[Paper Review] Intrinsic defect properties in halide double perovskites for optoelectronic applications

Tianshu Li, Xingang Zhao+3 more|arXiv (Cornell University)|Aug 16, 2018

Perovskite Materials and Applications44 references3 citations

TL;DR

This study investigates intrinsic defect properties in lead-free halide double perovskites (A2AgInCl6, A2AgBiCl6, A2AgBiBr6) using first-principles calculations to guide defect engineering for optoelectronic applications. It identifies optimal growth conditions that minimize trap states and enable p-type or semi-insulating behavior, achieving low defect density for efficient photovoltaic and radiation detection devices.

ABSTRACT

Lead-free halide double perovskites with the formula of quaternary A$_2^+$B'B'$^{3+}$X$_6^-$ have recently attracted intense interest as alternatives to lead-halide-perovskite-based optoelectronic materials for their non-toxicity and enhanced chemical and thermodynamic stability. However, the understanding of intrinsic defect properties and their effects on carrier transport and Fermi level tuning is still limited. In this paper, we show that, by exploring the phase diagram of a halide double perovskite, one can control the effects of intrinsic defects on carrier trapping and Fermi level pinning. We reveal the ideal growth conditions to grow p-type Cs$_2$AgInCl$_6$ and Cs$_2$AgBiCl$_6$ as well as semi-insulating Cs$_2$AgBiBr$_6$ with low trap density for targeted photovoltaic or visible-light/radiation detection application.

Motivation & Objective

• To understand the role of intrinsic defects in lead-free halide double perovskites for optoelectronic applications.
• To address the limited understanding of defect properties and their impact on carrier transport and Fermi level pinning.
• To identify growth conditions that minimize defect-induced carrier trapping and Fermi level pinning.
• To enable targeted design of Cs2AgInCl6, Cs2AgBiCl6, and Cs2AgBiBr6 for photovoltaic or visible-light/radiation detection applications.
• To establish phase diagram-based control of defect behavior for improved material stability and performance.

Proposed method

• Employed first-principles density functional theory (DFT) calculations to compute formation energies and transition levels of intrinsic defects.
• Mapped the phase diagram of halide double perovskites under varying chemical potentials to predict stable defect configurations.
• Analyzed defect levels and their impact on Fermi level pinning and carrier trapping in Cs2AgInCl6, Cs2AgBiCl6, and Cs2AgBiBr6.
• Evaluated defect concentrations as a function of Fermi level and chemical potential to identify optimal growth conditions.
• Used charge transition level analysis to assess the influence of defects on p-type and semi-insulating behavior.
• Correlated defect energetics with experimental growth parameters to guide synthesis for low trap density.

Experimental results

Research questions

• RQ1 How do intrinsic defects in halide double perovskites influence carrier transport and Fermi level pinning?
• RQ2 What are the dominant defect types and their formation energies under different growth conditions?
• RQ3 How can the phase diagram of double perovskites be used to control defect properties and minimize trap states?
• RQ4 Under what chemical potential conditions can p-type or semi-insulating behavior be achieved in Cs2AgInCl6 and Cs2AgBiCl6?
• RQ5 What defect configurations lead to low trap density in Cs2AgBiBr6 for radiation detection applications?

Key findings

• The study identifies that Ag and Bi vacancies are the dominant intrinsic defects in Cs2AgInCl6 and Cs2AgBiCl6, with formation energies strongly dependent on chemical potential.
• For Cs2AgInCl6 and Cs2AgBiCl6, p-type behavior is achievable under Ag-rich conditions, with Fermi level pinning near the valence band maximum.
• In Cs2AgBiBr6, the formation of Ag vacancies is suppressed under Ag-rich conditions, enabling semi-insulating behavior with low trap density.
• The phase diagram analysis reveals that growth under Ag-rich and Cl-rich conditions minimizes deep-level traps and suppresses defect formation.
• Optimal growth conditions are predicted to yield defect concentrations below 10^14 cm⁻³, suitable for high-performance optoelectronic devices.
• The results provide a roadmap for defect engineering in double perovskites, demonstrating that phase diagram control enables targeted tuning of electronic properties.