Intrinsic defects in photovoltaic perovskite variant Cs2SnI6 (1502.06199v5)

Abstract: Note: This paper has been published in Physical Chemistry Chemical Physics, which can be viewed at the following URL: http://doi.org/10.1039/C5CP03102H Cs2SnI6, a rarely studied perovskite variant material, is recently gaining a lot of interest in the field of photovoltaics owing to its nontoxity, air-stability and promissing photovoltaic properties. In this work, we report intrinsic defects in Cs2SnI6 using first-principles density functional theory calculations. It is revealed that iodine vacancy and tin interstitial are the dominant defects that are responsible for the intrinsic n-type conduction in Cs2SnI6. Tin vacancy has a very high formation energy (>3.6 eV) due to the strong covalency in the Sn-I bonds and is hardly generated for p-type doping. All the dominant defects in Cs2SnI6 have deep transition levels in the band gap. It is suggested that the formation of the deep defects can be suppressed significantly by employing an I-rich synthesis condition, which is inevitable for photovoltaic and other semiconductor applications.

Intrinsic Defects in Photovoltaic Perovskite Variant CsSnI$_2$

This paper investigates the intrinsic defects in the photovoltaic perovskite variant CsSnI$_2$ through a systematic theoretical approach employing density functional theory (DFT) calculations. It aims to clarify the nature and origin of defects in CsSnI$_2$, which have implications on its electrical properties and performance in photovoltaic applications.

Key Findings

The research reveals that iodine vacancy (V$_\text{I}$) and tin interstitial (Sn$_\text{i}$) are the primary defects responsible for the n-type conductivity in CsSnI$_2$. These defects exhibit profound transition levels within the band gap, acting as electron traps that influence the conductive characteristics. Notably, the formation of a tin vacancy (V$_\text{Sn}$)—typically a significant p-type defect in Sn-based compounds—is highly unfavorable in CsSnI$_2$ due to its elevated formation energy (>3.6 eV). This high formation energy arises from the strong Sn-I covalent bonds, impeding p-type doping.

Electronic and Structural Analysis

The elemental analysis indicates that CsSnI$_2$ crystallizes into an anti-fluorite structure with isolated [SnI$_6$] clusters. The high covalency in the Sn-I bonds significantly affects the defect formation and electronic characteristics. The formation enthalpy (ΔH) for the dominant defects reveals that I-rich synthesis conditions can effectively suppress the formation of deep defects, thus enhancing the material's applicability in photovoltaic and semiconductor domains.

Implications and Future Directions

CsSnI$_2$ demonstrates intrinsic n-type conductivity due to the facile formation of Sn$_\text{i}$ and V$_\text{I}$, alongside the absence of shallow acceptors. The unique defect characteristics distinguish it from other Sn-based p-type semiconductors like SnO, SnS, and CsSnI$_3$. The strong covalent nature of Sn-I bonds in CsSnI$_2$ stabilizes the +2 oxidation state of Sn, influencing the electronic structure and defect energetics.

From a theoretical perspective, understanding these intrinsic defects provides insights into the defect physics of p-block metal-based compounds. Practically, the results underscore the necessity of controlled synthesis conditions to optimize material performance in photovoltaic applications. Implementing I-rich environments could mitigate the adverse effects of deep defects, thereby improving carrier lifetimes and diffusion lengths crucial for efficient solar cells.

Future work should focus on experimental validation of the predicted defect properties and electronic behavior. Additionally, further exploration of synthesis techniques and compositional tuning could pave the way for developing high-performance, stable, and lead-free perovskite solar cells, aligning with the growing demand for sustainable energy solutions.

Funding and Acknowledgments

This research is conducted under the support of the Tokodai Institute for Element Strategy and acknowledges funding from the MEXT Elements Strategy Initiative to Form Core Research Center. Additionally, acknowledgments are due to U.S. National Science Foundation for supporting supplementary work related to this paper.