Triad-type, multi-functional compatibilizers for enhancing efficiency, stability and mechanical robustness of polymer solar cells

The operation stability of polymer solar cells (PSCs) is one of the most important prerequisites for their practical use. In this work, we report a new acceptor-donor-acceptor (A-D-A) triad-type small molecule, 5TRh-PCBM, as a compatibilizer for enhancing the thermal stabilities and mechanical properties of efficient PSCs while increasing their power conversion efficiencies (PCEs). This multifunctional 5TRh-PCBM molecule, consisting of an oligothiophene segment as the central core and fullerene derivatives as the end groups, is designed to enable strong interactions between the 5TRh-core with various types of efficient polymer donors containing thiophene or fused-thiophene units, while the end fullerene groups preferentially interact with PCBM acceptors. To examine the effectiveness of this molecular compatibilizer, PSCs with different donors (PTB7-Th, PBDB-T, and P3HT) have been fabricated and tested, with addition of various amounts of 5TRh-PCBM. The addition of 5 and 10 wt% of 5TRh-PCBM significantly enhances the thermal and mechanical stabilities of all tested PSCs. Importantly, unlike typical compatibilizers, the addition of 5TRh-PCBM can increase the PCEs of the PSCs due to its light harvesting capability. In particular, the PCE of PTB7-Th:PCBM-based PSCs is increased from 9.37% to 10.09% with the 5 wt% addition of 5TRh-PCBM. Our comprehensive investigations have revealed the effects of 5TRh-PCBM on the optical, morphological, photovoltaic, and mechanical properties of molecularly engineered PSCs.