Electroluminescence Spectra Research Articles

Preparation of Hybrid Junction Light Emitting Device from Cadmium Sulphite Quantum dot/ poly-TPD Junction

Stratums of ITO/TPD:PMMA/CdS/Alq3/Al were used to make the hybrid nanoparticles light emitting diode (NPs-LED), which were created using the phase segregation method and the spin coating procedure. Light-generating NPs-LED hybrid device is composed of 3 stratums stacked in a certain order on an Indium tin oxide(ITO) substrate made of transparent conducting oxide. The first stratum was (N, N'-bis (3-methylphenyl)-N, N'-bis (phenyl) benzidine) (TPD) polymer, and polymethyl methacrylate (PMMA) polymer. Cadmium Sulphite (CdS) is the second stratum, while Tris (8-hydroxyquinolinato) aluminum (Alq3) is the last stratum, which is one of the materials used as electron injector stratums.
 Various bias voltages (28, 30, and 32V) were used to examine the electroluminescence (EL) of NPs-LEDs at room temperature, depending on the CIE 1931 color spaces, `light was created at 32V, whereas blue light was generated at 28V. The transmission between sub levels in the energy gap of CdS NPs (surface state). Voltages are controlling the current change, and the knee voltage is 6.6 V, as seen by the current – voltage (I-V) distinctive. EL spectrum demonstrates a broadband emission spanning 300 nm to 700 nm. Finally, it is discovered that the correlated color temperature (CCT) is about (7450 to 11820k).

Circularly polarized deep-blue polymer light emitting diodes induced by a chiral small molecule dopant

The solution-processable molecule S-F4SO containing enantiomerically chiral side-chains was designed, and showed the weak circulardichroism (CD) and insignificant circularly polarized photoluminescence (CPPL) signal. Then the chiral perturbation strategy took effect when S-F4SO was blended with achiral polymer poly(9,9-dioctylfluorene) (PFO), the CD and CPPL signal can be enhanced dramatically. The circularly polarized light-emitting dioxidesshowed the deep blue circularly polarized electroluminescence with the maximal emission at 448 nm, |gEL|max value of 4.5 × 10−3, the LEmax of 1.8 cd A1 with a CIE coordinate of (0.16, 0.10) and good electroluminescence spectra stability.

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Ultra-broadband light-emitting diodes from co-evaporated lead-free CsCu2I3

• CsCu 2 I 3 films were prepared by vacuum co-evaporation method. • Co-evaporated CsCu 2 I 3 film exhibits self-trapped exciton behavior. • LEDs using the CsCu 2 I 3 films show red-shifted and ultra-broad EL. As an alternative to the conventional solution-processing, a vacuum co-evaporation method is developed to prepare CsCu 2 I 3 thin film, for fabricating light-emitting diodes (LEDs). LEDs using the co-evaporated lead-free CsCu 2 I 3 films exhibit ultra-broad warm-white emission. We found that the LEDs using the CsCu 2 I 3 thin films have red-shift and broadened electroluminescence (EL) with a full width at half-maximum of 245 nm in contrast to 132 nm from photoluminescence (PL) spectra for the CsCu 2 I 3 films. The current hysteresis of LEDs using the CsCu 2 I 3 films indicates that the migration of ions provides a new radiative-recombination center and further resulting in EL change and broadening. The experimental work presented here gains insight on broadening EL spectra for the LEDs with self-trapped exciton materials.

Triphenylethene-carbazole-based molecules for the realization of blue and white aggregation-induced emission OLEDs with high luminance

Molecules that use triphenylethene-carbazole moieties decorated with different substitutions were designed and synthesized, harvesting wide energy bandgaps and aggregation-induced emission (AIE) characteristics. The photophysical, thermal, and electroluminescence (EL) characteristics of the designed emitters were investigated to clarify the molecular structure-property-performance relationship. The photoluminescence spectra of the synthesized molecules measured using tetrahydrofuran-water solution presented strong blue emissions, confirming their AIE property. The doped blue-emitting organic light-emitting diodes (OLEDs) with compound 2 exhibited satisfactory peak efficiency of 8.7 cd/A and high maximum luminance of 18474 cd/m2. In comparison, the peak efficiency of the 2-based sky-blue OLEDs with a non-doped emitting layer was 10.2 cd/A with an even superior peak luminance of 39661 cd/m2. Blue-emitting AIE OLEDs have rarely presented such high luminance values, and this improvement can facilitate the development of white-emitting OLEDs (WOLEDs) for lighting applications. The fabricated WOLEDs with 2 achieved a maximum efficiency of 5.6% (8.0 cd/A and 7.5 lm/W), stable EL spectra, and high luminance of 29319 cd/m2. These results indicate that the triphenylethene-carbazole structure designs for AIE molecules could simultaneously harvest wide energy bandgap and high luminance, demonstrating their high potential for EL applications.

