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Enhancing the efficiency of polymer solar cells by incorporation of 2,5-difluorobenzene units into the polymer backbone via random copolymerization Zhe Zhang, Yahui Liu, Jicheng Zhang, Shiyu Feng, Liangliang Wu, Xue Gong, Xinjun Xu, Xuebo Chen, and Zhishan Bo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05787 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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ACS Applied Materials & Interfaces

Enhancing the efficiency of polymer solar cells by incorporation of 2,5-difluorobenzene units into the polymer backbone via random copolymerization Zhe Zhang,a,† Yahui Liu,b,† Jicheng Zhang,b Shiyu Feng,b Liangliang Wu,b Xue Gong,b Xinjun Xu,b,* Xuebo Chen,b,* Zhishan Bob,*

a

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key

Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China.

b

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry,

Beijing Normal University, Beijing 100875, China. Tel: +86-10-62206891.

Abstract

A series of conjugated polymers P0, P5 and P7 containing 0 mol%, 5 mol% and 7 mol% 2,5difluorobenzene units, respectively, were prepared and utilized as electron donors in polymer solar cells. Incorporation of a small amount of 2,5-difluorobenzene unit into the backbone of donor polymers can significantly increase their planarity and crystallinity as well as decrease their solubility. The improved molecular conformation can markedly affect the morphology of polymer:PC71BM blend films. After incorporation of 5 mol% 2,5-difluorobenzene unit into the

1 Environment ACS Paragon Plus


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backbone of donor polymers, the domain size of blend films became smaller and the hole mobility increased. Increasing the content of 2,5-difluorobenzene to 7 mol% can further decrease the solubility of resulting polymers and resulted in poor solution processability. As a result, P5 based devices achieved a power conversion efficiency (PCE) of 8.5%; whereas P0 based devices gave a PCE of 7.8%.

Keywords conjugated polymers, polymer solar cells, 2,5-difluorobenzene, fluorinated benzothiadiazole, random copolymerization, crystallinity.

Introduction Solar energy is the best choice among renewable energy sources that can independently meet the energy needs of industrialized society and may replace fossil energy sources.1-3 Polymer solar cells (PSCs) have potential applications due to various virtues, for example, lightweight, flexibility, solution-processable, and large-area manufacturing.4-10 For the sake of elevating power conversion efficiency (PCE) of PSCs, many methods have been dedicated to the preparation of various novel donor polymers.11-14 One of the most successful approaches is the widely adopted donor-acceptor strategy for polymer design,15-26 where electron accepting (A) and donating (D) units are arranged in a perfectly alternating way to achieve low band-gap polymers. Most of D-A alternating copolymers focus on the 1D-1A (one donor unit and one acceptor unit) polymers.16 However, sometimes fine tuning of the energy levels, light absorption, and molecular packing in such 1D-1A copolymers is hard to achieve. To overcome the limitation of 1D-1A type copolymers, random copolymers which consist of two different monomers


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including 2D-1A (two D and one A) or 1D-2A (one D but two A) type have been developed. However, a third component in random copolymers sometimes does harm to the device performance. For example, the incorporation of planar units such as anthracene, and pyrene

