Treating the PEDOT:PSS surface with hydroquinone enhances the

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Treating the PEDOT:PSS surface with hydroquinone enhances the performance of polymer solar cells Sujung Park, Myung Joo Cha, Jung Hwa Seo, Jinhee Heo, Dongchan Lim, and Shinuk Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15551 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Treating the PEDOT:PSS surface with hydroquinone enhances the performance of polymer solar cells Sujung Park, Myung Joo Cha, Jung Hwa Seo, Jinhee Heo, Dong Chan Lim and Shinuk Cho*

Sujung Park, Prof. Shinuk Cho Department of Physics and EHSRC, University of Ulsan, Ulsan 44610, Republic of Korea *E-mail: [email protected]

Myung Joo Cha, Prof. Jung Hwa Seo Department of Materials Physics, Dong-A University, Busan 49315, Republic of Korea

Dr. Jinhee Heo, Dr. Dong Chan Lim

ACS Paragon Plus Environment

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Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea.

Keywords: organic photovoltaic, surface modification, hole transport layer, PEDOT:PSS, hydroquinone

ABSTRACT

The

introduction

of

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

(PEDOT:PSS) as a standard hole transport layer greatly increased the efficiency of early organic solar cells. However, because PEDOT:PSS has a metallic property, it can still form a barrier by means of metal–semiconductor contact at its interface with the photoactive layer. In this study, we modified the PEDOT:PSS surface with hydroquinone (HQ) to remove that barrier. HQ treatment of the PEDOT:PSS surface lowered the hole


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transport barrier at the interface between the PEDOT:PSS and the active layer. In addition, because of the secondary doping effect of HQ, the sheet resistance of the PEDOT:PSS surface decreased by almost two orders of magnitude. As a result, the device fabricated with the HQ-modified PEDOT:PSS showed a 28% increase in efficiency compared to the device without HQ treatment. Modifying the PEDOT:PSS surface with HQ solution is an easy way to effectively boost the performance of polymer solar cells.


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INTRODUCTION

Polymer bulk heterojunction (BHJ) solar cells have attracted much attention for their potential applications in large-area flexible devices because they are lightweight, low cost, and simple to fabricate.1-5 Since the first report on polymer BHJ solar cells in 1995,6 substantial progress has been made, raising device efficiencies by more than 13% through improvements in the design and synthesis of active materials,7-10 the optimization of nanoscale morphology,11-13 and interface engineering.14-18 Of course, the most important component in high efficiency solar cells is high performing active materials, but developing new and better active materials requires a lot of time and effort. Therefore, a cost-effective approach to improving the performance of existing materials is maximizing charge collection efficiency through interface engineering. The insertion of an electron or hole transport layer is part of interface engineering in a broad sense. Numerous organic and inorganic materials that could function as hole and electron extraction layers have been developed and studied to enhance the performance of BHJ solar cells.19-23 The most widely used inorganic buffer layer materials are probably metal oxides. A variety of transition metal oxides, such as ZnO, TiO2, NiO and MoO3 have been developed as either hole or electron transport materials for fabricating high performance and cost-effective BHJ solar cells due to their advantages of environmental stability, high transparency and high carrier mobility.24-26 Organic-based buffer materials are much more diverse. Dozens of novel synthetic materials which can be classified in conjugated polyelectrolyte,27,28 small molecule transport materials,29,30 polymer buffer materials,31,32 etc., have been developed and successfully applied as a buffer layer in BHJ solar cells. Among the organic-based hole transport materials, however, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) remains the


