Materials For Light-emitting Devices Research Articles

Identification of Semiconductor Nanocrystals with Bright Ground-State Excitons.

While semiconductor nanocrystals provide versatile fluorescent materials for light-emitting devices, their brightness suffers from the "dark exciton"─an optically inactive electronic state into which nanocrystals relax before emitting. Recently, a theoretical mechanism, the Rashba effect, was discovered that can overcome this limitation by inverting the lowest-lying levels and creating a bright excitonic ground state. However, no methodology is available to systematically identify materials that exhibit this inversion, hindering the development of superbright nanocrystals and their devices. Here, based on a detailed understanding of the Rashba mechanism, we demonstrate a procedure that reveals previously unknown "bright-exciton" nanocrystals. We first define physical criteria to reduce over 500,000 known solids to 173 targets. Higher-level first-principles calculations then refine this list to 28 candidates. From these, we select five with high oscillator strength and develop effective-mass models to determine the nature of their lowest excitonic state. We confirm that four of the five solids yield bright ground-state excitons in nanocrystals. Thus, our results provide a badly needed roadmap for experimental investigation of bright-exciton nanomaterials.

Revealing defect-bound excitons in WS2 monolayer at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure

Abstract Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS2 monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS2 monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si3N4 layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS2 monolayer can be modified by the Si3N4/Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS2 monolayer attached on the Si3N4/Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers.

Improved exciton photoluminescence of Zn-doped quasi-2D perovskite nanocrystals and their application as luminescent materials in light-emitting devices

In this paper, we report Zn-doped quasi-two-dimensional (Q-2D) perovskite nanocrystals (NCs).

On the Determination of Elastic Properties of Indium Nitride Nanosheets and Nanotubes by Numerical Simulation

Among the semiconductors formed by a 13th group element and nitrogen, indium nitride (InN) has promising electronic and optical properties, which make it an appropriate material for light-emitting devices and high-speed electronic applications. One-dimensional and two-dimensional InN structures, such as nanotubes and nanosheets, respectively, are expected to present novel advanced characteristics different from those of bulk InN, bringing new prospects in the designs of electronic and optical nanodevices. Despite the difficulties in the synthesis and mass production of the indium nitride nanotubes and nanosheets, the understanding of their properties, including mechanical ones, deserves more research attention, taking into account future perspectives. In this context, the present work is an exploratory study on the numerical evaluation of elastic properties of InN nanosheets and nanotubes, using the nanoscale continuum modelling (also called molecular structural mechanics) approach. The results obtained constitute a solid base for further investigation on the mechanical behaviour of the InN nanostructures, where studies are at an early stage or almost absent.

(Invited) Synthesis and Characterizations of Highly Fluorescent Nanographene Molecules

Large polycyclic aromatic hydrocarbons (PAHs) correspond to nanostructures of graphene and have attracted increasing research attentions as nanographene molecules during the last decade. In contrast to the zero-bandgap graphene, nanographene molecules have open energy gaps that are dependent on their size, symmetry, and edge structure. Various nanographene molecules have been synthesized, demonstrating intriguing optical, electronic, and magnetic properties, which render them promising for a wide range of applications from optoelectronic and energy devices to spintronics and bioimaging. In particular, nanographene molecules have a high potential as fluorescent materials for light-emitting devices and bioimaging. However, large nanographene molecules with smaller optical energy gaps typically show low solubility due to the strong aggregation, and stable and processable nanographenes with red or near infrared (NIR) emission have been limited. We have synthesized dibenzo[hi,st]ovalene (DBOV) as a nanographene molecule with a combination of armchair and zigzag edges, which demonstrated strong red emission and stimulated emission [1]. DBOV derivatives with mesityl groups and/or other substituents showed good solubility in common organic solvents, allowing for facile processing. We have subsequently established regioselective bromination of DBOV, enabling the edge-functionalization to install various functional groups for the fine-tuning of the optical and electronic properties [2]. For example, fluoranthene imide groups were attached to DBOV, which led to red-shifted emission with larger Stokes shift [3]. Moreover, π-extension of DBOV was achieved through Pd-catalyzed alkyne benzannulation, affording circumpyrene, or a nanographene molecule with multiple zigzag edges [4]. Nitrogen-doped DBOV was also synthesized, displaying acid- and metal-sensitive fluorescence and potential for fluorescent imaging and sensing applications [5]. We have more recently also synthesized a wider variety of functionalized DBOVs as well as novel nanographene molecules with red to NIR emission, providing further insights into the structure-property relationship of such graphene nanostructures.

