Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.164
Publication date: 21st February 2018
Lead halide perovskites have developed from the initially used methylammonium lead iodide (MAPbI3) to structures containing anion (e.g. Br- and I-) and cation (e.g. formamidinium (FA), MA, Cs+ and Rb+) mixtures. Many aspects related to these materials still need to be understood including the materials’ photo-stability. Processes such as ion migration are likely to play a role in the photon to electron conversion mechanism and for the long-term stability. It is therefore of high importance to understand how the exact composition of the perovskite influences these properties.
We have recently developed a photoelectron spectroscopy-based methodology in which the chemical changes at the surface of the perovskite can be studied under visible light illumination [1]. These measurements are carried out at the LowDose PES beamline at the synchrotron Bessy II, where a X-ray low photon flux is combined with a highly efficient spectrometer. This enables us to study the electronic structure of the perovskite surface without any X-ray degradation. In our first study, we demonstrated that ion movement of halides can be observed through intensity changes of the halides at the surface for an MAFA-based perovskite during visible laser illumination [1]. Furthermore, we observed the reduction of a fraction of Pb2+ at the perovskite surface to Pb0, which was partially reversible in the dark.
We now have used the same methodology to study photo-induced processes in perovskites with different compositions including different I/Br ratios and Cs ions. This presentation will summarise the results of this study. We show that the presence of Cs ions can lead to a strong suppression of halide ion migration at high I/Br ratios, while other aspects of the photo-stability of the perovskite can be enhanced independent of I/Br ratio. Our results give therefore an indication of the mechanism with which Cs ions enhance the stability of perovskite solar cells.
[1] Ute B. Cappel, et al. ACS Appl. Mater. Interfaces 9, 34970-34978 (2017).
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