Interlayer Solid-Phase Reaction in Halide Perovskite Thin Films Fabricated Via Alternate Infrared Laser Deposition

It is now widely pursued to investigate the growth mechanism of halide perovskite thin films in detail to elucidate the degradation mechanism and to develop the thin films with superior photovoltaic properties. We have applied infrared laser molecular beam deposition (IRL-MBD), which can control the deposition of halide perovskite at the atomic layer level in ultra-high vacuum condition [1]. In our IRL-MBD process, CH3NH3PbI3 (MAPbI3) layers are formed by alternately depositing CH3NH3I (methylammonium iodide: MAI) and PbI2 layers, and subsequent interlayer solid-phase reaction at room temperature. In this study, we focus on the effect of the layer thickness on the interlayer solid-phase reaction and the properties of the formed MAPbI3 thin films. A synthetic silica substrate cleaned using organic solvents was introduced into the IRL-MBD chamber via a load lock. After that, PbI2 and MAI molecular beams were alternately irradiated onto the substrate by evaporating MAI and PbI2 sources using an infrared laser beam (wavelength: 808 nm, input power: 2~3 W). The deposition rate of PbI2 was controlled at 0.1 nm/s using a quartz crystal microbalance (QCM) thickness monitor and manual laser power control. The evaporation rate of MAI was manually adjusted so that the pressure in the chamber was kept at 4×10-4 Pa during the irradiation of MAI onto the substrate. Figure 1 shows a schematic layer structure of the samples A, B and C consisting of 1, 5 and 10 pair(s) of [MAI/PbI2] bilayer, respectively. The irradiation time of MAI and the nominal thickness of PbI2 were A: [80 min/300 nm]×1, B: [16 min/60 nm]×5 and C: [8 min/30 nm]×10. Figure 2 shows the X-ray diffraction (XRD) patterns obtained from the samples. The sample A shows two diffraction peaks corresponding to PbI2(001) and MAPbI3(110), while the samples B and C exhibits only single peak corresponding to MAPbI3(110) in the shown diffraction angle range. Figure 3 shows the schematic structure of the samples inferred from the results described above. In the sample A, PbI2 layer, which is not completely reacted with MAI, remains in the thin film. In contrast, in the samples B and C, only the phase of MAPbI3 is observed, indicating that PbI2 and MAI react completely by the interlayer solid-phase reaction. The results suggest that PbI2 has a critical film thickness that causes a complete interlayer solid-phase reaction with MAI at room temperature between 60 nm and 300 nm. The critical film thickness is likely comparable to the penetration depth of MAI into PbI2 and is supposed to be dependent on the temperature during the interlayer solid-phase reaction. [1] K. Kawashima, Y. Okamoto, O. Annayev, N. Toyokura, R. Takahashi, M. Lippmaa, K. Itaka, Y. Suzuki, N. Matsuki and H. Koinuma, Sci. and Tech. of Adv. Mater. 18, 307-315 (2017). Figure 1