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FLEXIBLE PEROVSKITE HYBRID SOLAR CELLS THROUGH ORGANIC

SALT TREATED CONDUCTING POLYMER AS THE TRANSPARENT

ELECTRODE

A Thesis

Presented to

The Department of Polymer Engineering of the University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Zixu Huang

April 2018

FLEXIBLE PEROVSKITE HYBRID SOLAR CELLS THROUGH ORGANIC

SALT TREATED CONDUCTING POLYMER AS THE TRANSPARENT

ELECTRODE

Zixu Huang

Thesis

Approved: Accepted: ______Advisor Dean of College Dr. Xiong Gong Dr. Eric J. Amis

______Faculty Reader Dean of the Graduate School Dr. Ruel McKenzie Dr. Chand K. Midha

______Department Chair Date Dr. Sadhan C. Jana

FLEXIBLE PEROVSKITE HYBRID SOLAR CELLS THROUGH ORGANIC

SALT TREATED CONDUCTING POLYMER AS THE TRANSPARENT

ELECTRODE

Zixu Huang

Thesis

Approved: Accepted: ______Advisor Department Chair Dr. Xiong Gong Dr. Sadhan C. Jana

______Committee Member Dean of College Dr. Ruel McKenzie Dr. Eric J. Amis

______Committee Member Dean of the Graduate School Dr. Jiahua Zhu Dr. Chand K. Midha

______Date

iii

ABSTRACT

Organic-inorganic hybrid perovskite solar cells (PSCs) have been widely researched due to its low fabrication cost and impressive power conversion efficiency (PCE) in the past 9 years. However, most of the PSCs are developed on transparent conductive

oxides (TCOs) like indium tin oxide (ITO), which fabrication process requires energy- consuming high-temperature sintering process. In addition, the high rigidity and brittleness of the ITO electrode also impede the PSCs to be fabricated on flexible substrates like

poly(ethylene terephthalate)(PET) film through solution processes.

In this study, we corroborated a facile routine to utilize highly electrically

conductive and highly transparent poly(3,4-ethylenedioxythiophene):poly(styrene

sulfonate) (PEDOT:PSS) thin films as the transparent electrode of the PSCs. To circumvent the problem of low electrical conductivity of the thin films prepared from the pristine

PEDOT:PSS solution, organic salt formamidinium iodide (FAI) was used as the treating

reagent to boost the conductivity of the thin films from 0.3 S/cm to about 1,600 S/cm. The

increment of the conductivity was achieved by the partial removal of the PSS segments and the phase segregation of PEDOT and PSS. As a result, a PCE of 13.36% was obtained for the devices fabricated on FAI-PEDOT:PSS/glass substrate and a PCE of 8.86% for that developed on FAI-PEDOT:PSS/PET substrate This thesis provides a facile way to fabricate flexible and efficient perovskite hybrid solar cells.

ACKNOWLEDGMENTS

First and foremost, I’d like to take this opportunity to show my sincere gratitude to my advisor Dr. Xiong Gong for his insightful guidance and invaluable suggestions in my studies and research project ever since I joined this group in 2016. I also acknowledge him for his constructive supervision on my way to achieve scientific goals which has laid a solid foundation for becoming an open-minded scientist in my long-term career.

In addition, I am grateful to my committee, Dr. Ruel McKenzie, and Dr. Jiahua Zhu for their attendance and fruitful discussion for my research proposal and defense.

Special thanks to Mr. Tianyu Meng for ESR measurement and Mr. Tao Zhu for the fabrication and characterization of the flexible quantum dot photodetectors, and other group members for their suggestions and assistance.

I also would like to express my greatest appreciation to my parents for everything they have offered me as well as their unconditional love and support.

Last but not the least, I take this chance to express my gratitude to all of the faculty members in the Department of Polymer Engineering and my classmates for their generous support.

TABLE OF CONTENTS

ABSTRACT ...... iv

ACKNOWLEDGEMENTS ...... v

LIST OF FIGURES ...... viii

LIST OF SCHEMES...... x

LIST OF TABLES ...... xi

CHAPTER 1. INTRODUCTION ...... 1

1.1 Solar energy and history of solar cells ...... 1

1.2 Crystal structure and history of organic-inorganic perovskite hybrid solar cells . 2

1.3 Fundamental physics of perovskite hybrid solar cells ...... 4

1.4 Characterization of perovskite solar cells ...... 5

1.4.1 J V characteristics ...... 5

1.4.2 External quantum efficiency (EQE) ...... 6

1.5 TCO-free perovskite solar cells with PEDOT:PSS as transparent electrode...... 7

1.5.1 Organic acid treated PEDOT:PSS as transparent electrode...... 7

1.5.2 Silver mesh and PEDOT:PSS hybrid transparent electrode...... 9

1.5.3 Roll-to-roll (R2R) printed flexible perovskite solar cells based on silver

nanowire/PH1000 electrode...... 11

1.6. Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) ..... 12

1.6.1 Overview of PEDOT:PSS ...... 12

1.6.2 Structure and conformation of PEDOT:PSS ...... 13

1.6.3 Methods to improve the electrical conductivity of PEDOT:PSS ...... 14

CHAPTER 2. HIGHLY ELECTRICALLY CONDUCTIVE PEDOT:PSS TREATED

BY ORGANIC SALT ...... 15

2.1 Introduction ...... 15

2.2 Experimental section ...... 16

2.3 Results and Discussion ...... 17

CHAPTER 3. TRANSPARENT CONDUCTIVE OXIDE-FREE PEROVSKITE

HYBRID SOLAR CELLS WITH ORGANIC SALT TREATED PEDOT:PSS AS

TRANSPARENT ELECTRODE...... 25

3.1 Introduction ...... 25

3.2 Experimental Section ...... 26

3.3 Results and discussion ...... 28

CHAPTER 4. CONCLUSION ...... 34

CHAPTER 5. FUTURE PLAN ...... 36

vii

LIST OF FIGURES

Figure Page

+ Figure 1.1 (a) The crystal lattice of CH3NH3PbI3 perovskite, cation A is CH3NH3 , cation 2+ - B is Pb , anion X is I . (b) unit cell of CH3NH3PbI3 perovskite. Reproduced from Ref. 1 with permission. Copyright © 2014 Elsevier Ltd...... 2

Figure 1.2 The device configuration of the perovskite hybrid solar cells (a) Meso- superstructured (MS) PSCs. (b) Planar heterojunction (PHJ) PSCs...... 4

Figure 1.3 The J V characteristics of a perovskite solar cell under illumination. The fill factor (FF) is defined as the ratio of the blue area determined by the maximum power point (mpp) over the green area defined by short circuit current density (JSC) and open circuit voltage. (VOC)...... 6

Figure 1.4 The external quantum efficiency (EQE) of the organic-inorganic hybrid perovskite solar cells...... 7

Figure 1.5. (a) The J V characteristics of the flexible perovskite solar cells fabricated on glass and PET substrates. (b) Photo of a device fabricated on PET substrate. Ref. 6 with permission. Copyright © 2015 American Chemical Society...... 9

