Using photovoltage to measure device-relevant exciton diffusion in luminescent and non-luminescent organic semiconducting materials

Exciton diffusion is a prominent component of many emerging organic optoelectronic devices and is commonly described by the exciton diffusion length (L$_{\mathrm{D}})$. Excitons are commonly tracked by measuring their end-of-life products: photons (from radiative decay) or charge carriers (from exciton dissociation). While tracking luminescence provides an accepted means of measuring L$_{\mathrm{D}}$ for many materials, non-luminescent (dark) materials and states are inaccessible via such techniques. For dark materials, the charge carriers generated from exciton dissociation are tracked to estimate L$_{\mathrm{D}}$ (e.g., by fitting device external quantum efficiency), despite the fact that photogenerated carriers are often subject to recombination events prior to collection as current. Here, we present an alternate method of measuring L$_{\mathrm{D}}$, equally applicable to luminescent and dark materials, that uses photovoltage instead of current to determine the number of excitons reaching the dissociating interface in an organic photovoltaic device. Use of the photovoltage sidesteps charge carrier recombination providing an unobscured measurement of L$_{\mathrm{D}}$. The technique is verified against previous luminescence-based methods, and measurements of L$_{\mathrm{D}}$ are presented for a variety of dark materials including fullerenes.