Mixed-halide vacancy-ordered double perovskite for photovoltaic and photocatalysis applications

Here, we report detailed first-principles calculations of the structural stability, optoelectronic properties, and interaction with water for a wide range of mixed-halide compositions of vacancy-ordered double perovskites ${\mathrm{Cs}}_2{\mathrm{Pt}(\mathrm{Cl}}_x{\mathrm{I}}_{1-x}{)}_6$. Our calculations reveal that lower halide dopant levels subdue phase segregation and enhance the stability. ${\mathrm{Cs}}_2{\mathrm{Pt}(\mathrm{Cl}}_x{\mathrm{I}}_{1-x}{)}_6$ demonstrate improved defect tolerance as compared to ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$ due to the covalent nature of the Pt—X bond. The chloride-rich ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_x{\mathrm{I}}_{1-x}$)${}_6$ exhibit notably improved stability against reaction with water, far surpassing ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$ due to the enhanced Cs—Cl bond strength and lower charge transfer between adsorbed ${\mathrm{H}}_2\mathrm{O}$ and surface Cs atoms. The spectroscopic limited maximum photovoltaic efficiency for the optimal composition of ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$ under 1 sun AM1.5G is determined to be $24$% for a 5-$\text{μ}\mathrm{m}$-thick film. Our calculations also suggest that the valence-band edge of this material might be positioned more positive than the standard potential of the oxygen-evolution reaction. These two factors combined with the high stability against reaction with water indicate that ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$ might be of considerable interest as a photovoltaic absorber, and possibly as a component of anodes for the photoelectrocatalytic water oxidation. Meanwhile, ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$ traverses relevant reduction and oxidation redox potentials, affirming it as a promising candidate for the overall photo(electro)catalyst water-splitting reaction.

Figure 1

Crystal structure of (a) pristine ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{Cl}}_6{/\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$, and (b) mixed-halide ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_x{\mathrm{I}}_{1-x}$)${}_6$ perovskites.

Figure 2

(a)–(i) Chemical phase diagrams of pristine and mixed-halide systems, which are stable in single phase, (a) ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$, (b) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.02}{\mathrm{I}}_{0.98}$)${}_6$, (c) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$, (d) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.08}{\mathrm{I}}_{0.92}$)${}_6$, (e) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.12}{\mathrm{I}}_{0.88}$)${}_6$, (f) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.92}{\mathrm{I}}_{0.08}$)${}_6$, (g) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$, (h) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.98}{\mathrm{I}}_{0.02}$)${}_6$, and (i) ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{Cl}}_6$. The grey shaded region represents the stability region of target system. (j) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.25}{\mathrm{I}}_{0.75}$)${}_6$, (k) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.5}{\mathrm{I}}_{0.5}$)${}_6$, (l) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.75}{\mathrm{I}}_{0.25}$)${}_6$, (m) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.88}{\mathrm{I}}_{0.12}$)${}_6$. The grey shaded region in these four diagrams are the projected phase diagrams showing co-existence of target phases with other secondary phases (i.e., multiple phase formation).

Figure 3

(i,ii) Simulated band structure and density of states using conventional unit cell (iii) unfolded band structure in the primitive unit cell for (a) ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$, (b) ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{Cl}}_6$, (c) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$, and (d) ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$, respectively, using SCAN functional.

Figure 4

(a) Simulated band gap versus Cl concentrations $x$ (at. %) in ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_x{\mathrm{I}}_{1-x}$)${}_6$. (b) Free enthalpy of mixing versus Cl concentrations $x$ (at. %) in ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_x{\mathrm{I}}_{1-x}$)${}_6$.

Figure 5

(a) Absorption coefficients and corresponding (b) SLME versus film thickness for ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$, ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$, ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$, and ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{Cl}}_6$ simulated using $G_0{W}_0$+BSE. (c) Band edge potentials of ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$, ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$, ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$, and ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{Cl}}_6$.

Figure 6

(i) ${\mathrm{H}}_2\mathrm{O}$ adsorption energies for different variants of perovskites. Ref: [65], §-[66], ¶-[58] (ii) (111) stable surface slabs (left) and charge-density difference plots (right) for ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6{/\mathrm{H}}_2\mathrm{O}$ [Fig. (a),(b)], ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{Cl}}_6{/\mathrm{H}}_2\mathrm{O}$ [Fig. (c),(d)], ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$/${\mathrm{H}}_2\mathrm{O}$ [Fig. (e),(f)], and ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$/${\mathrm{H}}_2\mathrm{O}$ [Fig. (g),(h)] systems. Cs, Pt, I, Cl, H, O atoms are represented by orange, grey, purple, green, peach, and red colors, respectively. Isosurfaces are displayed using yellow and cyan colours. Yellow represents positive electronic densities, which accept electrons (i.e., charge gain) and cyan represents negative densities, which donate electrons (i.e., charge loss) during chemical bonding.