A Theoretical Study on the Underlying Factors of the Difference in Performance of Organic Solar Cells Based on ITIC and Its Isomers

Recently, non-fullerene-based organic solar cells (OSCs) have made great breakthroughs, and small structural differences can have dramatic impacts on the power conversion efficiency (PCE). We take <b>ITIC</b> and its isomers as examples to study their effects on the performance of OSCs. <b>ITIC</b> and <b>NFBDT</b> only differed in the side chain position, and they were used as models with the same donor molecule, <b>PBDB-T</b>, to investigate the main reasons for the difference in their performance in terms of theoretical methods. In this work, a detailed comparative analysis of the electronic structure, absorption spectra, open circuit voltage and interfacial parameters of the <b>ITIC</b> and <b>NFBDT</b> systems was performed mainly by combining the density functional theory/time-dependent density functional theory and molecular dynamics simulations. The results showed that the lowest excited state of the <b>ITIC</b> molecule possessed a larger ∆<i>q</i> and more hybrid FE/CT states, and <b>PBDB-T/ITIC</b> had more charge separation paths as well as a larger <i>k</i>_CS and smaller <i>k</i>_CR. The reason for the performance difference between <b>PBDB-T/ITIC</b> and <b>PBDB-T/NFBDT</b> was elucidated, suggesting that <b>ITIC</b> is a superior acceptor based on a slight modulation of the side chain and providing a guiding direction for the design of superior-performing small molecule acceptor materials.