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Thiazolothiazole Donor–Acceptor Conjugated Polymer Semiconductors for Photovoltaic Applications

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journal contribution
posted on 08.07.2014, 00:00 by Selvam Subramaniyan, Hao Xin, Felix Sunjoo Kim, Nishit M. Murari, Brett A. E. Courtright, Samson A. Jenekhe
A series of eight new donor–acceptor conjugated copolymers based on thiazolo­[5,4-d]­thiazole (TT) were synthesized by Suzuki and Stille polycondensations, and their optical, electronic structure, field-effect charge transport, and photovoltaic properties were investigated. The number-average molecular weight was moderate to high (20–103 kDa) for four of the new copolymers and low (<10 kDa) for the rest. The optical band gap (1.7–2.2 eV) and the ionization potential (4.9–5.7 eV) of the copolymers could be tuned by varying the strength of the electron-donating comonomers. X-ray diffraction analysis of the neat TT polymer thin films showed that they exhibit a lamellar crystalline morphology with a (100) d-spacing of 1.449–1.881 nm. The polymers had field-effect hole mobilities of 2 × 10–3–3 × 10–2 cm2/(V s). Although fullerene (PC71BM)-based bulk heterojunction solar cells showed that most of the new polymers had low photovoltaic power conversion efficiency (<2.4%), the standout polymer PDEHTT had a high efficiency of 5.7%. X-ray diffraction analysis and TEM imaging of PDEHTT/PC71BM blend films showed a bicontinuous nanoscale morphology with a crystalline polymer nanofiber (15–20 nm width) network.