Highly Efficient Solar Cells Based on the Copolymer of

Article pubs.acs.org/JPCC

Highly Efficient Solar Cells Based on the Copolymer of Benzodithiophene and Thienopyrroledione with Solvent Annealing Bo Qu,†,* Di Tian,‡ Zhiyuan Cong,‡ Weiping Wang,‡ Zhongwei An,‡ Chao Gao,‡,* Zhi Gao,† Hongsheng Yang,† Lipei Zhang,† Lixin Xiao,† Zhijian Chen,† and Qihuang Gong†,* †

State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, Department of Physics, Peking University, 100871, People’s Republic of China; ‡ Xi’an Modern Chemistry Research Institute, Xi’an Shaanxi, 710065, People’s Republic of China S Supporting Information *

ABSTRACT: Highly efficient PBDTTPD-based photovoltaic devices with the configuration of ITO/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/PBDTTPD: methanofullerene (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) (weight ratio being from 1:1 to 1:4)/LiF (5 Å)/Al (100 nm), were realized with ortho-dichlorobenzene (DCB) solvent annealing treatment. It was revealed that the best photovoltaic device was obtained when the blend ratio of PBDTTPD:PC61BM was modulated to be 1:2 and processed with DCB solvent annealing for 12 h. The shortcircuit current density (Jsc) and power conversion efficiency (PCE) values were measured to be 10.52 mA/cm2 and 4.99% respectively, which were both higher than the counterparts treated with chlorobenzene (CB) solvent annealing or the thermal annealing. Atomic force microscopy measurements of the active layer after solvent annealing treatment were also carried out. The phase separation length scale of the PBDTTPD:PC61BM(1:2) layer was comparable to the exciton diffusion length when the active layer was treated under DCB solvent annealing, which facilitated effective exciton dissociation and carrier diffusion in the active layer. Therefore, highly efficient PBDTTPD-based photovoltaic devices could be achieved with DCB solvent annealing, which indicated that solvent annealing with proper solvent might be an easily processed, low-cost, and room-temperature alternative to thermal annealing for polymer solar cells.

1. INTRODUCTION The study of organic photovoltaic devices (OPVs) based on interpenetrating networks of conjugated polymers and fullerene derivatives have attracted considerable attention over the past decades,1−5 due to the unique advantages of low cost, lightweight, flexible renewable power sources, etc.6−9 The polymer solar cells (PSCs) employed bulk heterojunction (BHJ) played a significant role in the field of organic photovoltaics. The BHJ composed of poly(3-hexylthiophene) (P3HT) as the donor and methanofullerene (6,6)-phenyl-C61butyric acid methyl ester (PC61BM) as the acceptor was usually employed in OPVs, and the power conversion efficiency (PCE) reached ∼5% with optimization.10 However, P3HT had some drawbacks to hamper the improvement of photovoltaic behavior of OPVs. The highest occupied molecular orbital (HOMO) level of P3HT was relatively high, which resulted in low open-circuit voltage (Voc) of OPVs. Furthermore, the relatively broad bandgap of P3HT would limit spectrum response range and degrade absorption capability of P3HT. Therefore, novel conjugated polymer donors were designed and reported to boost the photovoltaic properties of OPVs.3,5,11−15 In order to achieve superior BHJ polymer solar cells, one efficient way is to design alternating donor−acceptor (D−A) copolymers combined electron-rich (donor) and electron© 2013 American Chemical Society

