Charge Selective Contacts to Metal Halide Perovskites Studied with Photoelectron Spectroscopy: X-Ray, Ultraviolet, and Visible Light Induced Energy Level Realignment

The electronic properties of metal halide perovskites (MHPs) are crucial for achieving the full potential of MHPs-based optoelectronic devices, and they are extensively studied by photoelectron spectroscopy (PES). However, there are numerous complexities during PES measurements, which can result in inconsistent experimental data or even misinterpretation of results. Here, it is demonstrated that in the presence of charge selective junctions with MHPs, significant electronic energy level realignment can occur during PES measurements due to the sample excitation with high-energy photons used in PES, amounting up to over 0.8 eV in the present case. This is caused by unbalanced charge carrier accumulation within the perovskite due to the charge-selective interface. X-ray photoelectron spectroscopy further reveals that photoexcitation due to bremsstrahlung, as produced in commonly employed twin-anode lab-sources, can readily produce sizable photoinduced shifts of core levels of MHPs films, whereas monochromatized X-ray lab-sources (irradiation flux reduced by >50 times by eliminating the bremsstrahlung) induce negligible shifts within the range of presently applied anode powers. The data and measurement conditions presented here are intended to enable others to obtain reliable MHP electronic property information from PES measurements.