Lanthanide(III)-Incorporating Polysiloxanes as Materials for Light-Emitting Devices

Lanthanide(III)-Incorporating Polysiloxanes as Materials for Light-Emitting Devices

Resonant Light Emission from Graphene/Hexagonal Boron Nitride/Graphene Tunnel Junctions

Single-layer graphene has many remarkable properties but does not lend itself as a material for light-emitting devices as a result of its lack of a band gap. This limitation can be overcome by a controlled stacking of graphene layers. Exploiting the unique Dirac cone band structure of graphene, we demonstrate twist-controlled resonant light emission from graphene/hexagonal boron nitride (h-BN)/graphene tunnel junctions. We observe light emission irrespective of the crystallographic alignment between the graphene electrodes. Nearly aligned devices exhibit pronounced resonant features in both optical and electrical characteristics that vanish rapidly for twist angles θ ≳3°. These experimental findings can be well-explained by a theoretical model in which the spectral photon emission peak is attributed to photon-assisted momentum conserving electron tunneling. The resonant peak in our aligned devices can be spectrally tuned within the near-infrared range by over 0.2 eV, making graphene/h-BN/graphene tunnel junctions potential candidates for on-chip optoelectronics.

Zinc ions doped cesium lead bromide perovskite nanocrystals with enhanced efficiency and stability for white light-emitting diodes

All-inorganic cesium lead halide perovskite nanocrystals (NCs) are considered as an excellent candidate material for light-emitting devices (LED) displays because of their great photo-physical properties. However, the efficiency and stability of these materials are still unsatisfactory, which is the main disadvantage hindering the commercialization of the perovskite NCs based LED displays. On the other hand, the poisonous element lead (Pb) restricted the large-scale application of the perovskite NCs. Here we reported a hot-injection method by doping zinc ions into the CsPbBr3 NCs with enhanced photoluminescence (PL) properties and stability in ambient air. The doped NCs exhibit the highest photoluminescence quantum yield (PLQY) of 91.3% and a narrow full width at half-maximum (FWHM) of 15.5 nm. The improved the optical properties and stability of the doped NCs may result from the enhanced formation energies of perovskite lattices and the surface passivation. Finally, a white light-emitting diode (WLED) was fabricated by combining the green-emitting CsPbBr3:Zn2+ doped NCs and red-emitting K2SiF6:Mn6+ phosphors along with a blue LED chip, which exhibits a luminous efficiency of 36 lm/W, a chromaticity coordinate of (0.327, 0.336), a color temperature (CCT) of 5760 K and a wide color gamut (137% of the National Television System Committee).

High Efficiency (41.85) of Br Perovskites base solar cells with ZnO and TiO2 comparable study as ETM

