DFT and TD-DFT calculations on thieno[2,3-b]indole-based compounds for application in organic bulk heterojunction (BHJ) solar cells

Eight novel organic compounds with donor–π–acceptor (D–π–A) structure were designed for use as donors in organic bulk heterojunction (BHJ) solar cells. The molecules have thieno[2,3-b]indole as donor, methylene malononitrile as electron acceptor group, and a π-spacer bridge based on thiophene and benzene or its derivatives. The designed compounds were studied using density functional theory (DFT) and time-dependent DFT approaches, to shed light on how the π-conjugation order influences the performance of corresponding photovoltaic solar cells. The study includes prediction of the energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, the energy gap, Voc (the open-circuit voltage), the λmax of absorption, and other quantum-chemical parameters. The results show that, the greater the number of thiophenes present in the bridge, the better the compound as a donor for BHJ solar cells. The theoretical power conversion efficiency calculated for the designed products reaches 6%.