29

27

perylene

28

has been demonstrated to enhance the π-π interaction of polymers, but the

dissimilar chemical structure of the third unit could cause an adverse effect on molecular packing and film morphology. To eliminate this negative effect, the incorporation of structurally similar third component into the polymer backbone has rarely been investigated. 30-34 Fluorine substitution on the polymer main chain is a successful strategy to improve the PCE of PSCs.35, 36 Because F has the highest electronegativity among all elements, fluoro substituted polymers usually show lower HOMO levels without drastic changing their bandgap. Consequently, fluorinated polymer based devices can achieve higher Voc value than nonfluorinated ones without sacrificing Jsc. Furthermore, fluorinated polymers are found to exhibit better oxidative stability.37-39 Lastly, improved inter/intramolecular interaction of polymer chains arisen from strongly induced dipole in O···F, S···F, or C···F bonds can enhance the π-π stacking of polymers, leading to increased charge mobility and forming an ideal fibrous structure in the blend thin-film of PSCs, thus helping in the improvement of fill factor (FF) and Jsc.23, 40-43 For instance, Kim and Woo et al. reported some dialkoxyphenylene-benzothiadiazole containing hemicrystalline small band-gap polymers (PPDTBT, PPDTFBT and PPDT2FBT). Noncovalent conformational locking (through C···F, O···F, or S···F bonds) were used to incresae planarity of the molecular backbone, intermolecular orderly staking and heat resistance of polymers without sacrificing the solubility. As a result, highly efficient PSCs with a thick-film single-junction configuration were fabricated.23



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Herein, we prepared three semiconducting copolymers P0, P5 and P7, comprising 0%, 5% and 7% mol. % of 2,5-difluorobenzene units in the polymer, respectively. These three polymers as shown in Scheme 1 serve as donors for PSCs. By random introduction of 2,5-difluorobenzene into the backbone of polymers, the planarity and crystallinity of polymers increase and their solubility decreases. The change of these factors can effectively tune the morphology of polymer:PC71BM blend films. As a result, P5:PC71BM blend films showed improved morphology with decreased domain size and well-developed fibrils network. P5 based PSCs demonstrated a PCEmax of 8.50% together with a Jsc of 16.70 mA cm-2, aVoc of 0.78 V, and an FF of 0.66 under AM 1.5G illumination, which showed an improvement relative to the PCE value of 7.80% for the reference polymer P0. Further increasing the content of 2,5-difluorobenzene unit to 7%, the resulting polymer P7 displayed poor solubility, which affords challenges for the preparation of PSCs using solution method.

Scheme 1 Syntheses of P0, P5 and P7. Results and Discussion


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Scheme 1 shows the syntheses of target polymers P0, P5 and P7, which were obtained by microwave heated one-pot Stille polycondensation. P0 is a donor and acceptor alternating conjugated polymer, which was synthesized by polymerization of 2,5-bis(2-hexylnonanyl)-1,4dibromobenzene

(1)

and

5,6-difluoro-4,7-bis(5-(trimethylstannyl)thiophen-2-

yl)benzo[c][1,2,5]thiadiazole (DTBT) (3). To tune the crystallinity and energy levels, 1,4dibromo-2,5-difluorobenzene (2) was used to partially replace of 1 in the polymerization. P5 and P7, which comprise 5% and 7% 2,5-difluorobenzene unit in the backbone of polymers, respectively, were obtained by copolymerization of three coponents (1, 2 and 3). These copolymers were achieved as black solids in 68% to 82% yields after standard purification. The molecular weight of polymers was measured by gel permeation chromatography (GPC) in 80 oC with chlorobenzene as a fluent phase and narrowly distributed polystyrenes as reference standards. The number-average molecular weights (Mn) for P0, P5 and P7 are 21, 19, and 14 kDa, respectively, and their polydispersity indices (PDI) are 2.25, 1.86 and 2.14, respectively. Under a nitrogen atmosphere, heat resistance of polymers P0, P5 and P7 were investigated by thermogravimetric analysis (TGA). The heating rate is 10 °C min-1. All these polymers showed the same 5% weight loss temperature at 386 °C (see Fig. S1), demonstrating a good thermal stability for them. No obvious glass transition for these polymers in the range of 80 to 250 oC was observed in differential scanning calorimetry (DSC) measurements (see Fig. S2). Fig. 1 displays UV-vis absorption characteristics of polymer solutions (in CHCl3) and thin films (spin-casted on quartz substrates). In chloroform solutions, they demonstrated very similar curves in the range of 350 to 700 nm with three absorption bands centered at 407, 592, and 642 nm, respectively. The weak absorption peak at 407 nm comes from the localized π-π transition and the intense absorption peaks at 592 and 642 nm arise from delocalized π-π transitions caused