Iridium(III) Complexes Based on Indolo[3,2,1‐jk]carbazole Derivatives for Narrowband Organic Light‐Emitting Diodes with External Quantum Efficiency of 32.3% and Low Efficiency Roll‐Off

Iridium(III) complexes have been widely used in organic light‐emitting diodes (OLEDs) due to their high efficiency and stability. However, the wide emission bands of most Ir(III) complexes affect the color purity and the device performance. Herein, three narrowband Ir(III) complexes (Ir1–Ir3) based on indolo[3,2,1‐jk]carbazole moiety with pyridine group bound to the 9‐position are synthesized. The trimethylsilyl and trifluoromethyl substituents tune the emission color from yellow (567 nm) to orange (577 nm) and red (620 nm) with small full width at half maximum (FWHM) of 38, 40, and 49 nm (1120, 1186, and 1262 cm−1) in toluene solutions, respectively. Furthermore, these complexes exhibit high photoluminescence quantum yields of up to 88% and short phosphorescent lifetimes of down to 0.59 μs in doped films. Employing three complexes as emitters, the electroluminescence spectra of devices also show narrow FWHMs of 43, 45, and 53 nm (1294, 1310, and 1379 cm−1), respectively. Furthermore, all devices show high efficiencies, accompanied by low efficiency roll‐offs. The results provide a reliable strategy for designing highly efficient Ir(III) complexes with narrowband emissions for OLEDs.

Deep‐Blue Electroluminescence of Perovskites with Reduced Dimensionality Achieved by Manipulating Adsorption‐Energy Differences

AbstractThe lagging development of deep‐blue perovskite light‐emitting diodes (PeLEDs) heavily impedes their practical applications in full‐color display due to the absence of spectrally stable emitters and the mismatch of carrier injection capacity. Herein, we report highly efficient deep‐blue PeLEDs through a new chemical strategy that addresses the dilemma for simultaneously constant electroluminescence (EL) spectra and high‐purify phase in reduced‐dimensional perovskites. The success lies in the control of adsorption‐energy differences between phenylbutylamine (PBA) and ethylamine (EA) interacting with perovskites, which facilitates narrow n‐value distribution. This approach leads to an increased exciton binding energy and enhanced surface potential, hence improving radiative recombination. As a result, an external quantum efficiency of 4.62 % is achieved in PeLEDs with a stable EL peak at 457 nm, demonstrating the best reported result for deep‐blue PeLEDs so far.

Deep-Blue Electroluminescence of Perovskites with Reduced Dimensionality Achieved by Manipulating Adsorption-Energy Differences.

The lagging development of deep-blue perovskite light-emitting diodes (PeLEDs) heavily impedes their practical applications in full-color display due to the absence of spectrally stable emitters and the mismatch of carrier injection capacity. Herein, we report highly efficient deep-blue PeLEDs through a new chemical strategy that addresses the dilemma for simultaneously constant electroluminescence (EL) spectra and high-purify phase in reduced-dimensional perovskites. The success lies in the control of adsorption-energy differences between phenylbutylamine (PBA) and ethylamine (EA) interacting with perovskites, which facilitates narrow n-value distribution. This approach leads to an increased exciton binding energy and enhanced surface potential, hence improving radiative recombination. As a result, an external quantum efficiency of 4.62 % is achieved in PeLEDs with a stable EL peak at 457 nm, demonstrating the best reported result for deep-blue PeLEDs so far.

Revealing the Evolution Processes of Excitons on High Energy Level in Anthracene‐Based OLEDs

AbstractIt is well‐known that the electrically generated excitons can perform the spin evolution between high‐lying excited states, providing an efficient way to utilize triplet excitons in organic light‐emitting diodes (OLEDs). Anthracene families offer an opportunity to deeply investigate the processes of triplet excitons on high‐lying excited states in detail. Here, a simplified model is proposed to study the exciton dynamics in anthracene derivatives‐based devices. The mechanism on the processes of high‐energy level intersystem crossing in anthracene derivatives is well revealed by theoretical calculation, transient electroluminescence, transient photoluminescence, and transient absorption spectrum measurements. Besides, doping strategy is proposed to suppress the exciton loss channel for improving the efficiency of devices. The studies establish an in situ method to evaluate the apparent singlet exciton formation ratio in devices due to the exciton evolution between high‐lying excited states and offer some clues to further utilize these triplet excitons, thus improving the efficiency of the resulting fluorescence OLEDs in the future.

