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

*S Supporting Information ABSTRACT: 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 HOMOsessential 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