An Insight into the Mechanism of Alkyl Side-Chain Engineering of BTCN on Its Photovoltaic Properties─A Theoretical Study

Organic photovoltaic materials featuring thiophene-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-T) units show great potential. However, the influence of alkyl side-chain engineering on BDT-T units in these materials remains elusive. In this study, we focused on a high-performance small-molecule BTCN series with an acceptor–donor–acceptor architecture, where BDT-T serves as the donor. We systematically explored how varying the number and positions of alkyl chains on lateral thiophene rings affects the photovoltaic properties. The geometric parameters and ground and excited state properties were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). The experimentally observed differences in photovoltaic performance between BTCN-M and BTCN-O due to different substituted positions of the two alkyl chains can be explained well by our calculated data. Furthermore, the results show that, out of the BTCN series considered, BTCN-S1 in which the single-alkyl substitution is next to the sulfur atom of the lateral thiophene molecule could be a promising donor since it has the most negative average electrostatic potential and strongest light absorption in the visible region. Lastly, compared to double-alkyl-chain substitutions, single-chain substitution on the BDT-T unit can decrease the exciton binding energy but may increase the singlet–triplet energy differences in BTCNs. These findings offer valuable insights into alkyl side-chain engineering for optimizing BDT-T-based organic materials.