Environmentally Friendly Plasma-Treated PEDOT:PSS as Electrodes

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Environmentally Friendly Plasma Treated PEDOT:PSS as Electrodes for ITO-free Perovskite Solar Cells Bjorn Vaagensmith, Khan Mamun Reza, Md Nazmul Hasan, hytham elbohy, Nirmal Adhikari, Ashish Dubey, Nick Kantack, Eman Gaml, and Qiquan Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10987 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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ACS Applied Materials & Interfaces

Environmentally Friendly Plasma Treated PEDOT:PSS as Electrodes for ITO-free Perovskite Solar Cells Bjorn Vaagensmith1, Khan Mamun Reza1, MD Nazmul Hasan1, Hytham Elbohy1, 2, Nirmal Adhikari1, Ashish Dubey1, Nick Kantack1, Eman Gaml1, 2, Qiquan Qiao1* 1

Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, USA E-mail: [email protected]

2

Department of Physics, Faculty of Science, Damietta University, Damietta 34511, Egypt

Abstract:

Solution

processed

poly(3,4-ethylenedioxythiophene):poly(styrene

sulfonate)

(PEDOT:PSS) transparent electrodes (TEs) offer great potential as a low cost alternative to expensive indium tin oxide (ITO). However, strong acids are typically used for enhancing the conductivity of PEDOT:PSS TEs, which produces processing complexity and environmental issues. This work presents an environmentally friendly acid free approach to enhance the conductivity of PEDOT:PSS using a light oxygen plasma treatment in addition to solvent blend additives and post treatments. The plasma treatment was found to significantly reduce the sheet resistance of PEDOT:PSS TEs from 85 to as low as 15 Ω sq-1, which translates to the highest reported conductivity of 5012 S/cm for PEDOT:PSS TE. The plasma treated PEDOT:PSS transparent electrode resulted in an ITO-free perovskite solar cell efficiency of 10.5%, which is the highest reported efficiency for ITO-free perovskite solar cells with a PEDOT:PSS electrode that excludes the use of acid treatments. This is the first demonstration of this technology. Moreover, the PEDOT:PSS TEs enabled better charge extraction from the perovskite solar cells and reduced the hysteresis in the current density-voltage (J-V) curves. Keywords: ITO-free, Transparent electrodes, PEDOT:PSS, Solution processed, Perovskite solar cells

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1. Introduction Transparent electrodes (TEs) represent an essential component within optoelectronic and photovoltaic devices. For perovskite solar cells, the TEs must have a high optical transparency, low sheet resistance, and smooth surface. Widely used transparent electrode materials include indium tin oxide (ITO), indium zinc oxide (IZO), and fluorine tin oxide (FTO). However, these TEs possess certain drawbacks, such as high deposition cost, use of scare and expensive indium in ITO and IZO, brittle nature of ITO and FTO

1-2

and high surface roughness of FTO.3

Therefore, low cost, environment friendly, and solution processable TEs are needed for thin film solar cells. Silver nanowires (SNWs), carbon nanotubes (CNTs), graphene, and poly(3,4ethylenedioxythio-phene):poly(4-styrenesulfonate) (PEDOT:PSS) remain prevalent as low cost solution processed TE alternates. The sheet resistance and optical transmittance of SNWs match ITO, but suffer from a highly rough surface along with poor electrode lifetime.4-6 SNW electrodes are not suitable as a bottom electrode due to the rough surface inhibiting perovskite crystal growth.7 CNTs and solution processed graphene exhibit poor performance compared to ITO in terms of transmittance and sheet resistance.8-11 Among all these TE materials, PEDOT:PSS has proven quite promising due to its smooth surface, low sheet resistance, and high transparency. PEDOT:PSS transparent electrodes have considerably improved with sheet resistances less than or equal to 50 Ω sq-1 and transmittances greater than 85% after formic acid, sulfuric acid, or methanesulfonic acid post treatments.12-14 However, these acid post treatments are not compatible with roll-to-roll fabrication techniques on plastic substrates due to the highly corrosive nature of strong acids. Therefore, an acid free method for enhancing the conductivity

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of PEDOT:PSS would be ideal for a cost effective continuous output fabrication and drastically simplify production safety precautions. The role of solvent additives in PEDOT:PSS inks have already been explored.15-18 Dimethyl sulfoxide (DMSO) and ethylene glycol (EG) have seen the most success in enhancing the conductivity of PEDOT:PSS films; however, little research has studied blend additives of DMSO and EG. Post treatments of common solvents for PEDOT:PSS films have also been investigated.19-20 Some of the best results were obtained by soaking PEDOT:PSS in ethanol, EG, or water blends with various solvents. Exploring more solvent blend combinations for post treatments other than with water alone could further enhance the conductivity of PEDOT:PSS films. Recently, in parallel to polymer solar cells,12-13 modified PEDOT:PSS electrodes have also been reported for application in perovskite solar cells.14 In the last several years, perovskite solar cells were of great interest due to a rapid increase in its power conversion efficiency (PCE) compared to polymer solar cells,21-33 dye sensitized solar cells,34-36 small molecule solar cells,37 and copper zinc tin sulfide (CZTS) solar cells.38 The high PCE for perovskite solar cells resulted from its high absorption coefficient and charge carrier diffusion length.39-41 Different device architectures of perovskite solar cells, such as planar and mesostructured cells, were used, but the p-i-n structured planar perovskite solar cells have shown flexibility and lower temperature processing. This work attempts to demonstrate an ITO-free perovskite solar cell using a PEDOT:PSS TE with enhanced optoelectronic properties though solvent blend additives, post treatments, and a mild oxygen plasma treatment. It was found that PEODT:PSS films with a DMSO:EG (1:1) additive and a post treatment in a water:EG:ethanol (1:1:1) mixture led to the greatest reduction in sheet resistance. The solvent blend additive and post treatment complement each other, as the



