The use of organic/inorganic perovskite and especially CH₃NH₃PbI₃ as solar cells material constitutes one of the main breakthrough of this last five years in solar energy research. A lot attention has since been devoted to the development and improvement of these organic/inorganic perovskite materials and alternative chemistry have been proposed i.e. different nature of the halide (X), the metal (B), or the cation group (A), combination with hole conductors as well as the development and optimization of novel fabrication/deposition techniques bringing efficiency up to 21% at the beginning of 2016. While these materials are usually characterized through their structure (XRD) and performance within solar cell communities, not so much attention is devoted to their chemical composition and, specifically at the surface. Photoelectron spectroscopy (PES) can easily fulfill this task, and, in addition to chemical information, PES provides an overall picture of the electronic structure of the perovskite and its relation to the scaffold layer used (e.g. TiO₂, Al₂O₃), when studied with hard X-rays. Through various recent examples, we will present how PES can be used to investigate perovskite solar cells materials based on different metals (Pb²⁺, Bi³⁺), anion (I^-, Br^-, Cl^-) or cation (Cs⁺, MA⁺, FA⁺). [1][2][3] By probing different surface sensitivity by coupling soft and hard X-ray PES, we will also show that the outermost surface (first few nanometers) can be significantly different from the bulk.  The main drawback towards a commercialization of these devices remains their poor stability. PES is well adapted to track this changes. We have exposed the classical CH₃NH₃PbI₃ to various environments, such as water, temperature, and long-time storage in air and argon, and followed changes of the surface composition with PES. The main result of the different exposures is that the perovskite is decomposed into PbI₂, but an important point is that this degradation seems to occur already at 100 °C and is not only related to large humidity level. Indeed, even in an inert atmosphere such as argon, a slow degradation to PbI₂ is observed. [4] 

References:

[1] R. Lindblad, N. K. Jena, B. Philippe, J. Oscarsson, D. Bi, A. Lindblad, S. Mandal, B. Pal, D. D. Sarma, O. Karis, H. Siegbahn, E. M. J. Johansson, M. Odelius, H. Rensmo J. Phys. Chem. C, 2014, 119, 1818-1825

[2] B. W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E. M. J. Johansson,Adv. Mater., 2015, 6806-6813

[3] S. M. Jain, B. Philippe, E. M. J. Johansson, B.-w. Park, H. Rensmo, T. Edvinsson, G. Boschloo, J. Mater. Chem. A, 2016, DOI: 10.1039/c5ta08745g

[4] B. Philippe, B.-W. Park, R. Lindblad, J. Oscarsson, S. Ahmadi, E. M. J. Johansson, H. Rensmo, Chem. Mater. 2015, 27, 1720−1731