deficient (acceptor) moieties, which can modulate the energy levels and absorption properties by controlling the intermolecular charge transferring from the donor to the acceptor. Recently, many kinds of D−A copolymers have been developed and excellent photovoltaic properties with PCE as high as 8% have been achieved.16 Among these conjugated copolymers, polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) unit as the electron donor and electron-deficient units (electron acceptor) like thieno[3,4-b]thiophene (TT),17,18 and 4,7dithiophene-2-yl-2,1,3-benzothia-diazole (DTBT),19 and so forth have attracted much attention in the PSCs field and promising photovoltaic properties have been achieved for this type of polymers. Recently, as a new electron-withdrawing comonomer, thieno[3,4-c]pyrrole-4,6- dione (TPD)-based copolymer have attracted much attention. Leclerc3 and Frechet,20 independently synthesized the same TPD-based polymers (PBDTTPD) and a high PCE as 5.5% and 6.3% were achieved with PC71BM and PC61BM as acceptor, respectively. Ye Tao21 synthesized a new alternating copolymer of dithienosilole and thienopyrrole4,6-dione (PDTSTPD) with a PCE as 7.3%. Reynolds22 also Received: November 8, 2012 Revised: January 27, 2013 Published: January 29, 2013 3272

dx.doi.org/10.1021/jp311059d | J. Phys. Chem. C 2013, 117, 3272−3278

The Journal of Physical Chemistry C

Article

Scheme 1. Synthesis Routine of PBDTTPD

powder (4.29 g, 66 mmol) were added to 90 mL of water, then 18 g of NaOH was added into the mixture. After the solution was refluxed for 1 h, 1-bromo-2-ethylhexane (17.4 g, 90 mmol) and a catalytic amount of tetrabutylammonium bromide were added and heated to be refluxed for another 6 h. Then, cold water (300 mL) was added and the mixture was extracted with ethyl acetate. The extract was dried over anhydrous magnesium sulfate. The residue was purified by column chromatography on silica gel to obtain 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4b]dithiophene (2) in a 65% yield. Compound 2 (6.1 g,13.6 mmol) was added into THF (200 mL) under nitrogen, nbutyllithium (36.3 mmol, 2.2 M) was added dropwise to the mixture at −80 °C and stirred for 1 h, the cooling bath was removed, and the reactant was stirred at ambient temperature for another 1 h. Trimethyltin chloride (8 g, 40.4 mmol) was added in one portion at −80 °C and the reactant was stirred at ambient temperature overnight. After 200 mL cold water was added and the mixture was extracted with hexane. The organic layer was dried over anhydrous magnesium sulfate. The residue was recrystallized by ethyl alcohol and 2,6-bis(trimethyltin)-4,8bis(2-ethylhexyl)benzo[1,2- b:3,4-b′]dithiophene (3) (6.15 g, 7.96 mmol) was obtained in a 58.3% yield. 1H NMR (CDCl3, 500 MHz) δ (ppm): 7.52 (s, 2H), 4.23 (d, 4H, J = 10 Hz), 1.79 (m, 2H), 1.54−1.36 (m, 16H), 1.06 (t, 6H, J = 7.5 Hz), 0.96 (t, 6H, J = 7.5 Hz), 0.40 (s, 18 H). 13C NMR (CDCl3, 500 MHz) δ (ppm): 143.41, 140.41, 133.91, 132.89, 128.03, 40.73, 30.60, 29.30, 23.97, 23.23, 14.24, 11.41, −6.97, −8.32, −9.95. Synthesis of PBDTTPD. In a 50 mL dry flask, Compound 3 (618 mg, 0.8 mmol), compound 4 (338 mg, 0.8 mmol) were dissolved in degassed toluene (10 mL), the mixture was flushed with argon for 30 min, then tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (14.7 mg) and tri(o-tolyl)phosphine (P(o-Tol)3) (26.2 mg) were added, and flushed with argon for another 30 min. Then the mixture was

