Lead-Free Perovskite Cs2AgBiX6 Nanocrystals with a Band Gap Funnel Structure for Photocatalytic CO2 Reduction under Visible Light

S-scheme Cs2AgBiBr6/Ag3PO4 heterojunction with efficient photocatalysis performance for H2 production and organic pollutant degradation under visible light

In this study, the electronic structure, optical, and electrical properties of double perovskite Cs2AgBiX6 (X = Br, Cl, and I) were investigated through the density functional theory within the Tran–Blaha modified Becker–Johnson approach. The optoelectronic properties such as the bandgap energy, density of states, charge density, and dielectric function are discussed. The physical parameters including the quantum efficiency, Voc, Jsc, were estimated for industrial photovoltaics (PV). Based on the optimized structure, it was found that the bandgap values are 1.12, 2.04, and 2.81 eV for Cs2AgBiI6, Cs2AgBiBr6, and Cs2AgBiCl6 respectively. Meanwhile, the electronic charge density predicts that the bounds between Cs and Br/I/Cl are covalent. Quantum efficiency shows high evolution, a very high spectrum of absorption coefficients is observed around the visible range, which proves the use of such double perovskite compound in the PV device.

Optoelectronic and photovoltaic properties of Cs2AgBiX6 (X = Br, Cl, or I) halide double perovskite for solar cells: Insight from density functional theory

Pb-free double perovskites, such as Cs2AgBiBr6, are alternatives to lead halide perovskites for photovoltaic applications due to superior stability, low toxicity, and promising optoelectronic properties. However, their performance is subpar. We combine nonadiabatic molecular dynamics and real-time time-dependent density-functional theory to show that the negatively charged Br vacancy in Cs2AgBiBr6 creates an extremely detrimental donor-yielded (DY) center, which is a typical defect in six-coordinated semiconductors. Ag+ and Bi3+ form a bond by attraction through the anisotropic vacancy charge, generating a midgap state that traps holes within tens of picoseconds. Substituting Ag with indium by doping produces a weak and long In-Bi bond, lifting the defect energy level to the conduction band. Hole trapping slows down by an order or magnitude, and trap-assisted charge recombination decreases 4-fold. The simulations bring atomistic insights into defects of Pb-free double perovskites and provide a defect mitigation strategy for rational design of high-performance optoelectronic devices.

Identifying and Passivating Killer Defects in Pb-Free Double Cs2AgBiBr6 Perovskite.

Photocatalytic hydrogen generation paves a promising way to mitigate the global energy crisis and deteriorative environmental issues. Among different materials, metal halide perovskites (MHPs) have recently emerged as a promising class of inexpensive and easy-to-make semiconductors for various photocatalytic applications such as organic contaminant degradation, CO2 reduction, H2 evolution, and N2 fixation. Although MHPs-based standalone photocatalysts offer architectural simplicity, they provide restricted control over recombination processes and spatial separation of redox half-reactions. Meanwhile, heterojunction systems are of growing interest due to their ability to control energy-consuming redox processes and effectively suppress charge carrier recombination. In this review, the authors elaborately discuss perovskite-based heterojunction photocatalysts for hydrogen generation both from a material chemistry point of view and band alignment perspective. They discuss the design principle of MHP heterostructures necessary for H2 evolution and the underlying photo-physics behind process optimization. Moving forward, they conclude by meticulously outlining the ongoing challenges, opportunities, and future outlooks for MHP-based heterojunction photocatalysts for H2 evolution. 

Metal Halide Perovskite Heterojunction for Photocatalytic Hydrogen Generation: Progress and Future Opportunities

High visible-light photocatalytic performance of stable lead-free Cs2AgBiBr6 double perovskite nanocrystals

To improve the performance of solar heat storage and reduce the energy consumption of the fresh air system in areas with poor solar conditions, this study investigates the thermal storage and release performance of a low-temperature rectangular thermal energy storage unit (RTESU). A validated Computational Fluid Dynamics (CFD) model is used to address the impacts of configuration parameters on the performance of the thermal storage process in the Phase change material (PCM) module, including the number and eccentric arrangement of tubes. Subsequently, the effect of the air channel width on the air outlet temperature and the heat transfer rate is analyzed for the heat-releasing process. The numerical results show that the number and the eccentricity of the heat exchanger tube have significant impacts on the heat storage performance, and a minimum duration of 117 min can be achieved with a tube pitch of 30 mm (corresponding to 5 water tubes) and an eccentric of 5 mm, respectively. The air channel width of 20 mm is found as the optimal width due to its higher heat release efficiency (83.24 %) and lower pressure drop (4.3 Pa). The identification of key structural impacts helped to optimize the RTESU configuration, which performs at a low heat storage time of 117 min and achieves a heat release efficiency as high as 83.24 % to satisfy the need for poor solar conditions. 

Yan Tao

Papers

High visible-light photocatalytic performance of stable lead-free Cs2AgBiBr6 double perovskite nanocrystals

Thermal performance improvement of a low-temperature thermal energy storage with configuration optimization