Optoelectronic Properties and Defect Physics of Lead-Free Photovoltaic Absorbers ${\mathrm{Cs}}_{2}{\mathrm{Au}}^{\mathrm{I}}{\mathrm{Au}}^{\mathrm{III}}{X}_{6}$ ($X=\mathrm{I},\phantom{\rule{0.2em}{0ex}}\mathrm{Br}$)

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Optoelectronic Properties and Defect Physics of Lead-Free Photovoltaic Absorbers ${Cs}_{2} {Au}^{I} {Au}^{III} X_{6}$ ( $X = I, Br$ )

Jiban Kangsabanik, Supriti Ghorui, M. Aslam, and Aftab Alam

Phys. Rev. Applied 13, 014005 – Published 6 January 2020

Abstract

Stability and toxicity issues with the hybrid lead iodide perovskite ${MAPbI}_{3}$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites ${Cs}_{2} {Au}_{2} X_{6}$ ( $X = I, Br, Cl$ ). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very high short-circuit current density and hence higher simulated efficiency compared with ${MAPbI}_{3}$ . However, careful investigation of defect physics reveals the possibility of deep-level defects (such as $V_{X}$ , $V_{Cs}$ , $X_{Au}$ , $X_{Cs}$ , ${Au}_{i}$ , and ${Au}_{X}$ , $X = I, Br$ ), depending on the growth conditions. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help in taking necessary precautions in synthesizing these compounds in a controlled chemical environment, which should minimize performance-limiting defects and pave the way for future studies on this class of materials.

Received 26 March 2019
Revised 25 October 2019

DOI:https://doi.org/10.1103/PhysRevApplied.13.014005

Physics Subject Headings (PhySH)

Optoelectronics Photovoltaic absorbers Point defects

Perovskites Solar cells

Band structure methods Density functional calculations Hybrid functionals

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jiban Kangsabanik, Supriti Ghorui, M. Aslam, and Aftab Alam ^*

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

^*aftab@iitb.ac.in

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Vol. 13, Iss. 1 — January 2020

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Figure 1
(a) Crystal structure and (b) 3D Brillouin zone of ${Cs}_{2} {Au}_{2} X_{6}$ , $X = I, Br, Cl$ . (c)–(e) show the Heyd-Scuseria-Ernzerhof (HSE06) electronic band structures of ${Cs}_{2} {Au}_{2} X_{6}$ , $X = I, Br, Cl$ , respectively, with the band gap shifted to the HSE06- $G_{0} W_{0}$ calculated value. Orange, turquoise, and red symbols indicate $I$ $p$ , $Au$ $d$ , and $Au$ $s$ orbital character, respectively.
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Figure 2
(a) Absorption coefficient vs incident photon energy and (b) SLME vs film thickness at 298 K for ${Cs}_{2} {Au}_{2} X_{6}$ , $X = I, Br, Cl$ , in the $x y$ plane and $z$ direction. For comparison, simulated results for state-of-the-art ${MAPbI}_{3}$ are also plotted.
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Figure 3
Phase diagram (allowed chemical-potential regions for constituents) for (a) ${Cs}_{2} {Au}_{2} I_{6}$ and (b) ${Cs}_{2} {Au}_{2} {Br}_{6}$ , with respect to competing elemental, binary, and ternary compounds. The brown-shaded regions show the stable region for these double perovskite halides (a zoomed view is shown in the inset). The stable regions for the competing phases are shown by the differently colored lines and the spaces on the side of those lines in the direction opposite to the shaded region.
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Figure 4
Defect formation energy as a function of Fermi level ( $E_{F}$ ) for ${Cs}_{2} {Au}_{2} I_{6}$ , for three different growth conditions: (a) cation poor and anion rich, (b) cation rich and anion poor, and (c) moderate cations and anions. In the phase diagram, these conditions are represented by points $A$ , $E$ , and $G$ , respectively. According to Eq. (1), the charge state $q$ of the defect is denoted by the slope of the function, and the Fermi level at the turning point gives the charge-transition energy level. Here each charge-transition point for a donor (acceptor) defect is indicated via filled (open) symbols. The regions to the left of $E_{VBM}$ and to the right of $E_{CBM}$ represent the valence band below the VBM and the conduction band above the CBM, respectively. The green arrows point toward the Fermi-level pinning.
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Figure 5
Defect formation energy as a function of Fermi level ( $E_{F}$ ) for ${Cs}_{2} {Au}_{2} {Br}_{6}$ , for three different growth conditions: (a) cation poor and anion rich, (b) cation rich and anion poor, and (c) moderate cations and anions. In the phase diagram, these conditions are represented by points $A$ , $D$ , and $G$ , respectively. According to Eq. (1), the charge state $q$ of the defect is denoted by the slope of the function, and the Fermi level at the turning point gives the charge-transition energy level. Here each charge-transition point for a donor (acceptor) defect is indicated via filled (open) symbols. The regions to the left of $E_{VBM}$ and to the right of $E_{CBM}$ represent the valence band below the VBM and the conduction band above the CBM, respectively. The green arrows point toward the Fermi-level pinning.
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Physical Review Applied

Optoelectronic Properties and Defect Physics of Lead-Free Photovoltaic Absorbers Cs2AuIAuIIIX6 (X=I,Br)

Jiban Kangsabanik, Supriti Ghorui, M. Aslam, and Aftab Alam

Phys. Rev. Applied 13, 014005 – Published 6 January 2020

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Optoelectronic Properties and Defect Physics of Lead-Free Photovoltaic Absorbers ${Cs}_{2} {Au}^{I} {Au}^{III} X_{6}$ ( $X = I, Br$ )