Figure 1.6 (a)The device configuration of the flexible perovskite solar cell (PET/Ag mesh/PH1000/PEDOT:PSS/Perovskite/PCBM/Al). (b)The J V characteristics of the flexible solar cell measured in the dark and under illumination. Ref. 7 with permission. Copyright © 2016 Macmillan Publishers Limited...... 10

Figure 2.1 (a) Variation of the conductivities of PEDOT:PSS thin films treated with FAI/DMF solution with different concentrations. (b) Dependence of the conductivities of PEDOT:PSS thin films treated with FAI/DMF solution on the treating temperature. The concentration of MAI was 0.1 M in the solutions...... 18

Figure 2.2 GIWAXS 2D patterns of (a) pristine (b) FAI/DMF solution (c) FAI/GBL solution and (d) FAI/EG solution treated PEDOT:PSS thin films...... 20

Figure 2.3 The molecular packing pattern of PEDOT chains in PEDOT:PSS thin film. 21

viii

Figure 2.4 1D GIWAXS pattern along the (a) out-of-plane direction and (b) in-plane direction of the pristine and FAI solutions treated PEDOT:PSS thin films...... 22

Figure 2.5 (a) Absorption spectra (b) electron spin resonance spectra of pristine and FAI solutions treated PEDOT:PSS thin films...... 23

Figure 2.6 Contact angle of toluene on (a) FAI/DMF treated PEDOT:PSS thin film and (b) ITO transparent electrode...... 24

Figure 3.1 Transmittance spectra of FAI/DMF solution treated PEDOT:PSS thin film and ITO transparent electrode...... 28

Figure 3.3 (a) Nyquist plot of the PSCs based on either ITO or FAI-treated PEDOT:PSS thin film measured in the dark. Light intensity dependences of the steady-state (b) JSC and (c) VOC of the PSCs based on either ITO or FAI-treated PEDOT:PSS thin film...... 31

Figure 3.4 (a)Photo of the flexible PSC developed on a FAI-PEDOT:PSS/PET substrate. (b) The normalized PCE of the flexible PSCs with respect to the bending cycle...... 32

LIST OF SCHEMES

Scheme Page

Scheme 1.1 The chemical structures of poly(3,4-ethylenedioxythiophene)(PEDOT) and poly(styrene sulfonate)(PSS)...... 8

Scheme 2.1 (a) The chemical structure of PEDOT:PSS and formamidinium iodide (FAI), N,N-dimethylformamide (DMF), γ-butyrolactone (GBL), and ethylene glycol (EG). (b) Conductivities of PEDOT:PSS thin films treated with 0.1 M FAI solutions with various solvents...... 17

LIST OF TABLES

Table Page

Table 2.1 The molecular packing parameters and electrical conductivity of pristine and FAI solution treated PEDOT:PSS thin films...... 22

Table 3.1 Device performance parameters of the PSCs based on ITO/Glass, FAI- PEDOT:PSS/Glass and FAI-PEDOT:PSS/PET substrates...... 29

CHAPTER 1. INTRODUCTION

1.1 Solar energy and history of solar cells

In the development of modern humanization, industrialization and rising living

standards, our society will witness dramatic increment in the energy consumption in the coming decades. However, we still largely rely on fossil energy source which is of limited storage now, the greenhouse effect and emission issues caused by combustion of fossil fuel is also not likely to be solved in the near future. Therefore, exploring cheap, renewable, and environmental-friendly energy sources is still the most pivotal mission for long-term development of our society.

Up to date, the most promising clean, renewable, and cost-effective energy source

is still solar energy, the total amount of solar energy received by the surface of the earth is over 173,000 terawatts1, which is 10,000 times larger than the current world energy source demand. Solar cells have many advantages over other methods to utilize solar energy since it can directly convert solar energy into electricity, it has attracted great attention ever since the first solar cell was fabricated by Bell Lab in 1954. Typically, the solar cells can be categorized into three generations, including (i) the first generation solar cells like mono- and polycrystalline silicon solar cells. (ii) the second generation solar cells mainly consist of amorphous silicon, copper indium gallium selenide (CIGS), and cadmium telluride

(CdTe), (iii) the third generation solar cells usually refer to small molecule, polymer, dye- sensitized, quantum dots, and perovskite hybrid solar cells. This classification is mainly based on the type of the photoactive layers. Currently, due to the shelf stability issue and

the scarcity of the elements, the second generation solar cells are not been widely used compared with first generation. On the other hand, organic photovoltaics (OPVs) like

polymer and small molecule based solar cells suffer from the issue of low power

conversion efficiency. In the past few years, perovskite solar cells have been considered as the most promising candidate to replace traditional first generation solar cells due to its

1.2 Crystal structure and history of organic-inorganic perovskite hybrid solar cells

The terminology “perovskite” originally denotes metal oxide compound consists of calcium titanate (CaTiO3), which was named after Russian mineralogist L. A. Perovski

(1792-1856). Latter on, the term was adopted to other compounds which have the same

XII 2+VI 4+ 2− crystal lattice of A B X 3, including the knopite ((Ca, Ce, Na)(Ti, Fe)O3) and

dysanalyte.

+ + + + + The cation on the A site is MA (CH3NH3 ), FA (CH(NH2)2) , or Cs . The anion

on the X site is halogen ion like I-, Cl-, or Br-. As for lead-based perovskite materials, lead

cation occupies the B site. Different crystal structures can be obtained by tuning the radius

size of the cations and anions in the perovskite crystal lattice, including cubic, tetragonal,

and orthorhombic structures. The structure of the perovskite crystal can be calculated by

RRB + X the tolerance factor t , t = , where RRAB,, and RX are the radii of the cations 2(RRA + X ) and anion, respectively. When 0.89<

The first utilization of organic-inorganic lead halide perovskite material into

photovoltaics dates back to 2009, Miyasaka et al. innovatively introduced CH3NH3PbI3

into dye-sensitized solar cells (DSSC) to substitute dye pigment and eventually achieved a

power conversion efficiency of 3.8%3. However, the hole transport layer (HTL) is liquid

electrolyte, which resulted in low stability and insufficient charge transport, the PCE is

thus lower than expected. An unprecedented breakthrough was made in 2012, Kim et al.

evolutionarily used spiro-MeOTAD and mp-TiOx as the hole transport layer (HTL) and

electron transport layer (ETL) to extract the charge carriers generated by the perovskite

photoactive layer. As a result, the PCE was drastically improved to 9.7%, their work marks

the milestone of the mesoscopic heterojunction perovskite solar cells4. In the year of 2013,

Jeng et al. developed planar heterojunction (PHJ) perovskite hybrid solar cells, which eliminated the utilization of the metal oxide insulating scaffold. This novel configuration

is beneficial to a higher PCE and environmental-friendly low-temperature processing5.

