Introduction: Perovskites

Air-Stable Self-Driven UV Photodetectors on Controllable Lead-Free CsCu2I3 Microwire Arrays

The rapid evolution of the Internet of Things has engendered increased requirements for low-cost, self-powered UV photodetectors. Herein, high-performance self-driven UV photodetectors are fabricated by designing asymmetric metal-semiconductor-metal structures on the high-quality large-area CsCu2I3 microwire arrays. The asymmetrical depletion region doubles the photocurrent and response speed compared to the symmetric structure device, leading to a high responsivity of 233 mA/W to 355 nm radiation. Notably, at 0 V bias, the asymmetric device produces an open-circuit voltage of 356 mV and drives to a short-circuit current of 372 pA; meanwhile, the switch ratio (Iph/Idark) reaches up to 103, indicating its excellent potential for detecting weak light. Furthermore, the device maintains stable responses throughout 10000 UV-light switch cycles, with negligible degradation even after 90-day storage in air. Our work establishes that CsCu2I3 is a good candidate for self-powered UV detection and thoroughly demonstrates its potential as a passive device.

Dopant engineering for ZnO electron transport layer towards efficient perovskite solar cells

The conventional electron transport layer (ETL) TiO2 has been widely used in perovskite solar cells (PSCs), which have produced exceptional power conversion efficiencies (PCE), allowing the technology to be highly regarded and propitious. Nevertheless, the recent high demand for energy harvesters in wearable electronics, aerospace, and building integration has led to the need for flexible solar cells. However, the conventional TiO2 ETL layer is less preferred, where a crystallization process at a temperature as high as 450 °C is required, which degrades the plastic substrate. Zinc oxide nanorods (ZnO NRs) as a simple and low-cost fabrication material may fulfil the need as an ETL, but they still suffer from low PCE due to atomic defect vacancy. To delve into the issue, several dopants have been reviewed as an additive to passivate or substitute the Zn2+ vacancies, thus enhancing the charge transport mechanism. This work thereby unravels and provides a clear insight into dopant engineering in ZnO NRs ETL for PSC.

Composition Dependent Strain Engineering of Lead-Free Halide Double Perovskite: Computational Insights

The critical photophysical properties of lead-free halide double perovskites (HDPs) must be substantially improved for various applications. In this regard, strain engineering is a powerful tool for enhancing optoelectronic performance with precise control. Here, we employ ab initio simulations to investigate the impact of mild compressive and tensile strains on the photophysics of Cs2AgB'X6 (B' = Sb, Bi; X = Cl, Br) perovskites. Depending on the pnictogen and halide atoms, the band gap and band edge positions of HDPs can be tuned to a significant extent by controlling the applied external strain. Cs2AgSbBr6 has the most substantial strain response under structural perturbations. The subtle electronic interactions among the participating orbitals and the band dispersion at the edge states are enhanced under compressive strain, reducing the carrier effective masses. The exciton binding energies for these Br-based HDPs are in the range 59-78 meV and weaken in the compressed lattices, suggesting improved free carrier generation. Overall, the study emphasizes the potential of lattice strain engineering to boost the photophysical properties of HDPs that can ultimately improve their optoelectronic performance.

High-Pressure and High-Temperature Dion-Jacobson Layered Perovskite Polymorphs of KWO3 F

Two new Dion-Jacobson layered perovskite polymorphs of the known oxyfluoride compound KWO3 F are reported. A high-pressure modification was synthesized using a multianvil setup and subsequently transformed into a high-temperature phase at ∼311 °C. The crystal structures of both polymorphs were determined by use of single-crystal X-ray diffraction and are described in detail herein. Differential thermal analyses and thermogravimetric analyses were carried out to further investigate the phase transition characteristics. Bond valence (BV) and charge distribution (CHARDI) calculations confirm the occupancy of mixed O|F anion positions, and Rietveld refinements as well as MAPLE calculations support the structure models.

