Theoretical Approach Towards Rational Design and Characterization of Benzo[1,2-b:5-B’]dithiophene (BDT)-Based (A-D-A) Small Molecules of Relevance for High Performance Solar Cells

Benzo[1,2-b:5-B’]dithiophene (BDT)–based small molecules with acceptor-donor-acceptor (A-D-A) structure were designed based on the experimental system BDTT-S-TR (1) for use as potential donor materials for organic photovoltaic (OPV) devices. Their geometry structures, electronic properties and other key parameters related to OPVs such as absorption spectra, energetic driving forces ΔE L-L, power conversion efficiencies (PCEs) and intramolecular charge transfer properties have been investigated by means of density functional theory (DFT) and time dependent density functional theory (TDDFT) methods. These have been exploited as donor materials for a heterojunction with [6,6] phenyl-C71-butyric acid methyl ester (PC70BM) as acceptor material. Based on Marks model, an excellent agreement between the experimental and predicted PCE was obtained for the reported system 1/PC70BM and a significant improvement in PCEs of BHJ devices based on 2-4/PC70BM was manifested. The charge transfer rates of the interfacial charge transfer Kint er−CT and recombination Kint er −CR in 1-4/PC70BM heterojunctions have been calculated using Marcus-Levich-Jortner rate equation. The calculations show that the ratios Kint er-CT / kint er-CR for the 2-4/PC70BM heterojunctions are ~104 times higher than that of the 1/PC70BM. From these predictions, we reached our purpose to provide rational design of three novel molecules that will be more promising candidates for high-efficiency SMs OPVs materials.