International Journal of Molecular Sciences

Review Materials for Photovoltaics: State of Art and Recent Developments

José Antonio Luceño-Sánchez 1 , Ana María Díez-Pascual 1,* and Rafael Peña Capilla 2

1 Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Faculty of Sciences, Alcalá University, 28871 Madrid, Spain; [email protected] 2 Department of Signal Theory and Communication, Polytechnic High School, Alcalá University, 28871 Madrid, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-918-856-430



Received: 22 January 2019; Accepted: 19 February 2019; Published: 23 February 2019


Abstract: In recent years, photovoltaic cell technology has grown extraordinarily as a sustainable source of energy, as a consequence of the increasing concern over the impact of fossil fuel-based energy on global warming and climate change. The different photovoltaic cells developed up to date can be classified into four main categories called generations (GEN), and the current market is mainly covered by the first two GEN. The 1GEN (mono or polycrystalline silicon cells and gallium arsenide) comprises well-known medium/low cost technologies that lead to moderate yields. The 2GEN (thin-film technologies) includes devices that have lower efficiency albeit are cheaper to manufacture. The 3GEN presents the use of novel materials, as well as a great variability of designs, and comprises expensive but very efficient cells. The 4GEN, also known as “inorganics-in-organics”, combines the low cost/flexibility of polymer thin films with the stability of novel inorganic nanostructures (i.e., metal nanoparticles and metal oxides) with organic-based nanomaterials (i.e., carbon nanotubes, graphene and its derivatives), and are currently under investigation. The main goal of this review is to show the current state of art on photovoltaic cell technology in terms of the materials used for the manufacture, efficiency and production costs. A comprehensive comparative analysis of the four generations is performed, including the device architectures, their advantages and limitations. Special emphasis is placed on the 4GEN, where the diverse roles of the organic and nano-components are discussed. Finally, conclusions and future perspectives are summarized.

Keywords: photovoltaics; generations; polymers; carbon nanotubes; graphene; efficiency

1. Introduction Electricity is a core resource for the development of human civilizations, and it is possible to link the living standard and the electricity consumption of a society. Electricity can be obtained from diverse resources and with different production methods, ranging from the combustion of raw materials (such as coal, natural gas, biomass, etc.) to complex nuclear reactors systems. Over the last 50 years, electricity production has continually increased, with a strong presence of fossil fuels [1] (see Figure1). However, with concern about climate change nowadays, the production must be reoriented towards renewable resources [2,3], such as solar energy, to the detriment of other fossil energies such as coal. The use of this primary energy source entails not only serious polluting emissions, but also a very high consumption of water, at a time when the scarcity of this element has become, for many countries, a key issue of concern.

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Figure 1.1. World energyenergy consumption.consumption. TakenTaken fromfrom [[1]1]

Solar energy is the energy obtained from solar radiation, and it is regarded as renewable since Solar energy is the energy obtained from solar radiation, and it is regarded as renewable since the the Sun expected life is still between 5000 and 10,000 billion years; furthermore, this kind of energy Sun expected life is still between 5000 and 10,000 billion years; furthermore, this kind of energy is is available in most of the Earth places. Photovoltaic energy (PV) is the electric energy produced available in most of the Earth places. Photovoltaic energy (PV) is the electric energy produced directly directly from the sun radiation by applying the photovoltaic effect [4], which was discovered in 1839 from the sun radiation by applying the photovoltaic effect [4], which was discovered in 1839 by the by the French physicist Alexandre-Edmond Becquerel. This effect is found in semiconductor materials, French physicist Alexandre‐Edmond Becquerel. This effect is found in semiconductor materials, characterized by their intermediate in electrical conductivity between a conductor and an insulator. characterized by their intermediate in electrical conductivity between a conductor and an insulator. When the incident radiation in the form of photons reaches the material, these are captured by electrons, When the incident radiation in the form of photons reaches the material, these are captured by electrons, resulting in higher energy content, and if a threshold value called “band gap” is exceeded, they can resulting in higher energy content, and if a threshold value called “band gap” is exceeded, they can break their nucleus links and circulate through the material. This electron flow generates a difference break their nucleus links and circulate through the material. This electron flow generates a difference of of potential between the terminals, and upon application of an electric field on the semiconductor, potential between the terminals, and upon application of an electric field on the semiconductor, the the electrons move in the direction of the field, generating an electrical current [4]. electrons move in the direction of the field, generating an electrical current [4]. Photovoltaic cells (PVCs) are devices used to convert solar radiation into electrical energy through Photovoltaic cells (PVCs) are devices used to convert solar radiation into electrical energy the photovoltaic effect. PVCs present an architecture based on the union of two semiconductor regions through the photovoltaic effect. PVCs present an architecture based on the union of two with different electron concentration (Figure2); these materials can be type n (semiconductors with semiconductor regions with different electron concentration (Figure 2); these materials can be type n excess of electrons) or type p (semiconductors with an excess of positive charges, called holes), though (semiconductors with excess of electrons) or type p (semiconductors with an excess of positive in both cases the material is electronically neutral. charges, called holes), though in both cases the material is electronically neutral. When both p and n regions are in contact, holes flow from the p region and electrons from the When both p and n regions are in contact, holes flow from the p region and electrons from the n n region through the p-n junction (diffusion current). In addition, the fixed ions near the junction region through the p‐n junction (diffusion current). In addition, the fixed ions near the junction generategenerate an electric field field in in the the opposite opposite direction direction to to the the diffusion, diffusion, which which leads leads to toa drift a drift current. current. At Atequilibrium, equilibrium, the the diffusion diffusion current current is balanced is balanced with with the the drift drift current, current, so sothat that the the net net current current is zero. is zero. In Inthis this condition, condition, a potential a potential barrier barrier is isestablished established at atthe the p‐ p-nn junction. junction. AsAs thethe light light strikes strikes the the cell, cell, the energythe energy contribution contribution of the of photons the photons can be absorbedcan be absorbed by the electrons, by the whichelectrons, can breakwhich their can bonds,break their producing bonds, hole-electron producing hole pairs.‐electron These charge pairs. carriersThese charge are pushed carriers by theare electricpushed fieldby the and electric conducted field and through conducted the p-n through junction. the p If‐n an junction. external If loadan external is connected, load is anconnected, electric currentan electric and current a potential and a difference potential between difference the between cell terminals the cell will terminals be established. will be established.

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Figure 2. Schematic representation of a photovoltaic cell, showing the n-type and p-type layers. Figure 2. Schematic representation of a photovoltaic cell, showing the n‐type and p‐type layers. The different PVCs that have been developed up to date can be classified into 4 main categories The different PVCs that have been developed up to date can be classified into 4 main categories calledcalled generations generations [5] (Figure [5] (Figure3): 3):  • First-generationFirst‐generation (1GEN): (1GEN): It is basedIt is based on crystalline on crystalline silicon silicon technologies, technologies, both both monocrystalline monocrystalline and and polycrystalline, and on gallium arsenide (GaAs); polycrystalline, and on gallium arsenide (GaAs);  Second‐generation (2GEN): It includes amorphous silicon (a‐Si) and microcrystalline silicon • Second-generation(μc‐Si) thin films (2GEN): solar cells, It includes cadmium amorphous telluride/cadmium silicon (a-Si) sulfide and (CdTe/CdS) microcrystalline and copper silicon (µc-Si)indium thin films gallium solar selenide cells, cadmium (CIGS) solar telluride/cadmium cells; sulfide (CdTe/CdS) and copper indium gallium Third selenide‐generation (CIGS) (3GEN): solar cells; It involves technologies based on newer compounds including • Third-generationnanocrystalline (3GEN): films, active It involves quantum technologiesdots, tandem or based stacked on multilayers newer compounds of inorganics including based nanocrystallineon III–V materials, films, active such quantum as GaAs/GaInP, dots, tandem organic or(polymer) stacked‐based multilayers solar cells, of inorganics dyed‐sensitized based on III–V materials,solar cells, etc.; such and as GaAs/GaInP, organic (polymer)-based solar cells, dyed-sensitized solar  Fourth‐generation (4GEN): Also known as “inorganics‐in‐organics”, it combines the low cells, etc.; and cost/flexibility of polymer thin films with the stability of novel inorganic nanostructures such as • Fourth-generationmetal nanoparticles (4GEN): and metal Also oxides known or asorganic “inorganics-in-organics”,‐based nanomaterials like it carbon combines nanotubes, the low cost/flexibilitygraphene and of polymer its derivatives. thin films with the stability of novel inorganic nanostructures such as metalFigure nanoparticles 4 shows a diagram and metal of oxidesthe three or first organic-based generations nanomaterialsof PVCs in terms like of carbon their costs nanotubes, and grapheneefficiencies and [6], its and derivatives. Figure 5 shows the best research efficiencies attained for the different types of solar cells. The aim of each generation is to reduce costs and simultaneously improve efficiency Figurecompared4 shows to the a diagramprevious one(s). of the In three this firstregard, generations calculations of as PVCs well inas termseconomic of theirand funding costs and efficienciesfeasibility [6], and features Figure should5 shows be made the best prior research to the design efficiencies of a PV attainedsystem [7]. for On the the different other hand, types there of solaris cells. Thelack aim of agreement of each generation in the literature is to reduce regarding costs the and classification simultaneously of PVCs, improve and several efficiency authors compared sort to the previousthem into one(s). different In thisgenerations regard, like calculations occurs with as well GaAs as and economic polycrystalline and funding silicon feasibility [8] or silicon features shouldnanotubes be made [8,9]. prior In to addition, the design there of is a PVcontroversy system [concerning7]. On the the other existence hand, of there the 4GEN, is lack since of agreement some in theauthors literature include regarding it in the the forefront classification of the 3GEN of PVCs, [6], while and others several believe authors that it sortis a different them into one different [5,8]. generations like occurs with GaAs and polycrystalline silicon [8] or silicon nanotubes [8,9]. In addition, there is controversy concerning the existence of the 4GEN, since some authors include it in the forefront of the 3GEN [6], while others believe that it is a different one [5,8].

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Figure 3. Timeline of the four GEN of photovoltaic cells with the associated materials that comprise eachFigure generation. 3. Timeline Taken of thefrom four [5]. GEN of photovoltaic cells with the associated materials that comprise Figure 3. Timeline of the four GEN of photovoltaic cells with the associated materials that comprise each generation. Taken from [5]. each generation. Taken from [5].

Figure 4. Efficiency and cost projections for first-(I), second-(II), and third-(III) generation PV Figure 4. Efficiency and cost projections for first‐(I), second‐(II), and third‐(III) generation PV technologies. Taken from [6]. technologies. Taken from [6]. Figure 4. Efficiency and cost projections for first‐(I), second‐(II), and third‐(III) generation PV technologies. Taken from [6].

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Figure 5. Best research-cell efficiencies. Taken from [10]. Figure 5. Best research‐cell efficiencies. Taken from [10]. The aim of this article is to illustrate the current state of art on photovoltaic cell technology in The aim of this article is to illustrate the current state of art on photovoltaic cell technology in terms terms of the materials used for the device fabrication, its efficiency and associated costs. A detailed of the materials used for the device fabrication, its efficiency and associated costs. A detailed comparative comparative analysis on the four solar cell generations is performed, focusing on the different analysis on the four solar cell generations is performed, focusing on the different architectures, their architectures, their benefits and drawbacks. Particular attention is placed on the 4GEN, where benefits and drawbacks. Particular attention is placed on the 4GEN, where the functions of the organic the functions of the organic and nano-components will be depicted, together with the difficulties and nano‐components will be depicted, together with the difficulties related with the fabrication of such related with the fabrication of such devices. A summary and a future outlook will be presented in the devices. A summary and a future outlook will be presented in the last section. last section.

2. First2. First-Generation‐Generation Photovoltaic Photovoltaic Solar Solar Cells Cells TheThe 1GEN 1GEN comprises comprises photovoltaic photovoltaic technology technology basedbased onon thick thick crystalline crystalline films, films, namely namely cells cells based basedon Si,on whichSi, which is the is the most most widely widely used used semiconductor semiconductor material material for for commercial commercial solar solar cells cells (~90%(~90% of of the current current PVC PVC market market [8]), [8]), and and cells cells based based on on GaAs, GaAs, the the most most commonly commonly applied applied for for solar solar panels panels manufacturing.manufacturing. These These are are the the oldest oldest and and the the most most used used cells cells due due to their to their reasonably reasonably high high efficiencies, efficiencies, albeitalbeit are are relatively relatively expensive expensive to toproduce. produce. TheThe use use of Si of in Si the in the PVC PVC production production has has some some advantages: advantages:  It is the second most abundant material in the earth’s crust [11], which implies that the • availabilityIt is the second of the most raw abundantmaterial may material be durable in the earth’s in the crust future [11 and], which its acquisition implies that cost the could availability be reduced;of the raw material may be durable in the future and its acquisition cost could be reduced;  • It isIt isa astable stable and non-toxicnon‐toxic chemical chemical element, element, characteristics characteristics that delaythat delay the contamination the contamination processes processesand the and loss the of durability loss of durability that may that occur may when occur used when as used a cell as material; a cell material; and and  • Si PVCsSi PVCs are are easily easily compatible compatible with with Si‐ Si-basedbased microelectronics microelectronics industry industry (i.e., (i.e., integrated integrated circuits, circuits, transistors,transistors, etc.) etc.) [8], [ 8thus], thus allowing allowing the the use use of ofwell well-known‐known and and well well-developed‐developed technologies. technologies. On the other hand, GaAs is especially useful for multi‐junction cells, those comprising multiple On the other hand, GaAs is especially useful for multi-junction cells, those comprising multiple p–n junctions made of different semiconductor materials, and high‐performance PVCs for several p–n junctions made of different semiconductor materials, and high-performance PVCs for several reasons [12]: reasons [12]:  It has a band gap of 1.43 eV, which is quite close to the ideal value for single‐junction PVCs;

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• It has a band gap of 1.43 eV, which is quite close to the ideal value for single-junction PVCs; • It presents a very high absorptivity, hence a cell of only a few microns thick is enough to absorb the usable sunlight spectrum corresponding to its band gap, while crystalline Si requires cells of 100 microns or even thicker; • It allows a versatile cell design, since the incorporation of different dopant substances and the combination with other III-V materials within the cell structure significantly change the optoelectronic properties; • It is very resistant to radiation degradation, which combined with its high efficiency makes it ideal for space applications; and • Unlike Si-based cells, those based on GaAs have low temperature coefficients, thus their performance is less affected by temperature. 1GEN PVCs can be further divided into three categories: monocrystalline and polycrystalline Si, as well as GaAs cells.

2.1. Monocrystalline Silicon (m-Si) The m-Si technology has achieved efficiencies of the order of 24.4% [13]. The m-Si cells are manufactured by the Czochralski process [8], which consists in the growth of Si ingots from small monocrystalline silicon seeds [14], and subsequently cut them to obtain m-Si wafers. This process allows the production of crystals with diameters ranging from 10 up to 300 mm, and lengths from 50 cm up to 2 m [14,15]. However, the Czochralski process involves a high production cost owing to several reasons: • It requires Si of very high purity (known as solar grade silicon) to avoid contamination by the raw material [8], since it would lead to defects in the structure and worsening of the electric properties; • Energy consumption is high, due to heat losses via conduction and radiation through the seed [8]; and • Temperature must be controlled to maintain the crystal growth during the long production times [8].

2.2. Polycrystalline Silicon (p-Si) The p-Si cells are manufactured on polysilicon wafers, which consist of small Si crystals randomly oriented. They present several advantages compared to the m-Si ones: they involve less energy in their production and less associated greenhouse effects. However, this type of PVCs can only reach efficiencies of 19.9% [13], smaller than m-Si based cells. The main reason for this smaller efficiency is the lower material quality due to grain boundaries and defects, and the higher concentration of impurities. Therefore, the influence of recombination in p-Si cells is higher than in m-Si cells, which leads to a slightly lower voltage. Current is also lower due to incomplete carrier collection in these devices. p-Si can be obtained at an industrial scale by the Siemens process, initially developed for electronic applications in the 1950s [16]. In general, the process consists in a gasification of the metallurgical grade Si, a distillation of the product, and a final deposition to obtain ultrapure silicon.

