Underwater Organic Solar Cells via Selective Removal of Electron

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Letter

Underwater Organic Solar Cells via Selective Removal of Electron Acceptors near the Top Electrode Jaemin Kong, Dennis Nordlund, Jong Sung Jin, Sang Yup Kim, Sun-Mi Jin, Di Huang, Yifan Zheng, Christopher Karpovich, Genevieve Sertic, Hanyu Wang, Jinyang Li, Guoming Weng, Francisco Antonio, Marina Mariano, Stephen Maclean, Tenghooi Goh, Jin Young Kim, and Andre D. Taylor ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00274 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Energy Letters

Underwater Organic Solar Cells via Selective Removal of Electron Acceptors near the Top Electrode Jaemin Kong,†‡ Dennis Nordlund,# Jongsung Jin,§ Sang Yup Kim,∥ Sun-Mi Jin,§ Di Huang,‡ Yifan Zheng,‡ Christopher Karpovich,‡ Genevieve Sertic,∇ Hanyu Wang,‡ Jinyang Li,‡ Guoming Weng,†‡ Francisco Antonio,‡ Marina Mariano,‡ Stephen Maclean,‡ TengHooi Goh,‡ Jin Young Kim*⊥, and André D. Taylor*† †Department

of Chemical and Biomolecular Engineering New York University Tandon School of Engineering 6 MetroTech Center Brooklyn, NY 11201 E-mail: [email protected] ‡Department

of Chemical and Environmental Engineering Yale University New Haven, CT 06511, USA #Stanford

Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park, CA 94025, USA §Division

of High Technology Research Korea Basic Science Institute Busan, South Korea ∥Department

of Mechanical Engineering and Material Science Yale University New Haven, CT 06511, USA ∇Department

of Electrical Engineering Yale University New Haven, CT 06520, USA ⊥Department

of Energy Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 44919, South Korea E-mail: [email protected]

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Abstract Electron acceptor degradation of organic solar cells is considered a main contributor to performance instability and a barrier for the commercialization of organic solar cells. Here, we selectively remove the electron acceptors on the surface of donor:acceptor blend films using a tape stripping technique. The near edge X-ray absorption fine structure (NEXAFS) spectrum reveals that only 6% of the acceptor component is left on the blend film surface after the tape stripping, creating a polymer-rich surface. The optimized morphology avoids direct contact of electron acceptors to the oxygen and water molecules from the film surface. Moreover, the polymer-rich surface dramatically enhances the adhesion between the photoactive layer and the top metal electrode, which prevents delamination of the electrode. Our results finally demonstrate that the selective removal of electron acceptors near the top electrode facilitates the realization of highly durable organic solar cells that can even function under water sans encapsulation.

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ACS Energy Letters

The emergence of new technologies such as flexible and ubiquitous devices demands that peripheral device units must conform to their various geometrical form factors while enduring harsh environments such as physical impact and water ingression.1-7 The same holds true for next generation solar cells which are deployable onto building surfaces or appliances without detracting from the main body aesthetics. From this perspective, organic solar cells (OSCs) are a promising solution because the main photoactive components, such as conjugated polymers, are intrinsically flexible and lightweight compared to those of conventional inorganic materials.8-11 In addition, the band gap of organic materials can be synthetically tuned by introducing various moieties in the backbone and/or the side chains of the molecules, which consequently enable the solar cells to be tuned in color.12-15 Despite these advantages, the practical application of OSCs to existing and/or upcoming technologies has been stymied due to the low reliability under ambient conditions where solar cells typically are deployed.16-18 One of the main culprits is the photochemical instability of fullerene derivatives which are widely used as electron acceptors in the donor:acceptor blend photoactive layers.19-21 Anselmo et al found that drastic changes at the surface of fullerene film happen when the film is exposed to ambient air with the presence of light, which could cause serious issues on solar cell performance.22 Recently, Lee et al identified the effects of photooxidation of fullerene derivative on its chemical structure, and connected the result to specific changes in its electronic structure, which consequently influence the electron transport and recombination kinetics of the solar cells.23 Bao et al also noticed that when fullerene molecule is exposed to water under light irradiation, its electronic structure is irreversibly changed.24 Conversely, however, the electron acceptor is beneficial for the electron donor stability, 3



