Solution Based Synthesis of Low Dimensional Pnictogen Chalco-Halides based on (Sb,Bi)(S,Se)(Br,I) for Sustainable Energy Applications

Quasi one-dimensional (Q1-D) structures based on van der Waals materials are gaining increasing interest in the search for novel energy applications such as photocatalysis, energy storage or photovoltaics. This attention is driven by the high stability and low environmental impact of chalcogenides and chalco-halides in contrast to third generation solar cells, especially Pb-based perovskites. The optoelectrical properties of these materials can be easily tuned through compositional engineering. A wide bandgap range (from 1 to 3.5 eV) can be achieved with available candidates for single-junction solar cells. The Sb/Bi chalco-halide compounds, generally formulated as MChX (M = Sb, Bi; Ch = S, Se; X = I, Br), crystallise in an orthorhombic Pnma space group consisting of covalently bonded ribbons along one crystallographic direction, while they are stacked together by van der Waals interactions in the other directions. This feature gives rise to unique optoelectronic properties when the material is properly oriented, such as enhanced carrier transport and increased mobility. In addition, it has been suggested that these compounds might possess a defect-tolerant electronic structure due to the trivalent metal cation, which is encouraging their development.<br/>Despite these encouraging properties, the correct synthesis of these materials containing a halogen and a chalcogen in the structure is still a big challenge. Binary materials such as (Sb,Bi)₂(S,Se)₃can occur as secondary phases, as well as sub-optimal morphologies. Uneven and uncompact films with uncontrolled ribbon orientation can form. This presentation will introduce a new methodology to synthesize mixed chalco-halides developed by the authors of this work. Thin films of the complete set of ternary van der Waals (Sb,Bi)(S,Se)(Br,I) chalco-halide semiconductors have been synthesized by dissolving MX₃ and selenourea/thiourea in N,N-dimethylformamide (DMF), followed by multiple spin-coating and hot-plate annealings at low temperatures (&lt; 400 °C). Additionally, antisolvent incorporation during spin-coating, followed by a drying step at a temperature below the solvent’s boiling point, have been applied to ensure that all organic residues are removed. This one step methodology provides less complexity compared to previous sequential procedures relying on diffusion of one precursor in a previously deposited film, which can lead to a gradient in the composition. Different surface treatments over the molybdenum substrate are explored to selectively orient texture in the (00l) direction by improving the solution adhesion and increasing the nucleation sites.<br/>In the second part of the presentation, the fundamental properties of these compounds obtained by electrical and optical characterisation will be discussed. The measurement of the bandgaps, band structures, optical and transport properties will be shown.<br/>The last part of the presentation will be dedicated to the challenges and possible technological solutions for the fabrication of solar cell devices using this innovative absorber synthesis route. The use of different electrons and holes transport layers, and architectures inspired in bulk-heterojunction devices using blended ribbons of BiSI and BiSBr/SbSI as donor and acceptor, respectively, will be discussed. These solar cell prototypes will be shown, with very encouraging open circuit voltages around 500 mV, and conversion efficiencies exceeding 1%. Finally, the perspective of these materials and the possible advantages with respect to current chalcogenide and halide technologies, will be presented and discussed.