Improving the Electron Mobility of ITIC by End-Group Modulation: The Role of Fluorination and π-Extension

Nonfullerene organic solar cells (OSCs) using ITIC derivatives as electron acceptors have achieved power conversion efficiencies up to 14%, yet optimal active-layer thicknesses are still limited to approximate to 100 nm, ascribed mainly to the low electron mobilities (approximate to 10(-4) cm(2)/Vs) of these acceptors. Because of the large steric hindrance of the bulky side chains on the fused-ring core, ITIC favors a local pi-pi stacking between the electron-withdrawing end groups (IC), which provides the main electron transport channel across the bulk volume. Here, the influence of different fluoro-substituted and pi-extended (i.e., benzene-fused) positions in the phenyl moiety of IC on the electron transport properties is systematically investigated by multiscale theoretical simulations. It is found that the electron mobility can be remarkablely improved by proper fluorination and pi-extension, especially by pi-extension, due to the lower reorganization energy and stronger end-group pi-pi interaction. Moreover, a judicious combination of pi-extension and fluorination can lead to a nearly six-fold increase of the electron mobility with respect to ITIC. This work shows that the electron mobility of A-D-A nonfullerene acceptors can be effectively improved by end-group engineering, paving the way toward higher-performance organic solar cells.