The Effect of Additives on the Stability of Mixed-Cation Perovskites Studied by Electron Spectroscopy

Abstract

Perovskite solar cells (PSC) based on mixed A-site cations which include formamidinium (CH(NH2)2+) and caesium (Cs+) have attracted considerable interest owing to their high power conversion efficiency (PCE). Lab-scale PSCs have achieved a highest PCE of >25% within a short time since their development, suggesting a promising future for PSCs as an alternative to other commercialized thin-film solar cells. However, despite the high efficiency of PSCs, the lack of stability under environmental conditions, such as heat, moisture, and light, challenges their commercialization. Hence understanding the degradation of perovskite and improving long-term stability has become an important topic in the development of PSCs. In this thesis, different strategies (such as use of additives and surface passivation) have been applied to improve the stability, and are investigated using multiple photoelectron spectroscopies, including near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and hard X-ray photoelectron spectroscopy (HAXPES). A novel Ga Kα HAXPES instrument was commissioned at the University of Manchester (UoM) during this work. The effect of incorporating rubidium iodide (RbI) on the stability of mixed-cation perovskites is investigated. Inclusion of a small amount of RbI is shown to play a major role in improving perovskite stability by filling iodide ion vacancies at the surface, reducing the possibility of ion migration under annealing. Furthermore, the perovskite exhibits superior moisture stability compared to the conventional perovskite, methylammonium lead iodide. It is shown that inclusion of an ionic liquid (IL) additive in the mixed-cation perovskite precursor can enhance the device performance and the stability of the material to moisture at room temperature and to annealing in ultra-high vacuum (UHV). The incorporated IL is found to increase the grain size by retarding the crystal growth, leading to improved device performance with reduced hysteresis. The enhanced stability is ascribed to enlarged grains and accumulation of IL at the surface, preventing the volatilization of organic molecules from the lattice. The surface passivation of mixed-cation perovskite with phenylethylammonium (PEA+) cations that can effectively suppress the defect sites is also shown to result in improved stability against annealing in dry conditions. This work provides insights into designing stable perovskites for photovoltaic application, and also incorporates additional results from the commissioning and benchmarking of the Ga Kα HAXPES instrument.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrew Thomas (Supervisor) & Wendy Flavell (Supervisor)