Quantum‐Chemical Calculations of Structure, Electronic Properties, and Spectra of PBDB‐T:ITIC Interfaces

Herein, interfaces between poly[(2,6‐(4,8‐bis(5‐(2‐hexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PBDB‐T) and 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2,3‐d]‐s‐indaceno[1,2‐b:5,6‐b]‐dithiophene) (ITIC) are modeled through density functional theory (DFT) applied to dimers constructed from various conformers of a PBDB‐T segment and the ITIC molecule. These variations represent nonuniformity at the donor–acceptor interface commonly observed in organic solar cells. Based on this model, electronic and optical properties relevant to charge separation are calculated using DFT and time‐dependent (TD‐) DFT. For nearly half of the dimers, band offsets relative to pristine donor and acceptor materials result in charge trapping, leading to inefficient charge separation and nongeminate recombination. Exciton‐binding energy is evaluated by TD‐DFT using Becke 3‐parameter Lee Yang Parr (B3LYP) and Heyd‐Scuseria‐Ernzerhof (HSE06) hybrid functionals, revealing significant qualitative and quantitative differences. These results are compared with experimental data from selected characterization techniques. Finally, the impact of these findings on key photovoltaic parameters, especially open‐circuit voltage and short‐circuit current, is discussed, providing insights into optimizing device performance.