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.

Automated summary

We study the effect of extending the length of the pi-conjugated central core in non-fused-ring electron acceptors on their energy levels and miscibility with donor polymers in organic solar cells. Increasing the length of the conjugated core raises the lowest unoccupied molecular orbital (LUMO) energy level. Solids with the longest conjugated core have the highest LUMO levels. The open-circuit voltage (V-OC) of the solar cells increases as the length of the conjugated core increases. The miscibility of the donor polymer with the acceptor is improved by increasing the length of the conjugated core. The narrower light-absorption range of the blend containing the acceptor with the longest conjugated core is compensated by the improved miscibility, resulting in comparable short-circuit current densities (J(SC)).