Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap Organic Semiconductor Acceptors with Their Absorption Edge Reaching 900 nm

Three n-OS acceptors with E-g values of <1.4 eV were synthesized by introducing double-bond pi-bridges into ITIC (ITVIC) and ITIC with monofluorine (ITVfIC) or bifluorine (ITVffIC) substituents on its end groups, and the structure-property relationships of the acceptors were systematically studied. The three n-OS films show broad absorption covering the wavelength range of 550-900 nm with narrow E-g values of 1.40 eV for ITVIC, 1.37 eV for ITVfIC, and 1.35 eV for ITVffIC. Additionally, the fluorine substitution downshifted the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the compounds. The photovoltaic properties of the n-OS acceptors were investigated by using a medium bandgap conjugated polymer J71 as a donor. The optimized polymer solar cells (PSCs) based on J71:ITVffIC demonstrated a power conversion efficiency (PCE) of 10.54% with a high J(sc) of 20.60 mA cm(-2) and a V-oc of 0.81 V, and the highest J(sc) reached 22.83 mA cm(-2). The high J(sc) values of the devices could be attributed to the broad absorption and lower-lying HOMO energy levels of the acceptor. Considering the V-oc of 0.81 V and the narrow bandgap of 1.35 eV for the acceptor ITVffIC, we found the energy loss (E-loss) of the ITVffIC-based PSCs was reduced to 0.54 eV, which is the lowest value in the PSCs with a PCE of >10%. The results indicate that ITVffIC is a promising narrow E-g acceptor for application in tandem and semitransparent PSCs.