Formation and composition-dependent properties of alloys of cubic halide perovskites

Distinct shortcomings of individual halide perovskites for solar applications, such as restricted range of band gaps, propensity of ABX3 to decompose into AX+BX2, or oxidation of 2ABX3 into A2BX6 have led to the need to consider alloys of individual perovskites. This creates a non-trivial material-selection problem, spanning a continuum of three sets of compositions (one for each sub lattice), and requiring control of phase-separation or ordering in each alloyed subfield. Not surprisingly, material and structure choices were made thus far mostly via trial-and-error explorations. We use ideas from solid state theory of semiconductor alloys to analyze the behaviors of the (FA,Cs)(Pb,Sn)I3 alloys system. Density functional calculations utilizing specially constructed supercells (SQS) are used to calculate the composition dependence of band gaps, energy of decomposition and alloy mixing enthalpies. A number of trends are observed for A-site as well as for B-site alloys. To understand the physical reasons that control these trends we decompose the alloy properties into distinct physical terms: (i) the energies associated with removing the octahedral deformations (tilting, rotations, B site displacements), (ii) the energies of compressing the larger component and expanding the smaller one to the alloy volume, (iii) the charge transfer energies associated with placing the alloyed units onto a common lattice, and finally, (iv) structural relaxation of all bonds within the cells., This analysis clarifies the origin of the observed trends in bang gaps, decomposition energies and mixing enthalpies. Unlike a number of previous calculations we find that the a proper description of alloy physics requires that even the pure, non-alloyed end-point compounds be allowed to develop local environment dependent octahedral deformation that lowers significantly the total energy and raises their band gaps.
Comment: 11 pages, 9 figures, 2 tables, 42 references