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

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 pi-pi 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 center dot center dot center dot O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar Ordering, but closer pi-pi 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 V-oc > 0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V-oc 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.