Core level and valence band analysis of in-situ cleaved perovskite single crystals
Alberto García-Fernandez a, Sebastian Svanström b, Cody M. Sterling c, Abhijeet Gangan c, Axel Erbing c, Chinnathambi Kamal c d e, Tamara Sloboda a, Birgit Kammlander a, Gabriel J. Man b, Håkan Rensmo b, Michael Odelius c, Ute B. Cappel a
a Division of Applied Physical Chemistry, Department of Chemistry, KTH - Royal Institute of Technology.
b Condensed Matter Physics of Energy Materials, Division of X-ray Photon Science, Department of Physics and Astronomy, Uppsala University
c Department of Physics, Stockholm University
d Theory and Simulations Laboratory, HRDS, Raja Ramanna Centre for Advanced Technology
e Homi Bhabha National Institute, Training School Complex
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Alberto García-Fernandez, presentation 155
Publication date: 20th April 2022

Lead halide perovskites are considered one of the largest breakthroughs in the optoelectronic research field in recent years due to their versatility and their multiple applications including solar cells, LEDs and X-ray detectors among others. All perovskite based optoelectronic devices are made by contacting different materials to the perovskite itself. Therefore, a detailed understanding of the surface and interface properties of lead halide perovskites is crucial for creating more stable and efficient devices. Due to the difficulty of obtaining clean perovskite surfaces for study, up to date, most of surface research was done theoretically. It is worth to note that some experimental studies have been carried out on cleaved surfaces of single crystals, but most of these have focussed on ARPES and STM. [1-3]

In our work, we investigate the surface properties and electronic structure of in-situ cleaved perovskite single crystals with photoelectron spectroscopy at the FlexPES beamline at MAX IV.[4] The use of synchrotron-based soft X-ray photoelectron spectroscopy enables high surface sensitivity and nondestructive depth-profiling. We present a detailed core level and valence band analysis of methylammonium lead triiodide (MAPbI3) and cesium doped formamidinium lead triiodide (CsxFA1-xPbI3). Additionally, we can distinguish two carbon 1s contributions at the surface of MAPbI3. With support of density functional theory and molecular dynamics simulations, we can assign these to MA+ ions in an MAI-terminated surface and to MA+ ions below the surface. We estimate that the surface is predominantly MAI-terminated but up to 30% of the surface could be PbI2-terminated. Our results can be used to optimize future perovskite-based device interfaces and our measurements can be used as a reference for future photoelectron spectroscopy investigations on hybrid perovskite materials.

The authors acknowledge funding from the Swedish Research Council (Grant Nos. VR 2018-04125, VR 2018-06465 and VR 2018-04330), the Göran Gustafsson Foundation, the Swedish Foundation for Strategic Research (Project No. RMA15-0130), and the Carl Tryggers Foundation (Grant No. CTS 18:59). The authors acknowledge MAX IV Laboratory for time on Beamline FlexPES under proposal 20200451. Research conducted at MAX IV, a Swedish national user facility, was supported by the Swedish Research Council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018- 04969, and Formas under contract 2019-02496. The authors also thank Stephan Appelfeller, Alexei Preobrajenski, and Alexander Generalov for support during the FlexPES beamtime. M.O. acknowledges support from the Swedish Energy Agency (Grant No. 2017-006797). The calculations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC), partially funded by the Swedish Research Council through Grant Agreement No. 2018-05973.

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