Scalable and Highly Efficient Antimony Selenide Thin Film Solar Cells by Close Spaced Sublimation
Sb2Se3 has emerged as a promising absorber material for solar cells owing to its superior optical and electronic properties, such as high absorption coefficient and suitable bandgap. It is a simple binary compound comprised of environmentally friendly and earth-abundant constituents, with one stable phase under normal conditions. More importantly, the crystal structure of Sb2Se3 is the unique one-dimensional ribbons that are stacked together by weak van der Waals force. The grain boundary is inert without dangling bonds, leading to significantly reduced combination centers. Furthermore, based on the S-Q limit, the theoretical maximum power conversion efficiency of Sb2Se3 can achieve as high as ~32%, implying the great potential to be employed for highly efficient solar cells. The goal of this dissertation work is to fabricate Sb2Se3 thin-film solar cells using close-spaced sublimation (CSS), which falls into three parts: (1) growth of high-quality Sb2Se3 thin films with CSS for thin-film solar cells, (2) investigation of the various buffer layer, and (3) hole-transport layer exploration for Sb2Se3 thin-film solar cells. In the first study, we systematically studied the growth behavior of Sb2Se3 with the CSS, particularly the dependence of the grain orientation on growth conditions, e.g., substrate temperature, thickness, etc. After optimization, we obtained Sb2Se3 films with desired crystal texture, i.e., (211)- and (221)-preferred orientation, thereby realizing efficiency of 4.27%. This work lays the foundation for our future work further optimizing the Sb2Se3 solar cells. The second study focuses on buffer layer engineering for the Sb2Se3 solar cells. Here we tried three different buffer layers: the oxygenated CdS via chemical bath deposition, oxygenated CdS via sputtering and sputtered CdSe. We carefully studied the optical response, band alignment with Sb2Se3, element diffusion, and the impact on grain orientation for each buffer layer. After careful engineering, we achieved 6.3%, 7.01%, and 4.5% for each buffer layer, respectively. Lastly, we applied NiOx as the hole-transport layer (HTL) on Sb2Se3 devices to construct a p-in configuration. The addition of NiOx HTL benefits charge extraction and helps reduce recombination at the backside via surface passivation. The thickness of the NiOx is a critical parameter for device performance. After the systematic screening, a decent efficiency enhancement from 6.6% to 7.3% was achieved with optimized NiOx HTL.