Inhomogeneous Trap-State-Mediated Ultrafast Photocarrier Dynamics in CsPbBr3 Microplates

Direct synthesis of high quantum yield lead-free CsCu2I3 powder in water and its application in yellow LED

Yellow light-emitting diodes (LEDs) with a wavelength of 570-590 nm can reduce the excitability of peripheral nerves and the sensitivity of the skin, stimulate collagen synthesis, and tighten the skin, which plays an important role in skin rejuvenation. In general, commercial LEDs are made of phosphor excited by ultraviolet chips. It is very important for the development of yellow light emitters with high luminous efficiency, good stability, and environmental protection. For the first time, a simple organic structural unit (2-methylimidazole, 2-MIM) was used to collect a mixture of two metal precursors (CsI and CuI) and successfully synthesized an all-inorganic lead-free yellow light CsCu2I3 powder in water. The prepared CsCu2I3 powder exhibited excellent optical properties and considerable stability. Finally, a phosphor-converted LED (pc-LED) device was fabricated via the CsCu2I3 phosphor coated on a 310 nm ultraviolet chip. The pc-LED device's electroluminescence spectra may be a good fit for the blood's absorption regions. Therefore, this work provides a facile method for the synthesis of novel lead-free metal halide CsCu2I3 powder in eco-friendly solvents. In addition, the stable and efficient CsCu2I3 powder shows promising exciting potential applications in photoluminescence and phototherapy fields.

Improved self-powered perovskite CH3NH3PbI3/SnO2 heterojunction photodetectors achieved by interfacial engineering with a synergic effect

Lead halide perovskite heterojunctions have been considered as important building blocks for fabricating high-performance photodetectors (PDs). However, the interfacial defects induced non-radiative recombination and interfacial energy-level misalignment induced ineffective carrier transport severely limit the performance of photodetection of resulting devices. Herein, interfacial engineering with a spin-coating procedure has been studied to improve the photodetection performance of CH3NH3PbI3/SnO2 heterojunction PDs, which were fabricated by sputtering a SnO2 thin film on ITO glass followed by spin-coating a CH3NH3PbI3 thin film. It has shown that spin-coating of a SnO2 layer on the sputtered SnO2 thin films suppressed the surface oxygen vacancies of SnO2 thin films and up-shifted their conduction band, which suppressed the interfacial non-radiative recombination and enhanced the carriers transport at the CH3NH3PbI3/SnO2 interface, respectively. Accordingly, improved photodetection performance, such as the reduced dark current and increased photocurrent, has been observed in the CH3NH3PbI3/SnO2 heterojunction PDs, where the responsivity and detectivity of 0.077 A W-1 and 2.0 × 1011 jones, respectively, at the zero bias have been demonstrated. These results show a simple way to suppress the interfacial non-radiative recombination and enhance the carrier transport at the interface to fabricate improved perovskite heterojunction PDs in the future.

Modulating hot carrier relaxation and trapping dynamics in lead halide perovskite nanoplatelets by surface passivation

Two-dimensional (2D) lead halide perovskite (LHP) nanoplatelets (NPLs) have recently emerged as promising materials for solar cells and light-emitting devices. The reduction of LHP dimensions introduces an abundance of surface defects, which can strongly influence the photophysical properties of these materials. However, an insightful understanding of the effect of surface defects on hot carrier (HC) relaxation, one of the important properties of LHP NPLs, is still inadequate. Herein, the HC relaxation and trapping dynamics in pristine and surface passivated two-layer (2L) CsPbBr3 NPLs have been investigated by using time-resolved spectroscopy. The results reveal that surface defects can trap HCs directly before they relax to the band edge, which accounts for the absence of the hot-phonon bottleneck (HPB) effect in LHP NPLs. After healing surface defects with a passivation agent, the relaxation time of HCs is extended from ∼73 to ∼130 fs in 2L CsPbBr3 NPLs, indicating that the channel of HCs trapped by the surface defects can be effectively blocked. Accordingly, the HPB effect is activated in surface-passivated CsPbBr3 NPLs. The finding of surface defect-related HC relaxation dynamics is important for guiding the development of high-performance LHP NPL devices related to HCs through surface defect engineering.

