Construction of Perovskites Thin Films of Black Orthorhombic Phased Cesium Tin Iodide Using Pulsed Laser Deposition
Authors: Ikeda Kazuha , Chikamatsu Akira , Kondo Toshihiro 
Published: December 2024

Introduction: Perovskite solar cells have the potential to achieve higher efficiency than current mainstream silicon-based solar cells. Black orthorhombic (B-γ) phased cesium tin iodide (CsSnI3) perovskite, which contains no lead and organic content leading to be environmentally friendly and thermally stable, respectively, has attracted much more attention than methylammonium lead iodide (MAPbI3), which is the most commonly used photosensitive layer in the perovskite solar cells. However, its photoelectric efficiency is much lower than that of MAPbI3. This low efficiency is attributed to the high defect density in CsSnI3, such as Cs or Sn vacancies which have very low defect formation energies. Thus, it is essential to control the atomic arrangement at the interface and the crystal growth process. The CsSnI3 perovskite exists in two orthorhombic polymorphs at room temperature: one is a yellow 1D double-chain structure and the other is a black 3D perovskite structure (B-γ). The B-γ phase has a band gap of ca. 1.3 eV, which is desirable for the photosensitive layer of the solar cells, in contrast to the Y phase with ca. 2.6 eV band gap. However, the orthorhombic B-γ phase rapidly transitions to the Y phase when some external perturbation (oxygen, moisture, solvent) is applied, and subsequently transfers to photoinactive Cs2SnI6 because of self-doping from Sn2+ to form Sn4+ under ambient-air conditions [1]. Therefore, it is necessary to construct defect-free B-γ CsSnI3 layers on solid substrates. In this study, we attempted to construct defect-free CsSnI3 perovskite thin films on various solid substrates at room temperature using the pulsed laser deposition (PLD) method, which enables the control of crystal growth process at an atomic level.

Experiment:To prepare the precursor target for the PLD, stoichiometric amounts of SnI2 and CsI powders were mixed and pressed to a tablet shape. Then it heated to 300ºC and held for 18 h under Ar atmosphere. The target was loaded into the PLD chamber, and CsSnI3 thin films were deposited onto n-typed Si(100), TiO2(100), (110), (001) single crystal and glass substrates at room temperature using a Nd:YAG laser. An Ar working pressure of 1.0 × 10-3 Torr was kept constant during deposition. Following CsSnI3 deposition, an amorphous Tin-doped Indium oxide (ITO) capping layer was applied by PLD to prevent the film oxidation.

Result and Discussion: X-ray diffraction (XRD) analysis was conducted for the precursor target and the obtained thin films. It revealed that the precursor target contains only pure B-γ CsSnI3 polycrystals. For the obtained thin films, B-γ CsSnI3 was obtained and it oriented mainly in the [010] direction normal to the surface regardless of the substrate type. In addition, the 2D diffraction images indicated that the orientation ranged within ±10º, also regardless of the substrate type. In the previous study by Kiyek et al. [2], polycrystalline films were obtained by the PLD method with the precursor containing the mixture of the raw materials (CsI and SnI2). On the other hand, in this study, the oriented films were obtained with B-γ CsSnI3 precursor, suggesting that the precursor condition may affect the orientation because the laser plume constantly contains strictly Cs:Sn:I = 1:1:3 composition during ablation. We will discuss this phenomenon in detail.

References:

[1] A. G. Kontos et al., Inorg.Chem.
56, 1 (2017).

[2] V. M. Kiyek et al., Adv. Mater. Interfaces
7, 11 (2020).