Due to its ease of processing, low production costs, superb light-harvesting characteristics, and high efficiency, organic-inorganic perovskite solar cells have attracted great attention in the photovoltaic research community in recent years, making it more preferable than other existing solar cell materials. Lead-based perovskites (CH3NH3PbX3, X=Cl, I, Br) solar cells have recently achieved high efficiency of ~19.3 percent, well exceeding most thin-film and organic solar cells’ efficiencies despite its potential for photovoltaic applications, organometal halide perovskites have attracted considerable attention recently and are also considered as promising materials in light-emitting devices. In particular, photovoltaic devices with efficiencies above 20 percent have been prepared using organometal halide perovskites as absorbent materials in the last few years A planar. The standard design for a planar hetero-junction perovskite-based solar cell is: Back electrode/Hole Transport Material (HTM)/Perovskite absorber / Electron Transport Material (ETM) / Transparent electrode. The hole mobility and acceptor concentration of the HTM, interface trap density, and work-function of back contact metal have shown a significant influence on the device performance. Also with these good merits.In this research, two types of ETM (TiO2and ZnO) were used with the Perovskite CH3NH3PbBr3and was found that the use of TiO2 with perovskite achieved high efficiency of 41.85% while the use of ZnO with perovskites we obtained an efficiency 17.67%. Method/Analysis: Solar cell architecture assessment is conducted using Solar Cell Power Simulator(SCAPS). This is a computer-based software tool and is well equipped to research photovoltaic structures with barriers to homo and heterojunctions, multi-junctions, and Schottky. This model optimizes various parameters such as thickness, absorber layer density, Electron Transport Material (ETM),Density of accepter and donor (ND and NA), and Hole Transport Material (HTM) doping concentrations. Achievement and simulates electrons and holes based on the Poisson’s and continuity equation The effected thickness of CH3 NH3 PbBr3 different from 0.05μm to 1.2μm and the finest results are observed at 0.05 μm.

Enhancing the efficiency of green perovskite-QDs-based light-emitting devices by controlling interfacial defects with diamine molecules

Inorganic perovskite quantum dots (QDs) have emerged as promising materials for light-emitting devices by merits of their high photoluminescence quantum yield (PLQY), narrow full width at half-maximum and tunable emission spectrum. However, fluorescence quenching and trap-mediated nonradiative recombination between hole transport layer (HTL) and QD films are prejudicial to the performance of perovskite-QDs-based light-emitting devices (QLEDs). Here, HTL was modified with diamine molecules (1,4-butanediamine (4-DA), 1,8-octyldiamine (8-DA) and 1,11-aminoundecylamine (11-DA)) through a simple self-assembly method. QD films based on modified poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine] (Poly-TPD) exhibited higher PL intensity and lower fluorescence quenching properties. Meanwhile, diamine molecules do favor to the flatness of Poly-TPD film and contribute to the smoothness of QD films. Moreover, the exciton quenching and the density of defects at the interface between HTL and QD films were effectively reduced. Particularly, QLEDs based on HTL modified with 8-DA was endowed with a maximum luminance of 78634 cd/m2, a peak current efficiency of 30.77 cd/A, a maximum external quantum efficiency of 8.90% and T50 of 16.8 min when the initial brightness is about 1000 cd/m2.

Exciton photoluminescence of CsPbBr3@SiO2 quantum dots and its application as a phosphor material in light-emitting devices

In this report, we mainly investigate the optical property differences between CsPbBr3@SiO2 quantum dots (QDs) and CsPbBr3 QDs. The photoluminescence demonstrates that CsPbBr3@SiO2 QDs and CsPbBr3 QDs have similar exciton binding energy. Both CsPbBr3 and CsPbBr3@SiO2 QDs present optical bandgaps and photoluminescence (PL) linewidth broadening as the temperature increases from 10 K to room temperature, which is attributed to the thermal expansion and electron-phonon coupling. The fitting results show that CsPbBr3 and CsPbBr3@SiO2 QDs have the similar bandgap thermal expansion coefficient, but the CsPbBr3@SiO2 QDs have weaker electron-phonon interaction. Temperature-dependent time-resolved photoluminescence (TRPL) demonstrates that the PL lifetime increases with the temperature and CsPbBr3@SiO2 QDs have longer PL lifetime than CsPbBr3 QDs after 110 K. In addition, the CsPbBr3@SiO2 QDs integrated on the blue light-emitting diode chip as green phosphor material show better thermal stability in ambient air.