because the electron acceptor quickly quenches the excited states of the electron donor within several hundred femtoseconds. As a result, the presence of electron acceptor slows down the photo-oxidation progress of electron donor polymer.18,20,25 Therefore we hypothesized that the donor and acceptor components should be well mixed inside the blend film while on the surface the acceptor component should be minimized in order for electron acceptors to avoid direct contact with oxygen and/or water from the surface. Here, we demonstrate that a facile tape stripping technique promotes the selective removal of the acceptor component from the surface of a donor:acceptor blend film, creating an optimized morphology which significantly enhances device performance and durability. Peeling after sticking adhesive tape onto the photoactive blend film surface selectively removes more than 60 % of acceptor component (i.e., PCBM) from the blend film surface, resulting in a high concentration of donor component (~94%) on the surface. The strategic removal of PCBM from the surface circumvents the direct contact of PCBM to oxygen and/or water molecules in air and enhances the adhesion between the photoactive layer and the top electrode. We demonstrate that this enhanced adhesion inhibits delamination of the top electrode from the photoactive layer, and enables the solar cell to retain ca. 70 % of its initial power conversion efficiency (PCE) after 10,000 bending cycles under the water. Moreover, upon performing ultrasonic fatigue in a water bath, no macroscopic damage was observed on the electrode. Further, we show that this new approach is also applicable to non-fullerene based organic solar cells, which showed the enhanced PCE and better underwater durability. Figure 1 illustrates the tape stripping process applied to a photoactive blend film consisting of electron-donating conjugated polymer, Poly(3-hexylthiophene) (P3HT), and electron4


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ACS Energy Letters

accepting conjugated small molecule, PCBM. First, the adhesive tape is applied onto the photoactive layer, and then the substrate is transferred to a hot plate set at 60 °C where a roller is used to gently press the tape onto the photoactive layer. After the substrate is cooled down to room temperature, the adhesive tape is slowly removed from the film surface. Further details of the process can be found in the experimental section. Figure 1b and c are the UV-Vis absorption data for the substrate and tape before and after the application of tape stripping process, respectively. We show that the absorbance of the photoactive layer in the 200 nm to 450 nm range is reduced after application of the adhesive tape (Fig. 1b), which corresponds to the increased absorption of the tape side in the same wavelength range (Fig. 1c). The spectral absorbance change of the blend film (tape side) largely matches with the PCBM absorption spectrum (Fig. S1). To quantify the surface composition of the pristine and tape-peeled-off photoactive layer films, carbon K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of the neat PCBM and P3HT films as well as the blend films before and after the tape stripping process were measured.26 Because NEXAFS spectroscopy is an effective technique capable of detecting sub-nanometer depth variation in atomic concentrations,27 we used this technique to accurately analyze the composition of the topmost surface within a few nanometers of the sampling depth. The carbon K-edge NEXAFS spectra for the neat P3HT and PCBM are provided in the supplementary information and used to assign characteristic resonance peaks for each component in the blend films (Fig. S2a). The surface composition of each film was achieved from the linear superposition of the pure P3HT and PCBM spectra, and fit numerically to the NEXAFS spectra of the blend films.26,27 The surface composition of the 5



pristine blend film was found to be a sum of 82% P3HT and 18% PCBM, (Fig. 1d). Noticeably, after applying the adhesive tape on the blend film, the PCBM ratio on the surface significantly reduces to 6%, while the P3HT content on the surface increases to 94% (Fig. 1e). NEXAFS spectra collected at a different angle, 35°, also confirmed the selective removal of the PCBM component on the surface (Fig. S2b and c). Phase-contrast AFM analysis also supports that the PCBM is preferentially removed from the blend film surface after tape peel off (Fig. 2a and b). We observe dark dotted spots on the surface of the pristine blend film (Fig. 2a), which are estimated to be PCBM molecules and clusters. In contrast, the surface where the adhesive tape was applied and removed rarely exhibits any dark dotted spots on it (Fig. 2b). Thus, we speculate that the PCBM molecules and clusters are removed from the surface, leaving small pits in their place as shown in AFM topology images (Fig. 2c and d). In Figure 2e and f, the depth profile shows that shallow dips are created at low depth range for the film after the tape stripping process. At the same time, the root mean square roughness reduces from 3.01 nm to 2.74 nm, and the bearing ratio profile curve also shifts toward a lower depth (Fig. 2e and f). Thus we interpret these results with respect to the process as follows: (1) the pressure exerted during the rolling process flattens the photoactive layer, while enabling the tape to intimately adhere to the blend film surface, and (2) the part having a higher degree of freedom (e.g. PCBM) and/or higher adhesion force to the tape is removed from the photoactive layer. Indeed, the adhesion force between the tape and the neat PCBM film is about 5 times higher than that between the tape and the neat P3HT film (Fig. S3). The adhesion of an electrode on an OSC device is crucial for reliability, especially for the 6


