this assumption to be true certain conditions are required, which I will discuss. In Figure 4.18 the two processes are presented schematically. The VB and trap states are sketched with the same representation used in Paper IV Figure 8. On the left side of each box, the ﬁlling of the diﬀerent Ni

Figure 4.18. Scheme of trap annihilation, A, vs. hole relaxation, B.Ineachbox the scheme on the left represents the electron occupation of the trap states and the VB states (the pink area represents the VB, and the squares and triangles are the traps, and the gray area gives the experimental reference of the DOS); the schemes on the right represent the shape of the distributions of population of the species participating in the reaction respect to the potential. states (squares and triangles) and the VB location (pink area) are represented. A white symbol represents an electron vacancy while a black one represents an electron occupancy. On the right side of each box, the green scheme represents the ﬁlling of the surface DOS of NiO found in the two cases; trap annihilation and hole relaxation. The white and dark areas on top of the green DOS represent, respectively, the trapped h+ and trapped e− population distributions created by BG ex. In the trap annihilation, i.e. when these two populations are created by the BG ex., the Fermi level is down-shifted with respect to the edge of the VB DOS12.Thetwopop- ulations have the same size and are at opposite sides of the Fermi level, see Figure 4.18 left. This is also represented in the scheme A-left by the

12As discussed previously, this is due to the high concentration of Ni3+ states present in bare NiO

62 annihilation of one electron/hole couple. In the case of trap relaxation, there is only the white area since there are no trapped e−. The holes are injected from the dye to the VB and trapped in the Ni3+ states, represented by a few empty triangles in the scheme B-left. The empty deep hole traps, i.e. Ni2+ states, are needed for the relaxation to occur. They are represented by the filled squares in B-left and by the dotted green area in the DOS, scheme B-right. In the presence of Ni2+ states, the Fermi level is up-shifted, see left schemes. This should be the case for a fully assembled DSC, where the redox couple should reduce part of the Ni3+ states during the equilibration of the cell, moving up the Fermi level. The two kinetics, of the annihilation and the relaxation, will be similar when the distributions of the respective empty states and filled states have similar sizes and shape. The kinetics that was measured for the trap annihilation was obtained producing roughly 5 to 10 holes per particle. In Paper II, instead, where the trap relaxation was probed, the excitation produced about 1 to 5 holes per particle. The rates of the two reactions should be comparable. More precisely the trap relaxation, in an unbiased cell, could occur with faster rate since the concentration of empty traps, the Ni2+ states, is probably larger than the one of the injected holes. The trap relaxation rate that was estimated in Paper II was of about 10-100 ns. The trap annihilation measured in Paper IV featured a half-life time of about 200 ns, which is close to the expected range. 4+ As final proof that the Ni(h+) hole trapping and a following hole relaxation can occur also in a sensitized NiO film, in Paper IV a TA experiment was performed exciting Ru-NMI adsorbed on NiO nanoparticles, see Figure 4.19. Clearly, 20 ns after excitation, the spectral features of the reduced dye together with characteristic peaks of Ni4+ were observed. Instead after 5 μs the features of the Ni4+ disappeared leaving only the TA spectrum of the reduced dye proving the hole relaxation. It is not really clear how these holes can feature so different recombination kinetics. Assuming that both of them are localised states, one hypoth- 4+ 3+ esis is that the Ni(h+) is the mobile species on the NiO surface while Ni(h+) 4+ is not. This would make Ni(h+) much more efficient in finding the reduced 4+ dye site. Other possibilities are that the Ni(h+) is not a localized state, 4+ or that the hole in Ni(h+) can easily be excited in the valence band while 3+ Ni(h+) cannot. This can be due to the different redox potential position of 4+ 3+ the traps: Ni(h+) is very close to the VB, 0-100 mV; Ni(h+) is placed far 4+ upwards, -400 mV. If the Ni(h+) holes are in communication with the VB, the delocalization increases the probability of the hole to react with the reduced dye. These concepts are only hypothesis that have not yet been tested, a future study might reveal the real mechanism.

63 Figure 4.19. Transient absorption of Ru-NMIsensitized on NiO and excited at 460 nm. The green trace is the 5 μs trace normalized to the 20 ns one at 500 nm. -Reprint from Figure 9(IV)

Summarising, in Paper IV we have then demonstrated that the hole relaxation proposed in Paper II can actually occur in NiO nanoparticles. 4+ 3+ It involves the conversion of a Ni(h+) species to a Ni(h+) species by the oxidation of a Ni2+ state. This conversion occurs in 10-300 ns time scale and its kinetics is dependent on the concentration of Ni3+ and Ni2+ on the NiO surface.

