Li Wenjie, Li Weimin, Chen Guo, Wu Liyun, Zhang Jun, Chen Ming, Zhong Guohua, Zhu Junyi, Feng Ye, Zeng Hao, Yang Chunlei
Shenzhen Institute of Advanced Technology, Chinese Academy of Science, Shenzhen 518055, China.
School of Physical Science and Technology, Lingnan Normal University, Zhanjiang 524048, China.
ACS Nano. 2023 Aug 22;17(16):15742-15750. doi: 10.1021/acsnano.3c03299. Epub 2023 Aug 14.
Conducting atomic force microscopy has been performed for a fundamental understanding of the mechanism responsible for the lower power conversion efficiency (PCE) of CuZnSnS (CZTS) solar cells than that of CuInGaSe (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.
为了从根本上理解铜锌锡硫（CZTS）太阳能电池的功率转换效率（PCE）低于铜铟镓硒（CIGS）太阳能电池的原因，已经开展了原子力显微镜研究。效率差异部分归因于这两种材料的晶界（GBs）和晶粒内部（GI）之间明显不同的能带排列。虽然CIGS呈现II型能带排列，但发现CZTS表现出I型能带排列，在晶界处导带向下移动而价带向上移动。CZTS中的I型能带排列导致电子和空穴被俘获，增强了它们的复合，并降低了光电转换效率（PEC）。通过对CZTS进行适度的氧化退火实现了能带工程。优先的晶界氧化将能带排列转变为反向I型（即晶界处导带向上弯曲而价带向下弯曲）。晶界处载流子复合的阻断导致光电转换效率提高30%。我们的工作揭示了晶界和晶粒内部之间的能带排列在多晶薄膜太阳能电池中所起的关键作用，并表明能带排列工程是提高光电转换效率的一种实用方法。此外，原子力显微镜已被证明是定性和半定量表征多晶薄膜中能带排列的有力工具。