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most widely used material even though it is the oldest. In early studies of organic BHJ solar cells, PEDOT:PSS offered superior performance in many ways. First, PEDOT:PSS showed excellent hole extraction because of a well-matched work function (~5.1 eV) for the highest occupied molecular orbital (HOMO) level of the donor polymer.33-35 Second, PEDOT:PSS delivered better adhesion between the inorganic ITO electrode and the organic active layer.36, 37 In addition, PEDOT:PSS induced uniform deposition of the active layer by smoothing the ITO surface.38 Now, various hole transport materials have been newly developed and used to replace PEDOT:PSS in BHJ solar cells. Nonetheless, PEDOT:PSS is still the best hole transport material in terms of process simplicity and performance consistency. In fact, PEDOT:PSS is the standard hole transport material; the performance of each newly developed hole transport material is judged by whether it is better or worse than PEDOT:PSS. Until now, interface engineering for organic BHJ solar cells has focused mainly on problems that occur at the interface between the organic active layer and the charge transport layer, which uses a metal-oxide such as ZnO, TiO2, or MoO3. In solar cells with a metal-oxide charge transport layer, many surface treatment methods, such as polar solvent treatment,39, 40 surface doping,41-44 or the insertion of a thin conjugated polyelectrolyte layer,45, 46 are applied to improve the interfacial properties between the inorganic metal-oxide layer and the organic BHJ layer. However, the literature contains only a few reports of surface treatments for the PEDOT:PSS hole transport layer. Recently, surface modifications of PEDOT:PSS were reported by adding a p-type dopant47 or treating it with either an alcoholic polar solvent48, 49 or a high boiling point solvent.50 However, those studies focused on increasing the conductivity of PEDOT:PSS. Furthermore, most studies about modifying the PEDOT:PSS surface have considered only inverted planar perovskite solar cells, which have interfacial problems between the inorganic


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metal-oxide transport layer and organic active layer similar to those in organic BHJ solar cells. Although several investigations have considered the interfacial problem between PEDOT:PSS and the active layer in organic BHJ solar cells,51-53 it is true that the interfacial problem between PEDOT:PSS and the active layer was much less considered compared to the interfacial problem between metal oxides and the active layer, probably, because PEDOT:PSS was itself understood to be a modifying material for the ITO surface. Conventionally, single-layer BHJ cells, which have a cathode/active/anode structure, were treated as metal-insulator-metal diodes. However, a careful investigation of dark current found that the fundamental unit is a semiconductor–metal Schottky junction.54-56 Because PEDOT:PSS also has a metallic property (though with relatively low conductivity),57,58 it still carries the possibility of forming a Schottky-type contact barrier at the interface. Therefore, in this work, we have modified the PEDOT:PSS surface with hydroquinone (HQ) by spin-coating to reduce the Schottky-type contact barrier that can interfere with hole transport. The conjugated polymer is an electron-rich system, and ultraviolet photoelectron spectroscopy (UPS) measurements showed that PEDOT:PSS and the conjugated polymer form an n-type Schottky contact. Surface treatment of PEDOT:PSS with HQ changed the energy level bending properties in a direction more favorable to hole extraction. In addition, HQ treatment significantly reduced surface resistivity. Those HQ-derived surface property improvements nearly doubled the density of the extracted holes. Consequently, a power conversion efficiency (PCE) of up to 10.18% was achieved from a conventionally structured BHJ solar cell using poly[[4,8-bis[5(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid


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methyl ester (PC71BM) as the active layer, a ~28% increase in efficiency compared to solar cells without HQ treatment. RESULTS AND DISCUSSION

Figure 1a shows a schematic of the conventional BHJ solar cell architecture and the chemical structures of the materials used in this study (PTB7-Th, PC71BM, PEDOT:PSS, and HQ). HQ treatment was simply done by spin-coating HQ solution diluted with isopropyl alcohol (IPA) on the PEDOT:PSS layer. HQ is weak polar solvent with a dipole moment of 1.4 Debye. Its effects are similar to those of alcoholic solvents such as methanol and ethanol, which are often used for surface treatment,48,49,59 because they all have two hydroxyl groups (-OH). Because HQ has a high boiling point (287 °C), it combines the effects of an alcoholic polar solvent and a high boiling point solvent. Furthermore, HQ can lose an H+ from one of the hydroxyls to form a monophenolate ion or lose an H+ from both to form a diphenolate ion. Therefore, we expected HQ to provide additional protonation doping on the PEDOT:PSS surface. Figure 1b shows the energy level diagrams of each material used in our PTB7Th:PC71BM solar cells. The work-function of the HQ-treated PEDOT:PSS was


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determined by UPS measurement (Figure S1). Other energy level values, such as the HOMO/LUMO level of active materials and conduction band minimum level of the ZnO nanoparticles, were obtained from the literature.