Later on, in the November of 2014, KRICT achieved a PCE of 20.1%, setting a new benchmark for defining high-performance perovskite solar cells. In December 2017,

KRICT again hit a record high PCE of 22.7%, which has surpassed the efficiency of CdTe

(Cadmium telluride) solar cells.

1.3 Fundamental physics of perovskite hybrid solar cells

Inspired by the dye-sensitized solar cells (DSSCs), the first development stage of

perovskite hybrid solar cells (PSCs) incorporated an insulating scaffold layer to facilitate

the extraction of the electrons. The PSCs employed mesoporous scaffold-like TiOx or

Al2O3 are usually referred to as mesoscopic heterojunction perovskite solar cells (MS-

PSCs). As is depicted in Figure 1.2(a), the porous structure of the insulating scaffold can

efficiently transport electrons from the perovskite absorber to the underlying transparent electrode, leading to a high power conversion efficiency (PCE)6. However, the fabrication

of the insulating scaffold requires high-temperature sintering process. Generally speaking,

a temperature of 500 °C is demanded to form a high quality mesoporous structure of the

layer. Also, the insulating scaffold possesses high rigidity and hardness, which hinders the

MS-PSCs to be compatible with printing fabrication technique like roll-to-roll (R2R), the

Figure 1.2 The device configuration of the perovskite hybrid solar cells (a) Meso-superstructured (MS) PSCs. (b) Planar heterojunction (PHJ) PSCs.

high temperature process also impedes the PSCs to be manufactured on flexible substrates

like PET or PES. A facile routine to circumvent the drawbacks of the MS-PSCs is employing solution -processable electron extraction material like (phenyl C61 butyric acid 4

methyl ester (PC61BM)) and hole extraction materials like (poly(3,4-

ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), or polytriarylamine

(PTAA)). The low-temperature solution processable feature of PHJ-PSCs imparts the

feasibility of cost-effective and energy-saving printing fabrication routine. However, the

utilization of transparent conductive oxides (TCOs) like tin-doped indium oxide (ITO) or

fluorine-doped tin oxide (FTO) as the electrodes impedes the fulfillment of the printing

fabrication process due to its rigidity and brittleness. In the past decade, some researchers

proposed using conducting polymers like PEDOT:PSS (1:2.5 wt%) to replace the TCO

layer, since PEDOT:PSS is compatible with common printing technique and commercially

available.

1.4 Characterization of perovskite solar cells

1.4.1 J V characteristics

In most cases, the performance of the solar cells is characterized by two important parameters derived from the J V characteristics under illumination which is short-circuit current density (JSC) and open circuit voltage (VOC). The short circuit current density JSC is

defined when the load between the terminals of the equivalent circuit is zero, the open circuit voltage VOC is defined when the load between the two terminals is close to infinite,

where there is no current flowing over the load. As is depicted in Figure 1.3. the fill factor

(FF) is the ratio of the blue area determined by the maximum power point (mpp) over the

green area defined by JSC and VOC. These three parameters are the most commonly used figures of merit to describe the performance of the solar cells, once the J V characteristics are determined by a setup of source meter and solar simulator, we can directly obtain the

values of the figures of merit. The power conversion efficiency (PCE) can be subsequently calculated by the following equation,

V⋅ J⋅ FF PCE = OC SC Pin

Power conversion efficiency (PCE) is defined as the ratio of the multiplication of

JSC, VOC, and FF to the incident radiation power (Pin). Generally, one sun (AM 1.5G) is considered as the standard testing environment for characterizing the performance of the photovoltaics. The intensity of the standard sun light resembles that of North America.

1.4.2 External quantum efficiency (EQE)

The term “quantum efficiency” characterizes the ratio of charge carriers collected

by the solar cells to the incident photons on the device. Usually, external quantum

efficiency (EQE) and internal quantum efficiency (IQE) are used to characterize the efficiency of the solar cells to convert incident photons.

electron/ sec current/(charge of one electron) EQE = = photons/ sec (total power of photons)/(energy of one photon)

Typically, the external quantum efficiency (EQE) spectra is a function of the incident light wavelength, if all of the charge carriers generated by the absorption of the incident light

Figure 1.4 The external quantum efficiency (EQE) of the organic- inorganic hybrid perovskite solar cells.

was collected by the electrode, the quantum efficiency at this certain wavelength is 100%.

In real cases, as is shown in Figure 1.4. the quantum efficiency can be diminished to some extent due to the limitation of diffusion length or the surface recombination effects.

1.5 TCO-free perovskite solar cells with PEDOT:PSS as transparent electrode.

1.5.1 Organic acid treated PEDOT:PSS as transparent electrode.

In the past few years, planar heterojunction perovskite solar cells (PHJ-PSCs) have drawn much attention due to its cost-effective fabrication process and solution-processing 7

compatibility. Since the PHJ structure doesn’t require any insulating scaffolds to facilitate

the electrons transport. The viability of utilizing mass production technique like roll-to-roll

(R2R) printing is promising. Also, the ambipolar charge transport and high charge carrier mobility enable a thick perovskite photoactive layer, making the printing of PSCs more feasible. However, most research groups develop perovskite solar cells on transparent conductive oxides (TCOs) like indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), which is not compatible with R2R printing technique due to the brittleness and high rigidity of the TCO electrodes. Moreover, the fabrication cost of the alternative transparent electrodes like metal grids and metal nanowires is also relatively high.

PSS

- - SO3 SO3H SO3H SO3H SO3

O O O O * PEDOT * S S S S S m

O O O O O O

Scheme 1.1 The chemical structures of poly(3,4- ethylenedioxythiophene)(PEDOT) and poly(styrene sulfonate)(PSS).

One promising alternative material is poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). It can be readily processed on flexible substrates like PET or PES by R2R or spin coating, and it also has high mechanical flexibility and high transmittance in both the visible and near-infrared (NIR) range. As is depicted in

Figure 1.5., the PEDOT chain and PSS chain are bounded together through Coulombic attraction. To obtain a homogeneous dispersion, PSS is used as the counterion to increase

the solubility of PEDOT in aqueous solution. Typically, the PEDOT:PSS transparent electrode is prepared from commercialized product (Heraeus Clevios PH1000), the

electrical conductivity of the PEDOT:PSS thin film prepared from pristine solution is circa.

0.3 S/cm, which is way lower than that of the TCOs. It is reported that, the conductivity of

PEDOT:PSS thin film can be drastically enhanced by treating with strong acid like sulfuric acid or mild organic acid like methanesulfonic acid (MSA). In 2015, Ouyang et al. fabricated flexible perovskite solar cells on MSA-treated PEDOT:PSS transparent electrodes. As a result, an average PCE of 8.1% was obtained for devices fabricated on

PET substrate7.

