Electroluminescence Spectra Research Articles

Through-Space Conjugated Molecule with Dual Delayed Fluorescence and Room-Temperature Phosphorescence for High-Performance OLEDs

Purely organic materials with room-temperature phosphorescence usually have much longer triplet state lifetimes than noble-metal containing phosphorescent materials, which lead to severe exciton quenching and inferior electroluminescence (EL) performance when applied in organic light-emitting diodes (OLEDs). Herein, a novel through-space conjugated molecule (2,3-PICz-XT) with dual delayed fluorescence and room-temperature phosphorescence is developed. Owing to suitable energy levels and strong spin–orbit coupling among excited states, 2,3-PICz-XT has short phosphorescence lifetimes at microsecond scale and high photoluminescence quantum yields in neat and doped films. 2,3-PICz-XT can function as efficient emitters in OLEDs, providing high external quantum efficiencies (ηexts) of up to 32.73% with small efficiency roll-offs. It can also efficiently sensitize various phosphorescent and thermally activated delayed fluorescence (TADF) materials to yield excellent EL performances. Impressively, the hyperfluorescence OLEDs using 2,3-PICz-XT as sensitizer for multiple resonance TADF materials attain narrow EL spectra and greatly improved ηexts over 35%. To the best of our knowledge, 2,3-PICz-XT is the most efficient emitter and sensitizer ever reported for dual delayed fluorescence and phosphorescence materials.

Long-Wavelength Luminescence of InSb Quantum Dots in Type II Broken-Gap Heterostructure

The features of the electroluminescence spectra of narrow-gap type II InAs/InSb/InAs heterostructures containing a single layer of InSb quantum dots placed into the p-n-InAs junction were studied. The luminescent properties of the heterostructures under a forward and reverse bias in the temperature range of 77–300 K were investigated as a function of the surface density of nano-objects buried in the narrow-gap matrix. When applying the reverse bias to the heterostructures under study, the suppression of negative interband luminescence and the dominance of interface recombination transitions at the InSb/InAs type II heterojunction were observed at room temperature. The radiation, which corresponded to recombination transitions involving localized electron-hole states of the InSb quantum dots, was revealed and recorded at low temperatures.

Spacer cation engineering in Ruddlesden-Popper perovskites for efficient red light-emitting diodes with recommendation 2020 color coordinates

Ruddlesden-Popper perovskites (RPPs) have been demonstrated as a very promising approach for tuning the emission color of perovskite light-emitting diodes (PeLEDs). However, achieving high-performance red PeLEDs with recommendation 2020 color coordinates is still challenging due to the lack of reasonable control over the properties of RPP films. Here, we demonstrate that the judicious selection of spacer cations in RPPs affords a lever for engineering their film properties, including phase distribution, energy funneling process, trap density, and carrier mobility. Four structurally related spacer cations, benzylammonium (BZA), phenylethylammonium (PEA), 3-phenyl-1-propylammonium (PPA), and phenoxyethylammonium (POEA), are studied. Owing to narrow phase distribution, efficient energy funneling, and low trap density, the POEA-based RPP films enable efficient red PeLEDs with a peak external quantum efficiency of 10.3%, a maximum brightness of 1052 cd m−2, and excellent spectral stability. Significantly, the electroluminescence spectrum represents CIE 1931 color coordinates of (0.71, 0.29), which meets the recommendation 2020 standard (0.708, 0.292). The findings provide useful guidelines for the rational design of new organic spacer cations for RPPs with high performance.

A Deep‐Blue‐Emitting Heteroatom‐Doped MR‐TADF Nonacene for High‐Performance Organic Light‐Emitting Diodes**

AbstractWe present a p‐ and n‐doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder‐type multiresonant thermally activated delayed fluorescence (MR‐TADF) emitter. This compound shows efficient narrow deep blue emission, with a λPL of 410 nm, full width at half maximum, FWHM, of 38 nm, photoluminescence quantum yield, ΦPL of 71 %, and a delayed lifetime, τd of 1.18 ms in 1.5 wt % TSPO1 thin film. The organic light‐emitting diode (OLED) using this compound as the emitter shows a comparable electroluminescence spectrum peaked at 409 nm (FWHM=37 nm) and a maximum external quantum efficiency (EQEmax) of 8.5 % at Commission Internationale de l’Éclairage (CIE) coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2 % at 3 wt % doping of the emitter within the emissive layer of the device. At this concentration, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

A Deep-Blue-Emitting Heteroatom-Doped MR-TADF Nonacene for High-Performance Organic Light-Emitting Diodes.

