# Hexagonal MASnI$_3$ exhibiting strong absorption of ultraviolet photons

## TL;DR

This study uses first-principles calculations to show that hexagonal MASnI$_3$ has strong ultraviolet absorption and unique electronic properties, making it promising for advanced perovskite solar cell applications.

## Contribution

The paper reveals the structural, electronic, and optical properties of hexagonal MASnI$_3$, highlighting its potential for ultraviolet photon absorption in solar cell technology.

## Key findings

- Hexagonal MASnI$_3$ has a larger indirect band gap than cubic and tetragonal phases.
- It exhibits high ultraviolet photon absorption and visible photon transmission.
- Enhanced electronic coupling and localization are observed in the hexagonal phase.

## Abstract

MASnI$_3$, an organometallic halide, has great potential in the field of lead-free perovskite solar cells. Ultraviolet photons have been shown to generate deep trapping electronic defects in mesoporous TiO$_2$-based perovskite, affecting its performance and stability. In this study, the structure, electronic properties, and optical properties of the cubic, tetragonal, and hexagonal phases of MASnI$_3$ were studied using first-principles calculations. The results indicate that the hexagonal phase of MASnI$_3$ possesses a larger indirect band gap and larger carrier effective mass along the \emph{c}-axis compared with the cubic and tetragonal phases. These findings were attributed to the enhanced electronic coupling and localization in the hexagonal phase. Moreover, the hexagonal phase exhibited high absorption of ultraviolet photons and high transmission of visible photons, particularly along the \emph{c}-axis. These characteristics demonstrate the potential of hexagonal MASnI$_3$ for application in multijunction perovskite tandem solar cells or as coatings in mesoporous TiO$_2$-based perovskite solar cells to enhance ultraviolet stability and photon utilization.