Efficient PEDOT:PSS-Free Polymer Solar Cells with an Easily

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Efficient PEDOT:PSS-free polymer solar cells with an easily accessible polyacrylonitrile polymer material as a novel solution-processable anode interfacial layer Yong-Jin Noh, Sae-Mi Park, Jun-Seok Yeo, Dong-Yu Kim, Seok-Soon Kim, and Seok-In Na ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07841 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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ACS Applied Materials & Interfaces

Efficient PEDOT:PSS-free polymer solar cells with an easily accessible polyacrylonitrile polymer material as a novel solutionprocessable anode interfacial layer Yong-Jin Noh,a Sae-Mi Park,a Jun-Seok Yeo,b Dong-Yu Kim,b Seok-Soon Kim,*c SeokIn Na*a

a

Professional Graduate School of Flexible and Printable Electronics, Polymer Materials

Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjingu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea b

Heeger Center for Advanced Materials (HCAM), School of Material Science and

Engineering, Gwangju Insititute of Science and Technology, Gwangju, 500-712, Republic of Korea c

Department of Nano and Chemical Engineering, Kunsan National University, Kunsan,

Jeollabuk-do, 753-701, Republic of Korea

Corresponding authors *Seok-Soon Kim, E-mail: [email protected]; *Seok-In Na, E-mail: [email protected]

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ABSTRACT

We demonstrate that an easily accessible polyacrylonitrile (PAN) polymer can efficiently function as a novel solution-processable anode interfacial layer (AIL) to boost the device-performances of polymer:fullerene based solar cells (PSCs). The PAN thin film was simply prepared with spin-coating of a cost-efficient PAN solution dissolved in dimethylformamide on indium tin oxide (ITO), and the thin polymeric interlayer on PSC-parameters and -stability were systemically investigated. As a result, the cell-efficiency of the PSC with PAN was remarkably enhanced compared to the device using the bare ITO. Furthermore, with the PAN, we finally achieved an excellent power conversion efficiency (PCE) of 6.7% and a very high PSC stability in PTB7:PC71BM systems, which constitute a highly comparable PCE and superior device life-time

to

conventional

PSCs

ethylenedioxythiophene):poly(styrenesulfonate)

with

(PEDOT:PSS).

poly(3,4These

results

demonstrate that the inexpensive solution-processed PAN polymer can be an attractive PEDOT:PSS alternative and is more powerful for achieving better cell-performances and lower cost PSC-production.

KEYWORDS

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Polymer Solar Cells, Bulk-heterojunction, Polymeric Interlayer, Polyacrylonitrile, Anode Interfacial Layer, Work Function Modification

1. INTRODUCTION

Polymer-based bulk-heterojunction (BHJ) solar cells (PSCs) have been regarded as a next-generation solar energy source due to their flexible, solution-processable, and mass-production ability.1-8 However, for real commercialization, the power conversion efficiency (PCE) and cell-stability of PSCs will need to be further improved.6-8 To this end, developing interfacial layers between electrodes and BHJ is crucial because the PSC-performances such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and cell life-time are highly dependent on the interlayers.9-12 For these reasons, significant and challenging studies on electrode modification have been conducted by introducing an interfacial layer, and in particular, the metal oxides and carbon-based materials have been actively studied as a promising interfacial layer in PSCs due to their solution-processability, low-temperature processes, and excellent selective carrier extraction.12-22

Recently, besides metal oxides and carbon-based materials, solution-processable polymeric materials could also be employed as an efficient electrode interfacial modifier to realize low-cost printable solar cells via a simple and solution-processed

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non-vacuum process.23-31 In particular, it has been well demonstrated that conjugated polyelectrolyte based thin polymeric interlayers with a thickness of a few nanometers can form an interfacial dipole and induce a vacuum level shift of adjacent metal electrodes,32,33 resulting in an increase of the built-in potential, Voc, and PCE.24-26 This excellent work-function tunability of the conjugated polyelectrolytes could render them widely applicable as interfacial materials. However, the conjugated polyelectrolytes still have some disadvantages. For examples, in order to synthesize such ionic conjugated polymeric materials, relatively time-consuming multi-step synthesis processes are typically required.26 In addition, most of the solution-processed polymer materials have been used as a cathode interfacial layer in PSC as mentioned in the recent paper;28 studies on the uses of the polymeric materials as anode interfacial materials are very limited, and the subject has rarely been addressed, despite the urgent need to replace the widely used poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) with a high hydroscopic material having an acidic nature. Therefore, studies on costefficient polymeric layers prepared using a simple synthetic process as well as studies on their use as anode interfacial layers could be highly valuable for low-cost and highperformance PSCs. In this study, we demonstrate that an easily accessible polyacrylonitrile (PAN)

