Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor

Recently, the fused-ring based low band gap（LBG） small molecule acceptors（SMAs） have emerged as efficient nonfullerene acceptors. So far, these LBG SMAs are mainly designed with IC(2-methylene-(3-（1,1-dicyanomethylene）indanone)) or its analogs, the benzo-type electron-accepting（A） units. Compared to benzene, thiophene is less aromatic and thus the thiophene-involving semiconducting molecule has more quinoidal character, which effectively reduces the energy gap between the highest occupied molecular orbit（HOMO） and the lowest unoccupied molecular orbit（LUMO）. Herein, we show that replacing the IC units in ITIC with the CT(cyclopenta[c]thiophen-4-one-5-methylene-6-（1,1-dicyanomethylene）), a thiophene-fused A unit, the quinoidal character is enhanced from 0.0353 on ITIC to 0.0349 on ITCT, the CT-ended SMA. The increase in the quinoidal character reduces the optical band gap and enhances the near IR absorptivity. When blended with the wide band gap（WBG） polymer donor, PBDB-T,an average power conversion efficiency of 10.99% is obtained with a short-circuit current-density(J;) of17.88 mA/cm;and a fill-factor（FF） of 0.723. For comparisons, the J;is of 16.92 mA/cm;, FF is of 0.655 and PCE is of 9.94% obtained from the ITIC:PBDB-T device. This case indicates that the replacement of the benzene ring on the IC unit with a more polarizable five-member ring such as thiophene is an effective way to enhance the absorption of the near IR solar photons towards designing high-performance nonfullerene polymer solar cells.