Blue sensitive sub-band gap negative photoconductance in SnO2/TiO2 NP bilayer oxide transistor

Large negative photoconductance (NPC) of SnO2/TiO2 nanoparticles (NPs) heterostructure has been observed with thin film transistor (TFT) geometry and has been investigated using sub-bandgap light (blue) illumination. This negative photoconduction has been detected both in accumulation and depletion mode operation, which effectively reduces the carrier mobility (μ) of the TFT. Moreover, the threshold voltage (Vth) widely shifted in the positive direction under illumination. The combined effects of the reduction of mobility and Vth shifting led to a faster reduction of On (or Off) state current under illumination. The negative photosensitivity of this system is as high as 3.2 A W-1, which has been rarely reported in the earlier literature. Moreover, the variation of On (or Off) current, μ and Vth shift is linear with low-intensity blue light. This SnO2/TiO2 NP bilayer channel has been deposited on top of an ionic dielectric (Li-Al2O3) that reduces its operating voltage of this TFT within 2 V. Furthermore, the device has achieved a saturation mobility of 0.4 cm2 V-1 s-1 with an on/off ratio of 7.4 × 103 in the dark. An energy band diagram model has been proposed based on the type-II heterostructure formation between SnO2/TiO2 semiconductors to explain this NPC mechanism. According to the energy band diagram model, adsorbed H2O molecules of TiO2 NPs created a depleted layer in the heterostructure that accelerated the recombination process of photo-generated carriers rather than its transport.

Detecting Visible to Near-Infrared II Light via CsPbBr3 Nanocrystals/Y6 Heterojunctions

In the endeavor to develop advanced photodetectors (PDs) with superior performance, all-inorganic perovskites, recognized for their outstanding photoelectric properties, have emerged as highly promising materials. Due to their unique electronic structure and band characteristics, the majority of all-inorganic perovskite materials are not sensitive to near-infrared (NIR) light. Here, we demonstrate the fabrication of a high-performance broadband PD comprising CsPbBr3 perovskite NCs/Y6 planar heterojunctions. The incorporation of Y6 not only facilitates charge transfer from CsPbBr3 NCs to Y6 for enhancing photodetection performance under visible illumination but also broadens the absorption spectrum range of the whole device toward the NIR regime. As a result, the heterojunction PD exhibits a photo-to-dark-current ratio above 105, a dynamic range of 149.5 dB, and an impressive lowest detection limit of incident power density of 1.6 nW/cm2 under 505 nm illumination. In the NIR regime, where photon energy is below the bandgap of CsPbBr3, electron-hole pairs can still be produced in the Y6 layer even when illuminated at 1120 nm. Consequently, photodetection is uniquely possible in PDs that incorporate heterojunctions when the illumination wavelength is longer than 565 nm. At 850 nm, the heterojunction device is capable of detecting light with power densities as low as 1.3 μW/cm2 corresponding to a LDR of 99.8 dB. The exceptional performance is attributed to the creation of a heterojunction between CsPbBr3 NCs and Y6. These findings propose a novel approach for developing broadband PDs based on perovskite NC materials.

Light-Controlled Multiphase Structuring of Perovskite Crystal Enabled by Thermoplasmonic Metasurface

Halide perovskites belong to an important family of semiconducting materials with electronic properties that enable a myriad of applications, especially in photovoltaics and optoelectronics. Their optical properties, including photoluminescence quantum yield, are affected and notably enhanced at crystal imperfections where the symmetry is broken and the density of states increases. These lattice distortions can be introduced through structural phase transitions, allowing charge gradients to appear near the interfaces between phase structures. In this work, we demonstrate controlled multiphase structuring in a single perovskite crystal. The concept uses cesium lead bromine (CsPbBr₃) placed on a thermoplasmonic TiN/Si metasurface and enables single-, double-, and triple-phase structures to form on demand above room temperature. This approach promises application horizons of dynamically controlled heterostructures with distinctive electronic and enhanced optical properties.

