Microwave‐Assisted Preparation of White Fluorescent Graphene Quantum Dots as a Novel Phosphor for Enhanced White‐Light‐Emitting Diodes

Graphene quantum dots (GQDs) with white fluorescence are synthesized by a microwave‐assisted hydrothermal method using graphite as the precursor. A solution‐processed white‐light‐emitting diode (WLED) is fabricated using the as‐prepared white fluorescent GQDs (white‐light‐emitting graphene quantum dots, WGQDs) doped 4,4‐bis(carbazol‐9‐yl)biphenyl as the emissive layer. White‐light emission is obtained from the WLED with 10 wt% doping concentration of WGQDs, which shows a luminance of 200 cd m^?2 at the applied voltage of 11–14 V. Importantly, an external quantum efficiency of 0.2% is achieved, which is the highest among all the reported WLED based on GQDs or carbon dots. The results demonstrate that WGQDs as a novel phosphor may open up a new avenue to develop the environmentally friendly WLEDs for practical application in solid‐state lighting.     

Construction of White‐Light‐Emitting Silk Protein Hybrid Films by Molecular Recognized Assembly among Hierarchical Structures

The fabrication of bio‐hybrid functional films is demonstrated by applying a materials assembly technique. Based on the hierarchical structures of silk fibroin materials, functional molecular/materials, i.e., quantum dots (QDs), can be fixed to amino acid groups in silk fibroin films. It follows that white‐light‐emitting QD silk hybrid films are obtained by hydrogen bond molecular recognition to the –COO^– groups functionalized to blue luminescent ZnSe (5.2 nm) and yellow luminescent CdTe (4.1 nm) QDs in a molar ratio of 30:1 of ZnSe to CdTe QDs. Simultaneously, a systematic blue shift in the emission peak is observed from the QD solution to QDs silk fibroin films. The significant blue shift hints the appearance of the strong interaction between QDs and silk fibroins, which causes strong white‐light‐emitting uniform silk films. The molecular recognized interactions are confirmed by high resolution transmission electron microscopy, field scanning electron microscope, and attenuated total internal reflectance Fourier transform infrared spectroscopy. The QD silk films show unique advantages, including simple preparation, tunable white‐light emission, easy manipulation, and low fabrication costs, which make it a promising candidate for multicomponent optodevices.     

New Ternary Europium Aluminate Luminescent Nanoribbons for Advanced Photonics

Developing novel one‐dimensional (1D) luminescent nanostructures (e.g., nanowires and nanoribbons) is highly desired for enabling progress in nanophotonics and other emerging optical technologies. Previous studies on 1D luminescent nanostructures were mostly focused on elemental and binary semiconductor materials, the light emission of which originates from the radiative recombination of electrons and holes via either intrinsic states or extrinsic defect states. Herein, three kinds of ternary europium aluminate nanoribbons are reported that have localized Eu²⁺ luminescent centers and exhibit new compositions, new crystal lattice structures, and new luminescence properties and mechanisms. These three europium aluminate nanoribbons are: blue luminescent EuAl₆O₁₀ with a new composition and a new tetragonal lattice structure, green luminescent EuAl₂O₄ with a monoclinic lattice structure, and orange luminescent EuAl₂O₄ with a new hexagonal lattice structure and extremely large band width and Stokes shift of emission. These materials have promising applications as nanometer‐scale light generators and waveguides in nanophotonics and as light converting phosphors in warm white light‐emitting diodes.     

Resonant‐Enhanced Full‐Color Emission of Quantum‐Dot‐Based Display Technology Using a Pulsed Spray Method

Colloidal quantum‐dot light‐emitting diodes (QDLEDs) with the HfO₂/SiO₂‐distributed Bragg reflector (DBR) structure are fabricated using a pulsed spray coating method. Pixelated RGB arrays, 2‐in. wafer‐scale white light emission, and an integrated small footprint white light device are demonstrated. The experimental results show that the intensity of red, green, and blue (RGB) emission exhibited considerable enhancement because of the high reflectivity in the UV region by the DBR structure, which subsequently increases the use in the UV optical pumping of RGB QDs. A pulsed spray coating method is crucial in providing uniform RGB layers, and the polydimethylsiloxane (PDMS) film is used as the interface layer between each RGB color to avoid cross‐contamination and self‐assembly of QDs. Furthermore, the chromaticity coordinates of QDLEDs with the DBR structure remain constant under various pumping powers in the large area sample, whereas a larger shift toward high color temperatures is observed in the integrated device. The resulting color gamut of the proposed QDLEDs covers an area 1.2 times larger than that of the NTSC standard, which is favorable for the next generation of high‐quality display technology.     