64 4.7 The role of the NiO surface in DSCs operation The results presented in the previous section are probably the most important ﬁndings of my doctorate research. The discovery of the exceptional hole trapping activity of the Ni3+ states emphasizes the importance of the role of the surface in the hole recombination, stabilization and transport.

It is now clear that the Ni3+ states, normally present on the surface of the NiO nanoparticles, are surprisingly active in trapping a VB h+ as 4+ Ni(h+). Moreover, these trapped holes can move on the surface of the NiO 3+ quite rapidly and possibly convert to a Ni(h+). The stabilization of the trapped particles is crucial for the existence of this mechanism. In bare NiO the stabilisation of the trapped charges is offered by the edge of the crystal. As described in Paper III,theNi3+ sites can be found in the form of Ni2O3 or NiOOH. The latter could offer a further stabilization to 4+ the Ni(h+) by distortion of the hydroxide bond. The hydroxyl group could distance the proton from the metal center or sharing it with a neighbouring oxygen. Furthermore, this hydroxyl distortion could also mediate hole hopping between the two Ni3+ sites. This stabilization mechanism is not really applicable when the dye is adsorbed on the surface. In a working cell, the electrolyte is wetting the nanoparticle surface, that most likely will affect the movement and stabilisation of the traps. In Paper III an experiment of electrochemical dye desorption suggested that the dye easily detaches when linked to Ni3+ sites. Thus, the space on the NiO surface that is left open from the dye, is exposed to the electrolyte and the redox couple. Since these exposed sites are mainly Ni3+ they can quickly trap the injected hole with the mechanism proposed above and work as a funnel for electrolyte recombination. This hypothesis is supported by the findings of Paper III where the removal of 3+ the Ni states resulted in an increase of the Voc. This empathises that elimination or the destabilization of the Ni3+ surface state is then crucial for preventing, besides dye-hole recombination, also electrolyte recombination.

In Paper III emerged that the cations adsorbed on the surface are fundamental for charge stabilisation. It has been proposed that the electrostatic neutralisation of the trapped hole might occur by expulsion of the surface adsorbed ion into the electrolyte bulk, see Paper III and [161, 175]. The ion adsorbed on the surface of the nanoparticle, in the Helmholtz layer, can detach and pass through the diﬀusion layer when the injected charge reaches the surface. This mechanism was used to explain the large diﬀerence in charge extraction observed in the DSCs of Paper III. In that study two methods for reducing the Ni3+ impurities in NiO were used: a chemical reduction

65 ◦ by NaBH4 and a physical reduction by heating the film at 200 C. The film were sensitized with P1 dye and the DSCs were tested. The cells prepared with the heating method showed one order in magnitude less extracted charges than the cells prepared with the other treatments, see Figure 5(III). This was explained by a lacking of trap stabilization in the heated-DSC. In fact, the two methods, chemical and physical reductions, modifies the NiO surface differently. The chemical method leaves cations adsorbed on the surface while the physical method does not. The cation 2+ 3+ left from the chemical reduction allow the Ni state to host a Ni(h+) hole stabilising it by ion expulsion. The physical method instead does not allow any of the surface states to host any hole. On this topic, it should be mentioned that the amount of charges extracted from the untreated and chemically treated samples in Paper III is much larger than the one usually extracted in n-type DSC (1019-1020 h+/cm3 vs. 1018-1019 e−/cm3 [43]). The amount of traps found in NiO are about ten times more than the one found in TiO2. This exception- ally large amount of traps might locate a large amount of injected holes in surface states promoting recombination and possibly preventing charge accumulation and a good hole transport. This can be connected with the discussion on the conductivity made in Section 4.2. Precluding the hole to reach the surface, should force the spacial distribution of holes to be more shifted towards the inner part of the nanoparticle. This should facilitate the accumulation of the charge carrier and increase the film conductivity, which is needed for a high current density, see Section 4.2. In TiO2 DSC this last condition is essential for a successful cell.

On this topic, the exceptional conversion efficiencies found in the studies reporting the records of efficiency, can be explained in terms of NiO surface engineering [58, 59]. In these studies, efficiencies of 1.3 and 2.5 % are reached, featuring in the best case, a Voc and Jsc of 645 mV and 7.65 mA/cm2, respectively. New redox mediators are used, iron and cobalt complexes, that feature a redox potential about 700-900 mV above the VB of NiO. This allows the cells to show high Voc, 650 and 730 mV. The NiO used in these works annealed prior dye loading. This, together with the strong reducing redox couple, greatly helped avoiding the formation of the surface Ni3+ impurities. In fact, the dye-hole recombination of these cells is very long, 100 μs, meaning that the cell is in a slow recombination regime. The efficient dye regeneration and good surface insulation performed by the dye, caused a large charge carrier accumulation. This probably led to a good film charge transport and eventually to the good conversion efficiencies.