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al₂O₃ by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al₂O₃ layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al₂O₃ treatment reaches a remarkable level of 2.5 nW cm^-2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 10⁸% and 1.85 × 10⁶ A W^-1, respectively, outperforming all other polymer-based PM-OPDs.

A study on theoretical models for investigating time-resolved photoluminescence in halide perovskites

Time-resolved photoluminescence (TRPL) is an effective experimental technique to study charge carrier dynamic processes in halide perovskites on different time scales. In the past decade, several models have been proposed and employed to study the TRPL curves in halide perovskites, but there is still a lack of systematic summarization and comparative discussion. Here, we reviewed the widely employed exponential models to fit the TRPL curves, and focused on the physical meaning of the extracted carrier lifetimes, as well as the existing debates on the definition of the average lifetime. Emphasis was placed on the importance of the diffusion process in the carrier dynamics, especially for the halide perovskite thin films having transport layers. The solving of the diffusion equation, using both analytical and numerical methods, was then introduced to fit the TRPL curves. Furthermore, the newly proposed global fit and direct measurement of radiative decay rates were discussed.

Controlled on-chip fabrication of large-scale perovskite single crystal arrays for high-performance laser and photodetector integration

Metal halide perovskites possess intriguing optoelectronic properties, however, the lack of precise control of on-chip fabrication of the large-scale perovskite single crystal arrays restricts its application in integrated devices. Here, we report a space confinement and antisolvent-assisted crystallization method for the homogeneous perovskite single crystal arrays spanning 100 square centimeter areas. This method enables precise control over the crystal arrays, including different array shapes and resolutions with less than 10%-pixel position variation, tunable pixel dimensions from 2 to 8 μm as well as the in-plane rotation of each pixel. The crystal pixel could serve as a high-quality whispering gallery mode (WGM) microcavity with a quality factor of 2915 and a threshold of 4.14 μJ cm^-2. Through directly on-chip fabrication on the patterned electrodes, a vertical structured photodetector array is demonstrated with stable photoswitching behavior and the capability to image the input patterns, indicating the potential application in the integrated systems of this method.

Photoluminescence Modulation of Ruddlesden-Popper Perovskite via Phase Distribution Regulation

The intrinsic chaotic phase distribution in Ruddlesden-Popper Perovskite (RPP) hinders its further improvement of photoluminescence (PL) emission and limits its application in optical devices. In this work, we achieve the phase distribution regulation of RPP by varying the composition ratio of organic bulky spacer cations 1-naphthylmethylamine (NMA) and phenylethyl-ammonium (PEA), which is controllable and nondestructive for structures of RPP. By suppressing the small n-phase, the PL intensity emission of RPP is further improved. Through the time-resolved PL (TRPL) measurements, we find the PL lifetime of the sample with 66% PEA concentration increases with the temperature initially and possesses the highest values of τ1 and τ2 at ~255 K, indicating the immediate state assisting exciton radiative recombination, and it can be modulated by phase manipulation in RPP. The immediate state may outcompete other non-radiative decay channels for excited carriers, leading to the PL enhancement in RPP, and broadening its further application.

