Tuning Band Alignment at Grain Boundaries for Efficiency Enhancement in Cu2ZnSnS4 Solar Cells

Wenjie Li; Weimin Li; Guo Chen; Liyun Wu; Jun Zhang; Ming Chen; Guohua Zhong; Junyi Zhu; Ye Feng; Hao Zeng; Chunlei Yang

doi:10.1021/acsnano.3c03299

Tuning Band Alignment at Grain Boundaries for Efficiency Enhancement in Cu₂ZnSnS₄ Solar Cells

ACS Nano. 2023 Aug 22;17(16):15742-15750. doi: 10.1021/acsnano.3c03299. Epub 2023 Aug 14.

Authors

Wenjie Li¹, Weimin Li¹, Guo Chen¹, Liyun Wu¹, Jun Zhang², Ming Chen¹, Guohua Zhong¹, Junyi Zhu³, Ye Feng¹, Hao Zeng⁴, Chunlei Yang¹

Affiliations

¹ Shenzhen Institute of Advanced Technology, Chinese Academy of Science, Shenzhen 518055, China.
² School of Physical Science and Technology, Lingnan Normal University, Zhanjiang 524048, China.
³ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong 999077, China.
⁴ Department of Physics, University at Buffalo, the State University of New York, Buffalo, New York 14303-1262, United States.

PMID: 37578321
DOI: 10.1021/acsnano.3c03299

Abstract

Conducting atomic force microscopy has been performed for a fundamental understanding of the mechanism responsible for the lower power conversion efficiency (PCE) of Cu₂ZnSnS₄ (CZTS) solar cells than that of CuIn_1-xGa_xSe₂ (CIGS) solar cells. The difference in efficiency is partly attributed to the distinctly different band alignment between the grain boundaries (GBs) and grain interior (GI) for the two materials. While CIGS shows type-II band alignment, CZTS was discovered to demonstrate type-I band alignment with the conduction band shifting downward while the valence band shifting upward at the GBs. The type-I band alignment in CZTS leads to both electron and hole trapping, enhancing their recombination, and lowers the PEC. Band engineering was realized by moderate oxidative annealing of CZTS. The preferential GB oxidation changes the band alignment into inverse type-I (i.e., the conduction band upward bending and valence band downward bending at GBs). The blocking of carrier recombination at GBs leads to 30% enhancement in PCE. Our work reveals the critical role that band alignment between the grain boundary and interior plays in polycrystalline thin film solar cells and suggests band alignment engineering as a practical approach to enhance PCE. Furthermore, conducting AFM has been shown to be a powerful tool for qualitative and semiquantitative characterization of band alignment in polycrystalline films.

Keywords: CZTS; conducting probe atomic force microscope; grain boundaries; grain interior; solar cells.