Thermally stable organic bulk heterojunction photovoltaic cells incorporating an amorphous fullerene derivative as an electron acceptor

Solar Energy Materials & Solar Cells 95 (2011) 432–439

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Thermally stable organic bulk heterojunction photovoltaic cells incorporating an amorphous fullerene derivative as an electron acceptor Seul-Ong Kim a,1, Dae Sung Chung b,1, Hyojung Cha b, Jae Wan Jang a, Yun-Hi Kim c, Jae-Wook Kang d, Yong-Soo Jeong d, Chan Eon Park b,n, Soon-Ki Kwon a,n a

School of Materials Science & Engineering and Engineering Research Institute (ERI), Gyeongsang National University, Jin-ju 660-701, Republic of Korea Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea c Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea d Department of Material Processing, Hybride coating Group Korea Institute of Materials Science (KIMS), 531 Changwondaero,Changwon, Gyeongnam 641-831, Korea b

a r t i c l e in f o

a b s t r a c t

Article history: Received 12 March 2010 Received in revised form 9 August 2010 Accepted 10 August 2010 Available online 20 October 2010

A highly soluble amorphous fullerene derivative substituted with dihexylfluorene (DHFCBM) was synthesized and used as an electron acceptor material for P3HT-based bulk heterojunction solar cells. By fitting the experimental J–V curves with space charge limited current equation, the electron mobility of DHFCBM was determined to be 4  10  4 cm2/Vs, possibly leading to balanced charge transport with P3HT. From structural and morphological analysis using X-ray diffraction, UV–vis absorption, and atomic force microscopy, we found that the amorphous nature of DHFCBM stabilized the nanomorphology of P3HT:DHFCBM blend films under high temperature annealing. By optimizing blend ratios and annealing conditions, P3HT:DHFCBM-based solar cells yielded power conversion efficiencies in excess of 3%. In addition, the fabricated cells maintained their initial performances even after high temperature annealing for long times, as predicted from the stable nanomorphology. We believe that the use of thermally stable amorphous fullerene as an electron acceptor can be a promising strategy for commercialization of organic solar cells. & 2010 Elsevier B.V. All rights reserved.

Keywords: Photovoltaics Electron acceptor PCBM Thermal annealing Organic solar cell

1. Introduction Recent research devoted to the development of organic solar cells (OSCs) has produced power conversion efficiencies (PCE) of 5–6% with bulk heterojunction (BHJ) device architectures [1–7]. It has been used for many scale-up process such as roll to roll, screen printing, etc. [8–9]. Although these results are very promising and constitute considerable progress, these relatively low PCEs, together with stability issues, are drawbacks hindering the commercialization of organic solar cells. A great deal of effort has been devoted to the optimization of solar cells using poly(3-hexylthiophene; P3HT) as an electron donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as an electron acceptor [10–13]. This donor–acceptor combination gives rise to good PCEs due to the nanomorphologies achievable under thermal or solvent annealing. However, diffusion and recrystallization of the component materials makes this combination unsatisfactory in terms of stability, especially upon exposure to the high temperatures that are indispensable for solar cell n

Corresponding authors. E-mail addresses: [email protected] (S.-K. Kwon), [email protected] (C.E. Park). 1 Seul-Ong Kim and Dae Sung Chung contributed equally to this work. 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.08.009

operation [14,15]. Recently, several strategies have been reported for suppressing diffusion and crystallization of the P3HT and fullerene components, resulting in stable OSCs. For example, disordered polythiophene [14] and high-Tg polymers [15] have been used to stabilize the nanoscale phase segregation. Modifying crystalline PCBM in such a way as to stabilize the nanomorphology is another stabilization solution; however, those efforts have not been reported frequently. Here, we report a new electron acceptor material, 9,9dihexylfluorenyl C61-butyric acid methyl ester (DHFCBM), which forms a stable nanomorphology when blended with P3HT, thanks to low structural diffusion due to its amorphous nature. Space charge limited current (SCLC) mobility measurements, X-ray diffraction, UV–vis absorption, and atomic force microscopy were employed to investigate the morphological and structural changes of P3HT:DHFCBM blend films under elevated temperatures.