Figure 2a shows the current density (J)-voltage (V) characteristic curves of conventionally structured PTB7-Th:PC71BM solar cells. Details of the performance of these solar cells are listed in Table 1, and external quantum efficiency (EQE) spectra are presented in Figure S2 in the Supporting Information. To find the proper dilution ratio for the HQ solution, we varied the ratio of HQ to IPA from 0.2 wt% to 2.0 wt%. Solar cells with a pristine PEDOT:PSS layer (without HQ treatment) yielded a PCE of 7.98% with a Jsc of 15.6 mA/cm2, a Voc of 0.782 V, and a fill factor (FF) of 0.654. The performance of solar cells with an HQ-modified PEDOT:PSS layer gradually increased along with the HQ concentration up to 1.0 wt%. The most efficient solar cell had a PCE of 10.2%, with a Jsc of 18.9 mA/cm2, a Voc of 0.793 V, and an FF of 0.680. However, when the HQ concentration exceeded 1.0 wt%, cell efficiency decreased rapidly. Since we have used HQ solution diluted with alcohol solvent such as methanol or ethanol, it


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has possibility that this performance enhancement may originated by alcohol solvent because alcohol solvent also can modify the PEDOT:PSS surface a little. However, the performance increase was not observed when the PEDOT:PSS surface was treated with a simple alcohol solvent. Thus, the performance enhancement was obviously an effect of the HQ (Figure S3). In addition, the performance enhancements from the HQ treatment were universal (clearly observable in other polymer BHJ systems). Figure S4a and S4b show the J-V characteristic results from poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), poly[[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7) and PC71BM, respectively. Note that the optimal HQ concentration varied a bit depending on the BHJ active material. The device with the PTB7:PC71BM mixture showed the highest efficiency at the same optimal dilution ratio of HQ and IPA as the PTB7-Th:PC71BM device, whereas the P3HT:PC61BM device showed the best efficiency at 0.8 wt%.


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As clearly shown in the J-V results, the PCE enhancement in devices with HQ treatment stemmed from improvements in Jsc. In an organic BHJ solar cell, factors that significantly influence Jsc are improvements in the charge extraction efficiency through energy level changes or morphology changes at the interface. First, to investigate the influence of HQ treatment on energy level changes in both the PEDOT:PSS and PTB7Th donor polymer, we performed UPS measurements, as shown in Figure 3a. To observe the band bending degree of the PTB7-Th polymer caused by the change in the PEDOT:PSS surface properties, we measured the UPS with various thicknesses of PTB7-Th polymer, which we controlled by changing the solution concentration from 0.003 wt% to 0.3 wt%. When PTB7-Th was deposited on pristine PEDOT:PSS, the secondary edges shifted toward higher binding energies as the concentration of the PTB7-Th solution increased. The total vacuum level (VL) shift at the saturated coverage was 0.25 eV for PTB7-Th deposited on pristine PEDOT:PSS and 0.35 eV for PTB7-Th deposited on HQ-treated PEDOT:PSS. The right side of the figure shows the evolution of the HOMO onsets for PTB7-Th. Figures 3c and 3d show the C 1s and S 2p emission lines from the PTB7-Th layers on PEDOT:PSS. These XPS analyses show the energies


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of the core levels, allowing us to probe the band bending. As the concentration of PTB7Th solution increased to 0.3 wt%, the S 2p emissions increased in intensity, and the PSS emissions became attenuated because of the thicker PTB7-Th layer. Using that S 2p and C 1s peak shift between the thin and thick PTB7-Th layers, we determined the band bending energies: 0.28 eV for PTB7-Th on pristine PEDOT:PSS and 0.32 eV for the PTB7-Th on PEDOT:PSS with HQ treatment. Figures 3e and 3f show energy level diagrams obtained by summarizing the UPS and XPS results. The hole injection barrier (φh) was estimated using the energy difference between the EF and HOMO levels extracted from the UPS results. Although the band bending degree of PTB7-Th on the HQ-treated PEDOT:PSS was slightly larger than that of PTB7-Th on pristine PEDOT:PSS, the φh of the PTB7-Th on the HQ-treated PEDOT:PSS was smaller than on pristine PEDOT:PSS. Thus, in terms of hole transport, HQ treatment improved the interfacial contact barrier and facilitated hole transfer.

