Energy-level modulation of non-fullerene acceptors to achieve high-efficiency polymer solar cells at a diminished energy offset

Efficient fullerene-free polymer solar cells (PSCs) are fabricated with a polymer donor PBDB-T1 and a non-fullerene acceptor ITTIC. With the incorporation of one thiophene bridge between the indacenodithienothiophene (IDTT) core and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) terminal, the new acceptor ITTIC exhibits a higher lying lowest unoccupied molecular orbital (LUMO), and a narrower bandgap than the pristine ITIC acceptor. The resultant PSCs with PBDB-T1:ITTIC blends exhibit a power conversion efficiency of 9.12%, with an increased open circuit voltage (V-OC) and broader photoresponse, compared with the PBDB-T1:ITIC based devices. Interestingly, it is shown that the charge transfer remains effective at a diminished highest occupied molecular orbital (HOMO) difference of 0.02 eV between PBDB-T1 and ITTIC, leading to a mitigated energy loss of 0.54 eV in PBDB-T1:ITTIC based devices. Overall, this work provides new insights into further improvement of fullerene-free PSCs.