2.3. Gallium Arsenide (GaAs) GaAs cells currently reach efficiencies in the range of 18.4–28.8% in the laboratory, depending on whether they have a crystalline structure or they consist in a thin layer [13]. GaAs is obtained by direct combination of Ga and As via a vapour-phase reaction at low pressure and high temperature. The production of GaAs can be summarized into 4 stages: growth of the ingot, wafer processing, epitaxy, and manufacture of the device [17,18]. In the first step, a mixture of Ga, As, and small amounts of dopant materials (Si, Te, Cr) are reacted under high temperature conditions to form crystalline ingots of doped GaAs. The processing consists Int. J. Mol. Sci. 2019, 20, 976 7 of 42 in characterizing the structure of the GaAs by means of X-rays, and in eliminating the ends of the ingot with a diamond saw. Subsequently, the ingot is separated in wafers with a saw, its surface is washed with strong acids (H2SO4) or H2O2/H2O mixture, cleaned in a lapping machine, rinsed with a soapy solution, dried and finally polished with a polisher. The GaAs wafers are used as a substrate for the growth of very thin sheets of the same or another compound (belonging to groups III–V) possessing the desired electronic or optical properties. This growth is carried out by epitaxy, a process in which the crystallinity of the substrate (GaAs) conditions the crystalline growth of the compound that is incorporated on top [19]. Epitaxy can be performed following different techniques: • In vapour phase (VPE): a stream of heated vapor from the material interacts with the surface of the wafer; • In liquid phase (LPE): a hot and saturated metal solution is brought in contact with the wafer; and • In metalorganic vapour-phase epitaxy (MOCVD or OMVPE): the growth of the crystals is produced by a chemical reaction between the surface and the compound to be deposited. Currently, it is the most used at an industrial level. Finally, the device manufacture consists in the incorporation of the metallic contacts and the antireflective layer, the isolation of the devices, their encapsulation the cell and other auxiliary processes. One of the main advantages of GaAs for PVC applications is that it offers a wide range of potential design options. GaAs-based cells can have several layers with a slightly different composition that allow a more accurate control of the generation and collection of electrons and holes than silicon cells, which are limited to changes in the level of doping to achieve the same results. This higher degree of control allows to achieve efficiencies closer to the theoretical limit. For instance, one of the most common GaAs cell structures has a very thin window layer of AlGaAs that enables electrons and holes to be generated close to the electric field at the junction. Further, GaAs is frequently used in multi-junction solar cells, where each p-n junction produces an electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorption of a broader range of wavelengths, thus improving the energy conversion efficiency of the cell [20].

3. Second-Generation Photovoltaic Solar Cells The 2GEN focuses on thin-film technologies with the aim of reducing the high costs associated with the 1GEN by using lower amount of material and of poorer quality, deposited on cheap substrates. It is based on materials identified as potentially useful during the development of the 1GEN and was extended to include a-Si, µc-Si, CIGS, and CdTe [5]. The 2GEN PVCs present the following general advantages [20,21]: • Cheaper compared to Si-based solar cells • Drastic reduction in the amount of materials needed. Sometimes only one-micron thick layer is required. • High absorption coefficient. • Can use both vacuum and non-vacuum process. • Most of technologies allow direct integration into a higher voltage module (i.e., a-Si), which reduces the number of production stages compared to the 1GEN-PVCs. However, they also present some disadvantages: • Lower efficiency: the best efficiency reached in the laboratory is 20.3% for CIGS [10]. • Light-induced degradation in first stages of outdoor usage. Higher degradation in outdoor uses: the semiconductor deposited onto glass can generate a flow of ions in the glass. In the case of amorphous silicon, this problem can occur even if the substrate is not glass. Environment contamination starts from fabrication process. • In some technologies, the availability of manufacturing materials may not be abundant. Int. J. Mol. Sci. 2019, 20, 976 8 of 42

3.1. Amorphous Silicon (a-Si) and Micro-Crystalline Silicon (µc-Si) a-Si is widely used in thin-film solar cells owing to several reasons [22]: • Raw materials are abundant and non-toxic; • It requires low-temperature processes, allowing the manufacturing of modules with a wider and a cheaper range of substrates; Int. J. Mol. Sci. 2019, 20, x 8 of 43 • It presents a high absorption coefficient, hence cells are thinner (of the order of 1–2 µm thick) and require It requires less amount low‐temperature of material processes, per cell; allowing the manufacturing of modules with a wider • Largeand area a cheaper deposition range technologies of substrates; can be applied [23]; • Laboratory It presents efficiencies a high absorption of a-Si arecoefficient, 10.2% forhence single-junction cells are thinner cells (of andthe order 12.7% of for 1–2 multi-junction μm thick) cellsand [13 require]. less amount of material per cell;  Large area deposition technologies can be applied [23]; However, Laboratory a-Si efficiencies also presents of a some‐Si are limitations: 10.2% for single‐junction cells and 12.7% for multi‐junction • Thecells non-crystalline [13]. structure of a-Si reduces the life cycle, due to the formation of hole-electron However, a‐Si also presents some limitations: recombination centres; and  The non‐crystalline structure of a‐Si reduces the life cycle, due to the formation of hole‐electron • The absence of crystalline structure hinders doping treatments with n-type and p-type compounds. recombination centres; and Hydrogen The absence is required of crystalline to dope thestructure material, hinders leading doping to hydrogenated treatments with amorphous n‐type siliconand p‐type (a-Si:H). Thecompounds. use of a-Si:H Hydrogen can lead is torequired operational to dope problems the material, since leading it can to be hydrogenated degraded by amorphous sunlight: the thicknesssilicon of the (a‐ layerSi:H). would decrease over time, and thicker layers would be required to ensure long-termThe operation. use of a‐Si:H However, can lead single-junction to operational andproblems multi-junction since it can devices be degraded with highby sunlight: efficiency the and moderatelythickness good of the stability layer would have decrease been developed over time, [and22]. thicker In addition, layers would if the be process required involves to ensure a high long‐term operation. However, single‐junction and multi‐junction devices with high efficiency and concentration of H, µc-Si can be formed, which has fewer defects than a-Si and is more stable in the moderately good stability have been developed [22]. In addition, if the process involves a high presence of solar radiation; the lab efficiencies of µc-Si are in the range of 11.9% for single-junction concentration of H, μc‐Si can be formed, which has fewer defects than a‐Si and is more stable in the cellspresence and 14.0% of solar for multi-junction radiation; the lab cells efficiencies [13]. of μc‐Si are in the range of 11.9% for single‐junction cellsThe and typical 14.0% production for multi‐junction process cells of a-Si:H [13]. cells is a roll-to-roll process [24], as schematically shown in FigureThe6. Firstly, typical aproduction cylindrical process sheet, usuallyof a‐Si:H stainless cells is a steel, roll‐to is‐roll unrolled process to [24], be used as schematically as a deposition surface.shown The in Figure sheet is6. washed,Firstly, a cutcylindrical to the desiredsheet, usually size, and stainless printed steel, with is unrolled an insulating to be used layer. as Then,a the a-Si:deposition H is depositedsurface. The on sheet the is reflector. washed, cut Subsequently, to the desired the size, transparent and printed conductive with an insulating oxide layer. (TCO) is depositedThen, the on a the‐Si: silicon H is deposited layer. Laser on the cuts reflector. are then Subsequently, made to interconnect the transparent the differentconductive layers oxide and, (TCO) finally, the moduleis deposited is encapsulated. on the silicon layer. Laser cuts are then made to interconnect the different layers and, finally, the module is encapsulated.

FigureFigure 6. 6. SchematicSchematic representation representation of of the the manufacturing manufacturing process process of a‐ ofSi a-Sibased based photovoltaic photovoltaic cell. cell.

3.2. Copper Indium Gallium Selenide (CIGS)

CIGS is a semiconductor material with general formula of Cu (InxGa1‐x)Se2 that varies its band gap value between 1.0–1.7 eV depending on the proportion of the elements in the compound [25]; it

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3.2. Copper Indium Gallium Selenide (CIGS)

Int.CIGS J. Mol. is Sci. a semiconductor 2019, 20, x material with general formula of Cu (InxGa1-x)Se2 that varies9 of its 43 band gap value between 1.0–1.7 eV depending on the proportion of the elements in the compound [25]; it is synthesizedis synthesized by preparing by preparing a molten a molten mixture mixture containing containing the the desired desired amount amount of of each each element element [ 26[26].]. CIGS CIGS cells are usually manufactured following five steps [27]: 1) A substrate such as Na2CO3–CaO, a cells are usually manufactured following five steps [27]: (1) A substrate such as Na CO –CaO, a metal, metal, a ceramic or a polymer sheet is placed to support the rest of the cell. 2) The2 substrate3 is a ceramic or a polymer sheet is placed to support the rest of the cell. (2) The substrate is covered with covered with the back contact, which is usually pulverized molybdenum in the form of MoSe2. 3) the backThe CIGS contact, layer which (p‐type) is usually is grown pulverized by a co‐evaporation molybdenum process. in the4) A form buffer of layer MoSe (n2‐.type), (3) The currently CIGS layer (p-type)formed is grown by a TCO by a such co-evaporation as zinc oxide process. (ZnO) with (4) or A bufferwithout layer doping (n-type), is deposited currently [25]. formed5) Finally, by an a TCO suchanti as zinc‐reflective oxide coating (ZnO) withis applied or without to improve doping the iscell deposited efficiency. [ 25The]. (5)cross Finally,‐section an structure anti-reflective of a CIGS coating is appliedcell is todepicted improve in Figure the cell 7. efficiency. The cross-section structure of a CIGS cell is depicted in Figure7.

FigureFigure 7. 7Cross-section. Cross‐section of a CIGS CIGS cell. cell. Taken Taken from from [27]. [27 ].

AlthoughAlthough co-evaporation co‐evaporation is theis the most most widespread widespread production production technique, other other techniques techniques such such as a one-stageas a one hot‐stage wall hot deposition wall deposition process process are currently are currently being being investigated investigated as anas alternativean alternative to to reduce productionreduce costsproduction [26]. Forcosts instance, [26]. For theinstance, third the step third can step be replaced can be replaced by a spraying by a spraying of the of CIGS the CIGS precursor elementsprecursor followed elements by selenization followed by andselenization sulphuration, and sulphuration, which results which in laboratoryresults in laboratory efficiency efficiency of 22.3% [27]. Thisof would 22.3% aid [27]. to reduceThis would the productionaid to reduce cost, the since production the co-evaporation cost, since the processes co‐evaporation require processes a great deal require a great deal of energy. of energy. 3.3. Cadmium Telluride (CdTe) 3.3. Cadmium Telluride (CdTe) CdTe is a semiconductor compound with a band gap of 1.45 eV, which makes it a good candidateCdTe is a semiconductorfor converting sunlight compound into withelectricity a band in gap single of‐ 1.45junction eV, whichcells. CdTe makes cells it a achieve good candidate lab for convertingefficiencies sunlightof around into 21% electricity[13], and can in be single-junction obtained mainly cells. by three CdTe different cells achieve routes: lab efficiencies of around 21%Direct [13 reaction], and can of Cd be and obtained Te at high mainly temperature by three in different a sealed routes:empty quartz tube;  Exposure of a Cd solution to gaseous H2Te under an inert atmosphere; and • Direct Addition reaction of ofCd Cd in an and alkaline Te at highmetal temperature telluride solution. in a sealed empty quartz tube; • ExposureCdTe ofcells a Cdare solution manufactured to gaseous by a multiple H2Te under deposition an inert process atmosphere; that lasts and less than 2.5 h, as • Additionschematized of Cdin Figure in an alkaline8: Firstly, metal a layer telluride of cadmium solution. sulfide (CdS) is vapour deposited onto a transparent conductive oxide film, front contact [−], which is supported on a heat‐treated glass. Then,CdTe a cells CdTe are layer manufactured is deposited on by the a CdS multiple layer. A deposition laser cut that process goes through that laststhe three less layers than is 2.5 h, as schematizedmade to introduce in Figure the insulator8: Firstly, into a layerthe module. of cadmium Afterwards, sulfide several (CdS) cuts is are vapour made with deposited the laser, onto a transparentcrossing conductive only the CdS oxide and CdTe film, layers, front contact in order [ −to ],add which the rear is supported contact [+] onby sputter a heat-treated deposition glass. and Then, a CdTelater layer crossing is deposited only the back on the contact CdS layer. Finally, A laser the cut cell that is goesencapsulated, through the the wires three are layers connected is made to introduceand the the tempered insulator rear into glass the is module.placed. Afterwards, several cuts are made with the laser, crossing only the CdS and CdTe layers, in order to add the rear contact [+] by sputter deposition and later crossing only the back contact layer. Finally, the cell is encapsulated, the wires are connected and the tempered rear glass is placed.

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Figure 8. Cross-section of a CdTe-based solar cell. Figure 8. Cross‐section of a CdTe‐based solar cell. CdTe PVCs can withstand high temperatures better than c-Si cells and capture radiation better in CdTe PVCs can withstand high temperatures better than c‐Si cells and capture radiation better humid environments. However, the elements that make up the CdTe are scarcer than Si, and CdTe is a in humid environments. However, the elements that make up the CdTe are scarcer than Si, and potentially toxic material. CdTe is a potentially toxic material. 4. Third-Generation Photovoltaic Solar Cells 4. Third‐Generation Photovoltaic Solar Cells The 3GENThe 3GEN arises arises from from the the idea idea of of increasing increasing device efficiency efficiency and and reducing reducing the thedistance distance to the to the CarnotCarnot limit, limit, which which is ~62% is ~62% above above the Shockley-Queisserthe Shockley‐Queisser limit limit (33%) (33%) [28 ].[28]. Its Its aim aim is to is developto develop devices withdevices high efficiencies with high usingefficiencies the thin using layer the deposition thin layer deposition techniques techniques employed employed for the 2GEN for the and/or 2GEN new architecturesand/or new or architectures materials [6 or]; materials this may [6]; lead this to may an incrementlead to an increment in the area in the cost, area but cost, the but cost the percost watt peakper would watt be peak reduced. would be In reduced. addition, In like addition, Si-based like cells, Si‐based 3GEN-PVCs cells, 3GEN use‐PVCs non-toxic use non and‐toxic very and abundantvery materials,abundant hence materials, are suitable hence for are the suitable large-scale for the implementation large‐scale implementation of photovoltaic of photovoltaic cells [6]. Further,cells [6]. they may employFurther, newthey nanostructuredmay employ new or organicnanostructured materials or thatorganic could materials achieve that high could conversion achieve efficiencies high conversion efficiencies (greater than 60%) using phenomena such as the hot carriers collection [28], (greater than 60%) using phenomena such as the hot carriers collection [28], the generation of multiple the generation of multiple carriers (impact ionization), or new semiconductor architectures that carrierscontain (impact multiple ionization), energy orlevels. new Considerable semiconductor attention architectures is paid thatto charge contain and multiple energy energy transfer levels. Considerableprocesses, attention and routes is to paid optimize to charge charge and collection energy and transfer improve processes, the energy and capture routes within to optimize the solar charge collectionspectrum and [29]. improve the energy capture within the solar spectrum [29]. The mostThe most important important technologies technologies included included inin the 3GEN 3GEN-PVs‐PVs are: are:  Dye‐sensitized solar cells (DSSCs); • Dye-sensitized Organic and solar polymeric cells (DSSCs);solar cells; • Organic Perovskite and polymeric cells; solar cells; • Perovskite Quantum cells; dot cells; and • Quantum Multi‐junction dot cells; cells. and The main advantages of 3GEN‐PVCs are: • Multi-junction cells.  Solution‐processable technologies; The mainSuitable advantages for large‐scale of 3GEN-PVCs production; are:  Mechanical robustness; and • Solution-processable High efficiencies at technologies; high temperatures. • SuitableHowever, for large-scale their key challenge production; is reducing the cost/watt of delivered solar electricity. • Mechanical robustness; and 4.1. Dye‐Sensitized Solar Cells • High efficiencies at high temperatures. DSSCs are low‐cost solar cells in the form of thin films based on a semiconductor formed However,between a photo their‐ keysensitized challenge anode is and reducing an electrolyte. the cost/watt They comprise of delivered five different solar electricity. layers (Figure 9) [30]: 4.1. Dye-Sensitized A transparent Solar anode Cells manufactured with a glass sheet, treated with a transparent conductive oxide layer (TCO glass); DSSCs are low-cost solar cells in the form of thin films based on a semiconductor formed between a photo-sensitized anode and an electrolyte. They comprise five different layers (Figure9)[30]:

• A transparent anode manufactured with a glass sheet, treated with a transparent conductive oxide layer (TCO glass); Int. J. Mol. Sci. 2019, 20, 976 11 of 42

• A layer of mesoporous oxide (usually TiO2) deposited onto the anode to improve electronicInt. J. Mol. conduction; Sci. 2019, 20, x 11 of 43 • A monolayer A layer of of charge mesoporous transfer oxide dye (usually covalently TiO2) deposited bonded onto to the the surface anode to of improve the mesoporous electronic oxide layer toconduction; enhance the absorption of light; • An electrolyte A monolayer containing of charge a redox transfer mediator dye covalently in an organic bonded solvent,to the surface which of the improves mesoporous the regeneration oxide of the dye;layer and to enhance the absorption of light;  An electrolyte containing a redox mediator in an organic solvent, which improves the • A cathoderegeneration made with of the a crystal dye; and coated with a catalyst (usually platinum) to facilitate the collection of electrons. A cathode made with a crystal coated with a catalyst (usually platinum) to facilitate the collection of electrons.