The effect of the energy level change from HQ treatment was consistently confirmed in several experiments, as shown in Figure 4. After HQ treatment, although the hole


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extraction barrier was reduced, the electron injection barrier became larger. Normally, electrons injected from the anode electrode generate undesirable leakage current. Within the operation range between 0 V and Voc, the flow of leakage current is opposite to the photocurrent, thereby reducing the device efficiency. Therefore, we expected that the enlarged electron injection barrier created by the HQ treatment would reduce the leakage current, and indeed we found significantly reduced leakage current in the device with HQ-modified PEDOT:PSS, as shown in the J-V characteristics measured under dark conditions (Figure 4a). The effect of reducing the interfacial contact barrier by HQ treatment was confirmed by impedance spectroscopy (IS), which is a useful method for analyzing the interfacial transport and recombination properties of solar cells. Figure 4b shows the Nyquist plots of the IS results measured under illumination at

Voc. Devices both with and without HQ treatment appear on one semicircle without a transmission line (TL). The absence of the TL indicates that solar cells with and without HQ treatment can be interpreted using the Gerischer impedance model.60, 61 Because neither transport resistance nor recombination resistance can be unambiguously determined in the Gerischer impedance model, only relative comparisons are possible,


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and the combined effective resistance is called Gerischer resistance. Because all device fabrication parameters were identical for both solar cells (except for the HQ treatment of the PEDOT:PSS surface), the difference in the IS spectra likely originated from that interface modification. The real impedance decreased significantly in the device with the HQ-modified PEDOT:PSS, which indicates that the surface modification improved the interfacial contact. The enhanced charge extraction (CE) property from the HQ treatment was confirmed by measurement, as shown in Figure 4c. Using the CE measurement, we calculated the charge extraction density directly, as shown in Figure 4d. Direct comparison between the devices with and without HQ treatment indicated that the device with HQ treatment had almost double the charge extraction density of the untreated device.

One more thing to note in the energy diagrams obtained through UPS measurements (Figures 3e and 3f) is the VL shift. When an organic layer contacts a metallic layer, the organic layer can be affected by the potential of the surface dipole. In general, VL shifts can yield the magnitude and direction of the interfacial dipole that originates from the


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polarization of the charge carrier density at the metallic surface. Therefore, the larger VL shift indicates that the PEDOT:PSS with HQ treatment has more free charge carriers than the pristine PEDOT:PSS, which indicates the possibility that the HQ allows secondary doping of the PEDOT:PSS surface. The conjugated PEDOT is positively doped by the sulfonate anionic groups of the PSS.62 Because the PEDOT:PSS is not fully doped, it can be doped to a higher level by an additional protonating source. In fact, many papers have reported on secondary doping of PEDOT:PSS.63-65 To investigate the possibility that the HQ treatment was responsible for secondary doping, we measured the conductivity sheet resistance of the PEDOT surface (Table 2). Pristine PEDOT:PSS had a conductivity of 0.18 (±6.35×10-3) Ω-1cm-1 and sheet resistance of 1,340±9.87 kΩ/□. The PEDOT:PSS with HQ treatment, however, showed a significantly reduced sheet resistance of 59.1±0.92 kΩ/□, almost two orders of magnitude lower. Such reduction of surface resistance also can be originated by the reduction of PSS.66 However, in our case, no PSS reduction was observed in XPS study (see Figure S9). Furthermore, if there was a reduction of PSS by HQ treatment, there would have been a significant morphology change. However, we couldn’t get any significant change of surface morphology of


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PEDOT:PSS by HQ treatment. Based on these facts, we concluded that additional doping by hydroxyl group of HQ would be main mechanism rather than PSS reduction.

We also found evidence of secondary doping in our Fourier-transform infrared spectroscopy (FTIR) measurements. Figure 5a shows the mid-IR absorption spectra obtained from the pristine PEDOT:PSS and HQ-modified PEDOT:PSS. Because the PEDOT:PSS was dispersed in water, we used ZnSe as a substrate instead of KBr, which has a weak resistance in water. Direct comparison between the absorption peaks of pristine PEDOT:PSS and HQ-modified PEDOT:PSS indicates that the intensity of all the absorption peaks in the HQ-modified PEDOT:PSS increased slightly. A conducting polymer in a doped state exhibits very strong doping-induced infrared bands, so called infrared active vibration (IRAV) modes, in the mid-IR region (typically between 400 cm-1 and 1500 cm-1, the fingerprint region). All the peaks observed between 500 cm-1 and 1500 cm-1 in both the pristine PEDOT:PSS and HQ-modified PEDOT:PSS are IRAV modes. Normally, the intensity of the IRAV modes increases slightly with the doping


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level.67 Therefore, slightly increased IRAV modes in the HQ-modified PEDOT:PSS clearly indicate that the PEDOT:PSS was secondarily doped by the HQ.

