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, we present synchrotron- and lab-based hard X-ray photoelectron spectroscopy (HAXPES) experiments to address the particular chemistry of MHP, beginning with the interface between (CH₂(NH₂)₂)_0.7Cs_0.3PbI_0.9Br_0.1)₃ and SnO₂, grown by atomic layer deposition on top, using photon energies of 2 and 6 keV. We find evidence for the formation of new chemical species (PbO, N and halide-containing compounds) and changes in the energy level alignment at the MHP/SnO₂ interface. The spectra exhibit binding energy shifts that may 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. We targeted films deposited on the organic hole transport layer PEDOT:PSS. For solar cells with optimized performance, the local SnF₂ distribution in the perovskite bulk and possible chemical reactions at the (buried) interface between PEDOT:PSS and the perovskite film are analyzed by photoelectron spectroscopy using soft (Al K-alpha) as well as hard (Ga K-alpha) X-ray photons. Our measurements revealed that SnF₂ significantly improves the layer morphology, but preferably precipitates at the PEDOT:PSS/MHP interface where it forms a SnS interlayer of approximately 1.2 nm thickness 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, where alternatives to PEDOT:PSS are gaining growing interest [2].

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Références

1. S. Béchu, M. Ralaiarisoa, A. Etcheberry, P. Schulz, Adv. Energy Mater. 2020, 201904007

2. J. Zillner, H.-G. Boyen, P. Schulz, J. Hanisch, N. Gauquelin, J. Verbeeck, J. Küffner, D. Desta, L. Eisele, E. Ahlswede, M. Powalla, Adv. Funct. Mater.2022, 2109649 (DOI: 10.1002/adfm202109649)