A-Site Cation in Inorganic A₃Sb₂I₉ Perovskite Influences Structural Dimensionality, Exciton Binding Energy, and Solar Cell Performance

Abstract

Inspired by the rapid rise in efficiencies of lead halide perovskite (LHP) solar cells, lead-free alternatives are attracting increasing attention. In this work, we study the photovoltaic potential of solution-processed antimony (Sb)-based compounds with the formula A 3Sb2I9 (A = Cs, Rb, and K). We experimentally determine bandgap magnitude and type, structure, carrier lifetime, exciton binding energy, film morphology, and photovoltaic device performance. We use density functional theory to compute the equilibrium structures, band structures, carrier effective masses, and phase stability diagrams. We find the A-site cation governs the structural and optoelectronic properties of these compounds. Cs3Sb2I9 has a 0D structure, the largest exciton binding energy (175 ± 9 meV), an indirect bandgap, and, in a solar cell, low photocurrent (0.13 mA cm–2). Rb3Sb2I9 has a 2D structure, a direct bandgap, and, among the materials investigated, the lowest exciton binding energy (101 ± 6 meV) and highest photocurrent (1.67 mA cm–2). K3Sb2I9 has a 2D structure, intermediate exciton binding energies (129 ± 9 meV), and intermediate photocurrents (0.41 mA cm–2). Despite remarkably long lifetimes in all compounds (54, 9, and 30 ns for Cs-, Rb-, and K-based materials, respectively), low photocurrents limit performance of all devices. We conclude that carrier collection is limited by large exciton binding energies (experimentally observed) and large carrier effective masses (calculated from density functional theory). The highest photocurrent and efficiency (0.76%) were observed in the Rb-based compound with a direct bandgap, relatively lower exciton binding energy, and lower calculated electron effective mass. To reliably screen for candidate lead-free photovoltaic absorbers, we advise that faster and more accurate computational tools are needed to calculate exciton binding energies and effective masses.

Lead‐Free Metal Halide Perovskite Nanocrystals: Challenges, Applications, and Future Aspects

“…The experimental and theoretical work on Cs 3 Sb 2 I 9 shows that this material possesses a layered 2D structure with a direct bandgap of 2 eV, suggesting a suitable material for photovoltaics. The preferred formation of 0D and 2D structure depends on the sample preparation technique as well as an addition of additives ,,. The bulk phase of Sb based perovskites are explored in the specific form of Cs 3 Sb 2 Cl 9 (hexagonal P321), Cs 3 Sb 2 Br 9 (trigonal ), MA 3 Sb 2 I 9 (hexagonal P 6 3 / mmc ), MA 3 Sb 2 Br 9 (trigonal ) and (NH 4 ) 3 Sb 2 I x Br 9− x (0≤ x ≤9) (Monoclinic P121/n1) ,.…”

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

“…[120] The VBM consists mainly of I 5p orbitals slightly hybridized with Bi 6s orbitals, while the CBM is derived from the antibonding states of Bi 6p and I 5p orbitals. [118,[123][124][125][126] Figure 11c shows the Tauc plot of Cs 3 Bi 2 I 9 crystals at 10 K. Notably, the sharp excitonic peak at 2.56 eV is well separated from the electronic bandgap at 2.86 eV, corresponding to an excitonic binding energy, E b X = 300 meV. [118,121,122] The 0D structure also causes large exciton binding energies.…”

“…Taking Cs 3 Bi 2 I 9 (Figure 11a) as an example, the calculations using hybrid Heyd-Scuseria-Ernzerhof (HSE) [119] functional with SOC indicated that Cs 3 Bi 2 I 9 exhibits an indirect bandgap of 2.10 eV from the Γ point to the K point (Figure 11b). [118,121,122] The 0D structure also causes large exciton binding energies. Because of the isolation of [Bi 2 I 9 ] bioctahedra, both the VBM and the CBM are relatively localized, leading to large carrier effective masses.…”

Two‐Dimensional Halide Perovskites in Solar Cells: 2D or not 2D?

“…Investigations to replace lead in 2D PSCs focused on the use of layered bismuth [52]‐ and antimony‐based perovskites . Unlike most of the others “B‐site” metals discussed in this Minireview, antimony and bismuth have predominantly a 3+ oxidation state, which precludes the formation of traditional 3D halide perovskites with these metals.…”