Lastly, to confirm whether the improvement of solar cell performance was caused by the morphology change at the interface, we took atomic force microscopy (AFM) measurements for the PEDOT:PSS layers with and without HQ treatment. Figure 5b shows the AFM image of the pristine PEDOT:PSS, which had a root mean square (RMS) roughness value of ~2.1 nm. Figure 5c shows the AFM image of the HQmodified PEDOT:PSS, and it had a RMS roughness value of ~1.8 nm. Although the RMS roughness of the HQ-modified PEDOT:PSS layer was slightly lower than that of the pristine PEDOT:PSS, the surface morphology did not change significantly. Therefore, we conclude that the improvement in the performance of the solar cell with the HQ-modified PEDOT:PSS was not due to a morphology change at the interface between the PEDOT:PSS and the active layer. In conducting AFM (C-AFM) measurements, which can probe local current levels, we found that the overall current level improved significantly in the HQ-modified PEDOT:PSS, as shown in Figures 5d


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and 5e. This C-AFM result is consistent with our previous results and confirms that the performance improvement in the solar cell with HQ-modified PEDOT:PSS resulted from enhancements to the charge transfer property between the PEDOT:PSS and the active layer.

CONCLUSION

In summary, we have enhanced the PCE of PTB7-Th:PC71BM BHJ solar cells by modifying the PEDOT:PSS surface with HQ. Although the band bending degree of PTB7-Th on the HQ-modified PEDOT:PSS was slightly larger than on pristine PEDOT:PSS, the HQ treatment lowered the hole transport barrier at the interface between the PEDOT:PSS and the active layer. In addition, because the HQ had a secondary doping effect, the sheet resistance of the PEDOT:PSS surface decreased by almost two orders of magnitude. IS studies clearly show reduced interfacial resistance after HQ treatment of the PEDOT:PSS surface. By lowering the hole transport barrier and reducing interfacial resistance, the HQ treatment nearly doubled the hole extraction


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density calculated from the CE measurement. An increase in the current level of the HQ-modified PEDOT surface was also clearly seen in our C-AFM results. Thus, the device fabricated with HQ-modified PEDOT:PSS showed a 28% increase in efficiency compared to the device without HQ treatment. The best solar cell with HQ-modified PEDOT:PSS exhibited a maximum PCE of 10.18%. Modifying the PEDOT:PSS surface using an HQ solution is thus an easy way to effectively boost the performance of polymer solar cells.

EXPERIMENTAL SECTION

Device fabrication. ITO substrates were cleaned with detergent and ultrasonicated in acetone and IPA for 15 min each. A hole transport layer of PEDOT:PSS (Clevios PH) was spin-coated at 4000 rpm for 40 s after UV-ozone surface treatment and annealed at 150 °C for 10 min. The thickness of the PEDOT:PSS was 40–50 nm. Solutions with between 0.2 wt% and 2 wt% of HQ in IPA were spin-coated on top of the PEDOT:PSS layer at 5000 rpm for 40 s and annealed at 100 °C for 10 min. The substrates were then transferred to a N2-ﬁlled glovebox. The active layer solution was made with a 1:1.5 ratio


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of PTB7-Th and PC71BM in chlorobenzene with 3% DIO as a processing additive to get a better phase-separated morphology. The prepared solution was stirred overnight. The active layer was deposited on top of the PEDOT:PSS layer by spin-coating at 1000 rpm for 60 s and then annealed at 80 °C for 10 min. A ZnO nanoparticle solution (2.5 wt% in IPA) diluted to an IPA ratio of 1:10 was spin-coated at 3000 rpm for 30 s on top of the PTB7-Th:PC71BM layer as an electron transport layer, and it was then annealed at 80 °C for 10 min. Finally, 100 nm of aluminum was deposited by thermal evaporation at a vacuum condition of 2×10-6 Torr. The device area is 0.13 cm2.