White‐Light‐Emitting Diodes Based on Iridium Complexes via Efficient Energy Transfer from a Conjugated Polymer

Efficient white‐light‐emitting diodes (WLEDs) have been developed using a polyfluorene‐type blue‐emitting conjugated polymer doped with green and red phosphorescent dyes. The emission spectrum of the conjugated polymer, which has a very high luminescent efficiency, shows a large spectral overlap with the absorbance of green and red iridium complexes. Also, efficient energy transfer from the conjugated polymer to the iridium complexes is observed. Poly(N‐vinyl carbazole) is used to improve the miscibility between conjugated polymer and iridium complexes because of their poor chemical compatibility and phase separation. A white emission spectrum is easily obtained by varying the contents of the three materials and controlling the phase morphology. Moreover, these WLEDs show a voltage‐independent electroluminescence owing to the threshold and driving voltage of the three materials being similar as a result of energy transfer.     

Band‐Tail Recombination in Hybrid Lead Iodide Perovskite

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low‐temperature photoluminescence of formamidinium lead triiodide (HC(NH₂)₂PbI₃) is investigated. A power‐law time dependence in the emission intensity and an additional low‐energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate‐equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap‐density tail with characteristic energy scale of ≈3 meV. Charge‐carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open‐circuit voltage losses commonly observed in these materials. In this high‐quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band‐structure picture is moderated by a modest degree of energetic disorder.     

Porous NIR Photoluminescent Silicon Nanocrystals‐POSS Composites

Near infrared photoluminescent porous silicon nanocrystals(ncSi)‐polyhedral oligomeric silsesquioxanes (POSS) polymer composites are synthesized using a combination of thermal hydrosilylation and polymerization between Vinyl‐POSS and hydrogen‐terminated silicon nanocrystals (ncSi:H). The synthesized materials are characterized by IR, powder X‐ray diffraction and solid‐state nuclear magnetic resonance (NMR) (¹³C and ²⁹Si). The results demonstrate that the hydrosilylation–polymerization reaction proceeded to create chemically crosslinked Vinyl‐POSS‐ncSi composites in which the integrity of the POSS cages is maintained intact. Scanning electron microscope (SEM) results demonstrate that morphology of these materials depends on the weight ratio of ncSi:H to Vinyl‐POSS. Brunauer–Emmett–Teller surface area analyses establish that the composites have high surface areas ranging from 290.5 to 1047.2 m² g^?1 and pore volumes from 0.64 to 1.17 cm³ g^?1. The pore sizes range from 6.08 to 3.54 nm and are dependent on the weight ratio of Vinyl‐POSS to ncSi:H. Photoluminescence spectroscopy shows that the absolute quantum yield of the nanocomposites is not affected by the weight ratio of ncSi:H to Vinyl‐POSS. Thermal gravimetric analysis results show that the POSS polymer composites with ncSi have lower thermal stability in nitrogen atmosphere as compared with the pure Vinyl‐POSS polymer. It is envisioned that future applications for these composites will likely be found in the fields of advanced materials, gas adsorption media, and biomedicine.     

Evidence for the Band‐Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large‐Area Luminescent Solar Concentrators

Ternary I‐III‐VI₂ nanocrystals (NCs), such as CuInS₂, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS₂ NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS₂ NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm²) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices.     