66 4.8 Future work The work done in these years raised many questions to which I would like to give an answer.

4+ One of the most important results was the discovering of the Ni(h+)- 3+ Ni(h+) conversion and surface diffusion. The mechanism at the basis of the diffusion process was not really understood. It was hypothesised that the hopping of the trapped charges could occur mediated by a hydroxyl group distortion. This could be studied with a band-gap TAS experiment in the IR region where the hydroxyl absorbs. Also it would be interesting to sub- stitute the surface protons with another cation and study the annihilation kinetics. If the trap diffusion is really mediated by protons this should slower the diffusion.

4+ Another important open question is the reason why the Ni(h+) is much 3+ more reactive than the Ni(h+). In the former section two reasons were proposed to justify this behaviour. One is the diﬀerent mobility of the two 4+ holes and the other one is the partial delocalization of the Ni(h+).Inthe 4+ case of the partial delocalization of the Ni(h+) the spectroscopic signal of this species should show some features of the VB hole. Typically a very good technique that is able to distinguish delocalised species from localized species is the THz spectroscopy. A THz study, which analyses the two re- 3+ 4+ combination paths through Ni(h+) or Ni(h+), could give information about 4+ the Ni(h+) character. Recent developments of THz transparent electrode can make this study possible. My suggestion is basically to repeat the measurement done in Paper II, monitoring the recombination reaction by transient THz spectroscopy as a function of the applied potential.

Even though the mechanism of hole hopping and hole conversion is fasci- nating it is quite established that for a working NiO-based DSC the presence of surface hole should be avoided, thus the former projects would have just a fundamental purpose. I have a sort of Christmas wish list for what I would do, if I had more time, to improve the performances of the p-type DSC. First of all it is clear that the formation of surface holes needs to be avoided. The removal of the adsorbed ions could be one possible way but also other strategies might be used. One example is the use of a surface additive to prevent the stabilization of surface holes. Another possibility is the use of a large cation in the supporting electrolyte. This would inhibit the mechanism of ion expulsion destabilizing the surface hole. A large cation would also shift the valence band upwards which decreases the Voc, therefore a more negative redox mediator should be used. This would be

67 beneficial also for the reduction, hence elimination, of eventual Ni3+ states present on the surface. The elimination of the surface states should force the injected hole to populate the states that are located internally to the NiO which should be the ones active in the charge transport. The problem of NiO is that the bulk states are not so many and are not very mobile. Populating those states might just increase the dye-hole recombination. There is a need for stabilisation of the holes in the bulk of the nanoparticle. One possibility is by doping, like with lithium, but, as already seen, doping nanoparticle is tricky since the doping sites would probably end up in the surface defeating their purpose. The rock salt structure of NiO is the main cause of this problem. One possible solution could be to make NiO nanoparticles more amorphous, for example creating the film in a much faster way. Then however, elimination of the surface state could be much more difficult. A general configuration should be an amorphous core able to store and transport holes together with a inert nanoparticle overlayer that helps to avoid the recombinations. Such overlayers could be build by atomic layer deposition (ALD). Obviously, another solution is to try different semiconductor materials without a rock salt structure. Alternative semiconductors for p-type DSC have been tried with limited success. My suggestion would be to move not so far from NiO, to use instead Ni(OH)2. It has a similar characteristic than NiO but it presents a much more labile structure and it has been demonstrated to be sufficiently conductive. Unfortunately Ni(OH)2 decompose into NiO and water upon heating above 200◦ C thus the nanoparticle needs to be sintered by pressure [176]. The position of the valence band of

Ni(OH)2 is about the same as for NiO, 0.3-0.4 V vs. NHE, thus it should work with the same dyes and redox mediator used for NiO.