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additive helps rearrange the film and post treatment helps remove excess insulating PSS within the PEDOT:PSS film. Using multiple layers of PEDOT:PSS, a typical sheet resistance of 85 Ω sq-1 with an average transmittance of 73% in the range of 350-900 nm were achieved. Plasma treatment improved wetting of additional layers and reduced the sheet resistance to an impressive 36 Ω sq-1 (with the best result reaching 15 Ω sq-1). The plasma treated PEDOT:PSS TE resulted in a perovskite device efficiency of 10.5%, which is the highest reported efficiency for perovskite solar cells with a PEDOT:PSS electrode that excludes the use of acid treatments.42 The control sample using an ITO TE exhibited a very comparable power conversion efficiency (PCE) of 12.4% to the PEDOT:PSS TE and matched the highest efficiencies of previous reports using a similar device structure with an ITO TE.43 2. Experimental Procedures 2.1 Materials Soda-lime glass substrates were purchased from Hartford Glass Co. Inc., PbI2 (99%) and chlorobenzene (>99%) were purchased from Acros organics, methylammonium iodide (CH3NH3I) was obtained from Dyesol, and PC60BM was acquired from Nano-C. Anhydrous dimethyl sulfoxide (DMSO) (>99.9%), γ-butyrolactone (>99%), ethylene glycol (>99%), and Rhodamine were purchased from Sigma Aldrich. Ethanol (99%), isopropanol alcohol (IPA), and acetone were acquired from Fisher Scientific. Clevios™ P VP AI 4083 and PH1000 PEDOT:PSS were ordered from Heraeus. All the materials were used as received. 2.2 Fabrication and characterization of PEDOT:PSS electrodes Glass substrates were cleaned by sequentially ultrasonicating for 20 minutes in detergent water, deionized (DI) water, acetone, and lastly isopropanol alcohol (IPA). The substrates were then plasma cleaned for 20 minutes. PEDOT:PSS (Clevios PH1000) solutions having 0, 5, 10, or 4 ACS Paragon Plus Environment

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15 wt.% of EG, DMSO, DMSO:EG (1:2 v/v), DMSO:EG (2:1 v/v), or DMSO:EG (1:1 v/v) additives were spin coated onto the glass substrate at 4000 rpm for 1 minute and allowed to air dry for 15 minutes, followed by annealing at 120 °C for 10 minutes. The resulting films were approximately 45 nm thick. For post treatments, films were then soaked in water, EG, DMSO, ethanol, or various blends thereof in equal ratios by volume for 2 minutes followed by soaking in water for 20 s and annealing at 120 °C for 10 minutes. The transmittance and absorbance of films were measured using an Agilent 8453 UV-visible Spectroscopy System. To measure the absorption in the ultraviolet region, PEDOT:PSS films were spin coated onto fused silica substrates at 2000 RPM. Sheet resistance was determined using the transfer length method or a four-point probe (Guardian SRM-232-100). The transfer length method 44 was executed by thermally evaporating 80 nm thick silver electrodes at a base pressure of 10-5 mbar. The resistance between the electrodes was measured with an Agilent 2000 digital multimeter and the sheet resistance was calculated in Excel. Raman spectroscopy was taken using Renishaw RM2000 with an argon laser of wavelength 514.5 nm, 300 grating, D3 filter, and 60 s detector integration time. Film topography was obtained using an Agilent 5500 AFM in tapping mode with a 75 kHz SSS-SEIH-SPL tip from NanoSensors. Current sensing AFM (CSAFM) was measured using Cr/Pt coated tips (budget sensors ContE-G, 0.2 N m-1). A Budget Sensors Multi 300-EG Cr/Pt coated tip was used for kelvin probe force microscopy (KPFM) measurement. The first resonance frequency of the tip was 300 kHz, which was used in the first lock-in amplifier (LIA1). The second frequency (f2) was 5 kHz and used in the second lock-in amplifier (LIA2) to perform the KPFM measurement. An electric oscillation to the tip was provided at 5 KHz with the drive offset of -3 V. The drive of LIA2 was around 15% to reach an amplitude of 0.2 V.