1.5.2 Silver mesh and PEDOT:PSS hybrid transparent electrode.

Even the PEDOT:PSS (PH1000) coated PET substrate can be readily used to develop flexible perovskite solar cells, the power conversion efficiency (PCE) is still lowered by one-fold compared with the glass/ITO based counterparts, leading to

insufficient performance of the devices. To circumvent this problem, further increase the

electrical conductivity of the transparent electrode is inevitable. However, even strong acid

like sulfuric acid can only increase the conductivity of the PEDOT:PSS thin film to over

3000 S/cm which is still lag behind those of the TCOs. In this scenario, a novel way to dramatically boost the conductivity by embedding silver mesh into the flexible substrate was investigated by some groups. Li et al. integrated silver mech with PET substrate by imprinting technique, the as-prepared PET/Ag-mesh/PH1000 flexible electrode shows an average transmittance over 80% in the visible range and a sheet resistance of 3 Ω/sq8. As

a result, the flexible perovskite solar cell in the configuration of PET/Ag-mesh/PH1000/

PEDOT:PSS/Perovskite/PC61BM/Al achieved a PCE of 13.7%, the PCE remains over 95% after 5000 bending cycles. The device configuration and J V characteristics are shown in

Figure 1.7. This work provides a facile way to fabricate highly electrically conductive transparent electrode with superior transmittance and bending durability.

1.5.3 Roll-to-roll (R2R) printed flexible perovskite solar cells based on silver nanowire/PH1000 electrode.

Roll-to-roll(R2R) printing technique has been considered as the most promising method for upscaling the fabrication of photovoltaics from laboratory level into industry

level. The utilization of R2R technique demands high quality inks for printing of both the

transparent electrode and the successive layers. In 2017, Sears et al. employed a

commercially available transparent electrode ink Clevios HY-E for the R2R slot-die

printing equipment. The ink is formulated of PEDOT:PSS (PH1000) and silver nanowires.

To diminish the shorting effect of the silver nanowires, a layer of PEDOT:PSS was coated

Figure 1.7 (a)The working mechanism of the roll-to-roll (R2R) printing machine, the inset is a photo of the slot die head for printing ink feeding. (b) SEM image of the cross- sectional view of the printed flexible perovskite solar cell. Ref. 8 with permission. Copyright © 2017 John Wiley & Sons, Inc. to planarize the surface of the as-prepared transparent electrode, they develop the flexible perovskite solar cells in the configuration of PET/AgNW/PH1000/PCBM/Al, and a PCE

of 11% was achieved for the printed solar cell9.

1.6. Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)

1.6.1 Overview of PEDOT:PSS

Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) was first

introduced into light emitting diodes (LEDs) as the hole injection layer due to its suitable

work function and its high intrinsic electrical conductivity. With the burgeoning research

of photovoltaics, PEDOT:PSS was more utilized as the hole extraction material (HTM)

and the transparent electrode of polymer and perovskite solar cells. Despite the intrinsic advantages of the PEDOT, it can not be processed through common solution process like spin coating and roll-to-roll slot die printing due to its poor solubility in common solvents like water. To form a stable dispersion of PEDOT, water-soluble PSS chains were combined with PEDOT segments through Coulombic attraction, making PEDOT:PSS a well-dispersed solution which can be readily used through solution processes10. As for the

PEDOT:PSS solution used for fabricating transparent electrodes, the molar ratio of PEDOT over PSS is 1:2.5 wt%, the molar ratio of PSS is lower than that of being used as the hole transport materials due to the demand for a high electrical conductivity of the transparent

electrodes11-12. The electrical conductivity of the thin film prepared from pristine

PEDOT:PSS solution is usually about 0.3 S/cm since PSS chain is an insulating material.

As is depicted in Figure 1.5, the electrostatic between PEDOT chain and PSS chain can be

screened by forming new Coulombic attractions, then the insulating PSS segment can be

thus removed from the highly electrically conductive PEDOT chains. Generally speaking,

an electrical conductivity of more than 1000 S/cm can be easily obtained through the

enhancement of the contacts between PEDOT chains13-14.

1.6.2 Structure and conformation of PEDOT:PSS

It is reported that, the electrical conductivity and transmittance of the PEDOT:PSS thin film is highly dependent on the chemical structure and conformation of PEDOT and

PSS chains. Usually, the structure of PEDOT:PSS is considered as a core-shell structure

which consists of highly conductive inner PEDOT core and amorphous insulating PSS

outer shell. The average radius of the core-shell structure is 20 nm which means the thin

film can only hold one layer of PEDOT:PSS core-shell clusters. In 2012, Takano et al.

reported that PEDOT is randomly oriented in the aqueous solution before they become

highly oriented nanocrystal in the core-shell cluster in the thin film. Also, the electrical

conductivity of the thin film can be enhanced by forming a more orderly and larger grain

size of the PEDOT nanocrystal. Doping the pristine PEDOT:PSS aqueous solution with

binary secondary dopant like ethylene glycol (EG) or dimethyl sulfoxide (DMSO) can lead

to a larger crystal size of PEDOT which is close to the size of the inner core of the

PEDOT:PSS core-shell cluster15.

1.6.3 Methods to improve the electrical conductivity of PEDOT:PSS

In the past twenty years, the academia has witnessed an unprecedented on the

research of boosting the electrical conductivity of the PEDOT:PSS thin film. In the year of

2002, Kim et al. has corroborated that binary secondary dopants like dimethyl sulfoxide

(DMSO) and tetrahydrofuran (THF) can effectively enhance the electrical conductivity of the PEDOT:PSS thin film by two orders of magnitude16. A plethora of additives have been introduced to facilitate the separation between PEDOT and PSS phase and the

conformational change of PEDOT chains. Other treating agents like ionic liquid, surfactant,

zwitterion, salt solution, and acid can also enhance the conductivity. The increment induced

by strong acid like sulfuric acid is most noticeable, which can reach a champion

conductivity of 4000 S/cm17.

To date, the electrical conductivity induced by pre- and post-treatment is still not

clear, the most recognized explanation is the removal of PSS segments and the phase

segregation between PEDOT and PSS chains, the increased crystallinity of PEDOT grain

can also contribute to a higher conductivity. Despite the fact that, the conductivity of the

sulfuric acid treated PEDOT:PSS thin film is comparable to that of the transparent

conductive oxides (TCOs) like ITO, the utilization of sulfuric acid is not environmental-

friendly and will cause safety issue due to its strong corrosivity. Exploring new neutral pH

value, cost-effective, and high effectiveness treating agent to increase conductivity is still

the center topic for the utilization of the PEDOT:PSS to be the transparent electrode of the

photovoltaics18-19.

CHAPTER 2. HIGHLY ELECTRICALLY CONDUCTIVE

PEDOT:PSS TREATED BY ORGANIC SALT

2.1 Introduction

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most

commonly used conducting polymer ever since it was invented by Bayer GmbH in 1988.

With the burgeoning research of photovoltaics, PEDOT:PSS was more utilized as the hole

extraction material (HTM) and the transparent electrode of polymer and perovskite solar

cells. As for the electrical conductivity of the thin film prepared from pristine PEDOT:PSS

solution, it’s usually about 0.3 S/cm since PSS chain is an insulating material. To boost the electrical conductivity of the PEDOT:PSS thin film, the electrostatic attraction between

PEDOT chains and PSS chains has to be screened.