Enriched-Bromine Surface State for Stable Sky-Blue Spectrum Perovskite QLEDs With an EQE of 14.6.

Halogen vacancies are of great concern in blue-emitting perovskite quantum-dot light-emitting diodes because they affect their efficiency and spectral shift. Here, an enriched-bromine surface state is realized using a facile strategy that employs a PbBr2 stock solution for anion exchange based on Cd-doped perovskite quantum dots. It is found that the doped Cd ions are expected to reduce the formation energy of halogen vacancies filled by the external bromine ions, and the excess free bromine ions in solution are enriched in the surface by anchoring with halogen vacancies as sites, accompanied with the shedding of surface long-chain ligands during the anion exchange process, resulting in a Br-rich and "neat" surface. Moreover, the surface state exhibits good passivation of the surface defects of the controlled perovskite QDs and simultaneously increases the exciton binding energy, leading to excellent optical properties and stability. Finally, the sky-blue emitting perovskite quantum-dot light-emitting diodes (QLEDs) (490nm) are conducted with a record external quantum efficiency of 14.6% and current efficiency of 19.9cd A-1 . Meanwhile, the electroluminescence spectra exhibit great stability with negligible shifts under a constant operating voltage from 3 to 7V. This strategy paves the way for improving the efficiency and stability of perovskite QLEDs.

Lower threshold current density of GaN-based blue laser diodes by suppressing the nonradiative recombination in a multiple quantum well.

The influence of the nonradiative recombination in a multiple quantum well of GaN-based blue laser diodes (LDs) has been are studied experimentally and theoretically by analyzing the optical and electrical properties of LDs with various thickness and indium content of quantum wells (QWs). It is found that when keeping the LD emission wavelength nearly unchanged, the LD device performance with thinner QW and higher indium content of InGaN QWs is much better than the LD with thicker QW and lower indium content, having smaller threshold current density, higher output optical power and larger slope efficiency. Typically, the threshold current density is as low as 0.69 kA/cm2, and the corresponding threshold current is only 250 mA. The lifetime is more than 10,000 hours at a fixed injection current of 1.2 A under a room-temperature continuous-wave operation. Characteristics of photoluminescence (PL) microscopy images, temperature dependent PL spectra, time-resolved PL and electroluminescence spectra demonstrate that a reduction of the nonradiative recombination centers and an improvement of homogeneity in QWs are the main reason for the performance improvement of GaN-based LD using thinner QW layers with a higher indium content in a certain range. Moreover, theoretical calculation results demonstrate that using a thinner quantum well is also helpful for improving the device performance if the change of alloy material quality is considered during the calculation.

Self-formation of InAs/InGaAsSb type-II superlattice structures on InP substrates by MBE and their application to mid-infrared LEDs

InAs/InGaAsSb type-II superlattice structures (SLSs) were spontaneously formed by the molecular beam epitaxy of InAs/GaAs0.86Sb0.14 SLSs on InP substrates. The strain due to lattice mismatch between InAs and GaAs0.86Sb0.14 induced two exchange reactions of In–Ga and As–Sb at both InAs/GaAs0.86Sb0.14 heterointerfaces, resulting in the strain relaxation and the self-formation of InAs/InGaAsSb type-II SLS. By energy dispersive x-ray spectroscopy analysis, the mixed crystal composition of the InGaAsSb layer was determined to be approximately In0.8Ga0.2As0.9Sb0.1. Electroluminescence spectra of LEDs, including the self-formed InAs/In0.8Ga0.2As0.9Sb0.1 2.5-periodic SLS, showed double peaks of 2.6 and 3.2 µm at 15 K. The luminescence spectrum was based on two transition mechanisms of type-I transition in InAs and type-II transition between InAs and InGaAsSb. The experimental results of luminescence spectra were supported by theoretical calculations. The 3.3 µm emission was maintained above 220 K.

