Simulating Self-Trapped Excitonic Emission in Halide ‘Defect Perovskites’

Inorganic lead halide perovskites have demonstrated comprehensive promise in LEDs, however lower dimensional perovskites and perovskite-like materials, have shown similar self-trapped excitonic behaviour and resultant broadband white light emission.¹ The Pb-free ‘defect perovskite’ Cs₂M^(IV)X₆ (M = Zr, Hf, Sn, X = Cl, Br) compounds,^2,3 in which half the perovskite ‘B’ site cations are absent, are among those in demonstrating broadband emission – although quantum efficiencies vary between compounds, and the precise mechanism of emission is unclear. Here, we demonstrate how ab initio calculations can shine some light on these issues.<br/>In this study, we simulate an ansatz of the emitting excitonic state using a supercell method with hybrid Density Functional Theory, avoiding the need for the explicit solution of the Bethe-Salpeter equation, while still obtaining accurate estimates of the photoluminescence of the halide defect perovskites – previously used in the simulation of other halide emitters.⁴ Beginning with the successful double perovskite emitter Cs₂AgInCl₆⁵ and its experimental photoluminescence as a benchmark, we move on to the defect perovskite families, and demonstrate the influence of both cation and anion to the excitonic properties – whether the luminescence energy or the barrier to the exciton self-trapping. We also relate these results to the degree of electronic, as well as structural, localization in these systems, and use additional QSGW+BSE calculations available through the code Questaal⁵ to support our results. Through these simulations, we hope to guide the understanding and future synthesis of Pb-free efficient light emitters.<br/>(1) Smith, M. D.; Karunadasa, H. I. <i>Acc. Chem. Res.</i> <b>2018</b>, <i>51</i>, 619.<br/>(2) Abfalterer, A.; Shamsi, J.; Kubicki, D.J.; Savory, C.N.; Xiao, J., <i>et al</i><i>.</i> <i>ACS Materials Letters</i>, <b>2020</b>, <i>2</i>, 1633-1652<br/>(3) Jing, Y.; Liu, Y.; Zhao, J. and Xia, Z. <i>J. Phys. Chem. Lett.</i> <b>2019,</b> <i>10</i>, 7439-7444<br/>(4) Jung, Y-K., Kim, S.; Kim, Y.C. and Walsh, A. <i>J. Phys. Chem. Lett.</i>, <b>2021</b>, <i>12</i>, 8447-8452<br/>(5) Luo, J.; Wang, X.; Li, S.; Liu, J.; Guo, Y. <i>et al</i>. <i>Nature,</i> <b>2018</b>, <i>563</i>, 541<br/>(6) Pashov, D.; Acharya, S.; Lambrecht, W.R.L.; Jackson, J.; Belashchenko, K.D.; Chantis, A.; Jamet, F.; van Schilfgaarde, M. <i>Comp. Phys. Commun., </i><b>2020</b><i>, 249, </i>107065