reported a similar polymer using dithienogermole as the donor and thienopyrrole-4,6-dione as the acceptor, and the same PCE of 7.3% was obtained. In addition, inverted device based on PBDTTPD and PC71BM was also investigated5 and a PCE of 4.2% was achieved. It is well-known that the morphology of active layers plays a critical role in determining the performance of PSCs, because charge carrier generation, recombination, and transport are all influenced by spontaneously formed micro/nanostructure of the active layers during solution-processing.10,23−25 Through thermal annealing10 or solvent annealing,25 the morphology of the active layer could be largely improved. However, the activelayer morphology and the annealing condition of PBDTTPDbased photovoltaic cells were seldom investigated. In this work, the solvent annealing and the morphology of the active layer of PBDTTPD:PC61BM-based photovoltaic devices were studied. By changing the weight ratio of the polymer with PC61BM and the solvent, a high PCE of 4.99% was achieved in comparing with that of thermal annealing devices (4.08%), which indicated that annealing treatment with a proper solvent might be an effective technique to achieve highly efficient PBDTTPD-based PSCs.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. 1,3-Dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (4) was purchased from Beijing Allmers Chemical S&T Co., Ltd. Unless otherwise stated, all of the chemicals were purchased from Alfa Aesar Co. and used as received. The copolymers were synthesized according to Scheme 1. Synthesis. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyl)benzo[1,2-b:3,4-b′]dithiophene (3). In a flask, 4,8-dihydrobenzo[1,2b:4,5-b′]dithiophen-4,8-dione (1) (6.6 g, 30 mmol) and zinc 3273



Article

vigorously stirred at 100 °C for 24h under argon atmosphere. After being cooled, the solution was poured into methanol. The polymer was collected by filtration and was Soxhlet-extracted in order with methanol, hexane, and then with chloroform. The chloroform solution was concentrated to a small volume, and the polymer was precipitated by pouring this solution into methanol. Finally, the polymer was collected by filtration, dried under vacuum at 50 °C overnight, and afforded PBDTTPD (525 mg) as a solid in a 92.8% yield. 1H NMR (CDCl3, 500 MHz) δ (ppm): 8.25 (br, 2H), 3.95 (br,6H), 0.9−2.3 (br, 45H). GPC (tetrahydrofuran, polystyrene standard): Mn = 40.188 kDa, Mw = 75.014 kDa, PDI = 1.87. Anal. Calcd for (C42H59NO4S3)n (%): C 68.34, H 8.06, N 1.90. Found (%): C 68.28, H 8.13, N 1.99. 2.2. Characterization. 1H NMR spectra were recorded at 500 MHz on a Bruker DRX-500 spectrometer. Molecular weights and distributions of the copolymers were gauged by gel permeation chromatography (GPC) and THF was used as eluent and polystyrene as the standard. The absorption spectra were taken with a Unico UV-2102 scanning spectrophotometer. Differential scanning calorimetry (DSC) measurements were performed on DSC Q20 V24.9 Build 121 with a heating rate of 20 °C/min in nitrogen. The electrochemical cyclic voltammetry (CV) was carried out on a CHI 660D Electrochemical Workstation. Platinum (Pt) wire, glassy carbon and Ag/Ag+ were used as the counter electrode, working electrode, and reference electrode, respectively, in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. Polymer thin films were obtained by drop-casting polymer chloroform solutions (analytical reagent, 2.5 mg/mL) onto the working electrode, and then dried in air. Atomic force microscopy (AFM) phase images of organic films with solvent (chlorobenzene, ortho-dichlorobenzene) and thermal annealing were recorded in air under ambient conditions using tapping mode of the multimode scanning probe microscope (Agilent Technologies 5500). 2.3. OPVs Fabrication and Characterization. In order to study the photovoltaic characteristics of PBDTTPD, OPVs with the configuration of ITO/Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS)/PBDTTPD:PC61BM (weight ratio being from 1:1 to 1:4)/LiF (5 Å)/Al (100 nm) were fabricated and characterized, in which PBDTTPD performed as the donor and PC61BM as the acceptor in these devices. The device architecture was presented in Figure 1. ITO was used as the anode, and ITO substrates with the sheet resistance and thickness of 20 Ω per square (Ω/□) and 120 nm respectively, were cleaned in deionized water, acetone and anhydrous ethanol in sequence by an ultrasonic cleaner, and then they were treated with UV irradiation and oxygen plasma for 60 s to remove the contamination on ITO substrates. Diluted PEDOT:PSS aqueous solution was spin-casted onto ITO substrates, and then annealed at 200 °C immediately in vacuum for about 15 min. PBDTTPD:PC61BM with different blend ratios was dissolved in chlorobenzene (CB) and orthodichlorobenzene (DCB) solvents, respectively. The blend ratios of PBDTTPD:PC61BM varied from 1:1 to 1:4, and the PBDTTPD concentration in all solutions was fixed to be 6 mg/mL. The photosensitive layers were fabricated using a spincoating technique and left in glovebox at room temperature for ∼12 h without thermal annealing treatment. The thickness of the active layers was around 89 nm according to AFM measurement. Finally, a 5-Å thick buffer layer (LiF) and 100-

