Heptacyclic S,N-Heteroacene-Based Near-Infrared Nonfullerene Acceptor Enables High-Performance Organic Solar Cells with Small Highest Occupied Molecular Orbital Offsets
Shanshan Ma, Shihao Wu, Jie Zhang, Yu Song, Haoran Tang, Kai Zhang, Fei Huang, Yong Cao
Abstract
The reduction of energy offsets between donors and acceptors is a direct way to improve the open-circuit voltage (VOC) and overall performance of organic solar cells (OSCs). In this work, two nonfullerene acceptors (NFAs) (BDTBO-4F and BDTBO-4Cl) were synthesized, which were composed of a heptacyclic S,N-heteroacene core and terminal units with halogen atoms, where the latter modulates the energy level of the frontier molecular orbital. Consequently, BDTBO-4Cl exhibited a deeper highest occupied molecular orbital level (EHOMO) and lowest unoccupied molecular orbital level (ELUMO) than BDTBO-4F. Moreover, these two NFAs exhibited high electron mobility and strong absorption at 700–900 nm. The polymer donor PM6 was combined with BDTBO-4F and BDTBO-4Cl, and the resulting OSCs exhibited outstanding power conversion efficiencies of 14.83% for the PM6:BDTBO-4F device and 13.87% for the PM6:BDTBO-4Cl device. More encouragingly, these OSCs exhibited efficient hole transfer from NFAs to PM6, despite small ΔEHOMO(D–A) values (<0.10 eV). These results prove that modulation of EHOMO of acceptors to decrease ΔEHOMO(D–A) is an efficient strategy for high-performance OSCs.