Fluorine Substitution on Donor Unit in A–D–A Type Small Molecule for High VOC Organic Solar Cells

Abstract

Since it has generally been accepted that the VOC in organic photovoltaics (OPVs) is proportional to the energy difference between the HOMO of donor and the LUMO of acceptor, it is indispensable for achieving high VOC to lower the HOMO energy level of donor material. For the purpose, we synthesized two different A–D–A type small molecules (SMs), where A is thiophene-capped DPP (TDPP) and D is fluorinated bithiophene (FT2) or fluorinated biphenylene (FPh2), and thus the two fluorinated SMs are denoted by FT2(TDPP)2 and FPh2(TDPP)2, respectively. Since fluorine has strong electron-withdrawing property, it is expected that the introduction of fluorine atom onto the D unit in A–D–A type SMs lowers its HOMO energy level, resulting in enhancement of VOC of OPV cell. The HOMO energy levels of fluorinated SMs were deeper than those of non-substituted ones by 0.07 eV: FT2(TDPP)2 and FPh2(TDPP)2 exhibit high VOC of 0.87 and 0.95 V, respectively.