A heterophase containing both the optically active $\alpha \text{−}{\mathrm{CsPbI}}_3$ and nonactive $\delta \text{−}{\mathrm{CsPbI}}_3$ has been demonstrated as an efficient white light emitter. This has challenged the conventional perspective that nonactive phases of perovskites are undesirable in any metal halide perovskite-based optoelectronic devices. To understand the role that yellow phase $\delta \text{−}{\mathrm{CsPbI}}_3$ plays in the light-emission process, we performed a systematic computational study on its electronic and optical properties, which are relatively unexplored in the literature. Using the Fröhlich model we showed that both the electron and hole exhibit moderate coupling to longitudinal optical phonons. Explicit density functional theory calculations show that small hole polarons exist with a formation energy of $-96\mathrm{meV}$, corresponding to the contraction of the Pb-I bonds within a $\left[{\mathrm{PbI}}_6\right]$ octahedron. Nudged elastic bands calculations show that the hole polaron can hop into neighboring $\left[{\mathrm{PbI}}_6\right]$ octahedral sites with a small activation barrier of $2.1\mathrm{meV}$. Molecular dynamics simulations also show that the hole polaron exhibit periodic localization and delocalization behavior similar to carrier hopping with a characteristic lifetime of $0.3\mathrm{ps}$. Our results have elucidated the role that $\delta \text{−}{\mathrm{CsPbI}}_3$ plays in the self-trapped emission in perovskite-based white light emitting diodes by supporting the presence of the localized small hole polaron.

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• Excitons
• Polarons
• Metal halides
• Density functional calculations
• First-principles calculations
• Molecular dynamics