Impact of Mesoscale Order on Open-Circuit Voltage in Organic Solar Cells
Structural order in organic solar cells is paramount: It reduces energetic disorder boosts charge and exciton mobilities and assists exciton splitting. Due to spatial localization of electronic states microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at a mesoscale. Long-range electrostatic interactions nevertheless probe this ordering making local properties depend on the mesoscopic order. To account for this a technique that addresses spatially \em aperiodic excitations in a \em periodic polarizable environment is developed. We show that structural order can reverse the role of donor and acceptor as conditioned by gas-phase energy levels. This finding resolves the controversy between experimental and theoretical results for the band shape and level alignment in efficient photovoltaic systems. Furthermore we rationalize the acceptor-donor-acceptor paradigm for molecular design of the successful DCVnT series of dyes which makes optimal use of these long-range effects. Comparing atomistic simulations to UPS experiments we provide an alternative interpretation for the empirical link between molecular energy levels and open-circuit voltage.