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Computational Screening and Discovery of Silver-Indium Halide Double Salts

Christos Tyrpenou, G. Krishnamurthy Grandhi, Paola Vivo, Mikaël Kepenekian, George Volonakis

Abstract

Perovskite-inspired materials have emerged as promising candidates for both outdoor and indoor photovoltaic applications owing to their favorable optoelectronic properties and reduced toxicity. Here, we employ the experimentally realized AgBiI$_4$ double salt as a structural prototype and replace Bi$^{3+}$ with In$^{3+}$ to design a novel lead-free halide compound, AgInI$_4$. First-principles calculations predict that AgInI$_4$ is both chemically and dynamically stable, exhibiting a direct band gap of 1.72 eV, comparable to its bismuth analogue. However, its predicted photovoltaic performance, evaluated using the spectroscopic limited maximum efficiency metric, is lower under both solar and LED illumination. This reduction arises primarily from symmetry-forbidden optical transitions and the absence of Bi-derived 6s$^2$ lone-pair states at the valence band maximum. High-throughput screening of the Ag-In-I ternary phase-space reveals several more stable and metastable compounds that fall into two structural families: tetrahedrally and octahedrally coordinated, with characteristic band gaps near 3.0 eV and 2.0 eV, respectively. Despite multiple synthetic attempts, the predicted AgInI$_4$ phase could not be experimentally realized, underscoring the challenges of stabilizing indium-based halide double salts. While these materials are unlikely to serve as efficient photovoltaic absorbers, their tunable band gaps and stability make them promising candidates for charge transport and other optoelectronic applications.

Computational Screening and Discovery of Silver-Indium Halide Double Salts

Abstract

Perovskite-inspired materials have emerged as promising candidates for both outdoor and indoor photovoltaic applications owing to their favorable optoelectronic properties and reduced toxicity. Here, we employ the experimentally realized AgBiI double salt as a structural prototype and replace Bi with In to design a novel lead-free halide compound, AgInI. First-principles calculations predict that AgInI is both chemically and dynamically stable, exhibiting a direct band gap of 1.72 eV, comparable to its bismuth analogue. However, its predicted photovoltaic performance, evaluated using the spectroscopic limited maximum efficiency metric, is lower under both solar and LED illumination. This reduction arises primarily from symmetry-forbidden optical transitions and the absence of Bi-derived 6s lone-pair states at the valence band maximum. High-throughput screening of the Ag-In-I ternary phase-space reveals several more stable and metastable compounds that fall into two structural families: tetrahedrally and octahedrally coordinated, with characteristic band gaps near 3.0 eV and 2.0 eV, respectively. Despite multiple synthetic attempts, the predicted AgInI phase could not be experimentally realized, underscoring the challenges of stabilizing indium-based halide double salts. While these materials are unlikely to serve as efficient photovoltaic absorbers, their tunable band gaps and stability make them promising candidates for charge transport and other optoelectronic applications.

Paper Structure

This paper contains 6 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Results and Discussion
  3. Structural and Electronic Properties
  4. Optical Properties and Photovoltaic Performance
  5. Ag/In phase space screening
  6. Conclusion

Figures (5)

  • Figure 1: a) Crystal structure of AgInI$_4$ based on the reduced symmetry model of AgBiI$_4$, b) alongside the distorted AgI$_6$ and InI$_6$ octahedra (OC-6) of the AgInI$_4$ structure. The OC-6 abbreviation refers to the IUPAC symbol for octahedron coordination environment.
  • Figure 2: Electronic band structure of AgInI$_4$ based on the recuded symmetry model and the corresponding density of states, green arrow represent the first allowed transition at $\Gamma$. (All details about symmetry analysis for In double salt can be found in the Supporting Information)
  • Figure 3: a) The theoretical absorption coefficient and corresponding first allowed dipole transitions for AgInI$_4$ (light blue) and AgBiI$_4$ (red). b) The SLME of double halide materials under standard solar and LED illumination. c) The corresponding J-V curves under indoor conditions.
  • Figure 4: Formation energy of AgInI$_4$ polymorphs in respect to AgI and In$_3$ salts and the correspoding coordination environment of each polymorph (left). Band gap (E$_g$) of each polymorph employing PBE0 functional (right). p1 corresponds to polymorph obtained by Wyckoff position splitting.
  • Figure 5: Electronic structure of the most stable polymorphs of each family. a) Presents the electronic structure of the tetrahedral coordinated polymorph (Polymorph-2), while b) panel displays the electronic structure of the octahedral coordinated polymorph (Polymorph-6).(Crystal structure insets depic the conventional cell of each polymorph).