A new low band gap donor–acceptor alternating copolymer containing dithienothiophene and fluorenone unit

Abstract

A low bandgap copolymer poly(3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene-alt-9-fluorenone) (PDTTFO) consisting of dithieno[3,2-b:2′,3′-d]thiophene (DTT) and 9-fluorenone (FO) was synthesized as the donor material for the polymer solar cells via Stille coupling polymerization. Both donor and acceptor units were confirmed by FT-IR and 1H-NMR. Optoelectronic properties of the PDTTFO copolymer were investigated and observed by UV-vis, photoluminescence (PL) spectrum, and cyclic voltammogram (CV). UV-vis spectrum of the film exhibited two absorption peaks centered at 358, 474 nm with a broad absorption band in the range of 300–700 nm and showed a low bandgap of 1.68 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be −5.09 and −3.41 eV, respectively. Based on the ITO/PEDOT:PSS/PDTTFO:PCBM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm2) was 0.374 %. The effects of annealing temperature on the device performance were studied. At annealing temperature of 175 °C/30 min, the device demonstrated an optimal efficiency of 0.923 %. The improved device efficiency under the optimal condition was confirmed by the higher light harvest in UV-vis spectra, the enhanced quenching of photoluminescence (PL) emission, the improved nanoscale morphology by atomic force microscopy (AFM) examination, and the increase in external quantum efficiency.