Tuning the LUMO levels of non-fullerene acceptors via extension of π-conjugated cores for organic solar cells

We investigate how extending the length of a pi-conjugated central A ' core in Acceptor-Donor-Acceptor '-Donor-Acceptor (A-D-A '-D-A)-type non-fused-ring electron acceptors (NFREAs) affects their energy level and the miscibility of donor polymer:NFREA organic solar cells (OSCs). The extended pi-conjugated central A ' core increases their lowest unoccupied molecular orbital (LUMO) energy level. We designed and synthesized three NFREAs: BT-4F, which has a benzothiadiazole (BT) core; DTBT-4F, which has two fused thiophene rings on the BT core; and BTST-4F, which has two fused thienothiophene rings on the BT core. In these NFREAs, the LUMO level is upshifted in the order of increasing length of the extended pi-conjugated core. The open-circuit voltage (V-OC) is related to the gap between the LUMO of the NFREAs and the highest occupied molecular orbital (HOMO) of the donor polymer, thus, the V-OC increased from 0.66 V in a BT-4F-based OSC to 0.81 and 0.84 V in OSCs based on DTBT-4F and BTST-4F, respectively. The PBDB-T donor polymer is more miscible with BTST-4F than with BT-4F and DTBT-4F. The BTST-4F-based blends have a narrower light-absorption range than the PBDB-T:BT-4F blends; however, because of the miscible morphologies of the PBDB-T:BTST-4F blends, the BTST-4F-based devices exhibit a comparable short-circuit current density (J(SC)) to that of BT-4F-based devices. These results indicate that the introduction of extended pi-conjugation of the central core in A-D-A '-D-A-type NFREAs could be an effective method to increase the V-OC of NFREA-based devices while maintaining a decent J(SC). These results further indicate that introducing an extended pi-conjugated central core is a promising design strategy to achieve highly efficient OSCs based on NFREAs.