Effects of disorder on carrier transport in Cu$_2$SnS$_3$

TL;DR

This study investigates how cation disorder affects carrier transport in Cu$_2$SnS$_3$, revealing that even ordered samples exhibit disordered transport properties due to defects, impacting photovoltaic performance.

Contribution

The paper develops techniques to control disorder in Cu$_2$SnS$_3$ and combines experimental and theoretical methods to analyze its effects on carrier transport.

Findings

Disorder influences majority carrier (hole) transport.

Minority carrier (electron) transport remains similar regardless of disorder.

Extended planar defects are present even in ordered samples.

Abstract

In recent years, further improvements in the efficiency of Cu$_2$ZnSn(S,Se)$_4$ photovoltaic devices have been hampered due to several materials issues, including cation disorder. Cu$_2$SnS$_3$ is a promising new absorber material that has attracted significant interest in recent years. However, similar to CZTS, Cu$_2$SnS$_3$ displays cation disorder. In this work, we develop synthetic techniques to control the disorder in Cu$_2$SnS$_3$ thin films. By manipulating the disorder in this material, we observe crystal structure changes and detect improvements in the majority carrier (hole) transport. However, when the minority carrier (electron) transport was investigated using optical pump terahertz probe spectroscopy, minimal differences were observed between the ordered and disordered Cu$_2$SnS$_3$. By combining these results with first-principles and Monte Carlo theoretical calculations, we are able to conclude that even ostensibly "ordered" Cu$_2$SnS$_3$ displays minority carrier transport properties corresponding to the disordered structure. The presence of extended planar defects in all samples, observed in TEM imaging, suggests that disorder is present even when it is not detectable using traditional structural characterization methods. The results of this study highlight some of the challenges to the further improvement of Cu$_2$SnS$_3$-based photovoltaics, and have implications for other disordered multinary semiconductors such as CZTS.