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by the internal charge transfer (ICT) between electron-donating (bis(2-hexylnonanyl)-benzene and difluorobenzene) and electron-withdrawing DTBT units in the polymer backbone.16 The absorption spectra of P0, P5 and P7 as thin films are exactly the same shape like their solution ones. As films, these three polymers exhibited broader absorption spectra with the three absorption peaks red-shifted to 417, 598 and 652 nm, respectively, because of the π-π interaction of polymer backbones in the thin-film state.44 P0, P5 and P7 exhibited the identical bandgap of 1.75 eV calculated according to the equation: Eg,opt=1240/λonset. The electrochemical characteristics of P0, P5 and P7 were tested by cyclic voltammetry (CV) using the classic threeelectrode cell and the curves are presented in Fig. 2a. According to the formula EHOMO = -e(Eox + 4.71), HOMO levels of P0, P5 and P7 were determined to be -5.35, -5.38 and -5.39 eV, respectively, indicating that the increase of the fluorine atom number will reduce the HOMO energy level of polymers, being helpful for improving Voc. The LUMOs of P0, P5 and P7 were determined (by the equation: ELUMO = Eg,opt + EHOMO) to be −3.60, −3.63 and −3.64 eV, respectively. To make a clear comparison, energy diagrams of all materials employed for the fabrication of devices are listed in Fig. 2b. A LUMO level offset of 0.7 eV between polymers and PC71BM could guarantee an efficient exciton dissociation at their interface. The corresponding physical, electrochemical and optical parameters are listed in Table 1. Table 1. Physical, Optical and Electronic Properties of P0, P5 and P7. Polymer

Mn [kg/mol]

PDI

T da (oC)

λb (nm) solution

λb(nm) film

Eg,optc (eV)

HOMO (eV)

LUMO (eV)

P0

21

2.25

386

407, 592, 642

417, 598, 652

1.75

-5.34

-3.59

P5

19

1.86

386

407, 592, 642

417, 598, 652

1.75

-5.36

-3.61

P7

14

2.14

386

407, 592, 642

417, 598, 652

1.75

-5.36

-3.61

a

Td is the temperature for losing 5% weight in N2.

b

Maximum absorption wavelength.


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Determined by the absorption edge (λonset) of P0, P5 and P7, Eg,opt = 1240/λonset.

1.0

b 1.0

P0 P5 P7

0.8

Absotbance (a.u.)

a

Absotbance (a.u.)

c

0.6 0.4 0.2 400

500 600 700 Wavelength (nm)

P0 P5 P7

0.8 0.6 0.4 0.2 0.0

0.0

800

400

500 600 700 Wavelength (nm)

800

Fig. 1 UV-visible absorption curves of P0, P5 and P7. (a) CHCl3 solutions; (b) films (spin-cast). a 0.05

0.04 Current (mA)

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0.03

P0 P5 P7

0.02 0.01 0.00

-0.01 -0.02 0.0

0.2 0.4 0.6 + Potential (V vs Ag/Ag )

0.8

Fig. 2 Cyclic voltammetry plots of P0, P5 and P7 thin films (a) and energy levels of all materials (b).



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Fig. 3 Optimized geometries of 2, 5-bis(alkoxy)benzen (a) and 2,5-difluorobenzene (b) based molecular backbone by DFT at the B3LYP/6-31G(d) level. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level were also used to investigate the geometries of polymer main chains and the dihedral angles between 2,5-bis(alkoxy)benzene and thiophene (a) as well as 2,5-difluorobenzene and thiophene (b) shown in Fig. 3. For simplicity, methoxy groups are utilized to replace the alkoxy side chains. The dihedral angle between thiophene unit and 2,5-difluorobenzene unit is 0.1o in P5 and P7; whereas the dihedral angle between thiophene unit and 2,5dialkoxylphenylene unit is 16.9o. It could be clearly seen from side view that polymer backbone containing 2,5-difluorobenzene has a planar conformation, which is beneficial to strengthen intermolecular π-π interactions and will further improve the charge transport. Moreover, the closer of the π-π stacking, the more beneficial for the formation