2. Experimental details 2.1. Synthesis of 5-(9,9-dihexyl-fluoren-2-yl)-5-oxopentanoic acid (1) Aluminum chloride (15.9 g, 120 mmol) was added in one portion to glutaric anhydride (8.9 g, 78 mmol) and 9,9-dihexyl-9H-fluorene

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(20.0 g, 60 mmol) in 200 mL of dichloromethane with cooling in an ice–ethanol bath and slowly allowed to warm to room temperature. The reaction mixture was stirred overnight and then quenched with 2 M HCl; the mixture was then extracted into dichloromethane, washed with brine, and dried. The crude product was chromatographed on silica using 30% ethyl acetate in hexane with 1% acetic acid followed recrystallization from hexane. Yield: 26.7 g (41%) as a white solid. 1H NMR (CDCl3, 300 MHz; ppm): aromatic (C–H), 7.97– 8.00 (m, 2H), 7.76–7.80 (m, 2H), 7.34–7.39 (m, 3H); aliphatic (C–H), 3.15–3.20 (t, 2H), 2.56–2.58 (d, 2H), 2.16–2.18 (d, 2H), 1.99–2.04 (m, 4H), 1.04–1.12 (m, 12H), 0.74–0.79(t, 6H), 0.59–0.61(d, 4H). 2.2. Synthesis of 5-(9,9-dihexyl-fluoren-2-yl)-5-oxo-pentanoic acid methyl ester (2) A solution of the acid compound (1) (11.0 g, 20 mmol) in MeOH (120 mL) with catalytic amount of sulfuric acid (96%, five drops) was refluxed for 5 h. The solvent was evaporated, and the residue was extracted into dichloromethane, washed with brine, and dried. The crude product was chromatographed on silica using 2% ethyl acetate in hexane as eluent. Yield: 11.2 g (99%) as a light yellow liquid. 1H NMR (CDCl3, 300 MHz; ppm): aromatic (C–H), 7.97–8.00 (m, 2H), 7.75–7.79 (m, 2H), 7.37–7.39 (t, 3H); aliphatic (C–H), 3.72 (s, 3H), 3.11–3.16 (t, 2H), 2.49–2.54 (t, 2H), 2.12–2.16 (t, 2H), 1.99– 2.01 (t, 4H), 1.03–1.12 (m, 12H), 0.74–0.79 (t, 6H), 0.56–0.59 (d, 4H).

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room temperature for 30 min. To the mixture, a solution of C60 (2.74 g, 4 mmol) in o-dichlorobenzene (o-DCB) (150 mL) was added, and the homogeneous reaction mixture was stirred at 70 1C under nitrogen overnight. The solution was heated to reflux, and the reaction was allowed to continue overnight again. The resulting mixture was concentrated in vacuo to 100 mL and preeluted with o-DCB (100 mL) and chlorobenzene (200 mL) and then toluene on SiO2/toluene, 40  10 cm. The first fraction, containing unreacted C60, was collected. After an intermediate fraction, the fraction containing DHFCBM was collected. The product was precipitated with MeOH. The product was 3 times treated with MeOH in the same manner and washed with a mixture solution (MeOH:diethylether, 1:1% v/v). MP: 158 oC, Yield: 2.5 g (42%) as a dark brown solid. 1H NMR (CDCl3, 300 MHz) (ppm): aromatic (C–H), 7.86–7.94 (m, 3H), 7.77–7.80 (m, 1H), 7.36–7.40 (m, 3H); aliphatic (C–H), 3.68 (s, 3H), 3.00 (s, 2H), 2.56–2.61 (t, 2H), 2.38 (s, 2H), 2.15 (s, 4H), 1.14 (s, 8H), 0.85–0.90 (m, 12H), 0.78–0.80 (d, 2H). FT-IR (KBr) (Cm  1): 3108 (aromatic C–H), 2919, 2852 (aliphatic C–H), 1730 (C¼O), 1583, 1540 (C¼C), 1167 (C–O). HRMS: calcd for C91H42O2, 1167.3263; found, 1167.3258. The synthesis scheme of DHFCBM is shown in Fig. 1. P3HT (95%) and PCBM (99%) used in this study were obtained from Rieke Metals and Aldrich, respectively. The P3HT:PCBM