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Gwyddion software was used to analyze all AFM, CSAFM, and KPFM images. Film thicknesses were measured using the Veeco Dektak 150. 2.3 Fabrication and characterization of perovskite solar cells Solar cells were fabricated on patterned glass\ITO or glass substrates. Substrates were cleaned by sequential ultrasonication for 20 minutes each in soapy deionized (DI) water, DI water, acetone, and lastly isopropanol alcohol (IPA), followed by 20 minutes of oxygen-plasma treatment (Harrick Plasma PDC-32G) with 10.5 W applied. The vacuum pump (BOC Edwards XDS5) was run for 5 minutes to form a good vacuum within the chamber prior to plasma cleaning and O2 gas was slowly released into the chamber manually using the three-way valve for 10 seconds every five minutes during the plasma cleaning. PEDOT:PSS (Clevios PH 1000) with a 15 wt.% DMSO:EG (1:1) additive was spin coated at 2000 RPM on the glass substrates, air dried for 15 minutes, and annealed at 120 °C for 10 minutes in air. The post treatment was then performed by soaking the PEDOT:PSS films in a mixed solution of DI water:EG:ethanol (1:1:1 vol. ratio) for 2 minutes. After soaking, films were dipped in DI water for 20 s to remove the EG and then annealed at 120 °C for 15 minutes. The PEDOT:PSS electrodes were exposed to oxygen plasma for 2 minutes using the same method described for cleaning the substrates. For PEDOT:PSS electrodes not exposed to plasma, the PEDOT:PSS ink was blended 2:1 v/v with IPA prior to spin coating to improve wetting. Contact angle was measured using an Advanced Surface Technology, Inc. VCA 2000 system. Three additional layers of PEDOT:PSS were coated using the same process except with an increased spin coating speed of 4000 RPM to obtain a four layer PEDOT:PSS electrode, now approximately 133 nm thick. Silver paste (PELCO colloidal silver 16031) was applied on the edge of the PEDOT:PSS electrode to provide a contact during the I-V characterization. PEDOT:PSS (Clevios AI 4083) was spin coated at 6 ACS Paragon Plus Environment

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4500 rpm onto the plasma cleaned PEDOT:PSS PH1000 TE or patterned glass\ITO substrates and annealed at 120 °C for 10 minutes. For plasma cleaned PEDOT:PSS electrodes, the PEDOT:PSS (AI 4083) hole transport layer was also plasma cleaned for 2 minutes prior to coating the perovskite layer. The perovskite solution was prepared by mixing 581 mg of lead iodide (PbI2) and 209 mg of methylammonium iodide (CH3NH3I) in 1 ml mixture of γbutyrolactone:DMSO (7:3 v/v), which was stirred for 2 hours at 70 °C inside a N2-filled glove box before spin coating. The perovskite solution was spin coated inside a N2 gas filled glovebox at 750 rpm for the first 20 s, followed by 4000 rpm for an additional 60 s. Toluene (160 µl) was dropped on the perovskite film after 40 s of spinning to remove excess DMSO solvent. Spin coated perovskite films were then annealed at 100 °C for 20 minutes. A PC60BM solution in chlorobenzene (20 mg ml-1) was spin coated at 2000 rpm for 40 s and dried for 15 minutes. Rhodamine (0.5 mg ml-1 in IPA) was then spin coated at 4000 rpm for 40 s to serve as an interfacial layer between the PCBM and top silver electrode.43 Silver (100 nm) was thermally evaporated at a base pressure of 10-5 mbar. Devices were tested using a Xenon arc lamp with a filter under AM 1.5 conditions, which were calibrated using a standardized silicon solar cell from NREL. All perovskite solar cells were characterized in the same conditions with 0.5V s-1 scan rate with a hold time of 0.5 s in forward scan, sweeping from 0 to 1 V or -0.2 to 1.2 V (the reverse scan performed with no hold time and in the opposite directions) using an Agilent 4155C semiconductor parameter analyzer. Transient measurements were carried out using a OBB’s OL-4300 Nitrogen Laser coupled with a dye laser which produced a 532 nm wavelength pulse for less than 1 ns. The transient response was recorded with an oscilloscope. Transient photocurrent was measured using a low input oscilloscope impedance of 50 Ω to simulate short circuit conditions. The



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transient photovoltage was measured by illuminating the perovskite solar cell at approximately 1-2 suns and with a 1M Ω oscilloscope input impedance to simulate open circuit conditions. The charge carrier lifetime and collection time were calculated by fitting a monoexponetially decaying function to the transient photovoltage and photocurrent, respectively. X-ray diffraction (XRD) measurements were executed using a Rigaku SmartLab system (2.2 kW and Cu-Kα (1.54 Å) radiation). Crystallite size of the perovskite films was calculated in PDXL using the Scherrer equation ߬ =

௄ఒ ఉ ୡ୭ୱ ఏ

, where ߬ is the average size of crystallite domain,

‫ ܭ‬is a unitless shape factor, ߣ is the wavelength of the X-ray, ߚ is the width of a peak at half of its maximum intensity, and ߠ is the Bragg angle. 3. Results and Analysis Figure 1 shows a bar graph of the average sheet resistance versus additive wt.% for ~45 nm thick PEDOT:PSS films. The lowest sheet resistance of 297 Ω sq-1 was observed for the PEDOT:PSS film with DMSO:EG (1:1) 15 wt.% additive, which was much lower than the 500 kΩ sq-1 observed for pristine PEDOT:PSS films. The optimal wt.% of EG and DMSO for obtaining low sheet resistance were found to be 10 wt.% and 15 wt.%, respectively, which agreed with previous reports,19, 45 and were found to be repeatable. Some reports suggest that 5 wt.% is optimal for both EG and DMSO.20 This difference was attributed to the slow drying time of PEDOT:PSS films used in this study prior to annealing. Excess solvent additive may evaporate off the film prior to annealing, as shown in Figure S1. To further understand the mechanism behind the observed conductivity enhancement, the topography images of PEDOT:PSS films with additives was taken.


