Proceedings of Perovskite Thin Film Photovoltaics (ABXPV16)
Publication date: 14th December 2015
The degradation mechanism of solution processed CH3NH3PbI3 (MAPbI3) compact layers to PbI2 was studied by X-Ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HR-TEM), Scanning Electron Microscopy (SEM) and Spectroscopic Ellipsometry (SE). The path of MAPbI3 degradation is characterised by an early stage, which can be described as a purely inner structural modification of the material. This internal transformation drives the starting tetragonal lattice towards a cubic-like atomic arrangement with the contraction of the c-axis length, without any significant change in the other lattice parameters and without any PbI2 appearance in both air as well as vacuum storage conditions. After this early stage a degradation process that generates PbI2 at the expenses of MAPbI3 occurs. This degradation process was investigated in air, nitrogen and vacuum conditions. In particular, the kinetic curves were measured in the range between 90°C and 135°C and the related activation energies were indeed extracted. The kinetic parameters were used to have important indications about the degradation mechanisms that drive the MAPbI3 transition to PbI2. Density Functional Theory (DFT) was indeed used to evaluate the formation energy of various defects in the MAPbI3 lattice to be related to the experimental results. We argue that even in the absence of humidity, a decomposition of the perovskite structure can take place through the statistical formation of molecular defects with a non-ionic character that, thanks to their volatility, can escape the surface of the perovskite layer and consequently break the thermodynamic defect equilibrium. Our results suggest that after the volatilization of HI + CH3NH2 or MAI, slight octahedral rearrangements with a relatively low energy cost take place. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material. We finally discuss the strategies that can limit such phenomenon thus prolonging the lifetime of the material.
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