FigureFigure 9. Schematic 9. Schematic representation representation of a DSSC. DSSC. Taken Taken from from [30]. [ 30].

When theWhen DSSC the DSSC is exposed is exposed to sunlight, to sunlight, the the sensitizing sensitizing dye dye is is excited excited and and an electron an electron moves moves to the to the conductionconduction band of band the of mesoporous the mesoporous oxide oxide film. film. The The dye dye promotes promotes the diffusion the diffusion of electrons of electrons towards the towards the anodeanode and and is is then then reused reused in in the the external external charge charge (anode) (anode) before becoming before becoming part of the cathode part of (see the the cathode changes experienced by the species I− / I3− in Figure 9), thus completing the dye cycle. (see the changes experienced by the species I−/I − in Figure9), thus completing the dye cycle. To improve the electrical conductivity and3 the capture of light at the back layers, a conductive To improvecrystal is used, the electrical the most common conductivity being tin and oxide the doped capture with of indium light at (ITO) the and back zinc layers, oxide a doped conductive crystal iswith used, fluorine the most(FTO). common The crystal being choice tin depends oxide on doped the cell with configuration indium and (ITO) its materials and zinc [31]. oxide The doped with fluorinesemiconductor (FTO). electrode The crystal is usually choice a thin depends‐film layer on (~5–30 the μ cellm) configurationof nanocrystalline and titanium its materials dioxide [31]. The semiconductor(TiO2) deposited electrode onto the is conductive usually aglass, thin-film and it layerplays (~5–30an importantµm) ofrole nanocrystalline both in the exciton titanium separation and in the electron transfer process. The porosity and the morphology of the TiO2 layer dioxide (TiO2) deposited onto the conductive glass, and it plays an important role both in the exciton are significant factors that condition the amount of dye molecules absorbed on its surface [30], thus separation and in the electron transfer process. The porosity and the morphology of the TiO layer allowing to collect more or less incident light. 2 are significantSince factors the first that DSSC condition was introduced, the amount many ofartificial dye molecules dye molecules absorbed have been on synthesized, its surface and [30 ], thus allowingsome to collect of them more have or been less successfully incident light. launch into the market such as the so‐called N3, N719, and SinceZ907 the [30]. first The DSSC dye molecules was introduced, must fulfil some many requirements, artificial dye such molecules as matching have the solar been spectrum, synthesized, and somelong of‐term them operational have been stability, successfully and strong launch attachment into to the the marketsemiconductor such surface. as the so-calledIn addition, N3, the N719, redox potential must be high enough to facilitate the regeneration reaction with the redox mediator and Z907 [30]. The dye molecules must fulfil some requirements, such as matching the solar spectrum, [32]. This type of cells is ideal in low density applications like rooftop solar collectors, where the long-termmechanical operational robustness stability, and light and weight strong of attachment the glass‐less tocollector the semiconductor is a major advantage. surface. Conversely, In addition, the redoxthey potential are not useful must for be large high‐scale enough deployments to facilitate where higher the regeneration‐cost higher‐efficiency reaction cells with are more the redox mediatorsuitable. [32]. This type of cells is ideal in low density applications like rooftop solar collectors, where the mechanical robustness and light weight of the glass-less collector is a major advantage. Conversely, they are not useful for large-scale deployments where higher-cost higher-efficiency cells are more suitable. Int. J. Mol. Sci. 2019, 20, 976 12 of 42

DSSCs present a number of advantages; for instance, they work even in low-light conditions, hence are able to work under cloudy skies and indirect sunlight, while conventional designs would experience a “cut-out” at some lower limit of illumination. Another advantage is the cell mechanical robustness that results in higher efficiencies at higher temperatures, whilst traditional silicon cells suffer significant decreases in efficiency as the cells heat up internally. DSSCs typically comprise only a thin conductive plastic as the front layer, which enables a simple and fast dissipation of heat; hence operating at lower internal temperatures. The foremost disadvantage of the DSSCs is the use of a liquid electrolyte that has temperature stability issues. The electrolyte can freeze at low temperatures, ending power production and frequently resulting in physical damage. Conversely, higher temperatures cause the liquid to expand, making the sealing of panels an important issue. Replacing the liquid electrolyte by a solid is an important field of research. Trials with solidified melted salts are promising, albeit these are not flexible and experience higher degradation during continuous operation. An additional drawback is that the electrolyte solution contains volatile organic compounds that are toxic to human health and the environment, which combined with the permeability of the solvents through the plastics, has prevented from large-scale outdoor applications and integration into flexible structure. Advances in DSSCs are mainly based on the following research lines [33]:

• Ruthenium complexes (Ru); • Organic compounds free from metals; • Sensitizers made of quantum dots; • Sensitizers based on perovskite; • Mordanting dyes and • Natural dyes.

Ru complexes are classified into Ru polypyridyl carboxylate dyes, Ru phosphonate dyes, and bipyridyl polynuclear Ru dyes [33]. They have enhanced the efficiency from ~7.1% in 1991 to 11.2% in 2005 [34,35], and present a number of advantages including excellent stability, high absorption in the visible spectrum, excellent electron injection, and good charge transport efficiency between the metal and the ligand. However, Ru is toxic and expensive since it is a rare metal, hence current research is aimed at searching for dyes without Ru [33], such as organic dyes without metals and complex metal-porphyrin dyes. Further, Ru is a non-renewable resource and tends to undergo degradation in presence of water, therefore, is of limited use in large-scale applications. On the other hand, metal-free organic dyes are cheaper and present a wider range of molecular structures. The success of these dyes lies in their tunable physicochemical properties, as well as their good coefficient of extinction in the visible spectrum [36]. They are usually classified into donors, unions or acceptors [33], and achieve energy conversion efficiencies in the range of 5–9%, although the best efficiency attained by union dyes is ~10% [37,38], and that of the best donor reaches 10.3% [39]. In 2011, a method for obtaining metal-free sensitizers with porphyrin dyes was proposed, resulting in a very high efficiency of 12.3% [40]. Numerous porphyrin dyes have been identified as light-gathering components for DSSCs due to their powerful absorption in the visible spectrum and their redox properties for the sensitization of TiO2 sheets [33]. DSSCs with quantum dots (QDs) are based on the substitution of the dye by inorganic nanoparticles of quantum dots, which will be described below. This approach is currently being developed since enables to carry out the exciton transition with higher energy and at lower optical transition rates owing to the tunable size, high optical absorption and good magnetic properties of QDs [41]. Most of the research has been performed with cadmium compounds, reaching efficiencies close to 6.76% [42]. DSSCs based on perovskite sensitizers began to be applied in 2009 with efficiencies in the range of 3.1–3.8% [43], although currently the efficiency is close to 20% [33,44], since these sensitizers are excellent light collectors. Int. J. Mol. Sci. 2019, 20, 976 13 of 42

Natural dyes are isolated from natural compounds such as fruits, vegetables (i.e., chlorophyll and anthocyanins [45]), bacteria, flowers, etc. Betalain pigments are another class of dyes present in Caryophyllales plants that have been subjected to a detailed photoelectrochemical study since have high molar extinction coefficients in the visible region and pH dependent redox properties [46]. The pigmentsInt. are J. Mol. present Sci. 2019 in, 20, the x different part of the plant including flowers petals, fruits, leaves,13 of stems, 43 and roots. The main advantages of the natural dyes are their absorption in the visible spectrum, easiness of preparationCaryophyllales and ecological plants that viabilityhave been [subjected47,48], together to a detailed with photoelectrochemical their low production study since cost have due to the absencehigh of noble molar metalsextinction [33 coefficients]. The efficiencies in the visible obtained region and using pH dependent these dyes redox are properties low, between [46]. The 0.03 and pigments are present in the different part of the plant including flowers petals, fruits, leaves, stems, 1.50%, although in 2008 a device reached 1.7% [46]. Thus, more research is needed before natural-dyed and roots. The main advantages of the natural dyes are their absorption in the visible spectrum, SSCs caneasiness become of preparation a cost-effective and ecological and environmentally viability [47,48], friendly together with large-scale their low choice. production cost due Mordantto the absence dyes are of noble a type metals of synthetic [33]. The colorantsefficiencies appliedobtained tousing textile these fibres dyes are with low, an between enhanced 0.03 affinity for the fibresand 1.50%, due to although the addition in 2008 of a mordants, device reached which 1.7% are [46]. typically Thus, compoundsmore research such is needed as sodium before chloride, chromiumnatural alum,‐dyed or SSCs metals can such become as a iron, cost‐effective chromium, and environmentally copper, etc. [33 friendly]. large‐scale choice. TheseMordant dyes are dyes much are a cheaper type of synthetic than Ru colorants complexes, applied are to textile free fromfibres heavywith an metalsenhanced and are affinity for the fibres due to the addition of mordants, which are typically compounds such as manufacturedsodium chloride, at an industrial chromium scale. alum, Millington or metals such et as al. iron, [49] chromium, studied the copper, performance etc. [33]. of 49 commercial mordant dyesThese as sensitizers dyes are much in DSSCs cheaper and than compared Ru complexes, them to are that free of N3from ruthenium heavy metals complex. and are Although N3 led tomanufactured the highest at output, an industrial six mordant scale. Millington dyes produced et al. [49] studied photocurrents the performance higher of than 49 commercial 0.2 mA, and the UV–visiblemordant spectra dyes of as the sensitizers dye-complexed in DSSCs photoanodes and compared suggested them to that that of someN3 ruthenium mordant complex. dyes are more Although N3 led to the highest output, six mordant dyes produced photocurrents higher than 0.2 strongly bound to the TiO2 surface than N3. Further, photocatalytic oxidation of these dyes does not mA, and the UV–visible spectra of the dye‐complexed photoanodes suggested that some mordant occur in a DSSC environment. Additional research in this direction is required to determine whether dyes are more strongly bound to the TiO2 surface than N3. Further, photocatalytic oxidation of these the use ofdyes purified does not dye occur samples in a DSSC would environment. improve theirAdditional performance research in for this commercial direction is uses. required to determine whether the use of purified dye samples would improve their performance for 4.2. Quantumcommercial Dot Solar uses. Cells Quantum dots (QDs) are nano-scale semiconductor materials, belonging to groups II–VI, III–V, 4.2. Quantum Dot Solar Cells or IV–VI of the periodic table, that have a discrete spectrum of quantized energy, since the movement of the electronsQuantum and holesdots (QDs) is confined are nano‐ inscale the semiconductor three directions materials, of the belonging space. to Owing groups to II–VI, their III–V, nanoscale or IV–VI of the periodic table, that have a discrete spectrum of quantized energy, since the dimensions,movement typically of the between electrons 2–10and holes nm [is50 confined,51], they in the exhibit three propertiesdirections of that the space. are intermediate Owing to their between those ofnanoscale bulk semiconductors dimensions, typically and those between of discrete 2–10 atomsnm [50,51], or molecules. they exhibit In aproperties typical semiconducting that are material,intermediate electrons jumpbetween from those the of valence bulk semiconductors to the conduction and those band of discrete when atoms energy or higher molecules. than In the a band gap is provided.typical semiconducting However, owingmaterial, to electrons the quantum jump from confinement the valence effect,to the conduction the two bands band when are so close togetherenergy that can higher be consideredthan the band as gap continuous is provided. bands However, [50]. owing Due to to thethe smallquantum size confinement of the QDs, effect, the values the two bands are so close together that can be considered as continuous bands [50]. Due to the small of these bands are quantized, in a way that a modification of the QDs’ size implies a change in the size of the QDs, the values of these bands are quantized, in a way that a modification of the QDs’ size band gapimplies value a andchange in in the the absorption band gap value spectrum and in the (Figure absorption 10). spectrum (Figure 10).

FigureFigure 10. Absorption 10. Absorption spectra spectra of of CdSe CdSe QDs QDs with different different diameters. diameters. Taken Taken from from[51]. [51].

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Typically, QDs have a structure consisting of a core on which layers of different compounds are deposited with the aim to improve efficiency by reducing interaction forces between the exciton and the surface of the nanoparticle [50]. It is also possible to embed QDs in a matrix of other material. The conversion efficiency of this type of cells has increased over recent years, being currently above 11% [52]. This value is difficult to be increased due to the diffusion of charge carriers, thus new cell structures are required, or the combination of QDs technology with other types of cells [53–59]. It is also feasible to increase efficiency through doping with other materials [60]; for instance, Si doping can increase efficiency from 11.3% to 17.0%. The amount of QDs is another key factor in the transport and recombination of charges, that strongly affects the cell efficiency; thus, variations up to 40% have been reported depending on the QDs concentration [60,61]. As discussed earlier, current QDs research is combined with other technologies. such as DSSCs [60]. In this regard, the addition of QDs has improved the cell efficiency from approximately 1% to 11.6% [10,62–64]. Nonetheless, the theoretical efficiency limit of QDs technology is estimated to be 63% [65]. Overall, the main potential benefits of QDs-based PVCs are [66,67]:

• Favorable power to weight ratio; • High efficiency; • Mass and area savings as well as flexibility leads to miniaturization; • Lower power consumption; • Versatility; • Increased electrical performance at low production costs; and • Can be used in complete buildings including windows and not just rooftops.

It is important to note that this technology is still at the laboratory scale, hence the indicated advantages need to be tested in an industrial context. Conversely, the drawbacks of these cells are derived from the technological immaturity and the lack of experience in the development and manufacturing processes. Further, some QDs are highly toxic in nature (i.e., CdSe) and require very stable polymer shell [68]. Additionally, shells can alter optical properties, and degradation increases under aqueous and UV conditions. The particle size is also difficult to control.