Two-Dimensional Layered Organic-Inorganic Hybrid Perovskite Thin-Film Fabrication by Langmuir-Blodgett and Intercalation Techniques

We demonstrate the formation of a well-organized thin film of two-dimensional (2D) layered (C₁₈H₃₇NH₃)₂PbI₄ hybrid perovskite by immersing octadecyl amine (ODA) Langmuir-Blodgett (LB) films in an aqueous solution of PbI₂/HI. The immersed films exhibit a sharp absorption band at 486 nm (2.55 eV), which is assigned to the excitonic absorption. The film exhibits a bright green emission under ultraviolet light at room temperature. The photoluminescence spectrum has a distinct peak at 497 nm (2.49 eV) and is a mirror image of the absorption spectrum. X-ray diffraction (XRD) analyses reveal that the film has a bilayer-like structure with a d-spacing of 6.4 nm, which is equal to that of a (C₁₈H₃₇NH₃)₂PbI₄ perovskite single crystal with a quantum well (QW) structure. Only intense peaks of the (0 0 l) (l = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24) reflections are observed in the out-of-plane XRD pattern, indicating that the c axis is vertically oriented with respect to the substrate surface, and the orientational order is remarkably high. Fourier transform infrared spectroscopy reveals that the ODA molecules are protonated in the PbI₂/HI solution. These results suggest that the nitrogen atoms of the ODA molecules in the film are protonated in the PbI₂/HI solution, and then, inorganic layers of the PbI₆ octahedra are intercalated in the alkyl ammonium film to neutralize the positive charge and form a QW structure. Fluorescence microscopy observation reveals that the 2D layered (C₁₈H₃₇NH₃)₂PbI₄ film has a relatively uniform surface, reflecting the well-organized layered structure of the base material (ODA LB film). Because the intercalation process can be applied to various metal cations and halogen anions, we believe that the proposed technique will aid in the development of highly efficient 2D layered organic-inorganic hybrid perovskite materials.

Impact of dynamic co-evaporation schemes on the growth of methylammonium lead iodide absorbers for inverted solar cells

A variety of different synthesis methods for the fabrication of solar cell absorbers based on the lead halide perovskite methylammonium lead iodide (MAPbI3, MAPI) have been successfully developed in the past. In this work, we elaborate upon vacuum-based dual source co-evaporation as an industrially attractive processing technology. We present non-stationary processing schemes and concentrate on details of co-evaporation schemes where we intentionally delay the start/end points of one of the two evaporated components (MAI and PbI2). Previously, it was found for solar cells based on a regular n-i-p structure, that the pre-evaporation of PbI$$_2$$ 2 is highly beneficial for absorber growth and solar cell performance. Here, we apply similar non-stationary processing schemes with pre/post-deposition sequences for the growth of MAPI absorbers in an inverted p-i-n solar cell architecture. Solar cell parameters as well as details of the absorber growth are compared for a set of different evaporation schemes. Contrary to our preliminary assumptions, we find the pre-evaporation of PbI2 to be detrimental in the inverted configuration, indicating that the beneficial effect of the seed layers originates from interface properties related to improved charge carrier transport and extraction across this interface rather than being related to an improved absorber growth. This is further evidenced by a performance improvement of inverted solar cell devices with pre-evaporated MAI and post-deposited PbI2 layers. Finally, we provide two hypothetical electronic models that might cause the observed effects.

Application of perovskites in bioimaging: the state-of-the-art and future developments

BACKGROUND

Recently, the development of perovskite-based nanocrystals for sustainable applications in bioimaging and clinical diagnostics have become a very active area of research. From 2D hybrid to zero-dimensional quantum dots (QDs), perovskites along with a variety of characteristic features, specifically non-linear optoelectronics properties, have attracted enormous research attention. These characteristics can be tuned by the type of cations or anions and their ratio used in host perovskites. Carrier doping and chemical modifications are additional alternatives to control optical and magnetism in radiodiagnostics.

AREA COVERED

This review begins by explaining the physical phenomena associated with luminescence or optical features of novel perovskites in diagnostic applications. Moreover, reported oxide, halide, doped, and QDs-based nanoprobes were elaborated. At last, the need for novel perovskite development, for example, persistent luminescent and low cytotoxicity is discussed, and the futuristic perspective of perovskites in clinical diagnostics with real-time demonstration is explained.

