Perovskites
Hybrid perovskites are versatile materials, adopting an ABX3 structure, as shown for the case of CH3NH3SnI3 and CH3NH3PbI3 in Figure 1. A divalent B-site metal such as Pb or Sn, is octahedrally coordinated in a BX6 configuration with X being a halide anion, for example iodine. A monovalent A-site cation is positioned in between the octahedra, with typical organic cations being methylammonium or formamidinium. The perovskite material is considered to be extremely defect-tolerant, highly tunable in terms of band gap, and easy to manufacture. Low processing conditions allow for deposition on nearly any substrate as no to very little heating is required to achieve high optoelectronic properties.
In the SPM group we have developed evaporation routines to synthesize perovskite absorbers with high optoelectronic quality [1,2]. The choice of the deposition technique is obvious considering that our core analysis tools, such as atomic force microscopy, Kelvin probe force microscopy, scanning tunneling microscopy and X-ray photoelectron spectroscopy all need clean contaminant free surfaces.
With evaporation, we can fulfill these requirements. The physical vapor deposition system is attached to a nitrogen-filled glovebox to transfer samples without any contact to ambient conditions from the synthesis chamber to the ultra-high-vacuum.
An important result is that we can grow Sn-based perovskites without detrimental Sn in the oxidation state 4+. This is shown in Figure 2 where a CH3NH3SnI3 (MASI) film is compared to a solution-based Sn-perovskite. The identification of the oxidation states was carried out with the help of a clean and oxidized Sn-plate. We find that PVD-grown perovskites do not contain measurable Sn(4+).
These materials are ideal for fundamental research and for deliberate degradation studies. In fact, we find that the degradation pathway depends strongly on the external stimuli. An example of light induced degradation can be found in the reference [3].
References:
[1] Gallet, T., Poeira, R. G., Lanzoni, E. M., Abzieher, T., Paetzold, U. W., & Redinger, A. (2021). Co-evaporation of CH3NH3PbI3: How growth conditions impact phase purity, photostriction, and intrinsic stability. ACS Applied Materials and Interfaces, 13(2), 2642–2653. https://doi.org/10.1021/acsami.0c19038
[2] Singh, A., Hieulle, J., Machado, J. F., Gharabeiki, S., Zuo, W., Farooq, M. U., Phirke, H., Saliba, M., & Redinger, A. (2022). Coevaporation Stabilizes Tin-Based Perovskites in a Single Sn-Oxidation State. Nano Letters, 22(17), 7112–7118. https://doi.org/10.1021/acs.nanolett.2c02204
[3] Ferreira Machado J., Hieulle J., Vanderhaegen A., & Redinger A. (2024), Light-induced degradation of methylammonium tin iodide absorber layers, Journal of Materials Chemistry A, accepted
Figure 1: The halide perovskites we synthesize in the laboratory
Figure 2: (a) co-evaporated MASnI3 compared to solution processed FASnI3, a clean Sn-plate and an oxidized Sn-plate [2].