Exciton, Biexciton, and Hot Exciton Dynamics in CsPbBr3 Colloidal Nanoplatelets.

Lead halide perovskites have emerged as promising materials for light-emitting devices. Here, we report the preparation of colloidal CsPbBr3 nanoplatelets (3 × 4 × 23 nm3) experiencing a strong quasi-one-dimensional quantum confinement. Ultrafast transient absorption and broadband fluorescence up-conversion spectroscopies were employed to scrutinize the carrier and quasiparticle dynamics and to obtain a full description of the spectroscopic properties of the material. An exciton binding energy of 350 meV, an absorption cross section at 3.2 eV of 5.0 ± 0.3 × 10-15 cm-2, an efficient biexciton Auger recombination lifetime of 9 ± 1 ps, and a biexciton binding energy of 74 ± 4 meV were determined. Moreover, a short-lived emission from hot excitons was observed, which is related to the formation of band-edge excitons. The time constant of both processes is 300 ± 50 fs. These results show that CsPbBr3 nanoplatelets are indeed quite promising for light-emitting technological applications.

Fluorescent Dye-Based Metal–Organic Framework Piezochromic and Multicolor-Emitting Two-Dimensional Materials for Light-Emitting Devices

The dye encapsulated metal–organic frameworks (MOFs) are of interest for their practical applications such as light-emitting diodes, temperature sensing, and microlasers. However, very few MOFs have been reported to date where the dye moiety is utilized as one of the components of the framework. Fluorescein dye (FSD) doped materials have an ability to emit various colors such as red, green, and yellow depending upon their tautomeric form and external stimuli. In anticipation of light-emitting devices, three FSD-based novel MOFs, namely, {[Co2(FSD)2(H2O)4]·H2O·MeOH}n, FSDCo; {[Mn2(FSD)2(H2O)4]·H2O·MeOH}n, FSDMn; and {[Cd2(FSD)2(H2O)3(MeOH)]·2(H2O)}n, FSDCd, have been designed and synthesized. These crystalline materials were shown to exhibit multicolor light-emitting behavior and reversible green to brown piezochromism, implying their potential to act as fluorescent switches, mechanosensors, and security papers. Further, these crystalline MOFs were also shown to exhibit a rare optical phenomenon called pleochroism. The green colored crystals displayed multiple colors under polarized light due to the agglomeration of nano- and microcrystals at different angles on the surface of macrocrystals.

Halogen-Bond-Assisted Photoluminescence Modulation in Carbazole-Based Emitter

Halogen bonding between a carbazole-based, pyridine-substituted organic semiconductor and a common halogen-bond donor (pentafluoroiodobenzene) yields efficient halogen-bond-driven fluorescence modulation in solution. Steady-state, time-resolved emission and absorption spectroscopy as well as density functional theory studies demonstrate that the fluorescence modulation arises from halogen-bond-induced intramolecular charge transfer. Fluorescence modulation offers a range of possibilities both in solution and in the solid state, for instance providing a potential pathway for the design of tunable luminescent materials for light-emitting devices.

Colloidal nanocrystals for quality lighting and displays: milestones and recent developments

AbstractRecent advances in colloidal synthesis of nanocrystals have enabled high-quality high-efficiency light-emitting diodes, displays with significantly broader color gamut, and optically-pumped lasers spanning the whole visible regime. Here we review these colloidal platforms covering the milestone studies together with recent developments. In the review, we focus on the devices made of colloidal quantum dots (nanocrystals), colloidal quantum rods (nanorods), and colloidal quantum wells (nanoplatelets) as well as those of solution processed perovskites and phosphor nanocrystals. The review starts with an introduction to colloidal nanocrystal photonics emphasizing the importance of colloidal materials for light-emitting devices. Subsequently,we continue with the summary of important reports on light-emitting diodes, in which colloids are used as the color converters and then as the emissive layers in electroluminescent devices. Also,we review the developments in color enrichment and electroluminescent displays. Next, we present a summary of important reports on the lasing of colloidal semiconductors. Finally, we summarize and conclude the review presenting a future outlook.

