The effect of structural modification of novel D-π-A-π-D molecules on their performance as donors in bulk heterojunction solar cells: Theoretical investigation

•New small efficient donor molecules (SMi, i = R, 1–7) are being investigated for organic solar cell applications.•Molecules using Thiazolothiazole as a π-bridge have remarkable optoelectronic properties compared with the SMR reference.•Changing the Carbazole to Pyranylidene results in a significant reduction in the energy gap and a concomitant red shift in the absorption spectra.•All the molecules designed are recommended to experimentalists for future solar cell developments.