4.3. Organic and Polymer Solar Cells Organic photovoltaic cells (OPVCs) are those that use conductive organic polymers or small organic molecules for light absorption and charge transport to produce electricity from sunlight. The mechanism of electricity generation in OPVCs differs from that of inorganic cells, since no free charge carriers are generated. They comprise electron donor and electron acceptor materials: the donor absorbs the photons from the solar radiation, where excited states (or excitons) are created and confined [69]. An exciton is a bound state of an electron and an electron hole attracted to each other by electrostatic interactions, which can be separated into free electron-hole pairs by effective electric fields. The acceptor is the material acquiring the electrons from the dissociated electron-hole pairs. The charge separation of an exciton into a free electron/hole pair at the donor-acceptor interface is schematized in Figure 11. Int. J.Int. Mol. J. Mol. Sci. Sci.2019 2019, 20, ,20 976, x 15 of 43 15 of 42

Figure 11. Charge separation of an exciton into a free electron/hole pair at a donor-acceptor interface. Figure 11. Charge separation of an exciton into a free electron/hole pair at a donor‐acceptor interface. Exciton dissociation is effective at the interface between materials with different electron affinity (EA) andExciton ionization dissociation potential is effective (IP) (Figure at the 11 interface). The band between gap materials in these with materials different is defined electron as the energyaffinity difference (EA) and between ionization the potential HOMO and(IP) (Figure LUMO 11). levels. The Theband open gap in circuit these voltage materials V ocis definedis the difference as betweenthe energy the HOMO difference of thebetween donor the material HOMO andand theLUMO LUMO levels. of The the acceptoropen circuit material. voltage TheVoc is differences the betweendifference the EA between and the the IP HOMO of both of materialsthe donor generatematerial and electrostatic the LUMO forces of the at acceptor the interface material. which The allow to generatedifferences stronger between electric the EA fieldsand the that IP of can both break materials the excitons generate moreelectrostatic efficiently. forces Consequently,at the interface each which allow to generate stronger electric fields that can break the excitons more efficiently. exciton generates a load-free carrier that can be collected by the electrode of the material with higher Consequently, each exciton generates a load‐free carrier that can be collected by the electrode of the EA,material while the with hole higher is accepted EA, while by the the hole material is accepted with by lower the material IP. with lower IP. OPVCsOPVCs can can be furtherbe further classified classified into into two two groups groups according according to to the the chemical chemical structure structure of of the the electron donorelectron (p-type) donor semiconductor: (p‐type) semiconductor: polymer polymer solar cells solar (PSCs) cells (PSCs) and small and small molecule molecule solar solar cells. cells. PSCs are usuallyPSCs made are usually of an indiummade of tinan oxideindium (ITO) tin oxide conductive (ITO) conductive glass covered glass bycovered a polymeric by a polymeric hole transporting hole layer,transporting an active layer, layer, an an active electron layer, transport an electron layer transport and a low layer work and functiona low work metal function electrode metal like Al (Figureelectrode 12). Inlike contrast, Al (Figure an 12). inverted In contrast, device an inverted comprises device an ITOcomprises glass withan ITO a bufferglass with layer a buffer as cathode andlayer the anode as cathode is a high and work-function the anode is a metal high like work Ag‐function or Au [70metal]. A highlike Ag work-function or Au [70]. metalA high (higher work‐function metal (higher than that of the hole transport layer) and a low work‐function metal than that of the hole transport layer) and a low work-function metal (lower than that of the electron (lower than that of the electron transport layer) are required in order to implement ohmic contacts, transport layer) are required in order to implement ohmic contacts, preventing the formation of preventing the formation of Schottky junctions (which show rectifying behaviour, increasing the Schottkydifficulty junctions of current (which extraction, show rectifyingand decreasing behaviour, device efficiency). increasing the difficulty of current extraction, and decreasing device efficiency).

Int. J. Mol. Sci. 2019, 20, 976 16 of 42 Int. J. Mol. Sci. 2019, 20, x 16 of 43 Int. J. Mol. Sci. 2019, 20, x 16 of 43

Figure 12. Schematic representation of a typical (a) and inverted (b) polymer solar cell (PSC). Taken fromFigure [69 12.]. Schematic representation of a typical (a) and inverted (b) polymer solar cell (PSC). Taken Figure 12. Schematic representation of a typical (a) and inverted (b) polymer solar cell (PSC). Taken from [69]. from [69]. The order and chemical nature of the layers and the metal electrode strongly condition the The order and chemical nature of the layers and the metal electrode strongly condition the performanceThe order of PSCs, and and chemical consequently nature of their the layers efficiency and [the71 ].metal electrode strongly condition the performance of PSCs, and consequently their efficiency [71]. performancePSCs can be of configured PSCs, and consequently as single-junction their efficiency or hetero-junction, [71]. as schematized in Figure 13[ 72]. PSCs can be configured as single‐junction or hetero‐junction, as schematized in Figure 13 [72]. The hetero-junctionPSCs can be architectureconfigured as consists single‐junction in a sandwich or hetero connection‐junction, as ofschematized organic materials in Figure between 13 [72]. two The hetero‐junction architecture consists in a sandwich connection of organic materials between two metallicThe conductors,hetero‐junction typically architecture ITO, consists separated in a by sandwich a metal connection coating such of organic as Al, Mg, materials or Ca; between the organic two layer metallic conductors, typically ITO, separated by a metal coating such as Al, Mg, or Ca; the organic metallic conductors, typically ITO, separated by a metal coating such as Al, Mg, or Ca; the organic withlayer the with highest the highest EA and EA IPand value IP value would would be be the the acceptor, acceptor, while while the the other would would be be the the donor. donor. A A more layer with the highest EA and IP value would be the acceptor, while the other would be the donor. A efficientmore efficient option option is the useis the of bulkuse of hetero-junctions bulk hetero‐junctions (BHJ) (BHJ) [73], formed[73], formed by blending by blending an electron-rich an more efficient option is the use of bulk hetero‐junctions (BHJ) [73], formed by blending an conjugatedelectron‐rich polymer conjugated as donorpolymer and as donor an electron-deficient and an electron‐deficient fullerene fullerene as acceptor as acceptor with a with bicontinuous a electron‐rich conjugated polymer as donor and an electron‐deficient fullerene as acceptor with a bicontinuous nanoscale interpenetrating network; since the area of the D‐A interface is significantly nanoscalebicontinuous interpenetrating nanoscale interpenetrating network; since network; the area since of the the D-A area interface of the D‐A is interface significantly is significantly increased, the increased, the exciton dissociation efficiency is improved compared to the other architectures. excitonincreased, dissociation the exciton efficiency dissociation is improved efficiency compared is improved to thecompared other architectures.to the other architectures.

Figure 13. Schematic representation of (A) single-junction and (B) hetero-junction solar cells. Taken Figure 13. Schematic representation of single‐junction and hetero‐junction solar cells. Taken from fromFigure [72]. 13. Schematic representation of single‐junction and hetero‐junction solar cells. Taken from [72]. [72]. A very wide range of polymers have been used for the manufacture of PSCs [74–76]; the chemical structure of some of the most representative conjugated polymers employed in BHJ PSCs is shown in Figure 14. The alternating carbon–carbon double bonds are common to all of them, and are responsible for their electronic properties, low-energy optical transitions and high-electron affinities. Int. J. Mol. Sci. 2019, 20, x 17 of 43

A very wide range of polymers have been used for the manufacture of PSCs [74–76]; the Int.chemical J. Mol. Sci. structure2019, 20, 976 of some of the most representative conjugated polymers employed in BHJ PSCs17 of is 42 shown in Figure 14. The alternating carbon–carbon double bonds are common to all of them, and are responsible for their electronic properties, low‐energy optical transitions and high‐electron affinities. Raw conjugated polymers are typically insulators, and become conductive via oxidation (p-doping) or Raw conjugated polymers are typically insulators, and become conductive via oxidation (p‐doping) reduction (n-doping). or reduction (n‐doping).

FigureFigure 14. 14.Chemical Chemical structurestructure ofof conjugatedconjugated polymerspolymers typically used in polymer solar solar cells. cells. Taken Taken fromfrom [ 69[69].]. The developments in PSCs can be classified according to the layer in which the polymers The developments in PSCs can be classified according to the layer in which the polymers are are introduced. For the substrate design, little thickness and flexibility are desirable [77]. introduced. For the substrate design, little thickness and flexibility are desirable [77]. Thus, polyethylene Thus,terephthalate polyethylene (PET) terephthalateor polyethylene (PET) naphthalate or polyethylene (PEN) are naphthalate frequently employed (PEN) are since frequently they have employed good sincethermal they and have mechanical good properties thermal and and good mechanical stability propertiesduring solvent and treatments good stability [78]. during solvent treatmentsRecently [78]. poly(3,4‐ethylenedioxythiophene)‐poly(styrene sulfonate) (PEDOT:PSS) has captured the Recentlyinterest of poly(3,4-ethylenedioxythiophene)-poly(styrene scientists due to its flexibility and processing capacity sulfonate) in (PEDOT:PSS)solution [79]. hasIn addition, captured thePEDOT:PSS interest of has scientists good transparency, due to its flexibility mechanical and processingresistance, ability capacity to intransport solution holes, [79]. Inhigh addition, work PEDOT:PSSfunction, and has good good stability transparency, at room temperature. mechanical resistance, It has been ability applied to as transport an anode, holes, cathode, high and work as function,both electrodes and good simultaneously. stability at room In this temperature. regard, Hau It haset al. been developed applied solution as an anode,‐processed cathode, functional and as bothdevices, electrodes although simultaneously. they exhibited In low this efficiency regard, Hau [80]. et On al. developedthe other hand, solution-processed PEDOT:PSS has functional several devices,shortcomings although including they exhibited hygroscopic low character efficiency and [80 inhomogeneous]. On the other electrical hand, PEDOT:PSS properties hasthat several result shortcomingsin low durability including [81]. hygroscopic character and inhomogeneous electrical properties that result in low durabilityPoly(3‐hexylthiophene) [81]. (P3HT), with a wide band gap of 1.9 eV, is also broadly used in PSCs owingPoly(3-hexylthiophene) to its superior electronic (P3HT), properties with combined a wide band with gap its ability of 1.9 eV,to self is also‐assemble broadly and used to be in easily PSCs owing to its superior electronic properties combined with its ability to self-assemble and to be easily dissolved in organic solvents. Polyaniline (PANI) is also very attractive owing to its low price, simple synthesis, and high environmental stability. In addition, it shows an acid/base doping response and Int. J. Mol. Sci. 2019, 20, x 18 of 43 Int. J. Mol. Sci. 2019, 20, 976 18 of 42 dissolved in organic solvents. Polyaniline (PANI) is also very attractive owing to its low price, itssimple electrical synthesis, properties and canhigh be environmental tuned by changing stability. its oxidationIn addition, and it protonation shows an stateacid/base [82]. doping On the otherresponse hand, and other its electrical polymers properties display low can solubility be tuned like by poly(p-phenylene changing its oxidation vinylene) and (PPV), protonation polypyrrole state (PPy),[82]. On and the poly(p-phenylene) other hand, other (PPP), polymers which restrictsdisplay theirlow applicability.solubility like Further, poly(p PPV‐phenylene suffers fromvinylene) poor absorption(PPV), polypyrrole and photodegradation (PPy), and poly(p [83].‐phenylene) (PPP), which restricts their applicability. Further, PPV PSCssuffers suffer from from poor differentabsorption types and ofphotodegradation stresses, atmospheric [83]. exposure and electrical polarization [meterPSCs REF: suffer Improving from different efficiency types of and stresses, stability atmospheric of perovskite exposure solarand electrical cells with polarization photocurable [meter fluoropolymers].REF: Improving efficiency To improve and thestability stability of perovskite of the PSCs, solar fluoropolymeric cells with photocurable coatings fluoropolymers]. can be applied onTo bothimprove sides the of stability the cell. Recently,of the PSCs, fluoropolymers fluoropolymeric have coatings been incorporated can be applied in PSCs on both due tosides their of excellent the cell. opticalRecently, and fluoropolymers mechanical properties have been and incorporated their extraordinary in PSCs due stability to their against excellent ultraviolet optical radiation.and mechanical Thus, polytetrafluoroethyleneproperties and their extraordinary (PTFE) has stability been usedagainst as ultraviolet the ITO buffer radiation. layer Thus, owing polytetrafluoroethylene to its low chemical reactivity,(PTFE) has high been thermal used as stability, the ITO excellentbuffer layer moisture owing resistance,to its low chemical flexibility, reactivity, light weight, high andthermal low coststability, [84]. excellentAnother moisture aspect resistance, that should flexibility, be considered light weight, for the and development low cost [84]. of electrodes is the work function, sinceAnother it must aspect decrease that gradually should be for considered a correct charge for the transport. development Currently, of electrodes methods is tothe reduce work electrodefunction, valuessince it using must ethoxylated decrease gradually polyethyleneimine for a correct (PEIE), charge and polyethyleniminetransport. Currently, (PEI) methods have been to proposedreduce electrode [79]. values using ethoxylated polyethyleneimine (PEIE), and polyethylenimine (PEI) haveOPVCs been proposed present lower[79]. efficiencies than cells based on inorganic compounds [73], owing to their technologicalOPVCs present and market lower maturityefficiencies and than other cells factors based on such inorganic as their compounds wide band [73], gap. owing The highestto their efficienciestechnological reported and market range betweenmaturity 9.7–11.2% and other [13 factors], and taking such intoas their account wide the band increasing gap. The trend highest found withinefficiencies the last reported years, range it is expected between that 9.7–11.2% they will [13], improve and taking further into (Figure account 15). the The increasing best laboratory trend performancefound within in the OPVCs last years, (22.4%) it has is beenexpected attained that withthey P3HT will improve as donor materialfurther (Figure [73]. 15). The best laboratory performance in OPVCs (22.4%) has been attained with P3HT as donor material [73].

Figure 15.15. Power conversion efficienciesefficiencies vs. time for OPVCs. Taken from [[79].79].

Overall, OPVCs display important advantages compared to previousprevious generationsgenerations includingincluding flexibility,flexibility, lightweight,lightweight, lower processing costs and lessless environmentalenvironmental impactimpact [[85,86].85,86]. In addition, the cellcell layers layers can can be be deposited deposited via via solution-based solution‐based methods methods like spin-coatinglike spin‐coating or printing, or printing, which enablewhich deviceenable fabricationdevice fabrication at a large at a scale large at scale low at temperatures, low temperatures, hence reducinghence reducing the associated the associated costs. costs. PSCs canPSCs also can be also transparent be transparent at the expense at the of expense a lower efficiency,of a lower which efficiency, is interesting which foris applicationsinteresting for in windows,applications walls, in windows, flexible electronics, walls, flexible and electronics, so forth. Their and main so forth. disadvantages Their main are disadvantages their low efficiencies are their andlow thatefficiencies are susceptible and that to are photochemical susceptible to degradation photochemical [83 ].degradation [83]. 4.4. Perovskite Solar Cells 4.4. Perovskite Solar Cells Perovskite solar cells (PVSCs) include a perovskite structured compound (see Figure 16[ 87]), Perovskite solar cells (PVSCs) include a perovskite structured compound (see Figure 16 [87]), most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting most commonly a hybrid organic‐inorganic lead or tin halide‐based material, as the light‐harvesting active layer, which is placed between the electron transport layer (ETL) (usually mesoporous active layer, which is placed between the electron transport layer (ETL) (usually mesoporous material or flat TiO2) and the hole transport layer (HTL) [87]. In the standard device configuration, material or flat TiO2) and the hole transport layer (HTL) [87]. In the standard device configuration,

Int. J. Mol. Sci. 2019, 20, 976 19 of 42 Int. J. Mol. Sci. 2019, 20, x 19 of 43 the front transparent electrode is a fluoride fluoride-doped‐doped tin oxide sheet (FTO), and the back electrode is a thermallythermally-evaporated‐evaporated gold layer.

FigureFigure 16. Perovskite 16. Perovskite structure: structure: A) monovalent (A) monovalent cations, cations, B) lead, (B) X) lead, iodine. (X) iodine. Taken Takenfrom [87]. from [87].

