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

Abstract

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 C60 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 (Voc). 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, ECT, obtained by including electronic polarization effects and electron–hole interactions, exhibits a near-quantitative linear relationship with Voc. ECT 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 Voc OPVs, and suggests that our approximate approach can enable computational design of Voc for a broad class of molecular-based OPVs.