Hybrid organic inorganic metal halide perovskites (MHPs) denote a family of compound semiconductors, which established a novel class of optoelectronics, most prominently known for the perovskite solar cell. While the power conversion efficiency of these photovoltaic devices saw a steep rise in the past decade, tailoring the interfaces between the MHP film and charge transport layer became the major control lever to enhance performance. The use of photoemission spectroscopy to analyze the chemical and electronic properties of these interfaces has been challenging due to many possible chemical reactions at the buried interfaces [1].

Here, I will discuss the use of synchrotron- and lab-based X-ray photoelectron spectroscopy (XPS) experiments to address the particular chemistry of MHP interfaces to adjacent oxide charge transport layers (CTL). At the example of oxide ALD-SnO2 layer grown on top of a double cation mixed halide perovskite film investigated by hard X-ray photoelectron spectroscopy (HAXPES), we find evidence for the formation of new chemical species and changes in the energy level alignment at the interface. The spectra exhibit binding energy shifts that indicate upward band bending of the MHP energy levels. Assuming flat band conditions at the MHP surface prior interface formation, this upward band bending may form an electron transport barrier detrimental to cell performance.

We also employed the methodology to evaluate lead-free halide perovskite films based on formamidinium tin iodide (FASnI₃), for which tin fluoride (SnF₂) is a commonly used additive enabling a retardation of tin oxidation and a reduction of tin vacancies. Our measurements reveal that SnF₂ significantly improves the layer morphology, but preferably precipitates at the PEDOT:PSS/MHP interface where it forms an ultrathin SnS interlayer induced by a chemical reaction with sulfur-containing groups at the PEDOT:PSS surface. Our work adds a new aspect to the discussion of high-efficiency Sn-based perovskite solar cells which still commonly make use of PEDOT:PSS as HTL material in contrast to Pb-based solar cells [2].

I will conclude my talk with a general discussion about the use of PES methods for the analysis of MHP layers and in particular the effect of irradiation-induced beam damage via synchrotron and lab-based X-ray sources.³ By using complementary photoluminescence measurements we are able to reveal beam-induced changes to the optoelectronic properties and track unique physicochemical phenomena such as stimulated self-healing in formamidinium lead bromide (FAPbBr₃).^4,5