Figure 1. Structure sketch of the devices.

nm thick cathode (Al) were successively thermally evaporated onto the active layer under the vacuum pressure of 4.0 × 10−6 Torr, and the evaporation rates of Al and LiF were 2.2 Å/s and 0.1 Å/s, respectively. The thicknesses of the LiF and Al layers was monitored with a quartz crystal microbalance in the vacuum chamber, and a shadow mask with 2 mm diameter openings was used to define the cathodes. The current density−voltage (J−V) curves were recorded simultaneously using a Keithley 2611 source meter. All OPVs worked under AM 1.5G illumination at 100mW/cm2 (Newport Thermal Oriel 69911 300W). Moreover, a calibrated mono silicon diode was used as the reference and all of the measurements were carried out at room temperature without encapsulation.

3. RESULTS AND DISCUSSION 3.1. Thermal Properties. The thermal property of PBDTTPD was determined by using DSC (Figure 2). There was no obvious thermal transition found between 50 and 250 °C, which was consistent with the literature.4 3.2. Absorption Properties of PBDTTPD. The normalized UV−vis absorption spectra of PBDTTPD in chloroform

Figure 2. DSC scan of PBDTTPD at a heating rate of 20 °C/min in nitrogen. 3274



Article

Figure 3. Absorption spectra of PBDTTPD in film and solution states.

Figure 6. Current density vs voltage curves of OPVs. (a) Using CB solvent annealing and (b) DCB solvent annealing.

Table 1. Photovoltaic Characteristics of the OPVs

Figure 4. Cyclic voltammogram of PBDTTPD film on glassy carbon electrode. (0.1 mol/L Bu4NPF6, acetonitrile solution with the scan rate of 100 mV/s.).

solvents PBDTTPD:PC61BM CB

DCB

1:1 1:2 1:3 1:4 1:1 1:2 1:3 1:4

Voc (V) Jsc (mA/cm2) 0.93 0.88 0.82 0.75 0.84 0.89 0.85 0.81

8.82 9.11 9.00 8.25 9.55 10.52 7.95 6.84

FF

PCE (%)

0.54 0.61 0.45 0.45 0.51 0.53 0.58 0.57

4.41 4.92 3.31 2.78 4.03 4.99 3.88 3.14

Figure 5. The energy levels of the materials used in OPVs.

(CHCl3) solution and film state were shown in Figure 3, and the spectra were almost identical, implying the similar rigid-rod conformation in both states. The absorption covered from 400 nm to 650 nm, which was ascribed to both the efficient packing of PBDTTPD and planarization of the conjugated backbone.26 The broad absorption range approximately matched the solar spectrum, which would benefit the photovoltaic properties of PBDTTPD-based OPVs. The optical band gap (Egopt) of PBDTTPD was evaluated to be 1.80 eV according to the absorption onset of the film as shown in Figure 3. The narrow band gap of PBDTTPD facilitated effective absorption in solar spectrum and hence might enhance the PCE of PBDTTPDbased OPVs. 3.3. Electrochemical Properties. The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels could be estimated from the CV curve measured by an electrochemical workstation as shown in Figure 4. The energy level of ferrocene/ferrocenium (Fc/Fc+) was −4.8 eV below the vacuum level27 and the former potential of Fc/Fc+ was 0.08