Current-controlled electroluminescence from light-emitting diodes based on Tm, Er, and Eu codoped Ga2O3 thin films

In this work, we report on the demonstration of an LED based on Tm, Er, and Eu codoped Ga2O3 thin film. Current-controlled electroluminescence (EL) has been observed from the Ga2O3:(Tm+Er+Eu)-based LED with a relatively low driven voltage of ∼9 V. The EL spectra reveal that the observed light emission results from the combination of simultaneous blue, green, and red emissions from Tm3+, Er3+, and Eu3+ ions, respectively. This strategy for LED shows great potential to achieve full color (white light) by intentionally changing the concentrations of Tm3+, Er3+, and Eu3+ ions in Ga2O3 without using any other phosphors.

White electroluminescence from cross relaxation-free CaSiO3:Tb3+ film on silicon wafer and its concentration dependent white-color tunability

A full-color white light source is continuously required for general lighting applications. Herein, the white electroluminescence (EL) from Tb3+-singly doped CaSiO3 (CSO) thin film on a silicon substrate has been reported for the first time. The EL device has a metal-oxide-semiconductor (MOS) structure which consists of two CSO and SiOx oxide layers on silicon wafer. Above the threshold voltage in AC power, it shows a full-color white EL spectrum with two dominant emission groups of ultraviolet-blue and green-red peaks from the f-f intra-transition of Tb3+ ions without the cross-relaxation effect in the four blue peaks which suffers from their severe cross-relaxation quenching in its photoluminescence spectrum. It is attributed to the strong electric field splitting (Stark effect) which prevents the resonance energy transfer due to energy perturbation. With increasing Tb3+ concentrations, the blue bands get more dominant along with increasing the threshold voltage. Furthermore, we demonstrate the EL-voltage transient behavior with a large hysteresis in a positive voltage polarity.

Efficient and stretchable organic light-emitting devices based on spontaneously formed disordered wrinkles.

We propose a facile, scalable strategy to introduce spontaneously formed disordered wrinkles into organic light-emitting devices (OLEDs) to enhance light extraction and realize stretchability of the devices. The luminance and current efficiency of the wrinkled OLEDs are improved by 37% and 18%, respectively, compared to the planar device. Meanwhile, broadband light scattering induced by the disordered wrinkles results in angle-stable electroluminescent spectra at wide viewing angles for the wrinkled OLEDs. The disordered wrinkles enable the OLEDs to be stretchable and withstand hundreds of stretching-releasing cycles at strain between 0% and 5%. This study provides a simple method to realize stretchable OLEDs with high efficiency.

Demonstration and modeling of unipolar-carrier-conduction GaN Schottky-pn junction diode with low turn-on voltage

By introducing a thin p-type layer between the Schottky metal and n-GaN layer, this work presents a Schottky-pn junction diode (SPND) configuration for the GaN rectifier fabrication. Specific unipolar carrier conduction characteristic is demonstrated by the verification of temperature-dependent current–voltage (I–V) tests and electroluminescence spectra. Meanwhile, apparently advantageous forward conduction properties as compared to the pn diode fabricated on the same wafer have been achieved, featuring a lower turn-on voltage of 0.82 V. Together with the analysis model established in the GaN SPND for a wide-range designable turn-on voltage, this work provides an alternative method to the GaN rectifier strategies besides the traditional solution.

Management of exciton recombination zone and energy loss for 4CzTPN-based organic light-emitting diodes via engineering hosts

The exciton recombination zone and energy loss of organic light-emitting diodes (OLEDs) with thermally activated delayed fluorescence (TADF) emitter 4CzTPN (1, 2, 4, 5-Tetrakis(carbazol-9-yl)-3, 6-dicyanobenzene) as the dopant are explored by engineering hosts. The results demonstrate that the exciton recombination zone is determined by the matching condition between hole and electron mobility of hosts. The shift of the recombination zone from cathode to anode side induces a blue-shift of the electroluminescent spectra. And energy loss can be reduced by well-matched carrier mobility and TADF feature of host as well as lower energy level offset between host and guest. Specifically, the exciplex-co-hosted device delivers the maximum current/power efficiency/external quantum efficiency of 29.06 cd·A−1/28.89 lm·W−1/8.86%, respectively, enhanced by more than 50% than the single-component-hosted devices. This study illuminates that controlling exciton recombination zone and energy loss via engineering hosts is a feasible strategy for developing high performance TADF OLEDs.

