On the Relation between the Open Circuit Voltage and Quasi Fermi Level Splitting in Efficient Perovskite Solar Cells

Today s perovskite solar cells PSCs are limited mainly by their open amp; 8208;circuit voltage VOC due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity amp; 8208;dependent measurements of the quasi amp; 8208;Fermi level splitting QFLS and of the VOC on the very same devices, including pin amp; 8208;type PSCs with efficiencies above 20 , are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley amp; 8208;Queisser theory. This has far amp; 8208;reaching implications for the applicability of some well amp; 8208;established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift amp; 8208;diffusion simulations, the intensity dependence of the QFLS, the QFLS amp; 8208;VOC offset and the ideality factor are consistently explained by trap amp; 8208;assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS amp; 8208;VOC relation is of great importanc