Towards Stable Free Lead Mixed Halide Perovskite Thin Films on FTO-Coated Glass Substrate

Abstract

All-inorganic cesium (Cs) lead perovskites have better thermal and chemical stability than organic–inorganic hybrids. They therefore represent a hope for stability and increased performance of perovskites as absorber layers in photovoltaic solar cells. In the present work, we have deposited different layers on FTO-coated glass substrates using the one-step spin-coating method. The results of the lead substitution are presented and critically discussed. The X-ray diffraction (XRD) results show four peaks for all three samples. The main peaks of the different films are located at the 2θ angles of 26.45° and 51.50° for the Muller indices (220) and (242), respectively. These two main peaks indicate that the prepared thin films all have two preferred crystallographic orientations. Beyond these two main peaks, we have two other smaller peaks at 2θ of 33.67° and 37.70° corresponding to the Muller indices of (210) and (211), respectively. The smoother the surface of the thin films, the more light they reflect, resulting in poor light absorption by the films. It is therefore important to obtain a surface image of the prepared films, as the larger the surface image, the better the film. With this in mind, we carried out a scanning electron microscope (SEM) analysis, which gave us the surface images. Figure 3 shows the SEM images of thin CsPb1−xSnxI1.5Br1.5 layers (x = 0, 0.5, and 1) grown on FTO-coated glass substrate with different [Pb]/[Sn] ratios. As can be seen from the figure, the effect of the [Pb]/[Sn] ratio is visible in the surface images of the different thin films. The smallest grain size is that of the unsubstituted CsPbI1.5Br1.5, while the largest grain size corresponds to the partially Pb-substituted layer (CsPb0.5Sn0.5I1.5Br1.5). The fully Pb-substituted layer (CsSnI1.5Br1.5) has an intermediate grain size. The surface images of the films show that the surfaces are well coated with grain sizes that vary greatly depending on the layer. The best grain size is that of the thin film with partial lead substitution (CsPb0.5Sn0.5I1.5Br1.5). Regarding the UV-visible absorption of the different films, we can say that the films absorb the maximum amount of light in the wavelength range of 350–550 nm. Above 550 nm, the absorption coefficients drop significantly. The absorption coefficients of the tin-free (Sn) layer remain higher than the coefficients of the other layers throughout the UV-visible spectrum. The degradation study revealed that the Sn-free layer retains good light absorption compared to the other layers after 4 weeks of exposure to the ambient environment. The crystal structure of all the layers shows good resistance to the elements during the 4 weeks, as shown by the renewed XRD results after the 4 weeks of exposure.