First-Principles Modeling of Interfacial Charge Transfer Exciton States in Organic Solar Cell

Charge‑transfer (CT) states enable the separation of excitons created when photons are absorbed into free charges that can be harvested as electric current.  In a CT exciton, the electron and hole sit on different molecules at the interface but are still bound together by Coulomb attraction. By modeling this state directly, we link bulk excitons to separated polarons and calculate the CT binding energy, a key factor that controls how efficiently organic solar cells generate current and avoid voltage losses. Our method uses a tight‑binding model, in which physical parameters are computed by first-principles quantum methods; this approach is physically interpretable, and at the same time computationally efficient and scalable. Our model describes the electron–hole wavefunction, and represents the energy as a sum of kinetic, Coulomb, and dielectric terms, which allows us to analyze how contact geometry and screening change charge binding and dissociation. This simple, transferable framework provides a practical way to study charge transfer at donor–acceptor interfaces, enabling the design of materials and interfaces that yield more free charge and better device performance.