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800 Sheet Resistance (Ω sq-1)

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DMSO EG 1:1 DMSO:EG 2:1 DMSO:EG 1:2 DMSO:EG

700 600 500 400 300 200 100 0 5

Figure 1.

10 Additive wt.%

15

Sheet resistance of PEDOT:PSS films versus wt.% of various additives in the

PEDOT:PSS inks prior to spin coating. The sheet resistance of the pristine PEDOT:PSS film (0% additive) is 500 kΩ sq-1. Figures 2a-e show the topography images of PEDOT:PSS films with different concentrations of additives and additive mixtures. Figure 2b shows that with the optimal wt.% of EG, small spherical features appear. These diminutive spherical features were previously described as highly ordered grain structures composed of PEDOT and PSS.18, 46 Current sensing AFM (CSAFM) was conducted to investigate the conductivity of these spherical features shown in Figures 3a-b. CSAFM topography images typically suffer from low resolution due to the Cr/Pt coating on AFM tips, which increases the AFM cantilever tip radius. The CSAFM image in Figure 3b shows small granular features of higher current compared to the pristine film (Figure 3d). The line scan in Figure 3e shows that regions of higher current correspond to peaks within the topography. This result indicates that these small spherical grains (attributed to the peaks within the topography line scan) possess high conductive and supports the idea that they are composed of ordered PEDOT and PSS blends with a thinner PSS shell barrier.47 The CSAFM 9 ACS Paragon Plus Environment


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image (Figure 3d) of pristine PEDOT:PSS shows less current compared to the PEDOT:PSS film with a 10 wt.% EG additive. This result indicates the rolling hill morphology in pristine films (Figure 3c and Figure 2a) presents many insulating PSS barriers, which impede charges from hopping between PEDOT chains. a

c

b

e

d

Figure 2. Topography images of (a) pristine PEDOT:PSS, PEDOT:PSS with (b) 10 wt.% EG, (c) 15 wt.% DMSO, (d) 15 wt.% DMSO:EG (1:1), and (e) 15 wt.% DMSO:EG after being soaked in water:EG:ethanol (1:1:1).

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Figure 3.

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1

(a) Topography and (b) current-sensing AFM images of 10% EG additive

PEDOT:PSS with 0.1 V bias, (c) topography and (d) current-sensing AFM images of pristine films with 0.3 V bias, and (e) line scan from the topography images. Note: (a) and (b) line scan location is indicated by a white line.

Fibrous structures are observed with an optimal wt.% of DMSO (Figure 2c). This fibrous morphology has also been reported after PEDOT:PSS films are treated with strong acids.12, 14, 48 Figures 2d and e (15 wt.% DMSO:EG at the 1:1 ratio) show the most fibrous structures and correspond to the lowest sheet resistance produced (Figure 1). This outcome suggests that fibrous morphologies are more optimal than small spherical grain morphologies for charge



transport. CSAFM images of PEDOT:PSS films with 15 wt.% DMSO:EG (1:1) additive (Figures S3a and b) exhibited a constant 10 nA current reading under the same voltage bias used for Figure 3b, which also demonstrates their higher conductivity compared to films using only EG additives. The RMS roughness of the 15% DMSO:EG PEDOT:PSS film after soaking in water:EG:ethanol (1:1:1) (Figure 2e) was 2 nm and consistently maintained after depositing three additional layers (Figure S3c). The RMS value of the PEDOT:PSS film was comparable to an ITO RMS roughness of ~1 nm (Figure S4, supplementary information).

0.8

Abs. (a.u.)

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0 310 610 910 Wavelength (nm)

b 0% DMSO 5% DMSO 10% DMSO 15% DMSO

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0% EG 5% EG 10% EG 15% EG Before post treatment After post treatment 0.1

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0.4

0 310 610 910

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0 190

230 270 Wavelength (nm)

310

190

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310

Figure 4. Absorbance spectra of PEDOT:PSS with (a) an increasing wt.% of EG (solid line), before and after the post treatment of soaking PEDOT:PSS films with 15 wt.% DMSO:EG (1:1) in water:EG:ethanol (1:1:1) (dashed line), and (b) an increasing wt.% of DMSO. The inset shows the absorbance of the films in the range 310-1100 nm. Figure 4a shows the absorption spectra for PEDOT:PSS films (with inset showing the absorbance range from 310-1100 nm) having different concentrations of EG spin coated on a quartz substrate. Two distinct absoption peaks at 193 nm and 225 nm are attributed to PSS.47 The absorption spectrum shows a decrease in PSS absorption peaks with an increasing wt.% of EG 12 ACS Paragon Plus Environment

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before and after the post treatment without causing any significant change in the PEDOT absorption region (310-1100 nm). Because no PSS could have been removed from the PEDOT:PSS film during spin coating, the addition of EG causes the PSS to rearrange itself by disrupting the static charge interaction between the PEDOT and PSS chains.15 This disruption led to the formation of small spherical grains of PSS as observed in our topography images (Figures 2b-d) and less uniform PSS film coverage. The decrease in absorbance due to a nonuniform film coverage has been explained by Xia et al. using the Beer-Lambert law.19 The dashed lines in Figure 4a represent the absorption of PEDOT:PSS films with a 15 wt.% additive of DMSO:EG (1:1) before and after soaking in a water:EG:ethanol (1:1:1) blend. It was found that soaking the films in a water:EG:ethanol (1:1:1) blend resulted in the lowest sheet resistance compared to other blends (Figure S2 supplementary information). The absorption peak intensity of the PSS significantly dropped after soaking, which indicates that excess PSS was removed from the PEDOT:PSS film. For films with DMSO additives, no significant change was observed for the 225 nm absorption peak with increased wt.% of DMSO. Although the absorption peak at 193 nm exibited a small change in peak intensity, no discernable trend was evident. This suggests that DMSO does not significantly influence PSS in the same way as EG does.