68 5. Summary

The finite nature of fossil fuels and their effect on the global climate, raised the need to find an alternative source of energy that is climate and environment compatible, cheap and easy to build, efficient and abundant. The light coming from the Sun seems to be a promising alternative. It is defi- nitely abundant. The energy that arrives to Earth from the Sun in one day is more than what mankind consumes in one year. Also it is potentially climate and environmentally friendly. There are several ways to transform the solar energy into a usable form of energy and in Chapter 1 an overview of the most common strategies is presented. In general solar energy can be converted into electrical or chemical energy. The research discussed in this thesis can be applied to two solar energy conversion devices called dye-sensitised solar cell (DSC), which can perform solar-to-electrical energy conversion, and dye-sensitised solar fuel cell (DSFC), which performs the solar-to-chemical conversion. The mechanism of these two devices is described in Chapter 1. In few words, in DSC and DSFC, a dye is used as a light harvester which sensitises a mesoporous semiconductor. Together they construct a so called photocathode or photoanode, depending of the kind of the used semiconductor, p-type or n-type respectively. The energy conversion efficiency of the p-type based device is normally very poor and the reason is attributed to the semiconductor, NiO. The work of this thesis is focused on the characterisation of the NiO mesoporous film with the aim to understand the causes of the poor cell performances. The main results and the relation between them is presented in Chapter 4. In Paper I the electrical conductivity of the material was studied. Im- pedance spectroscopy revealed that the conductivity of NiO normally used for DSC can sustain a large current making it compatible with high cell efficiencies. In Paper I NiO was doped with lithium with the aim of increasing the material conductivity. This scope was achieved together with an improved electrical insulation of the surface of NiO. The Li cations con- centrate on the surface building a passivation layer. This can be useful in a DSC device helping to reduce the process of the charge recombination that is considered the main source of power losses in NiO-based DSC. The recombination reaction was studied in Paper II. The relation between the dye-NiO recombination rate and the position of the Fermi level in the DOS of NiO was studied by transient absorption spectroscopy. It was discovered that the NiO can feature biphasic recombination kinetics with

69 two components: a fast one, t1/2 =100 ns, and a slow one, t1/2 =10 ms. The relative amplitude of each component was a function of the Fermi level position of the NiO. This indirectly indicated the presence of two kind of holes in NiO, that can perform the recombination with two different rates. The reaction of conversion between these two holes was hypothesised. The 10 ms recombination restored the hope for a working NiO-based DSC. A better characterisation of the biphasic recombination kinetics and the hole conversion reaction was then needed. In Paper III the identity of the Ni states present on the surface of NiO was determined. The surface can host, besides the obvious Ni2+ states, Ni3+ and Ni4+ states. It was also possible to assign the UV-vis spectrum to these high valence states of Ni present on NiO surface. In Paper III two methods for reducing these high valence Ni states were demonstrated. The effect of the removal of these states was analysed in working DSC observing a substantial improvement of the performances, especially regarding the Voc that was increased by 100 mV by one of the reducing methods. The spectral characterisation of the Ni species was used in Paper IV to study the dynamics of the trapping of holes and electrons of the NiO surface. By band-gap absorption an electron-hole couple was created in NiO nanoparticles and the surface dynamics of electron trapping, hole trapping and trap annihilation followed by fs- and ns-transient absorption spectroscopy. The characteristic spectra of Ni3+ and Ni4+ discovered in Paper III were found in the fs experiment. In this way it was possible to discover that the Ni3+ surface states can work as either electron or hole traps produc- 2+ 4+ ing, respectively, Ni(e−) and Ni(h+). The annihilation of the electron and hole traps was studied and correlated with the hole relaxation observed in sensitised NiO in Paper II. Summarising, in Paper I a good electrical conductivity of NiO mesoporous films was proven which directed the attention to the recombination issue. In Paper II the presence of two kinds of holes was formulated and their effect on the recombination was measured. In Paper III the NiO surface states were assigned to specific chemical species and characteristic spectra were associate to them. In Paper IV the spectra were used to study the dynamics of electron and hole trapping due to these surface states. In the thesis these findings are presented and discussed treating them as a whole, explaining old results with new ones trying to deliver the most important knowledge they hold for a better design of p-type DSC and DSFC.

70 6. Sommario divulgativo

La natura finita degli idrocarburi fossili e l’effetto che il loro uso ha sul clima sprona la comunità scientifica a cercare una fonte di energia alterna- tiva al petrolio. L’intero fabbisogno energetico dell’umanità si fonda quasi esclusivamente sugli idrocarburi fossili. Per intero fabbisogno energetico si intende l’energia usata per i trasporti, il riscaldamento, la produzione industriale e alimentare, l’elettricità. Insomma tutto. Sembra impossibile trovare una fonte di energia così abbondante, non finita e che sia al con- tempo priva di impatti nocivi sul clima. Eppure una possibile soluzione è lì davanti a noi tutti i giorni: il Sole. Il nostro sole fornisce alla Terra, in un solo giorno, una quantità di energia superiore al nostro fabbisogno energetico di un intero anno. L’energia solare è abbondante, rinnovabile, ed è facilmente usabile in modo rispettoso per il clima. Il primo che si chiese perché l’uomo si ostinasse ad usare il petrolio come fonte di energia puttosto che l’energia solare fu un chimico italiano, Giacomo Ciamician, che sognava “zone industriali senza fumo e ciminiere”. Era il 1912. È difficile usare l’energia solare direttamente, bisogna convertirla prima in una forma di energia più fruibile come l’energia elettrica o chimica. La conversione in energia elettrica è quella effettuata nelle celle fotovoltaiche, la conversione in energia chimica invece è ancora ad uno stadio di sviluppo. In quest’ultimo tipo di celle solari l’energia elettromagnetica è utilizzata per trasformare una sostanza a basso potenziale chimico, per esempio l’acqua, in una ad alto potenziale chimico, per esempio l’idrogeno (e ossigeno). La ricerca che ho condotto durante gli studi di dottorato riguarda un materiale, l’ossido di nichel, che può essere impiegato in entrambe le conversioni. Le celle solari a cui la mia ricerca è indirizzata sono chiamate “celle solari sensitizzate ai coloranti”, in inglese dye-sensitized solar cell (DSC), oppure celle di Grätzel, dal nome di colui che le ha ideate. Nella versione originale queste celle utilizzano un film mesoporoso di ossido di titanio, TiO2,un semiconduttore trasparente che viene deposto su un elettrodo di vetro. Sul TiO2 viene adsorbito un mono strato di un colorante che funge da collettore di luce. L’elettrodo così formato viene messo a contatto, faccia a faccia, con un contro elettrodo platinizzato. Il contatto elettrico tra i due elettrodi è assicurato da un soluzione elettrolitica contenente una coppia redox. Le celle studiate nel mio dottorato vengono costruite allo stesso modo ma utilizzano il NiO al posto del TiO2. Il meccanismo di funzionamento delle DSC è rappresentato in Figura 7.1. Quando il colorante assorbe un fotone viene portato in uno stato di eccitazione che è in grado di accettare un elettrone dal semiconduttore, in questo modo si crea una separazione di carica:

