Stability Characterization of PbI₂-Added CH₃NH₃PbI_3–xCl_x Photovoltaic Devices

The stability of TiO2/CH3NH3PbI3–xClx-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.