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polymeric film can effectively act as a novel anode interfacial layer (AIL) to enhance overall PSC-performances. The PAN simply synthesized via free radical polymerization of acrylonitrile has been well known as one of the least expensive precursors for the fabrication of various carbon materials such as carbon fiber and carbon-based transparent electrodes, and basically, the PAN also has desirable properties for use as interfacial layers such as good solubility to common solvents, good formability of thin film, and good chemical stability.34-38 In addition, Summan demonstrated that the electrical conductivity of PAN materials can be improved with increasing treatment temperature owing to enhanced conjugation of carbon-nitrogen in PAN materials.39 Recently, Jiao et al. reported that the work functions of PAN materials were controlled from 4.66 to 4.48 eV with increasing temperature, indicating that the work functions of the PAN can be suitable for carrier injection and extraction in organic devices.40 From these observations, in particular considering that the PAN work-function and conductivity can be easily tunable and also the PAN already possesses the basic requirements for use as anode interfacial materials, it was believed that the PAN could be a promising AIL candidate in PSCs. Nevertheless, the study on the PAN as the interfacial materials and its real application in electronic devices have seldom been addressed.37,39,41 In addition, no studies have been conducted on the PAN material as an

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anode interfacial layer in polymer solar cells. Herein, we actively investigated the PAN materials as AILs and systemically studied theirs effects on PSC-parameters and stability. As a result, the cell-efficiencies of the PSCs with PAN AILs were remarkably enhanced compared with the PSCs using bare ITO. Furthermore, with the PAN, we finally achieved an excellent power conversion efficiency of 6.7% and a very high PSC stability based on PTB7:PC71BM, which constitute a highly comparable cell-efficiency and superior PSC life-time to reference PSCs with the conventional PEDOT:PSS. These observations demonstrate that the inexpensive solution-processed PAN polymer can be a viable PEDOT:PSS alternative material and more beneficial for achieving better cellperformances and lower cost PSC-production.

2. EXPERIMENTAL DETAILS

For the fabrication of the polymer:fullerene based PSCs, typically with the ITO/AIL/BHJ/Ca(20 nm)/Al (100 nm) multilayer stacking shown in Figure 1(a), the patterned ITO (~10 ohm/sq.) substrates were first cleaned with acetone, deionized water, and isopropyl alcohol. The substrates were then dried at 100 °C for 30 min and UVozone treatments were applied for 30 min. The PAN powder (150,000 Mw, Sigma

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Aldrich) was dissolved in dimethylformamide (DMF) to fabricate 0.01, 0.1, 0.5, and 1 wt% PAN solutions. The PAN solutions were spin-coated at 5000 rpm for 40 s onto ITO substrates and dried at 350 °C for 30 min under an air atmosphere for a typical stabilization process.37,42 The thickness-values of PAN films prepared with 0.01, 0.1, 0.5, and 1 wt% PAN solution were ~0.5, ~1, ~2, and ~5 nm, respectively. PEDOT:PSS (Clevios P VP AI 4083, Heraeus) as the reference AIL was also prepared by spin coating at 5000 rpm for 40 s, followed by drying at 120 °C for 10 min. The photoactive layers (~230 nm) based on poly(3-hexylthiophene) (P3HT, 4002-EE, 50000~70000 Mw, ~ 96% regioregularity, Rieke Metals) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM, 99.5% purity, Nano-C) were then fabricated by spin-coating of the blend solution mixed with P3HT (25 mg) and PCBM (25 mg) in 1,2-dichlorobenzene (DCB) (1ml) at 700 rpm for 60 s. This was followed by solvent annealing for 120 min and thermal annealing at 110 °C for 10 min in N2. For the formation of another active layer, a BHJ mixed solution composed of 10 mg of thieno[3,4-b]thiophene/benzodithiophene (PTB7, ~97000 Mw, 1-Material), 15 mg of [6,6]-phenyl C71-butric acid methyl ester (PC71BM, >99% purity, Nano-C), and 1,8-diiodictance (5 vol.%, Tokyo Chemical Industry Co. Ltd.) in 1 ml of chlorobenzene (CB) was also prepared, and the resulting blend solution was spin-coated at 2000 rpm for 60 s for the fabrication of a PTB7:PC71BM layer (~100