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ACS Energy Letters

devices created on flexible substrates. We first examined the electrode adhesion for the pristine and the surface PCBM removed blend films created on the basic device structures: Glass/ITO/ZnO/(P3HT:PCBM or single component)/Ag or Glass/ITO/ZnO/(P3HT:PCBM or single component)/V2O5/Ag (Fig. S4). To qualitatively compare the electrode adhesions on the pristine and surface PCBM removed blend films, we applied a strong adhesive tape (3M™ Super Bond Film Tape 396) to the top surface of the samples and quickly peeled the tape to see if the top electrodes are detached from the photoactive layers. In the pristine P3HT:PCBM samples (Fig. S4a), we observe partial damage on the top electrodes (Fig. 3a; top left corner), whereas no apparent damages was found on the top electrodes of the surface PCBM removed P3HT:PCBM samples (Fig. 3a; top right corner, Fig. S4b). This result implies that removing the surface PCBM enhances the electrode adhesion regardless of having an intermediate layer (V2O5), which also infers that PCBM may be a major contributor for weakening the electrode adhesion. To clearly demonstrate the impact of PCBM on the electrode adhesion we prepared samples with neat PCBM and neat P3HT, respectively. For the neat PCBM samples (Fig. 3a; bottom left corner) all the top electrodes were easily peeled off, being completely transferred to the adhesive tape. To identify the cleavage separating the two parts, we analyzed the surfaces using XPS spectra. In the XPS spectra of the Ag 3d region for the Glass/ITO/ZnO/PCBM/Ag sample (Fig. S4c), it was found that the silver electrode was completely transferred to the tape side after the tape peel test (Fig. S5a). For the Glass/ITO/ZnO/PCBM/V2O5/Ag sample (Fig. S4c), the silver and vanadium signals were only found on the tape side while no silver and vanadium peaks were detected on the surface of PCBM substrate side (Fig. S5b and c). Thus, Glass/ITO/ZnO/PCBM/Ag and 7



Glass/ITO/ZnO/PCBM/V2O5/Ag samples were cleaved along the interface between the PCBM and Ag (or V2O5/Ag). To obtain a quantitative adhesion strength between the top electrode and the PCBM, 180° peel tests were conducted using a uni-axial tensile machine Instron 3345 (See details in experimental section). We show that the peel strength between the PCBM and the Ag electrode is about 19.2 N/m and that between the PCBM and V2O5/Ag electrode, it is about 7.87 N/m (Fig. 3b). On the other hand, the neat P3HT samples (Fig. S4d) show no apparent damage on the electrodes (Fig. 3a; bottom right corner), and peel strengths obtained from the neat P3HT samples are two orders of magnitude higher (~1300 N/m) than those from the PCBM samples (Fig. 3b and Fig. S6). The drastic difference in between the cases of P3HT and PCBM might be attributed to the Lewis acid-based interaction. In the XPS data for silver (Fig. S5a and b), we found that the silver electrode surface incorporates a certain portion of silver oxide (Ag2O, 367.8 eV), which confirms that the natural oxide can be formed at the silver surface or interface.28 In organic chemistry, the silver oxide is used as a mild oxidizing agent which oxidizes aldehydes to carboxylic acids,29 and it’s also considered a Lewis acid. Since the Lewis acid-base interaction is the major molecular interaction for bonding and adhesion,30 Lewis acidic silver oxide will interact with Lewis basic moiety which is the thiophene group in the P3HT polymer chains, and in turn the interaction gives a strong bonding in between P3HT and silver electrode.31 Vanadium oxide which is used for a hole transport layer used to be also used as an oxidizing agent, so it would play a same role in the bonding between P3HT and silver electrode.32 In contrast, since PCBM contains butyric acid methyl ester group as a side chain where its chemical property might be closer to the Lewis acid, silver oxide or vanadium oxide would not preferentially react with the same type of Lewis acidic moiety of the PCBM, or would rather repulse each other. Therefore, the silver electrode is tightly attached 8


