Energetic Stability and Band-Edge Orbitals of Layered Inorganic Perovskite Compounds for Solar Energy Applications

Halide and oxide perovskite semiconductors (e.g., CsPbI3 and SrTiO3) have been widely studied for solar energy conversion applications. The optoelectronic properties and performance of these compounds can be tuned through the growth of layered perovskite superstructures. While oxides are quite varied in the compositions and geometries taken up by layered perovskites, halides have proven much more limited. In this paper, we use density functional theory calculations and chemical intuition to explore why this is the case. We show that, in general, the thermodynamic stability or instability of layered perovskite superstructures depends on the interplay of their ionic and covalent character and, just as importantly, on the features of other competing phases.

VASP input files.For all calculations discussed in this paper, the VASP input files (POSCAR, INCAR, and KPOINTS) are available on the authors' research group GitHub page (https://github.com/bergerlab-wwu).
Pseudopotentials.In order to produce high-quality results, PBE pseudopotentials in VASP are chosen such that as many electrons as possible are treated as valence.Table S1 shows which electrons are treated as valence.Tabulated results.Tables S2 through S8 show the structural and orbital energies of the DFT-PBE-optimized structures discussed throughout in body of the paper.S3: Results analogous to Table S2, with the key difference that Table S3 includes van der Waals corrections using the D3 method of Grimme.All energies relative to AX and ABX 3 are very similar (within 0.006 eV per atom) with and without van der Waals corrections, justifying our use of DFT without van der Waals corrections throughout the body of the paper.

Table S1 :
Electrons treated as valence in the VASP PBE pseudopotentials of each of the elements computed in this paper.

Table S2 :
Comparisons of the DFT-PBE structural energy per atom of phases within the Sr-Ti-O, Ca-Ti-O, Cs-Pb-I, and Cs-Ge-I systems, as in Figure 2 in the body of the paper.Phases compared are NaCl-type AX, A 4 BX 6

Table S4 :
Structural energies and energy differences per atom of A 4 BX 6 phases and combinations of the respective n = 1 Ruddlesden-Popper (RP) phases and NaCl-type AX phases, as in Figure3ain the body of the paper.A 4 BX 6 relative energy (i.e., the last column) is defined as the difference between A 4 BX 6 and a combination of A 2 BX 4 and AX, where a negative value means A 4 BX 6 is favored.

Table S5 :
Valence band maximum (VBM) energies and energy differences of A 4 BX 6 and n = 1 Ruddlesden-Popper (RP) phases, as in Figure3bin the body of the paper.For each compound, the VBM energy is referenced to the energy of the flat, lowest-energy, semi-core band at k-point Γ = (0, 0, 0).The difference between A 4 BX 6 and A 2 BX 4 VBM energies is defined such that a negative value means the VBM of A 4 BX 6 is lower in energy.

Table S6 :
Results analogous to TableS5, with the key difference that TableS6includes spin-orbit coupling.All VBM energy differences are very similar (within 0.1 eV per atom) with and without spin-orbit coupling, justifying our use of DFT without spin-orbit coupling in the body of the paper.

Table S7 :
Comparisons of the DFT-PBE structural energy per atom of phases within the Sr-W-O and Cs-As-Cl systems, as in Figure 4a,b in the body of the paper.Phases compared are NaCl-type AX, [111]-layered A 3 B 2 X 9 , [011]-layered ABX 4 , and BX 3 .

Table S8 :
Structural energies and energy differences per atom of [011]-layered ABX 4 phases and combinations of the respective [111]-layered A 3 B 2 X 9 phases and AX 3 phases, as in Figure4cin the body of the paper.ABX 4 relative energy (i.e., the last column) is defined as the difference between ABX 4 and a combination of A 3 B 2 X 9 and AX 3 , where a negative value means ABX 4 is favored.