Impact of space charge polarization on light-soaking phenomena in non-fullerene organic solar cells

Abstract

Revealing the microscopic origin of the unstable nature in organic solar cells (OSCs) is a key issue for further device optimization. Here, we identify that the light-soaking process seen in OSCs could be caused by the effect of space charge polarization in the photoactive layer. The impact of the space charge polarization on the light-soaking is traced experimentally through the determination of dielectric spectra and explored by recombination characterization through charge carrier dynamics study. On the basis of the analysis of dielectric spectra, we find the dielectric changes are closely related to the degradation and stability of the devices. The Maxwell-Wagner relaxation time is determined from the dielectric spectra. Hence, important information on factors that govern the light-soaking in OSCs is obtained. The results reveal the relationship between dielectric spectra and stability of the devices under illumination, and establishes a benchmark for the performance comparison of a set of different cells. Moreover, study of charge carrier dynamics in the device under operation shows that light-soaking and the subsequent space charge polarization result in improved mobility and charge carrier lifetime, thus responsible for the improved device performance. The findings presented here are important for the understanding and optimization of organic solar cells and we expect the presented methods to be generally applicable to other OPV material sets.