Conjugated polymers with 2,7-linked 3,6-difluorocarbazole as donor unit for high efficiency polymer solar cells

Abstract

A novel donor–acceptor (D–A) copolymer PDFCDTBT with 3,6-difluoro substituted carbazole as the donor unit and dialkoxy substituted benzothiadiazole as the acceptor unit has been synthesized and used as a donor material for bulk heterojunction polymer solar cells (BHJ PSCs). PDFCDTBT possesses a band gap of 1.75 eV, a low-lying HOMO energy level of −5.23 eV, and a good thermal and electrochemical stability. In comparison with the corresponding non-fluoro substituted HXS-1, which has a HOMO energy level of 5.21 eV, a LUMO energy level of 3.35 eV, and an optical band gap of 1.86 eV, the incorporation of two fluoro atoms in the carbazole donor unit lowers the HOMO and the LUMO energy levels of the polymer, which results in simultaneously decreasing the band gap of the polymer and increasing the V_oc of polymer solar cells. The fluoro-containing polymer PDFCDTBT also shows strong intramolecular interactions and forms close packing in the solid state. Polymer solar cells based on PDFCDTBT and (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) demonstrate a power conversion efficiency (PCE) of 4.8% with a V_oc of 0.91 V, a J_sc of 9.5 mA cm⁻², and an FF of 0.55. In comparison with HXS-1, the better stability, higher V_oc, and narrower band gap indicate that PDFCDTBT is a very promising donor material for high efficiency polymer solar cells.