Research Article pubs.acs.org/journal/ascecg
All-Polymer Solar Cells with Bulk Heterojunction Films Containing Electron-Accepting Triple Bond-Conjugated Perylene Diimide Polymer Sungho Nam,†,‡,# Suk Gyu Hahm,§,∥,# Hyemi Han,† Jooyeok Seo,† Changsub Kim,§ Hwajeong Kim,†,⊥ Seth R. Marder,∥ Moonhor Ree,§ and Youngkyoo Kim*,† †
Organic Nanoelectronics Laboratory, Department of Chemical Engineering, School of Applied Chemical Engineering, and Research Institute of Advanced Energy Technology, Kyungpook National University, Daegu 702-701, Republic of Korea ‡ Center for Plastic Electronics, Department of Physics, Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom § Department of Chemistry, Division of Advanced Materials Science, Pohang Accelerator Laboratory, Polymer Research Institute, and BK School of Molecular Science, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea ∥ School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ⊥
S Supporting Information *
ABSTRACT: A triple bond-linked perylene diimide (PDI) conjugated polymer, poly{[N,N′-dioctylperylene-3,4,9,10-bis(dicarboximide)-1,7(6)-diyl]-alt-[(2,5-bis(2-ethylhexyl)-1,4phenylene)bis(ethyn-2,1-diyl]} (PDIC8-EB), was examined as an electron-accepting component in all-polymer solar cells. As an electron-donating component, poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) were introduced in order to investigate the feasibility of PDIC8-EB because of their similarity. Results showed that the power conversion efficiency (PCE) was higher for the PTB7-Th:PDIC8-EB solar cells (PCE = 3.58%) than the PTB7:PDIC8-EB solar cells (PCE = 2.81%). The better performance of the PTB7-Th:PDIC8-EB solar cells has been attributed to the formation of a well-defined nanodomain morphology in the PTB7Th:PDIC8-EB bulk heterojunction layer, as measured with transmission electron microscopy (TEM), atomic force microscopy (AFM), and synchrotron radiation grazing incidence X-ray diffraction (GIXD). KEYWORDS: All-polymer solar cells, Polymer:polymer bulk heterojunction, Triple bond-conjugated perylene diimide polymer, Nanodomains
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INTRODUCTION Polymer solar cells have been extensively investigated over the past decade because of their potential for lightweight, ultrathin, and flexible plastic solar modules, which can be achieved by employing continuous roll-to-roll (R2R) processes at low temperatures.1−5 The power conversion efficiency (PCE) of single-stack polymer solar cells has recently reached 8−10%, while more than 11% PCE has been also reported for tandem polymer solar cells.6−15 Interestingly, such high PCE polymer solar cells are fabricated with bulk heterojunction (BHJ) films of conjugated polymers and fullerene derivatives, so-called polymer:fullerene solar cells, of which principles and nanostructures have been intensively studied in the early works.16−25 However, polymer:fullerene solar cells have drawbacks in terms of poor light absorption in the visible range and © XXXX American Chemical Society
morphological instability in the BHJ layers, which are caused by fullerene derivatives that have a strong tendency for aggregation and recrystallization owing to their small molecular weight and concomitant diffusion within the film.26−30 To overcome the issue of fullerene diffusion and aggregation in polymer:fullerene solar cells, polymer:polymer solar cells, so-called all-polymer solar cells, have been introduced by replacing fullerene derivatives with electron-accepting polymers that are believed to have a lower tendency to aggregate due to their large molecular size and better mechanical endurance.31−35 In addition, all-polymer solar cells could benefit from increased light absorption through by virtue of many of them having Received: July 22, 2015 Revised: November 23, 2015
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DOI: 10.1021/acssuschemeng.5b00732 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 1. (a) Chemical structure of electron-accepting (PDIC8-EB) and electron-donating (PTB7 and PTB7-Th) polymers. (b, c) Absorption coefficient (α) and photoelectron yield spectra (PEYS) of pristine polymer films. (d, e) Device structure and flat energy band diagram for all-polymer solar cells with polymer:polymer bulk heterojunction (BHJ) films. Note that the HOMO and LUMO levels of all materials used in this study is displayed together in part e for comparison.