Figure 7. Photovoltaic comparison of OPVs (PBDTTPD:PC61BM = 1:2) in various annealing condition.

eV against Ag/Ag+. Therefore, the HOMO and LUMO energy levels of PBDTTPD could be evaluated to be −5.35 eV and −3.17 eV according to the onset oxidation potential (Eonsetox) and onset reduction potential (Eonsetred), respectively. The energy levels of the materials used in this work were depicted in Figure 5.28 The electrochemical energy band gap (Egec) of PBDTTPD was calculated to be 2.18 eV, which seemed 3275



Article

somewhat larger than Egopt (1.80 eV). The higher electrochemical bandgap than optical bandgap is a common phenomenon for the conjugated polymers29,30 because of the energy barrier of the charge transfer at electrodes during the electrochemical measurement. The energy gap between the HOMO level of PBDTTPD and LUMO level of PC61BM was as large as 1.44 eV, which could facilitate high Voc of the photovoltaic devices. In addition, the LUMO level of PBDTTPD were ∼0.74 eV higher than that of PC61BM, then

Table 2. Photovoltaic Characteristics of the OPVs (PBDTTPD:PC61BM = 1:2) under Various Annealing Condition solvents

annealing condition

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

CB DCB CB DCB

150 °C, 10 min 150 °C, 10 min solvent annealing, 12 h solvent annealing, 12 h

0.75 0.86 0.88 0.89

9.55 8.12 9.11 10.52

0.45 0.58 0.61 0.53

3.22 4.08 4.92 4.99

Figure 8. The AFM phase images of PBDTTPD:PC61BM (1:2) films with various annealing condition (left, 5 × 5 μm; right, 0.5 × 0.5 μm). (a) CB, thermal annealing 150 °C, 10 min; (b) DCB, thermal annealing 150 °C, 10 min; (c) CB, solvent annealing, 12 h; and (d) DCB, solvent annealing, 12 h. 3276



Article

annealing) were systematically studied. With the weight ratio of the polymer with PC61BM being 1:2 and DCB solvent annealing (12 h), the high PCE (4.99%) and Jsc (10.52 mA/ cm2) of PBDTTPD-based device was achieved in comparison with those of thermal annealing devices (4.08%, 8.12 mA/cm2) and CB solvent-annealing devices (4.92%, 9.11 mA/cm2), which indicated that the solvent annealing treatment with a proper solvent might be an effective technique to achieve highly efficient PSCs.

