Relating Trends in First-Principles Electronic Structure and Open-Circuit Voltage in Organic Photovoltaics

Using first-principles density functional theory, and accounting for solid-state polarization effects and electron-hole interactions, we calculate excited electronic states at interfaces between C-60 and a series of functionalized boron(subphthalocyanine) molecules, a class of donor materials for organic photovoltaic (OPV) devices, and correlate energetics with their measured open-circuit voltages (V-oc). For isolated donor and acceptor molecules, a staggered (type-II) interface energy alignment is predicted with an energy offset of several tenths of an electron volt, capable of promoting charge separation. The solid-state charge transfer excited state energy, E-CT, obtained by including electronic polarization effects and electron-hole interactions, exhibits a near-quantitative linear relationship with V-oc. E-CT depends sensitively on interface morphology, resulting in a predicted 0.2-0.6 eV spread in energy for the geometries studied here. The agreement between theory and experiment provides insight into possible routes to higher V-oc OPVs, and suggests that our approximate approach can enable computational design of V-oc for a broad class of molecular-based OPVs.