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On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells
Author(s) -
Caprioglio Pietro,
Stolterfoht Martin,
Wolff Christian M.,
Unold Thomas,
Rech Bernd,
Albrecht Steve,
Neher Dieter
Publication year - 2019
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201901631
Subject(s) - materials science , perovskite (structure) , recombination , open circuit voltage , diffusion , saturation (graph theory) , atomic physics , fermi level , non radiative recombination , voltage , optoelectronics , condensed matter physics , thermodynamics , physics , semiconductor , chemistry , nuclear physics , quantum mechanics , biochemistry , mathematics , combinatorics , electron , gene , crystallography , semiconductor materials
Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage ( V OC ) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the V OC on the very same devices, including pin‐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 V OC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the V OC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐ V OC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the V OC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the V OC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS ‐V OC relation is of great importance.