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

Abstract

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 non‐geminate recombination during the charge extraction processes is neglected. This assumption becomes questionable for low‐mobilities, e.g. at low temperatures and in case of high energetic disorder. In this study, we theoretically investigate the extraction process in BACE measurement by incorporating both drift and bimolecular recombination of charges during charge extraction into a unified framework. We propose an explicit analytical model to describe the time dependence of the transient photocurrent in BACE measurement and found excellent agreement with drift‐diffusion simulations. We show 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.This article is protected by copyright. All rights reserved.