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

Abstract

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

Characterization of Methylammonium Lead Iodide Thin Films Fabricated by Exposure of Lead Iodide Layers to Methylammonium Iodide Vapor in a Closed Crucible Transformation Process

“…This position is assumed to be the onset of the fundamental absorption and accordingly serves as a measure of the optical band gap. With 1.57–1.58 eV for MAPI and 2.35 eV for PbI 2 it fits well to the literature values …”

“…Figure shows that the contact between MAPI and MAI will be hole blocking. The role of remnant PbI 2 in perovskite solar cells and its impact on their performance is still subject to scientific discussions . The alignment of the vacuum levels of MAPI and PbI 2 shows that the contact from MAPI to PbI 2 would be hole blocking while the conduction bands do not show an electron barrier.…”

Detrimental Effect of Unreacted PbI2 on the Long‐Term Stability of Perovskite Solar Cells

“…[ 18 ] Thus, the presence of PbI 2 crystals in films is a potential source of perovskite's instability. However, how PbI 2 crystals affect long‐term stability of perovskites like CsFAMA under continuous light irradiation has been rarely studied [ 19–21 ] and, therefore, the clarification of basic degradation mechanisms is needed to further improve the stability of PSCs.…”

Effective Preparation of Nanoscale CH3NH3PbI3 Perovskite Photosensitizers for Mesoporous TiO2‐Based Solar Cells by Successive Precursor Layer Adsorption and Reaction Process

“…In addition, the peak corresponding to PbI 2 at 12.7° was observed to appear slightly from the second cycle, meaning that some of the PbI 2 had not completely changed to MAPbI 3 during the consecutive deposition cycles. Recently, there have been a few reports on the role of excess PbI 2 remaining in the formation of perovskite crystalline films . This is still a controversial issue in bulk perovskite films and much more studies are needed at the nanoscale MAPbI 3 photosensitizers.…”