2.3. Synthesis of 5-(9,9-dihexyl-fluoren-2-yl)-5-(2tosylhydrazono)pentanoate methyl ester (3) A mixture of the methyl ester (2) (11.0 g, 20 mmol), p-toluenesulfonyl hydrazide (5.68 g, 30 mmol), and MeOH (120 mL) with catalytic amount of HCl was stirred and refluxed for 7 h. The mixture was then extracted into dichloromethane, washed with brine, and dried. The crude product was chromatographed on silica by using 2% acetone in dichloromethane. The product purified by recrystallization from petroleum ether. Yield: 12.8 g (84.5%) as a light yellow solid. 1H NMR (CDCl3, 300 MHz) (ppm): aromatic (C–H), 7.98–8.00 (d, 2H), 7.57–7.72 (m, 4H), 7.92–7.36 (m, 5H); aliphatic (C–H), 3.83 (s, 3H), 2.68–2.70 (d, 2H), 2.40–2.43 (d, 3H), 2.96–2.38 (d, 2H), 1.96–2.01 (m, 4H), 1.70 (s, 2H), 1.05–1.14 (m, 12H), 0.75–0.80 (t, 6H), 0.59–0.63 (t, 4H). 2.4. Synthesis of DHFCBM (4) [16] A mixture of alkoxyfluorene substitued benzoyl p-tosylhydrazone (3.0 g, 5 mmol), sodium methoxide (0.27 g, 5 mmol), and dry pyridine (30 mL) was placed under nitrogen and stirred at

Fig. 2. Current density versus voltage plot, corrected for the built-in voltage of PCBM and DHFCBM electron only devices. Experimental data (symbol) were fitted by a SCLC equation.

Fig. 1. Synthetic routes for DHFCBM.

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blend solutions were prepared in chlorobenzene at a concentration of 40 mg/mL. After cleaning the prepatterned ITO-coated glass, PEDOT–PSS (Baytron P TP AI 4083, Bayer AG) was spin-coated to a thickness of 30–50 nm and annealed at 120 1C (for 60 min) in air. The active layer was spin-coated on PEDOT:PSS layer for 60 s to a thickness of 200 nm. Finally, LiF (0.6 nm)/Al (100 nm) cathodes were thermally deposited. The current density–voltage (J–V) characteristics were measured using Keithley 4200 source/ measure units in the dark and under AM 1.5 solar illumination (Oriel 1 kW solar simulator) with respect to the reference cell PVM 132 (calibrated at the National Renewable Energy Laboratory, NREL, at an intensity of 100 mW/cm2). The effective area of film was measured to be 0.09 cm2. UV–vis and PL measurements were carried out on a UV– vis–NIR spectrophotometer (Cary 5000: Varian Co. and FR 6500, JASCO Co.). Cyclic voltammetry measurements of the polymer films were performed on a BAS 100 B/W electrochemical analyzer in anhydrous acetonitrile with Bu4NClO4 (0.1 M) as the

electrolyte. The potentials were measured against an Ag/AgCl reference electrode with ferrocene as the internal standard. The onset potentials were determined from the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram. GIXD studies were performed using the 10C1 and 4C2 beamlines at the Pohang Accelerator Laboratory (PAL). AFM (Multimode IIIa, Digital Instruments) operating in tapping mode was used to image the surface morphology of the photoactive layer.