We present a p- and n-doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder-type multiresonant thermally activated delayed fluorescence (MR-TADF) emitter. This compound shows efficient narrow deep blue emission, with a λPL of 410 nm, full width at half maximum, FWHM, of 38 nm, photoluminescence quantum yield, ΦPL of 71 %, and a delayed lifetime, τd of 1.18 ms in 1.5 wt % TSPO1 thin film. The organic light-emitting diode (OLED) using this compound as the emitter shows a comparable electroluminescence spectrum peaked at 409 nm (FWHM=37 nm) and a maximum external quantum efficiency (EQEmax ) of 8.5 % at Commission Internationale de l'Éclairage (CIE) coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2 % at 3 wt % doping of the emitter within the emissive layer of the device. At this concentration, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

Biometric authentication security enhancement under quantum dot light-emitting diode display via fingerprint imaging and temperature sensing

We improved biometric authentication security using dual recognition based on fingerprint image detection and skin-temperature-change sensing under quantum dot light-emitting diode (QLED) displays. QLEDs are more advantageous than organic light-emitting diodes (OLEDs) in terms of the contrast classification of patterns such as those in fingerprint recognition, owing to their narrow full-width-half-maximum. In this work, scattered, transmitted, and reflected light was captured from the top of the QLED, improving the digital luminance by 25%, as compared with that of OLEDs, because the electroluminescence spectra of the QLED were sustained, whereas those of the OLED were distorted by the generated noise peaks. A QLED with eight apertures sized up to tens of micrometers, mimicking the actual wiring structure of commercialized smartphones, was implemented to detect human fingerprints. The QLED using reduced graphene oxide as the temperature sensor detected temperature changes instantaneously upon finger touch, showing a 2% temperature response based on the human body temperature; however, the temperature change was less than 0.1% for spoof fingerprints printed on paper. Thus, this study successfully enhanced biometric authentication security, through fingerprint recognition based on image sensing using an optical system with micrometer-sized apertures and skin-temperature detection under QLED displays.

Detach GaN-Based Film to Realize a Monolithic Bifunctional Device for Both Lighting and Detection.

Due to the emerging requirements of miniaturization and multifunctionality, monolithic devices with both functions of lighting and detection are essential for next-generation optoelectronic devices. In this work, based on freestanding (In,Ga)N films, we demonstrate a monolithic device with two functions of lighting and self-powered detection successfully. The freestanding (In,Ga)N film is detached from the epitaxial silicon (Si) substrate by a cost-effective and fast method of electrochemical etching. Due to the stress release and the lightening of the quantum-confined Stark effect (QCSE), the wavelength blueshift of electroluminescent (EL) peak is very small (<1 nm) when increasing the injection current, leading to quite stable EL spectra. On the other hand, the proposed monolithic bifunctional device can have a high ultraviolet/visible reject ratio (Q = 821) for self-powered detection, leading to the excellent detection selectivity. The main reason can be attributed to the removal of Si by the lift-off process, which can limit the response to visible light. This work paves an effective way to develop new monolithic multifunctional devices for both detection and display.

Manipulating Crystallization Dynamics for Efficient and Spectrally Stable Blue Perovskite Light‐Emitting Diodes

AbstractReduced‐dimensional perovskite light‐emitting diodes (PeLEDs) have shown great potential in solution‐processed high‐definition displays. However, the inferior electroluminescent (EL) performance of blue PeLEDs has become a huge challenge for their commercialization. The inefficient domain control [number of PbX6− layers (n)] and deleterious phase segregation make the blue PeLEDs suffer from low EL efficiency and poor spectral stability. Here, a rational strategy for perovskite crystallization control by adjusting the precursor concentration is proposed for improving phase distribution and suppressing ion migration in reduced‐dimensional mixed‐halide blue perovskite films. Based on this method, efficient sky‐blue PeLEDs exhibit a maximum external quantum efficiency (EQE) of 8.5% with stable EL spectra at 482 nm. Additionally, spectrally stable pure‐blue PeLEDs at 474 and 468 nm are further obtained with maximum EQEs of 4.0% and 2.4%, respectively. These findings may provide an alternative scheme for manipulating perovskite crystallization dynamics toward efficient and stable PeLEDs.

