Understanding effects of two different acceptors in one small molecule for solution processable organic solar cells

Abstract

Introducing two different electron-withdrawing groups into the molecular backbone is one of the effective methods to extend absorption and lower the highest occupied molecular orbital (HOMO) energy level for a high open-circuit voltage (V_oc). However, examples of two-acceptor type small molecules are less studied. In order to understand the effect of two acceptors, two kinds of small molecules named as BDT(dFBT-TT)₂ with a single acceptor unit and BDT(dFBT-ON)₂ with two acceptor units have been synthesized, respectively. However, the power conversion efficiency (PCE) of the BDT(dFBT-ON)₂ device is lower (3.54%) than that of BDT(dFBT-TT)₂ (5.55%). Grazing incidence wide angle X-ray scattering (GIWAXS) and the intensity dependence of current–voltage characteristics were used to study the spatially periodic order and charge recombination of these small molecules. Although the V_oc of BDT(dFBT-ON)₂ is higher than that of BDT(dFBT-TT)₂, the short circuit current (J_sc) of BDT(dFBT-ON)₂ is lower due to a higher ratio of charge carrier recombination. For the sake of getting a high V_oc and J_sc simultaneously in the future, this work would supply valuable insights for designing multi-acceptor-type small molecules.