Reduced exciton binding energy and diverse molecular stacking enable high-performance organic solar cells with V_OC over 1.1 V

High-voltage organic solar cells (OSCs) have received increasing attention because of their promising applications in tandem devices and indoor photovoltaics, but the trade-off between energy loss and charge generation induced by exciton binding energy (E_b) has become one of the biggest bottlenecks limiting the development of this field. Here, a wide bandgap (WBG) nonfullerene acceptor BTA503 with reduced E_b is designed by changing the phenyl side chain on the central core of Cl-BTA5 to an alkyl chain. The diverse π-π interactions and enhanced molecular stacking of BTA503 are responsible for its reduced E_b. Furthermore, both the diminished charge recombination and the fast exciton dissociation caused by the small E_b favor the generation of more charge carriers for the PTQ10:BTA503 combination. The efficient Forster resonance energy transfer (FRET) and multiple π-π stacking patterns provide additional charge transfer and transport pathways. Ultimately, the PTQ10:BTA503-based OSC device achieves a V_OC of 1.112 V and a PCE of 12.70%, which is higher than that of PTQ10:Cl-BTA5 (PCE=10.92%). Simultaneously, the thick film (~300 nm) binary device of PTQ10:BTA503 achieves a PCE of 10.13% with a V_OC of 1.102 V, which is the best result for thick film high-voltage OSCs. More importantly, the ternary device of PTQ10: BTA503:Cl-BTA5 (1:0.9:0.1) realizes a champion PCE of 13.12% with a V_OC of 1.126 V. Our study demonstrates that it is an effective strategy to reduce E_b of A₂-A₁-D-A₁-A₂ type WBG acceptors by modulating the side chains on D unit, which further favors the corresponding devices to obtain world-record PCE and improves their potential for commercial applications.