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

Abstract

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 ( V oc) of the devices decreased, while hole and electron mobilities of the active layers increased by one order of magnitude. Contributed to the most balanced V oc, short-circuit current ( J sc) and fill factor (FF), the device based on BDTF-CA/IDIC-2F achieved the highest power conversion efficiency of 9.11%.