Device characterization. The power conversion efficiencies of the organic solar cells were measured by J-V curves using a Keithley 2401 source measurement unit under AM 1.5G 100 mW/cm2 spectra from a solar simulator (Newport Co., Oriel). To calibrate the intensity of the solar simulator, a standard Si-photodiode detector with a KG-3 filter (Newport Co., Oriel) was used. The EQE was measured using a quantum efficiency measurement system solar cell spectral response/QE/IPCE (Newport Co., Oriel IQE200B). The light intensity at each wavelength was calibrated using a standard, single-


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crystal Si photovoltaic cell. The absorption spectra of the device films were measured by a UV/Vis spectrophotometer (Varian, Cary5000).

The surface properties and morphologies of pristine PEDOT:PSS, HQ-modified PEDOT:PSS, and PTB7-Th:PC71BM films were characterized by AFM (NanoNavi II, SII Nano Technology Inc.) in the tapping mode. The current levels of the pristine PEDOT:PSS and HQ-modified PEDOT:PSS were measured using a C-AFM system (HITACHI NanoNavi E-Sweep) with +3V applied on the ITO bottom electrode under dark conditions. The pristine PEDOT:PSS film was fabricated by spin-coating PEDOT:PSS at 4000 rpm for 40 s on an ITO substrate, and the HQ-modified PEDOT:PSS film was further coated with 1 wt% HQ solution. IS measurements were taken using an impedance analyzer (IVIUM Tech., IviumStat) under dark conditions at an open circuit voltage in the frequency range between 0.1 Hz and 1.0 MHz. CE was conducted using the CE analyzer function of an organic semiconductor parameter test system (McScience T4000) under 1.0 sun at Voc conditions. To determine the valence band and work function of pristine PEDOT:PSS and HQ-modified PEDOT:PSS, UPS


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measurements were carried out with an ESCALAB 250-XI surface analysis system equipped with a He-discharge lamp providing He-I photons of 21.22 eV. The XPS investigation used a monochromatic Al-Kα X-ray gun with a photon energy of 1486.6 eV. The base vacuum pressure of the analysis system was ≈10-7 Torr. The Fermi edge was calibrated using a clean Au film, and all spectra presented were plotted with respect to the determined Fermi level. All XPS measurements were calibrated with reference to the Au 4f7/2 core level (83.98 eV) of a freshly deposited Au film. The measured samples were prepared by spin coating a PTB7-Th solution of 0.003 to 0.3 wt% on both pristine PEDOT:PSS and HQ-modified PEDOT:PSS films. The surface conductivity of pristine PEDOT:PSS and HQ-modified PEDOT:PSS was determined using a hall effect measurement system (Ecopia model No. HMS-5000). The thickness of the film was measured using a Dektak XT surface profiler. These samples were fabricated in the same way as the AFM samples. Mid-IR absorption spectra from pristine PEDOT:PSS and HQ-modified PEDOT:PSS were measured using an FTIR spectrometer (Varian, Varian 670). These samples were fabricated in the same way as the AFM samples, except ZnSe was used as the substrate.


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ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details on characterization and the results of device performances for other polymer devices; full UPS spectra of pristine PEDOT:PSS and modified PEDOT:PSS; external quantum efficiency (EQE) spectra; J-V characteristics of PTB7-Th:PC71BM solar cells with/without IPA surface treatment; J-V characteristics of P3HT:PCBM and PTB7:PC71BM solar cells; absorption spectra of PEDOT:PSS and PTB7-Th:PC71BM layer; contact-angle measurements; XPS spectra of PEDOT:PSS (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

ORCID Shinuk Cho: 0000-0002-6855-833X


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Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This research was supported by a National Research Foundation of Korea grant (2017R1A2B4003583, 2014R1A4A1071686, 2017M2A2A6A01018599).

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39


(b)

(a)

3.8 3.9

PTB7-Th PC71BM

PTB7-Th

E (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

Al

OMe

O

4.95

ITO Hydroquinone (HQ)

4.2

HQ modified PEDOT:PSS

5.3 5.9 PC71BM

ZnO NPs

Figure 1. (a) Schematic illustration of the device structure and chemical structure of the photoactive materials (donor: PTB7-Th, acceptor: PC71BM) and hydroquinone (HQ) surface modifier for PEDOT:PSS, (b) Energy level diagram of a BHJ solar cell based on a PTB7Th:PC71BM blend.

Figure 2. (a) J-V characteristics of PTB7-Th:PC71BM solar cells with pristine PEDOT:PSS and PEDOT:PSS modified with various ratios of HQ. (b) Jsc, (c) Voc, and (d) PCE distributions for the solar cells with pristine PEDOT:PSS and PEDOT:PSS modified with various ratios of HQ. The data were obtained from 20 devices for each case.


