A benzo[1,2-b:4,5-b′]difuran- and thieno-[3,4-b]thiophene-based low bandgap copolymer for photovoltaic applications

Abstract

A new low bandgap conjugated polymer—PBDFTT-C was synthesized from thieno-[3,4-b]thiophene and benzo[1,2-b:4,5-b′]difuran units by Stille coupling reactions. The structure was verified by ¹H NMR and elemental analysis, the molecular weight was determined by gel permeation chromatography (GPC) and the thermal properties were investigated by thermogravimetric analysis (TGA). PBDFTT-C showed a low HOMO energy level of −5.27 eV. The polymer film displayed broad absorption in the wavelength region from 300 nm to 840 nm with a low bandgap of 1.48 eV. The field effect hole mobility of PBDFTT-C reached 5.4 × 10⁻³ cm² V⁻¹ s⁻¹. By using 1,8-diiodooctane (DIO) as the solvent additive, photovoltaic cells with the structure of ITO/PEDOT:PSS/PBDFTT-C:PC₇₁BM (1 : 1.5, w/w)/Ca/Al demonstrated a power conversion efficiency of 4.4% with a short circuit current of 10.45 mA cm⁻², open circuit voltage of 0.66 V and a fill factor of 0.64, under the illumination of AM 1.5G, 100 mW cm⁻².