Jun Xing

Vapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature Nanolasers

Semiconductor nanowires have received considerable attention in the past decade driven by both unprecedented physics derived from the quantum size effect and strong isotropy and advanced applications as potential building blocks for nanoscale electronics and optoelectronic devices. Recently, organic-inorganic hybrid perovskites have been shown to exhibit high optical absorption coefficient, optimal direct band gap, and long electron/hole diffusion lengths, leading to high-performance photovoltaic devices. Herein, we present the vapor phase synthesis free-standing CH3NH3PbI3, CH3NH3PbBr3, and CH3NH3PbIxCl3(-x) perovskite nanowires with high crystallinity. These rectangular cross-sectional perovskite nanowires have good optical properties and long electron hole diffusion length, which ensure adequate gain and efficient optical feedback. Indeed, we have demonstrated optical-pumped room-temperature CH3NH3PbI3 nanowire lasers with near-infrared wavelength of 777 nm, low threshold of 11 μJ/cm(2), and a quality factor as high as 405. Our research advocates the promise of optoelectronic devices based on organic-inorganic perovskite nanowires.

High-Efficiency Light-Emitting Diodes of Organometal Halide Perovskite Amorphous Nanoparticles.

Organometal halide perovskite has recently emerged as a very promising family of materials with augmented performance in electronic and optoelectronic applications including photovoltaic devices, photodetectors, and light-emitting diodes. Herein, we propose and demonstrate facile solution synthesis of a series of colloidal organometal halide perovskite CH3NH3PbX3 (X = halides) nanoparticles with amorphous structure, which exhibit high quantum yield and tunable emission from ultraviolet to near-infrared. The growth mechanism and photoluminescence properties of the perovskite amorphous nanoparticles were studied in detail. A high-efficiency green-light-emitting diode based on amorphous CH3NH3PbBr3 nanoparticles was demonstrated. The perovskite amorphous nanoparticle-based light-emitting diode shows a maximum luminous efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W, and an external quantum efficiency of 3.8%, which is 3.5 times higher than that of the best colloidal perovskite quantum-dot-based light-emitting diodes previously reported. Our findings indicate the great potential of colloidal perovskite amorphous nanoparticles in light-emitting devices.

Solution-processed highly bright and durable cesium lead halide perovskite light-emitting diodes

Recently, CsPbBr3 perovskites have been emerging as very promising green emission materials for light-emitting diodes (LEDs) due to their high color purity, low cost and high photoluminescence quantum yield (PLQY). However, the corresponding LED performance is still low and far behind CH3NH3PbBr3; it is due to the lack of proper perovskite film preparation methods and interfacial engineering. Herein, we report highly bright and durable CsPbBr3-based LEDs fabricated using a one-step solution method. The precursor solution is prepared by simply dissolving CsPbBr3 powder and a CsBr additive in dimethyl sulfoxide (DMSO). We find that the CsBr additive not only significantly enhances the PLQY but also induces directional crystal growth into micro-plates, forming a smooth perovskite film for LEDs. LEDs employing such high quality films show a high luminance of 7276 cd m-2 and high color purity with a full width at half maximum of 18 nm. Furthermore, the as-fabricated LEDs reveal an outstanding ambient stability with a decent luminance output (>100 cd m-2, steady increase without any degradation trend) for at least 15 h under a constant driving current density (66.7 mA cm-2). And we propose two reasons for this unique luminance increasing behavior: (1) the CsPbBr3 perovskite is thermally stable and can survive from joule heat; and (2) on the other hand, the joule heating will induce interface or crystalline film annealing, reduce device resistance and then enhance the luminance output.

Highly Efficient Visible Colloidal Lead-Halide Perovskite Nanocrystal Light-Emitting Diodes

Lead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron-hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a maximum external quantum efficiency of 12.9% and a power efficiency of 30.3 lm W-1 at luminance levels above 1000 cd m-2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays.

Ultrafast Photogenerated Hole Extraction/Transport Behavior in a CH3NH3PbI3/Carbon Nanocomposite and Its Application in a Metal-Electrode-Free Solar Cell

Aligned and flexible electrospun carbon nanomaterials are used to synthesize carbon/perovskite nanocomposites. The free-electron diffusion length in the CH3 NH3 PbI3 phase of the CH3 NH3 PbI3 /carbon nanocomposite is almost twice that of bare CH3 NH3 PbI3 , and nearly 95 % of the photogenerated free holes can be injected from the CH3 NH3 PbI3 phase into the carbon nanomaterial. The exciton binding energy of the composite is estimated to be 23 meV by utilizing temperature-dependent optical absorption spectroscopy. The calculated free carriers increase with increasing total photoexcitation density, and this broadens the potential of this material for a broad range of optoelectronics applications. A metal-electrode-free perovskite solar cell (power conversion efficiency: 13.0 %) is fabricated with this perovskite/carbon composite, which shows great potential for the fabrication of efficient, large-scale, low-cost, and metal-electrode-free perovskite solar cells.