40

Page 41 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Summarized device performance characteristics of PTB7-Th:PC71BM solar cells with pristine PEDOT:PSS and PEDOT:PSS modified with various amounts of HQ. HQ

Voc

Jsc

Jsc-EQE§

η

ηave*

(V)

(mA/cm2)

(mA/cm2)

(%)

Pristine

0.782

15.6

15.4

0.654

HQ 0.2wt%

0.798

16.8

15.7

HQ 0.4wt%

0.795

17.6

HQ 0.6wt%

0.792

HQ 0.8wt%

(%)

Rs (Ω/cm2)

Rsh (Ω/cm2)

7.98

7.87

4.03

509

0.658

8.82

8.63

4.33

372

16.2

0.657

9.19

9.06

3.95

339

18.1

16.3

0.649

9.30

9.23

3.81

347

0.796

18.6

17.1

0.681

10.1

9.95

4.01

477

HQ 1.0wt%

0.793

18.9

17.4

0.680

10.2

10.1

3.81

518

HQ 1.5wt%

0.779

18.5

17.1

0.639

9.21

9.16

3.31

382

HQ 2.0wt%

0.781

17.9

16.4

0.611

8.54

8.48

3.84

251

concentration

FF

*Average PCE values were obtained from 20 devices. §

Jsc values calculated from EQE measurements.


41


Page 42 of 45

Figure 3. Secondary electron cut-off and HOMO onset UPS spectra of (a) pristine PEDOT:PSS and (b) HQ-modified PEDOT:PSS with increasing concentrations of PTB7-Th. The XPS spectra for S 2p and C 1s of (c) pristine PEDOT:PSS and (d) HQ-modified PEDOT:PSS with increasing concentrations of PTB7-Th. Schematic energy-level diagrams of PTB7-Th on (e) pristine PEDOT and (f) HQ-modified PEDOT:PSS. The energy unit is eV. (EF: Fermi energy level, Evac: Vacuum level, Δ: Interfacial dipole, IP: Ionization potential, Vb: Energy level relaxation, φe: Electron injection barrier, φh: Hole injection barrier).


42

Page 43 of 45

1

(a) 100

(b)

Pristine PEDOT:PSS HQ modified PEDOT:PSS

-Z'' (k )

2

J sc (mA/cm )

10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0


C2

(c)

40 30

R3

R2

0.5

1.0

1.5

2.0

Z' (k )

2.5

3.0

11

8x10

(d) 7x1011 -2

n ext (cm )

35 25 20 15

11

6x10

11

5x10

11

4x10

11

3x10

11

10

2x10


5 0 0

C3

R1

Voltage (V)

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2

3

4

Time (s)

5

11

1x10 6

0 0

2

4

6

Time (s)

8

10

Figure 4. (a) Dark J-V characteristics of PTB7-Th:PC71BM solar cells with pristine PEDOT:PSS and HQ-modified PEDOT:PSS. (b) Nyquist plot of impedance spectroscopy measurements at

Voc under light irradiation and the corresponding equivalent circuit model. (c) CE current transient under 1 sun Voc conditions. (d) Calculated charge extraction density from the CE measurements.


43


Page 44 of 45

Figure 5. (a) FTIR absorption spectra of IRAV modes in mid-IR region in pristine PEDOT:PSS and HQ-modified PEDOT:PSS. AFM topo images of (b) pristine PEDOT:PSS and (c) HQmodified PEDOT:PSS. C-AFM images of (d) pristine PEDOT:PSS and (e) HQ-modified PEDOT:PSS on an ITO substrate in the dark at +3V.

Table 2. Conductivity and sheet resistance of pristine PEDOT:PSS and HQ-modified PEDOT:PSS.

Pristine PEDOT:PSS

Thickness

Conductivity

Sheet Resistance

(nm)

(Ω-1cm-1)

(kΩ/□)

52.5

0.18

1,340

(±1.35×10-3)

(±9.87)


44

Page 45 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HQ-modified PEDOT:PSS

51.7

3.38

59.1

(±5.17×10-3)

(±0.92)

ToC figure


45

Treating the PEDOT:PSS surface with hydroquinone enhances the

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