intrinsically higher absorptivities and tunable optical gaps, through careful choice of the monomers used in their synthesis. Unfortunately, as reported in several early studies, the efficiency of all-polymer solar cells has been limited by the presence of charge blocking resistances in the polymer:polymer BHJ layers on account of an intimate mixing of electrondonating and electron-accepting polymers.35 In addition, the relatively low electron mobility of conventional electronaccepting polymers, including benzothiadiazole-containing polymers, was one of the major hurdles in all-polymer solar cells.31−33,35 Hence various attempts including chemical doping have been explored, resulting in improvements in the efficiency of all-polymer solar cells.36−40 In recent years, encouraging PCE improvement up to ca. 5−6% has been achieved by employing electron-accepting conjugated polymers with naphthalene diimide (NDI) and/or perylene diimide (PDI) moieties, which feature high electron mobilities and appropriate electron affinities when paired with various “donor” polymers.41−45 In addition to the PDI-based polymers, PDI-based small molecules have been also introduced as an electron acceptor.46 However, a triple bond-conjugated PDI polymer, which has alkyne linkers leading to planar and rigid 1D nanostructures leading to a high electron mobility of 10−1 cm2/ V·s in field-effect transistors as reported in our previous study,47
has not been examined as an electron-accepting component in all-polymer solar cells. In this work, we have attempted to use a triple bondconjugated PDI polymer, poly{[N,N′-dioctylperylene-3,4,9,10bis(dicarboximide)-1,7(6)-diyl]-alt-[(2,5-bis(2-ethylhexyl)-1,4phenylene)bis(ethyn-2,1-diyl]} (PDIC8-EB), as an electronaccepting component in all-polymer solar cells, because PDIC8EB has a tendency to self-assembly driven by strong interchain interactions leading to 1D nanostructures that may help a desirable nanoscale phase segregation in polymer:polymer BHJ layers. To examine the performance of PDIC8-EB as an electron-accepting polymer, two electron-donating polymers, poly[4,8-bis[(2-ethylhexyl)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 poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-3fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th), were employed because of their suitable energy band structures compared to PDIC8-EB as well as high performances reported recently.48−51
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RESULTS AND DISCUSSION As shown in Figure 1a, PDIC8-EB consists of PDI and phenyl units, which are linked by the triple bond unit of ethyne group B
DOI: 10.1021/acssuschemeng.5b00732 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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intensity of the PTB7:PDIC8-EB film was remarkably lower than that of the pristine films (see Figure S3). The similar PL quenching was measured for the PTB7-Th:PDIC8-EB film (see Figure 2b). A close look into the change of PL intensity provides that the degree of PL quenching was slightly higher for the PTB7-Th:PDIC8-EB film (98.2%) than the PTB7:PDIC8EB film (97.7%). This result suggests that PTB7-Th is better than PTB7 as an electron donor in terms of charge separation in the BHJ films with PDIC8-EB as an electron acceptor, which may be partly attributable to the slightly higher LUMO energy level of PTB7-Th (see Figure 1e). On the basis of the information on energy band structure and PL quenching, all-polymer solar cells with the device structure (ITO/ZnO/BHJ/MoO3/Ag) designed in Figure 1d were fabricated and their performances were examined under 1 sun condition. As shown in Figure 3a, a reasonable current