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15% DMSO:EG (1:1) 0% DMSO 5% DMSO 10% DMSO 15% DMSO

0.6 0.4 0.2 0 1200 1250 1300 1350 1400 1450 Wavenumber (cm-1)

Figure 5. Raman spectroscopy of PEDOT:PSS films with various wt.% of (a) EG and (b) DMSO additives.

Figure 5 shows the Raman spectra of PEDOT:PSS films with varying concentrations of EG and DMSO. The peaks occurring between 1350 cm-1 and 1425 cm-1 are attributed to Cα=Cβ symmetric stretching in the PEDOT.49 An increasing redshift was observed with increasing wt.% of DMSO, whereas no peak shift was observed with increasing wt.% of EG. This result implies that the addition of EG does not modify the structure of the PEDOT species and that its primary role is to rearrange the PSS. The redshift was observed for all films with DMSO additives and appeared highest for films with 15 wt.% DMSO:EG (1:1) additive (see Figure S5 for other DMSO:EG blend ratios). The redshift indicates the presence of PEDOT in its quinoid structure 49-50

. The more stable benzoid structure has a single bond between monomers and exhibits a

coiled morphology, making charge transport more difficult due to a suppressed conductivity along the polymer backbone (intra-chain conductivity) and a dominant reliance on hopping between polymer chains (inter-chain conductivity).51-52 The quinoid type structure tends to form 14 ACS Paragon Plus Environment

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more linear chains due to the presence of double bonds between the monomers. The linear conjugated structure enables better charge transport because charges can move further distances along the polymer backbone before needing to jump to a neighboring polymer chain 12.

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PEDOT: PSS:

Figure 6. Schematic illustration of PEDOT:PSS film morphology and flow of electrons in (a) a pristine film, (b) a film with EG additive, (c) a film with DMSO additive, (d) a film with DMSO:EG additive before post treatment, and (e) a film with DMSO:EG additive after post treatment. Figure 6 depicts a schematic illustration of the transformation in PEDOT:PSS morphology for (a) a pristine film, (b) a film with EG additive, (c) a film with DMSO additive, (d) a film with DMSO:EG additive before post treatment, and (e) a film with DMSO:EG additive after post treatement. The attraction between the positively charged PEDOT and the negatively 15 ACS Paragon Plus Environment


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charged PSS in the pristine films led to coiled PEDOT and PSS chains surrounded by a PSS shell shown in Figure 6a. The PSS species which surrounded the coiled PEDOT polymer created a barrier that blocked charge transport within the PEDOT network inside the film and correlates to the rolling hill morphology within Figure 2a. The EG additive may have disrupted the static charge interaction between the PEDOT and PSS, enabling the film to reorganize itself into more ordered spherical grains (Figure 6b) as shown in Figure2b and Figure 3a. The PEDOT remained coiled with the PSS in its benzoid structure, but charge transport became more efficient due to an increase in continuous pathways and smaller PSS barriers for charge carriers as shown by the improved current flow in Figures 3b and 3d. By using DMSO as an additive, PEDOT polymer chains are transformed from a coil-like benzoid structure to a more linear or extended quinoid structure (Figure 6c). The linear structure of the PEDOT correlates to the fibrous topography in Figure 2c. This result allows for better intra-chain conductivity, but these films still have PSS insulating barriers, which resulted in higher sheet resistances than films with optimal EG additives, as seen in Figure 1. By incorporating DMSO:EG (1:1), the DMSO helped to form fibrous structures with more linear PEDOT chains, which correlates to the fibrous morphology in Figure 2d, while EG facilitated the reduction of PSS barriers within the film (Figure 6d) and resulted in lower sheet resistance. The post treatment in the water:EG:ethanol (1:1:1) blend washed away excess PSS species from the film as shown in Figure 6e to further reduce PSS insulating barriers and correlates to Figure 2e.


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(nm)

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Figure 7. Contact angle a-c, phase images d-f, and topography g-h for PEDOT:PSS films with 15 wt.% DMSO:EG (a, d, and g), after post treatment (b, e, and h), and after plasma treatment (c, f, and i).

a

b

Ave SP -0.545 V



d

c

Ave SP -0.705 V e 40 35 30 25 20 15 10 5 0

f 0.8

Before plasma

Absorbance (a.u.)

