Evolution of the formation of AgBiS₂ colloidal nanocrystals for optoelectronic devices

AgBiS₂ has emerged as a promising environmentally friendly nanocrystal (NC) for optoelectronic applications due to its broad absorption spectrum in the visible and near-infrared regions. However, a performance ceiling has hindered the development of AgBiS₂-based solar cells, with power conversion efficiencies (PCE) remaining stagnant at 9-10%. This limitation arises from a lack of understanding of its fundamental properties, such as its crystal structure and electronic properties, which are crucial for further optimisation. Here, we focus on a comprehensive investigation of the synthesis and formation of AgBiS₂ NCs. Utilising Rietveld refinement, the crystallographic structure and phase purity are meticulously analysed, ensuring precise insights into the material's formation pathways. Furthermore, the constructed phase diagram of the Ag-Bi-S system reveals the threshold temperature at 160 °C to obtain pristine AgBiS₂ NCs, while below that, Ag₂S NCs are dominant. Density of states (DOS) analysis shows a similar bandgap with an experimental result of 0.8 eV, confirming the semiconductor properties. High-performance photodetectors of AgBiS₂ NCs are demonstrated, exhibiting high responsivity, although only using a few monolayers of the nanocrystal assemblies. These results bridge the knowledge gap in AgBiS₂ formation mechanisms and electronic behaviour, establishing a robust foundation to enhance its optoelectronic performance and integrate it into advanced device architectures.