71 un elettrone (-) nel colorante e una vacanza (+) nel semiconduttore (nella banda di valenza). Se questa separazione dura abbastanza a lungo, la coppia redox accetta l’elettrone dal colorante, rigenerandolo. Quindi, la coppia redox trasporta l’elettrone ﬁno al contro elettrodo, mentre la vacanza, tra- mite il semiconduttore, raggiunge l’elettrodo di vetro chiudendo il circuito. Purtroppo questi non sono i soli processi chimici che possono avvenire in una DSC. Ogni volta che l’elettrone e la vacanza si riaccoppiano prima di raggiungere i rispettivi elettrodi, l’energia accumulata nella separazione di carica viene persa. Questo processo viene chiamato ricombinazione. Nelle

Figura 6.1. Rappresentazione schematica di una DSC. Nel diagramma di sinistra: il fotocatodo, l’elettrodo sulla sinistra, è composto dall’elettrodo di vetro con sopra il ﬁlm mesoporoso di NiO; il colorante legato al semiconduttore è in contatto con la coppia redox che tocca il controelettrodo (CE) sulla destra. Nello diagramma di destra è schematizzato il meccanismo delle DSC: 1, fotoec- citazione del colorante; 2, trasferimento della vacanza nella banda di valenza (BV) del semiconduttore; 3, rigenerazione del colorante per opera della coppia redox; 4, ricombinazione colorante-NiO; 5, ricombinazione redox-NiO.

DSC a base di TiO2, la ricombinazione non è un grosso problema. Infatti l’efficienza (potenza generata dalla cella su potenza luminosa fornita alla cella) delle celle a base di TiO2 può raggiungere il 13-14% Invece, le celle a base di NiO non sono affatto efficienti, e la causa è attribuita ad una veloce ricombinazione. Nonostante i numerosi sforzi della comunità scientifica, l’efficienza delle DSC a base di NiO supera in pochissimi casi lo 0.5%. Negli articoli discussi in questa tesi ho cercato di studiare le cause di questo malfunzionamento arrivando ad avere una comprensione piuttosto profonda dei meccanismi alla base di queste celle. Infatti ho scoperto, insieme ai miei collaboratori, che i film mesoporosi di NiO possono ospitare due tipi di vacanze. Que- ste sono in equilibrio tra di loro e sono responsabili di due diversi tipi di

72 ricombinazone: una veloce (100 ns) ed una molto lenta (10 ms). La ricombinazione veloce potrebbe essere la causa della scarsa efficienza delle DSC al NiO. Sono anche riuscito ad identificare le specie chimiche associate a queste vacanze e a capire il meccanismo di conversione tra loro. Brevemen- te, la superficie del NiO non è stechiometrica e ospita dei difetti sotto forma di Ni3+. Quando una vacanza elettronica si forma nel semiconduttore viene intrappolata da uno stato Ni3+ formando uno stato Ni4+ molto reattivo e responsabile della ricombinazione veloce. Queste trappole di Ni4+ si possono convertire in Ni3+ reagendo con degli stati adiacenti di Ni2+ presenti anch’essi sulla superficie del NiO. Le trappole di Ni3+ sono responsabili della ricombinazione lenta. Questi concetti, oltre a spiegare le cause del malfunzionamento delle DSC basate sul NiO, rivelano che l’ossido di nichel può essere usato per preparare DSC ad alta efficienza. Nella tesi questi concetti sono discussi enfatizzando i risultati più importanti. Il primo capitolo motiva la mia ricerca con una riflessione sull’effetto dell’estrazione del petrolio sul clima e introduce il mio campo di ricerca. I capitoli secondo e terzo riassumono i concetti teorici più importanti e i me- todi sperimentali utilizzati nella tesi. Il quarto capitolo presenta i risultati dei miei articoli discutendone i significati. La tesi si conclude con la mia personale opinione sul futuro delle DSC a base di NiO proponendo alcune soluzione ai problemi scoperti.

