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Enhancing Photovoltaic Performance of Copolymers Containing Thiophene Unit with D–A Conjugated Side Chain by Rational Molecular Design

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journal contribution
posted on 23.12.2013, 00:00 by Ping Shen, Haijun Bin, Lu Xiao, Yongfang Li
Rational molecular design of conjugated polymers and cautious optimization of morphologies of the active layer are critical for developing high performance polymer solar cells (PSCs). In this work, we designed and synthesized a new thiophene monomer TBTF attaching donor–acceptor (D–A) conjugated side chain with fluorinated 4,7-dithien-5-yl-2,1,3-benzodiathiazole (BTF) as acceptor unit, and synthesized two new two-dimension-conjugated (2D-conjugated) copolymers, P­(BDT-TBTF) and P­(BDT-TBTF/DPP), for the application as donor materials in PSCs. P­(BDT-TBTF) is a new side chain D–A copolymer of benzodithiophene (BDT) and TBTF units, and P­(BDT-TBTF/DPP) is a ternary D–A copolymer of BDT, TBTF and pyrrolo­[3,4-c]­pyrrole-1,4-dione (DPP) units. The introduction of TBTF unit with D–A conjugated side chain and DPP unit forming the ternary copolymer provides the opportunity to tune the optoelectronic properties of the resulting polymers. As expected, the binary copolymer P­(BDT-TBTF) shows an enhanced absorption coefficient and lower-lying HOMO energy level, and the ternary copolymer P­(BDT-TBTF/DPP) possesses a small bandgap and quite broad absorption band matched well with solar spectrum. These features are beneficial to achieving reasonable high short-circuit current (Jsc) and high open-circuit voltage (Voc). Bulk-heterojunction PSCs based on P­(BDT-TBTF) showed an initial power conversion efficiency (PCE) of 5.66% with a high Voc of 0.88 V and a Jsc of 11.23 mA cm–2, whereas P­(BDT-TBTF/DDP) gave a PCE of 3.51% along with a higher Jsc of 13.15 mA cm–2. The Jsc and PCE of the devices were further improved by a simple methanol treatment, to 13.21 mA cm–2 and 6.21% for P­(BDT-TBTF) and 14.56 mA cm–2 and 5% for P­(BDT-TBTF/DPP), respectively. To the best of our knowledge, the PCE of 6.21% is the highest value reported for PSCs based on side chain D–A copolymers to date. This is a good example for a subtle tuning absorption properties, energy levels, charge transport and photovoltaic properties of the polymers by rational molecular design.