so that the PDI units can rotate relatively easily to make planar alignment leading to better interchain stacking (see Figure S1), compared to conventional PDI polymers without the triple bond unit.40,44,52−54 In a similar sense, PTB7 and PTB7-Th can have a distorted molecular structure owing to the steric hindrance between comonomer units (see Figure S2). However, optical absorption spectra of pristine films in Figure 1b disclose that PDIC8-EB has weaker absorption in deep red and/or near-infrared parts absorption spectrum relative to PTB7 and PTB7-Th. Here it is noted that the absorption of PTB7-Th was red-shifted by ca. 30 nm from that of PTB7 due to the extended conjugation by the presence of additional thiophene rings in the benzodithiophene (BDT) unit. Interestingly, as shown in Figure 1c, the ionization potential of PTB7-Th was marginally (by ∼0.1 eV) higher than that of PTB7, whereas a significantly higher ionization potential was measured for PDIC8-EB. These ionization potentials corresponds roughly to the highest occupied molecular orbital (HOMO) energies of 5.1 (PTB7), 5.2 (PTB7-Th), and 6.1 eV (PDIC8-EB), after calibration.45 Finally, the energy of the LUMO for each polymer was calculated as 3.5 (PTB7), 3.6 (PTB7-Th), and 4.3 eV (PDIC8-EB) by subtracting optical band gaps, 1.6 (PTB7), 1.6 (PTB7-Th), and 1.8 eV (PDIC8EB), from the HOMO energy values. Considering the LUMO energy offset between PDIC8-EB and PTB7 or PTB7-Th, an efficient charge separation is expected in the inverted-type allpolymer solar cells (see Figure 1d and e). Note that zinc oxide (ZnO) electron-collecting buffer layers were coated on the indium−tin oxide (ITO) bottom electrodes, while molybdenum oxide (MoO3) hole-collecting buffer layers were deposited on the BHJ layers just before forming silver (Ag) top electrodes (see Figure 1d). The possibility of efficient charge separation between PDIC8-EB and PTB7 or PTB7-Th components in polymer:polymer BHJ films has been examined by photoluminescence (PL) quenching experiments. As shown in Figure 2a, the PL
Figure 3. Current density−voltage (J−V) curves for all-polymer solar cells with two different BHJ layers (PTB7:PDIC8-EB and PTB7Th:PDIC8-EB) under a simulated solar light (air mass 1.5 G, 100 mW/cm2) (a) and in the dark (b).
density−voltage (J−V) shape was measured for both devices. However, a higher short circuit current density (JSC) was obtained for the PTB7-Th:PDIC8-EB solar cells than for the PTB7:PDIC8-EB solar cells, while the open circuit voltage (VOC) was quite similar (see Table 1). The resulting PCE was Table 1. Summary of Solar Cell Performances for AllPolymer Solar Cells with the Polymer:Polymer BHJ Layers (PTB7:PDIC8-EB and PTB7-Th:PDIC8-EB)a VOC (V) JSC (mA/cm2) FF (%) PCE (%) RS (kΩ·cm2) RSH (kΩ·cm2)
Figure 2. Photoluminescence (PL) spectra of pristine polymer and BHJ films: (a) PTB7 and PTB7:PDIC8-EB BHJ films (excitation wavelength = 690 nm) and (b) PTB7-Th and PTB7-Th:PDIC8-EB BHJ films (excitation wavelength = 700 nm). Note that the PL intensity was normalized by the film thickness.
a
C
PTB7:PDIC8-EB
PTB7-Th:PDIC8-EB
0.71 7.92 50.1 2.81 0.40 4.10
0.70 9.98 51.0 3.58 0.12 13.31
All data were extracted from the light J−V curves in Figure 3a. DOI: 10.1021/acssuschemeng.5b00732 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering 3.58% and 2.81% for the PTB7-Th:PDIC8-EB solar cells and the PTB7:PDIC8-EB solar cells, respectively. The pronounced PCE difference can be mainly attributed to the JSC difference because other parameters including fill factor (FF) were quite similar. The higher JSC for the PTB7-Th:PDIC8-EB solar cells can be supported by lower series resistance (RS = 0.12 kΩ·cm2) compared to RS = 0.4 kΩ·cm2 for the PTB7:PDIC8-EB solar cells (see the dark J−V curves in Figure 3b). In addition, the higher shunt resistance (RSH = 13.3 kΩ·cm2 for the PTB7Th:PDIC8-EB solar cells and RSH = 4.1 kΩ·cm2 for the PTB7:PDIC8-EB solar cells) might contribute to the higher PCE for PTB7-Th:PDIC8-EB solar cells (also note that high rectification ratio of >104 for the PTB7-Th:PDIC8-EB solar cells in Figure 3b). As shown in Figure S4 and Table S1, the performance of PTB7-Th:PDIC8-EB solar cells became slightly more poor when the content of PDIC8-EB was increased or decreased from the PTB7-Th:PDIC8-EB = 5:5 composition. As shown in Figure 4, external quantum efficiency (EQE) spectra show that the monochromatic efficiency of the PTB7-
Figure 5. TEM images for the BHJ films: (a) PTB7:PDIC8-EB and (b) PTB7-Th:PDIC8-EB. Note that the magnification was 4500 times for the left images and 32 000 times for the right images. “PDIC8-EB NP” denotes the nanoparticles formed with PDIC8-EB (electronaccepting polymer), which are marked with circles.