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ACS Energy Letters

to the P3HT polymer layer while it’s easily peeled off the PCBM layer. However, there are a lot of factors which could affect the conclusion of the bonding mechanism; for instance, electrostatic, polarization, charge transfer, exchange repulsion and coupling of any of the four interactions in between the two media. Therefore, this is one of the possible mechanisms which are open to be investigated theoretically and experimentally in the future. Besides that, in order to get the actual adhesion strength between P3HT and a top electrode, stronger tape or bond needs to be used since the top electrodes were not stripped off with the tape, inferring that the actual adhesion strength between the P3HT and the top electrode should be higher than 1300 N/m. To investigate the influence of the surface-PCBM removal on the device performance, we fabricated solar cells using the pristine and the surface PCBM-removed blend films based on the inverted structure, respectively (Fig. 4a). We observe a distinctive change in the fill factor enhanced from 0.56 to 0.69 (Fig. 4b) and attribute this to the enhanced charge carrier selectivity and Ohmic transition at the anode contact.33,34 Typically, high selectivity of charge carriers is accomplished by blocking undesired charge carriers while consistently collecting energetically favorable charge carriers.34 In our case, the P3HT-rich surface layer where the PCBM was mostly removed acts as an energetic barrier for electrons and at the same time ensures a better Ohmic contact toward the anode.35 The time-of-flight secondary ion mass spectrometry (ToFSIMS) depth profiles of S- which represents the P3HT also confirm that the blend film after tape stripping has better compositional distribution for charge collection in the vertical direction (Fig. S7). In the pristine photoactive layer (Fig. S7; dash line), we observe a dip near the top surface, which infers that this layer is relatively rich in PCBM near the top surface. 9



However, after the application of the tape stripping process (Fig. S7; solid line) the dip disappears, and the S- peak intensifies toward the top surface. Thus hole charge carriers can be efficiently collected by the anode without any traps. We confirm this finding by the fact that the charge carrier mobility becomes balanced after the surface PCBM removal (Fig. S8). Conversely, we observe a slight reduction in the JSC, which might be attributed to either the thickness decrease of the photoactive layer (i.e. the effective thickness decrease due to the lack of electron acceptor in the photoactive layer surface) or the V2O5-doped inactive P3HT in the photoactive layer surface, which is not contributing to photocurrent.32 To verify the reliability of the surface PCBM removed solar cell in practice, we conducted underwater constant load discharge tests. Here, the solar cells were dipped in 15-cm deep water bath consistent with IPX7 requirements,36 and the photovoltage was continually measured under a constant load (10 kΩ) with light irradiation. The 10 kΩ resistance started to be loaded on the cells right after the first 10 seconds at open circuit voltage condition. For the pristine solar cell the photovoltage measured under the constant load abruptly drops with stepwise decrease from 0.49 to 0.30 V over 30 minutes (Fig. 4c). For the surface PCBM removed solar cell, the photovoltage measured under constant load only decreases slightly from 0.56 V to 0.55 V over the same period of time (Fig. 4c). We note that after the underwater constant load discharge tests, the top electrode of the surface PCBM removed solar cell remained robust while the top electrode of the control solar cell was easily exfoliated (Table S1). We suggest that this might be a reason for the abrupt device failure of the control cell. After the constant load discharge tests under the water, we re-measured the J-V characteristic curves for the both solar cells, and find that surface PCBM removed solar cell still has a high 10


