Optimizing Charge Transport and Crystal Orientation in Antimony Sulfide Thin-Film Solar Cells—Strategies of Doping and Using the Seed Layer to Improve Device Performance

It has been less than a decade since Sb₂X₃ (where X = S, Se, S_xSe_1-x) has drawn attention as a promising absorber material for thin-film solar cells [1]. This material has a crystal structure composed of (Sb₄S(e)₆)_n ribbons with a tunable orientation resulting in favorable charge transport toward the solar cell device electrodes when ribbons are oriented to a high degree in [hk1] plane [2]. The tuning of this direction may be controlled by different approaches during either growth/deposition or annealing/crystallization processes. In this study, we focus particularly on ultrathin Sb₂S₃ deposited by Radio Frequency (RF) magnetron sputtering and crystalizing in the graphite box at 350oC in a nitrogen-filled tube furnace with sulfur powder. Here, the crystal orientation, charge transport, grain size, and crystal orientation are controlled by i) the seed transport layer for absorber growth in both superstrate and substrate configurations and ii) alloying/doping of the absorber layer.
A facile, oriented seed-assisted method was demonstrated once for TiO₂-based Sb₂S₃ planar solar cells where epitaxial single-crystalline Sb₂S₃ cuboids were grown [3]. To reveal and further study how the underlying layer affects the directional growth of Sb₂S₃, in this study, a ZnO quantum dot (QDs) layer in a superstrate configuration, and a MoS₂ layer in a substrate configuration, are applied. The influence of the underlying layer on the main device parameters (V_OC, J_SC, FF, PCE) and microstructure of the Sb₂S₃ absorber layer is communicated and analyzed using SEM, EDX, XPS, UPS, XRD, and JV.
Efficient doping strategies with Ti, alkali metals (Li, Na, K, Rb, and Cs), and Zn were also reported to enhance uniformity greatly and to increase both grain size and charge carrier concentration, thereby increasing solar cell PCE [4]. Herein, the Sb₂S₃:Ag layers are prepared by the same sulfurization process of an Ag/Sb bimetallic precursor film deposited using RF sputtering for Sb (100 or 200 nm) and thermal evaporation for a thin Ag layer (2-8 nm). The Ag layer is incorporated in different positions within the absorber layer stack (under, in the middle, and onto the Sb), showing the effect of Ag layer position and thickness on the physical and electrical properties of Sb₂S₃:Ag, as well as the efficiency of solar cells.

This work is supported by the M-ERA.NET Grant 112877 “LightCell” and DFF-Research Project - Inge Lehmann 71336 “New, tunable n-type material for emerging thin-film solar cells”.

References:
[1] Rokas Kondrotas, Chao Chen, and Jiang Tang, Sb₂S₃ Solar Cells, Joule 2, 857–878, May 16, 2018, https://doi.org/10.1016/j.joule.2018.04.003.
[2] Tang, R., Wang, X., Lian, W. et al. Hydrothermal deposition of antimony selenosulfide thin films enables solar cells with 10% efficiency. Nat Energy 5, 587–595 (2020). https://doi.org/10.1038/s41560-020-0652-3.
[3] Chen, J., Qi, J., Liu, R. et al. Preferentially oriented large antimony trisulfide single-crystalline cuboids grown on polycrystalline titania film for solar cells. Commun Chem 2, 121 (2019). https://doi.org/10.1038/s42004-019-0225-1.
[4] Lei, H., Lin, T., Wang, X. et al. Copper doping of Sb2S3: fabrication, properties, and photovoltaic application. J. Mater. Sci: Mater Electron 30, 21106–21116 (2019). https://doi.org/10.1007/s10854-019-02481-9.