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.