After plasma

ρ (V-1)

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Before plasma After plasma

0.6 0.4 0.2 0

-0.8

-0.7

-0.6 SP (V)

-0.5

-0.4

190 210 230 250 270 290 310 Wavelength (nm)

Figure 8. (a)(c) Topography, (b)(d) surface potential KPFM images of PEDOT:PSS films with 15 wt.% DMSO:EG additive and solvent post treatment (a)(b) before plasma treatment and (c)(d) after plasma treatment, (e) surface potential distribution from KPFM measurement, and (f) Absorption spectrum before and after plasma treatment. Insert in Figure 8(f) shows an extended absorption region from 310-1100nm. The final PEDOT:PSS TE was fabricated by depositing four layers of PEDOT:PSS (with optimized solvent additives and each layer subject to optimized solvent post treatments) on top of one another. The plasma treatment facilitated easy deposition of multiple PEDOT:PSS layers and additionally improved the perovskite layer formation due to better wetting as seen in Figure 7a-c. Lower degrees in the phase images observed in Figure 7d-f are found to correspond with lower contact angles. We propose that the lower degrees in the phase image are correlated to a 18 ACS Paragon Plus Environment

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more hydrophilic PSS rich surface and the higher degrees to a more hydrophobic PEDOT rich surface. Films with a 15 wt.% DMSO:EG solvent additive exhibit an average phase value of 51.5° and a contact angle of 23°. After the post treatment, the average phase value and contact angle increased to -50.4° and 32°, respectively, as more hydrophilic PSS was washed away, exposing more hydrophobic PEDOT chains on the film surface. This outcome coincided with the RMS roughness of the AFM films, which increased from 1.98 nm to 2.23 nm after the solvent post treatment and can be attributed to the removal of excess PSS species that smoothed the surface of the plentiful PEDOT under layer. After the plasma treatment, a PSS rich surface is uncovered, which again reduced the phase value and contact angle to -51.9° and 14°, respectively. After the plasma treatment, the surface morphology dramatically changed and resulted in a high surface roughness of 3.26 nm. These changes were likely the combination of the plasma etching the film and gas ions disrupting the static interaction between the PEDOT and PSS species to form small spherical structures. KPFM images from Figure 8b and d show the average surface potential (SP) dropped from -0.545 V to -0.705 V after exposure to Oxygen plasma, which suggests the PEDOT:PSS surface was oxidized.53 A shorter and wider surface potential distribution was observed after the plasma treatment in Figure 8e; attributed to an increased number of surface defects and consequently surface states. The increased number of surface states may be expected as the plasma fragments the polymer backbones.54 Unlike PEDOT, PSS has protective aromatic functional groups that provide an additional barrier against the plasma from breaking apart carbon backbone bonds. The absorption peaks (at 193 and 225 nm) attributed to PSS in Figure 8f are not significantly decreased after subject to oxygen plasma. Covertly, a significant decrease in the absorption region from 760-1100 nm attributed to PEDOT indicate it is much more 19 ACS Paragon Plus Environment


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vulnerable to plasma etching. The improved wetting after the plasma treatment was attributed to a combination of surface oxidation, etching of the PEDOT:PSS top layer (which likely increased the surface free energy by breaking apart the side chains backbones of the polymers and leaving unpaired bonds), and creating a more PSS rich hydrophilic layer.54-56 Without the plasma treatment, depositing multiple PEDOT:PSS layers proved to be extremely difficult due to poor wetting. First attempts to solve this problem involved blending the optimized PEDOT:PSS ink with IPA in a 2:1 volume ratio. These electrodes resulted in a decent average optical transmittance of 73% over the perovskite absorption region (350-850 nm) and a high average sheet resistance of 85 Ω sq-1. The best electrode exhibited 78% transmittance and 77 Ω sq-1 sheet resistance, as seen in Figure 9a. In addition, coating a uniform high quality perovskite active layer was very difficult in spite of using a PEDOT:PSS (AI 4083) hole transport layer, and typically resulted in little photovoltaic performance. A light plasma treatment on the PEDOT:PSS surface significantly improved the surface wetting without the aid of IPA blending and resulted in PEDOT:PSS TE with an average transmittance of 73% and an average sheet resistance of 36 Ω sq-1. The best plasma cleaned four layer PEDOT:PSS electrode exhibited an astonishingly low sheet resistance of 15 Ω sq-1 and moderate average transmittance of 76%. This low sheet resistance, to the best of the authors knowledge, translates to the highest ever reported conductivity of 5012 S cm-1 for PEDOT:PSS TE. The reduction in sheet resistance can be explained by improved interconnection between layers without any adverse effects from high dilution in IPA. A progression of the four layer deposition for PEDOT:PSS films with an IPA blend or plasma treatment is shown in Figure S6. With the added IPA, the topography changes considerably from the fibrous structures in Figure 2e to circular structures in Figure S6a, c, e, and g. The plasma treated films exhibit some pin holes attributed to plasma etching, 20 ACS Paragon Plus Environment

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but the final layer (Figure S6h) still retains the more conductive fibrous morphology. Without the use of strong acid treatments described in previous reports,12-14 these electrodes exhibited excellent optoelectronic properties with very comparable conductivities. For comparison, the glass\ITO substrates used in this study exhibited an average transmittance of 84% and a sheet resistance of 16 Ω sq-1 (conductivity of 6250 S cm-1). 86 84 82 80 78 76 74 72 70

ITO Plasma treated Non-plasma treated

b

1000 0 ITO

2000 1000 0

0 10 20 30 40 50 60 70 80 90 100 Sheet Resistance (Ω sq-1) c

PEDOT:PSS

2000 Intensity (a.u.)

a Transmittance (%)

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5

15

25 35 2θ (°)