PTB7-Th:PDIC8-EB film showed very small nanodomains (ca. 20 nm) that we suggest are due to PDIC8-EB given its demonstrated ability to self-assemble into highly order structures.46 This result reflects the fact that PTB7-Th is more compatible with PDIC8-EB than PTB7 in the present fabrication condition. Hence, the larger PDIC8-EB nanodomains in the PTB7:PDIC8-EB film may be responsible for the lower JSC for the PTB7:PDIC8-EB solar cells, because the larger the size of nanodomains, the smaller the interfacial area between electron-donating and electron-accepting components. The surface of the BHJ films was further investigated with atomic force microscopy (AFM) measurements. The surface of the PTB7:PDIC8-EB film was too rough to get proper AFM images so that the middle part among big nanodomains (>300 nm) was focused by adjusting the AFM tip (see Figures 6 and S6). The height-mode AFM image (Figure 6a left) measured randomly distributed nanoparticle-like domains with various sizes (ca. 20−200 nm) in the PTB7:PDIC8-EB film (see also the phase-mode image in Figure 6a right), which indicates poorly a defined nanomorphology leading to the low device efficiency. However, the PTB7-Th:PDIC8-EB film showed evenly distributed nanodomains (ca. 20 nm) (Figure 6b), which is consistent with the TEM image (Figure 5b) and supports well-controlled nanomorphology leading to the high PCE. Finally, the nanostructure of polymer:polymer BHJ films was investigated with synchrotron grazing incidence X-ray diffraction (GIXD) measurements. As shown in the 2D GIXD
Figure 4. External quantum efficiency (EQE) spectra of all-polymer solar cells with two different BHJ layers (PTB7:PDIC8-EB and PTB7Th:PDIC8-EB). (inset) Corresponding optical absorption spectra (absorption coefficient) of the BHJ layers.
Th:PDIC8-EB solar cells was higher than that of the PTB7:PDIC8-EB solar cells over the entire wavelength measured this work. This result supports the higher JSC for the PTB7-Th:PDIC8-EB solar cells. In particular, the shape of the two EQE spectra was quite similar to that of optical absorption spectra for corresponding BHJ films (see Figure 4 inset), indicative of photocurrent generation from both electron-donating (PTB7 or PTB7-Th) and electron-accepting (PDIC8-EB) components. It is worth to noting that the EQE at the wavelength between 370 and 550 nm should be far lower than the measured values, if the PDIC8-EB component did not contribute to the photocurrent generation (see Figure S5). This result does indeed support that photocurrents can be generated by charge separation from the excitons formed in the electronaccepting component (PDIC8-EB) of the BHJ layers. To understand the different solar cell performance according to the electron-donating polymers (PTB7 and PTB7-Th), the nanostructure of BHJ films was investigated with transmission electron microscopy (TEM) measurements. As shown in Figure 5a, nanoparticle-like PDIC8-EB domains with a diameter of ca. 300−500 nm were measured in the PTB7:PDIC8-EB film. In contrast, as shown in Figure 5b, the D
DOI: 10.1021/acssuschemeng.5b00732 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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CONCLUSIONS
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METHODS
A triple bond-conjugated PDI polymer (PDIC8-EB) was introduced as an electron-accepting component in all-polymer solar cells, while two similar electron-donating polymers (PTB7 and PTB7-Th) were employed to make polymer:polymer BHJ layers. The PTB7-Th:PDIC8-EB solar cells showed higher PCE (3.58%) than the PTB7:PDIC8-EB solar cells, which was attributed to the higher JSC thanks to better light harvesting property (light absorption at longer wavelengths) of PTB7-Th. Further insight into the origin of the higher PCE was provided by TEM, AFM, and GIXD measurements, which confirmed the well-defined nanodomain morphology for the PTB7Th:PDIC8-EB (BHJ) film, compared to poorly developed nanomorphology for the PTB7:PDIC8-EB film, which is of importance to secure high charge separation/transport characteristics. Finally, it is suggested that thienyl side chains (PTB7-Th) are more compatible and give better morphology with PDIC8-EB than just alkoxy side chains (PTB7).
