Asymmetrically Alkyl-Substituted Wide-Bandgap Nonfullerene Acceptor for Organic Solar Cells

An asymmetric wide-bandgap (WBG) nonfullerene acceptor (C-6-IDTT-T) is developed by shearing one alkyl side-chain from a symmetrically alkyl-substituted indacenodithieno[3,2-b]thiophene (IDTT) core of the fused-ring electron acceptor 2C(6)-IDTT-T. These two acceptors both exhibit wide optical bandgaps over 1.8 eV. Investigations on the optical, electrochemical, and active layer morphology are conducted to understand the effect of asymmetric side chains on the electrical and photovoltaic properties. Compared with symmetric 2C(6)-IDTT-T, asymmetric C-6-IDTT-T is found to exhibit redshifted absorption and higher electron mobility. As a result, the C-6-IDTT-T blend with a thienothiophene-benzodithiophene copolymer (PTB7-Th) presents higher electron mobility and more balanced charge carrier transport, which leads to an enhanced power conversion efficiency of 8.51% for C-6-IDTT-T-based device with a high open-circuit voltage of 1.052 V and a low energy loss of 0.60 eV.