A Closer Look into Antimony Sulfide (Sb₂S₃) with Tailored Crystallographic Orientation

Antimony sulfide (Sb₂S₃) has emerged as a promising absorber material for next-generation thin-film photovoltaics (PV) due to its attractive and robust properties. These include an absorption coefficient exceeding 10⁵ cm^-1, a bandgap ranging from 1.60 to 1.70 eV, good environmental stability, and a simple binary composition. Interestingly, Sb₂S₃ distinguishes itself from conventional solar absorbers like CdTe, Cu(In,Ga)Se₂ (CIGS), or Cu₂ZnSn(S,Se)₄ (CZTSSe) through its unique quasi-one-dimensional (Q1D) crystal structure. This structure, comprising [ Sb₄S₆]_n ribbons covalently bonded along the c-axis, leads to anisotropic carrier transport. While efficient along the [hk1] direction, transport in the [hk0] direction is limited due to interlayer hopping mechanisms. This inherent anisotropy offers unique advantages but also challenges optimizing Sb₂S₃ PV device performance, particularly in optimizing grain orientation. Moreover, inevitable grain boundaries (GBs) in polycrystalline materials and trap states in bulk and at interfaces can significantly impact overall efficiency. ¹

In this work, we will present several strategies for engineering the preferred [hk1] orientation of Sb₂S₃ solar absorbers via bulk engineering by controlling the grain orientation during film growth and substrate/back-contact engineering in substrate and superstrate device configurations. Furthermore, we will show that Polarized Raman Microscopy (PRM) can be utilized to map the local crystallographic orientation of Sb₂S₃ polycrystalline films at a spatial resolution of approximately 200 nm. PRM provides detailed insights into the structural quality and phase purity, with 3D orientation maps revealing differences between films with engineered [hk1] orientation and those with random grain orientations.

In the second part of the talk, we focus on the electronic structure and surface characteristics of Sb₂S₃ films and their interfaces using X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), and Reflection Electron Energy Loss Spectroscopy (REELS). Combined XPS/UPS can provide valuable information on the chemical states of Sb in Sb₂S₃ , along with the valence band maximum (VBM), work function (WF), and Fermi level position, all of which are essential for understanding the band alignment between Sb₂S₃ and adjacent layers in the photovoltaic device². One of the main challenges in Sb₂S₃ devices is the unfavorable band alignment when using the CdS buffer layer, which forms a "cliff-like" interface.³ Instead of the traditional CdS buffer layer, Mg_xZn_1-xO (MZO) thin oxide films were chosen in this study as alternative n-type layers. REELS is integrated into this study to examine further the surface properties of the wide band gap MZO films, namely the surface band gap. Our data reveal that the band gap and the conduction band minimum (CBM) of MZO can be tuned as a function of Mg content. Consequently, the conduction band offset (CBO) can be adequately tailored by tuning the Mg content, ensuring more efficient charge transfer at the Sb₂S₃/MZO interface.

Overall, with this combination of techniques, a detailed electronic and chemical picture of the bulk and interface is painted, which will be discussed in view of its impact on the performance of Sb₂S₃ PV devices.

References
1. Liu, X. et al. Grain Engineering of Sb₂S₃ Thin Films to Enable Efficient Planar Solar Cells with High Open-Circuit Voltage. Advanced Materials 36, 2305841 (2024).
2. Gansukh, M. et al. Energy band alignment at the heterointerface between CdS and Ag-alloyed CZTS. Sci Rep 10, (2020).
3. Zeng, Y. et al. Comparative Study of TiO₂ and CdS as the Electron Transport Layer for Sb₂S₃ Solar Cells. Solar RRL 6, 2200435 (2022).