Stability Characterization of PbI₂-Added CH₃NH₃PbI_{3- x}Cl _x Photovoltaic Devices

The stability of TiO₂/CH₃NH₃PbI_{3- x}Cl _x-based photovoltaic devices in ambient air was evaluated upon adding PbI₂ and/or PbCl₂. X-ray diffraction (XRD) peak intensities corresponding to the perovskite phase were increased by adding PbI₂. After 7 weeks, the XRD peak intensities corresponding to the perovskite phase decreased and those corresponding to PbI₂ increased. The reaction rate constants for the decomposition of perovskite and formation of PbI₂ were estimated from these data. Thermodynamic calculations of the reaction between PbCl₂ and I₂ suggested that the formation of PbI₂ was not related to the added PbCl₂ but rather to excess PbI₂. Open-circuit voltages and fill factors of the devices were improved with the 7 week time lapse because of the suppression of electron-hole recombination by the PbI₂. In addition, the decomposition of perovskite grains was suppressed by the added PbI₂. The I content of the perovskite phase decreased with the 7 week time lapse. However, the Cl content was largely constant after the 7 weeks, which suggested that Cl doping effectively stabilized the perovskite photovoltaic devices.