PVSCs also have have a a D D-A‐A interface. interface. However, However, unlike unlike OPVCs, OPVCs, the the excitons excitons do do not not need need to to have have a longa long lifetime lifetime since since the the absorption absorption of of photons photons practically practically results results in in the the generation generation of of free charge carriers [[87].87]. ThisThis non-generation non‐generation nature nature of excitonsof excitons is critical is critical for the for device the device performance. performance. The efficient The efficientgeneration generation of electrons of electrons and holes and in a holes single in stage a single is one stage of the is main one advantagesof the main of advantages the PVSCs, of since the PVSCs,the energy since losses the due energy to the losses generation due ofto excitons, the generation their movement of excitons, and their their dissociation movement are and avoided. their dissociationIn addition, theare raw avoided. materials In addition, used and the the raw fabrication materials methods used and (such the as fabrication printing techniques) methods (such are both as printinglow cost [techniques)88]. They are are simple both to low manufacture, cost [88]. andThey their are high simple absorption to manufacture, coefficient allowsand their to absorb high absorptionthe whole visible coefficient solar allows spectrum to absorb with ultrathin the whole films visible (~500 solar nm) [spectrum89]. These with characteristics ultrathin films make (~500 them nm)ideal [89]. candidates These characteristics to fabricate low make cost, them high ideal efficiency, candidates thin, lightweight,to fabricate low and cost, flexible high solar efficiency, modules. thin, lightweight,One the and other flexible hand, onesolar of modules. the main challenges for PVSCs is the short-term and long-term stability. TheirOne instability the other is mostly hand, associated one of the to environmentalmain challenges moisture for PVSCs and oxygen,is the short thermal‐term influence, and long heating‐term stability.under and Their applied instability voltage, is UV mostly and visible associated lights andto environmental mechanical fragility moisture [90– 92and]. oxygen, thermal influence,In particular, heating under the water-solubility and applied voltage, of the absorberUV and visible material lights makes and the mechanical cells highly fragility prone [90–92]. to fast degradationIn particular, under the moist water environments.‐solubility of the This absorber degradation material can makes be minimized the cells highly by optimizing prone to fast the degradationconstituent materials, under moist the cell environments. architecture, theThis interfaces degradation and the can environmental be minimized conditions by optimizing during the constituentfabrication stages.materials, For the instance, cell architecture, it has been reported the interfaces that the and encapsulation the environmental of the perovskite conditions absorber during withinthe fabrication a carbon stages. nanotube For composite instance, andit has an been inert polymerreported matrix that the hinders encapsulation the degradation of the inperovskite a humid absorberatmosphere within and a at carbon elevated nanotube temperatures composite [93]. and an inert polymer matrix hinders the degradation in a humidAnother atmosphere key challenge and forat elevated this type temperatures of cells is that [93]. their current-voltage curves show a hysteretic behaviourAnother depending key challenge on the for measurement this type of cells conditions is that their (i.e., scancurrent direction,‐voltage scan curves speed, show light a hysteretic soaking, behaviourbiasing). Several depending reasons on the have measurement been proposed conditions to explain (i.e., such scan behaviour direction, scan including speed, ion light movement, soaking, biasing).polarization, Several ferroelectric reasons effects,have been filling proposed of trap states,to explain albeit such the exactbehaviour origin including is not known ion movement, yet [94]. polarization,In PVSCS, ferroelectric the separation effects, of filling charges of trap can states, take placealbeit eitherthe exact by origin the injection is not known of the yet electrons [94]. photo-generatedIn PVSCS, the in theseparation ETL or byof charges the injection can take of the place holes either in the by HTL. the Further,injection the of freethe electrons photogenerated‐generated near the in perovskite/HTLthe ETL or by the interface injection should of the diffuse holes in through the HTL. the Further, full layer the thickness free electrons before generatedbeing extracted near atthe the perovskite/HTL ETL/perovskite interface interface, should which diffuse increases through the possibilities the full layer of recombination thickness before [95]. beingSimilar extracted phenomenon at the occurs ETL/perovskite to the holes interface, near the ETL/perovskitewhich increases interface. the possibilities Recent works of recombination have shown [95].that bothSimilar processes phenomenon take place occurs simultaneously to the holes [near87]. the ETL/perovskite interface. Recent works have shownPerovskite that both cells processes have improved take place their simultaneously efficiency from [87]. an initial value of 3.8% in 2009 [43] to 22.13% in 2018Perovskite in single-junction cells have architectures improved their [96,97 efficiency]. Current researchfrom an focusesinitial value on improving of 3.8% efficiency,in 2009 [43] while to 22.13%trying toin solve 2018 thein aforementionedsingle‐junction architectures challenges of [96,97]. this type Current of cells research [96]. It hasfocuses been on reported improving that efficiency, while trying to solve the aforementioned challenges of this type of cells [96]. It has been reported that solid‐state perovskite cells exhibit higher efficiency than those with liquid electrolytes,

Int. J. Mol. Sci. 2019, 20, 976 20 of 42

Int.solid-state J. Mol. Sci. 2019 perovskite, 20, x cells exhibit higher efficiency than those with liquid electrolytes, therefore20 of 43 the use of solid electrolytes is recommended. The improvement of the efficiency is usually approached thereforeby means the of use the of optimization solid electrolytes of the is perovskite recommended. composition, The improvement the deposition of the method efficiency applied, is usually and the approachedarchitecture by of means the device of the [96 ]:optimization of the perovskite composition, the deposition method applied,• Composition: and the architecture the highest of the efficiencies device [96]: achieved (21.1–21.6%) have been obtained by mixing  Composition:different cations the highest to form efficiencies the perovskite achieved [98,99 (21.1–21.6%)]. Further, band have gaps been are obtained tunable by and mixing can be differentoptimized cations for the to solarform spectrum the perovskite by altering [98,99]. the Further, halide content band gaps in the are film tunable (i.e., by and mixing can Ibe and optimizedBr); for the solar spectrum by altering the halide content in the film (i.e., by mixing I and Br); • Deposition method: a good interface between the different cell layers improves the efficiency.  Deposition method: a good interface between the different cell layers improves the efficiency. In In this regard, a value of 19.7% has been reached when using DMF and DMSO as solvents during this regard, a value of 19.7% has been reached when using DMF and DMSO as solvents during the spin-coating process [100]; and the spin‐coating process [100]; and • Cell architecture: this parameter strongly affects the final cell efficiency. For instance, the use of  Cell architecture: this parameter strongly affects the final cell efficiency. For instance, the use of TiO2 with fullerene as ETL enabled the fabrication of a hysteresis-free device, with an efficiency of TiO2 with fullerene as ETL enabled the fabrication of a hysteresis‐free device, with an efficiency 19.1% [101]. Additional layers should also be considered, such as the anti-reflective coating, as of 19.1% [101]. Additional layers should also be considered, such as the anti‐reflective coating, well as the thickness and strength of each layer [102,103]. as well as the thickness and strength of each layer [102,103]. 4.5. Multi-Junction Solar Cells 4.5. Multi‐Junction Solar Cells Multi-junction (MJ) solar cells comprise multiple p–n junctions made of different semiconductor materials,Multi‐junction and each of(MJ) them solar produces cells electric comprise current multiple in response p–n to differentjunctions wavelengths made of ofdifferent light, thus semiconductorincreasing the materials, conversion and of incident each of sunlight them intoproduces electrical electric energy current and the in device response efficiency. to different The idea wavelengthsof using different of light, materials thus increasing with different the conversion band gap of is incident proposed sunlight to take advantageinto electrical of the energy maximum and thepossible device photons.efficiency. The The whole idea cellof using can be different made of materials the same with material different or different band gap ones, is offering proposed a wide to takerange advantage of design of possibilities. the maximum A schematicpossible photons. representation The whole of the cell general can be structure made of of the a tandemsame material cell with ortwo different junctions ones, also offering known a as wide double-junction range of design cell, ispossibilities. shown in Figure A schematic 17[ 104]. representation They typically of include the generalthe following structure elements: of a tandem cell with two junctions also known as double‐junction cell, is shown in Figure 17 [104]. They typically include the following elements:  • A Atransparent transparent electrode electrode (i.e., (i.e., TCO), TCO), which which covers covers a larger a larger band band gap gap cell cell that that will will be be the the first first to to capturecapture the the radiation. radiation.  • A Arecombination recombination layer, layer, which which is isapplied applied either either as as a tunnel a tunnel connection connection or or a TCO a TCO depending depending on on whetherwhether a serial a serial or or a parallel a parallel connection connection of of the the cells cells is isrequired. required.  • A Acell cell with with lower lower band band gap gap than than the the previous previous one. one.  • A Aback back contact. contact.

Figure 17. (a) Irradiance (black line) and absorption coefficient (red and green lines) vs. wavelength. Figure 17. (a) Irradiance (black line) and absorption coefficient (red and green lines) vs. wavelength. (b) Schematic representation of a double-junction plastic solar cell. Taken from [104]. (b) Schematic representation of a double‐junction plastic solar cell. Taken from [104].

In general, there are two types of designs: the monolithically integrated tandem and the mechanically stacked tandem. Nevertheless, it should be noted that the mechanical stacking

Int. J. Mol. Sci. 2019, 20, 976 21 of 42

Int. J. Mol. Sci. 2019, 20, x 21 of 43 Int.In J. Mol. general, Sci. 2019 there, 20, x are two types of designs: the monolithically integrated tandem and21 the of 43 mechanicallytechnology is stacked difficult tandem. to scale Nevertheless, up and itrelatively should be notedexpensive that thenowadays. mechanical The stacking manufacture technology of ismulti difficulttechnology‐junction to scale isdevices difficult up and is relatively challengingto scale expensiveup since and the nowadays.relatively extraction expensive The of manufacturecurrents nowadays. is not of multi-junctiontrivial. The Themanufacture theoretical devices of isefficiencies challengingmulti‐junction for since MJ devices cells the extraction with is challenging different of currents number since is notofthe unions trivial.extraction are The plotted of theoretical currents in Figure efficienciesis not 18. trivial. for MJThe cells theoretical with differentefficiencies number for ofMJ unions cells with are plotteddifferent in number Figure 18of. unions are plotted in Figure 18.

Figure 18. Efficiencies for MJ cells as a function of the number of junctions. Taken from [105]. FigureFigure 18 18.. EfficienciesEfficiencies for for MJ MJ cells cells as as a a function function of of the the number number of of junctions. junctions. Taken Taken from from [105]. [105]. Clearly, as the number of junctions increases, the efficiency raises. Nevertheless, a high number of junctionsClearly,Clearly, asmakes theas the number the number choice of junctions of of junctions the different increases, increases, layers the efficiencythe difficult, efficiency raises. and raises. increases Nevertheless, Nevertheless, the complexity a high a high number ofnumber the of junctionsdevice;of junctions therefore, makes makes the the choice optimumthe choice of the balance differentof the betweendifferent layers difficult,these layers three difficult, and parameters increases and increases the has complexity to be the investigated. complexity of the device; of the therefore,device;This thetherefore,type optimum of cells the appears balanceoptimum to between balancebe the theseonly between realistic three these parameters option three toparameters has achieve to be extremely investigated. has to be highinvestigated. efficiencies, and ThistheThis monolithic type type of cellsof structurecells appears appears will to be tolead the be onlytothe the only realistic best realistic results option option[105]. to achieve However, to achieve extremely the extremely manufacturing high efficiencies, high efficiencies, of large and thesurfaceand monolithic the cells monolithic requires structure structureexpensive will lead will toproduction the lead best to resultsthe processes, best [105 results]. However,thus [105]. the However,combined the manufacturing the use manufacturing of multiple of large surfacesmaller of large cellscellssurface requires to cover cells expensive therequires same production expensivearea seems processes,production to be economically thus processes, the combined morethus theviable use combined of multiplenowadays. use smaller Anof multiple alternative cells to coversmaller to theincreasecells same to areaefficiency cover seems the is to samethe be combination economically area seems of moreto thesebe viableeconomically smaller nowadays. cells more with An concentrationviable alternative nowadays. to increasetechnologies, An efficiencyalternative which is to theconcentrateincrease combination efficiency hundreds of these is or the smaller thousands combination cells of with times of concentration these the sunlight, smaller technologies, cellsleading with to concentrationhigher which energy concentrate technologies, production hundreds [105]. which or thousandsCurrently,concentrate of40% times hundreds efficiency the sunlight, or thousandshas been leading surpassedof to times higher the with energy sunlight, multi production leading‐junction to [ 105 cellshigher]. Currently,[106–109], energy production 40%and efficiency they [105].are hasexpectedCurrently, been surpassed to even40% withefficiencysurpass multi-junction 50% has [110],been cells surpassedalthough [106–109 significantwith], and multi they are‐researchjunction expected cellsis tocarried even[106–109], surpass out inand 50% different they [110 ],are althoughdirections,expected significant asto depicted even research surpass in Figure is carried50% 19. [110], out in although different directions,significant as research depicted is in Figurecarried 19 out. in different directions, as depicted in Figure 19.

Figure 19. Technological tools and processes that are used to design new high-efficiency III-V multi-junctionFigure 19. Technological solar cells. Taken tools fromand [processes105]. that are used to design new high‐efficiency III‐V multiFigure‐junction 19. Technological solar cells. Taken tools from and [105]. processes that are used to design new high‐efficiency III‐V multi‐junction solar cells. Taken from [105].

Int. J. Mol. Sci. 2019, 20, 976 22 of 42

As can be observed in the figure, the substrates are often based on 1GEN and 2GEN compounds. For instance, an efficiency of 35.8% has been achieved with an InGaP/GaAs/InGaAs cell on a Ge substrate, and it is expected that with further improvements, efficiencies higher than 50% could be attained, hence becoming a promising alternative for terrestrial and space applications [111]. Several growth methods can be applied, and allow the synthesis of different structures focused on different applications. A well-studied alternative over recent years is the metamorphic growth, which is based on the progressive monolithic growth of materials with gradually higher lattice constants [107,112,113]. This growth would likely reduce the price of substrates used in cells based on chemical compounds of the III–V groups [105]. Currently, efficiencies close to 40% have been reached under sunlight concentration, although theoretical calculations indicate higher values. Similarly to OPSCs, inverted cell growth is feasible, as well as its combination with another techniques such as metamorphic growth to achieve better efficiencies [105]. One of the advantages of the inverted growth is that the growth of the buffer layer is delayed, hence the potential dislocations due to the transfer of the lattice constant do not affect the cells located on top. Another advantage is that the material employed to fabricate the bottom cell can be more flexible depending on whether an epitaxial growth is performed with or without Ge. In addition, the substrate can be recycled to manufacture several cells, which would reduce the large-scale production costs. By applying this inverted metamorphic growth, cells with efficiencies between 40–44.4% have been developed [114]. Overall, albeit MJ solar panels are highly efficient, are more expensive than other technologies, and this implies different applications: MJ solar cells are preferred in space whilst c-Si solar cells are better for terrestrial applications. In order to extend the use of MJ cells, large-area, cost-effective, and highly reproducible fabrication processes need to be developed. Nevertheless, MJ solar cells do have the potential to have an important penetration in terrestrial applications in concentrator systems.

5. Fourth-Generation Photovoltaic Solar Cells The 4GEN combines the low cost/flexibility of polymer thin-films with the good stability of nanomaterials like metallic nanoparticles, metal oxides, carbon nanotubes, graphene, and its derivatives. These architectures will maintain the advantage of solution processable devices, hence cheap manufacture, but also incorporate nanomaterials to improve the charge dissociation and charge transport within the cells. In particular, special emphasis is placed on graphene (G), which has become the nanomaterial with the highest scientific and technological expectations. Currently, some researchers consider G as the fundamental unit of graphitic structures [115]; the different arrangements of honeycomb like carbon atoms lead to different allotropes with different dimensionalities, as depicted in Figure 20: OD fullerenes, 1D carbon nanotubes, 2D graphene, and 3D graphite.