3. Results and discussions Prior to the investigation of the photovoltaic properties of the P3HT:DHFCBM films, it was indispensable to know the hole and electron mobilities of each component. Recently, much work has been focused on measuring hole mobilities in P3HT samples using time-of-flight (TOF) photocurrent measurements and space charge limited (SCL) current, in sandwich structures that model

Fig. 3. X-ray diffraction results of various P3HT:DHFCBM blends after thermal annealing at 80, 120, and 150 1C. Illustration denotes the (0 0 1) orientation of P3HT molecules.

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a solar cell configuration, yielding measured hole mobilities on the order of 10–4 cm2/Vs [17–19]. To measure the electron mobility of DHFCBM, a DHFCBM was sandwiched between a layer of ITO covered with PEDOT:PSS and a LiF/Al top electrode. Because the work function of PEDOT:PSS (5.2 eV) is significantly lower than the HOMO of DHFCBM (6.0 eV), hole injection into the active layer can be neglected and, thus, only electrons flow under forward bias. In Fig. 2 the experimental J–V characteristics of electron only devices of the PCBM and DHFCBM (film thickness L¼100 nm) corrected for the built-in voltage are shown. The typical SCLC follows well known Mott–Gurney equation: [18] J¼

9 V2 ee0 m 3 8 d

ð1Þ

where J is the current density, e0 the permittivity of vacuum, e the relative permittivity of polymer, which is assumed to be 3, m the mobility, V the voltage, and d is the thickness of the layer. Then the low-field electron mobility of each layer can be calculated by fitting above equation to experimental results of Fig. 2 The obtained electron mobility of DHFCBM (4  10–4 cm2/Vs) was three times lower than that of PCBM (1.2  10–3 cm2/Vs) and the values reported by other researchers [19,20], implying that the bulky fluorene group may hinder charge transport. However,

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considering the hole mobility of P3HT,  10–4 cm2/Vs, the observed electron mobility should result in balanced charge transport in bulk heterojunction configurations. To investigate the effect of the blend ratio of P3HT:DHFCBM on the nanomorphology and structure of the film, the weight fractions of DHFCBM in the blend solutions were gradually varied from 30% to 70% in 10% increments and spin-coated onto the ITO glass substrates coated with PEDOT:PSS. Thermal annealing was also performed at 80, 120, and 150 1C. X-ray diffraction patterns of the prepared blend films are presented in Fig. 3 All films showed diffraction patterns of crystalline P3HT with the Bragg peaks at 2y ¼5.41, 10.81, and 16.31 (with the (1 0 0) orientation of the polymer backbone as depicted in Fig. 2). No fullerene-related patterns were observed, confirming the amorphous nature of DHFCBM. Interestingly, when the film was rich in DHFCBM (60% and 70%), the peak intensity of P3HT was observed to increase up to 120 1C, but after further annealing, the peak intensity decreased. This indicated that at high temperatures ( 4120 1C), thermally activated diffusive motion of the fullerene effectively suppressed the crystallization of P3HT. On the other hand, for films with DHFCBM contents of 30%, 40%, and 50%, fullerene-induced limitations on P3HT crystallization were not observed, as shown in Fig. 3. For further analysis of

Fig. 4. UV–vis absorption spectra of various P3HT:DHFCBM blends after thermal annealing at 80, 120, and 150 1C.

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Fig. 5. AFM images of various P3HT:DHFCBM blends after thermal annealing at 80, 120, and 150 1C (lateral scale is zero (black) to 7 nm (white)).

Fig. 6. AFM images of P3HT:DC60BM blends after thermal annealing at 80, 120, 150, and 180 1C.

the nanostructure, the average size of P3HT crystallites was calculated using Scherrer’s relation [21] L

