On the Impact of Bimolecular Recombination on Time-Delayed Collection Field Measurements and How to Minimize Its Effect

Abstract

The time-delayed collection field (TDCF) technique is a popular method to quantify the field and temperature dependences of free charge generation in organic solar cells. Because the method relies on the extraction of photogenerated charge carriers, bimolecular recombination not only between the photogenerated carriers but also between the photogenerated and dark-injected carriers affects its accuracy, particularly at forward bias. In this work, drift–diffusion simulations are employed to quantify the recombination losses in conventional and modified TDCF measurements, where the latter technique intends to reduce the impact of dark injection. It is shown that parameters such as the generation profile, carrier mobilities, and effective density of states affect the recombination losses in both measurements. Importantly, modified TDCF enables to reduce the recombination losses at forward bias, especially beyond the open-circuit voltage. However, conventional TDCF is preferable for studies at reverse bias due to a better depletion of the active layer prior to the emergence of the photogenerated carriers. Measurements on a ZR1:Y6 blend with fast recombination are in good agreement with the simulation results. This work shows that artifacts in TDCF measurements related to non-geminate recombination can be accounted for and minimized through an informed choice of the experimental conditions.