A couple of methods have been used to increase the conductivity of PEDOT:PSS.

Recently, Xia et al. achieved a conductivity higher than 3000 S/cm by treating PEDOT:PSS

thin film with diluted sulfuric acid20. Ouyang et al. observed a conductivity of 1814 S/cm

through methanesulfonic acid treatment21. However, organic and inorganic acids are

certainly not environmental-friendly and can cause safety issues22. In this study, we

exemplified the application of formamidinium iodide (FAI) as a non-acidic agent to increase the electrical conductivity of the pristine PEDOT:PSS thin films by 4 orders of magnitude. The chemical structures of PEDOT:PSS and FAI are shown in Scheme 2.1a,

the positively charged PEDOT chain is Coulombically combined with the negatively

charged PSS chain.

2.2 Experimental section

Materials and Chemicals. PEDOT:PSS (Clevios PH1000) was purchased from Heraeus

Precious Metal North America., Inc. Formamidinium iodide (FAI) was synthesized in our

lab using the method published in other literatures. N,N-Dimethylformamide (DMF,

anhydrous, ≥99.9%), isopropyl alcohol (IPA, anhydrous, ≥99%), ethylene glycol (EG,

anhydrous, ≥99%), and γ-butyrolactone (GBL, anhydrous, ≥99%) were bought from

Sigma-Aldrich. All chemicals are used as received without any further purification.

FAI Treatment of PEDOT:PSS Thin Films. Glass substrates with an area of 1.5 ×1.5 cm2

were cleaned with detergent, DI water, acetone, and isopropanol in ultrasonic cleaning bath, respectively. Then pristine PEDOT:PSS aqueous solution (Clevios PH1000) were spun coated on the pre-cleaned glass substrates. The PEDOT:PSS thin films were heated at

140 °C for 10 min. The as-prepared thin films were treated by 100 μL of formamidinium iodide (FAI) solutions (DMF, EG, and GBL were used as the solvents). The thin films dried out after 10 min. They were then rinsed with DI water for three times and isopropanol once to remove the excess FAI salt on the surface.

Thin Film Characterizations: The thicknesses of the PEDOT:PSS thin films were measured by using a Bruker DektakXT Stylus Profilometer with a scan rate of 0.03 mm s-

1. The van der Pauw four-point probing system was used to measure the sheet resistances

of the thin films. The transmittance spectra of FAI-treated PEDOT:PSS thin film and ITO

were then characterized with a Lambda 900 UV-Vis-NIR spectrophotometer (PerkinElmer,

Waltham, MA, USA). The three-dimensional molecular packing patterns of the thin films were characterized with a grazing incidence wide angle X-ray scattering (GIWAXS) system (Beamline 8-ID-E, APS, Argonne National Laboratory). CV measurement was

conducted by using Gamry reference 3000 at the potential ranging from 0 to 1 V. The

electron paramagnetic resonance spectra (EPR) of the PEDOT:PSS thin films were

characterized by a Bruker Elexsys E 500 to measure the density of polaron states in the

PEDOT chain.

(a) * n* O

N,N-Dimethyl- PSS formamide (DMF) H N H - - SO3 SO3H SO3H SO3H SO3

O O O O * γ PEDOT * S S S -Butyrolactone (GBL) S S m O O

O O O O O O

H - I OH Formamidinium iodide (FAI) Ethylene glycol (EG) OH H2N NH2

(b) 1800

1600 1562

1400 1347

1200 1175

1000

800

600

400 Electrical Conductivity (S/cm) 200 0.3 0 w/o DMF GBL EG

Scheme 2.1 (a) The chemical structure of PEDOT:PSS and formamidinium iodide (FAI), N,N- dimethylformamide (DMF), γ-butyrolactone (GBL), and ethylene glycol (EG). (b) Conductivities of PEDOT:PSS thin films treated with 0.1 M FAI solutions with various solvents.

.2.3 Results and Discussion

The as-prepared pristine PEDOT:PSS thin films were treated by FAI solutions. As a

result, the electrical conductivity was enhanced from 0.3 S/cm up to 1562 S/cm by 4 orders

of magnitude. The enhancement of conductivity is correlated with the solvents. The electrical conductivities of the treated thin films are shown in Scheme 2.1b. The increment

(a)

103

102

101 Elecrical Conductivity (S/cm) Conductivity Elecrical

100 10-4 10-3 10-2 10-1 100 Concentration of FAI Solution (M)

(b) 1800

1600

1400

1200

1000

800 Elecrical Conductivity (S/cm) Conductivity Elecrical

600

60 80 100 120 140 160 180 200 Temperature of Treatment (°C)

Coulombic attraction between PEDOT and PSS23 and the conformational change of

PEDOT chains from benzoid to quinoid conformation24. 18

The characterization of electrical conductivity of PEDOT:PSS thin films treated by

DMF solutions of FAI was conducted due to the significant enhancements. The dependence

of the electrical conductivities on the concentration of FAI solutions is depicted in Figure

2.1a. When the concentration of the solution is 10-4 M, the conductivity was only slightly

increased to 2.3 ± 0.9 S/cm. With the increment of the concentration, the conductivities of

the thin films can be optimized to over 1000 S/cm. And it almost remained unchanged with a concentration higher than 0.1 M. To optimize the conductivities of the thin films, we

further carried out the measurements of the dependence of the conductivities on the treating

temperature. As is shown in Figure 2.1b, the conductivities increased drastically with the

increment of treating temperature in the range of 80 °C to 140 °C, then it reduced to 1275

± 36 S/cm when we further increase the treating temperature to 180 °C. The conformational

change of PEDOT is sensitive to the treating temperature. In the range of 80 °C to 140 °C,

the movement of PEDOT chains is facilitated by the increased thermal energy, which will

lead to a more linear conformation of PEDOT chains. When the treating temperature is

above 140 °C, the conductivities are diminished due to the degradation of PEDOT chains25.

Grazing incidence wide angle X-ray scattering (GIWAXS) is carried out to characterize

the molecular packing of the FAI treated PEDOT:PSS thin films, Figure 2.2. shows the

GIWAXS patterns of the FAI post-treated PEDOT:PSS thin films. The Bragg peak along

-1 the out-of-plane direction with the qz of 0.4725 Å indicates the backbone direction of the

PEDOT chain is parallel to the substrate with an interchain spacing of 13.30 Å. In the same

Figure 2.2 GIWAXS 2D patterns of (a) pristine (b) FAI/DMF solution (c) FAI/GBL solution and (d) FAI/EG solution treated PEDOT:PSS thin films.

manner, the Bragg peak along the in-plane direction corroborates that the packing pattern

of the PEDOT chain along the π-π direction is parallel to the substrate with a π-π distance

of 3.54 Å. 26-29 The scheme of the packing pattern of the pristine PEDOT:PSS thin film is depicted in Figure 2.3. according to the above observations. Since the packing patterns of the FAI treated PEDOT:PSS thin films resemble the molecular packing pattern of the

pristine PEDOT:PSS thin film, we can calculate the interchain spacing and the π-π distance of the PEDOT chains in the FAI-treated PEDOT:PSS thin film through the same procedure.

