First-Principles Modeling of Charge Transfer Excitons in Organic Solar Cells

Charge-transfer (CT) excitons formed at donor-acceptor interfaces play a central role in charge separation in organic solar cells. Their binding energy and spatial structure are highly sensitive to local molecular geometry, particularly in disordered morphologies. In this work, we present a multiscale computational framework to model CT excitons at the interface between poly(3-hexylthiophene) (P3HT) and the nonfullerene acceptor O-IDTBR, a representative polymer-nonfullerene blend. We extend a previously developed tight-binding model to include kinetic energy, Coulomb interaction and dielectric polarization, with all parameters derived from density functional theory (DFT). Using molecular dynamics simulations of an amorphous P3HT:IDTBR blend, we identify hundreds of donor-acceptor contact geometries and compute the CT exciton for each by minimizing the total energy with respect to electron and hole wavefunctions. We analyze the distribution of CT exciton energies, compare them with localized excitons on IDTBR, and estimate binding energies relative to free polarons in the bulk phases. By decomposing the CT energy into one-body kinetic and two-body (Coulomb + polarization) contributions, we show that the primary variation of CT energies is controlled by the underlying polaron energy landscape, while configuration-dependent Coulomb and polarization stabilization explains the residual scatter and distinguishes true CT states from quasi-separated polarons. Additionally, we place these results in the context of a full energy landscape from HOMO-LUMO gaps to excitons, CT states, and polarons, highlighting how interfacial structure tunes the balance between exciton formation and dissociation. This approach enables predictive evaluation with no adjustable parameters of CT excitons across realistic morphologies, and offers design principles for engineering interfaces that promote efficient charge separation in organic photovoltaics.