Energy Loss in Organic Photovoltaics: Nonfullerene Versus Fullerene Acceptors

Abstract

The energy loss ${E}_{l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}s}$ seen in organic photovoltaics sets a fundamental limit to their open-circuit voltage, and hence power conversion efficiency. This study compares molecular structures of fullerene and nonfullerene acceptors and quantifies the relationship between ${E}_{l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}s}$, exciton binding energy, and intra- and intermolecular electron-phonon couplings. Molecular design strategies derived from this analysis provide elementary approaches to reduce ${E}_{l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}s}$ yet also achieve efficient exciton dissociation. While addressing the source of ${E}_{l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}s}$ is particularly relevant for solar cells, it has wide-ranging implications for any system with an organic heterojunction.