A Systematic Approach to Achieving High Performance Hybrid Lighting Phosphors with Excellent Thermal‐ and Photostability

The authors have designed and synthesized a family of high‐performance inorganic–organic hybrid phosphor materials composed of extended and robust networks of one, two, and three dimensions. Following a bottom‐up solution‐based synthetic approach, these structures are constructed by connecting highly emissive Cu₄I₄ cubic clusters via carefully selected ligands that form strong Cu? N bonds. They emit intensive yellow‐orange light with high luminescence quantum efficiency, coupled with large Stokes shift, which greatly reduces self‐absorption. They also demonstrate exceptionally high framework‐ and photostability, comparable to those of commercial phosphors. The high stabilities are the result of significantly enhanced Cu? N bonds, as confirmed by the density functional theory (DFT) binding energy and electron density calculations. Possible emission mechanisms are analyzed based on the results of theoretical calculations and optical experiments. Two‐component white phosphors obtained by blending blue and yellow emitters reach an internal quantum yield as high as 82% and correlated color temperature as low as 2534 K. The performance level of this subfamily exceeds all other types of Cu–I based hybrid systems. The combined advantages make them excellent candidates as alternative rare‐earth element‐free phosphors for possible use in energy‐efficient lighting devices.     

Optically‐Pumped Lasing in Hybrid Organic–Inorganic Light‐Emitting Diodes

Here, the use of metal oxide layers both for charge transport and injection into an emissive semiconducting polymer and also for the control of the in‐plane waveguided optical modes in light‐emitting diodes (LEDs) is reported. The high refractive index of zinc oxide is used to confine these modes away from the absorbing electrodes, and include a nano‐imprinted grating in the polymer layer to introduce distributed feedback and enhance optical out‐coupling. These structures show a large increase in the luminescence efficiency over conventional devices, with photoluminescence efficiency increased by up to 45%. Furthermore, optically‐pumped lasing in hybrid oxide polymer LEDs is demonstrated. A tuneable lasing emission is also obtained in a single device structure by employing a graduated thickness of a zinc oxide inter‐layer. This demonstrates the scope for using such architectures to improve the external efficiency of organic semiconductor LEDs, and opens new possibilities for the realization of polymer injection lasers.     

Ultrathin Highly Luminescent Two‐Monolayer Colloidal CdSe Nanoplatelets

Surface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two‐monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes‐shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof‐of‐concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm W_opt^?1. The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 10⁶ cm^?1) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 10⁵ cm^?1) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next‐generation light‐emitting and light‐harvesting applications.     

Polymer-Encapsulated Halide Perovskite Color Converters to Overcome Blue Overshoot and Cyan Gap of White Light-Emitting Diodes

Currently, the most popular way to manufacture white light-emitting diodes (WLEDs) is based on blue-emitting InGaN LED chips (440–460 nm) and yellow-emitting phosphor coating (520–700 nm) to produce white light lighting. However, because conventional white WLEDs lack the uniformly distributed continuous emission spectrum compared to natural sunlight: “blue overshoot” (extra 440–460 nm blue-light can cause damage to the retina) and “cyan gap” (470–520 nm wavelength range). Here, a novel strategy to “kill two birds with one stone” is reported: using the stable and bright polymer encapsulated perovskite nanocrystals (PNCs) composite films as the cyan color converters that efficiently absorb the “blue overshoot” and effectively emit the cyan light to fill the “cyan gap”. A series of polymer-encapsulated PNC films is achieved that can reach very high photoluminescence quantum yield (PL QY) of 90–95% under 450 nm blue-light excitation. Importantly, both 370 nm UV-excited WLED and 455 nm blue-excited WLED devices are constructed that exhibit smoothly and evenly distributed white light without blue overshoot and cyan gap, which was not achieved before for blue-excited cyan-emissive materials. This study paves the way toward the application of PNC color converters in the next generation full-visible-spectrum WLED lighting that mimic the natural daylight.     

