Designing a thiophene-fused benzoxadizole as an acceptor to build a narrow bandgap polymer for all-polymer solar cells

In this work, a thiophene-fused benzoxadizole (BXT) unit was designed as a new acceptor and synthesized for the first time to build a D–A conjugated polymer (PBXT-IDT) with 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT) for all polymer solar cells (all-PSCs). Due to the strong electron-withdrawing ability of the BXT unit, PBXT-IDT exhibited a very narrow optical bang gap of 1.43 eV and very strong ICT absorption in the range of 500–850 nm that could be complementary with poly(3-hexylthiophene) (P3TH) in the visible absorption region. Moreover, PBXT-IDT showed relatively low HOMO–LUMO energy levels of −5.33 eV and −3.64 eV, respectively, which can be act as an electron-accepting material to match with P3HT as an electron-donating material for all-PSCs. Therefore, the all-PSC device with a blend of PBXT-IDT and P3HT as the active layer was fabricated and the photovoltaic performances were investigated. The device showed a PCE of 1.09% with a high Voc of 0.84 V and a relatively low energy loss (Eloss) of 0.59 V. This indicates that reasonable structural modification of benzoxadizole (BX) can pave a new way to design a polymer as an electron-accepting material in all-PSCs.