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nm). Finally, the thermal evaporation of a metal cathode having Ca (20 nm)/ Al (100 nm) was performed in a vacuum at 10-6 torr. The photocurrent density (J)–voltage (V) power curves of the fabricated solar cells were measured using Keithley2400 equipment and a solar simulator (Class AAA) under the standard AM 1.5G and 100 mW/cm2 measurement condition. The PSC stability was also recorded without encapsulation according to the ISOS-D-1 protocol measured as a function of exposure time in air under ambient humidity/temperature and AM 1.5G solar simulator.43 The surface images, work function, and UV-vis transmission data of each film were investigated using atomic force microscopy (AFM, Veeco Dimension 3100), a Kelvin probe (KP 6500, McAllister Technical Services. Co. Ltd.), X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha), and a Scinco S-3100 spectrophotometer, respectively. More detailed PSC fabrication and measurement were described in our previous reports.17,44

3. RESULTS AND DISCUSSION

To investigate the ability of PAN to work as AILs and the thickness effects of the PAN AIL on solar cell parameters, PSCs with and without PAN interlayers were fabricated

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and compared. As can be seen in Figure 1 and Tables 1 and 2, the FF and Jsc values in the PAN-based PSCs tended to decrease with increasing thickness of interfacial materials, with the exception of the FF of 0.5 nm-PAN, while Voc in PAN-based PSCs showed similar values, irrespective of the AIL thickness. This demonstrates that the overall device-parameters can be strongly affected by the AIL-film thickness; thus, the PAN-thickness for the use of AILs should be optimized. As shown in Figure 1(c), the 0.5nm-PAN based PSC showed poor cell-efficiency of ~2.5%, probably due to the fully uncovered PAN film on the ITO surface, which can result in a non-uniform contact at BHJ/electrode interfaces, thus producing higher series resistance (Rs), lower shunt resistance (Rsh), and lower FF,44,45 which can be confirmed in the J-V curve and film morphology of PAN in Figures 1(b) and S1. With increasing the PAN-thickness, the PAN-based PSCs showed more enhanced cell-efficiencies of up to an average PCE value of ~ 3.6%, probably because of a more uniform and full-covered PAN film.15,45 However, the further increase of AIL thickness beyond ~1 nm reduced the cell efficiencies, mainly due to the lower FF and Jsc, which could be induced by the reduced transmittance with an increase in AIL-film thickness and the increased surface roughness as shown in Figures S1 and S2;15,45 beyond 1 nm-PAN, the transmittance and rms roughness were changed from 84.59% and ~0.72 nm to 82.27% and ~2.94 nm (2

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nm-PAN) or 75.18% and ~13.9 nm (5 nm-PAN), respectively. In particular, as shown in the inset of Figure 1(b), as the PAN thickness increased up to ~5 nm, the Rs value was further increased, probably due to the charge blocking effect shown in a thicker insulating material than the monolayer,46 which resulted in the relatively low FF shown in the 5 nm-PAN based device. Consequently, the best cell-efficiency was obtained in the PSC with 1 nm-PAN, and more importantly, when considering that PSCs with PAN AILs showed dramatically enhanced device-characteristics compared with the PSCs with no PAN, it can be confirmed that the easily accessible and cost-efficient PAN material can work effectively as the anode/BHJ interfacial layer to provide a high-PCE based device. For a better feasibility test, we further investigated the PAN-based PSCs and directly compared them with the conventional reference PSCs having PEDOT:PSS anode interfacial materials. Variances in the PSC-parameters (PCE, Voc, FF, and Jsc) resulting from changing the AIL are summarized in Figure 2. With the use of AIL materials, all of the PSC performances were improved, which resulted from the significant enhancement in the Voc and FF values and the relatively small enhancement in Jsc values, as depicted in Figure 2; the solar cell with bare ITO had an average PCE of 1.148%, an average Voc of 0.423 V, an average FF of 30.66%, and an average Jsc of 9.067 mA/cm2,

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while the PAN raised the average PCE, Voc, FF, and Jsc up to 3.621%, 0.606 V, 59.49%, and 10.04 mA/cm2, respectively. More notably, the device performances in the PANbased PSCs closely approached those of the reference PEDOT:PSS-AIL based PSCs.