5.1. Graphene and Its Derivatives G is a two-dimensional material composed of carbon atoms that form an atomically-thick layer with a honeycomb lattice structure (Figure 20). The carbon atoms are linked by covalent bonds in the same layer and by Van der Walls forces between layers. Each layer presents a flat sheet appearance although, due to the carbon hybridization and the possible imperfections and impurities, it presents small undulations. The sp2 hybridization of carbon atoms also determines that the free electron of the 2px orbital can form π bonds with other atoms. This chemical configuration gives the material exceptional chemical, electronic and mechanical properties. G is an exceptional electrical and thermal conductor: its electrical conductivity is superior to that of Cu or Si [116], and it has such a high thermal conductivity (G: 3.000–5.000 W·m−1·K−1 vs. Cu: 400 W·m−1·K−1 [117,118]) that it is one of the best thermal conductors. G also presents very high electron mobility (15,000 cm2/V s) and very large specific surface area (SSA ≈ 2.630 m2/g [118]). In addition, G is one of the strongest materials on earth, with an elastic modulus close to 1 TPa, a tensile strength of 130 GPa and a breaking strength of ∼40 N/m [118,119]. The combination of these exceptional properties make G an excellent candidate for application in photovoltaic cells. Further, G sheets are flexible and chemically inert, which leads to Int. J. Mol. Sci. 2019, 20, 976 23 of 42

Int. J. Mol. Sci. 2019, 20, x 23 of 43 a double role application: as an electrode and as a protective layer. Nonetheless, some issues need toto be be solved. solved. G G is is almost almost transparent transparent since since it it just just absorbs absorbs 2.3% 2.3% of of the the radiation radiation [118]; [118]; this this can can be be a a problemproblem for for solar solar cell cell applications, applications, since since it it is is not not able able to to capture capture the the photons, photons, but but can can be be solved solved by by meansmeans of of doping. doping.

FigureFigure 20.20. DifferentDifferent arrangements arrangements of honeycomb of honeycomb like carbon like atomscarbon depending atoms ondepending their dimensionality. on their dimensionality.Taken from [115 Taken]. from [115].

G can be obtained by different production methods. The mechanical exfoliation of the graphite G can be obtained by different production methods. The mechanical exfoliation of the graphite allows to obtain clean surfaces of G, which present a high mobility of load carriers [120,121]. It can allows to obtain clean surfaces of G, which present a high mobility of load carriers [120,121]. It can also be obtained by chemical exfoliation of graphite, either by means of a dispersion with metal also be obtained by chemical exfoliation of graphite, either by means of a dispersion with metal ions ions [118,122], or a simple sonication process followed by a dispersion in an organic solvent. Another [118,122], or a simple sonication process followed by a dispersion in an organic solvent. Another approach is the epitaxial growth of G on SiC [118,121] by vacuum graphitization (thermal treatment of approach is the epitaxial growth of G on SiC [118,121] by vacuum graphitization (thermal treatment SiC at ~1300 ◦C under vacuum). Although the G obtained by this method has good properties and of SiC at ~1300 °C under vacuum). Although the G obtained by this method has good properties and can be molded by lithography methods, the applicability to large-scale production is limited due to can be molded by lithography methods, the applicability to large‐scale production is limited due to the high cost of monocrystalline SiC sheets [118]. In addition, a previous work has shown that the the high cost of monocrystalline SiC sheets [118]. In addition, a previous work has shown that the properties of G depend on the effects of the interface with the substrate sheet [121]; in this regard, properties of G depend on the effects of the interface with the substrate sheet [121]; in this regard, the the use of Ru has been investigated, and epitaxial G was successfully grown [121]. Chemical vapour use of Ru has been investigated, and epitaxial G was successfully grown [121]. Chemical vapour deposition (CVD) is the most widely used since allows obtaining large quantities of G in a simple and deposition (CVD) is the most widely used since allows obtaining large quantities of G in a simple fast way [123]. G grows on a substrate (usually Ni, Cu, Pd, Pt, or [121,124]) as a C gas steam (usually and fast way [123]. G grows on a substrate (usually Ni, Cu, Pd, Pt, or [121,124]) as a C gas steam methane) flows through the chamber; G can be doped by adding other gases during the process [118]. (usually methane) flows through the chamber; G can be doped by adding other gases during the The CVD method enables to obtain G with few structural defects at a large-scale. Some drawbacks of process [118]. The CVD method enables to obtain G with few structural defects at a large‐scale. Some drawbacksthis method of are this the method control ofare the the G filmcontrol thickness of the and G film the elevatedthickness cost and of the substrates.elevated cost of the substrates.Graphene oxide (GO) is a modified form of G that incorporates carboxylic groups at the edges of theGraphene sheet, as oxide well (GO) as epoxide is a modified and hydroxyl form of groupsG that incorporates within the basal carboxylic plane groups [125]. The at the chemical edges of the sheet, as well as epoxide and hydroxyl groups within the basal plane [125]. The chemical structure of GO is schematized in Figure 21. Some properties of GO are different from those of G. The anchored groups and lattice defects change the electronic structure, hence it presents

Int. J. Mol. Sci. 2019, 20, 976 24 of 42 structure of GO is schematized in Figure 21. Some properties of GO are different from those of G. TheInt. anchored J. Mol. Sci. 2019 groups, 20, x and lattice defects change the electronic structure, hence it presents considerably24 of 43 lower electron mobility and electrical conductivity, being usually insulating (sheet resistance of 1012considerablyΩ/sq). Conversely, lower electron it has mobility larger and surface electrical area andconductivity, better ability being to usually retain compoundsinsulating (sheet in the interlaminarresistance of space. 1012 Ω It/sq). can Conversely, be processed it has in aqueous larger surface solutions, area and has better amphiphilic ability to character retain compounds and surface functionalizationin the interlaminar capability, space. It it can is versatile,be processed biocompatible in aqueous solutions, and can formhas amphiphilic stable aqueous character colloids and to enablesurface the functionalization assembly of macroscopic capability, structures, it is versatile, which biocompatible is decisive for and large-scale can form applicationsstable aqueous [126 ]. Additionally,colloids to itenable can be the deposited assembly on aof substrate macroscopic and later structures, converted which into ais conductor, decisive for henceforth large‐scale being a perfectapplications candidate [126]. to Additionally, be used in solar it can cells. be deposited on a substrate and later converted into a conductor, henceforth being a perfect candidate to be used in solar cells. OH HOOC

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HO HO

COOH

HOOC OH Figure 21. Chemical structure of graphene oxide (GO). Figure 21. Chemical structure of graphene oxide (GO). GO can be synthesized from graphite by means of a chemical exfoliation of graphite oxide [127], GO can be synthesized from graphite by means of a chemical exfoliation of graphite oxide [127], oror from from GO GO using using four four main main methods methods [[121,126]:121,126]: Staudenmaier, Hofmann, Hofmann, Brodie, Brodie, and and Hummers. Hummers. TheThe most most usually usually employed employed is is the the Hummers’ Hummers’ method,method, which consists consists in in an an oxidation oxidation process process using using KMnOKMnO4, NaNO4, NaNO3,3 and, and H H22SOSO44 [[128].128]. ReducedReduced graphene graphene oxide oxide (rGO) (rGO) cancan bebe obtainedobtained by heat heat treatment treatment or or by by chemical chemical or or electrical electrical reductionreduction of GOof GO to eliminate to eliminate some some functional functional groups groups [125,129 [125,129].]. Different Different reagents reagents including including hydrazine monohydratehydrazine monohydrate (N2H4·H2O), (N2 NaBH4,H4∙H2O), hydroiodicNaBH4, hydroiodic acid (HI) acid and (HI) urea and urea have have been been employed employed for for the chemicalthe chemical reduction reduction of GO of into GO rGO into [ 130rGO,131 [130,131].]. rGO hasrGO better has better electrical electrical properties properties than than GO [GO126 ],[126], owing toowing its lower to degreeits lower of functionalization,degree of functionalization, hence it is alsohence a goodit is also candidate a good for candidate solar cell for applications. solar cell applications. 5.2. Graphene Applications Tendencies 5.2. Graphene Applications Tendencies G is a promising material for energy-related applications, both in inorganic and organic cells, hybridsG cells is a andpromising DSSCs material [118]. It for is mainlyenergy‐ usedrelated as applications, an additional both layer in ininorganic functional and cells organic of previous cells, generations,hybrids cells since and it DSSCs allows [118]. to obtain It is mainly cells with used improved as an additional properties. layer In in particular,functional cells it has of been previous mostly usedgenerations, in transparent since it conductive allows to obtain electrode cells (TCEs), with improved active layers properties. and interfacial In particular, layers, it has that been is, mostly the hole transportingused in transparent layer and conductive electron transporting electrode (TCEs), layer betweenactive layers the and active interfacial layer and layers, the anode/or that is, the cathode, hole respectivelytransporting [123 layer,132 ]:and electron transporting layer between the active layer and the anode/or cathode, respectively [123,132]: (a) G as a transparent conductive electrode: a) G as a transparent conductive electrode: GG and and its its derivatives derivatives have have been been usedused as TCEs owing owing to to their their good good flexibility flexibility and and transparency, transparency, to replaceto replace conventional conventional ITO ITO electrodes electrodes in in PSCs PSCs [ 132[132,133].,133]. The The efficiency efficiency ofof aa PSCPSC withwith a transparenttransparent G electrodeG electrode is 3.98% is 3.98% compared compared to 3.68%to 3.68% for for the the same same PSC PSC with with an ITO electrode electrode [123], [123], which which leaves leaves a a smallsmall margin margin of of improvement improvement in in the the cell cell manufacturing.manufacturing. Modern Modern micro micro fabrication fabrication techniques, techniques, using using roll-to-rollroll‐to‐roll printing printing techniques techniques [ 134[134],], enable enable thethe synthesis of of single single graphene graphene sheets sheets over over a large a large area area without defects or structural kinks, thus increasing conductivity. Nonetheless, in order to reach the sheet resistance desired for a TCE (15 Ω/sq), about 20 G monolayers need to be stacked, which, in turn, can result in a strong fall in light transmission, close to 50% (Figure 22). To overcome such

Int. J. Mol. Sci. 2019, 20, 976 25 of 42 without defects or structural kinks, thus increasing conductivity. Nonetheless, in order to reach the sheet resistance desired for a TCE (15 Ω/sq), about 20 G monolayers need to be stacked, which, in turn, can result in a strong fall in light transmission, close to 50% (Figure 22). To overcome such problems, the surfaceInt. J. Mol. of Sci. G 2019 can, 20 be, x chemically functionalized or combined with conducting polymers via25 of physical 43 interactionsproblems, [127 the]. surface For instance, of G can be a PEDOT:PSSchemically functionalized layer coated or onto combined a G-based with conducting electrode polymers fills the gaps betweenvia physical the G sheets,interactions thus [127]. improving For instance, conductivity, a PEDOT:PSS and layer acts coated as an onto energy a G‐based step electrode from the fills electrode the to thegaps active between materials. the G sheets, Further, thus improving a graphene/CNT conductivity, composite and acts as as an electrodeenergy step has from better the electrode performance to thanthe the active individual materials. components Further, a graphene/CNT [135]. The best composite approach as electrode is to integrate has better low performance amount of than G withinthe a transparentindividual conducting components host [135]. matrix, The best thus approach increasing is to transparency integrate low withoutamount of reducing G within conductivity a transparent [136]. Semitransparentconducting host PSCs matrix, with a CVD-grownthus increasing monolayer transparency G doped without with Aureducing nanoparticles conductivity and PEDOT:PSS[136]. as topSemitransparent TCE and ITO PSCs as the with bottom a CVD electrode‐grown monolayer have been G doped developed with Au (see nanoparticles Figure 23 [and137 PEDOT:PSS]). The use of a as top TCE and ITO as the bottom electrode have been developed (see Figure 23 [137]). The use of a doped G electrode strongly increased the conductivity (~400%) compared to raw G and led to a better doped G electrode strongly increased the conductivity (~400%) compared to raw G and led to a better transmittance. A very similar device with multilayer doped G as TCE on polyimide (PI) substrates transmittance. A very similar device with multilayer doped G as TCE on polyimide (PI) substrates was was alsoalso manufactured manufactured [138], [138 which], which exhibited exhibited outstanding outstanding flexibility flexibilityand stability. and Further, stability. air was Further, not able airto was not ablediffuse to diffuseacross the across interlayer the interlayer spacing of spacing the G oflayers, the Gthus layers, providing thus providing an excellent an packaging excellent effect. packaging effect.Multilayer Multilayer G can G act can as act a barrier as a barrier and protect and PSCs protect from PSCs air oxidation, from air making oxidation, easier making the cell fabrication easier the cell fabricationprocess processand diminishing and diminishing the associated the costs. associated costs.

FigureFigure 22. (22a). Dependence(a) Dependence of of the the sheet sheet resistance resistance on on the the film film thickness thickness and and (b) (dependenceb) dependence of solar of solar transmissiontransmission over over the the whole whole spectrum spectrum on on thethe sheet resistance resistance for for ITO, ITO, carbon carbon nanotubes, nanotubes, metal metal grids, grids, and reducedand reduced graphene graphene oxide oxide electrodes. electrodes. Taken Taken fromfrom [5]. [5].

Int. J. Mol. Sci. 2019, 20, 976 26 of 42 Int. J. Mol. Sci. 2019, 20, x 26 of 43

Figure 23. (A) Schematic representation and band structure of a PSC with the structure Glass/ITO/ ZnO/P3HT:PCBM/Au/PEDOT:PSS/G; (B) J–V characteristics measured from two sides of the PSC Figure 23. (A) Schematic representation and band structure of a PSC with the structure Glass/ITO/ with G top electrode and different active layer thicknesses. Reproduced with permission from [137]. ZnO/P3HT:PCBM/Au/PEDOT:PSS/G; (B) J–V characteristics measured from two sides of the PSC with G top electrode and different active layer thicknesses. Reproduced with permission from [137]. (b) G in active layers b) G in active layers MostMost OPSCs OPSCs are are based based on on thethe BHJBHJ architecture,architecture, in in which which the the ideal ideal active active layer layer consists consists in ina a bicontinuousbicontinuous interpenetrating interpenetrating network network of of donordonor and acceptor at at the the nanometer nanometer scale scale with with maximum maximum interfacialinterfacial area. area. The The efficiency efficiency enhancementenhancement in in BHJ BHJ solar solar cells cells compared compared to to other other architectures architectures is is attributedattributed to to the the more more efficient efficient exciton exciton dissociationdissociation enabled by by the the larger larger heterojunction heterojunction interface interface andand the the better better charge charge carrier carrier collection collection duedue toto thethe formation of of an an interpenetrating interpenetrating donor donor-acceptor‐acceptor networknetwork [139 [139].]. In In this this regard, regard, GG showsshows great potential to to be be used used in in active active layers layers owing owing to toits its2D 2D structurestructure and and large large specific specific surface surface area area thatthat wouldwould promote the the development development of of an an interpenetrating interpenetrating donor-acceptordonor‐acceptor network network at at the the nanoscale. nanoscale. TheThe first first use use of of solution-processable solution‐processable functionalized functionalized G G as as an an electron-acceptor electron‐acceptor material material in in PSCs PSCs was reportedwas reported by in 2008 by in by 2008 Liu by and Liu coworkers and coworkers (Figure (Figure 24[ 140 24]). [140]). The functionalized The functionalized G was G preparedwas prepared in two stages:in two firstly, stages: GO firstly, was synthesized GO was synthesized using a modified using a Hummer’smodified Hummer’s method, and method, then itand was then reacted it was with phenylreacted isocyanate, with phenyl leading isocyanate, to a hydrophobic leading to a materialhydrophobic that material could be that dispersed could be in dispersed organic solvents in organic used forsolvents PVCs applications. used for PVCs Then, applications. a composite Then, film a composite of poly(3-octylthiophene) film of poly(3‐octylthiophene) (P3OT) as a donor(P3OT) and as a the functionalizeddonor and the G asfunctionalized acceptor was G prepared as acceptor by was spin prepared coating. Theby spinπ-π interactionscoating. The between π‐π interactions P3OT and functionalizedbetween P3OT G madeand functionalized this composite G work made well this ascomposite active layer work in well BHJ as devices, active albeitlayer thein BHJ efficiency devices, was albeit the efficiency was low (~1.4% for a G concentration of 5 wt% optimized by an annealing low (~1.4% for a G concentration of 5 wt % optimized by an annealing process at 160 ◦C for 20 min). process at 160 °C for 20 min). The annealing process removes some functional groups, thus The annealing process removes some functional groups, thus improving the charge transport, and improving the charge transport, and increases the crystallinity of the donor matrix, consequently increases the crystallinity of the donor matrix, consequently enhancing the properties of the optical enhancing the properties of the optical active layer. active layer. A P3HT/solution processable G/functionalized multiwalled carbon nanotubes (f‐MWCNTs) compositeA P3HT/solution has also been processable used as an active G/functionalized layer in PSCs, multiwalled where G acted carbon as electron nanotubes acceptor, (f-MWCNTs) P3HT as compositedonor and has the also f‐MWCNTs been used provided as an active percolation layer inpaths PSCs, of holes where [141]. G acted However, as electron the efficiency acceptor, of P3HTthe as donor and the f-MWCNTs provided percolation paths of holes [141]. However, the efficiency of Int. J. Mol. Sci. 2019, 20, 976 27 of 42

Int. J. Mol. Sci. 2019, 20, x 27 of 43 the resulting device was also low, about 1.05%. Improved efficiency is expected by changing the G contentresulting and device processing was conditions,also low, about since 1.05%. theoretical Improved simulations efficiency envisage is expected efficiencies by changing higher thanthe G 12% forcontent G-based and PSCs. processing conditions, since theoretical simulations envisage efficiencies higher than 12% for G‐based PSCs.

