Electroluminescence Spectra Research Articles

Bias-controlled modulation for monolithic III-nitride optoelectronic integration.

III-nitride multi-quantum well (MQW) diodes can modulate the light emitted by another diode with the same MQW structure by varying the bias voltage owing to the spectral overlap between the electroluminescence spectrum and spectral responsivity curve of the MQW diodes. Here, we investigate bias-controlled modulation by monolithically integrating an optical transmitter, waveguide, electro-absorption modulator (EAM), and slot grating coupler on a silicon-based III-nitride platform using compatible fabrication processes. The modulated light is coupled into a fiber, which is direct to a photodiode for characterization. The bandwidths of forward-biased emission modulation and reverse-biased absorption modulation are of the same order of magnitude, with the latter exhibiting significant performance improvements. In addition, real-time video signal transmission was achieved using an EAM, which provides a meaningful reference for modulation applications of silicon-based GaN optoelectronic integrated systems.

Hetero-Nucleation Induced [111]-Oriented Mixed Halide Perovskite for Stable Pure Red Light-Emitting Diodes.

Mixed halide 3D perovskites are promising for bright, efficient, and wide-color gamut light-emitting diodes (LEDs) due to their excellent carrier transport, high luminescence, and easily tunable bandgaps. However, serious halide ion migration inside mixed halide 3D perovskite results in poor operational and spectral stability of the as-fabricated LEDs. Here, a hetero-nucleation crystallization strategy is reported to grow [111]-orientationpreferred mixed halide 3D perovskite CsPbI3-xBrx thin films for stable pure red LEDs. This hetero-nucleation crystallization is enabled by the addition of phosphoric acid (H3PO4) complexation, which promotes the growth of small perovskite grains into large grains with uniform [111]-orientation. The obtained [111]-orientation preferred film exhibits excellent stability under light or electric field stimulus as revealed by model analysis and experimental results compared to that of [001]-orientation preferred film. Thus, based on the [111]-orientationpreferred film, the fabricated LED exhibits an external quantum efficiency of 22.8%, a maximum brightness of 12000cdm-2, and a half-life time of 4080min under 1.5mA cm-2. More importantly, the electroluminescence spectrum of the device remains stable during the continuous operation of 4080min, showcasing the significant spectral stability improvement enabled by the hetero-nucleation induced [111]-orientation strategy.

Investigation into characteristics of blue iridium (III) pyrimidine complexes with suppressed shoulder peak emission

Three novel blue-emitting iridium (III) complexes (Ir1, Ir2 and Ir3), embedding pyrimidine group have been synthesized. Density functional theory (DFT) calculations demonstrate that pyrimidine complex series occupy less component of triplet ligand centered (3LC) could restrain the vibronic shoulders comparing to the pyridine series. As for Ir1, the relative intensity of shoulder in electroluminescence (EL) spectrum is ∼0.6 of the intensity for dominate peak, relevant full width at half maximum (FWHM) is merely 47 nm. Such parameter gradually zooms out from trifluoro-to monofluoro-substitution in EL spectra. Furthermore, three complexes could obtain efficient EL performance, in which, devices based on Ir3 achieve the peak external quantum efficiency (EQE) of 28.9 %. In addition, the photoelectric properties of the three complexes display a certain regularity and this work indeed provides a strategy to modulate shoulder peak emission for blue phosphorescence.

Structure regulation and luminescence improvement of Gd3Ga5O12:Cr3+: An anti-thermal quenching NIR phosphor for multifunctional applications

