When Poor Exciton Dissociation Limits Photocurrents in Organic Solar Cells: Why Low Offset Non-Fullerene Acceptor Blends Can't Be Efficient.

Abstract

It is a well-established fact that the energetic offset between the donor and the acceptor components in organic solar cells dictates the tradeoff between photovoltage and photocurrent losses. Yet, a deeper understanding of the photophysical processes that ultimately limit photocurrent generation in low offset blends is needed for more informed future molecular and device design. This Perspective addresses the most important issues surrounding this topic, from the difficulty of accurately determining state energies and driving forces to bottlenecks in free-charge generation arising from low energy offsets. Using experimental, analytical, and theoretical evidence, we then substantiate our view that the primary-if not the sole-cause of photocurrent losses in low offset non-fullerene acceptor blends is the decay of excitons that did not undergo charge transfer. We conclude that the offset between the local exciton and the interfacial charge transfer state must be at least 100 meV, which translates into a frontier orbital offset of approximately 300 meV, similar to what has been proposed for fullerene-based blends. We discuss the relevance of this finding with regard to the non-radiative voltage loss of current high-performance solar cells, and what measures should be taken to reduce this loss without impairing photovoltaic power generation.