Charge carrier mobility in disordered organic blends for photovoltaics

Charge transport in disordered organic blends is studied theoretically by numerically solving the Pauli master equation. The influence of morphology, disorder, electric field, and charge carrier concentration on blend mobility is assessed. Important differences between neat materials and blends are found. The dependence of mobility on charge carrier concentration is more pronounced in blends and it is influenced by the electric field strength. At low charge carrier densities, blend mobility is found to decrease with increasing field. Additionally, the impact of the volume ratio of the constituent materials and their domain size on the mobility is presented. Especially for strongly disordered materials charge transport is favored by relatively large domains. To compare these theoretical findings with existing experimental mobility data, the current density in a space-charge-limited device is computed. The author finds that, for the parameters and morphologies studied, the apparent mobility in such a device decreases with increasing bias voltage.