Nonlinear optical response of Au nanorods for broadband pulse modulation in bulk visible lasers

Due to the lack of suitable optical modulators, directly generated Pr3+- and Dy3+-doped bulk visible lasers are limited in the continuous-wave operation; yet, pulsed visible lasers are only sparingly reported recently. It has been theoretically predicated that Au nanorods could modulate the visible light operation, based on the nonlinear optical response of surface plasmon resonance. Here, we demonstrate the saturable absorption properties of Au nanorods in the visible region and experimentally realized the pulsed visible lasers over the spectral range of orange (605 nm), red (639 nm), and deep red (721 nm) with Au nanorods as the optical modulator. We show that Au nanorods have a broad nonlinear optical response and can serve as a type of broadband, low-cost, and eco-friendly candidate for optical switchers in the visible region. Our work also advocates the promise of plasmonic nanostructures for the development of pulsed lasers and other plasmonic devices.

Color-stable highly luminescent sky-blue perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) have shown excellent performance in the green and near-infrared spectral regions, with high color purity, efficiency, and brightness. In order to shift the emission wavelength to the blue, compositional engineering (anion mixing) and quantum-confinement engineering (reduced-dimensionality) have been employed. Unfortunately, LED emission profiles shift with increasing driving voltages due to either phase separation or the coexistence of multiple crystal domains. Here we report color-stable sky-blue PeLEDs achieved by enhancing the phase monodispersity of quasi-2D perovskite thin films. We selected cation combinations that modulate the crystallization and layer thickness distribution of the domains. The perovskite films show a record photoluminescence quantum yield of 88% at 477 nm. The corresponding PeLEDs exhibit stable sky-blue emission under high operation voltages. A maximum luminance of 2480 cd m−2 at 490 nm is achieved, fully one order of magnitude higher than the previous record for quasi-2D blue PeLEDs.Perovskite light-emitting diodes show promising color tunability and device performance but suffer from emission color shift at higher driving voltages. Here Xing et al. report color stable blue light-emitting diodes by drastically increasing the phase purity of the quasi-2D perovskite thin films.

Room temperature long-range coherent exciton polariton condensate flow in lead halide perovskites

Long-range coherent polariton condensate flow is observed in all-inorganic perovskite microcavities. Novel technological applications significantly favor alternatives to electrons toward constructing low power–consuming, high-speed all-optical integrated optoelectronic devices. Polariton condensates, exhibiting high-speed coherent propagation and spin-based behavior, attract considerable interest for implementing the basic elements of integrated optoelectronic devices: switching, transport, and logic. However, the implementation of this coherent polariton condensate flow is typically limited to cryogenic temperatures, constrained by small exciton binding energy in most semiconductor microcavities. Here, we demonstrate the capability of long-range nonresonantly excited polariton condensate flow at room temperature in a one-dimensional all-inorganic cesium lead bromide (CsPbBr3) perovskite microwire microcavity. The polariton condensate exhibits high-speed propagation over macroscopic distances of 60 μm while still preserving the long-range off-diagonal order. Our findings pave the way for using coherent polariton condensate flow for all-optical integrated logic circuits and polaritonic devices operating at room temperature.

Room Temperature Coherently Coupled Exciton–Polaritons in Two-Dimensional Organic–Inorganic Perovskite

Two-dimensional (2D) organic-inorganic perovskite semiconductors with natural multiquantum well structures and confined 2D excitons are intriguing for the study of strong exciton-photon coupling, due to their large exciton binding energy and oscillation strength. This strong coupling leads to a formation of the half-light half-matter bosonic quasiparticle called exciton-polariton, consisting of a linear superposition state between photonic and excitonic states. Here, we demonstrate room temperature strong coupling in exfoliated wavelength-tunable 2D organic-inorganic perovskite semiconductors embedded into a planar microcavity, exhibiting large energetic splitting-to-line width ratios (>34.2). Angular-dependent spectroscopy measurements reveal that hybridized polariton states act as an ultrafast and reversible energy oscillation, involving 2D perovskite exciton, cavity modes (CM), and Bragg modes of the distributed Bragg reflector. Meanwhile, sizable hybrid particles dominantly couple to the measured optical field through the CMs. Our findings advocate a considerable promise of 2D organic-inorganic perovskite to explore fundamental quantum phenomena such as Bose-Einstein condensation, superfluidity, and exciton-polariton networks.