Free Charge Photogeneration in a Single Component High Photovoltaic Efficiency Organic Semiconductor

Abstract

\n Organic photovoltaic cells promise cheap, flexible and scalable solar energy. Whereas light directly generates free charges in silicon photovoltaic cells, bound electron and hole pairs known as excitons are understood to be the primary excitations in organic semiconductors due to their low dielectric constants. These excitons must then be split apart at molecular heterojunctions in order to extract current. Recent record efficiency organic photovoltaics utilise the small molecule, Y6, as a key component in the photon-absorbing blend layer. This molecule and its analogues – unlike previous organic semiconductors – have both low band-gaps and high dielectric constants. Here we show that, in a neat film of Y6, these factors lead to intrinsic free charge generation without the need for a molecular heterojunction to split the exciton. We use a suite of intensity-dependent optical spectroscopy measurements to show that a significant (20-90%) fraction of free charges exist in equilibrium with bound states at light intensity equivalent to 1 sun. Rapid bimolecular charge recombination constrains single component Y6 organic photovoltaic devices to low efficiencies, but this recombination is reduced by the introduction of small quantities of donor polymer. Quantum-chemical calculations reveal charge generation pathways through strong coupling between exciton and CT states, and an intermolecular polarisation pattern that drives exciton dissociation. Our results challenge the understanding of how current record efficiency organic photovoltaics operate, and point towards new future possibilities – offering a molecular picture of intrinsic charge generation as a platform to improve charge yields, and renewing the possibility of efficient single-component organic photovoltaic devices.