An Analytical Model for Describing Transient Photocurrents in Bias‐Assisted Charge Extraction for Low‐Mobility Organic Solar Cells

Bias‐assisted charge extraction (BACE) is a powerful technique for measuring the carrier density in organic solar cells under operational conditions and to deduce information on the charge recombination properties. Hereby, the carrier density in the active layer device is determined by integrating the transient extraction current, while nongeminate recombination during the charge extraction processes is generally neglected. This assumption becomes questionable for low mobilities, for example, at low temperatures and in case of high energetic disorder. Herein, the extraction process in BACE measurement is investigated by incorporating both drift and bimolecular recombination of charges during charge extraction into a unified framework. An explicit analytical model is proposed to describe the time dependence of the transient photocurrent in BACE measurement and excellent agreement is found with drift‐diffusion simulations. It is shown that global fitting of transient photocurrents measured at various collection biases with this analytical model allows to determine the carrier density in the device with high accuracy. Furthermore, the analytical model is demonstrated to provide accurate mobilities for both the fast and slow carriers within the device, rendering it a valuable supplement to other mobility measurement techniques such as resistance‐dependent photovoltage and space–charge‐limited current.