Near-infrared (NIR) phosphors converted light emitting diodes (LEDs) are booming as next NIR light sources. But the quantum efficiency and thermal stability of the NIR phosphors need to be improved. Here, thermally stable garnet Gd3Ga5O12 is selected as the matrix and Cr3+ as the luminescent center to obtain Gd3Ga5O12:Cr3+ NIR phosphor. Equivalent smaller Lu3+ was adopted to regulate the structure of Gd3Ga5O12 to further improve the luminescence performance. By regulating the structure the emission spectra shift toward blue region and the intensity is improved. Optimized GdLu2Ga5O12:Cr3+ exhibits anti-thermal quenching behavior and the internal quantum efficiency reaches 73.69 %. Relative sensitivity value of the phosphor GdLu2Ga5O12:Cr3+ reaches 3.353 % K−1 at 423 K based on lifetime mode. LED device is fabricated by combining the phosphor GdLu2Ga5O12:Cr3+ and the commercial red phosphor with blue chip. Electroluminescence spectrum can cover the absorption spectra of the plant dyes well. The lettuce grow faster by adopting this LED device as compensating light source. This work provides an anti-thermal quenching NIR phosphor for multiple applications.

Thermoelectroluminescence in Anthracene and Anthracene Doped With Acridine Phosphors

AbstractIn the present study, an effort is made to record the thermoelectroluminescence (TEL) glow curves and electroluminescence (EL) spectra for anthracene, acridine, and anthracene doped with acridine to determine the trap depth and escape frequency in these materials. For recording the TEL glow curves, the EL cell is heated at the study rate after 500 Vrms (RMS) of AC is supplied between the plates. At various field frequencies, the light output is recorded. To comprehend the luminescence processes and mechanisms in these films, three methods have been employed to determine the trap energy and escape frequency.

Performance improvement of red, green and blue InGaN micro-LEDs with distributed Bragg reflector

Abstract Red-green-blue (RGB) micro light-emitting diodes (micro-LEDs) without distributed Bragg reflector (DBR), with air-separating DBR, and with integrated DBR, were demonstrated. The effect of the DBRs as reflectors on the external quantum efficiency (EQE) and electroluminescence (EL) spectra enhancement of RGB micro-LEDs was systematically investigated for realizing higher-performance micro-LEDs for display applications. At 5 A/cm2, the EQEs of the RGB micro-LEDs with integrated DBR were improved by 38%, 33%, and 32%, respectively, with comparison to the RGB DBR free micro-LEDs. Further, the full width at half maximum (FWHM) of the red micro-LEDs was reduced by 4.3 nm at 50 A/cm2 with the integrated DBR due to the higher enhancement of the central wavelength spectrum. The green and blue micro-LEDs with integrated DBR had higher EQE and the red micro-LEDs with integrated DBR had narrower FWHM compared to those with air-separating DBR. However, the peak wavelength of the RGB micro-LEDs with integrated DBR shifted, resulting in a lower color gamut in CIE 1931. The above work provides guidance for future full-color micro-display applications based on RGB InGaN micro-LED technology.

Different Optical Behaviors Revealed by Electroluminescence and Photoluminescence of InGaN/GaN Coupled Quantum Wells.

The optical properties of wurtzite violet InGaN/GaN coupled quantum well (QW) structures are experimentally studied using photoluminescence (PL) and electroluminescence (EL) spectroscopy. Two emission peaks, referred to as Peak H and Peak L, are observed in both PL and EL spectra, due to the ground state splitting induced by the well coupling. Experimental PL and EL results reveal that coupled QWs show different optical responses due to the different variation in the electric field inside the QW structure. Since the direction of the polarization electric field of the as-grown well/barrier layers is different, the external electric field applied by electrodes can change the energy band alignment between the well and the barrier layers, thus adjusting the coupling between the wells. Our results provide relevant information to improve our understanding of the optical properties of InGaN/GaN QWs and to develop novel optoelectronic devices.