Figure 2.3 The molecular packing pattern of PEDOT chains in PEDOT:PSS thin film.

Based on the 2D GIWAXS diffraction pattern, the extracted 1D diffraction patterns of the out-of-plane and in-plane direction are shown in Figure 2.4. The molecular packing pattern and the electrical conductivity of the FAI solution treated PEDOT:PSS thin films as well as the pristine PEDOT:PSS thin film are summarized in Table 2.1. As a result, the

electrical conductivity of the PEDOT:PSS thin films is impacted by the interchain spacing

of the out-of-plane direction, the lateral packing of the PEDOT chains with a more compact stacking pattern can facilitate a higher electrical conductivity30-31. Thus, it is exemplified

that the charge carrier hopping along the out-of-plane direction is ameliorated by the

treatment of the FAI solution.

Absorption spectroscopy is carried out to study the mechanism of the increased electrical

conductivities of the FAI-treated PEDOT:PSS thin films. As is shown in Figure 2.5a, the

(a) 400 (a) pristine 700 pristine 360 FAI/DMF FAI/DMF FAI/EG FAI/EG 320 600 FAI/GBL FAI/GBL

280 500

240 400 200 300 Intensity(a.u.)

160 Intensity(a.u.)

120 200

80 100

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 q (A-1) q (A-1) y z

Figure 2.4 1D GIWAXS pattern along the (a) out-of-plane direction and (b) in-plane direction of the pristine and FAI solutions treated PEDOT:PSS thin films. enhanced absorption peaks ranging from 750 nm to 850 nm and from 1800 nm to 2000 nm can be attributed to the increased densities of polaron state and bipolaron state, respectively.

Table 2.1 The molecular packing parameters and electrical conductivity of pristine and FAI solution treated PEDOT:PSS thin films. Electrical π-π qz Interchain qy conductivity distance (Å-1) spacing (Å) (Å-1) (Å) (S/cm)

Pristine 0.4725 13.30 1.7769 3.54 0.3

0.1 M FAI/DMF 0.5623 11.17 1.7769 3.54 1563

0.1 M FAI/EG 0.5225 12.03 1.7769 3.54 1175

0.1 M FAI/GBL 0.5538 11.35 1.7769 3.54 1347

Since both the enhanced densities of polaron state and bipolaron state can facilitate the transportation of charge carriers. The intensities of the absorption peaks of the PEDOT:PSS 22

thin films is in the order of FAI/DMF > FAI/GBL > FAI/EG > pristine, which is in

accordance with the observation of enhanced conductivities32-34.

Figure 2.5b. shows the electron spin resonance (ESR) spectra of the pristine and FAI

(a) 1.2 pristine 1.0 FAI/DMF FAI/EG FAI/GBL 0.8

0.6

0.4 Absorption (a.u.)

0.2

0.0 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Wavelength (nm)

(b) 1.2 pristine 1.0 FAI/DMF FAI/EG 0.8 FAI/GBL 0.6 0.4 0.2 0 -0.2 -0.4 Intensity (counts) -0.6 -0.8 -1.0 -1.2

3360 3365 3370 3375 3380 3385 3390 Magnetic field (Gauss)

Figure 2.5 (a) Absorption spectra (b) electron spin resonance spectra of pristine and FAI solutions treated PEDOT:PSS thin films.

treated PEDOT:PSS thin films. Since the polaron state consists of a spin of 1/2 and the bipolaron state is spinless, the ESR measurement only indicates the densities of the polaron states. To obtain the spin concentrations of the PEDOT:PSS thin films, double integration

can be used to characterize the area under each magnetic peak. The calculated spin

concentrations of the pristine PEDOT:PSS thin film and treated with FAI/DMF, FAI/GBL,

FAI/EG solutions are 8.53 ×1020 spin/cm3, 8.71 ×1020 spin/cm3, 3.84 ×1021 spin/cm3, and

Figure 2.6 Contact angle of toluene on (a) FAI/DMF treated PEDOT:PSS thin film and (b) ITO transparent electrode.

3.36 ×1021 spin/cm3, respectively. It is found that the densities of the polaron state is in

consistency with the absorption spectroscopy characterization. To investigate the surface

properties of both FAI-treated PEDOT:PSS thin film and ITO, we performed the surface

energy measurement by measuring the contact angle with toluene (Figure 2.6)35-36. The

PEDOT:PSS thin film treated with 0.1 M FAI/DMF solution displays a contact angle of

40.9°, which is much smaller than that of ITO. Since the hole transporting material poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) is dissolved in toluene. Such

hydrophobic characteristic of the FAI-treated PEDOT:PSS thin film is favorable to form a homogeneous and fully-covered hole extraction layer, which can facilitate the holes to be transported from perovskite photoactive layer to the anode.

CHAPTER 3. TRANSPARENT CONDUCTIVE OXIDE-FREE

PEROVSKITE HYBRID SOLAR CELLS WITH ORGANIC SALT

TREATED PEDOT:PSS AS TRANSPARENT ELECTRODE

3.1 Introduction

Recently, organic-inorganic hybrid perovskite solar cells (PSCs) have been widely researched due to their low-cost fabrication process and superior light harnessing efficiency.

There has been an unprecedented boom on the investigation of the methylammonium lead tri-iodide (CH3NH3PbI3) perovskite hybrid solar cells (PSCs) aiming to boost the performance and corroborate the feasibility of utilizing mass production system like roll- to-roll (R2R)37-41 to print the device. However, among the most efficient PSCs, transparent conductive oxides (TCO) like indium tin oxide (ITO) are used as the transparent electrodes of the devices. As a matter of fact, ITO has many drawbacks, for instance, due to the scarcity of the indium element and the high-temperature sintering process, the fabrication cost of ITO is usually high42, which renders it the most expensive part of the PSCs. In addition, ITO is not compatible with R2R technique due to its brittleness43-44.

To circumvent these problems, some researchers investigated a plethora of materials including carbon nanotubes (CNTs)45-46, silver nanowires (Ag NWs)47-48, graphene sheets

and conducting polymers23, 49-51 like poly(3,4-ethylenedioxythiophene):poly(styrene

sulfonate) (PEDOT:PSS) to be the substitutes for ITO. PEDOT:PSS thin film as the transparent electrode has many merits over the other materials, it’s highly mechanically flexible, highly transparent in the visible range and compatible with roll-to-roll (R2R)52

mass production. On the other hand, pristine PEDOT:PSS thin films suffer from an

electrical conductivity approximately 0.3 S cm-1 which is the main barricade for it to be

used as the electrode53.. The power conversion efficiency of PSCs incorporated FAI-treated

PEDOT:PSS electrode is over 13%, which demonstrates the feasibility of using FAI-

treated PEDOT:PSS as the substitute for ITO.

