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Energy level alignment of vacancy-ordered halide double perovskites

Ibrahim Buba Garba, George Volonakis

Abstract

Vacancy-ordered double perovskites have emerged as lead-free alternatives, offering remarkable stability and compositional tunability for optoelectronic applications. In this study, we provide first-principles insights into their electronic properties, surface stability, and energy level alignment using a non-empirical, dielectric-dependent hybrid functional. For a representative family of Cs$_2$MX$_6$ compounds, with M = Zr, Sn, Te, and X= Cl, Br, I, our calculations reveal that the predicted bulk electronic band gaps are in excellent agreement with those obtained using the state-of-the-art GW method, validating the accuracy of our approach. We investigate the stability of these materials under simulated experimental conditions, considering both the rich and poor chemical potentials of their precursor salts. Our results indicate distinct regions of surface energy stability that favor CsX terminations. In contrast, MX$_4$ terminations show in-gap surface states, which can act as trap states and reduce carrier lifetime. Finally, based solely on the intrinsic absolute energy levels, we identify promising candidates as charge transport and injection layers for typical photovoltaic and light-emitting applications. This study provides a detailed map of energy level alignment at Cs$_2$MX$_6$ surfaces, offering valuable design principles for the development of next-generation Cs$_2$MX$_6$-based optoelectronic devices.

Energy level alignment of vacancy-ordered halide double perovskites

Abstract

Vacancy-ordered double perovskites have emerged as lead-free alternatives, offering remarkable stability and compositional tunability for optoelectronic applications. In this study, we provide first-principles insights into their electronic properties, surface stability, and energy level alignment using a non-empirical, dielectric-dependent hybrid functional. For a representative family of CsMX compounds, with M = Zr, Sn, Te, and X= Cl, Br, I, our calculations reveal that the predicted bulk electronic band gaps are in excellent agreement with those obtained using the state-of-the-art GW method, validating the accuracy of our approach. We investigate the stability of these materials under simulated experimental conditions, considering both the rich and poor chemical potentials of their precursor salts. Our results indicate distinct regions of surface energy stability that favor CsX terminations. In contrast, MX terminations show in-gap surface states, which can act as trap states and reduce carrier lifetime. Finally, based solely on the intrinsic absolute energy levels, we identify promising candidates as charge transport and injection layers for typical photovoltaic and light-emitting applications. This study provides a detailed map of energy level alignment at CsMX surfaces, offering valuable design principles for the development of next-generation CsMX-based optoelectronic devices.

Paper Structure

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

Table of Contents

  1. Introduction
  2. Results and Discussion
  3. Electronic band gaps
  4. Surface energy and stability
  5. Ionization potential and electron affinity
  6. Conclusion

Figures (5)

  • Figure 1: Comparison of the calculated fundamental band gaps of A$_2$MX$_6$ vacancy-ordered double perovskites obtained with PBE, hybrid HSE and PBE0, and the dielectric-dependent hybrid functional DSH0. Literature G$_0$W$_0$ values from refs. Cucco_2021Cucco_2023.
  • Figure 2: Stability of Cs$_2$MX$_6$(001) surfaces with CsX and MX$_4$ terminations as a function of CsX chemical potential. The Cs$_2$MX$_6$ stoichiometric surfaces show constant energy under both CsX-rich and CsX-poor conditions.
  • Figure 3: Charge density of the VBM and CBM shown for Cs$_2$MBr$_6$ (M = Sn or Te) in the bulk, the (001) CsBr-terminated surface, and the (001) MBr$_4$-terminated surface. For CsBr-terminated surfaces, the CBM and VBM are quite similar to the bulk, while MBr$_4$-terminated surfaces have surface states localized on Br atoms in Sn-based systems above the VBM, and both VBM and CBM in Te-based systems.
  • Figure 4: Comparison of absolute energy levels using DSH0 functional and experiment for Cs$_2$SnI$_6$. Experimental values represented by a,b,c,d and e are from Refs. Lee_2017,Lee_2014,Maughan_2016,Zhang_2014, and Saprov_2016, respectively.
  • Figure 5: Energy levels of Cs$_2$MX$_6$ (M = Ti, Zr, Sn, Te; X = Br, I) computed using DSH0 functional for CsX-terminated surfaces. Experimental energy levels for MAPbI$_3$ and the error bars are from Ref. Schluz_2016Kim_2015 while that of CsPbX$_3$ nanocrystals are taken from ref.Ravi_2016