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

We have used density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to perform a theoretical study of seven novel small molecules SMi (i = 1-7) characterized by their D-n-A-n-D structures, these molecules have been designed based on a reference molecule composed mainly of Diketopyrrolopyrole as acceptor. In addition, to improve the optoelectronic and photovoltaic properties, the D electron donor (Carbazole) was replaced by another donor (Pyranylidene), and the n bridge (Thiophene) was also replaced by other bridges. Thus, replacing the Carbazole with Pyranylidene results in a significant reduction in the energy gap Egap and an accompanying red shift in the absorption spectra, in addition to the decrease in electron and hole reorganization energy. On the other hand, modification of the n-bridge increased the VOC and LHE values. Finally, a study was conducted on the SMi (i = R, 1-7)/ PC61BM blend to assess the effective charge transport at the D/A interface.