Efficient and Stable Blue Perovskite Light‐Emitting Diodes Enabled by Effective Hydrolysis of Dichloride

AbstractDespite the substantial progress in sky‐blue (480−495 nm) perovskite light‐emitting diodes (PeLEDs), the pure‐blue PeLEDs (<480 nm) merely show moderate performances. As a straightforward and effective facile way to obtain pure‐blue emission, the bromide‐chloride mixed perovskites have received considerable attention. However, the tricky issue of halide migration in the mixed halide perovskites makes a tough challenge to achieve efficient PeLEDs with stable electroluminescence (EL) spectra. Herein, the in situ treatment of Cl‐rich benzene phosphorus oxydichloride (BPOD) is proposed to achieve high‐quality pure‐blue perovskites by simultaneously enlarging the perovskite bandgap, passivating the halide vacancy defects, and immobilizing the halide ions through the hydrolysis products of chloride ions and phenylphosphonic acid. As a result, highly‐performed pure‐blue PeLEDs with a maximum external quantum efficiency (EQE) of 9.3% at 477 nm, an impressive lifetime of 34.7 min at 104 cd m−2, and very stable EL spectra are achieved, which are superior to the conventional mixed halide PeLEDs based on lead‐chloride precursor, representing one of the best performances for mixed halide pure‐blue PeLEDs. Therefore, this work contributes a feasible and effective method for efficient and stable pure‐blue PeLEDs.

Synthesis, structural characterization and intense far-red luminescence of Mn4+ -activated SrLaMgTa1-yAlyO6 oxide phosphors

The transition metal Mn4+ activated phosphors have attracted increasing attention due to the potential uses in phosphor-converted lighting emitting diodes (LEDs). Herein, the far-red emitting SrLaMgTa1-yAlyO6:Mn4+ (y = 0−0.15) oxide phosphors were successfully synthesized by the high-temperature solid state reaction, in which the Mn4+ acted as an activator. The monoclinic double perovskite crystal structure, chemical composition, and 4+ state of activator Mn were confirmed by means of X-ray diffraction Rietveld refinement, scanning electron microscopy elemental mapping, and X-ray photoelectron spectroscopy. The optical properties were characterized with photoluminescence excitation and emission spectra, temperature-dependent emission spectra, and electroluminescence spectra. The excitation spectra are interweaved with the ligand-to-metal charge transfer band of Mn−O and intrinsic transitions (4A2g→4T1g, 4A2g→2T2g, and 4A2g→4T2g) of Mn4+, locating at the ultraviolet and blue light region. The emission spectra mainly contain a dominant far-red emission band from 650 to 775 nm with peaks at 695 and 708 nm, which are ascribed to the 2Eg→4A2g transition of Mn4+. Moreover, the cationic substitution strategy with tiny Al3+ occupying Ta5+ octahedral site, promotes the improvement of luminescence intensity and optical thermal stability. The quantum yield of optimal phosphor reaches 88.2 % and the fluorescence intensity at 373 K (100 °C) retains 83.4 % in respect to that at ambient temperature, implying the phosphor with intense and thermally stable luminescence. The phosphor is packaged with 365 nm chip to fabricate an LED device, and the electroluminescence result is quite consistent with the photoluminescence, matching well with the absorption spectrum of the phytochrome. The Mn4+ activated oxide phosphors with intense far-red emission are promising for plant cultivation lighting.

Quasi-passive modulation for monolithic GaN photoelectronic circuit

Due to the overlap between the electroluminescence spectrum and spectral responsivity curve, gallium nitride (GaN)-based multi-quantum well (MQW) diodes can modulate and detect light emitted by another diode with the same MQW structure. This enables the realization of a monolithic integrated GaN optoelectronic circuit, integrating an MQW-based transmitter, waveguide, modulator, and receiver on a tiny GaN chip. It is well known that the active region of MQW absorbs high-energy photons within the plane, generating electron-hole pairs and forming photogenerated carriers. The change in free carrier concentration causes variations in the refractive index and absorption characteristics of the waveguide, thereby manipulating light propagation within the waveguide. Based on this physical phenomenon, a quasi-passive modulation scheme is proposed for a monolithic GaN photoelectronic circuit. This scheme achieves optical modulation by connecting a variable resistor between the modulator electrodes, avoiding the need for high-precision external electric fields and complex control circuits. The performance of the quasi-passive modulation was tested through on-chip data transmission and real-time audio signal transmission. The results indicate that quasi-passive modulation is highly feasible in optoelectronic systems and shows great promise as a competitive core module for future large-scale photonic integrated circuits.

