Stability, electronic, and optical properties of lead-free halide double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X$ = halogen)

Recently developed halide double perovskites $A_2{M}^{+}{M}^{3+}{X}_6$ have attracted attention as potential lead-free perovskite solar cell absorbers. Here, the stability, electronic, and optical properties of the lead-free inorganic-organic hybrid double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X$ = halogen) are studied based on density functional theory. By investigating the thermodynamic stability against decomposition, the ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ double perovskites are shown to be stable. Furthermore, the use of a small magnitude of hydrostatic pressure is proposed to improve the thermodynamic stability of double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$. Meanwhile, the calculated electronic band structures of ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ double perovskites exhibit suitable direct band gaps, small carrier effective masses, and three-dimensional electronic dimensionality. This, in turn, leads to strong optical absorption. Both the electronic and optical features make double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ suitable for applications in optoelectronics.

Figure 1

The schematic crystal structure of (a) ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X=\mathrm{halogen}$) hybrid halide double perovskites in four different phases, which are (b) cubic polar, (c) cubic nonpolar, (d) tetragonal, and (e) orthorhombic. The red dashed squares in (b)–(e) indicate the unit cells and the green and light blue balls in (b)–(e) indicate carbon and nitrogen atoms, respectively.

Figure 2

(a) Calculated decomposition energies $\mathrm{\Delta }{H}_{\mathrm{d}}$ of ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X=\mathrm{I}$, Br, Cl) double perovskites along three representative decomposition pathways. The pink bars indicate pathways involving only binary compounds. The blue and green bars indicate the lowest $\mathrm{\Delta }{H}_{\mathrm{d}}$ nonredox and redox pathways involving ternary compounds. (b–d) Calculated decomposition energies $\mathrm{\Delta }{H}_{\mathrm{d}}$ of ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X=\mathrm{I}$, Br, Cl) double perovskites as a function of hydrostatic pressure for the three decomposition pathways considered in (a).

Figure 3

Fluctuation of (a–c) total energy and (d–f) temperature as a function of testing time in AIMD at 300 K for ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X=\mathrm{I}$, Br, Cl) double perovskites.

Figure 4

Calculated band structures and corresponding dipole transition matrix elements (in arbitrary units) for (a), (e) ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2{\mathrm{InBiI}}_6$, (b), (f) ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2{\mathrm{InBiBr}}_6$, and (c), (g) ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2{\mathrm{InBiCl}}_6$. (d) Total and projected densities of states for ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2{\mathrm{InBiBr}}_6$.

Figure 5

Calculated optical absorption spectra of ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X=\mathrm{I}$, Br, Cl) double perovskites. The results of GaAs and ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ are shown for comparison. The standard AM1.5G solar spectrum is shown on the bottom as a reference with different colors from red to purple indicating the visible light spectrum.