Fluorination-substitution effect on all-small-molecule organic solar cells

Due to the strong crystallinity and anisotropy of small molecules, matched molecular photoelectric properties and morphologies between small molecules and non-fullerene acceptors are especially important in all-small-molecule organic solar cells (OSCs). Introducing fluorine atoms has been proved as an effective strategy to achieve a high device performance through tuning molecular energy levels, absorption and assembly properties. Herein, we designed a novel benzodithiophene-based small molecule donor BDTF-CA with deep highest occupied molecular orbital (HOMO) energy level. All-small-molecule OSCs were fabricated by combing non-fullerene acceptor IDIC with different fluorine-atom numbers. Two or four fluorine atoms were introduced to the end-capped acceptor of IDIC, which are named as IDIC-2F and IDIC-4F, respectively. With the increase of fluorination from IDIC to IDIC-4F, the open circuit voltage (Voc) of the devices decreased, while hole and electron mobilities of the active layers increased by one order of magnitude. Contributed to the most balanced Voc, short-circuit current (Jsc) and fill factor (FF), the device based on BDTF-CA/IDIC-2F achieved the highest power conversion efficiency of 9.11%.