The role of PbI₂ in CH₃NH₃PbI₃ perovskite stability, solar cell parameters and device degradation

We report a systematic investigation on the role of excess PbI₂ content in CH₃NH₃PbI₃ perovskite film properties, solar cell parameters and device storage stability. We used the CH₃NH₃I vapor assisted method for the preparation of PbI₂-free CH₃NH₃PbI₃ films under a N₂ atmosphere. These pristine CH₃NH₃PbI₃ films were annealed at 165 °C for different time intervals in a N₂ atmosphere to generate additional PbI₂ in these films. From XRD measurements, the excess of PbI₂ was quantified. Detailed characterization using scanning electron microscopy, X-ray diffraction, UV-Visible and photoluminescence for continuous aging of CH₃NH₃PbI₃ films under ambient condition (50% humidity) is carried out for understanding the influence of different PbI₂ contents on degradation of the CH₃NH₃PbI₃ films. We find that the rate of degradation of CH₃NH₃PbI₃ is accelerated due to the amount of PbI₂ present in the film. A comparison of solar cell parameters of devices prepared using CH₃NH₃PbI₃ samples having different PbI₂ contents reveals a strong influence on the current density-voltage hysteresis as well as storage stability. We demonstrate that CH₃NH₃PbI₃ devices do not require any residual PbI₂ for a high performance. Moreover, a small amount of excess PbI₂, which improves the initial performance of the devices slightly, has undesirable effects on the CH₃NH₃PbI₃ film stability as well as on device hysteresis and stability.