electrons could transfer automatically at the donor/acceptor interface. 3.4. Photovoltaic properties. Compared with the previous literature,5 the copolymer PBDTTPD synthesized in this work possessed an almost similar Mw value of 75.014 KDa, and relatively low PDI of 1.87, which might facilitate superior photovoltaic performance of PBDTTPD-based devices with proper solvent annealing. OPVs with the configuration of ITO/ PEDOT:PSS/PBDTTPD:PC61BM/LiF/Al were realized according to the description above. The J−V curves of the photovoltaic devices under 100 mW/cm2 AM 1.5G simulated solar irradiation were shown in Figure 6, and all of the photovoltaic data were summarized in Table 1. When CB was used as the solvent for PBDTTPD:PC61BM and the compound active layers were processed with CB solvent annealing for ∼12 h, OPVs with the blend ratio of PBDTTPD:PC61BM being 1:2 exhibited acceptable photovoltaic performance as shown in Figure 6(a), and the Voc, short-circuit current density (Jsc), fill factor (FF), and PCE values of the device were gauged to be 0.88 V, 9.11 mA/cm2, 0.61, 4.92%, respectively. Figure 6(b) exhibited the J−V curves of OPVs with DCB solvent annealing, and the best OPV was obtained when the blend ratio of PBDTTPD:PC61BM reaching 1:2. Furthermore, the corresponding Jsc and PCE values were achieved to be 10.52 mA/ cm2 and 4.99%, respectively. The photovoltaic characteristics of PBDTTPD:PC61BM(1:2)-based device under thermal annealing (150 °C, 10 min)min) and solvent annealing were also compared in Figure 7, and the photovoltaic data were summarized in Table 2. The Voc and PCE of the devices with DCB or CB solvent annealing were all enlarged in comparing with those of thermal annealing devices. In order to investigate the influence of the annealing condition on the morphology, AFM was used to depict the phase images of the active layers and shown in Figure 8. All of the films showed bicontinuous networks containing polymer phase and PC61BM phase. However, the large phase scales of the thermal annealing films were not favorable for efficient exciton dissociation and charge transport. For the solvent annealing films, the phase scales were decreased. In particular, after the DCB solvent annealing, the interpenetrating networks of PBDTTPD and PC61BM were formed (Figure 8(d)) and the phase separation length scale was determined to be ∼30 nm, which would benefit the exciton dissociation and carrier diffusion in the active layer. Herein, the optimized phase separation of PBDTTPD:PC61BM (1:2) films with DCB solvent annealing was realized, which facilitated the superior photovoltaic data (Table 2) of OPVs.31,32 All of the experimental data revealed that PBDTTPD was a promising donor candidate, and highly efficient PBDTTPDbased PSCs could be realized using DCB solvent annealing treatment. However, more work is required to substantially improve the photovoltaic properties of PBDTTPD-based devices, such as optimizing device structure (e.g., varying film thickness, employing novel electrodes, or utilizing novel electron-acceptors with high LUMO levels) or blending additives into the active layers.33−38

■

ASSOCIATED CONTENT

S Supporting Information *

Complete references. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-62767290. Fax: +86-10-62756567. E-mail: bqu@ pku.edu.cn (B.Q.); [email protected] (C.G.); qhgong@ pku.edu.cn (Q.G.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program under Grant No. 2009CB930504, the National Natural Science Foundation of China under Grant Nos. 60907015, 60907012, 61177031, 61177020, 10934001, and 11121091. B.Q. was also supported by the Research Fund for the Doctoral Program of Higher Education (RFDP) under Grant No. 20110001120124.

■

REFERENCES

(1) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron-Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (2) Tang, C. W. 2-Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, 183−185. (3) Zou, Y.; Najari, A.; Berrouard, P.; Beaupré, S.; Aich, B. R.; Tao, Y.; Leclerc, M. A Thieno [3,4-c] Pyrrole-4,6-Dione-Based Copolymer for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 5330−5331. (4) Zhou, H. X.; Yang, L. Q.; Price, S. C.; Knight, K. J.; You, W. Enhanced Photovoltaic Performance of Low-Bandgap Polymers with Deep LUMO Levels. Angew. Chem.-Int. Ed. 2010, 49, 7992−7995. (5) Zhang, Y.; Hau, S. K.; Yip, H. L.; Sun, Y.; Acton, O.; Jen, A. K. Y. Efficient Polymer Solar Cells Based on the Copolymers of Benzodithiophene and Thienopyrroledione. Chem. Mater. 2010, 22, 2696−2698. (6) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338. (7) Li, H.; Sun, S.; Salim, T.; Bomma, S.; Grimsdale, A. C.; Lam, Y. M. Conjugated Polymers Based on Dicarboxylic Imide-Substituted Isothianaphthene and Their Applications in Solar Cells. J. Polym. Sci., Polym. Chem. 2012, 50, 250−260. (8) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Yang, Y. Synthesis and Photovoltaic Properties of Two Benzo[1,2-b:3,4-b′] DithiopheneBased Conjugated Polymers. J. Phys. Chem. C 2009, 113, 21202− 21207. (9) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Efficient Photodiodes from Interpenetrating Polymer Networks. Nature (London) 1995, 376, 498−500. (10) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale

4. CONCLUSIONS BHJ polymer solar cells with PBDTTPD:PC61BM as the active layers were fabricated and investigated systematically. The photovoltaic properties of PBDTTPD:PC61BM-based OPVs under different annealing condition (thermal or solvent 3277



Article

Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (11) Qu, B.; Jiang, Z. L.; Chen, Z. J.; Xiao, L. X.; Tian, D.; Gao, C.; Wei, W.; Gong, Q. H. Synthesis of A Soluble Polythiophene Copolymer with Thiophene−Vinylene Conjugated Side Chain and Its Applications in Photovoltaic Devices. J. Appl. Polym. Sci. 2012, 124, 1186−1192. (12) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297−302. (13) Blouin, N.; Michaud, A.; Leclerc, M. A Low-Bandgap Poly(2,7Carbazole) Derivative for Use in High-Performance Solar Cells. Adv. Mater. 2007, 19, 2295−2300. (14) Wang, E.; Wang, L.; Lan, L.; Luo, C.; Zhuang, W.; Peng, J.; Cao, Y. High-Performance Polymer Heterojunction Solar Cells of A Polysilafluorene Derivative. Appl. Phys. Lett. 2008, 92, 033307. (15) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar CellsTowards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789−794. (16) Zhang, J.; Cai, W. Z.; Huang, F.; Wang, E. G.; Zhong, C. M.; Liu, S. J.; Wang, M.; Duan, C. H.; Yang, T. B.; Cao, Y. Synthesis of Quinoxaline-Based Donor−Acceptor Narrow-Band-Gap Polymers and Their Cyclized Derivatives for Bulk-Heterojunction Polymer Solar Cell Applications. Macromolecules 2011, 44, 894−901. (17) Huo, L. J.; Zhang, S. Q.; Guo, X.; Xu, F.; Li, Y. F.; Hou, J. H. Replacing Alkoxy Groups with Alkylthienyl Groups: A Feasible Approach to Improve the Properties of Photovoltaic Polymers. Angew. Chem.-Int. Ed. 2011, 50, 1−7. (18) Huo, L. J.; Guo, X.; Li, Y. F.; Hou, J. H. Synthesis of A Polythieno [3,4-b] Thiophene Derivative with A Low-Lying HOMO Level and Its Application in Polymer Solar Cells. Chem. Commun. 2011, 47, 8850−8852. (19) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer−Fullerene Solar Cells. J. Am. Chem. Soc. 2011, 133, 4625−4631. (20) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Frechet, J. M. J. Synthetic Control of Structural Order in n-Alkylthieno [3,4-c] Pyrrole-4,6-Dione-Based Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 7595−7597. (21) Chu, T. Y.; Lu, J. P.; Beaupre, S.; Zhang, Y. G.; Pouliot, J. R.; Wakim, S.; Zhou, J. Y.; Leclerc, M.; Li, Z.; Ding, J. F.; et al. Bulk Heterojunction Solar Cells Using Thieno [3,4-c] Pyrrole-4,6-Dione and Dithieno [3,2-b:20,30-d] Silole Copolymer with A Power Conversion Efficiency of 7.3%. J. Am. Chem. Soc. 2011, 133, 4250− 4253. (22) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.; Reynolds, J. R. Dithienogermole as A Fused Electron Donor in Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2011, 133, 10062− 10065. (23) Li, W. W.; Zhou, Y.; Andersson, B. V.; Andersson, L. M.; Thomann, Y.; Veit, C.; Tvingstedt, K.; Qin, R. P.; Bo, Z. S.; Inganäs, O.; et al. The Effect of Additive on Performance and Shelf-Stability of HSX-1/PCBM Photovoltaic Devices. Org. Electron. 2011, 12, 1544− 1551. (24) van Bavel, S. S.; Barenklau, M.; de With, G.; Hoppe, H.; Loos, J. P3HT/PCBM Bulk Heterojunction Solar Cells: Impact of Blend Composition and 3D Morphology on Device Performance. Adv. Funct. Mater. 2010, 20, 1458−1463. (25) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (26) Koeckelberghs, G.; De Cremer, L.; Persoons, A.; Verbiest, T. Influence of the Substituent and Polymerization Methodology on the Properties of Chiral Poly(Dithieno [3,2-b: 2′,3′-d] Pyrrole)s. Macromolecules 2007, 40, 4173−4181.