To further study why the PAN raised the PSC-performances, various investigations on the work-function (WF), XPS, transmittance, surface AFM morphology, Rs, and Rsh were recorded as shown in Figure 3. As shown in Figure 3(a), a considerable change was observed in the WFs. However, as shown in Figures 3(c) and (d), the corresponding transmittance and surface-morphology, which can also have a high impact on PSC performances,44 did not show a significant difference; the transmittance at 500 nm and the rms roughness of the bare ITO, PAN, and PEDOT:PSS were 87.66% and 0.69 nm, 84.59% and 0.72 nm, and 85.53% and 0.90 nm, respectively. The ITO-WF was 4.66 eV, but the PAN presented an increased WF value of 5.10 eV, very similar to that of the PEDOT:PSS of 5.09 eV. As shown in Figure 3(b), the C1s and N1s XPS data of the pristine PAN material showed a strong CN triple bond peak. On the other hand, the stabilized PAN exhibited various peaks with CN, CO, and NO bonds; the C1s peak of stabilized PAN exhibited C-C (285 eV), C-N (285.9 eV), C-O (286.9 eV), C=O (288.2 eV), and C-(O)-O (289.77 eV), and the N1s peak of stabilized PAN showed C-N (398.5 eV), C=N (401 eV), and N-O (403 eV).39,47-49 These changes shown in Figure 3(b) are

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in agreement with the previously reported stabilization and oxidation reaction of PAN.37,39,47,48 More importantly, considering that the CN, CO, and NO bonds with different electronegativity can induce a high internal dipole moment,17,26,50,51 it was believed that the larger WF of PAN could be due to the presence of the CN, CO, and NO bonds. Furthermore, when considering that the anode-electrode WF and the HOMO level of the donor conjugated polymer should be better matched to achieve a maximum built-in potential as shown in the inset of Figure 3(a),52,53 and thus improving the Voc, FF, and Jsc values,9-13 it was believed that the larger WF of PAN might be responsible for the largely increased PSC performances shown in the PAN-based PSCs. In addition, these observations concurred with the Rs and Rsh results shown in Figure 3(e); the introduction of PAN as an anode-interface modifier also had an impact on the Rs and Rsh values in solar cells. As shown in Figure 3(e), the average Rs and Rsh values of PSCs without AILs were 14.37 and 110.3 Ω cm2, respectively, while when the PAN AIL was employed, the Rs and Rsh were largely improved up to 4.637 and 1838 Ω cm2, respectively. Such values for Rs and Rsh closely approach those of the reference PSCs with PEDOT:PSS having an average Rs of 4.117 Ω cm2 and average Rsh of 2021 Ω cm2, indicating that the PAN film can also provide a better ohmic contact and effectively prevent charge-carrier recombination at the BHJ/ITO interface.25,45 Furthermore, from

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the overall results shown in Figure 3, the largely improved PSC characteristics obtained in the PAN based cells are believed to be mostly due to the increased built-in potential between the anode and cathode electrodes through the use of PAN AILs to provide a better ohmic contact and charge collection.9-13,25 More notably, the quantitatively similar device-parameters of PAN-based PSCs with the PEDOT:PSS-based cells effectively demonstrate that the PAN can be as effective as the representative PEDOT:PSS AIL typically used for providing high efficiency in PSCs. To expand the value of PAN as a PEDOT:PSS alternative, the PSC-stability of the PAN-based solar cells was investigated according to air-exposure time and the ISOS-D1 protocol,43 which was also compared with the reference PEDOT:PSS cell. As depicted in Figure 4, although the PEDOT:PSS provided an excellent device-efficiency similar to PAN, the conventional PEDOT:PSS-based device exhibited a rapid efficiency degradation and was not working after 3600 min, while the PAN-based PSC showed an excellent device life-time with the PCE maintaining ~ 60% of its initial PCE value, even after 13440 min. Considering that the PEDOT:PSS, well known with a high acidity and hydrophilicity, experimentally showed a pH of ~2 and a contact angle of 11.03 degree and that such PEDOT:PSS properties can result in a poor PSC-stability, the relatively neutral (pH = ~8) and hydrophobic (contact angle = 50.63 degree) properties of the

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PAN material could be responsible for the more enhanced PSC life-time compared with PEDOT:PSS.17,54 To further expand the application ability of PAN materials as anode interfacial layers in solar cells, we also studied the PAN AIL and applied it to different BHJ-based PSCs with PTB7:PC71BM. As described in Figure 5, the PTB7:PC71BMbased PSC with the PAN AIL exhibited excellent PSC-parameters with a PCE of 6.70%, a Voc of 0.713 V, a Jsc of 15.19 mA/cm2, and a FF of 61.83%, and their values were very similar to the reference PTB7:PC71BM based PSC with the widely employed PEDOT:PSS AIL even in the low band-gap polymer based PSC, which showed a PCE of 6.905%, a Voc of 0.790 V, a Jsc of 13.89 mA/cm2, and a FF of 62.87%. Similar device-stability results and trends were also detected in PTB7-based PSCs, as shown in the bottom inset of Figure 5. These observations indicate that the PAN is sufficient to be used as AILs in various PSCs and more beneficial than PEDOT:PSS for greater efficiency and stability of polymer photovoltaic cells.