45

55

d

Figure 9. (a) Average transmittance over the perovskite absorption region of 350-850 nm vs. sheet resistances for ITO and PEDOT:PSS TE, (b) the XRD spectrum of perovskite on an ITO electrode and a 4 layer plasma treated PEDOT:PSS TE, and a topography image of perovskite spin coated on (c) a plasma treated PEDOT:PSS TE and (d) an ITO electrode. 21 ACS Paragon Plus Environment


a

Current Density (mA cm-2)

b

0 PEDOT:PSS forward -4

PEDOT:PSS reverse

-8

ITO forward ITO reverse

-12 -16 -20 -24 0

105 90

d 27 24 21 18 15 12 9 6 3 0

PEDOT:PSS EQE ITO EQE PEDOT:PSS Jsc ITO Jsc

75 60 45 30 15 0 350 450 550 650 750 850 Wavelength (nm)

Current density (mA cm-2)

120

Integrated Current (mA cm-2)

c

EQE (%)

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0.2

1.0E+3

0.4 0.6 Voltage (V)

0.8

1

PEDOT:PSS ITO

1.0E+2 1.0E+1 1.0E+0 1.0E-1 1.0E-2 1.0E-3

PEDOT:PSS ITO

1.0E-4 0

Rsh (kΩ) 0.1385 18.1

0.2 0.4 0.6 0.8 Voltage (V)

Rs (Ω) 0.99 5.7

1

1.2

Figure 10. (a) Schematic showing the planar perovskite solar cell device structure, (b) J-V characteristics of perovskite solar cells using the PEDOT:PSS or ITO electrode, (c) EQE (solid lines) and integrated current (dashed lines) for the perovskite solar cell with the PEDOT:PSS or ITO transparent electrodes, and (d) the log plot of dark current for perovskite solar cells using the PEDOT:PSS or ITO electrodes with an inset of the shunt (Rsh) and series (Rs) resistance values. 22 ACS Paragon Plus Environment

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The XRD spectra of the perovskite active layer (Figure 9b) showed characteristic peaks at 14° and 28.5°, which confirmed the formation of a crystalline perovskite phase on the plasma treated PEDOT:PSS and ITO electrode.57 The perovskite crystallite sizes (calculated from the Scherr equation) on the PEDOT:PSS and ITO electrode were found to be 231 Å and 242 Å, respectively, which confirms that the perovskite crystallization is not significantly affected by plasma cleaning the PEDOT:PSS electrode. Topography images (Figures 9c and d) show the perovskite morphology on the PEDOT:PSS and ITO electrode. The average perovskite grain sizes on PEDOT:PSS and ITO electrodes from the AFM images were found to be ~223 nm and ~232 nm, respectively, and correlated to the crystallite size trend calculated from the XRD spectra. The planar perovskite solar cell device structure adopted to test the transparent electrode performance is shown in Figure 10a and the resulting device parameters for the best devices are shown in Table S1. The average performance of five perovskite solar cells with the PEDOT:PSS electrode exhibited a Jsc of 17.7 mA cm-2, a Voc of 0.82 V, FF of 0.57, and PCE of 8.2% for reverse scan and a Jsc of 17.5 mA cm-2, a Voc of 0.81 V, FF of 0.55, and PCE of 7.8% for forward. The Average performance of five perovskite solar cells with ITO electrode was Jsc of 19.0 mA cm-2, a Voc of 0.89 V, FF of 0.69, and PCE of 11.7% for reverse scan and a Jsc of 19.0 mA cm-2, a Voc of 0.87 V, FF of 0.65, and PCE of 10.7% for forward. The best PCE for the PEDOT:PSS and ITO TEs were found to be very comparable at 10.5% and 12.4%, respectively. The hysteresis for PEDOT:PSS devices was smaller compared to the ITO device (Figure 10b), which suggests that the PEDOT:PSS selective TE is more efficient at charge extraction than the ITO TE.58 Previous reports with similar perovskite device architecture also achieved the highest efficiency for an ITO control device of 12%.43 The Jsc for both PEDOT:PSS and ITO devices 23 ACS Paragon Plus Environment


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were found to be 19.2 mA cm-2 and 19.8 mA cm-2 for forward scan conditions, respectively, and 19.6 mA cm-2 for both devices under reverse scan. The Jsc for both devices were found to be very similar despite the ITO TE having a greater optical transmittance. From the EQE measurement shown in Figure 10c, the Jsc was calculated to be 18.3 mA cm-2 for the PEDOT:PSS device and 18.8 mA cm-2 for the ITO device, which agreed with the J-V curves. The main reason for the lower performance of the PEDOT:PSS electrode was due a low Voc of 0.81 V compared to the 0.95 V of the ITO device, which could be attributed to the smaller crystallite and grain size observed in the XRD spectrum and AFM topography (Figures 9b-d).59-60 The series and shunt resistances were calculated from the dark J-V curves shown in Figure 10d to better understand the diode properties of each device. The PEDOT:PSS device was found to have series and shunt resistances of 0.99 Ω and 138.5 Ω and for the ITO device 5.7 Ω and 18.1 kΩ, respectively. The low shunt resistance for the PEDOT:PSS devices helps explain the low Voc due to increased trap states and defects within the perovskite layer.60-62 Devices without the plasma treatment exhibited little photovoltaic performance due to a poor or patchy formation of the perovskite layer (visible to the naked eye) and exhibited extremely low shunt resistances around 12 Ohms. Thus, the plasma treatment improved the perovskite film uniformity and future optimizing to the plasma treatment time may increase the low shunt resistance and decrease the leakage current. The higher series resistance in the ITO device supports the idea that the PEDOT:PSS TE was more efficient at charge extraction. Despite having a lower quality perovskite layer, the PEDOT:PSS device notably produced a much lower series resistance and hysteresis while exhibiting similar Jsc and fill factor compared to the ITO device.


