Exploring the Synthesis and Optoelectronic Properties of Cs₂AgSb_xBi_1-xBr₆ Double Perovskites

Abstract

Perovskite photovoltaic (PV) cells have become one of the most highly researched topics in photovoltaics and have achieved unprecedented increases in device efficiencies, but their commercialization remains hindered by their low stability and high toxicity. The currently best performing perovskite PV cells contain lead, a neurotoxic material whose use is prohibited under many national consumer protection laws, thus impeding adoption by industry. A class of materials called double perovskites offer an elegant pathway to lead-free, low-toxicity perovskites for PV cell applications by replacing the Pb²⁺ cation in the perovskite with a mixture of charge 1+ and 3+ cations. A promising double perovskite, Cs₂AgBiBr₆, was first synthesized in 2016 and has been used in the fabrication of PV devices with efficiencies of ≤2.5%. While numerous research groups have attempted various synthesis routes and produced various final materials, little is known about the dynamics of the double perovskite synthesis or the effect of further metal substitution on the material’s optoelectronic properties. In this work, the solution phase synthesis of Cs₂AgBiBr₆ was studied via Density Functional Theory (DFT) and the optoelectronic properties of Cs₂AgSb_xBi_1-xBr₆ thin films were explored, with antimony substitution presented as a method to lower the band gap to make a more favorable perovskite for PV cell applications.

The synthesis method of the thin films involved mixing all of the precursors in DMSO solvent and spin coating. However, only BiBr₃ and SbBr₃ were found to dissolve individually in solution, indicating a sequential pathway to double perovskite crystallites in solution. Geometry optimizations of Bi-Br-DMSO complexes were performed via DFT using the BLYP functional, with COSMO used to approximate a solution phase system. While COSMO was found to be incompatible with the corrected method of calculating the interaction energy, the relatively low (~11%) basis set superposition error was accepted and the uncorrected calculation method was used to find the most stable Bi-Br DMSO complexes in solution. These complexes were analyzed using TD-DFT and the CAM-B3LYP functional to simulate absorbance spectra and match them to experimental solution spectra. While one of the transitions at ~3.9 eV may be ascribed to a larger cluster of [Bi₄Br₂₀]^8-, the source of the stronger experimental transition at ~3.5 eV could not be determined. The dominant electronic transition of the Bi-Br-DMSO system was a metal-to-ligand charge transfer from the 6푠 orbital of the central bismuth ion to the 3푝 orbital of the bromine ligand.

A facile synthesis method reported in literature was attempted for the synthesis of Cs₂AgSb_xBi_1-xBr₆ thin films, described briefly above. The method was found to produce thin films of high crystallinity but with a tendency to degrade upon exposure to ambient conditions, as evidenced by x-ray diffraction (XRD) measurements. A reduced annealing temperature of 90°C rather than 250°C led to the successful substitution of Sb³⁺ for Bi³⁺ in the double perovksite while simultaneously avoiding material degradation (at the cost of optoelectronic performance). Shifts in the lattice parameter of ~0.05 Å and shifts in the absorbance onset energy of ~0.2 eV were found by XRD and absorbance measurements, respectively, for antimony replacement of up to x = 0.7. The optoelectronic properties of the materials were studied using time-resolved microwave conductivity (TRMC) measurements, and showed a decrease in photoconductance of two orders of magnitude and a reduction of charge carrier lifetime as the annealing temperature was lowered from 250°C to 90°C. Low temperature absorbance measurements combined with TRMC measurements indicated that the peak in the absorbance spectra was most likely the result of an excitonic transition.