Bis(carboxylate) substituted benzodithiophene based wide-bandgap polymers for high performance nonfullerene polymer solar cells

Abstract

To develop wide bandgap donor materials for high performance nonfullerene polymer solar cells (NF PSCs), bis(carboxylate) substituted benzo[1,2-b:4,5-b]dithiophene (BDT) weak electron acceptor (A) unit was used to polymerize with 5-alkylthiophene-2-yl-substituted BDT or 5-alkylthiothiophene-2-yl-substituted BDT donor unit to construct donor-acceptor (D-A) alternative polymers PBDT-BDTC and PBDTS-BDTC, respectively. PBDT-BDTC and PBDTS-BDTC exhibit wide bandgaps over 1.90 eV and low lying highest occupied molecular orbital (HOMO) levels below −5.50 eV. The absorption and energy levels of these two polymers match well with those of the typical low bandgap acceptor ITIC-Th, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset of about 0.15 eV. NF PSCs with an inverted device structure were fabricated and power conversion efficiencies (PCEs) higher than 7% were achieve. Furthermore, PBDT-BDTC based PSCs exhibit much better photovoltaic performance than PBDTS-BDTC based ones after the treatment with 0.5% 1,8-diiodooctane (DIO). The optimized devices gave a PCE of 8.32% with an open-circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 13.91 mA/cm2, and a fill factor (FF) of 65.21%, indicating that PBDT-BDTC is a promising polymer donor for NF PSCs. To the best of our knowledge, a PCE of 8.32% is the highest record for bis(carboxylate) substituted BDT based polymers.