Apparent Field-Dependence of the Charge Carrier Generation in Organic Solar Cells as a Result of (Bimolecular) Recombination

Charge carrier generation in organic solar cells is often reported to depend on the electric field. This is, however, not measured directly but derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this work, numerical simulations of TDCF experiments are presented which - when analyzed in the same way as reported in literature - result in a field-dependence of charge carrier generation despite the fact that a field-independent generation is used. This discrepancy is shown to be caused by recombination of photogenerated charge carriers occurring in the time range prior to and during extraction. This apparent field-dependence becomes more pronounced for larger recombination coefficients and decreasing charge carrier mobilities, very much in accordance with experimental TDCF data from literature. Even an apparent voltage- and time-dependence of the bimolecular recombination coefficient is reproduced in the simulations although a constant, voltage-independent one is used. These findings strongly question whether TDCF is an appropriate method to detect a potential field-dependence of the photocurrent generation and the recombination coefficient. Our study shows that all experimental results can consistently be explained without the assumption of a field-dependence of the charge carrier generation and the bimolecular recombination coefficient.