Conformation flipping of asymmetric nonfullerene acceptors enabling high-performance organic solar cells with 77% fill factors

Considerable progress on high-performance organic solar cells (OSCs) has been achieved in the past due to the rapid development of nonfullerene acceptors (NFAs). Typically, two kinds of methods have been employed to manipulate energy levels and aggregation of NFAs, i.e., molecular engineering on alkyl side chains and modification of the heterocyclic rings in the backbone. Herein, a novel asymmetric thiophene[3,2-b] pyrrole (TP)-based NFA with flipped molecular conformation, named as PTBTT-4F, is designed and synthesized. The introduction of the pyrrole ring in the novel NFA would not only afford extra reaction sites for side chain modification, but also induce substantial intramolecular charge transfer, thus leading to elevated energy levels of the NFA and thereby lower energy loss of the OSCs. When pairing with polymer donor PBDB-TF to fabricate OSCs, concurrent improvement in open-circuit voltage, short-circuit current (J_SC), and fill factor (FF) is realized, which delivers an outstanding power conversion efficiency (PCE) of 14.49%. Benefitting from effective molecular stacking and optimized phase separation induced by molecular conformation variation, asymmetric PTBTT-4F fabricated OSCs exhibit much higher J_SCs and FFs than the symmetrical PTBTP-4F devices.