Gradient copolymers of thiophene and pyrrole for photovoltaics

[Display omitted] •We calculate the energy band gap of gradient copolymers of thiophene and pyrrol.•The optimal thiophene fraction range between zero and 40% gives the maximum band gap gradient.•It is suggested a route for enhanced charge separation in gradient copolymers.•Photovoltaic properties can be tuned by changing the composition of gradient copolymers. The electronic properties of copolymers can be tuned by controlling their monomer ratio, and therefore can potentially be used to improve charge separation in organic photovoltaic devices. Here we show evidence based on density functional theory calculations that it is possible to control the electronic structure of π-conjugated copolymers of thiophene and pyrrole to obtain a gradient in the band gap and both conduction and valence crystal orbital band levels by controlling their composition. Our calculations predict and optimal thiophene monomer fraction range between zero and 40% is needed in order to obtain the largest electronic structure gradients.