Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
The stoichiometry of hybrid perovskites is critical to determinate the stability and the optoelectronic properties of this class of materials. Their peculiar structure leads to a large number of compositional degrees of freedom and, in principle, allows to create solid solution intermixing different organic cations, halides or divalent metals.
In this study we focused on the mixed-halide compounds. We employed DFT ab-initio calculations with PBEsol exchange-correlation functional, to explore the thermodynamic properties with the generalized quasi-chemical approximation (GQCA). We initially applied this method to the solid solution MAPb(I1-xBrx)3 as a primary test case. We calculated the enthalpy of mixing of the alloy and built the first reported phase diagram for these type of materials.
From the analysis of the diagram we can conclude that the alloy is more stable in the Br-rich region and phase segregation occurs below the critical temperature of 343K. Our calculations confirm what has already been reported for the analogous inorganic perovskite CsPb(I1-xBrx)3, and agree with the experimental observation of bleaching occurring in hybrid perovskite thin films with intermediate halide composition.
These results shed light on the mechanisms behind the observed anion redistribution upon heating and/or illumination responsible for degradation and phase separation that have been reported in devices.
References:
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