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ACS Energy Letters

VOC of ~0.58 V with a good J-V characteristic curve shape (Fig. S9a). In contrast, the control solar cell shows a significantly decreased VOC of ~0.21 V with a linear-like J-V characteristic curve (Fig. S9b). To simulate extremely harsh conditions, we placed the solar cells in an ultrasonic bath (42 kHz, 70 W) for 6 hours. After underwater sonication, the top electrodes of the pristine solar cell were severely damaged and exfoliated, whereas there was no macroscopic damage on the top electrodes of the surface PCBM removed solar cell. We did however, observe some damage in several spots on the exposed photoactive layer where the top electrode was not deposited (Fig. 4d). Based on these findings, we conclude that the surface PCBM removed solar cell has a stronger electrode adhesion even under water, and also suggest that the electrode adhesion or mechanical robustness between the photoactive layer and the top electrode is a critical attribute for underwater organic solar cell durability. To practically assess the underwater mechanical durability, we fabricated flexible solar cells based on PET/ITO substrates, and carried out cyclic bending tests under the water. To avoid the cracking of the bottom ITO electrode during the bend cycle test, the bending radius was kept 15 mm (corresponding to ~ 0.4% of bending strain, εbending) until 9000 times of bend cycle, and then it was decreased up to 7 mm (εbending = ~9%) after 9000 times of bend cycle to accelerate the degradation. Figure 5a shows the decreases in PCE of the pristine and the surface PCBM removed solar cells with respect to the number of bending cycles. During the bending test up to 9000 cycles the difference in PCE between the pristine and the surface PCBM removed solar cells gradually increased by 3 times (from 6% to 18%), and after 11,000 bending cycles, the difference in PCE between the two groups increased up to 148%. Figure 5b and c apparently show the difference of the degradation tendencies in J-V characteristics of the two 11



groups. Notably, even after 10,000 bending cycles under the water, the surface PCBM removed solar cells maintained approximately 80 % of its initial PCE while the PCE of the pristine solar cells dropped 40% under the same conditions. To demonstrate a broad applicability of our approach, we applied the tape stripping technique to the non-fullerene based organic solar cell. For this, we used poly[(2,6-(4,8-bis(5(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)]) (PBDB-T) and 3,9bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene(IT-M) (Fig. S10a and b) to fabricate a representative non-fullerene based organic solar cell.37,38 In the PBDBT:IT-M solar cells, a similar tendency to the P3HT:PBCM solar cells was found. When the tape stripping process was applied to the PBDB-T:IT-M solar cells, the PCE increases from 10.6 ± 0.072% to 11.0 ± 0.153% with an enhanced fill factor of 0.76 ± 0.005 in the glass/ITO based solar cells (Fig. S11 and Table S2). In the flexible solar cells based on the blends, the PCE also increases from 7.36 ± 0.187% to 8.07 ± 0.198% upon the tape stripping process (Table S4). Using the flexible solar cells, we performed underwater cyclic bending tests as described previously. Fig. 5d shows decreasing tendencies in PCE of the pristine and the surface IT-M removed solar cells as the bending cycles increase. From the both groups, we observe a more rapid PCE decrease in comparison to the P3HT:PCBM solar cells. Yet even in this nonfullerene system, the PCE slope for the surface IT-M removed group is relatively less gradual compared to that for the pristine group. We note that as the bending cycle increases, the gap in 12


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ACS Energy Letters

the PCE between the pristine and the surface IT-M removed groups widens by 17 times from 9 % to 155 %. We further note that after 1000 bending cycles, the surface IT-M removed solar cell group maintains ca. 80 % of its initial PCE, whereas for the pristine solar cell group, more than half of the initial PCE is lost under the same conditions. Figure 5e and f are the representative J-V characteristic curves for the pristine and surface IT-M removed solar cells, which show that the significant PCE decrease in the pristine solar cell is attributed to the reduced current and fill factor with increased series resistance. We attribute the poor durability of the pristine PBDT-T:IT-M solar cells to the presence of the IT-M in the pristine PBDBT:IT-M film surface, which could weaken the electrode adhesion causing the exfoliation and disconnection. This speculation could be evidently supported by the fact that the peel strength between IT-M and the top electrode (14.8 N/m) is much weaker than that between PBDB-T and top electrode (>1258 N/m) (Fig. S12). Further, ultrasonic fatigue tests in the water bath reveal that the surface IT-M removed solar cell has higher electrode adhesion durability than the pristine solar cell (Fig. S13). In summary, we demonstrate that the selective removal of the electron acceptor component (e.g., PCBM or non-fullerene acceptor IT-M) from the photoactive blend film surface using a facile tape stripping technique not only minimizes the direct exposure of the electron acceptors to the oxygen or water molecules, but also significantly enhances the adhesion between the photoactive layer and the top electrode. The optimized blend films after the tape stripping exhibits higher durability under harsh ultrasonic conditions and bending stress tests under the water. Moreover, removing the surface electron acceptor component optimizes the blend film morphology in a vertical direction, which results in higher fill factors in the inverted solar cell 13



