One of the most important features of perovskites, which have been rapidly developed in recent years as materials for high-efficiency and low-cost thin-film solar cells, is the applicability of a simple process using solution coating under atmospheric pressure [1]. Crystal growth of perovskite by the solution method occurs through equilibrium chemical reactions, enabling the formation of thin films on the order of μm thickness in less than a few seconds; however, it is difficult to systematically control the crystal growth process at the molecular layer level by the solution method. On the other hand, in the physical vapor deposition (PVD) method, although the deposition rate is relatively low, ranging from sub-nanometer to several nm/s, the deposition can be controlled at the molecular layer level, as well as allowing different atoms to be readily introduced into the film. Therefore, the PVD method allows the addition of elements that are difficult to achieve with solution reactions and is suitable for studies that investigate the effects of such elements on physical properties.
In laser deposition (LD), one of the PVD methods, materials are irradiated with an infrared (IR) or an ultraviolet (UV) laser beam, and thin films are formed by evaporation through IR heating or ablation with plasma formation through UV electron excitation. The advantage of LD is that it can cause instantaneous heating or ablation by switching on and off laser irradiation only on the material surface, thus enabling the deposition of molecular layers of organic materials with high vapor pressure or high melting point materials such as inorganic oxides with minimal cross-contamination caused by evaporation. We have prepared perovskite thin films by alternate deposition of PbI2 and CH3NH3I (MAI) using an IR laser and solid-phase reaction at room temperature aiming at the detailed investigation of the crystal growth process and introduction of various dopants in perovskite materials. [2, 3]. In this study, we have also performed perovskite film deposition by UV pulsed laser and investigated the difference in the physical properties of the perovskite films compared to those deposited by IR laser.
Fig. 1 shows a schematic structure and appearance of the laser deposition system used in this experiment. The experimental method is described below. A synthetic quartz substrate was introduced into an ultra-high vacuum chamber with a base vacuum of 2×10-5 Pa., Then a semiconductor continuous IR laser (wavelength: 808 nm) or a UV pulse laser beam generated by the fourth harmonic of an Nd:YAG Q-Switch laser (wavelength: 266 nm, 10 Hz) was irradiated alternately onto PbI2 and MAI targets to form CH3NH3PbI3.
The optical band gap of the thin films deposited by continuous infrared laser deposition shows a change in optical band gap depending on the thickness of the alternately deposited PbI2 and MAI, which can be attributed to the different content ratios of the tetragonal and orthorhombic phases. Interestingly, in UV pulsed laser deposition, the deposition rate was about 0.35 nm/s under the high laser fluence condition with lens focusing, while a high deposition rate of 20 nm/s was obtained under the low laser fluence condition without lens focusing. This result suggests that the excess kinetic energy of the deposition precursor in the case of lens-focused deposition causes re-sputtering on the surface of the already deposited film, resulting in a reduction of the effective deposition rate.
In the presentation, we will demonstrate and discuss the results of crystallinity and optical absorption properties.
[1] T. Miyasaka, A. Kulkarni, G. M. Kim, S. Öz and A. K. Jena, Adv. Energy Mater. 2020, 11902500 (2020).
[2] K. Kawashima, Y. Okamoto, O. Annayev, N. Toyokura, R. Takahashi, M. Lippmaa, K. Itaka, Y. Suzuki, N. Matsuki and H. Koinuma, Science and Technology of Advanced Materials 18, 307 (2017).
[3] N. Matsuki, Y. Iida, T. Shimada, Yuta Abeand T. Sato, ECS Meeting abst. MA2020-02, 1860 (2020).