0:9l D2Y cos Y

ð2Þ

where l is the illumination wavelength (0.154 nm) and D2Y is the full-width at half-maximum of the peak. Similar to the trend observed for the Bragg peak intensity, the crystallite size in P3HTrich blends increased with thermal annealing temperatures up to 120 1C and maintained a constant average size (20 nm) for annealing temperatures above 120 1C. The PCBM-rich blends yielded a decrease in crystallite P3HT size for high thermal annealing temperatures. These results show that it was possible to limit the phase segregation of donor–acceptor molecules using amorphous fullerene. Blend film ratios with DHFCBM contents of 30%, 40%, and 50% needed only minimal heat treatment

at 120 1C to achieve the final morphology and showed strong heat resistance during further annealing, retaining its final morphology. The UV–vis absorption spectra of the blend films also revealed similar results (Fig. 4). In contrast to the DHFCBM-rich blends that showed a gradual decrease in absorption intensity by high temperature annealing, films with DHFCBM contents of 30%, 40%, and 50% showed almost uniform absorption intensities for all three vibronic absorptions after thermal annealing at 120 and 150 1C, consistent with XRD results. Uniform absorption intensities may be due to the reduced capacity for structural diffusion in amorphous fullerenes compared with PCBM, because they do not crystallize and, therefore, tend to maintain structures even at elevated temperatures. Structural stability can be an important merit because crystalline PCBM can diffuse and recrystallize at elevated temperatures, changing the nanomorphology of donor–acceptor bulk heterojunctions [22,23].

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Top surfaces of P3HT:DHFCBM blend films were studied (Fig. 5) by AFM to verify the structural analysis of the XRD studies. Films with 30% DHFCBM showed an increase in crystalline domain size induced by annealing at 120 1C; however, the crystalline domains rapidly decreased in size after annealing

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at 150 1C. Interestingly, for films with 50% and 70% DHFCBM, annealing at 120 and 150 1C did not show significant differences in morphology and contained similar domain sizes. These structural and morphological studies revealed that the P3HT:DHFCBM blend films were quite thermally stable due to the amorphous nature of DHFCBM. We also prepared P3HT:PCBM blends, showing thermal stablility upto 180 1C as shown AFM images (Fig. 6). The electrochemical measurements showed that DHFCBM has the HOMO and LUMO levels of  6.0 and  3.8 eV, respectively (Fig. 7). We found reduction onset at  0.64 eV and calculated the

Fig. 7. Cyclic voltammogram of PHFCBM and PCBM (+ ferrocene) in 0.1 M Bu4NClO4 in acetonitrile at a scan rate of 100 mV/s at room temperature.

Fig. 10. Current density versus voltage curves of P3HT:DHFCBM solar cells under illumination.

Table 1 Obtained photovoltaic parameters of P3HT:DHFCBM solar cells with various blend ratios and annealing temperatures and compared with P3HT:PCBM solar cell performance. DHFCBM (%)

Voc (V)

Jsc (mA cm  2)

FF (%)

Eff. (%)

30 40 50 60 70 P3HT:PCBM

0.67 0.68 0.67 0.66 0.66 0.65

8.1 8.9 8.9 4.5 3.4 10.3

42 52 51 18 13 47

2.55 3.16 3.1 1.1 0.8 3.1

Fig. 8. UV–vis absorption spectra of PCBM and DHFCBM.

Fig. 9. The energy level diagram of components used in this paper.

Fig. 11. Hole and electron mobility of P3HT:DHFCBM blends with various ratio.

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Fig. 12. Annealing temperature dependence of measured PCE of various blends (DHFCBM contents 30% to 70%).

