(Invited) Surface/Interface Structures of Perovskite Films Studied By Electron Spectroscopies

Interface of perovskite layer play a crucial role in the charge extraction and recombination process of perovskite solar cell. It is, therefore, of great importance to examine the geometrical and electronic structures of surface/interface of perovskite. Unfortunately, standard experimental techniques such as X-ray diffraction (XRD) and scanning tunneling microscopy (STM) are not suitable to examine practical perovskite films for solar cells. XRD observes the lattice periodicity and is not sensitive to the surface structure. Although STM can precisely observe the surface atoms, it cannot be applied to rough surface or thick films, e.g. solution-processed perovskite surface. We have demonstrated that a combination of electron spectroscopies is powerful and versatile tool to examine both geometrical and electronic structures of perovskite surfaces. Figure 1 shows principles of ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). In UPS, the valence electrons are excited by ultraviolet photons (typically with the energy of 21.22 eV) and kinetic energies of photoelectrons are analyzed to measure the valence electronic states as a difference between the photon and electron kinetic energies. The probing depth of UPS is limited by the mean free path of electrons which is usually several layers a few nanometers (several layers) in the energy range of UPS. In MAES, the excitation source is replaced with the metastable helium atom (with the energy of 19.82 eV). Although the process as electron spectroscopy is similar to that of UPS, MAES is extremely surface sensitive because the helium atom can only interact with the outermost atomic orbitals of the sample surface. By comparing the UPS and MAES spectra, we can obtain information about the surface structure as well as the electronic states. In this presentation, we will demonstrate the following two topics: 1. Surface termination of CH3NH3PbI3 [1] The properties of the interface of perovskite ABX3 should be strongly affected by the surface termination, i.e. whether the surface is terminated with AX or BX layer. We examined the surface termination of a CH3NH3PbI3 thin film prepared by the standard one-step solution process. The results show that surface of the perovskite film is terminated with the layer consisting of CH3NH3 and I. 2. Surface structure of quasi-2D perovskite films, PEA2m(CH3NH3)n-2mPbnI3n [2] A small amount of large organic cations incorporated in perovskite film (quasi-2D) passivates the surface and enhances the stability of the perovskite solar cell against moisture. We examined surface structure of PEA2m(CH3NH3)n-2mPbnI3n, and confirmed that the PEA segregates to the surface and that the phenyl group covers the outermost surface of the quasi-2D perovskite. From the concentration dependence, we also examined the plausible structure of the quasi-2D film. 1) A. Mirzehmet, T. Ohtsuka, S. A. Abd. Rahman, T. Yuyama, P. Krüger, H. Yoshida, Adv. Mater. 33, 2004981 (2021). 2) A. Mirzehmet, T. Ohtsuka, S. A. Abd. Rahman, T. Aihara, M. A. Kamarudin, S. R. Sahamir, S. Hayase, T. Yuyama, P. Krüger, H. Yoshida, Appl. Phys. Express 14, 031006 (2021).