73 7. Populärvetenskaplig sammanfattning

I dagens samhälle utnyttjas främst fossila bränslen för att mätta det mänsk- liga energibehovet, speciﬁkt inom sektionerna: transport, värme, elektricitet industri och matproduktion. Det vill säga, i princip överallt. Fossila bräns- len utgör inte endast en begränsad, men också en miljöskadlig energikälla, vilket har föranlett forskare att leta efter alternativ. Dessa alternativ öns- kas vara obegränsade och framförallt ge minsta möjliga påverkan på miljön. En sådan källa vore solen. Vår sol tillför jorden mer energi under en dag än mänsklighetens årliga energibehov. Den första att vända blicken mot skyn och fråga sig varför människan envist hållit sig kvar vid användandet av fossila bränslen var den italienska kemisten Giacomo Ciamician, 1912. Trots solens goda egenskaper så kan dess energi inte utnyttjas direkt, ut- an den måste omvandlas till mer användbara former så som elektricitet eller kemisk energi. I fotovoltaikceller omvandlas solens strålar till elektricitet, medan i bränsleceller erhålls kemiska slutprodukter. Bränsleceller är däre- mot fortfarande i utvecklingsstadiet. I dessa celler omvandlas, med hjälp av solljusets elektromagnetiska energi, ett ämne med låg kemisk potential så som vatten, till ett ämne med hög kemisk potential så som väte eller syre. I min forskning har jag studera ämnet nickeloxid, vilket kan utnyttjas i de två nämnda celltyperna. Solcellerna i fokus för min forskning kallas Grätzelsolceller eller färgäm- nessolceller, vilket på engelska benämns Dye Sensitized Solar Cells (DSCs). Det förstnämnda namnet kommer från innovatören av dessa celler, Grät- zel, som 1991 tillsammans med O’Regan, skapade sin solcell med mesoporös titandioxid, TiO2, vilket exempelvis används i vit målarfärg. En glaselek- trod täcks med den transparenta halvledaren TiO2 som i sin tur adsorberar ett tunt lager av ett ljusabsorberande färgämne. Elektroden placeras sedan i kontakt med en motelektrod täckt med ett lager av platinum. För att skapa kontakt mellan dessa elektroder tillförs en elektrolyt som innehåller ett redoxpar. I min forskning tillverkas cellerna på ett liknande sätt, dock med NiO istället för TiO2. I dessa celler exciterar solljuset färgämnet som då accepterar en elektron från halvledaren. Detta skapar en laddningsse- paration: elektronen (-) i färgämnet och ett hål (+) (en avsaknad av en elektron) i halvledaren (i valensbandet). Om separationen är långlivad kan redoxparet i elektrolyten återställa färgämnet genom att ta upp dess extra elektron. Därefter transporterar redoxparet elektronen till motelektroden, medan hålet i halvledaren transporteras i motsatt riktning till glaselektroden. Detta leder till en sluten cirkel. Tyvärr är inte dessa de enda kemiska

74 processerna som kan äga rum i en Grätzelsolcell. Den önskade laddnings- separationen, mellan elektronen och hålet, kan gå förlorad om dessa två istället återförenas i en process som kallas rekombination. I TiO2 baserade