Highly Luminescent Colloidal CdS Quantum Dots with Efficient Near-Infrared Electroluminescence in Light-Emitting Diodes

Quantum dots are of growing interest as emissive materials in light-emitting devices. Here first we report the formation of highly luminescent organic capped colloidal cadmium sulfide (CdS) nanoparticles having the highest photoluminescence quantum yield of 69% in solutions and 34% in neat thin films in the near-infrared range. Second, we also show efficient electroluminescence in the near-infrared from solution processed hybrid light emitting diodes (LEDs) based on such colloidal CdS quantum dots embedded in an organic semiconductor matrix forming a nanocomposite active layer. We also discuss the device structure and role of the doped active layer in efficiency improvement. With optimized active layer thickness and concentration of QDs, the device exhibits an external electroluminescence quantum efficiency of 0.62% at a peak emission wavelength of 760 nm, providing a route to solution processable flexible light sources for biosensors and medicine.

Organic-inorganic and all-inorganic lead halide nanoparticles [Invited

Organic-inorganic (hybrid) and all-inorganic lead halide perovskites, in particular APbX(3) where A is an organic cation (methylammonium or formamidinium) or cesium cation and X = Cl, Br, I, respectively, are of great interest in photovoltaic devices and as luminescent materials for light-emitting devices. It has recently been demonstrated that they can be prepared not only as nanoparticulate material by using the pores of mesoporous films, but also as colloidal nanoparticles, which exhibit enhanced optical properties with respect to the bulk material. We summarize here the methods reported for their preparation as well as their optical features. Experimental and theoretical studies on this class of materials are ongoing and there is still a demand for enhancing their emissive properties, stability in polar solvents, dispersibility in different media and/or photostability.

Thermally Activated Delayed Fluorescence in Polymers: A New Route toward Highly Efficient Solution Processable OLEDs.

Efficient intermonomer thermally activated delayed fluorescence is demonstrated for the first time, opening a new route to achieving high-efficiency solution processable polymer light-emitting device materials. External quantum efficiency (EQE) of up to 10% is achieved in a simple fully solution-processed device structure, and routes for further EQE improvement identified.

1,2-dihydrophosphete: a platform for the molecular engineering of electroluminescent phosphorus materials for light-emitting devices.

The discovery and molecular engineering of novel electroluminescent materials is still a challenge in optoelectronics. In this work, the development of new π-conjugated oligomers incorporating a dihydrophosphete skeleton is reported. Variation of the substitution pattern of 1,2-dihydrophosphete derivatives and chemical modification of their P atoms afford thermally stable derivatives, which are suitable emitters to construct organic light-emitting diodes (OLEDs). The optical and electrochemical properties of these new P-based oligomers have been investigated in detail and are supported by DFT calculations. The OLED devices exhibit good performance and current-independent CIE coordinates.

All-Solution-Processed Inverted Quantum-Dot Light-Emitting Diodes

Quantum dots are a promising new candidate for the emissive material in light-emitting devices for display applications. The fabrication of such devices by solution processing allows considerable cost reduction and is therefore very attractive for industrial manufacturers. We report all solution-processed colloidal quantum-dot light-emitting diodes (QLEDs) with an inverted structure. The red, green, and blue devices showed maximum luminances of 12 510, 32 370, and 249 cd/m(2) and turn-on voltages of 2.8, 3.6, and 3.6 V, respectively. We investigate the effect of a surfactant addition in the hole injection layer (HIL), with the aim of facilitating layer deposition and thereby enhancing device performance. We demonstrate that in the device structure presented in this study, a small amount of surfactant in the HIL can significantly improve the performance of the QLED.