White‐Emitting Conjugated Polymer/Inorganic Hybrid Spheres: Phenylethynyl and 2,6‐Bis(pyrazolyl)pyridine Copolymer Coordinated to Eu(tta)3

The synthesis of two cyan color (blue and green emission) displaying high molecular weight 2,6‐bis(pyrazolyl)pyridine‐co‐octylated phenylethynyl conjugated polymers (CPs) is presented. The conjugated polymers are solution‐processed to prepare spin coated thin films and self‐assembled nano/microscale spheres, exhibiting cyan color under UV. Additionally, the metal coordinating ability of the 2,6‐bis(pyrazolyl)pyridine available on the surface of the CP films and spheres is exploited to prepare red emitting Eu(III) metal ion containing conjugated polymer (MCCP) layer. The fabricated hybrid (CP/MCCP) films and spheres exhibit bright white‐light under UV exposure. The Commission Internationale de l'Eclairage (CIE) coordinates are found to be (x = 0.33, y = 0.37) for hybrid films and (x = 0.30, y = 0.35) for hybrid spheres. These values are almost close to the designated CIE coordinates for ideal white‐light color (x = 0.33, y = 0.33). This easy and efficient fabrication technique to generate white‐color displaying films and nano/microspheres signify an important method in bottom‐up nanotechnology of conjugated polymer based hybrid solid state assemblies.     

Screening Approach for the Discovery of New Hybrid Perovskites with Efficient Photoemission

Owing to quantum confinement, low‐dimensional hybrid perovskite materials have recently shown a great potential for applications in optoelectronics. Such compounds can exhibit broad‐ or narrow‐band light emission, low‐temperature solution processability, high thermal stability, and relatively high photoluminescence quantum yields (PLQY). However, the search for efficient phosphors with a specific set of characteristics remains difficult because the family of hybrid perovskites consists in an extremely large chemical system (i.e., different halides, metals, and organic molecules), and optical properties are not predictable prior to material synthesis and characterization. Here, is proposed a simple approach to screen a significant amount of new hybrid lead halide perovskites. The synthetic method by fast crystallization at low temperature enables the rapid identification of the materials exhibiting the targeted photoluminescence properties. This approach is tested for the discovery of hybrid lead halide perovskites with efficient white‐light emission. Among 100 newly synthesized compounds, 5 exhibit intense white emission, and the in‐depth characterization of a selected candidate shows high color rendering index (CRI) = 78 and a PLQY of 9%, which is equivalent to the record reported for hybrid perovskites. This compound exhibits a new structure type for warm white‐light emitting hybrid perovskites with chains of corner‐sharing PbX₆.     

The effect of annealing/quenching on the performance of polymer light-emitting electrochemical cells

We report the effect of thermal annealing and quenching on the film morphology and device performance of polymer light-emitting electrochemical cells (LECs). The polymer films of LECs consist of a luminescent polymer, an ion-conducting polymer, and a lithium salt. The LECs studied have an extremely large planar configuration, which enables time-resolved fluorescence imaging of both doping and emission profiles of the devices. Annealing at temperatures above 350 K leads to the disappearance of many visible “white dots” initially present in the LEC film, and a much smoother surface. Annealed and quenched devices exhibit dramatically improved initial and peak current, peak electroluminescence (EL) intensity, doping propagation speed and response time. In addition, the emission zone of annealed devices is more centered than un-annealed devices. These improvements are attributed to the melting of electrolyte domains in the LEC film, which leads to better film quality and enhanced ion conductivity. Our results demonstrate that the simple annealing/quenching technique can be used to achieve the desired phase morphology in LEC films, which are often severely phase-separated due to incompatibility between the luminescent polymer and the electrolyte polymer.     

Device Physics of White Polymer Light‐Emitting Diodes

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.     