a

1.2 1.0

ITO (0.99 µs) PEDOT:PSS (0.90 µs)

0.8 0.6 0.4 0.2 0.0

1.1

b Photovoltage Decay (V)

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Normalized Photocurrent (mA cm-2)

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1.0 0.9 0.8 0.7 ITO (1.7 µs) PEDOT:PSS (0.39 µs)

0.6 0.5

-0.2 -2.0

0.0

2.0 Time (µs)

4.0

-1.0

0.0

1.0 2.0 Time (µs)

3.0

4.0

Figure 11. (a) Transient photocurrent (TPC) and (b) transient photovoltage plots of mixed halide perovskite solar cells with a PEDOT:PSS and an ITO electrode. Figures 11a and b show transient photocurrent (TPC) and photovoltage measurements of perovskite solar cells using ITO and PEDOT:PSS as transparent electrodes. The short-lived transient photocurrent is generated by a nanosecond pulse of a dye laser incident on solar cells and can provide insightful information on charge transport within the device. Charges were found to have a faster decay time of 0.9 µs for PEDOT:PSS TEs compared to 0.99 µs decay time for the ITO device, which validated the supposition that the PEDOT:PSS TE has better charge extraction. This trend also helps explain the Jsc data observed in Figures 10b-c because, despite a higher transmittance, the ITO device does not transport charges as efficiently as the PEDOT:PSS device. Thus, both devices produce a similar Jsc value despite the respective TEs having different transmittances. This finding is significant as future enhancements in the transmittance of PEDOT:PSS TEs could result in superior Jsc performance over ITO TEs for perovskite devices. Transient photovoltage provides information about the electron lifetime within the perovskite



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layer.63 The ITO

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device demonstrated a longer decay time of 1.7 µs compared to the

PEDOT:PSS device (0.39 µs) and the measured Voc values were consistent with those found in Figure 10b. The longer decay times for the ITO devices indicate a superior perovskite layer formation over those formed over the PEDOT:PSS TE, which is also consistent with the calculated shunt resistances, crystallite size from the XRD spectra (Figure 10b), and grain size from the AFM topography (Figure 10d). The perovskite layer on top of the PEDOT:PSS TE could be improved by further optimizing the oxygen plasma treatment time for the hole transport layer or by incorporating a thin interfacial surfactant layer.64 Improvements in the perovskite layer formation over the PEDOT:PSS TE would raise the Voc and Rsh values and create a clear path to equal or better performance than ITO devices. 4. Conclusions In summary, an environmentally friendly acid free method for enhancing the conductivity of PEDOT:PSS films was demonstrated. By using a DMSO:EG (1:1) additive blend, we proposed a dual mechanism for conductivity enhancement. The EG serves to create an ordered grain structure and reduce insulating PSS barriers within the PEDOT:PSS film that allows for better charge transport. The DMSO induces the PEDOT polymer chain to transform from the coiled benzoid structure into the more conductive and linear quinoid structure. Soaking the PEDOT:PSS electrodes in the water:EG:ethanol blend removed excess non-conductive PSS species from the film. A light plasma cleaning resulted in an improved connection between multiple PEDOT:PSS layers. The best results with this acid free approach achieved a low sheet resistance of 15 Ω sq-1 and an average transmittance of 76% in the wavelength range of 350-850 nm for a 4 layer PEDOT:PSS TE. The PEDOT:PSS TE was demonstrated in a perovskite solar cell and obtained a PCE of 10.5%, which, to the best of the authors’ knowledge, is the highest 26 ACS Paragon Plus Environment

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reported PCE for an acid free PEDOT:PSS electrode in perovskite solar cells. The PEDOT:PSS TE was shown to have superior charge carrier extraction compared to ITO. With further improvements in perovskite layer formation and PEDOT:PSS TE transmittance, higher PCEs than ITO devices could be achieved. Our acid free approach could drastically simplify safety precautions for a fully printable device and create a larger variety of flexible substrates from which to choose. Associated Content Supporting Information. Time laps optical photographs of the PEDOT:PSS films slow drying process, sheet resistance results for optimizing solvent post treatment, AFM topography and CSAFM images of PEDOT:PSS (with optimized solvent additives) single layer film and four layer electrode (with optimized solvent post treatment), AFM topography of an ITO film, Raman spectroscopy of PEDOT:PSS films with various wt. % of solvent blend additives, and a table of the parameters for the J-V curves in Figure 10b. Acknowledgments This work has been supported by the NSF IGERT grant number program ‘Nanostructured Solar Cells: Materials, Processes, and Devices’ (DGE-0903685). This work also benefitted from the NSF MRI grant program (1428992) and U.S. - Egypt Science and Technology (S&T) Joint Fund. The authors would like to thank Amanda Vaagensmith for grammar edits.

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Graphical abstract

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Environmentally Friendly Plasma-Treated PEDOT:PSS as Electrodes

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