LiF film enhanced high brightness blue electroluminescence of diamond /CeF3 composite film

Diamond/CeF3 composite film (DCCF) and diamond/CeF3/LiF composite film (DCLCF) electroluminescent(EL)devices were deposited on n-type heavy-doped silicon substrates by using microwave plasma chemical vapor deposition system (MPCVD) and vacuum electron beam vapor deposition system (EBVD). A series of corresponding tests characterized the composition, microstructure, and EL characteristics of the devices. The experimental results show that the LiF layer can effectively enhance the blue light emission intensity of the composite film device. Blue light emission can be observed in the two types of devices when the forward bias is applied, and the EL spectra of both devices show a strong wide peak at 440 nm. Meanwhile there is no significant difference in the maximum luminous intensity between them. However, when the reverse bias voltage is applied to the two devices, it is found that DCCF device hardly observes visible light emission, while the blue light emission of DCLCF device is very strong. And the EL spectrum of DCLCF device still shows a strong wide peak at 440 nm, and the blue light emission can reach the maximum at 350 V reverse bias voltage. Its maximum luminous intensity is 3.5 times that of DCCF devices, and its brightness can reach 35 cd/m2.

MOVPE grown InGaAs quantum dots with emission near 1.3 [formula omitted]m for electrically driven single-photon sources

InGaAs quantum dots (QDs) with low areal density suitable for single-photon applications are fabricated on a GaAs substrate by MOVPE. AFM measurements used to investigate the dependency of QD density and size on the V/III ratio during growth reveal a low QD density of 3.2⋅107cm−2 for a high V/III ratio of 360. To allow for a targeted electrical excitation, the InGaAs QDs are embedded into a pin-LED structure containing an oxide aperture. Low temperature electroluminescence measurements show emission of a QD line at a wavelength of 1272.4 nm. Direct comparison of photoluminescence and electroluminescence spectra approve the electrical excitation of a single QD.

Atomic layer deposition of Er-doped yttrium aluminum gallium garnet nanofilms with tunable crystallization and electroluminescence properties.

Polycrystalline erbium-doped Y3(AlxGa1-x)5O12 (Er-YAGG) nanofilms with various Al/Ga compositions are deposited on silicon using atomic layer deposition followed by annealing at different temperatures. The Al/Ga ratios and the corresponding annealing temperatures required for crystallization are confirmed by investigating the diffraction patterns and micro-morphologies. The co-alloying of Al and Ga compositions controllably changes the lattice constant and impacts the grain growth. The crystal-field splitting of doped Er3+ ions is also modified, manifesting different electroluminescence (EL) spectra that also indicate the crystallization of garnet matrices. The EL performance of a device based on the Y3Al2Ga3O12 nanofilm (1.39 at% Er dopant) annealed at 900 °C is improved due to the adjustment of morphology and microstructural perturbations that are beneficial for radiative transition. The optimal EL device exhibits a low onset voltage of ∼25 V and a maximum external quantum efficiency of 3.29%. The excitation cross-section under electrical pumping is estimated to be 1.18 × 10-15 cm2. The carrier transport of these co-alloyed Er-YAGG devices conforms to the Poole-Frenkel mechanism. Both the EL decay lifetime and the device operation time increase with the incorporation of Ga within the Er-YAGG nanofilms. These Er-YAGG devices with tunable optoelectronic properties manifest promising potential for the engineering of light sources compatible with CMOS technology.

Излучательная рекомбинация в разъединенном гетеропереходе II типа InAs/InSb с квантовыми точками на интерфейсе

The electroluminescent properties of narrow-gap type II InAs/InSb/InAs heterostructures containing a single layer of InSb quantum dots placed at the interface of the p-n junction in InAs were studied. The features of the electroluminescence spectra depending on the surface density of nanoobjects at a broken-gap type II heterointerface were investigated both at forward and reverse bias. When applying a reverse bias to the heterostructures under study, the suppression of negative interband luminescence and the dominance of interface recombination transitions at the InSb/InAs type II heterojunction were observed at room temperature. The radiation, which corresponded to recombination transitions involving localized states of InSb quantum dots, was recorded at low temperature.