3.2 Experimental Section

Materials and Chemicals. PEDOT:PSS (Clevios PH1000) was purchased from Heraeus

Precious Metal North America., Inc. Formamidinium iodide (FAI) and methylammonium

iodide (MAI) were synthesized in our lab using the method published in other literatures.

[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was bought from Solenne BV.

Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) was obtained from Sigma-

Aldrich. Lead iodide (PbI2) was supplied by Alfa Aesar., Inc. The other materials,

including N,N-Dimethylformamide (DMF, anhydrous, ≥99.9%), isopropyl alcohol (IPA,

anhydrous, ≥99%), ethyl alcohol (≥99.9%), chlorobenzene (CB, anhydrous, ≥99.9%),

dimethyl sulfoxide, (DMSO, anhydrous, ≥99%), ethylene glycol (EG, anhydrous, ≥99%),

and γ-butyrolactone (GBL, anhydrous, ≥99%) were bought from Sigma-Aldrich. All

chemicals are used as received without any further purification.

Device Fabrication: The perovskite solar cells were fabricated on pre-cleaned glass

substrates with the configuration of ITO/PTAA/Perovskite/PC61BM/Al and FAI-treated

PEDOT:PSS/PTAA/Perovskite /PC61BM/Al. A PTAA hole extraction layer was prepared

by spin-coating at 6000 rpm for 25s with a thickness of 35 nm. The perovskite photoactive layers were fabricated by two-step procedures in the glovebox with nitrogen atmosphere, followed by thermal annealing at 100 °C for 90 min. The PC61BM layers were spun coated

on top of perovskite layers with a spin rate of 1500 rpm for 35s. The aluminum (Al) cathode was thermally evaporated under high vacuum (10-6 mbar) with a thickness of 100 nm.

Device Characterization: The performance of the PSCs was characterized under an AM

1.5 G solar simulator (Newport, 91160-1000), the light intensity was calibrated by a mono- silicon detector (with KG-5 visible color filter) of National Renewable Energy Laboratory

(NREL) to obtain a spectrum which in accordance with the standard. The current density vs. voltage (J-V) characteristics under illumination and in the dark were obtained by using a Keithley 2400 source meter. The external quantum efficiency (EQE) spectra were measured through the solar cell quantum efficiency measurement system (QEX10) with a

300 W steady-state Xenon lamp as the light source from PV Measurement., Inc. The impedance spectra (IS) of the perovskite solar cells were measured through an HP 4194A impedance analyzer in the dark, the devices were operated under open circuit voltage (VOC)

with an oscillating voltage frequency increased from 5 Hz to 105 Hz.

3.3 Results and discussion

The transmittance spectra of the FAI-treated PEDOT:PSS transparent electrode and ITO are depicted in Figure 3.1. The transmittance of the FAI-treated PEDOT:PSS is superior than that of ITO when the wavelength is below 470 nm, and its comparable to that of ITO transparent electrode in the visible range54, the incident light can thus easily reach the

100 90 80 70 60 50 40 30 Transmittance (%) Transmittance 20 FAI-treated PEDOT:PSS 10 ITO 0 300 400 500 600 700 800 900 1000 Wavelength (nm)

Figure 3.1 Transmittance spectra of FAI/DMF solution treated PEDOT:PSS thin film and ITO transparent electrode.

perovskite photoactive layer through the FAI-treated PEDOT:PSS electrode.

The configuration of the device is shown in Figure 3.2a, The current density versus voltage (J-V) characteristics characterize the efficiency of the PSCs. Figure 3.2b. displays the J-V curves of the device developed on rigid FAI-PEDOT:PSS/Glass and flexible FAI-

PEDOT:PSS/PET, and ITO/Glass substrates, the device performance of the solar cells is summarized in Table 3.1. The devices utilized FAI-PEDOT:PSS/Glass substrate have an

- open-circuit voltage (VOC) of 1.02 V, a short-circuit current density (JSC) of 18.40 mA cm

2, a fill factor (FF) of 71.2%, and a power conversion efficiency (PCE) of 13.36%, which

is slightly lower than that of the control device based on rigid glass substrate which

possesses an open-circuit voltage (VOC) of 1.06 V, a short-circuit current density (JSC) of

20.58 mA cm-2, a fill factor (FF) of 76.1%, and a power conversion efficiency (PCE) of

16.60%. To exemplify the possibility of fabricating flexible solar cells, the PSC is also

developed on FAI-PEDOT:PSS/PET flexible substrate. As a result, the devices show an

average open-circuit voltage (VOC) of 0.88 V, a short-circuit current density (JSC) of 16.30

Table 3.1 Device performance parameters of the PSCs based on ITO/Glass, FAI- PEDOT:PSS/Glass and FAI-PEDOT:PSS/PET substrates. a b Sh S J J VOC FF PCE R R SC SC 2 2 2 2 (%) (%) (mA/cm ) (mA/cm ) (V) (Ω cm ) (Ω cm )

ITO 20.58 19.51 1.06 76.1 16.60 2624 4.20

FAI- 18.40 17.53 1.02 71.2 13.36 1354 5.89 PEDOT:PSS/Glass FAI- 16.30 15.27 0.88 61.8 8.86 931 6.91 PEDOT:PSS/PET

(a): JSC obtained from J V characteristics. (b): JSC obtained by the integration of the EQE spectra.

mA cm-2, a fill factor (FF) of 61.8%, and a power conversion efficiency (PCE) of 8.86%.

The shunt resistance (RSh) and series resistance (RS) the of the devices are also calculated

from the reciprocal of the slopes of the J-V curves under illumination at 0 and 1 V7, 55-56.

The external quantum efficiency (EQE) spectra of the device utilized FAI-treated

PEDOT:PSS as the transparent electrode and the control device are displayed in Figure

3.2d. A lowered quantum efficiency was observed of the device with FAI-treated

PEDOT:PSS than the control device which is related to the transmittance and conductivity

of the transparent electrodes7.

Impedance spectroscopy is performed to research the electrical properties of the PSCs

which cannot be investigated by direct current measurement. Figure 3.3a. shows the IS

spectra of the PSCs. The RS consists of the sheet resistance (Rsheet) and the charge transport

resistance (RCT) at the interface between the charge carrier extraction layer and the

(a)

Table 3.2 (a) Device configuration of the ITO-free perovskite hybrid solar cell. (b) and (c) J- V characteristics of the PSCs based on either ITO or FAI-treated PEDOT:PSS thin film under illumination and in the dark. (d) external quantum efficiency spectra of the PSCs.

electrodes, the interface between the charge extraction layer and the perovskite layer as

57-58 well as inside the perovskite film . Since Rsheet is determined by the resistance of the

electrodes. The Rsheet of the device utilized FAI-treated PEDOT:PSS film as the electrode

shows a resistance of 30.7Ω, which is larger than that of the ITO based device due to the

lower conductivity of the PEDOT:PSS thin film. An enhanced RCT of 1065.3 Ω is also

observed for the device with PEDOT:PSS, which is higher than that of ITO based device

(842Ω). This indicates an inferior charge transport at the interfacial boundaries of the

device59. As a result, the observation of the IS spectra agrees with the J-V characteristics

under illumination.