4. CONCLUSIONS

We employed a cost-effective, solution processed, and easily accessible polymer material, polyacrylonitrile (PAN), as an interfacial layer between the polymer:fullerene

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active layer and ITO-anodes for boosting device-performances of polymer solar cells. The thin polymeric interlayer was systemically investigated to determine its feasibility as a new PEDOT:PSS alternative AIL and, as a result, the cell-efficiency of the PSC with PAN was remarkably enhanced compared with the device using the bare ITO, due to the enhanced built-in potential and effective shielding effect from the direct contact between ITO and BHJ through the use of PAN AIL to provide a better ohmic contact and charge collection. Furthermore, with PAN, we finally achieved a high power conversion efficiency of 6.7% and an excellent PSC stability in PTB7:PC71BM systems, which constitute a highly comparable PCE and superior PSC life-time to the conventional reference PEDOT:PSS-based PSCs. These observations suggest that the easily accessible and solution processed PAN is sufficient for use as AILs for polymeric solar cells and more beneficial than PEDOT:PSS for achieving better PSCperformances and lower cost PSC-production.

ACKNOWLEDGMENTS

This paper was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future

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Planning (MSIP) (2013R1A1A1011880).

ASSOCIATED CONTENT Supporting Information. AFM topographic images and optical transmission spectra of PAN with different concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *Seok-Soon Kim, E-mail: [email protected]; *Seok-In Na, E-mail: [email protected]

Notes The authors declare no competing financial interests.

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Table 1. Representative cell performances of P3HT-based PSCs with various PAN films. Voc

Jsc

FF

PCE

Rs

(V)

(mA/cm2)

(%)

(%)

(Ω cm2)

ITO

0.449

8.751

29.03

1.140

13.94

0.01 wt% PAN

0.571

10.38

42.51

2.52

4.109

0.1wt% PAN

0.606

10.09

58.48

3.575

3.857

0.5 wt% PAN

0.582

10.07

53.38

3.130

4.805

1 wt% PAN

0.577

9.543

42.34

2.331

8.674

PEDOT:PSS

0.608

9.901

61.13

3.681

3.204

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Table 2. Average and standard deviation values of photovoltaic parameters of P3HTbased PSCs with PAN interfacial layers as a function of concentration.

0.01 wt% PAN

0.1 wt% PAN

0.5 wt% PAN

1.0 wt% PAN

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.563

10.3725

42.905

2.504

(± 0.011)

(± 0.013)

(± 0.559)

(± 0.023)

0.606 (± 0.004)

10.04 (± 0.051)

59.49 (± 1.035)

3.621 (± 0.053)

0.583

10.040

52.185

3.055

(± 0.002)

(± 0.082)

(± 1.044)

(± 0.068)

0.566 (± 0.009)

9.414 (± 0.129)

37.313 (± 4.359)

1.993 (± 0.293)

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Figure 1. (a) Device structure of the tested polymer solar cells and (b) representative current density-voltage (J-V) curves of P3HT-based PSCs with various PAN films and PEDOT:PSS. The inset shows the Rs data of various PAN films in P3HT-based PSCs. Influence of the PAN thickness on (c) PCE and FF and (d) Jsc and Voc of P3HT-based PSCs.

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Figure 2. Influence of different interfacial layers on the (a) PCE and FF and (b) Jsc and Voc of P3HT-based PSCs.

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Figure 3. (a) Work-functions of bare-ITO, PAN, and PEDOT:PSS. The inset shows dark J-V characteristics of PSCs with and without interfacial layer. (b) C1s and N1s XPS spectra of the pristine and stabilized PAN. (c) Optical transmission spectra of PAN and PEDOT:PSS on ITO. (d) AFM topographic images of ITO, PAN, and PEDOT:PSS. (e) Rs and Rsh data of various AIL materials in P3HT-based PSCs.

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Figure 4. Changes in the photovoltaic parameters of P3HT-based PSCs with PAN and PEDOT:PSS films under an ambient atmosphere. The inset shows the initial J-V curve of P3HT-based PSCs with PEDOT:PSS and PAN.

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Figure 5. Representative J-V curves for PTB7-based PSCs with PAN and PEDOT:PSS films. The insets show the device-stability of PTB7-based PSCs with PAN and PEDOT:PSS.

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TOC GRAPHICS

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Efficient PEDOT:PSS-Free Polymer Solar Cells with an Easily

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