Above Ambient Pressure Synthesis of Antimony Chalcohalide Semiconductors

Antimony chalcogenides Sb₂X₃ (X=Se,S,Te) and its alloys have become a topic of active research through the past decade due to their interesting electrical and optical properties stemming from an intriguing quasi-1D crystal structure. Observation of anisotropic charge transport properties coupled with ease of synthesis at relatively lower temperature are important characteristic observed for this class of semiconductor compounds which have been studied both in bulk and thin film form. Amongst the various possible chemical compositions, Sb₂Se₃ has garnered particular attention from the PV community due its ideal 1.3 eV band gap and high absorption coefficient a &gt; 10^5-6cm^-1 in the visible region, positioning it as a promising absorber layer for thin film photovoltaic applications. State of the art Sb₂Se₃ based thin film solar cell devices have reached a record efficiency = 12% [1] and to the growing community is aiming for much higher values in the short term. Beyond the various strategies being implemented to improve performance, there is also a need to further expand the range of application beyond stand-alone PV by tuning the band gap of the parent Sb₂Se₃ material. The inclusion of halogen elements such as iodine and bromine has been proposed as an interesting strategy to widen the band gap of Sb₂Se₃ making it optically suitable for application in building integrated PV as well as top cell in tandem solar cells [2].<br/>We present here a novel method for the fabrication of antimony chalcohalide thin films and report on recently obtained results on SbSe(I/Br) absorber synthesis. The process relies on the high pressure (&gt; 1 atm) reactive thermal annealing under Se atmosphere and SbI₃ and SbBr₃ precursors at low temperature (≤ 450 degree celsius) for the preparation of SbSeI and SbSeBr semiconductor thin films. A series of optimizations for different experimental conditions such as temperature, pressure, duration etc. were performed leading to the formation of highly <i>c-axis</i> oriented chalcohalide semiconductor on Mo coated glass substrates with a unique micro columnar grain morphology. Detailed structural and compositional studies were performed on the as prepared absorber layers in order to study their physical characteristics as well as propose plausible conversion pathway starting from Sb₂Se₃ to SbSeI and SbSeBr. First proof of concept solar cell devices was completed in the substrate configuration of Mo/SbSe(I/Br)/CdS/i-ZnO/ITO and their photovoltaic properties as well as spectral response were investigated. Best performing devices with SbSeI (E_g = 1.6 eV) and SbSeBr (E_g = 2 eV) exhibited V_oc= 633 mV and 600 mV respectively, demonstrating the potential of these materials as wide bandgap absorbers. We postulate that the main limitation of the current devices comes from a poor carrier extraction due to the use of a CdS buffer layer with a cliff-like p-n interface and poor coverage due to micro-columnar morphology presenting shunt paths. The replacement of the n-partner layer by a material with a more suited band structure, or the use of electron selective contacts will be discussed as ways to markedly improve performance.<br/><br/>[1] Duan, Zhaoteng, et al. "Sb₂Se₃Thin Film Solar Cells Exceeding 10% Power Conversion Efficiency Enabled by Injection Vapor Deposition Technology." <i>Advanced Materials</i> 34.30 (2022): 2202969.<br/>[2] Nie, Riming, et al. "Efficient and Stable Antimony Selenoiodide Solar Cells." <i>Advanced Science</i> 8.8 (2021): 2003172.