Systematic control of heteroatoms in donor–acceptor copolymers and its effects on molecular conformation and photovoltaic performance

Abstract

Heteroatom substitutions are an effective means of tuning the optoelectronic, conformational, and molecular packing properties of donor–acceptor conjugated copolymers, with a view to efficient photovoltaic performance. We investigate the effects of systematic sulfur/selenium substitutions into thiophene (donor) and benzothiadiazole (acceptor) units using complementary Raman spectroscopy, density functional theory, and X-ray diffraction to characterise the resulting copolymers. We find that, in each case, the heavy atom substitution is detrimental to photovoltaic performance and undertake to understand this in terms of fundamental optoelectronic properties of the copolymers. Specifically, we find that, due to mesomeric effects, the selenium atom donates electron density into the donor unit (selenophene) more strongly than sulfur, but also withdraws electron density more strongly from the benzene ring in the acceptor unit (benzoselenadiazole). In both cases, the selenium substitution reduces the optical energy gap but is unfavourable for intermolecular packing in thin films and so results in poor charge carrier mobility. We identify a complex interplay between the electronic properties of the substituted donor and acceptor units relating to the frontier molecular orbital energy levels, molecular conformation, and intermolecular packing of the copolymers. In particular, we find that the pairing of a strong acceptor unit with a weak donor unit results in relatively low electron density on the conjugated backbone, leading to high inter-unit torsion and weak optical absorption in the visible range. The methods and insights developed here have broad applicability to the design of other donor–acceptor copolymers for optoelectronic device applications.