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Bithiopheneimide–Dithienosilole/Dithienogermole Copolymers for Efficient Solar Cells: Information from Structure–Property–Device Performance Correlations and Comparison to Thieno[3,4-c]pyrrole-4,6-dione Analogues

Guo, Xugang, Zhou, Nanjia, Lou, Sylvia J., Hennek, Jonathan W., Ponce Ortiz, Rocío, Butler, Melanie R., Boudreault, Pierre-Luc T., Strzalka, Joseph, Morin, Pierre-Olivier, Leclerc, Mario, López Navarrete, Juan T., Ratner, Mark A., Chen, Lin X., Chang, Robert P. H., Facchetti, Antonio, Marks, Tobin J.
Journal of the American Chemical Society 2012 v.134 no.44 pp. 18427-18439
X-radiation, composite polymers, microstructure, photovoltaic cells, semiconductors, solubility, spectroscopy
Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor–acceptor copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOsessential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R₂Si→R₂Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π–π spacings vs linear N-alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure–property–device performance correlations are provided by a thieno[3,4-c]pyrrole-4,6-dione (TPD)–dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD-based polymers achieve greater subunit–subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π–π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC₇₁BM as acceptor exhibit promising device performance with PCEs up to 6.41% and Vₒc > 0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher Vₒc with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure–property relationships in TPD, BTI, and related polymer semiconductors.