Nanoporous Silver Thin Films: Multifunctional Platforms for Influencing Chain Morphology and Optical Properties of Conjugated Polymers

Disordered nanoporous silver (NPAg) thin films fabricated by a thermally assisted dewetting method are employed as a platform to influence chain alignment, morphology, and optical properties of three well‐known conjugated polymers. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurements show that the porous structure of the metal induces close π–π stacking of poly(3‐hexylthiophene) (P3HT) chains and extended, planar chain conformations of poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (PFO) and poly[(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)] (F8BT). A greater degree of vertically‐oriented P3HT chains are found on NPAg compared with planar Ag. However, PFO and F8BT chain alignment is only affected when pore size is large. The optical properties of NPAg films are investigated by transmission and back‐scattering spectroscopies. Strong back‐scattering is observed for all NPAg morphologies, especially for NPAg with small pore sizes. Photoluminescence spectroscopy of conjugated polymer layers on NPAg showed pronounced emission enhancements (up to factors of 26) relative to layers on glass. The enhancements are attributed primarily to: 1) redistribution of conjugated polymer emission by Ag; 2) redirection of emission by polymer‐filled nanopores; and 3) local electromagnetic field effects. This work demonstrates the potential of NPAg‐thin films to influence molecular chain morphology and to improve light‐extraction in organic optoelectronic devices.     

Improving spectral response of monocrystalline silicon photovoltaic modules using high efficient luminescent down‐shifting Eu3+ complexes

One way to improve the spectral response of solar cells in the ultraviolet (UV) region is to convert high energy photons into lower energy ones via luminescent down‐shifting (LDS) technique. Eu³⁺ complexes are excellent LDS species because of their high luminescence quantum efficiency and large Stokes‐shift. In this paper, we aim to optimize the LDS property of Eu³⁺ complexes for monocrystalline silicon (c‐Si) photovoltaic (PV) modules by chemical modification of the UV absorbing antenna ligands. Our results show that the LDS performances of Eu³⁺ complexes are strongly dependent on their absorption and emission properties. By carefully modifying the absorption and emission features, the LDS performances of Eu³⁺ complexes can be significantly improved. The spectroscopic features of the Eu³⁺ complex with a bispinene‐containing bipyridyl ligand match well with the requirement of ideal LDS species for the c‐Si PV module. Simple coating of polyvinyl acetate film doped with this complex onto the surface of c‐Si PV module leads to increase of the external quantum efficiency in the UV region and enhancement of the PV module efficiency η (from 16.05% to 16.37%). Copyright © 2012 John Wiley & Sons, Ltd.     

Phosphorescent Polymeric Thermometers for In Vitro and In Vivo Temperature Sensing with Minimized Background Interference

Temperature plays a crucial role in many biological processes. Accurate temperature determination is important for diagnosis and treatment of diseases. Autofluorescence is an unavoidable interference in luminescent bioimaging. Hence, a large amount of research works has been devoted to reducing background autofluorescence and improving signal‐to‐noise ratio (SNR) in biodetection. Herein, a dual‐emissive phosphorescent polymeric thermometer has been developed by incorporating two long‐lived phosphorescent iridium(III) complexes into an acrylamide‐based thermosensitive polymer. Upon increasing temperature, this polymer undergoes coil‐globule transition, which leads to a decrease in polarity of the microenvironment surrounding the iridium(III) complexes and hence brings about emission enhancement of both complexes. Owing to their different sensitivity to surrounding environment, the emission intensity ratio of the two complexes is correlated to the temperature. Thus, the polymer has been used for temperature determination in vitro and in vivo via ratiometric luminescence imaging. More importantly, by using the long‐lived phosphorescence of the polymer, temperature mapping in zebrafish has been demonstrated successfully with minimized autofluorescence interference and improved SNR via time‐resolved luminescence imaging. To the best of our knowledge, this is the first example to use photoluminescent thermometer for in vivo temperature sensing.     

Soft Nanopatterning on Light‐Emitting Inorganic–Organic Composites

In this work we demonstrate the nanopatterning of nanocomposites made by luminescent zinc oxide nanoparticles and light‐emitting conjugated polymers by means of soft molding lithography. Vertical nanofluidics is exploited to overcome the polymer transport difficulties intrinsic in materials incorporating nanocrystals, and the rheology, fluorescence, absolute quantum yield, and emission directionality of the nanostructured composites are investigated. We study the effect of patterned gratings on the directionality of light emitted from the nanocomposites, finding evidence of the enhancement of forward emitted light, due to the printed wavelength‐scale periodicity. These results open new possibilities for the realization of nanopatterned devices based on hybrid organic‐inorganic systems.