Effect of Laser Type on Material Characteristics of Halide Perovskite Solar Cell Materials Fabricated Via Laser Molecular Beam Deposition
Authors: Matsuki Nobuyuki , Sato Tomomasa 
Published: February 2024

A key feature of halide perovskite solar cells is the ability to achieve conversion efficiencies in excess of 25% in a solution-coating process under atmospheric pressure [1, 2]. In the solution-coating process, equilibrium chemical reactions that promote crystal growth can produce thin films on the order of μm thickness in a few seconds. On the other hand, physical vapor deposition (PVD) has the advantage of controlled deposition at the molecular level of elements that cannot be obtained by solution reaction, although the deposition rate is relatively slow, generally from sub-nm/s to several tens of nm/s. Laser molecular beam deposition (LMBD), one of the PVD methods, can form thin films by heating and evaporation through infrared (IR) laser irradiation or by ablation through plasma formation by ultraviolet (UV) electron excitation with a pulsed UV laser. LMBD has the advantage of being able to switch the laser beam on and off only on the material surface, resulting in the instantaneous deposition of molecular layers of organic materials with high vapor pressure, as well as of high melting point materials such as inorganic oxides, with minimal cross-contamination. We have been developing halide perovskite solar cell materials based on these advantages in LMBD [3].

Experiments with IR laser LMBD were conducted using the following process: synthetic quartz substrates were placed in an ultra-high vacuum chamber with a base vacuum of 2×10-5 Pa. A semiconductor continuous IR laser (wavelength: 808 nm) or an Nd:YAG Q-Switch quadruple-wave UV pulsed laser (wavelength: 266 nm, 10 Hz) was employed to alternately irradiate material targets of PbI2 and MAI, forming CH3NH3PbI3 (MAPbI3) at room temperature. The electron transport layer, using a TiO2 target, and the hole transport layer, using a CuI target, were formed at room temperature. The optical, crystalline, and electrical properties of the films were then characterized by UV-visible spectroscopy, X-ray diffraction (XRD), and Hall measurements, respectively.

Four multilayer samples consisting of [PbI2/MAI]×n (n=1, 2, 5, 10) were prepared by IR-LMBD under conditions where the total thickness of PbI2 was fixed at 300 nm. The analysis of the prepared samples by XRD measurements revealed two distinct features: (1) The sample with n = 1 had unreacted PbI2 in addition to MAPbI3, while the samples with n = 2, 5, and 10 only showed MAPbI3, indicating a successful solid-phase reaction. (2) The full width at half maximum and peak position of the diffraction peak for MAPbI3 at 2θ = 14.0° ~ 14.1° varied among the samples, and thus peak separation was performed to determine the presence of both tetragonal (002) and orthorhombic (020) diffraction peaks.

In UV-LMBD (or pulsed laser deposition; PLD), a deposition rate of 20 nm/s was attained for the perovskite layer, even under low fluence conditions without lens focusing. However, under the high fluence condition with the lens focusing, the deposition rate dropped significantly to about 0.35 nm/s. The evaluation of the film surface roughness suggested that the effective deposition rate was considerably reduced under the high fluence condition with lens-focusing, due to the excess kinetic energy of the film precursor, which caused re-sputtering on the film surface during deposition.

The carrier transport layers were also fabricated by UV-LMBD. The optical and electrical properties of the TiO2 thin film fabricated as the electron transport layer showed an average optical transmittance of 85% and conductivity of 3.2 S/cm at 400 to 800 nm. The CuI thin film fabricated as the hole transport layer was found to have <111> preferential orientation of polycrystalline γ-CuI from the results of X-ray diffraction measurement and to display a conductivity of 24.5 S/cm, hole concentration of 2.7×1019 cm-3, and hole mobility of 6.0 cm2/ (V·s) in hole measurements.

We are currently optimizing the process for consistently fabricating the electron transport layer, halide perovskite layer, and hole transport layer in a vacuum through pattern fabrication using a movable shielding mask in the film fabrication area of each layer.

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