Theoretical and Experimental Characterization of Highly Anharmonic Chalcohalide Anti-Perovskites for Energy Applications

Silver chalcohalide antiperovskites (AH) are a family of materials with chemical formula Ag3BC where B=S, Se and C=Cl, Br, I. These are interesting energy materials that present suitable thermal stability and optoelectronic properties for applications in photovoltaics, solid-state batteries and fuel cells [1, 2]. Few experimental works have already reported the structural and ionic transport properties of Ag3sBr and Ag3SI [3], and recently a facile synthesis method has been proposed for them [4]. Nevertheless, a detailed description and understanding of the phase competition and thermodynamic properties of AH is still lacking, both at the experimental and theoretical levels, hence their thermodynamic and structural properties remain relatively unknown to date. Here, we present a comprehensive computational and characterization study of this family of materials, based on first principle calculations (DFT), as well as structural and optoelectronic measurements, which endorse the validity of the employed simulation approaches. Ag3SBr and Ag3SI polycrystalline thin films have been synthesized by a low-temperature solution-based methodology using thiol-amine molecular precursor inks. Then, structural and optical properties have been investigated by X-ray diffraction, microscopy and photothermal deflection spectroscopy experiments, demonstrating cubic anti-perovskite structure for both Ag3SBr and Ag3SI (Pm-3m and Im-3m respectively), and measured bandgaps in the 0 9-1.0 eV range. Interestingly, Ag3S(Br1-x Ix) solid solutions have been successfully synthesized, showing a bandgap bowing effect (bandgap increases, saturates after a certain value of x, and then decreases again). Thin film prototype solar cells have been prepared using Ag3SBr and Ag3SI absorbers (FTO/TiO2/AH/P3HT/Au superstrate device structure), showing photo-active response and Voc values up to 150 mV. For theoretical calculations, we performed a thorough exploration of the zero-temperature polymorphism of AH by using advanced crystal structure prediction and DFT methods. Several previously overlooked stable phases were thus predicted and studied for the six analysed parent compounds; for example, a cubic phase with space group P2_13 for Ag3SBr and an orthorhombic phase with space group P2_12_12_1 for Ag3SeBr. Ab initio molecular dynamics (AIMD) simulations were also performed at different temperatures to take into account the effects of anharmonicity (i.e., electron-phonon coupling) and ionic diffusivity on the optoelectronic properties of AH. Remarkably, it was found that temperature-renormalization effects on the band gap of AH are giant (i.e., decrease it around 40% decreasing for Ag3SBr) and necessary to provide good agreement with the experiments. Thus, these materials could be exploited for applications in which materials with strongly temperature-dependent electronic properties are required.<br/><br/>[1] PALAZON, Francisco. Metal chalcohalides: next generation photovoltaic materials?. <i>Solar RRL</i>, 2022, 6.2: 2100829.<br/>[2] LIU, Zhiyang, et al. Bandgap engineering and thermodynamic stability of oxyhalide and chalcohalide antiperovskites. <i>Ceramics International</i>, 2021, 47.23: 32634-32640.<br/>[3] HOSHINO, S., et al. Phase transition of Ag3SX (X= I, Br). <i>Solid State Ionics</i>, 1981, 3: 35-39.<br/>[4] SEBASTIÁ-LUNA, Paz, et al. Chalcohalide Antiperovskite Thin Films with Visible Light Absorption and High Charge-Carrier Mobility Processed by Solvent-Free and Low-Temperature Methods. <i>Chemistry of Materials</i>, 2023, 35.16: 6482-6490.