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of nanofibrils and interpenetrating networks for the blend films. To calculate ground state excitation energies, the time-dependent DFT (TD-DFT) was employed. The performance of TD-DFT depends on the approximate exchange and correlation functional (xcfunctional) and the B3LYP/6-31G(d) level basis set. For the fragment composed of 2,5difluorobenzene and thiophene (Fig. 3b), the calculated absorption wavelength is 621 nm contributed from π−π* transition, with theoretical oscillator strength f =1.49 and calculated excitation energy of 2.00 eV. X-ray diffractions (XRD) of P0, P5 and P7 films were measured to inquire into the packing and crystallinity of polymers in the thin-film state and the corresponding curves are displayed in Fig. 4. The diffraction peaks, reflecting the π−π stacking distance between conjugated main chains, are located in the wide angle region at 2θ of 24.46° for P0, 24.74° for P5 and 24.94° for P7, corresponding to a 3.64, 3.59 and 3.57 Å distance, respectively. As can be seen from X-ray diffraction results, the π-π stacking distance decreases after the incorporation of 2,5difluorobenzene unit into the polymer main chain, which is beneficial for hole transport in the solid state. The X-ray diffraction peak of P5 and P7 in the high-angle region became more acute than P0, reflecting that the crystallinity of polymers is improved when 2,5-difluorobenzene is incorporated into the polymer backbone, which is beneficial to enhance the PCE of PSCs. As expected, P5 based devices gave the best photovoltaic performance. Further increasing the content of 2,5-difluorobenzene unit, the solubility and molecular weight of polymers decreased. Therefore, the PCE of P7 based devices gave slightly reduced PCE in comparison with P5 (vide infra). 34, 45-48



30000 P0 P5 P7

25000 20000

P0 P5 P7

4000

Intensity

Intensity

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15000

2000

25

10000

2 Theta(Degree)

5000 5

10 15 20 2 Theta(Degree)

25

Fig. 4 XRD curves of P0, P5 and P7 Films. To assess their photovoltaic performances, PSCs based on these random copolymers were fabricated with a structure of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al. Chlorobenzene (CB) was used as the solvent for device preparation and diphenyl ether (DPE) as the additive. The composition (D:A ratio) and thickness of active layer and the content of additive were systematically screened. For all polymers, the optimized D:A ratio is 1:1.5 (w/w) and the content of additive is 2.0wt% DPE. The optimized thickness is about 220, 220 and 240 nm for P0, P5 and P7, respectively. The current density-voltage (J-V) curves of PSCs are illustrated in Fig. 5a and the results are listed in Table 2. The best PCEs of P0, P5 and P7 are 7.80%, 8.50% and 8.24%, respectively. The trend of Voc is reasonable for P0, P5 and P7, because the HOMO energy level is reduced when increasing the content of 2,5-difluorobenzene. The Voc is proportional to the offset between HOMO energy level of donor material and LUMO energy level of electron-accepter material (PC71BM), polymers with deeper HOMO energy level should exhibit higher Voc. For P0, P5 and P7, the HOMO energy levels do not show much differences, so their Voc are almost the same. The Jsc of P5 and P7 are higher than that of P0, since P5 and P7 have higher space charge limited current (SCLC) and better active layer morphology than P0, the


