A novel way of analyzing perovskite outdoor degradation: the S-Voc

Abstract

Perovskite mini-modules with two different electron transport layer compositions, ETL1 composed of a thick LiF/C60/BCP stack and ETL2 with a thinner LiF/C60/LiF stack, were located and measured outdoors for an extended period of more than three years. The analysis of the performance over time highlights a rapid degradation in the first 90 days of exposure, followed by a stabilization. Most of the initial degradation appears to be caused by a reduction in fill-factor (FF) while the open-circuit voltage (Voc) remains surprisingly constant over time. The analysis of the current-voltage (IV) curves reveals that by considering these figures of merit only, the presence of important features such as S-shaped curves can be overlooked, which could lead to a wrong interpretation of the degradation origin. To address this issue, two new figures of merits, S-Voc and S-FF, are introduced. The comparison between Voc and S-Voc shows to be an efficient and reliable way to identify S-shape formation, while enabling to quantify their contribution to performance loss. Finally, an attempt is made at understanding the origin of degradation in the perovskite mini-modules, likely an interface barrier formation in ETL1 samples and an increase in series resistance in ETL2 samples, possibly due to a partial delamination of the thinner C60 layer.