InGaN-based LEDs on convex lens-shaped GaN arrays toward multiwavelength light emitters

InGaN-based LEDs on convex lens-shaped GaN microstructures are fabricated. The gently sloping microstructures enable us to employ simple device processes similar to those for conventional planar LEDs. The fabricated LEDs show a clear rectifying property and broadband electroluminescence spectra ranging from ∼380 to ∼500 nm due to the multiwavelength emission properties of the InGaN quantum wells. Furthermore, the peak wavelength continuously changes from ∼470 to ∼410 nm within a particular current range.

Current injection and luminescence properties of wurtzite InP nanowires with crystal phase transition

We characterized the current injection and electroluminescence (EL) properties of wurtzite (WZ) InP nanowire (NW) light-emitting diodes (LEDs) with axial junctions. The EL spectra of two samples with the same LED junction structure exhibited two different behaviors. One showed a single EL peak originating from the zinc-blende (ZB)-InP bandgap. The other showed two EL peaks originating from the ZB and WZ phases. This difference in EL behavior is attributed to the difference in the contact position and to depletion layer spreading. Clarification of the origin of the different EL peaks is important for optimizing the NW-LED structure.

Effects of Thermal Treatment on DC Voltage-Driven Color Conversion in Organic Light-Emitting Diode.

A DC voltage-dependent color-tunable organic light-emitting diode (CTOLED) was proposed for lighting applications. The CTOLED consists of six consecutive organic layers: the hole injection layer, the hole transport layer (HTL), two emission layers (EMLs), a hole blocking layer (HBL), and an electron transport layer (ETL). Only one metal-free phthalocyanine (H2Pc) layer with a thickness of 5 nm was employed as the EML in the CTOLED on a green organic light-emitting diode (OLED) structure using tris (8-hydroxyquinoline) aluminum (III) (Alq3). The current density-voltage-luminance characteristics of the CTOLEDs before and after thermal treatment were characterized and analyzed. Several Gaussian peaks were also extracted by multipeak fitting analysis of the electroluminescent spectra. In the CTOLED before thermal treatment, green emission was dominant in the entire voltage range from low to high voltages, and blue and infrared were emitted simultaneously and at relatively low intensities at low and high voltages, respectively. In the CTOLED after thermal treatment, the dominant color conversion from blue to green was observed as the applied voltage increased, and the infrared emission was relatively low over the entire voltage range. By simulating the CTOLED with and without traps at the H2Pc interface using a technology computer-aided design simulator, we observed the following: 1. After thermal treatment, the CTOLED emitted blue light by exciton generation at the H2Pc-HBL interface because of the small electron transport through the H2Pc thin film due to the dramatic reduction of traps in the low-voltage regime. 2. In the high-voltage regime, electrons reaching the HBL were transferred to Alq3 by resonant tunneling in two quantum wells; thus, green light was emitted by exciton generation at the HTL-Alq3 interface.

Near-Infrared LEDs Based on Quantum Cutting-Activated Electroluminescence of Ytterbium Ions.

Cesium lead halide perovskite nanocrystals (PNCs) exhibit promising prospects for application in optoelectronic devices. However, electroactivated near-infrared (NIR) PNC light-emitting diodes (LEDs) with emission peaks over 800 nm have not been achieved. Herein, we demonstrate the electroactivated NIR PNC LEDs based on Yb3+-doped CsPb(Cl1-xBrx)3 PNCs with extraordinary high NIR photoluminescence quantum yields over 170%. The fabricated NIR LEDs possess an irradiance of 584.7 μW cm-2, an EQE of 1.2%, and a turn-on voltage of 3.1 V. The ultrafast quantum cutting process from the PNC host to Yb3+ has been revealed as the main mechanism of electroluminescence (EL)-activated Yb3+ for the first time via exploring how the trend between the EL intensity of PNC and Yb3+ varies with different voltages along with the tendency of temperature- and doping-concentration-dependent PL and EL spectra. This work will extend the application of PNCs to optical communication, night-vision devices, and biomedical imaging.

Microcavities integrated in metal halide perovskite light-emitting field-effect transistors

Metal halide perovskites are materials that show unique characteristics for photovoltaics and light emission. Amplified spontaneous emission and stimulated emission has been shown with these materials, together with electroluminescence in light-emitting diodes and light-emitting transistors. An important achievement that combine stimulated emission and electroluminescence could be the fabrication of electrically driven metal halide perovskite lasers.In this work, the integration of metal halide perovskite light-emitting field-effect transistors with photonic microcavities is proposed. This can lead to the engineering of electrically driven lasers. The microcavities have been designed in order to have the cavity mode at 750 nm, which is the peak wavelength of the electroluminescent spectrum of recently reported MaPbI3-based electroluminescent devices. The optical properties of the photonic microcavities have been simulated by means of the transfer matrix method, considering the wavelength dependent refractive indexes of all the materials involved. The material for the gate is indium tin oxide, while different materials, either inorganic or organic, have been considered for the microcavity architectures.