LUMO energy level to be 3.76 eV (4.4  0.64 eV). HOMO level is calculated using optical band gap from the UV absorption spectrum of pure DHFCBM (Fig. 8). The LUMO of DHFCBM is slightly shifted to higher values relative to that of PCBM, by approximately 100 meV. Because there is a linear relationship between the donor–HOMO and acceptor–LUMO energy differences and the VOC of the bulk-heterojunction devices [23], relatively larger VOC of P3HT:DHFCBM based cells, originated from the replacement of phenyl group with electron-donating dihexyl–fluorene, can be expected rather than that of P3HT:PCBM [24]. Also, for an efficient charge separation in the donor–acceptor interface, it is well known that a certain offset ( 0.3 eV) of the donor–LUMO and acceptor–LUMO levels is required [25]. In the P3HT:DHFCBM blend, this energy level matching can be successfully achieved (Fig. 9). Photoluminescence measurements on several films showed effective quenching of the P3HT emission due to the presence of DHFCBM in the film. This quenching can be attributed to sufficient LUMO level difference between donor and acceptor and effectively phase separated morphology of blend films. The J–V characteristics of the P3HT:DHFCBM blend-based photovoltaic cells, which were thermally annealed at 120 1C, are shown in Fig. 10, and device performance parameters are summarized in Table 1. The best photovoltaic performances were obtained from blends containing 40% DHFCBM, with a VOC of 0.68 V, JSC of 8.9 mAcm–2, FF of 52%, and PCE of 3.16%. Consistent with the expectations from the DHFCBM LUMO level, a relatively large VOC of 0.68 V was obtained. (VOC of P3HT:PCBM-based cells were measured to be less than 0.65 V [26].) Interestingly, changing the content of DHFCBM from 30% to 70% had a large influence on device performance. Devices containing 40% and 50% DHFCBM showed enhanced performances compared with the devices containing 60% and 70% DHFCBM. Devices containing 30% DHFCBM, however, performed slightly worse, possibly due to the presence of fewer percolation pathways for negative charge carriers. To elucidate the underlying physics of fullerene contents dependence of photovoltaic performances, we have measured hole and electron mobility for all the blend ratios. To fabricate hole and electron only devices we fabricated two kinds of cells, namely, ITO/PEDOT:PSS/P3HT:DHFCBM/Au for hole only device and Al/ P3HT:DHFCBM /LiF/Al for electron only device. The obtained hole and electron mobility are summarized in Fig. 11. The hole mobility of  10  4 cm2/Vs and an electron mobility of 10  5–10  3 cm2/Vs were obtained. In the case of devices containing 40% and 50% DHFCBM have most balanced mobility while device containing 30% DHFCBM has low electron mobility as predicted previously. Therefore we can conclude that there is an optimized blend ratio for both efficient light absorption and charge carrier collection, in this case, 60:40.

To investigate the thermal stability of P3HT:DHFCBM solar cells, thermal annealing was performed at 80, 120, and 150 1C, which were the temperatures used in the structure and morphological analysis. DHFCBM-poor devices (30%, 40%, and 50%) performed better with 120 1C annealing, achieving saturation at higher annealing temperatures (Fig. 12(a)). In addition, these solar cells even showed thermal stability over long periods of time. Their PCE values stayed above 90% of the initial values even after 10 h of annealing at 150 1C. On the other hand, DHFCBM-rich devices (60% and 70%) showed decreased performances for such high temperature annealing processes (Fig. 12(b)). This result suggests that by adjusting the blend ratio of P3HT:DHFCBM films, it is possible to achieve stable organic solar cells, even under long thermal exposure times, by suppressing diffusion and recrystallization motions of donor–acceptor molecules, as predicted from structural and morphological studies.

4. Conclusion In this study, we have synthesized a new amorphous fullerene derivative, DHFCBM, for use as an electron acceptor material in OSCs. Its amorphous nature enabled good thermal stability of the resulting devices by suppressing the diffusive motion of donor– acceptor molecules, as confirmed by various structure and morphology analyses.

Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (Grant no. 20100000826) and Korea Institute for Advancement in Technology (KIAT) through the Workforce Development Program in Strategic Technology. Mr. Seul-Ong Kim, and Dae Sung Chung equally contributed to this work. References [1] J.Y. Kim, S.H. Kim, H.-H.O. Lee, K. Lee, W. Ma, X. Gong, A.J. Heeger, New architecture for high-efficiency polymer photovoltaic cells using solution-based titanium oxide as an optical spacer, Adv. Mater. 18 (2006) 572–576. [2] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, 10.Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols, Nat. Mater. 6 (2007) 497–500. [3] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A.J. Heeger, Efficient tandem polymer solar cells fabricated by all-solution processing, Science 317 (2007) 222–225.

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