EXPERT OPINION

Our article concludes that hybrid perovskites, including metal-free, core-shell nanocomposites-based, and alloy-based perovskites, exhibit tunable bandgap and high photoluminescence quantum yields which ultimately result in high optical features. However, given limited understanding of ion transport mechanisms and dependency on environmental conditions of the perovskites, more research is needed.

Organic Cation Diffusion-Induced Heterogeneous Viscoelasticity in Organic-Inorganic Hybrid Perovskite Polycrystalline Films

Organic-inorganic hybrid perovskite (OIHP) polycrystalline films are the key light-absorbing layers of laminated-structure OIHP-based devices that have attracted increasing attention in photoelectronics and flexible electronics. Internal stresses induced by the mismatched responses of laminated layers to long-term and cyclic multiphysical fields generate time-dependent mechanical deformation in OIHP polycrystalline films, which makes the mechanical constitution relation of great significance. However, few studies focus on either the mechanical properties and behaviors of OIHP polycrystalline films or the underlying mechanism coupled with the grain structure and ion diffusion. Here, we uncovered the heterogeneous viscoelasticity of MAPbBr₃ films strongly correlated with the grain structure. Combining experiments and modeling, we revealed that the organic cation diffusion from grain interiors to grain boundaries leads to heterogeneity in the chemical distribution and viscoelastic modulus. Our work provides the nanomechanical understanding of the OIHP polycrystalline films that are crucial for safety design and performance optimization in OIHP-based electronics.

Photocatalysis and perovskite oxide-based materials: a remedy for a clean and sustainable future

The massive use of non-renewable energy resources by humankind to fulfill their energy demands is causing severe environmental issues. Photocatalysis is considered one of the potential solutions for a clean and sustainable future because of its cleanliness, inexhaustibility, efficiency, and cost-effectiveness. Significant efforts have been made to design highly proficient photocatalyst materials for various applications such as water pollutant degradation, water splitting, CO2 reduction, and nitrogen fixation. Perovskite photocatalyst materials are gained special attention due to their exceptional properties because of their flexibility in chemical composition, structure, bandgap, oxidation states, and valence states. The current review is focused on perovskite materials and their applications in photocatalysis. Special attention has been given to the structural, stoichiometric, and compositional flexibility of perovskite photocatalyst materials. The photocatalytic activity of perovskite materials in different photocatalysis applications is also discussed. Various mechanisms involved in photocatalysis application from wastewater treatment to hydrogen production are also provided. The key objective of this review is to encapsulate the role of perovskite materials in photocatalysis along with their fundamental properties to provide valuable insight for addressing future environmental challenges.

Coherent Two-Dimensional and Broadband Electronic Spectroscopies

Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO₃ exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO₃ in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu³⁺ ions stabilize at both Ba²⁺ and Sn⁴⁺ sites consistent with the highly negative formation energy of around -6.26 eV. Eu³⁺ doping leads to an intense ⁵D₀→⁷F₁ orange emission and a feeble ⁵D₀→⁷F₂ red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of Eu_Ba and Eu_Sn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn²⁺ in the PL of BaSnO₃ or Eu²⁺ in the PL of BaSnO3:Eu³⁺. Interestingly, when co-doped, Eu³⁺ stabilizes at Sn⁴⁺ sites whereas La³⁺ stabilizes at Ba²⁺ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La³⁺ ions lead to lowering of symmetry around Eu³⁺ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO₃ in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO₃ and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO₃ is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

Melting of hybrid organic-inorganic perovskites

Several organic-inorganic hybrid materials from the metal-organic framework (MOF) family have been shown to form stable liquids at high temperatures. Quenching then results in the formation of melt-quenched MOF glasses that retain the three-dimensional coordination bonding of the crystalline phase. These hybrid glasses have intriguing properties and could find practical applications, yet the melt-quench phenomenon has so far remained limited to a few MOF structures. Here we turn to hybrid organic-inorganic perovskites-which occupy a prominent position within materials chemistry owing to their functional properties such as ion transport, photoconductivity, ferroelectricity and multiferroicity-and show that a series of dicyanamide-based hybrid organic-inorganic perovskites undergo melting. Our combined experimental-computational approach demonstrates that, on quenching, they form glasses that largely retain their solid-state inorganic-organic connectivity. The resulting materials show very low thermal conductivities (~0.2 W m^-1 K^-1), moderate electrical conductivities (10^-3-10^-5 S m^-1) and polymer-like thermomechanical properties.

