Controlling open-circuit voltage in organic solar cells by terminal fluoro-functionalization of narrow-bandgap π-conjugated molecules

A series of narrow-bandgap π-conjugated small molecules composed of benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) chromophoric units with different electron-withdrawing fluorinated end groups was designed and synthesized as donor materials for systematically studying their structure-property relationship in organic solar cells (OSCs). The terminal fluoro-functionalization of the π-conjugated BDT-DPP backbone resulted in systematic changes in the HOMO and LUMO energy levels of the resulting materials, as well as in their subsequent OSC device performance. These materials possessed relatively low HOMO energy levels ranging from -5.23 to -5.47 eV, while simultaneously maintaining small bandgap energies of approximately 1.6 eV in their thin films. With such proper engineering of the HOMO energy levels, the bulk heterojunction OSCs based on these fluoro-functionalized molecules as donor materials and PC₇₁BM as an acceptor material demonstrated high open-circuit voltages of up to 0.94 V. In this series of donor materials, the 3,5-difluorophenyl and 4-fluorophenyl-substituted BDT-DPP molecules exhibited superior charge transport and self-organization properties, resulting in higher power conversion efficiencies.