Ultrafast Antisolvent Growth of Single-Crystalline CsPbCl3 Microcavity for Low-Threshold Room Temperature Blue Lasing

All-inorganic perovskite CsPbCl₃ has recently attracted considerable attention due to its great potentials for the development of high-efficiency, deep-blue optoelectronic devices. Particularly, single-crystalline CsPbCl₃ planar microstructures provide good platforms for both fundamental studies and nanophotonics applications from lasers and detectors to amplifiers. In this study, we report an ultrafast antisolvent deposition route to fabricate single-crystalline CsPbCl₃ microplatelets (MPs). The as-grown MPs exhibit uniform morphology, strong emission, and outstanding gain property. Room temperature photoluminescence lasing is realized at 428 nm with a low threshold of 11.5 μJ cm^-2 and high net optical gain up to 720 cm^-1. These findings advance fundamental understanding on the fabrication and optoelectronic applications of low-dimensional CsPbCl₃ perovskite structures.

Single Crystal Halide Perovskite Film for Nonlinear Resistive Memory with Ultrahigh Switching Ratio

Morre's law is coming to an end only if the memory industry can keep stuffing the devices with new functionality. Halide perovskite acts as a promising candidate for application in next-generation nonvolatile memory. As is well known, the switching ratio is the key device requirement of resistive memory to improve recognition accuracy. Here, the authors introduce an all-inorganic halide perovskite CsPbBr₃ single crystal film (SCF) into resistive memory as an active layer. The Ag/CsPbBr₃ /Ag memory cells exhibit reproducible resistive switching with an ultrahigh switching ratio (over 10⁹ ) and a fast switching speed (1.8 µs). It is studied that the Schottky barrier of metal/CsPbBr₃ SCF contact follows the tendency of Schottky-Mott theory, and the Fermi level pinning effect is effectively reduced. The interface S parameter of metal/CsPbBr₃ SCF contact is 0.50, suggesting a great interface contact is formed. The great interface contact contributes to the steady high resistance state (HRS), and then the steady HRS leads to an ultrahigh resistive switching ratio. This work demonstrates high performance from halide perovskite SCF-based memory. The introduction of halide perovskite SCF in resistive random access memory provides great potential as an alternative in future computing systems.

Enhanced Light Emission through Symmetry Engineering of Halide Perovskites

Metal-halide perovskites (MHPs) have attracted tremendous attention as active materials in optoelectronic devices. For light-emitting diode (LED) applications, nanostructuring of MHPs is considered to be inevitable, but its light-enhancement mechanism is still elusive because the particle (or grain) size is often beyond the quantum confinement regime. As motivated by the experimental finding that the nanostructuring can change the preferred crystalline symmetry from the orthorhombic phase to the high-symmetric cubic phase, we here investigated the carrier dynamics in various polymorphic phases of CsPbBr₃ using ab initio quantum dynamics simulation. We found that the cubic phase shows a smaller inelastic phonon scattering than the orthorhombic phase; the suppression of the octahedral tilt minimizes the longitudinal Br fluctuation and helps disentangle the A-site cation dynamics from the nonadiabatic carrier dynamics. We thus anticipate that our present work will offer a material design principle to enhance the quantum yield of MHPs via symmetry engineering, which will help develop highly luminescent LED technology based on MHPs.

Sub-Nanometer Thick Wafer-Size NiO Films with Room-Temperature Ferromagnetic Behavior

Adding ferromagnetism into semiconductors attracts much attentions due to its potential usage of magnetic spins in novel devices, such as spin field-effect transistors. However, it remains challenging to stabilize their ferromagnetism above room temperature. Here we introduce an atomic chemical-solution strategy to grow wafer-size NiO thin films with controllable thickness down to sub-nanometer scale (0.92 nm) for the first time. Surface lattice defects break the magnetic symmetry of NiO and produce surface ferromagnetic behaviors. Our sub-nanometric NiO thin film exhibits the highest reported room-temperature ferromagnetic behavior with a saturation magnetization of 157 emu/cc and coercivity of 418 Oe. Attributed to wafer size, the easily-transferred NiO thin film is further verified in a magnetoresistance device. Our work provides a sub-nanometric platform to produce wafer-size ferromagnetic NiO thin films as atomic layer magnetic units in future transparent magnetoelectric devices.