Figure 24. Schematic representation of a PSC with (P3OT)/functionalized G as the active layer. Taken fromFigure [142 24]. . Schematic representation of a PSC with (P3OT)/functionalized G as the active layer. Taken from [142]. (c) G and its derivatives as a hole transport layer (HTL): c) G and its derivatives as a hole transport layer (HTL): TheThe most most common common commerciallycommercially available available material material used used as asHTL HTL in organic in organic electronics electronics is isPEDOT:PSS. PEDOT:PSS. However, However, the the highly highly-acidic‐acidic and and hygroscopic hygroscopic nature nature of the of polymer the polymer mixture, mixture, its itsnon non-homogeneous‐homogeneous electrical electrical properties properties and and low low stability stability over over time time has has initiated initiated the the search search for for air-stableair‐stable effective effective replacement replacement materials. materials. GG is anis an interesting interesting material material to to be be used used as as HTL HTL [123 [123],], particularly particularly in in its its oxidized oxidized form, form, since since GO GO has a lowerhas a work lower function work function compared compared to pristine to G, pristine making G, it bettermaking for holeit better injection. for hole A GO/PEDOT:PSS injection. A compositeGO/PEDOT:PSS has been composite used as HTL has been in a BHJ used PSC, as HTL resulting in a BHJ in an PSC, efficiency resulting of in 4.28%, an efficiency higher than of 4.28%, those of deviceshigher with than only those GO of (2.77%) devices or with PEDOT:PSS only GO (3.57%), (2.77%) asor well PEDOT:PSS as in better (3.57%), reproducibility as well as and in better stability overreproducibility time. The enhanced and stability performance over time. isThe due enhanced to the well performance matched is work due to function the well between matched GOwork and PEDOT:PSSfunction between that increases GO and the mobilityPEDOT:PSS of charge that increases carriers. [the143 ].mobility In addition, of charge GO establishes carriers. [143]. a band-gap In ofaddition, 3.6 eV, making GO establishes it possible a toband block‐gap electrons, of 3.6 eV, and making avoids it possible the acid corrosionto block electrons, problems and in theavoids ITO the layer causedacid corrosion by PEDOT:PSS, problems thus in the increasing ITO layer the caused useful by lifePEDOT:PSS, of the cell. thus However, increasing the the homogeneity useful life of ofthe the cell. However, the homogeneity of the layer must be ensured and the GO thickness maintained layer must be ensured and the GO thickness maintained between 1–3 nm to optimize its behaviour. between 1–3 nm to optimize its behaviour. On the other hand, partial‐reduction of GO in order to On the other hand, partial-reduction of GO in order to rise conductivity both in vertical and lateral rise conductivity both in vertical and lateral directions has also been tested, leading to devices with directions has also been tested, leading to devices with efficiencies surpassing that of PEDOT:PSS [81]. efficiencies surpassing that of PEDOT:PSS[81]. Recently, a BHJ PSC with a HTL made of a Recently, a BHJ PSC with a HTL made of a PEDOT:PSS/GO composite (1:1 w/w) was developed [144], PEDOT:PSS/GO composite (1:1 w/w) was developed [144], resulting in an efficiency of 5.22%. The resultingimproved in anperformance efficiency ofwas 5.22%. ascribed The to improved the reduction performance of the HOMO was ascribed‐LUMO togap the by reduction GO and ofthe the HOMO-LUMOimpedance reduction, gap by GOthus and improving the impedance the charge reduction, transport. thus improving the charge transport. (d) G andc) G its and derivatives its derivatives as an as electron an electron transport transport layer layer (ETL): (ETL): ThereThere are are only only a a few few ETL ETL materials, materials, thethe mostmost widely used used being being highly highly reactive reactive alkali alkali earth earth metals metals likelike calcium calcium or or magnesium magnesium (with (with low low workwork functions),functions), or or lithium lithium fluoride. fluoride. These These materials materials have have to tobe be evaporated, requiring high vacuum and melting of metals to high temperatures for deposition. Owing to

Int. J. Mol. Sci. 2019, 20, 976 28 of 42 evaporated, requiring high vacuum and melting of metals to high temperatures for deposition. Owing Int. J. Mol. Sci. 2019, 20, x 28 of 43 to the energy-band structure of G that results in efficient charge transport and its superior mechanical properties,the energy G‐band and itsstructure derivatives of G arethat postulated results in efficient as suitable charge materials transport as ETL and [ 123its superior]. In particular, mechanical the use ofproperties, GO will enableG and its the derivatives full device are postulated fabrication as viasuitable roll-to-roll materials processes. as ETL [123]. Further, In particular, GO can the use tune of the energyGO will levels enable through the full chemical device modification,fabrication via allowingroll‐to‐roll to processes. function forFurther, electron GO injection can tune instead the energy of hole injection.levels through For instance, chemical the modification, esterification allowing of the to COOH function groups for electron of GO injection with Cs instead2CO3 in of water hole injection. reduced its workFor functioninstance, fromthe esterification 4.7 eV to 4.0 of eV. the The COOH resulting groups device, of GO that with used Cs2 theCO3 modified in water GOreduced as both its ETLwork and HTL,function showed from better 4.7 eV performance to 4.0 eV. The than resulting the reference device, that with used Cs the2CO modified3 [145]. GO as both ETL and HTL, showedThe Schottkybetter performance junction, than formed the reference by contacting with Cs a2 metalCO3 [145]. with a doped semiconductor, is a potential structureThe in Schottky solar cells junction, [146]. formed It presents by contacting the advantages a metal of with material a doped universality, semiconductor, performance is a potential stability inexpensivenessstructure in solar and cells easy [146]. of It fabrication presents the compared advantages to of the material p–n junction. universality, However, performance in this stability type of cell,inexpensiveness the metal layer and would easy of absorbfabrication most compared of the solar to the radiation p–n junction. and However, consequently in this limit type the of cell, energy conversionthe metal efficiency.layer would To solveabsorb this most drawback, of the ITOsolar film radiation has been and used consequently to replace thelimit metal the film,energy albeit thisconversion results in efficiency. high production To solve costs this drawback, and restricts ITO its film use has for flexiblebeen used devices. to replace Since the G metal presents film, very highalbeit conductivity this results and in high a near-zero production band-gap, costs and G/n-type restricts semiconductor its use for flexible heterojunction devices. Since can G bepresents used as a metal/semiconductorvery high conductivity Schottky and a junction.near‐zero The band mechanism‐gap, G/n‐ oftype such semiconductor solar cell can beheterojunction explained considering can be used as a metal/semiconductor Schottky junction. The mechanism of such solar cell can be explained the energy band diagram (Figure 25). Owing to the work function difference between G and the considering the energy band diagram (Figure 25). Owing to the work function difference between G semiconductor, a built-in potential is formed in the semiconductor near the interface. Under light, the and the semiconductor, a built‐in potential is formed in the semiconductor near the interface. Under photogenerated holes and electrons are driven towards the Schottky electrode (G) and semiconductor light, the photogenerated holes and electrons are driven towards the Schottky electrode (G) and layer, respectively, by the built-in electric field. When the solar cell is short-circuited, the extracted semiconductor layer, respectively, by the built‐in electric field. When the solar cell is short‐circuited, photogenerated carries will generate a short-circuited current. Compared to traditional Schottky the extracted photogenerated carries will generate a short‐circuited current. Compared to traditional junctionSchottky solar junction cells, moresolar lightcells, can more go throughlight can the go Schottky through G the electrode Schottky and G excite electrode electron–hole and excite pairs inelectron–hole the semiconductor, pairs in leading the semiconductor, to increased leading efficiency. to increased efficiency.

FigureFigure 25. 25(. a(a)) Energy Energy diagram diagram ofof a semiconductor/G Schottky Schottky junction junction solar solar cell cell under under illumination. illumination. ØG and ØS are the work functions of graphene and the semiconductor, respectively. eVi is the built‐in ØG and ØS are the work functions of graphene and the semiconductor, respectively. eVi is the built-in potential, V is the output voltage, EC, EV, and EF correspond to the conduction band edge, valence potential, V is the output voltage, EC,EV, and EF correspond to the conduction band edge, valence band edge, and Fermi level of semiconductor, respectively, and E0 is the vacuum level. (b) Schematic band edge, and Fermi level of semiconductor, respectively, and E0 is the vacuum level. (b) Schematic illustrationillustration of of G-based G‐based Schottky Schottky junctionjunction solar cell. cell. Photogenerated Photogenerated holes holes (h+) (h+) and and electrons electrons (e− (e) are−) are drivendriven towards towards the the G G and and semiconductorsemiconductor layers, layers, respectively. respectively. Taken Taken from from [146]. [146 ].

DifferentDifferent typestypes of of G G-based‐based Schottky Schottky junctions junctions have have been beenreported, reported, including including G‐on‐silicon G-on-silicon ones [147,148]. These devices include a silicon square window patterned by etching off a SiO2 layer on an ones [147,148]. These devices include a silicon square window patterned by etching off a SiO2 n‐Si wafer. Then, a G sheet contacts with the Si window to form a Schottky junction; the G layer also layer on an n-Si wafer. Then, a G sheet contacts with the Si window to form a Schottky junction; acts as anti‐reflection coating, reducing reflection, and the device efficiency was ~1.5%. A the G layer also acts as anti-reflection coating, reducing reflection, and the device efficiency was G‐on‐silicon Schottky junction cell with a PCE of 8.6% was developed by using G doped with ~1.5%. A G-on-silicon Schottky junction cell with a PCE of 8.6% was developed by using G doped bis(trifluoromethanesulfonylamide) [(CF3SO2)2NH] (TFSA)[149]. Overall, the G‐based Schottky with bis(trifluoromethanesulfonylamide) [(CF SO ) NH] (TFSA) [149]. Overall, the G-based Schottky junction cells reduce the cost of the traditional3 Si2‐based2 solar cells, their fabrication processes are junction cells reduce the cost of the traditional Si-based solar cells, their fabrication processes are easy easy and compatible with Si microelectronic technology, and can be used in flexible devices. and compatible with Si microelectronic technology, and can be used in flexible devices. 5.3. Current Research on G Applications

Int. J. Mol. Sci. 2019, 20, 976 29 of 42

5.3. Current Research on G Applications Current research on G and its derivatives is focused on two main aspects: (a) the replacement of materials used in previous cells to improve some properties or reduce costs; and (b) the incorporation into previously studied materials, leading to enhanced cell performance.

5.3.1. Replacement of Other Materials The replacement of some materials by G is motivated by its excellent properties that can improve the cell operation and its economic competitiveness; however, the performance improvement can lead to a detriment in other properties. For instance, the use of a thin G sheet as a substitute for Pt to manufacture conducting electrodes in DSSCs with PEDOT:PSS decreases the efficiency from 4.39% to 3.95%, albeit the cell durability is improved, and the efficiency is maintained between 2.37–3.23% after 100 cycles of flexion, values significantly higher than the one obtained using Pt (2.08%) [118,150]. Also in DSSCs, liquid electrodes have been replaced by rGO with electrolyte gels (such as polyethylene oxide (PEO) doped with gamma-butyrolactone, LiI and I2) to improve efficiency [151]. − In the case of PEO, efficiency values of 5.7% were achieved owing to the faster diffusion of the I3 ions and other ionic species and the plasticizing effect of the carbon sheets. In the field of inorganic cells, G can also be used to improve designs; for instance, a perovskite cell used an ETL based on G/polymer composite (a mesoporous layer of rGO/PANI deposited on an ITO substrate) to solve thermal and chemical stability issues [152]. This type of combination allows achieving an efficiency of 13.8% under solar lighting. Further, some authors proposed the substitution of ITO for CVD-grown G [153], or its use as a transparent anti-reflective coating [154]. Thus, a G/PEDOT:PSS layer was deposited by spray coating as a cathode in an inverted PSC, albeit the efficiency dropped slightly compared to the device with ITO, which indicates that an optimization is required. In addition, it was found that ITO worked better in the long wavelength region, while GO worked better with short wavelengths [153].

5.3.2. Incorporation into Other Materials Some authors proposed to use G sheets coated with ZnO nanocrystals for infrared radiation capture, since it corresponds to the 40% of the incident solar radiation [154]. In addition, if randomly-distributed gold nanoparticles were used as dopants and the G thickness was maintained at 20 nm, the overall yield would increase to 8.94%. The use of G sheets as a dopant has also been studied; in particular, it has been added to PEDOT:PSS solutions in isopropyl alcohol to obtain a material with a conductivity 10 times higher, and with a transparency comparable to ITO [155]. By dissolving PEDOT:PSS in the alcohol, the conductivity is improved due to charges excess, but the roughness worsens and the surface defects are increased. Upon addition of the G sheets, the mechanical strength and integrity of the whole layer are improved. The addition of rGO to PEDOT:PSS as HTL in hybrid Si-organic cells (HSCs) has also been studied [156]. The combination of rGO with PEDOT not only provides new routes for the charge transport in the HTL, that improve the mobility and efficiency in the collection of the carriers, but also suppresses the recombination of the electrons at the interface. Moreover, the rGO acts as an antireflection coating and reduces the reflectance of PEDOT, thus improving the performance of the cells. Overall, the addition of rGO improved the electric conductivity of the PEDOT:PSS by 35%, and with a concentration of 2 mg/mL, an efficiency of 11.95% was attained. However, an excess of rGO can induce a faster cell deterioration due to the formation of additional defects. G, GO functionalized with Li or rGO can be added to increase efficiency in perovskite cells. The idea is to either add or to replace the TiO2 by one of these nanomaterials, leading to an efficiency in the case of Li-GO of 11.8% [153]. The improvement is attributed to the presence of G that reduces the recombinations of the electron-holes [118]. Further, rGO/PEDOT:PSS composite can be used as a HTL layer in this type of cells [157,158]. The use of rGO hardly changes the surface roughness, thereby Int. J. Mol. Sci. 2019, 20, 976 30 of 42 favouring the deposition of the perovskite absorber and improving the interface. With an optimal rGO:PEDOT:PSS 1:1 ratio, a maximum efficiency of 10.3% was obtained [157,158]. In addition, the rGO can be used as a heat sink to extend the useful life by avoiding mechanical stresses caused by heat. Other authors added rGO to PMMA-based cells to form rGO/PMMA nanocomposites [159], and a maximum efficiency of 25% was obtained for an optimum G concentration of 1.0 wt %.