Electrically pumped random laser device based on Pd/SiO2/ZnO nanorods MIS structure

Electrically pumped random lasing based on Pd/SiO2/ZnO nanorods (NRs) structures is demonstrated. Chemical bath deposition (CBD) technique was used to synthesize the ZnO NRs. Then, an insulating layer of SiO2 was deposited via radio frequency (RF) magnetron sputtering. After that, direct current (DC) magnetron sputtering was used to deposit palladium (Pd) metal contacts on the sample through a shadow mask. The electrical, morphological as well as optical characteristics of the ZnO NRs were analyzed. The device exhibits typical Schottky diode current–voltage (I-V) characteristics at a turn-on voltage of 6.18 V. The electroluminescence (EL) spectra shows good random lasing behavior with lasing peaks centered at approximately 380 nm and exhibited a threshold current of 37.5 mA. This report reveals Pd as a relatively cheaper material and low-cost CBD fabrication method is sufficient to fabricate electrically pumped random laser devices. Furthermore, this report also present a new way to interpret the mechanism of random lasing in the device.

Reduction of green-gap effect for light-emitting diodes using InGaN-ZnGeN2-InGaN/GaN type-II MQW

In this work, the design of two multiple-quantum-wells (MQWs) green LEDs are investigated − (i) LED A with standard InGaN/GaN type I band alignment quantum wells (QWs), and (ii) LED B with InGaN-ZnGeN2-InGaN/GaN type II band alignment QWs. Using numerical program we obtain energy band diagram, carrier concentration, electric field, electroluminescence spectra and radiative-recombination-rate (RRR). Owing to presence of strong polarization field and compositional inhomogeneity in InGaN/GaN heterostructure, it is hard to obtain the high efficiency in wavelength of 500–600 nm, termed as ‘green-gap’ effect. Introduction of InGaN-ZnGeN2-InGaN QWs reduces the piezoelectric polarization field and enhances overlapping between electron-hole wave functions thereby increasing RRR. Moreover, holes are distributed uniformly in all QWs in the active region of LED that leads to improved emission efficiency. The proposed device, LED B exhibits 710 % more output optical power and less wall-plug efficiency droop is only 3.1 % in contrast to 82.4 % in LED A.

Molecular Engineering Towards Efficient Aggregation-Induced Delayed Fluorescence Luminogens as Emitters and Sensitizers for High-Performance Organic Light-Emitting Diodes.

Exploring efficient thermally-activated delayed fluorescence materials having maximum external quantum efficiencies (ηext,maxs) exceeding 30 % for organic light-emitting diodes (OLEDs) still remains challenging because it generally requires efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (ΦPL), and large optical out-coupling efficiency (Φout) simultaneously. Herein, two green aggregation-induced delayed fluorescence (AIDF) luminogens, named XTCz-2 and XTCz-3, are designed and constructed by using xanthone (XT) as electron acceptor and phenylcarbazole-substituted carbazole as donors. XTCz-2 and XTCz-3 exhibit distinguished advantages of high thermal stability (439-560 °C), excellent ΦPLs (84-88 %) and fast RISC rates (1.9×105-4.2×105 s-1), and prefer horizontal dipole orientation and thus have high Φouts. Consequently, they can achieve the state-of-the-art electroluminescence (EL) performances with ηext,maxs of up to 35.0 %. Moreover, XTCz-3 is selected as a sensitizer for sky-blue multi-resonance delayed fluorescence emitter in hyperfluorescence OLEDs, providing narrow EL spectra and excellent ηext,maxs of up to 33.8 % with low efficiency roll-offs. The splendid comprehensive performances demonstrate the significant application potential of these AIDF luminogens as both light-emitting materials and sensitizers for OLEDs.