Materials and Solutions. PTB7 (Mw = 92 kDa and PDI = 2.6) and PTB7-Th (Mw = 126 kDa and PDI = 2.5) were purchased from 1Material (Canada) (note that Mw and PDI denote weight-average molecular weight and polydispersity index, respectively). PDIC8-EB (Mw = 67 kDa and PDI = 4.2) was synthesized as reported in our previous study.46 Binary blend (PTB7:PDIC8-EB and PTB7Th:PDIC8-EB = 5:5 by weight) solutions were prepared using cosolvents of chlorobenzene (CB) and chloroform (CF) (CB:CF = 1:1 by volume) at a solid concentration of 10 mg/mL. These solutions were vigorously stirred at room temperature for 12 h prior to spincoating. ZnO precursor solutions were prepared by dissolving zinc acetate dihydrate (1 g) in cosolvent of 2-methoxyethanol (10 mL) and ethanolamine (0.28 mL), followed by stirring at 60 °C for 3 h and at room temperature overnight prior to spin-coating. Thin Film and Device Fabrication. Prepatterned ITO-coated glass substrates were cleaned with ultrasonication processes in acetone and isopropyl alcohol, followed by drying with a nitrogen blow. The wet-cleaned ITO-glass substrates were subject to the UV-ozone treatment for 20 min. Then ZnO precursor films were spin-coated on the ITO-glass substrates and annealed at 200 °C for 1 h to make 30 nm-thick ZnO layers, which were transferred into a nitrogen-filled glovebox. The 60 nm-thick BHJ layers were spin-coated onto the ZnO-coated ITO-glass substrates and dried inside the glovebox for 20 min, which were transferred to a vacuum chamber installed in the glovebox. Next, 10 nm-thick MoO3 layers and 80 nm-thick Ag top electrodes were subsequently deposited onto the BHJ layers at a pressure of 2 × 10−6 Torr. The active area of final devices was 0.09 cm2. All samples for morphology and nanostructure measurements were prepared in the same way as for the device fabrication, while quartz substrates were used for the preparation of thin films for optical measurements. Measurements. The film thickness was measured using a surface profiler (Alpha Step 200, Tencor Instruments). Optical absorption and photoluminescence (PL) spectra were measured using a UV−visible spectrometer (Optizen 2120, MECASYS) and a PL spectrometer (FS2, SCINCO), respectively. The ionization potential of pristine films was carried out using a photoelectron yield spectrometer (AC2, RikenKeiki). An atomic force microscope (AFM, Nanoscope IIIa, Digital Instruments) was used for the measurement of surface morphology. The crystalline structure was measured using a synchrotron radiationgrazing incidence X-ray diffraction (GIXD) system (X-ray wavelength = 0.1213 nm, incidence angle = 0.12°, 3C SAXS I beamline, Pohang Accelerator Laboratory), and a field-emission transmission electron microscope (FE-TEM, Titan G2 ChemiSTEM Cs Probe, FEI Company). The J−V characteristics of devices were measured using a specialized solar cell measurement system equipped with an electrometer (Keithley 2400) and a solar simulator (100 mW/cm2,
Figure 6. AFM images (left height-mode, right phase-mode) for the BHJ layers coated on the ZnO/ITO-glass substrates: (a) PTB7:PDIC8-EB and (b) PTB7-Th:PDIC8-EB. Note that nanocluster-like domains are marked with circles.
