High‐Performance Ternary Polymer Solar Cells Enabled by a New Narrow Bandgap Nonfullerene Small Molecule Acceptor with a Higher LUMO Level

Obtaining a large open‐circuit voltage ( V OC ) and high short‐circuit current density ( J SC ) simultaneously is important in improving power conversion efficiency (PCE) of organic photovoltaics. The ternary strategy with using a higher lowest unoccupied molecular orbital (LUMO) level nonfullerene acceptor (NFA) guest can achieve increased V OC , yet J SC is decreased or maintained, so it's still a challenge to offer increased V OC and J SC values concurrently via the newly presented V OC ‐increased ternary strategy. To overcome this issue, a new narrow bandgap NFA TT‐S‐4F is reported by introducing 3,6‐dimethoxylthieno[3,2‐b]thiophene (TT) as π‐spacers to connect electron‐rich core with terminal groups, so as to upshift the LUMO level and extend π‐system. When adding 10% TT‐S‐4F into binary system based on PTB7‐Th:IEICO‐4F, the higher‐LUMO‐level of TT‐S‐4F, the increased charge mobilities, the reduced trap‐assisted combination loss, and a finer nanofiber structure and increased phase separation size are obtained, which simultaneously promotes J SC , V OC , and fill factor (FF), thus obtaining an optimal PCE (12.5% vs 11.5%). This work illustrates that an extending conjugated backbone with large π‐spacers and inclusion of alkylthiophenyl side‐chains is a concept to synthesize NFA guests for use on the V OC ‐increased ternary strategy that enables to realize simultaneously increased J SC , V OC , and FF.