Near-infrared small molecule acceptors based on 4H-cyclopenta[1,2-b:5,4-b']dithiophene units for organic solar cells

Abstract Two small-molecule non-fullerene acceptors (NFAs) CPDT-Me-2Eh and CPDT-4Cl-2Eh based on 4H-cyclopenta[1,2-b:5,4-b′] dithiophene as the electron-donating unit 2-(5/6-methyl-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the end group respectively were developed. Compared with CPDT-Me-2Eh, CPDT-4Cl-2Eh possesses red-shift absorption spectrum, lower HOMO and LUMO energy level, and higher electron mobility. Non-fullerene polymer solar cells based on PBDB-T: CPDT-4Cl-2Eh gave a power conversion efficiency (PCE) of 7.52%, with a fill factor (FF) of 64.46%, and a short-circuit current (Jsc) of 16.90 mA/cm2. As contrast, the cell based on PBDB-T: CPDT-Me-2Eh got a PCE value of 4.39%. Our results indicate that the small molecule acceptor containing the CPDT core can effectively extend the absorption spectrum to the near-infrared (NIR) region by chlorination of the capping group. Our work provides an easy routine to broaden the field of new NIR NFAs with high PCE based on non-fullerene OSCs.