All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10%

We synthesized two wide bandgap A-D-A structured p-type organic semiconductor (p-OS) small molecules with weak electron-withdrawing ester end groups: SM1 with cyano group (CN) on the ester group and SM2 without the CN group. SM1 showed stronger absorption, lower-lying HOMO energy level, and higher hole mobility in comparison with that of SM2 without the CN groups. The all-small molecule organic solar cell (SM-OSC) with SM1 as donor and a narrow bandgap n-OS IDIC as acceptor demonstrated a high power conversion efficiency (PCE) of 10.11% and a high fill factor (FF) of 73.55%, while the PCE of the device based on SM2:IDIC is only 5.32% under the same device fabrication condition. The PCE of 10.11% and FF of 73.55% for the SM1-based device are the highest values for the nonfullerene SM-OSCs reported in the literature so far. The results indicate that the cyano substitution in SM1 plays an important role in improving the photovoltaic performance of the p-OS donors in the nonfullerene SM-OSC. In addition, the photoinduced force microscopy (PiFM) was first used in OSCs to characterize the morphology of its donor/acceptor blend active layer.