Figur 7.1. Illustration av en DSC/Grätzelsolcell. Till vänster: en fotokatod med glaselektroden till vänster, täckt med en mesoporös NiO film. Denna film är färgad med ett färgämne, som i sin tur är i kontakt med redoxparet i elektrolyten och därmed även motelektroden till höger. Till höger: illustration av mekanismen i en Grätzelsolcell: 1. fotoexcitation av färgämnet; 2. transport av hålet inom halvledarens valensband (VB); 3. regenerering av färgämnet genom reduktion av redoxparet; 4. färgämne-NiO rekombination; 5. Redoxpar-NiO rekombination. solceller är rekombinationen oftast liten, vilket leder till dess höga effektivitet upp emot 13-14%. NiO solceller däremot visar låg effektivitet på grund av hög rekombination. Trots år av forskningsförsök har effektiviteten hos NiO solceller endast överstigit 0.5% ett fåtal gånger. Artiklarna som presenteras i denna avhand- ling har försökt synliggöra de underliggande problemen och ge en fördjupad förståelse för de processer som äger rum i dessa celler. Faktum är att jag har upptäckt, tillsammans med mina kollegor, att mesoporösa NiO filmer kan inneha två olika typer av hål. De står i jämvikt med varandra och är skyldiga till två olika typer av rekombination; en snabb (100 ns) och en långsam (10 ms). Den snabba rekombinationen kan ligga bakom den låga effektiviteten hos NiO solceller. De kemiska ämnena som besitter dessa hål har kunnat karakteriseras och omvandlingsmekanismen dem emellan har klargjorts. Sammanfattningsvis är NiO ytan icke-stökiometrisk och består av flertalet defekter i form av Ni3+. När elektronen lämnat ett hål i halvledaren oxideras Ni3+ till Ni4+.Ni4+ är väldigt reaktiv och är anledningen till den snabba rekombinationen. Dessa Ni4+ tillstånd kan reagera med

75 ytplacerade Ni3+ tillstånd, från vilka den långsammare rekombinationsme- kanismen utgår ifrån. I den här avhandlingen diskuterar jag dessa koncept och försöker un- derstryka de viktigaste resultaten. Avslutningsvis ges min personliga åsikt gällande framtiden för NiO solceller och vilka möjliga lösningar som ﬁnns på de problem som uppdagats.

76 8. Acknowledgments

This thesis and what it represents would not be possible without the help and support of some people.

First of all I would like to thank my main supervisor and mentor, Leif Hammarström, who gave me the possibility to play with science and pa- tiently guided me to become a physical chemist. A big thanks Leif.

I would like to thank my co-supervisor, Gerrit Boschloo, for the nice discussion about my experiments and for introducing me to the world of solar cells.

I am also grateful to my former co-supervisor, Anders Hagfeldt,who supervised me only for a short period. I will remember those discussions as fun and stimulating.

I am very thankful to the people I work, and I worked, together with every day. Without them the lab would be nothing more than a work place. A thanks to: Jens Föhlinger, for sharing with me these years of doctoral studies, for proof reading my thesis and for the great job in taking care of the lab; Liisa Antila, for the hard work done together, for the intense discussion and the advices (but also for the nail, the sauna in front of the lake and your laughs); Mohammad Mirmohades, for the help with the Quanta Ray, for being a point of reference in the lab for many years and a kind source of advices; Mohamed Qenawy, for always being super positive towards me and for calling us to order every time we mess up the lab; Mariia Pavliuk, for moving in all the available apartments of Uppsala and for being a SI standard of cuteness; Starla Glover, for the help with the Brilliant B, the advices for the experiments and for being a researcher model: clever, handy and fun; Robin Tyburski for the help with the Brilliant B and the teaching we did together; Vincent Wang for proof reading my thesis and for the sparkling scientiﬁc and non-scientiﬁc discussions; Tianfei Liu, for the help with the glovebox. Thanks to my Asian colleagues, Shihuai Wang, Lei Tian and Lei Zhang for bringing a piece of Cina in Sweden. I am also thankful to all the previous members of our lab for the help in the lab and the nice time spent together, in particular: Todd Markle, Jonas Lissau, Daniel Streich, Erik Göransson, Allison Brown, James Gardner and Jonathan Freys.

77 Thanks to Reiner Lomoth and Burkhard Zeitz, for helping me with the instrumentations and with the teaching in the lab.

I am very grateful to all the researcher, postdocs, and Ph.D. students I have collaborated with: Roger Jiang, Ute Cappel, Libby Gibson, Chris Wood, Haining Tian, Marina Freitag, Patrícia Raleiras.

The PhD. studies are full of all kinds of problems on top of a pile of formal documents that needs to be ﬁlled and signed. My doctorate would be way longer if I did not have the help of our administrator staﬀ. Thanks to: Jessica Stålberg, Anna Fahlén, Susanne Söderberg, Ulrika Jans- son and Åsa Furberg; for helping me always in a fun way. Thanks to Sven Johansson for being always available and for giving a useful hand with my projects.

I am sure that I would have not reached the same achievements if I was not supported and encouraged by those people that are around the lab or not even related to the lab. I want thank you all. A special thanks goes to my “naitch” buddy, Keyhan Keykoon,for helping every day in keeping my mood up, thank you for all our fun, surreal discussions and also for the serious ones. Thanks to Edgar Mijangos, for being simultaneously a good researcher, a ninja and a high school kid. Thanks for the help in the lab, for all the talks, the non-talks, and the shoulder pounces. Thanks to Michele Bedin, for helping me to raise the Italian ﬂag in our department and for stilling that chair.

