Theoretical investigation by DFT and TDDFT the extension of π-conjugation of novel carbazole-based donor materials for bulk heterojunction organic solar cell applications

In this study, we have proposed seven designed symmetrical compounds ( C 2 -C 8 ) having a D-π-A-π-D structure based on derivative carbazole as a donor by introducing various π-spacer groups into the reference compound C 1 -Ref having a D-A-D structure in order to understand the influence of different π-spacers on their efficiency in BHJ solar cells. Various parameters such as geometrical structures, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), nonlinear optical properties (NLO), optical properties, light-harvesting efficiency (LHE), reorganization energy, chemical reactivity indices, exciton binding energy ( E b ), open-circuit voltage ( V OC ), and fill-factor (FF) have been investigated using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The results show that the extended π-conjugation of the designed compounds ( C 2 -C 8 ) produces a lower energy gap ( E g ), a stronger and broader absorption spectrum, lower reorganization energies and exciton binding, and higher nonlinear optical properties compared to C 1 -Ref , indicating that these designed compounds are promising as electron donors in BHJ-OSCs. Additionally, the calculated V oc , FF, and LHE of all compounds showed that the C 2 , C 3 , C 4 , C 5 , and C 7 compounds have the best performance in BHJ solar cells compared to the others. In particular, C 4 and C 5 are excellent candidates for the effective donor materials of BHJ solar cells due to their large V oc , FF, and LHE than the other compounds. This theoretical investigation is expected to provide new strategies to synthesize efficient donor materials for BHJ-OSCs.