Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

Abstract Atomic level understanding of the materials used in optoelectronic devices can be obtained using the element specificity of X-ray based spectroscopy techniques. Photoemission or Photoelectron Spectroscopy (PES) is one such technique that yields information on both the chemical composition and the electronic structure of a material. One of its key attribute is the high surface sensitivity which perfectly complements more bulk-sensitive characterization techniques such as X-ray diffraction. This surface sensitivity can easily be tuned by varying the energy of the photo-excitation, allowing the first few atomic layers down to a few tenths of nanometers to be probed. This chapter introduces the principles of PES and gives some basis for the interpretation of a PES spectrum. After a description of the differences between traditional in-house set-ups and synchrotron-based approaches, we describe how these techniques can be applied to the investigation of halide perovskite materials used as light absorbers in solar cells. Some precautions are given regarding sample preparation, sample handling, and the PES measurements themselves. Finally, examples of the application of PES to lead-based halide perovskite materials are summarized with emphasis on the chemical characterization, determination of the electronic structure of the halide perovskite, and the energy level alignment at the perovskite interfaces.