image for the pristine PDIC8-EB film (Figure 7a), a broad (100) diffraction ring was only measured in the out-of-plane (OOP) direction but three distinctive Debye rings ((100), (001), and (002)) were measured in the in-plane (IP) direction. This result indicates that the PDIC8-EB polymer chains are more likely to be better ordered in the direction parallel to the film plane, which is clearly observed from the 1D diffractograms in Figure 7b. In the case of the pristine PTB7 and PTB7-Th films, the diffraction rings were relatively more distinctive in the IP direction than the OOP direction because of their face-on stacking structures as reported previously.55,56 The d-spacing for the (100) diffraction was slightly larger for the pristine PTB7-Th film (23.71 Å) than the pristine PTB7 film (19.95 Å) (see Table S2). Looking into the 1D diffractograms of the BHJ films in Figure 7b, the major characteristic diffraction peaks of the pristine PDIC8-EB film are found in the BHJ films as well, implying that both PDIC8EB and PTB7 or PTB7-Th components were not intimately mixed but phase-segregated as supported from the TEM and AFM images (see Figures 5 and 6). However, more pronounced diffractions were measured for the PTB7Th:PDIC8-EB film than the PTB7:PDIC8-EB film, which can be attributable to the well-defined (controlled) nanodomain morphology for the PTB7-Th:PDIC8-EB film (leading to high PCE) but the poorly developed nanomorphology (random and broad distribution of various sized nanodomains) for the PTB7:PDIC8-EB film (leading to low PCE). E
DOI: 10.1021/acssuschemeng.5b00732 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 7. 2D GIXD images (a) and 1D GIXD profiles (b) for the pristine and BHJ layers coated on the ZnO/ITO-glass substrates. Note that “D1”, “D2”, and “A” stand for PTB7, PTB7-Th, and PDIC8-EB, respectively. The geometry for GIXD measurement is illustrated in part a: IIN and θIN are the incident synchrotron X-ray beam and the incident angle with respect to the film surface, respectively; qxy and qz represent scattering vectors (q) in the direction of in-plane (IP) and out-of-plane (OOP), respectively.
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model 92250A-1000, Newport-Oriel). The EQE spectra of devices were measured using a home-built EQE rig with a light source (Tungsten-Halogen lamp, 150W, ASBN-W, Spectral Products), a monochromator (CM110, Spectra Products), and a calibrated (certified) Si-photodiode.
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ACKNOWLEDGMENTS This work was financially supported by grants from the Korean Government (NRF_2015R1A2A2A01003743, Basic Science Research Program_2009-0093819, NRF2014R1A6A3A03055861, NRF_2014R1A1A3051165, Basic Research Laboratory Program_2011-0020264, Human Resource Training Project for Regional Innovation_MOE (NRF_2014H1C1A1066748)) and the US Office of Naval Research through support of the MURI Center for Advanced Organic Photovoltaics. In addition, S.G.H. and M.R. appreciate financial supports from the National Research Foundation (NRF) of Korea (Doyak Program 2011-0028678), the Ministry of Education, and the Ministry of Science, ICT & Future Planning (MSIP) (BK21 Plus Program and Global Excel Program). S.N. and Y.K. thank Prof. Donal D. C. Bradley for help with experiments in London.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00732. Summary of PTB7-Th:PDIC8-EB solar cell performances according to compositions and d-spacing values, 2D chemical and 3D energy minimized structures of polymers, PL spectra, J−V curves of PTB7-Th:PDIC8EB solar cells according to composition ratio, absorption coefficient and EQE spectra of all-polymer solar cells, and illustration for the AFM measurement zone from the TEM image (PDF)
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REFERENCES
(1) Scharber, M. C.; Sariciftci, N. S. Efficiency of bulk-heterojunction organic solar cells. Prog. Polym. Sci. 2013, 38, 1929−1940. (2) Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Organic ternary solar cells: A Review. Adv. Mater. 2013, 25, 4245−4266. (3) Kim, H.; Nam, S.; Jeong, J.; Lee, S.; Seo, J.; Han, H.; Kim, Y. Organic solar cells based on conjugated polymer: History and recent advances. Korean J. Chem. Eng. 2014, 31, 1095−1104. (4) Wu, J.-S.; Cheng, S.-W.; Cheng, Y.-J.; Hsu, C.-S. Donor-acceptor conjugated polymers based on multifused ladder-type arenes for organic solar cells. Chem. Soc. Rev. 2015, 44, 1113−1154. (5) Facchetti, A. Polymer donor−polymer acceptor (all-polymer) solar cells. Mater. Today 2013, 16, 123−132.
AUTHOR INFORMATION
Corresponding Author
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[email protected]. Tel: +82-53-950-5616. Fax: +8253-950-6615. Author Contributions #
S.N. and S.G.H. contributed equally to this work.
Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acssuschemeng.5b00732 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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