Abstract
Three copolymers based on poly(dialkoxybenzo[1,2‐b:4,5‐b′]dithiophene‐biselenophene) P3a and its heteroatom(s) analogues P3b and P3c are synthesized by Stille cross‐coupling to investigate the effects of the heteroatom on the optoelectronic and photovoltaic properties. Detailed characterizations of the copolymers are carried out by 1H NMR, gel permeation chromatography, UV–vis absorption spectroscopy, and cyclic voltammetry. Optical absorption spectra reveal that the presence of biselenophene moiety in the copolymer P3a shows red‐shifted absorption (band gap 1.82 eV) compared to bithiophene containing copolymer P3b (band gap 1.91 eV), while band gap of bifuran containing copolymer P3c is 2.05 eV. The photovoltaic devices are fabricated using the copolymers P3a–c blended with phenyl‐C71‐butyric acid methyl ester as an acceptor material with the device structure of indium tin oxide (ITO)/MoO3/active layer/Al. The power conversion efficiency is gradually increased with the replacement of furan by thiophene and furthermore thiophene by selenophene, respectively, due to increment in J sc. Density functional theory calculations are studied on these three copolymers to understand the trends of energy levels and throughout compared with experimental data where applicable. The strategy has advantage of different aromaticities of heterocyclic rings combining with electronegativities and polarizabilities of the heteroatom(s) to allow the band gap engineering and tunability of photovoltaic performances. image
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