Cs$_2$InAgCl$_6$: A new lead-free halide double perovskite with direct band gap (1611.05426v2)

Abstract: A$2$BB$^\prime$X$_6$ halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally-friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs$_2$BiAgX$_6$ (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B$^{3+}$ and B$^{+}$ cations are In$^{3+}$ and Ag$^{+}$, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs$_2$InAgX$_6$ (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs$_2$InAgCl$_6$ and Cs$_2$InAgBr$_6$, and we succeed to form the hitherto unknown double perovskite Cs$_2$InAgCl$_6$. X-ray diffraction yields a double perovskite structure with space group $Fm\overline{3}m$. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs$_2$InAgX$_6$ and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs$_2$InAg(Cl${1-x}$Br$_{x}$)$_6$ for $x<1$. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

• The paper reports the synthesis and X-ray diffraction confirmation of a stable Cs₂InAgCl₆ double perovskite with a direct band gap essential for optoelectronic devices.
• The study employs hybrid functional calculations to reveal a tunable band gap ranging from 2.1 to 3.3 eV, indicating potential for UV light applications.
• The paper positions Cs₂InAgCl₆ as an eco-friendly alternative for lead-based perovskites, paving the way for advanced solar cell research.

Overview of Cs InAgCl: A Lead-Free Halide Double Perovskite with Direct Band Gap

The paper introduces a newly synthesized lead-free halide double perovskite, Cs InAgCl. The paper proposes an alternative material for photovoltaic applications, addressing environmental concerns associated with lead-based perovskites. By replacing Pb with In, the authors aim to generate double perovskites with direct band gaps which are crucial for thin-film photovoltaics. The synthesis and characterization of Cs InAgCl present significant progress in the development of sustainable perovskite solar cells.

Synthesis and Structural Characteristics

The researchers successfully synthesized Cs InAgCl using a reaction in hydrochloric acid. X-ray diffraction analysis confirmed that this compound has a double perovskite crystal structure with the cubic space group Fm3m. The stability of this structure and compound was confirmed through empirical analysis based on Goldschmidt’s rules, which indicated that Cs InAgCl falls within the structural stability region typical for halide perovskites. This stability is noteworthy, primarily as synthetic efforts toward forming Cs InAgBr or other halide variants have not yet been as successful.

Electronic Properties and Band Gap Analysis

This paper emphasizes the direct band gap nature of Cs InAgCl, an attribute that enhances its potential for optoelectronic applications. Using hybrid functionals, the authors calculated a band gap ranging from 2.1 to 3.3 eV depending on structural optimization methods, ultimately estimating it as 2.7 ± 0.6 eV. The direct band gap arises from the absence of occupied In-s orbitals at the valence band top, a notable contrast with Bi/Ag-based compounds that typically exhibit an indirect band gap. The reported band gap value of 3.3 eV is consistent with calculated absorption properties, indicating potential application in ultraviolet light devices. Additionally, the compound undergoes a reversible color change upon UV exposure, from white to orange, suggesting photochromic behavior likely due to photo-induced defect states at 2.1 eV.

Implications and Future Directions

The realization of Cs InAgCl opens avenues for advancements in lead-free perovskite photovoltaics. Notably, the inclusion of mixed halides is expected to provide an additional lever for tuning the band gap to achieve better photovoltaic efficiencies. The optical properties of mixed-halide perovskites could rival those of established photovoltaic materials like GaAs and Si, paving the way for tandem solar cell integration. This compound's synthetic routes and analytical insights establish a foundation for further experimental research to develop a broader range of environmentally benign, lead-free perovskites.

The paper suggests a pivotal direction in materials science, combining theoretical prediction with empirical synthesis to innovate new solar cell materials. This meticulous approach not only enhances the understanding of electronic properties of halide perovskites but also proposes a structured pathway towards realizing high-efficiency, stable, and eco-friendly solar solutions.