(27) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bässler, H.; Porsch, M.; Daub, J. Efficient 2-Layer LEDs on A Polymer Blend Basis. Adv. Mater. 1995, 7, 551−554. (28) Lee, K. H.; Kumar, B.; Park, H. J.; Kim, S. W. Optimization of An Electron Transport Layer to Enhance the Power Conversion Efficiency of Flexible Inverted Organic Solar Cells. Nanoscale Res. Lett. 2010, 5, 1908−1912. (29) Yang, R.; Tian, R.; Hou, Q.; Zhang, Y.; Li, Y.; Yang, W.; Zhang, C.; Cao, Y. Light-Emitting Copolymers Based on Fluorene and Selenophene-Comparative Studies with Its Sulfur Analogue: Poly(Fluorine-co-Thiophene). J. Polym. Sci., Polym. Chem. 2005, 43, 823− 836. (30) Hou, J.; Tan, Z.; He, Y.; Yang, C.; Li, Y. Branched Poly(Thienylene Vinylene)s with Absorption Spectra Covering the Whole Visible Region. Macromolecules 2006, 39, 4657−4662. (31) Scully, S. R.; McGehee, M. D. Effects of Optical Interference and Energy Transfer on Exciton Diffusion Length Measurements in Organic Semiconductors. J. Appl. Phys. 2006, 100, 034907. (32) Kim, B. J.; Miyamoto, Y.; Ma, B. W.; Fréchet, J. M. J. Photocrosslinkable Polythiophenes for Efficient, Thermally Stable, Organic Photovoltaics. Adv. Funct. Mater. 2009, 19, 2273−2281. (33) Wang, Y.; Chen, X. H.; Zhong, Y. L.; Zhu, F. R.; Loh, K. P. Large Area, Continuous, Few-Layered Graphene as Anodes in Organic Photovoltaic Devices. Appl. Phys. Lett. 2009, 95, 063302. (34) Jeong, J. A.; Kim, H. K. Low Resistance and Highly Transparent ITO−Ag−ITO Multilayer Electrode Using Surface Plasmon Resonance of Ag Layer for Bulk-Heterojunction Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2009, 93, 1801−1809. (35) Matsuo, Y. Development of Fullerene Derivatives with High LUMO Level Through Changes in Pi-Conjugated System Shape. Pure Appl. Chem. 2012, 84, 945−952. (36) Meng, X. Y.; Zhang, W. Q.; Tan, Z. A.; Du, C.; Li, C. H.; Bo, Z. S.; Li, Y. F.; Yang, X. L.; Zhen, M. M.; Jiang, F.; et al. Dihydronaphthyl-Based [60] Fullerene Bisadducts for Efficient and Stable Polymer Solar Cells. Chem. Commun. 2012, 48, 425−427. (37) Alley, N. J.; Liao, K. S.; Andreoli, E.; Dias, S.; Dillon, E. P.; Orbaek, A. W.; Barron, A. R.; Byrne, H. J.; Curran, S. A. Effect of Carbon Nanotube-Fullerene Hybrid Additive on P3HT:PCBM BulkHeterojunction Organic Photovoltaics. Synth. Met. 2012, 162, 95−101. (38) Liu, Z. Y.; He, D. W.; Wang, Y. S.; Wu, H. P.; Wang, J. G. Graphene Doping of P3HT:PCBM Photovoltaic Devices. Synth. Met. 2010, 160, 1036−1039.

3278


Highly Efficient Solar Cells Based on the Copolymer of

Recommend Documents