(a) 1000 900 FAI-treated PEDOT:PSS ITO 800

700

600

500

400 -ImZ''(Ohm) 300

200

100

0 0 200 400 600 800 1000 1200 ReZ'(Ohm)

1.20 (b) (c) FAI-treated PEDOT:PSS FAI-treated PEDOT:PSS ITO 1.15 ITO

1.10 ) 2 1.05 10

1.00 mA/cm ( Voc(V) 0.95 Jsc

0.90

0.85

1 0.80 10 100 10 100 Light intensity (mW/cm2) Light intensity (mW/cm2)

Figure 3.2 (a) Nyquist plot of the PSCs based on either ITO or FAI-treated PEDOT:PSS thin film measured in the dark. Light intensity dependences of the steady-state (b) JSC and (c) VOC of the PSCs based on either ITO or FAI-treated PEDOT:PSS thin film.

To investigate the underlying physics of the inferior performance of the device with FAI-

treated PEDOT:PSS, the light intensity dependence of JSC and VOC was measured in order

to evaluate the impact of the charge carrier recombination within the solar cells60-62. As

shown in Figure. 3.3b, PSCs based on either FAI-treated PEDOT:PSS or ITO exhibit a

power-law dependence of JSC on the light intensity. The JSC can be correlated with the light

α intensity by JIsc ∝ (α ≤ 1) , where I is the light intensity and α is the coefficient. An α

of 0.936 is observed from the devices based on ITO, and an α of 0.887 is observed from

(b) 1.2

0.8

0.6

Normalized PCE Normalized 0.4

0.2

0 0 100 200 300 400 500 Bending cycles (times)

Figure 3.3 (a)Photo of the flexible PSC developed on a FAI-PEDOT:PSS/PET substrate. (b) The normalized PCE of the flexible PSCs with respect to the bending cycle. the devices with FAI-treated PEDOT:PSS. The reduced α value indicates a stronger non-

geminate recombination process63, which is due to the poor morphology of the PTAA layer

induced by the FAI-treated PEDOT:PSS thin film. Figure. 3.3c displays the light intensity dependence of Voc. At the open-circuit condition, VOC can be correlated with the light

kT intensity by VI∝⋅ ln( ) , where k is the Boltzmann constant, T is the temperature in oc q

Kelvin, and q is the elementary charge. The VOC of the PSCs with FAI-treated

kT PEDOT:PSS shows a stronger dependence of the light intensity with a slope of 1.96 , q

kT whereas a slope of 1.42 is observed from the devices using ITO. The enhanced slope of q

the devices with FAI-treated PEDOT:PSS suggests an augmented trap-assisted charge

recombination process63-64.

To investigate the flexibility of the PSCs, the devices based on the FAI-

PEDOT:PSS/PET flexible substrate was bent to a radius of 3 mm without any

encapsulation. As is depicted in Figure 3.4b, The efficiency was only slightly diminished

from 8.85% to 6.93% for over 500 cycles of bending. which demonstrates the novel

configuration for flexible perovskite solar cells is capable of enduring frequent and small

bend radius bending It’s been reported that PSCs developed on ITO/PET flexible substrate

can endure several tens of bending cycles and thus can be used for R2R printing technique65.

However, if the device is further bended for more cycles, the cracked ITO sharps could

substantially damage the above layers which will certainly lead to an inferior PCE of the

PSCs44, 66.

CHAPTER 4. CONCLUSION

In conclusion, we demonstrated a simple yet effective method to enhance the electrical

conductivity of the PEDOT:PSS thin film through FAI organic solution treatment. The

electrical conductivity was substantially increased from 0.3 S/cm to 1562 S/cm by 4 orders of magnitude. The conductivity increment depends on the concentration of the FAI solutions and also the treating temperature. Through the observation of GIWAXS, we found that the organic solutions of FAI can induce a more compact molecular packing of

the PEDOT chains in the verticle direction, which can effectively facilitate the charge

carriers to be transported in the PEDOT domains. Moreover, the organic salt can also

increase the doping level of the PEDOT chains by enhancing the densities of both polaron

and bipolaron states, rendering a higher concentration of the charge carriers in the PEDOT

domain. The high transmittance property of the PEDOT:PSS thin film can be retained after the treatment, which ensures sufficient amount of photons can reach the perovskite photoactive layer.

ITO-free PSCs based on FAI-treated PEDOT:PSS transparent electrode were fabricated through solution process. The power conversion efficiency of the devices based on FAI- treated PEDOT:PSS/glass substrate is above 13.36%, which is still sufficient for industry use. The PSCs were also developed on FAI-treated PEDOT:PSS/PET substrate to corroborate the feasibility of utilizing this configuration for mass printing production. As a result, a PCE of 8.85% was obtained for the flexible PSC. The mechanical flexibility of the device was tested by bending the solar cells to a radius of 3 mm without any encapsulation. The efficiency was only slightly diminished from 8.85% to 6.93% for over

500 cycles of bending. which demonstrates the novel configuration for flexible perovskite

solar cells is capable of enduring frequent and small bend radius bending

The treatment of FAI organic solutions offers us a facile route to utilize PEDOT:PSS thin films as the transparent electrode of the PSCs, which is promising to substitute ITO

for large-scale and flexible photovoltaics.

CHAPTER 5. FUTURE PLAN

In addition to the results above, there is still a long way for the flexible PSCs to be

upscaled to mass production for industry use, the feasibility of fabricating flexible devices of PEDOT:PSS/PET substrate is only demonstrated by spin coating in this work. As for

industry use solution process technique like roll-to-roll (R2R) slot-die printing, the

suitability of the organic treated PEDOT:PSS transparent electrode still needs further

investigation. The challenge of utilizing the PEDOT:PSS transparent electrode for large

scale high- efficiency PSCs can be mainly classified into two categories:

(1)Enhance the electrical conductivity of the transparent electrode:

Even though the electrical conductivity of the organic salt treated PEDOT:PSS thin

film is about 1600 S/cm, it’s still much lower than that of ITO,which is about 6000S/cm.

To dramatically boost the conductivity, the insulating counterion phase must be removed

from the highly electrically conductive PEDOT phase. Small molecules like tosylate (Tos-)

is a promising candidate to replace PSS segments. Since small molecules can be easily removed by post-treatment solution, the transparent electrode based on PEDOT:Tos thin film usually shows a superior conductivity compared with PEDOT:PSS counterpart.

(2)Improve the quality of the transparent electrode:

The post-treated PEDOT:PSS transparent electrodes usually have an inferior

morphology compared with untreated as-prepared thin films. The uneven distribution of the treating solution on the PEDOT:PSS thin film could lead to an uneven topography and a large deviation of the local conductivity. To circumvent this problem, we may want to transfer the substrate to a shaker when is being treated. As for large-scale electrode manufacture, doctor blading and inkjet casting may solve this issue as well.

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