structure. We anticipate that our approach and results will initiate a new frontier for the development of highly durable, all-weather organic solar cells that can be operable even under the water. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. >> Experimental methods, UV-Vis, NEXAFS spectra, peel strength test, XPS, device structure, ToF-SIMS, charge carrier mobility, J-V characteristics, photos of solar cells after sonication, and tables (PDF) >> Videos showing tape stripping process and cyclic bending test (MP4) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgments The authors gratefully acknowledge the National Science Foundation (DMR-1410171), NSF14


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CAREER award (CBET-0954985), the Yale Climate and Energy Institute (YCEI), and the NASA (CT Space Grant Consortium) for partial support of this work. J. K. also acknowledge Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1A6A3A03020005).

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Figure 1. Flow diagram of the tape stripping process and the effect on P3HT:PCBM blend film surface. (a) A schematic illustration for the application of a tape stripping method. The adhesive tape is a regular transparent Scotch tape (model #: S-9782). (b) UV-Vis absorption data for the blend film before and after tape application. (c) UV-Vis absorption data for the tape before and after tape stripping process. NEXAFS spectra collected at 55° for (d) a pristine blend film and (e) a blend film after the tape stripping process, and the fitting curves with scaled component spectra. 16


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ACS Energy Letters

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Glass/ITO/ZnO/(P3HT:PCBM or Single component)/Ag or Glass/ITO/(ZnO/P3HT:PCBM or Single component)/V2O5/Ag. The four samples on the upper row were prepared based on P3HT:PCBM blend films. Among them, the two samples on the upper left corner were prepared with pristine BHJ films, whereas the other two samples on the upper right corner were prepared using surface PCBM removed blend films. The four samples on the bottom row have a single component layer such as PCBM or P3HT, instead of P3HT:PCBM blend film. (b) Peel strengths for the four samples on the bottom row in Fig. 3a were plotted, and huge differences in peel strength are found between PCBM and P3HT groups.

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(a)

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0 P3HT:PCBM organic solar cells

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Pristine BHJ Surface PCBM removed BHJ

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After sonication (70 W) for 6 hours

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Current density (mA cm-2)

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0.4 0.3 Under water with 10 k constant load Pristine BHJ Surface PCBM removed BHJ

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Figure 4. Solar cell performance and electrode adhesion strength under the water. (a) Device structure of organic solar cells. (b) J-V characteristic curves for pristine and surface PCBM removed P3HT:PCBM blend films, respectively. (c) Constant load discharge technique was introduced for tracking the operating photovoltage variations of the solar cells respectively based on the pristine and the surface PCBM removed blend films. During the measurement, the devices were kept in 15-cm deep water with the constant load of 10 kΩ resistance under light irradiation (100 mW cm-2). (d) Photographs of P3HT:PCBM solar cells after sonication (42 kHz, 70 W) under the water for 6 hours.

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PCE (%)

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PCE (%)

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Figure 5. Bend cycle test under the water. (a) Trends in power conversion efficiency of flexible solar cells based on P3HT:PCBM with respect to the number of bending cycle. Changes in JV characteristic curves of flexible solar cells based on (b) pristine P3HT:PCBM and (c) surface PCBM removed P3HT:PCBM with respect to the number of bending cycle. (d) Trends in power conversion efficiency of flexible solar cells based on PBDB-T:IT-M with respect to the number of bending cycle. Changes in J-V characteristic curves of flexible solar cells based on (e) pristine PBDB-T:IT-M and (f) surface IT-M removed PBDB-T:IT-M with respect to the number of bending cycle.

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Underwater Organic Solar Cells via Selective Removal of Electron

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