Exploring the Synthesis and Optoelectronic Properties of Cs₂AgSb_xBi_1-xBr₆ Double Perovskites: A Combined Computational and Experimental Study

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 Cs2AgSbxBi1-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 Cs2AgSbxBi1-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

An experimentally validated model for anodic H₂O₂ production in alkaline water electrolysis and its implications for scaled-up operation

The anodic co-production of hydrogen peroxide (H2O2) during alkaline water electrolysis has gained interest as a sustainable alternative for anthraquinone oxidation. However, electrochemical H2O2 production is often studied with idealized laboratory setups to determine the H2O2 formation kinetics. In this work, we perform the reaction with industrially relevant operating principles using a flow cell with separately recirculating anolyte and catholyte. We then fit the data to an analytical model that we derive based on mole balances that accounts for anodic generation, anodic oxidation, and bulk disproportionation of H2O2, as well as electrolyte volumes and electrode surface area. We performed experiments at 100, 200, and 300 mA cm-2 to derive values for the reaction system. At 200 mA cm-2, we found a generation rate of 0.037 mmol min-1 cm-2 (FEH2O2 = 59%) and an anodic decomposition rate constant of 0.304 cm min-1, with a bulk disproportionation rate constant of 1.85 × 10-3 min-1. We successfully applied our model to two sources in literature to derive values for their systems as well. In all cases, the contribution of anodic oxidation of H2O2 was found to be the larger loss mechanism in comparison to bulk disproportionation. Using the analytical model, we show that decreasing the reservoir volume is a simple way to increase the H2O2 concentration over time. Further refinement of the model can be achieved through the use of mass transfer relationships based on electrolyzer geometries to describe the anodic oxidation of H2O2 in the mole balance equations.Large Scale Energy StorageEnergy Technolog

Traps in the spotlight: How traps affect the charge carrier dynamics in Cs₂AgBiBr₆ perovskite

Suitable optoelectronic properties of lead halide perovskites make these materials interesting semiconductors for many applications. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs2AgBiBr6. However, efficiencies of Cs2AgBiBr6-based photovoltaics are still modest. To elucidate the loss mechanisms, in this report, we investigate charge dynamics in Cs2AgBiBr6 films by double-pulse excitation time-resolved microwave conductivity (DPE-TRMC). By exciting the sample with two laser pulses with identical wavelengths, we found a clear photoconductance enhancement induced by the second pulse even 30 μs after the first laser pulse. Modeling the DPE-TRMC results, complemented by photoluminescence and transient absorption, we reveal the presence of deep emissive electron traps, while shallow hole trapping is responsible for the long-lived transient absorption signals. These long-lived carriers offer interesting possibilities for X-ray detectors or photocatalysis. The DPE-TRMC methodology offers unique insight into the times involved in charge trapping and depopulation in Cs2AgBiBr6.ChemE/Opto-electronic MaterialsChemE/O&O groepLarge Scale Energy Storag