Benzimidazole based electron-transport hosts for efficient pure blue narrowband OLED device

Two electron-transport host materials are synthesized by linking anthracene with benzimidazole derivatives through a unit of methyl substituted phenyl. The methyl substitution generates a large dihedral angle between the benzimidazole plane and bridging phenyl, keeping the excited state distribution and energy levels of T1 and S1 unchanged. The electron-only devices reveal that the two hosts have obviously enhanced electron transport ability than that of Ph-AN-MBP without benzimidazole. As a result, the devices with Ph-AN-BIZ and N1-AN-BIZ as hosts and a blue multiple resonance material of tBuCz-DABNA as emitter exhibit high external quantum efficiency of 10.1 % and 9.1 %, respectively, and low turn-on voltage of 2.7 V, which is better than those of Ph-AN-MBP based device (7.9 %, 3.4 V). And their electroluminescence spectra have a very small full-width at half-maximum of 16.2 nm and pure blue color with CIE coordinates of (0.12, 0.11).

Synthesis, Photoluminescence, and Electroluminescence of Phosphorescent Dipyrido[3,2-a;2'3'-c]phenazine-Platinum(II) Complexes Bearing Hole-Transporting Acetylide Ligands.

In this study, novel phosphorescent dipyrido[3,2-a;2'3'-c]phenazine (dppz)-platinum(II)-phenylacetylide complexes were developed to fabricate non-doped organic light-emitting diodes (OLED) by solution-processing. To facilitate the charge carrier injection into the emitting layer (EML), 3,6-di-tert-butylcarbazole-functinalized phenylacetylides were employed. As for the dppz ligand, 9,9-dihexylfluoren-2-yl and 4-hexylthiophen-2-yl side-arms were introduced to the 2,7-positions, which led to reddish orange and red photoluminescence (PL), respectively, in solution and film states (PL wavelength: ca. 600 and ca. 625 nm, respectively). The carbazole-appended phenylacetylide ligands hardly affected the emission color, although unsubstituted phenylacetylides gave rise to aggregate- or excimer-based near-infrared PL with a low quantum yield. Two types of non-doped OLEDs were fabricated: single-layer and multilayer devices. In both devices, the organic layers were fabricated by spin-coating, and the EML consisted of a neat film of the corresponding platinum(II) complex. Therein, electroluminescence spectra corresponding to those of PL were observed. The single-layer devices exhibited low device efficiencies due to a deteriorated charge carrier balance. The multilayer devices possessed hole- and electron-transporting layers on the anode and cathode sides of the EML, respectively. Owing to an improved charge carrier balance, the multilayer devices exhibited higher device performance, affording considerably improved values of luminance and external quantum efficiency.

Achieving white electroluminescence and low current consumption in devices based on graphene oxide and silicon

This work reports the emission of white electroluminescence (EL) in devices based on silicon. These devices are formed by a heterostructure of porous silicon (PSi), graphene oxide (GO), silicon-rich-oxide (SRO) thin films, and zinc oxide (ZnO) as a transparent conductor oxide (contact). The current consumption in these devices has been reduced by approximately three orders of magnitude compared to the current currents commonly reported for electroluminescent devices. The white EL emission is corroborated by the EL spectra, which are very broad (from 400 to 750 nm) and are attributed to the different nanostructured defects present in the materials that make up the heterostructures. The devices exhibited EL only in negative bias. According to our analysis of the I–V curves, the space-charge limited current conduction mechanism is the dominant carrier conduction mechanism in the region where the EL takes place. The electroluminescence exhibited by these devices starts at a current of 1.1 μA and is the entire area electroluminescent at approximately 3 μA reaches its maximum value at 9 μA. We demonstrated that producing LEDs on a Si substrate can result in significant energy savings.

