Processing of perovskite thin films by excimer laser (invited)

Objective In recent years, organic-inorganic hybrid perovskites have attracted the attention of the photovoltaic industry for their excellent photoelectric properties and efficient solar cell processing technology. The current highest efficiency record for single-layer perovskite solar cell device certification is 26.1%, and the highest efficiency record for series structure perovskite solar cell device certification is 29.1%. However, bringing perovskite thin-film photovoltaic technology to the market still faces some problems that need to be solved. At present, the high-efficiency perovskite solar cell devices are small size devices obtained in the laboratory, when the device area is enlarged, the series resistance of perovskite solar cells will increase linearly with the increase of the area (especially the TCO substrate), and eventually cause huge performance loss. Therefore, in order to commercialize perovskite solar cells, a larger area of solar cell series modules must be obtained. The excimer laser wavelength is in the deep ultraviolet band, the perovskite material has high absorption rate and shallow action depth, which is expected to trigger the ablation mechanism and accurately remove the perovskite material by layer, obtain a clean bottom and boundary, and effectively protect the underlying device. In this paper, the processing mechanism of perovskite thin films (MAPbI₃) by excimer laser is studied. The chemical composition residue and surface morphology of perovskite thin films treated by excimer laser and fs laser were compared.

Methods Due to the low transparency of the glass substrate aligned molecular laser, excimer laser processing uses the film side irradiation. The effects of excimer laser and fs laser on the processing of perovskite thin films were compared by two irradiation methods: film side and glass side. Field emission scanning electron microscope (FE-SEM), energy dispersive spectrometer (EDS) and white light interference microscopy (WLIM) were used to characterize the surface morphology and residual elements of the samples after laser processing.

Results and Discussions The damage threshold of perovskite material irradiated by 248 nm excimer laser was studied. The damage threshold of 248 nm excimer laser irradiation on perovskite materials was 12 mJ/cm²(Fig.5), based on the WLIM 3D graphics and depth data of materials processed with different energy densities and 1-3 pulses. Significant damage begins to occur when the energy density reaches 18 mJ/cm².The surface processing effect of perovskite thin film was studied by 1-6 pulsed excimer lasers with different energy densities of 20-160 mJ/cm²(Fig.7, Fig.8), Fig.9 is the energy spectrum diagram given by EDS. It can be seen that the intensities of I and Pb in the processed sample are significantly reduced. Due to the gradual exposure of the underlying substrate material, the strength of Sn in the TCO component and Si in the glass component increased significantly, and the processed sample curve was very close to the substrate curve. Atomic number ratio of I and Pb in EDS graph is shown in Fig.10. With the increase of energy density, at low energy density, the atomic numbers of I and Pb almost drop linearly with the increase of energy density, which is consistent with the increase of depth linear shape at low energy. At high energy density, the substrate is gradually exposed, and the atomic number of I and Pb is maintained at an extremely low level, which proves that the excimer laser can effectively remove the perovskite layer. Finally, the perovskite film was processed by 517 nm fs laser to compare the difference between excimer laser and 517 nm fs laser for perovskite film processing. Inside the excimer spot, the substrate is completely exposed and the bottom is clean, with no perovskite layer remaining. The outer grain of the spot is clear, and there is no obvious heat-affected zone and redeposition. The edge of femtosecond laser film side processing area produces obvious melting and material modification. The perovskite layer inside the processing area was effectively removed, but some flake remained near the processing area. The femtosecond laser base side processing area near the edge remains partially melted material. Some flakes are attached to the edges of the membrane.

Conclusions The processing mechanism of perovskite (MAPbI₃) film irradiated by a 248 nm excimer laser was investigated. The surface morphology of the sample after laser processing was characterized by WLIM, and the damage threshold of perovskite thin film was determined based on the surface morphology characteristics and depth curve. On this basis, the influence of different energy density and pulse number on machining depth was investigated by depth curve. The morphology and removal effect of perovskite films were studied by SEM and EDS aligned molecules. It was proved that perovskite materials can be effectively removed by excimer laser. Finally, the perovskite thin film materials processed by excimer laser and fs laser (517 nm) were compared. Excimer laser triggered a unique ablative mechanism on perovskite materials, while fs laser from the substrate surface processing is a combination of ablative mechanism and stripping mechanism. Excimer laser processing of perovskite materials can obtain good boundary and clean substrate, which provides a new technical means for the future processing of perovskite films.