Living outside your own mother country for many years requires a special support. I want to thank all my Italian friends living here for giving me injections of “italianess”. In particular Giuseppe, for your sharp point of view, for helping me with the English during my ﬁrst year and for the amazing food. Matteo, for sharing with me the experience of leaving a small town to arrive here, for enjoying together kids music in the morning and for being damn good at foosball. Ricky, for being not so good at foosball, for calling me “Luca” twice and for driving nuts a tricycle in Venice beach. Dieghino, for building our friendship on the passion of meteorites and for being an awesome guy.

A great thanks goes to the person that are far away from me but are always present in my life. My parents, for supporting me and pushing me in doing always better. My little brother Simone, to keeping me at the phone for hours and for being the best brother ever. My friends: Dott. Prof. Roberto, who transformed this doctorate from an achievement to

78 a great victory; Anto, for sharing with me an blind passion for chemistry.

Last but not least, I am very thankful to my girlfriend, Belinda,who proof read my thesis (twice) and was close to me in the hard moments. For ﬁlling my days with jokes and sweet moments. For being, substantially, awesome. Appendix A. Earth’s energy balance

As said in the introduction the system Earth’s surface-atmosphere can be rationalised as a closed system. Even though the system exchange energy with the external space the balance between the energy that is received and released is fairly close to zero. The surface of Earth receive energy from the space, mainly from the Sun, and from the core of the planet while it releases the energy by radiative heat. The majority of the energy arrives on Earth in the form of electromagnetic waves, light, which is absorbed by the surface that convert it to heat which is then released into space. In the process of converting light in heat a small part of this energy takes a longer path which we call life. Namely, the light is absorbed from plants that convert the electromagnetic energy to chemical energy which is used by living organisms that eventually will convert it to heat as well. In this energy balance of our planet the atmosphere plays an essential role for the existence of life. It increases the albedo of Earth so that the energy of the Sun does not fully reach the surface keeping the temperature excursion small. Moreover the composition of the atmosphere allows the so called “green house effect” which keep the surface temperature of Earth warm enough for the existence of life. Without the atmosphere the temperature profile of Earth would be about the one of the Moon, an average temperature of -23◦Candan excursion two times the one of the Earth. As deducible from above, the relation between the composition of the atmosphere and the presence of life on the planet is really strong. Fortunately for us the fact that the atmosphere and the surface of Earth form a closed system leaves to only few external factors the possibility to compromise the entire system. This lead life to have a long history, hundreds millions of years. The only few times when its survival was threatened, the so called “big five” mass extinctions, were due to external event like the impact of asteroids or exceptional volcanoes eruptions. The system is very dynamic and can hold small or slow perturbation, like geological changes of Earth surface, but this kind of event perturbs the system so quickly and so much that triggers what is called a mass extinction. As said in the beginning of the chapter another perturbation is ongoing in the last hundred years: the fossil hydrocarbons extraction. From about 150 years, but intensively in the last 50 years, mankind ex- ploited fossil hydrocarbons as the main source of energy. From the point

80 of view of the thermodynamic system discussed above this means an in- jection1 of external matter into the system since this material come from a deep part of the Earth’s crust. Fossil hydrocarbons are extracted in the form of methane gas, more or less fluid oil (petrol) and solid coal. After extraction the hydrocarbons are burned to produce energy. Basically humans are transferring carbon from inside of our planet directly into the atmosphere in the form of CO2. Adding material into the system can alter its internal equilibria com- promising it. In this specific case adding CO2 in the atmosphere means injecting new material into the carbon cycle. The carbon cycle is one of the dynamic processes happening on Earth. It accounts for the fluxes between the various reservoirs of carbon, it can be divided in the organic and inorganic cycles. The organic cycle accounts for the exchange of carbon between the atmosphere and the living organisms while the inorganic cycle accounts for the fluxes between the carbon in the atmosphere and the one present in the carbonates of rocks and dissolved in the oceans. Increasing the concentration of CO2 in the atmosphere alters the carbon cycle that reacts mainly trying to reabsorbing the excess of CO2 into the inorganic cycle, in particularly in the oceans. This has obvious consequence on the pH of the oceans water and thus living species. The main problem of the increased CO2 in the atmosphere is its consequence on the green house effect. The higher CO2 atmospheric concentration increased the green house effect that in turn is increasing the average temperature of the planet. The effect of this warmer environment has already the first effects on the globe and its ecosystems. The scientific community is debating on the effect of the temperature rising on the cycles of matter. Though it is reasonable to think that a rising of the ocean temperature would produce a further release of the CO2 in the atmosphere with a consequent worsening of the overall situation.

1Actually it is an exchange of matter since the extracted petrol is normally substituted by water to ﬁll the empty space left by the extraction.

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