Automatic light-adjusting electrochromic device powered by perovskite solar cell

Electrochromic devices can modulate their light absorption under a small driving voltage, but the requirement for external electrical supplies causes response-lag. To address this problem, self-powered electrochromic devices have been studied recently. However, insensitivity to the surrounding light and unsatisfactory stability of electrochromic devices have hindered their critical applications. Herein, novel perovskite solar cell-powered all-in-one gel electrochromic devices have been assembled and studied in order to achieve automatic light adjustment. Two alkynyl-containing viologen derivatives are synthesized as electrochromic materials, the devices with very high stability (up to 70000 cycles) serves as the energy storage and smart window, while the perovskite solar cell with power-conversion-efficiency up to 18.3% serves as the light detector and power harvester. The combined devices can automatically switch between bleached and colored state to adjust light absorption with variable surrounding light intensity in real-time swiftly, which establish significant potentials for applications as modern all-day intelligent windows.

Isosymmetric compression of cubic halide perovskites $$\mathrm{ABX}_{3}$$ ($$A=K, Rb, Cs$$; $$B=Ge, Sn, Pb$$ and $$X=Cl,Br,I$$)-influence of cation–anion exchange: a first principle study

Halide perovskites are potential candidates for their use in solar cells. In this study, an investigation of the structural, mechanical and electro-optical properties of $$\mathrm{ABX}_{3}$$ ABX 3 ($$A=K, Rb, Cs$$ A = K , R b , C s ; $$B=Ge, Sn, Pb$$ B = G e , S n , P b and $$X=Cl,Br,I$$ X = C l , B r , I ), under isosymmetric compression, is presented. All the calculations are performed with generalized gradient approximation schemes. We have examined the effect of cation and anion substitution on their overall properties.

Synergistic Cascade Carrier Extraction via Dual Interfacial Positioning of Ambipolar Black Phosphorene for High-Efficiency Perovskite Solar Cells

2D black phosphorene (BP) carries a stellar set of physical properties such as conveniently tunable bandgap and extremely high ambipolar carrier mobility for optoelectronic devices. Herein, the judicious design and positioning of BP with tailored thickness as dual-functional nanomaterials to concurrently enhance carrier extraction at both electron transport layer/perovskite and perovskite/hole transport layer interfaces for high-efficiency and stable perovskite solar cells is reported. The synergy of favorable band energy alignment and concerted cascade interfacial carrier extraction, rendered by concurrent positioning of BP, delivered a progressively enhanced power conversion efficiency of 19.83% from 16.95% (BP-free). Investigation into interfacial engineering further reveals enhanced light absorption and reduced trap density for improved photovoltaic performance with BP incorporation. This work demonstrates the appealing characteristic of rational implementation of BP as dual-functional transport material for a diversity of optoelectronic devices, including photodetectors, sensors, light-emitting diodes, etc.

Synthesis of triphenylpyridines via an oxidative cyclization reaction using Sr-doped LaCoO3 perovskite as a recyclable heterogeneous catalyst

An La_0.6Sr_0.4CoO₃ strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the cyclization reaction between ketoximes and phenylacetic acids. The transformation proceeded via the oxidative functionalization of the sp³ C–H bond in phenylacetic acid. The La_0.6Sr_0.4CoO₃ catalyst demonstrated a superior performance to that of the pristine LaCoCO₃ as well as a series of homogeneous and heterogeneous catalysts. Furthermore, the La_0.6Sr_0.4CoO₃ catalyst was facilely recovered and reused without considerable decline in its catalytic efficiency. To the best of our knowledge, the combination of ketoximes with easily available phenylacetic acids is novel.

An La_0.6Sr_0.4CoO₃ strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the cyclization reaction between ketoximes and phenylacetic acids.