Properties of Luminescent Materials and Photoelectronic Devices

In this paper, an organic light emitting device is designed, and its fabrication and performance are tested. During the stacking process, the relative thickness ratio of each luminescent layer is changed. It is found that the devices show different luminescent characteristics under the condition that the total thickness is the same but the thickness ratio of each luminescent layer is different. Among them, x varies by 8, 6, 4 and 2 nm, while y varies by 2, 4, 6 and 8 nm. Four devices are made. It is found that the electroluminescence spectrum of the device has obvious changes, the color of the device changes significantly from blue to near white, and the brightness and efficiency of the device also have different degrees of change. This paper mainly studies the role of luminescence layer thickness for the brightness in the device. The experimental results show that when x = 6 nm, y = 4 nm, the maximum brightness of the device is 4238 cd/m2, color coordinate (0.31,0.38).

Red-emitting IrIII(C^N)2(P-donor ligand)Cl-type complexes showing aggregation-induced phosphorescent emission (AIPE) behavior for both red and white OLEDs

Two red-emitting IrIII(C^N)2(P-donor ligand)Cl-type complexes bearing C^N ligands with carbazole functional group have been successfully prepared with different P-donor ligands of triphenylphosphine and triethylphosphine, respectively. The investigation of their phosphorescent behavior in the mixture of THF and water to reveal their aggregation induced phosphorescent emission (AIPE) ability, which is also indicated by their much higher phosphorescent quantum yield (ΦP) in doped film than those in the dilute solution. Mainly, their AIPE are induced by the blocked stretching motion of aromatic segments in C^N ligand and restrained the deformation of their coordinating skeletons. The AIPE complexes can possess AIE factor (αAIE) of ca. 7.4. In addition, the carbazole group can effectively promote hole transporting ability of the concerned AIPE emitters, which can benefit their electroluminescent ability. Hence, the IrIII(C^N)2(P-donor ligand)Cl-type complexes can show decent EL efficiencies in the solution-processed red-emitting organic Light-emitting diodes (OLEDs) with a maximum external quantum efficiency (ηext) of 8.5%, a maximum current efficiency (ηL) of 22.0 cd A−1 and a maximum power efficiency (ηP) of 15.9 lm W−1. Furthermore, as long-wavelength emitter, solution-processed white OLEDs (WOLEDs) have been constructed based on the red-emitting AIPE IrIII(C^N)2(P-donor ligand)Cl-type complexes, which can play critical role in achieving stable white electroluminescent spectra at high luminescence. The concerned WOLEDs can show attractive EL efficiencies of 6.6%, 23.7 cd A−1 and 16.0 lm W−1. All these results can provide valuable information for developing new AIPE materials with high EL ability.

Quantum Dot-Based White Organic Light-Emitting Diodes Excited by a Blue OLED

In this study, white organic light-emitting diodes (OLEDs) consisting of red quantum dots (RQD) and green quantum dots (GQD) were investigated. These are the most exciting new lighting technologies that have grown rapidly in recent years. The white OLED development processes used consisted of the following methods: (a) fabrication of a blue single-emitting layer OLED, (b) nanoimprinting into QD photoresists, and (c) green and red QD photoresists as color conversion layers (CCL) excited by blue OLEDs. To fabricate the blue OLED, the HATCN/TAPC pair was selected for the hole injection/transport layer on ITO and TPBi for the electron transport layer. For blue-emitting material, we used a novel polycyclic framework of thermally activated delayed fluorescence (TADF) material, ν-DABNA, which does not utilize any heavy metals and has a sharp and narrow (FWHM 28 nm) electroluminescence spectrum. The device structure was ITO/HATCN (20 nm)/TAPC (30 nm)/MADN: ν-DABNA (40 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al (150 nm) with an emitting area of 1 cm × 1 cm. The current density, luminance, and efficiency of blue OLEDs at 8 V are 87.68 mA/cm2, 963.9 cd/m2, and 1.10 cd/A, respectively. Next, the bottom emission side of the blue OLED was attached to nanoimprinted RQD and GQD photoresists, which were excited by the blue OLED in order to generate an orange and a green color, respectively, and combined with blue light to achieve a nearly white light. In this study, two different excitation architectures were tested: BOLED→GQD→RQD and BOLED→RQD→GQD. The EL spectra showed that the BOLED→GQD→RQD architecture had stronger green emissions than BOLED→RQD→GQD because the blue OLED excited the GQD PR first then RQD PR. Due to the energy gap architectures in BOLED-GQD-RQD, the green QD absorbed part of the blue light emitted from the BOLED, and the remaining blue light penetrated the GQD to reach the RQD. These excited spectra were very close to the white light, which resulted in three peaks emitting at 460, 530, and 620 nm. The original blue CIE coordinates were (0.15, 0.07). After the excitation combination, the CIE coordinates were (0.42, 0.33), which was close to the white light position.