Herein, we demonstrate a vacuum-based evaporation approach to fabricate organic-inorganic perovskite thin films by using phase-formed halide perovskite nanoparticles (NPs) as a precursor/source. We are able to consistently obtain MAPbX3 (MA=CH3 NH3 and X=Cl, Br or I) thin films at various substrates (e. g., glass, ITO, or plastic). The perovskite phase formation in thin film form is confirmed by x-ray diffraction (XRD) studies. Small micro-strain (tensile) values of 1.64×10-3 , 1.42×10-3 and 6.85×10-4 obtained for MAPbCl3 , MAPbBr3 and MAPbI3 films respectively from Williamson-Hall equation indicate low structural distortions in perovskite thin films. The absorption spectra of thin films show sharp band edge having direct band gap, which is followed by narrow full width at half maxima (FWHM ∼0.1 eV) of the emission peak. Thin films of MAPbCl3 , MAPbBr3 and MAPbI3 show direct band gap of 3.1 eV, 2.4 eV and 1.6 eV, respectively. Small Urbach energy values of 33 meV, 44 meV and 66 meV for MAPbCl3 , MAPbBr3 and MAPbI3 films respectively indicates low defect density in various perovskite films. Scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDS) shows high surface coverage and uniform chemical composition of MAPbX3 (X=Cl, Br and I) thin films deposited by the present method. We have successfully controlled the film thickness from 250 nm to 1 μm by varying the nanoparticle precursor amount. The perovskite thin films deposited by the present method are highly stable against the degradation under ambient conditions. Systematic XRD studies along with absorption data demonstrate that the MAPbCl3 and MAPbBr3 films stored under ambient conditions remained stable for more than 30 days and MAPbI3 films for more than 7 days.
Keywords: Organic-inorganic perovskite nanoparticles; perovskite thin films; room-temperature film deposition; single-source thermal deposition; thermal vapor deposition method.
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