5.4. Carbon Nanotubes Carbon nanotubes (CNTs) are allotropes of carbon discovered by Iijima in 1991 that present a tubular structure formed by curling G sheets [160]. There are two main types of CNTs: single walled CNTs (SWCNTs), which consist of a single tube of graphene, and multi-walled CNTs (MWCNTs) that are composed of several concentric tubes of G. SWCNTs can be further classified into metallic and low band gap semiconductors, while MWCNTs are metallic in nature [161]. Different techniques have been developed to synthesize CNTs, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation and CVD processes [162]. CNTs have unique electronic, chemical and mechanical properties that make them interesting materials for a range of applications [163]. They present very high length-to-diameter ratio, in the range of 103–105, and are among the strongest and stiffest materials known, with an elastic modulus close to 1 TPa [164]. They also display very high electrical conductivity; metallic nanotubes can carry an electric current density of 4 × 109 A/cm2, which is more than 1000 times greater than those of metals such as Cu [165,166]. Further, they are very good thermal conductors along the tube, showing ballistic conduction at room temperature. SWCNTs present a thermal conductivity of ~3500 W·m−1·K−1 [167], significantly higher than that of Cu (385 W·m−1·K−1), and possess very high thermal stability under both air and inert conditions. Mainly, CNTs can be applied in PVSCs in three ways: as TCEs, as transport layer or in active layers.

5.4.1. CNTs as Transparent Conductive Electrodes CNTs have a low percolation threshold, hence just a low concentration is needed to attain a low sheet resistance suitable for the development of TCEs. However, the contact resistance between individual nanotubes is very high, hence CNT films alone need to be more than 100 nm thick to attain a sheet resistance [168]. Even so, CNT films manufactured by an inexpensive screen printing technique on glass and on elastic polymer substrates displayed high optical transmittance, and were tested as TCEs in a standard silicon cell. The best efficiency obtained (8.6%) was lower than that of a cell with conventional ITO, owing to the elevated resistance among CNTs [169]. To overcome the issue of conduction between individual nanotubes, researchers have looked at adding functionality to the CNT surface that can act as a conducting bridge.

5.4.2. CNTs in Transport Layers CNTs can be used as HTL, since have excellent energy-level matching for injecting holes and are environmentally-friendly materials. Further, they present high thermal and chemical stability, making them perfect candidates to replace conventional PEDOT:PSS HTLs. Owing to the possibility to functionalize CNTs with a variety of functional moieties, the work function and affinities can be modified for either electron or hole transport/injection. Achieving a good dispersion of CNTs or G within the polymer matrix is the key to fabricate composites with enhanced mechanical, electrical, and thermal properties [161]. However, raw CNTs have a strong tendency to aggregate and form bundles, and G sheets to remain stacked together via π-π stacking interactions, hence new approaches to homogeneously disperse these nanomaterials and keep a large specific surface area when blended with polymers have been reported including the use of surfactants [170] or polymer functionalization by means of covalent [171] and non-covalent approaches [172]. Covalent functionalization takes place via chemical modification of the CNT sidewall, either by acid treatments that generate hydroxyl and carboxyl groups, or via anchoring of small molecules or polymeric chains [173]. The covalent Int. J. Mol. Sci. 2019, 20, 976 31 of 42 Int. J. Mol. Sci. 2019, 20, x 31 of 43 polymericfunctionalization chains damages [173]. The the covalent nanotube functionalization sidewalls, while damages the non-covalent the nanotube functionalization sidewalls, maintainswhile the π π nonthe integrity‐covalent of functionalization the nanotubes since maintains the polymers the integrity interact of the via nanotubes van der Waals since forces the polymers and - stacking interact viainteractions van der Waals [174]. forces and π‐π stacking interactions [174]. Using non non-covalent‐covalent functionalization of of P3HT P3HT onto onto SWCNTs, SWCNTs, a thin film film with transparency equivalent toto PEDOT:PSSPEDOT:PSS was was obtained, obtained, leading leading to to a PSCa PSC with with higher higher fill fill factor factor and and efficiency efficiency than than the thecell cell with with PEDOT:PSS PEDOT:PSS as HTL as HTL (Figure (Figure 26[127 26 ]).[127]).

Figure 26.26. ((aa)) Optical Optical transmission, transmission, ( (bb)) atomic force micrographs, and ( c) J–VJ–V characteristics of P3HT/SCWNT composites composites as as hole hole extraction extraction layers. layers. Taken Taken from from [127]. [127].

One ofof thethe advantages advantages of of the the carbon-based carbon‐based nanomaterials nanomaterials is the is abilitythe ability to control to control their energytheir energy levels levelsby chemical by chemical modification, modification, allowing allowing them to them act as to ETL act insteadas ETL ofinstead HTL. of The HTL. first The trial first step trial was step reported was reportedby Kymakis by Kymakis and Amaratunga and Amaratunga [175] who [175] used who a P3OT/SCWNT used a P3OT/SCWNT composite composite as ETL. Theas ETL. addition The additionof SCWNTs of SCWNTs led to a strong led to increase a strong in increase efficiency in attributed efficiency to attributed the enhanced to the exciton enhanced dissociation exciton dissociationat the polymer-nanotube at the polymer interface.‐nanotube Carbon interface. nanotubes Carbon nanotubes were tip sonicatedwere tip sonicated in the solvent in the priorsolvent to polymerprior to addition,polymer addition, in order to in improve order to their improve dispersion. their Similarly,dispersion. Pradhan Similarly, et al. Pradhan [176] functionalized et al. [176] functionalizedMWCNTs to achieve MWCNTs a better to achieve dispersion a better in a P3HT dispersion matrix in and a P3HT used thematrix resulting and used composite the resulting as ETL. compositeImproved efficiencyas ETL. Improved was also efficiency attained, was ascribed also attained, to the hole ascribed transporting to the hole nature transporting of the CNTs nature which of werethe CNTs expected which to were act either expected as high to act mobility either charge as high pathways mobility orcharge as bridging pathways sites or for as betterbridging percolation sites for betterin the polymerpercolation phase. in the polymer phase. There is controversy regarding the actual role of CNTs. While certain groups claim them to be a hole transportertransporter [177 [177–179]–179] others others assert assert that they that are they electron are transporters electron transporters [175,180,181 ].[175,180,181]. Furthermore, Furthermore,there exists opposing there exists proofs opposing for the preferredproofs for charge the preferred transportation charge via transportation nanotubes under via nanotubes the same undermechanisms. the same Understanding mechanisms. their Understanding charge transport their properties charge transport is crucial priorproperties to incorporating is crucial prior them asto incorporatingdonors or acceptors them as in donors PSCs. or acceptors in PSCs.

5.4.35.4.3. CNTs CNTs in in Active Active Layers Layers CNTs—in particular,particular, MWCNTs—have MWCNTs—have been been used used in active in active layers layers of PSCs. of PSCs. One of One the majorof the reasons major reasonsfor the use for of the MWCNTs use of MWCNTs instead of instead SWCNTs of is SWCNTs their more is uniform their more electronic uniform properties. electronic Additionally, properties. Additionally,controlled doping controlled of MWCNTs doping by of suitable MWCNTs treatments by suitable improves treatments the cell improves performance. the cell N- performance. or B-doped NMWCNTs‐ or B‐doped uniformly MWCNTs dispersed uniformly in the activedispersed layer in of the a bulk-heterojunction active layer of a bulk solar‐heterojunction cell consisting solar of P3HT cell consistingand a fullerene of P3HT derivative, and a PC71BM, fullerene selectively derivative, enhanced PC71BM, electron selectively or hole enhanced transport electron and eventually or hole transportaid carrier and collection. eventually The dopingaid carrier using collection. thermal treatments The doping with using an ammonia/argon thermal treatments gas introduces with an ammonia/argonnitrogen ions into gas the introduces wall, lowering nitrogen their ions work into function the wall, from lowering 4.6 eV for their the pristinework function MWCNTs from to 4.6 4.4 eV;eV forin contrast, the pristine B is addedMWCNTs to make to 4.4 the eV; CNTs in morecontrast, p-type. B is In added particular, to make the addition the CNTs of 1.0 more wt %p‐ B-dopedtype. In particular,MWCNTs resulted the addition in balanced of 1.0 electron wt% B and‐doped hole MWCNTs transport, leadingresulted to in an balanced efficiency improvementelectron and fromhole transport,3.0% (without leading CNTs) to toan 4.1% efficiency [182]. improvement Further, unlike from other 3.0% common (without electron CNTs) transport to 4.1% materials, [182]. Further, CNTs unlikedo not other degrade common upon exposure electron transport to water or materials, oxygen. Further,CNTs do Lu not and degrade co-workers upon [exposure183] added to N-dopedwater or oxygen. Further, Lu and co‐workers [183] added N‐doped MWCNTs into active layers based on polythieno[3,4‐b]‐thiophene‐co‐benzodithiophene (PTB7) and PC71BM, leading to an efficiency

Int. J. Mol. Sci. 2019, 20, 976 32 of 42

MWCNTs into active layers based on polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) and PC71BM, leading to an efficiency increase from 7.3% to 8.6% by raising the MWCNT concentration. The improvement was ascribed to better light coupling, exciton dissociation and charge transport due to the presence of the nanotubes into the active layer. Overall, the recent developments in the dispersion techniques for CNTs and the better understanding of their role in PVCs has led to a strong increase in efficiency, from 0.04% in 2002 for a PSC with SWCNTs dispersed in P3OT as active layer to 8.6% for N-doped MWCNTs in PTB7/PC71BM as active layer. Nonetheless, preliminary works on PSCs incorporating either GO or CNTs in the active layer indicated better performance for those comprising GO. For instance, the effect of a thiophene derivative, PTM21, covalently grafted to the surface of GO and MWCNTs on the cell efficiency was investigated [184]. Albeit the performance was improved upon addition of both carbon nanofillers, the highest efficiency was attained with 0.3 wt % GO, ascribed to the larger surface contact area, improved charge transport, and better dispersibility of the GO layers compared to the CNTs. Moreover, the amphiphilic character of GO facilitates the device processability in aqueous solutions, while CNTs present a hydrophobic nature and are insoluble in most of the common solvents.

6. Conclusions and Future Perspectives Currently, photovoltaic technology is regarded as a part of the solution to the growing energy challenge and as a key component of future global energy production. In this work, a brief description of the state of art on photovoltaic cells has been provided. The different technologies developed up to date have been divided into four generations, and the characteristics, advantages and limitations of each generation along with the most recent investigations have been discussed in detail. A summary of these technologies, production methods, characteristics, and efficiencies attained is given in Table1.

Table 1. Summary of PV technologies.

GEN Technology Production Method Characteristics Efficiency (%) Reference 1GEN m-Si Czochralski Expensive, stable 24.4 [8,15] Low cost, high defect 1GEN p-Si Siemens 19.9 [15,16] content Expensive, good design 1GEN GaAs Expitaxial growth 18.4–28.8 [15,19,20] control Large-area 2GEN a-Si Non-toxic, short life cycle 10.2–12.7 [15,24] deposition Low defect content, good 2GEN µc-Si Roll-to-roll 11.9–14.0 [13,22] degradability Deposition, 2GEN CIGS Tuneable band gap 22.3 [13,26,27] co-evaporation High temperature tolerance, 2GEN CdTe Deposition 21 [13] low fooling Work in low-light 3GEN DSSC Roll-to-roll, 5.0–20.0 [30] conditions, robustness 3GEN QDs Solution casting Efficient conductivity 11.0–17.0 [52] High work function, 3GEN OPSCs Solution casting 9.7–11.2 [13,77] thermally stable PVSC 3GEN Sputtering/ printing Cheap, simple 21.1–21.6 [13,87] (CH3NH3PbI3) Wide range of design, 3GEN MJ 1 Stacking 35.8 [13,105,111] Challenging manufacture 3GEN IMM 2 Monolithic growth Cheap, high band gap 40–44.4 [13,105,114,185] BHJ 3 PSC 4 with 4GEN Solution casting Reproducible and stable 4.28 [123] GO/PEDOT:PSS PSC with 4GEN Solution casting Good functionality 2.82–11.8 [121,153] G/PEDOT:PSS 4GEN PVSC 5 with Li-GO Spray deposition Stable, long lifetime 1.07–11.14 [186] PVSC with Long lifetime, reduced 4GEN Solution casting 5.7–11.95 [129,152,157,158] rGO/PEDOT:PSS elec.-hole recombination PSCs with B-doped 4GEN Solution casting Improved electron transport 4.1–8.6 [182] CNTs 1 MJ: multi-junction; 2 IMM: inverted methamorphic multijunction; 3 BHJ: Bulk heterojunction; 4 PSC: polymer solar cell; 5 PVSC: perovskite. Int. J. Mol. Sci. 2019, 20, 976 33 of 42

Most 1GEN (m-Si, p-Si and GaAs) and 2GEN (a-Si, µc-Si, CdTe/CdS, and CIGS) technologies are highly standardized and have undergone few changes in recent years; they exhibit high efficiencies (20–25%) and are typically expensive, though there has been a reduction in the cost of silicon-based cells. On the other hand, the majority of 3GEN (QDs, perovskite, PSCs, DSSCs), as well as 4GEN (polymers combined with metal nanoparticles, CNTs, G or its derivatives) technologies are in states very close to the so-called “basic research”; laboratory prototypes that lead to good results have been developed though they have not been implemented at an industrial scale yet (efficiencies 10–15%). However, the 3GEN multi-junction cells are already commercial, and have achieved very high energy conversion rates (>40%), thus becoming the best alternative if efficiency is sought. 4GEN cells based on CNTs, G or its derivatives are in a state of early-research, hence constitute a very promising field for investigation. The versatile nature of such carbon nanostructures allowing to incorporate them throughout the PSC architecture, including transport layers, active layer, and electrodes, with the aim to attain inexpensive stable devices with improved performance. Both G and CNTs have been shown to be an effective, solution processable, replacement for traditional transport layers such as PEDOT:PSS. Furthermore, CNT doping has been proved to be effective for tuning the charge transport within the active layers. Additionally, there is also promise in the fabrication of hybrid architectures involving metal oxide/carbon nanostructures as transport layers in DSSCs. Despite the increasingly evident enhancements in PSC performance due to the addition of the carbon nanostructures, they have not been introduced into the market yet, since several issues have to be addressed. (1) New approaches that enable to synthesize high-purity and high-quality CNT or G thin films with controlled morphology and electronic properties need to be developed, since the purity, quality, band-gap, and morphology of the carbon nanostructures conditions the PSC performance. In the case of TCEs, solution processable films with an optimum balance between sheet resistance and transparency are required. Further, the functionalization and ultrasonication processes used during the fabrication of PSCs lead to a remarkable drop in the electrical conductivity of the carbon nanomaterials. Thus, the electrical properties of most conjugated polymer/carbon nanotube composites do not fulfil the requirements for TCEs and counter electrodes in PVCs. (2) The actual specific surface area of carbon-based nanomaterials is smaller than the predictions due to their strong agglomeration tendency by means of π-π stacking interactions, and the mixture with polymers makes the issue worse. Hence, novel synthetic methods to prevent aggregation are sought. (3) Novel doping or functionalization approaches compatible with the fabrication process of PSCs have to be designed to attain higher stability, improved charge transport and tunable energy levels in carbon-based nanomaterials. (4) New inexpensive techniques that enable the synthesis of CNTs and G at a large scale are need. Despite some improvements have been attained in this direction, the current methods are gravely limited by their low yields, and this needs to be addressed prior to their use in commercial applications. It is envisaged that in the next future and after comprehensive research on the field, 4GEN PSCs incorporating carbon-based nanomaterials would offer high performance levels to rival those of traditional silicon-based cells, thus providing a new outlook for the solar energy industry.

Author Contributions: The three authors performed the literature survey. A.M.D.-P. designed the manuscript and J.A.L.-S. wrote the paper; R.P.C. and A.M.D.-P. critically reviewed the manuscript. Funding: This work was funded by University of Alcalá, project reference CCG2018/EXP-011. Acknowledgments: We thank University of Alcalá for making possible this study. Conflicts of Interest: The authors declare no conflict of interest.

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