Effects of including electron-withdrawing atoms on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene-based donor–acceptor polymers: towards an acceptor design for efficient polymer solar cells

Abstract

Three new D–A polymers PIDTT-DTBO, PIDTT-DTBT and PIDTT-DTFBT, using indacenodithieno[3,2-b]thiophene (IDTT) as the electron-rich unit and benzoxadiazole (BO), benzodiathiazole (BT) or difluorobenzothiadiazole (FBT) as the electron-deficient unit, were synthesized via a Pd-catalyzed Stille polymerization. The included electron-withdrawing atoms of the acceptor portion were varied between O, S, and F for tailoring the optical and electrochemical properties and the geometry of structures. Their effects on the film topography, photovoltaic and hole-transporting properties of the polymers were thoroughly investigated via a range of techniques. As expected, the stronger electron-withdrawing BO unit affords red-shifted absorption, low-lying HOMO and LUMO levels for the polymer PIDTT-DTBO. However, it depicts lower hole mobility and a less efficient charge collection in the active layer compared to the polymer PIDTT-DTBT. In addition, degradation of the solubility is observed in the fluorinated polymer PIDTT-DTFBT. As a result, a BHJ PSC (ITO/PEDOT:PSS/polymer:PC₇₁BM/interlayer/Al) fabricated with PIDTT-DTBT attains the best power conversion efficiency (PCE) of 4.91%. These results thus demonstrate the potential effects of electronegative atoms on IDTT-based polymers and the structure–function correlations of such electron-donor materials for efficient PSCs.