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detailed analysis will be described in terms of hole mobility and film morphology. The PCE of P0 (7.8%) is lower than that of PPDT2FBT reported in the references ( 8.64% in Ref. 23 and 8.28% in Ref. 41). The reason is that the Mn value of our polymer P0 (21.0 k) is lower than that reported in references (42.6 k). External quantum efficiency (EQE) characterizations were carried out to examine the response of devices to monochromatic light and the Jsc is obtained by the integration of J-V curves. As demonstrated in Fig. 5b, devices of the three polymers exhibited similar EQE shape ranging from 300 to 750 nm, in which the response before 450 nm is due to the absorption of PC71BM and the response in long wavelengths (after 450 nm) comes from the absorption of polymers. The highest EQE value of P5 and P7 is close to 80%, which is quite high for PSCs and much higher than that of P0. The Jsc values calculated by the integration of EQE curves were consistent to those acquired by J-V measurements. Table 2. Photovoltaic Parameters and SCLC Mobilities of Polymer:PC71BM Blend Films Polymer a

Voc(V)

Jsc (mA cm-2)

FF

0.768 15.79 0.636 ±0.003 ±0.19 ±0.009 0.773 16.57 0.654 P5 ±0.002 ±0.156 ±0.011 0.775 16.50 0.630 P7 ±0.003 ±0.248 ±0.015 a Dissolved in CB with 2% DPE, by volume; P0

b

PCE (%) best/ave b 7.80 7.69±0.099 8.50 8.38±0.100 8.24 8.08±0.160

The average PCEs (based on 10 devices).


Thickness (nm)

Mobility (cm2 v-1 s1 )

220

7.62×10-4

220

2.18×10-3

224

9.08×10-4


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Fig.5 (a) J−V and (b) EQE curves of P0, P5 and P7 based devices.

Fig.6 AFM images (5 µm ×5 µm) of polymer (P0, P5, or P7):PC71BM blend films fabricated under optimized conditions.


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Fig.7 TEM images of polymer (P0, P5, or P7):PC71BM blend films fabricated under optimized conditions. The hole mobility in devices are obtained by measuring the J-V curve of hole-only device in dark using SCLC method (Fig. S3, Table 2). The photovoltaic devices of P5 and P7 showed higher hole mobilities than P0-based devices. The hole mobilities of P0, P5 and P7 were determined to be 7.62×10-4, 2.18×10-3 and 9.08×10-4 cm2 v-1 s-1, respectively. Generally, the higher hole mobility, the larger Jsc of PSCs. The SCLC mobility trends are quite in accordance to the tested Jsc and PCEs. The morphology of blend films fabricated under optimized conditions was probed by atomic force microscopy (AFM) in tapping mode and transmission electron microscopy (TEM). Fig. 6 and Fig. 7 show the corresponding images, respectively. The morphologies of P0:PC71BM, P5:PC71BM and P7:PC71BM blend films are of big difference. P5 has an optimal balance between the crystallinity and the solubility, so the blend film shows a well-developed and interconnected network structure. The optimal morphology is beneficial for charge carrier generation and transportation. As for the blend film of P0:PC71BM, larger aggregates have been observed. The reason is P0 has a better solubility in comparison with P5, PC71BM is prone to precipitate first and form larger aggregates when the film is dried. As for P7, it displayed much poorer solubility, leading to apparent phase separation when blending with PC71BM.49 The



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marked morphology difference of the blend films showed an affect on the photovoltaic device performance. So the AFM and TEM results can well explain Jsc and FF of these three polymersbased devices. Conclusions We have developed an effective way of tuning the film morphology by incorporation of 2, 5difluorobenzene for the first time as the third component into the backbone of D-A alternating polymers. Three conjugated polymers P0, P5 and P7 that containing 0%, 5% and 7% 2,5difluorobenzene units, respectively, have been synthesized by Stille polycondensation. The incorporation of 2, 5-difluorobenzene can improve the flatness and crystallinity of polymers and decrease their solubility. The content of 2,5-difluorobenzene unit in the polymers showed an apparant impact on the morphology of polymer:PC71BM blend films. As observed by AFM, the domain size of polymers in the blend film significantly decreased after the incorporation of 5% 2, 5-difluorobenzene unit. Further increasing the content of 2,5-difluorobenzene could further reduce the solubility of polymers and made the polymers not suitable for solution processing. The formation of a fibrous network for the P5:PC71BM blend film could facilitate the vertical charge transport, leading to higher Jsc for the devices. As a result, the PCE of P5 (8.50 %) is improved relative to P0 (PCE: 7.80%). Experimental Section Compound 1 was synthesized via the reported method.23,50 Monomer 2 was purchased from commercially suppliers and recrystallized from n-hexane in at room temperature and least twice before using. Monomer 3 was acquired from Derthon Optoelectronic Materials Science Technology Co LTD and the purity is 98%.


