In recent years, further improvements in the efficiency of Cu2ZnSn(S,Se)4 photovoltaic devices have been hampered due to several materials issues, including cation disorder. Cu2SnS3 is a promising new absorber material that has attracted significant interest in recent years. However, similar to CZTS, Cu2SnS3 displays cation disorder. In this work, we develop synthetic techniques to control the disorder in Cu2SnS3 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 Cu2SnS3. By combining these results with first-principles and Monte Carlo theoretical calculations, we are able to conclude that even ostensibly "ordered" Cu2SnS3 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 Cu2SnS3-based photovoltaics, and have implications for other disordered multinary semiconductors such as CZTS.
@article{arxiv.1504.01327,
title = {Effects of disorder on carrier transport in Cu$_2$SnS$_3$},
author = {Lauryn L. Baranowski and Kevin McLaughlin and Pawel Zawadzki and Stephan Lany and Andrew Norman and Hannes Hempel and Rainer Eichberger and Thomas Unold and Eric S. Toberer and Andriy Zakutayev},
journal= {arXiv preprint arXiv:1504.01327},
year = {2015}
}