A Study on Varying the Number of Fluorene Units in 2, 1, 3‐Benzothiadiazole‐Containing Oligomers and the Effect on OLED Performance and Stability

AbstractSolution‐processing has the potential to reduce the cost and energy requirements in the fabrication of organic light‐emitting diode (OLED) devices. In non‐doped emissive layers, polymers are commonly used but often suffer from batch‐to‐batch variation. Oligomers, which have precise molecular structures, avoid this problem but maintain the favourable film‐forming properties that polymers possess. We present three benzothiadiazole‐containing fluorene‐based oligomers, BT‐F32, BT‐F42 and BT‐F52, which have terfluorene, quaterfluorene and pentafluorene arms, respectively, attached to a benzothiadiazole core. The materials show very high photoluminescence quantum yields, with BT‐F42 exhibiting the highest PLQY of 0.96 in the thin film. Although there is little change in the electroluminescence spectra, the oligomer size influences device performance. BT‐F32 had an external quantum efficiency of 1.9 % and high luminance whilst BT‐F42 reached an external quantum efficiency of 2.3 %. Furthermore, the stability of the OLED devices was evaluated and showed that the device lifetime decays with increasing molecular length.

Unique Role of Hot-Electron Induced Self-Heating in Determining Gate-Stack Dependent Dynamic R ON of AlGaN/GaN HEMTs Under Semi-on State

In this work, we report a unique dependence of dynamic RON on gate-stack of AlGaN/GaN high electron mobility transistors (HEMTs) on carbon (C)-doped GaN buffer under semi-on condition. Unlike the OFF-state stress, semi- ON state stressing of the devices revealed a significantly lower dynamic RON in the Schottky HEMTs when compared to SiNx-gated HEMTs. Detailed experiments including substrate bias dependence, electro-luminescence (EL) spectrum, and electric field analysis established electron trapping in the GaN buffer to be the source of dynamic RON in both the HEMT types. An on-the-fly thermal analysis, using thermoreflectance technique, revealed Schottky HEMTs to have a higher hot electron-induced self-heating near the field plate (FP) edge as compared to SiNx-gated devices. The higher lattice temperature was found to result in a significantly higher electron de-trapping from the buffer traps. The higher de-trapping reduced the dynamic RON in the Schottky HEMTs as compared the SiNx-gated devices.

Effect of extra added Mg2+ and Si4+ on the microstructure and luminescence properties of Ce:YAG ceramic phosphors for high power LED/LD lighting

The study deals with the use of different amounts of MgO or tetraethyl orthosilicate (C8H20O4Si, TEOS), from 0.04 wt %, 0.06 wt %, 0.08 wt %, 0.4 wt %, 0.6 wt % to 0.8 wt %, respectively, as additional sintering aids during the vacuum sintering of transparent Ce3+ doped Y3Al5O12 (Ce:YAG) ceramics to investigate their effect on microstructure and luminescence properties. A comparative study was carried out by using X - ray diffraction, scanning electron microscopy/energy dispersive X - ray spectroscopy, optical and photo-luminesce spectra, as well as electroluminescent spectra. The dominant garnet phase was obtained even when adding 0.8 wt % of MgO or TEOS, while adding MgO led to finer average grain size (∼ 5 - 10 μm) and more homogenous distribution compared to TEOS (∼ 20 - 25 μm) in Ce:YAG ceramics. Adding a large amount of MgO was found to result in the segregation of MgO in Ce:YAG ceramics, while an excess of TEOS led to the segregation of Al2O3. A strong absorption peak at 308 nm was observed in Ce:YAG ceramics with MgO sintering aids, which was ascribed to the existence of Ce4+ induced by a charge compensation effect of Mg2+. The optimum transmittance reached 80% @ 800 nm in Ce:YAG ceramics when adding 0.6 wt % TEOS, which also exhibited a maximum luminous efficacy of 106 lm/W. Si4+ was experimentally proved to have a better optimization effect on luminous efficacy compared to Mg2+.