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Synthesis of Polymers The three polymers were prepared by microwave heated one-pot Stille polycondensation. For the synthesis of P5, a mixture of 2,5-dialkoxylphenylene (0.20000 g, 0.279 mmol), 2,5difluorobenzene (0.00400 g, 0.015 mmol), 4,7-bis(5-trimethylstannylthiophen-2-yl)-5,6-difluoro2,1,3-benzothiadiazole (0.19500 g, 0.294 mmol), tris(dibenzylideneacetone)dipalladium (0) (4 mol%), tri(o-tolyl)phosphine (8 mol%) was flushed with N2 for 0.5 h, followed by dissolution in CB (1.0 mL) in a glovebox. The polymerization reaction mixture was heated at 180 oC (80 W, 1 h) in a special microwave equipment (Model: DISCOVER SP, CEM Corporation). Then 2(tributylstannyl)thiophene (0.1 mL) and 2-bromothiophene (0.1 mL) were successively added for end-capping in time interval of 20 min at 180 oC. After reaction, the crude P5 was poured into CH3OH (100 mL), and the resulted precipitates were collected by filtration. Then, Soxhlet extraction with methanol, acetone, hexane, dichloromethane and chloroform (each 5 h) was successively performed to remove impurities and oligomers. The residue was dissolved into a small amount of chloroform (about 30 mL) and precipitated into CH3OH (100 mL), the resulting precipitates were collected and dried under vacuum to afford P5 as a dark solid (73%). Anal. Calcd for C52H74F2N2O2S3 (95% mol) and C20H8F4N2S3 (5% mol): C 68.91, H 7.87, N 3.14. Found: C 69.09, H 8.02, N 3.29. P0 and P7 were also synthesized by following the same procedure as P5. P0 was obtained as a dark solid (82%). Anal. Calcd for C52H74F2N2O2S3: C 69.61, H 8.05, N 3.12. Found: C 69.91, H 8.35, N 3.14. P7 was obtained as a dark solid (68%). Anal. Calcd for C52H74F2N2O2S3 (93% mol) and C20H8F4N2S3 (7% mol): C 68.62, H 7.83, N3.31. Found: C 68.76, H 7.81, N 3.36. AUTHOR INFORMATION



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Corresponding Author * E-mail: [email protected], [email protected], [email protected] †

Zhe Zhang and Yahui Liu contributed equally to this work.

Author Contributions This manuscript was written from the contribution of all authors. All have given approval to the manuscript’s final version. †These authors contributed equally to this paper. Notes The authors declare no competing financial interest. Acknowledgment Financial support from the NSFC (91233205, 21574013, and 51673028), Program for Changjiang Scholars and Innovative Research Team in University and the Fundamental Research Funds for the Central Universities are gratefully acknowledged, Innovation Team Basic Scientific Research Project of Gansu Province (1606RJIA324). Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of the materials and instruments, hole mobilities, polymer solar cell fabrication and characterization process, TG and DSC curves. References (1) Armaroli, N.; Balzani, V. Towards an Electricity-Powered World. Energy Environ. Sci. 2011, 4, 3193−3222.


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Enhancing the Efficiency of Polymer Solar Cells by ... - ACS Publications

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