Fabrication of Stable CsPbI2Br Perovskite Solar Cells in the Humid Air

Inorganic perovskite materials have gained considerable attention owing to their good thermal stability, high absorption coefficient, adjustable bandgap, and simple preparation. However, most inorganic perovskites are sensitive to water and need to be prepared under inert environments in a glove box, which increases their preparation cost. In this study, we used a simple one-step spin coating anti-solvent process to prepare CsPbI2Br, which was then annealed in humid air (relative humidity < 35%) at 300 degrees C for 5 min with isopropanol as the anti-solvent. An inorganic perovskite solar cell with fluorine-doped tin dioxide/compact TiO2/mesoporous TiO2/CsPbI2Br/hole transport materials/Ag structure was prepared. By varying the concentration of the mesoporous precursor, we controlled the thickness of mesoporous TiO2 in order to investigate its effect on the properties of the perovskite films and devices. The X-ray diffraction (XRD) results confirmed the successful synthesis of CsPbI2Br in humid air. Moreover, the thickness of the substrate affected the crystal growth orientation. The scanning electron microscopy results revealed that the thickness of the mesoporous titanium dioxide substrate affected the crystallization processing of CsPbI2Br, resulting in the formation of compounds with different morphologies and phases. The ultraviolet-visible (UV-Vis) and photoluminescence spectra of the perovskite materials revealed that the substrate thickness affected their optical properties. With a decrease in the thickness of the mesoporous TiO2 substrate, the bandgap of CsPbI2Br increased slightly. At the substrate thickness of 145 nm, the defect density of state of CsPbI2Br increased. At the optimum mesoporous titanium dioxide substrate thickness of 732 nm, the device showed the best power conversion efficiency of 8.16%. The electrochemical impedance spectroscopy measurements revealed that the devices prepared on thicker mesoporous layers showed better carrier extraction and transmission capabilities but higher interfacial charge recombination resistance, leading to a lower open-circuit voltage but higher current density. Thus, an increase in the thickness of the mesoporous substrate improved the photovoltaic performance of the devices. The stability of the CsPbI2Br perovskite film improved with an increase in the mesoporous substrate thickness. The stability test results along with the UV-Vis and XRD analysis results showed that the perovskite film prepared on the 732 nm-thick substrate showed no significant structure change after being placed in humid air for 144 h. The stability of the perovskite solar cells was also investigated. The device with the 732 nm-thick substrate could maintain its original efficiency of 73% after exposure to air with relative humidity less than 35% for 72 h. Thus, inorganic perovskite solar cells could be successfully prepared in the humid air environment.