Thermal stability of CH3NH3PbIxCl3-x versus [HC(NH2)(2)](0.83)Cs0.17PbI2.7Br0.3 perovskite films by X-ray photoelectron spectroscopy
Abstract
The thermal stability of CH3NH3PbIxCl3-x and [HC(NH2)(2)](0.83)Cs0.17PbI2.7Br0.3 perovskite films was studied insitu by X-ray photoelectron spectroscopy. It was found that below 85 degrees C both of them are relatively stable.
After annealing above 85 degrees C, we observe a clear perovskite surface decomposition, i.e., a release of organic cations and creation of "metallic lead". The mixed cation lead mixed halide perovskite, however, decomposes at a much lower rate.
For both perovskite films, the metallic to the total lead ratio changes with the same rate for the same annealing temperatures. The release of A-site cations from the ABX(3) crystal structure of perovskite and/or creation of "metallic lead" causes also a small shift of the valence band maximum towards the Fermi level.
The release of [HC(NH2)(2)] (+/-) or Cs (+/-) is not as significant as the release of CH3NH3 +/- ; therefore, it may explain why [HC(NH2)(2)](0.83)Cs0.17PbI2.7Br0.3 solar cells are thermally more stable. Therefore, as the stability of CH3NH3 PbIxCl3-x is same as the stability of [HC(NH2)(2)](0.83)Cs0.17PbI2.7Br0.3 below 85 degrees C, there must be more severe degradation pathways that are currently underappreciated on the solar cell level.