Novel (Sb,Bi)(S,Se)(Br,I) van der Waals Chalco-Halide Thin Film Photovoltaics—Current Status and Future Perspectives

Quasi one-dimensional (Q1-D) structures based on van der Waals materials such as (Sb,Bi)₂(S,Se)₃ have demonstrated impressive progresses over the past 5 years. These types of materials typically form nano/micro-ribbons with strong covalent bonding in one direction, and at the same time, weak van der Waals bonding in the others two. This fingerprint feature confers unusual optoelectronic properties to these materials when properly oriented, and has allowed fast progresses in terms of conversion efficiency of solar cell devices, with a current record exceeding 10%. Beyond Sb and Bi-chalcogenide compounds, there is a limited body of knowledge on other Q1-D systems such as mixed chalco-halide van der Waals compounds, that combine chalcogens (S,Se) with halogens (Br,I) in the same structure. In fact, recent theoretical studies suggests that mixed chalco-halides can simultaneously achieve the robustness and stability of chalcogenide materials, along with the excellent optical and electrical properties of halides, and in particular showing high defect tolerance similarly to halide-perovskites.<br/><br/>This presentation will introduce a novel family of materials based on mixed Sb and Bi chalco-halides [(Sb,Bi)(S,Se)(Br,I)]. The first part will be devoted to reviewing the most relevant results reported so far for the few examples available in the literature, that have reached encouraging conversion efficiencies around 5% within a very limited timeframe, with a meso-porous solar cell configuration. In addition, the fundamental properties of these compounds obtained by DFT modelling will be discussed. The calculation of the bandgaps, band structures, optical and transport properties will be presented, discussing the trends of these properties depending on the chalcogen or the halogen introduced into the structure. The results will show that all these compounds can be relevant for different thin film photovoltaic applications, with bandgaps ranging from 1.6 eV for SbSeBr up to 2.3 eV for BiSBr.<br/><br/>In the second part of the presentation, the complexity of the synthesis of mixed chalco-halides will be discussed, and a new methodology for their synthesis, developed by the authors of this work, and based on the combination of co-evaporation of chalcogenides and high-pressure reactive annealing under halogen atmosphere, will be presented. This methodology allowed for the first time the demonstration of working solar cells with absorbers produced by vapor deposition techniques with these compounds. The tunability of the Q1-D structures by changing the synthesis temperature and pressure will be demonstrated. It will be shown that the synthesis of bromine-based compounds requires higher temperatures and pressures than iodine ones, due to the thermodynamics associated with the incorporation of Br and I in the chalcogenide phase. A detailed analysis of the mechanisms behind the formation of the different compounds will be presented by implementing interrupted-synthesis processes, and supported by the combination of thermo-gravimetric analysis, differential scanning calorimetry, and advanced structural/compositional characterizations.<br/><br/>The last part of the presentation will be devoted to the challenges and possible technological solutions for the fabrication of planar-heterojunction solar cell devices with these innovative photovoltaic absorbers. The use of different electrons and holes transport layers will be discussed, demonstrating for the first-time conversion efficiencies with these architectures between 1-5% and with very encouraging Voc values above 600 mV in some cases. Finally, the perspective of these materials and the possible advantages with respect to current chalcogenide and halide technologies, will be presented and discussed.