A2AgCrBr6 (A = K, Rb, Cs) and Cs2AgCrX6(X = Cl, I) Double Perovskites: A Transition-Metal-Based Semiconducting Material Series with Remarkable Optics

High Open-Circuit Voltage Cs2 AgBiBr6 Carbon-Based Perovskite Solar Cells via Green Processing of Ultrasonic Spray-Coated Carbon Electrodes from Waste Tire Sources

Costs and toxicity concerns are at the center of a heated debate regarding the implementation of perovskite solar cells (PSCs) into commercial products. The first bottleneck could be overcome by eliminating the top metal electrode (generally gold) and the underlying hole transporting material and substituting both with one single thick layer of conductive carbon, as in the so-called carbon-based PSCs (C-PSCs). The second issue, related to the presence of lead, can be tackled by resorting to other perovskite structures based on less toxic metallic components. An interesting case is that of the double perovskite Cs₂ AgBiBr₆ , which at present still lacks the outstanding optoelectronic performances of the lead-based counterparts but is very stable to environmental factors. In this work, the processing of carbon electrodes onto Cs₂ AgBiBr₆ -based C-PSCs was reported, starting from an additive-free isopropanol ink of a carbon material obtained from the hydrothermal recycling of waste tires and employing a high-throughput ultrasonic spray coating method in normal environmental conditions. Through this highly sustainable approach that ensures a valuable step from an end-of-life to an end-of-waste status for used tires, devices were obtained delivering a record open circuit voltage of 1.293 V, which might in the future represent ultra-cheap solutions to power the indoor Internet of Things ecosystem.

Recent Advancements in Nontoxic Halide Perovskites: Beyond Divalent Composition Space

Since the inception of organic-inorganic hybrid perovskites of ABX3 stoichiometry in 2009, there has been enormous progress in envisaging efficient solar cell materials throughout the world, from both the theoretical and experimental perspectives. Despite achieving 25.5% efficiency, hybrid halide perovskites are still facing two main challenges: toxicity due to the presence of lead and device stability. Two particular families with A3B2X9 and A2MM'X6 stoichiometries have emerged to address these two prime concerns, which have restrained the advancement of solar energy harvesting. Several investigations, both experimental and theoretical, are being conducted to explore the holy-grail materials, which could be optimum for not only efficient but also stable and nontoxic photovoltaics technology. However, the trade-off among stability, efficiency, and toxicity in such solar energy materials is yet to be completely resolved, which requires a systematic overview of A3B2X9- and A2MM'X6-based solar cell materials. Therefore, in this timely and relevant perspective, we have focused on these two particular promising families of perovskite materials. We have portrayed a roadmap projecting the recent advancements from both theoretical and experimental perspectives for these two exciting and promising solar energy material families while amalgamating our critical viewpoint with a future outlook.

Dielectric polarization effect and transient relaxation in FAPbBr3 films before and after PMMA passivation

In organic-inorganic hybrid ionic lead halide perovskites with a naturally arranged layered structure, the dielectric polarization effect caused by the dielectric mismatch between the organic and inorganic layers takes effect in their optical responses. But this effect has received little attention. Here we used infrared transient spectroscopy to study FAPbBr3 perovskite polycrystalline films before and after PMMA film passivation and found that there is a dielectric polarization effect at the interface between the organic cation layer and the inorganic lattice layer inside the perovskite lattice, and also at the interface between the PMMA film and perovskite film. Due to the dielectric polarization effect and the spatial confinement of the surface electronic (or polaron) state, the luminescence intensity of the passivated perovskite film is significantly enhanced, and the exciton lifetime is greatly increased. Dielectric polarization enhances their efficient transient absorption (TA) and leads to the intramolecular vibration frequency red-shifts, which exhibited the combined relaxation kinetics of the large polaron with dielectric polarization in the perovskite film. Dielectric polarization between the internal lattice and the nanocrystal surface of the perovskite film shows different relaxation processes. The polarization-dependent TA spectrum reveals that the dielectric polarization field causes light-induced anisotropy by changing the chemical bond configurations. These direct TA experimental observations help us to understand the influence of the dielectric polarization effect on the electronic state in various organic-inorganic nanocomposite perovskites.

A2AgCrCl6 (A = Li, Na, K, Rb, Cs) halide double perovskites: a transition metal-based semiconducting material series with appreciable optical characteristics

We have theoretically examined the geometries, electronic density of states and band structures of cubic and hexagonal A₂AgCrCl₆ (A = Cs, Rb, K, Na, Li) using meta-GGA SCAN-rVV10. The optimized lattice density was found to vary between 2.68 and 4.08 g cm^-3 for cubic-A₂AgCrCl₆, with the fundamental electronic bandgap (direct) in the range of 0.66-0.69 eV. The cell density of hexagonal A₂AgCrCl₆ was between 2.97 and 3.93 g cm^-3, but with an indirect bandgap of 0.93-1.02 eV. The valence band maximum and the conduction band minimum of A₂AgCrCl₆ were confirmed to be essentially of Cr(3d) character, but the contributions from the orbital states of Cl(3p) to the VBM were also appreciable. Cubic A₂AgCrCl₆ (A = Cs, Rb, K) was identified to possess genuine perovskite stoichiometry, evaluated using various geometry-based indices (viz. octahedral factor, tolerance factor, and global instability index). This was not so for A₂AgCrCl₆ (A = Na, Li), and was due to the small size of Na and Li cations that caused the critical strain of CrCl₆ octahedra and a significant decrease in the cell volume. However, all the five A₂AgCrCl₆ displayed nearly similar optical properties, including the nature of the oscillator peaks in the dielectric function, absorption coefficient, photoconductivity, reflectivity, and Tauc spectra. The zero-limit of the refractive index was calculated around 2.25 and 2.00 for cubic and hexagonal A₂AgCrCl₆, respectively, and the extinction coefficient was very small for all cases. The nature of the optical bandgap and transition peaks discussed in this study of cubic and hexagonal Cs₂AgCrCl₆ agreed well with the experiment. The examination of phonon band dispersion led to the conclusion that cubic-A₂AgCrCl₆ (A = Cs, Rb) are the only halide double perovskites of the entire series that are dynamically stable.