Role of electron withdrawing moieties in phenoxazine–oxadiazole-based donor–acceptor compounds towards enriching TADF emission

Herein, we reported the effect of electron withdrawing units, such as trifluoromethyl (CF3) and cyanide (CN), substitution on a thermally activated delayed fluorescent (TADF) molecule (10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10H-phenoxazine (PXZ-OXD)). The addition of electron withdrawing units has increased the acceptor strength and reduced the interaction between PXZ and OXD, thus reducing the gap between the singlet and triplet states (ΔEST) of excitons. To understand the variation in the acceptor strength of PXZ-OXD as a function of its molecular structure and density of states, density functional theory (DFT) and time-dependent DFT (TDDFT) were conducted. Transition density matrix investigations revealed that the addition of −CF3 and –CN to PXZ-OXD triggers CT excitons, while inducing lower ΔEST values. Particularly, the lowest ΔEST (0.05 eV) with a notable red shift in the emission spectrum was observed with 4′-CF3PXZOXD. The delayed component lifetime of PXZOXD is found to be reduced after the substitution of −CNwhm and −CF3. Despite the weak charge transfer transitions, the substitution of conjugative –CN group is found to be beneficial in improving the HOMO-LUMO overlap with a moderate decrease in reverse intersystem crossing (KRISC), which attained enhancement in the photoluminescent quantum yield. Additionally, the substitution of the above electron withdrawing units on PXZ-OXD yielded a red shift in the electroluminescence spectrum. Furthermore, the external quantum efficiency (at 100 cd/m2, i.e., EQE100) of the 4′-CNPXZOXD-based organic light-emitting diode is found to improve by 21.13 % against the PXZOXD (16.9 %).

Simulation strategy for organic LEDs with spectral dissimilarity between photoluminescence and electroluminescence spectra

Simulation strategy for organic LEDs with spectral dissimilarity between photoluminescence and electroluminescence spectra

Influence of low-temperature cap layer thickness on the structure and luminescence of InGaN/GaN multiple quantum wells

In order to effectively regulate the luminescence performance of MQW and enhance its optical quality, it is crucial to investigate the InGaN/GaN MQW's structure and luminescence properties. In this study the focus is how they are affected by low-temperature cap (LT-cap) layer's thickness which is grown above each InGaN well layer during growth process of InGaN/GaN multiple quantum well (MQW) samples in MOCVD system. This was achieved by analyzing high resolution X-ray diffraction (HRXRD) spectra, electroluminescence (EL) spectra, temperature-dependent photoluminescence (TDPL) spectra, and micro-area fluorescence imaging of these samples. The results show that changes in LT-cap layer's thickness even have no significant impact on some structural parameters of MQW, such as the thickness of the well layer, but have an influence on the In component of the well layer. Due to the existence of LT-cap layer, dissociation of InGaN can be effectually reduced. In addition, the augmentation of LT-cap layer's thickness will make polarization effect of the QW sample more remarkable, so that the blue shift of the EL peak with the augmentation of current injection increases. The change of LT-cap layer's thickness will also influence distribution of the tail states of the quantum wells, which leads to a different localization states for injected carriers. As LT-cap layer becomes getting thicker, the material's internal quantum efficiency (IQE) tends to decrease, which may result from an increase in non-radiative recombination centers.

High-performance sky-blue quasi-2D perovskite light-emitting diodes via synergistic defect passivation and phase narrowing strategies

Quasi-2D perovskite light-emitting diodes (PeLEDs) are considered as one of the most promising candidates for next-generation displays. Yet, the unstable electroluminescent spectra and inferior performance of blue PeLEDs hinder their commercial use. Herein, we demonstrate highly efficient quasi-2D pure bromide (QPB) sky-blue PeLEDs based on an oxygen-rich triethyl 2-acetylcitrate (TEAC) additive. The TEAC additive contains four C = O functional groups, offering several coordination sites to simultaneously passivate multiple bromine vacancy defects. Furthermore, it inhibits the growth of large n (n ≥ 5) phases to achieve narrow phases distribution. As a result, the TEAC-modified PeLEDs show sky-blue emission at 494 nm, with maximum current efficiency and external quantum efficiency up to 29.1 cd/A and 18.5 %, respectively. Moreover, the T50 lifetime of TEAC-modified PeLED is about 25 times higher than that of pristine PeLEDs. Our results provide a reliable approach to realizing high-efficiency blue quasi-2D PeLEDs.