Warm White Emission Research Articles

High Efficiency Non‐Doped White Organic Light Emitting Diodes Based on a Bilayer Interface‐Exciplex Structure

High efficiency non‐doped blue and white organic light emitting diodes (OLEDs) are realized by using a bilayer interface‐exciplex (i‐Exc) structure. Since the neutral exciplex are formed at the interface, the interface charge accumulation is reduced, thereby suppressing exciton quenching in the device. With the i‐Exc structure, a maximum external quantum efficiency (EQE) of 23.1% is achieved in the non‐doped blue phosphorescent OLED. Moreover, the white emission can be obtained by inserting an ultrathin orange‐red phosphorescent emitter into the blue devices. The charge trapping in the orange‐red emitter can be suppressed by adjusting the position of the ultrathin orange‐red emitting layer in the device, contributing to superior spectral stability. The WOLED achieves an EQE of 20.0% and exhibits a stable warm white emission. The Commission Internationale de l'Eclairage (CIE) coordinates of WOLED are (0.41, 0.40) at 5 V and slightly change to (0.40, 0.41) at 9 V.

White Photoluminescent Ti3C2 MXene Quantum Dots with Two-Photon Fluorescence.

A recently created class of inorganic 2D materials, MXenes, has become a subject of intensive research. Reducing their dimensionality from 2D to 0D quantum dots (QDs) could result in extremely useful properties and functions. However, this type of research is scarce, and the reported Ti3C2 MXene QDs (MQDs) have only shown blue fluorescence emission. This work demonstrates a facile, high‐output method for preparing bright white emitting Ti3C2 MQDs. The resulting product is two layers thick with a lateral dimension of 13.1 nm. Importantly, the as prepared Ti3C2 MQDs present strong two‐photon white fluorescence. Their fluorescence under high pressure is also investigated and it is found that the white emission is very stable and the pressure makes it possible to change from cool white emission to warm white emission. Hybrid nanocomposites are then fabricated by polymerizing Ti3C2 MQDs in polydimethylsiloxane (PDMS) solution, and the bright white emitting hybrid materials in white light‐emitting diodes are used. This work provides a facile and general approach to modulate various nanoscale MXene materials, and could further aid the wide development of applications for MXene materials in various optical‐related fields.

Improving the color-rendering index of a tandem warm white organic light-emitting device by employing a simple fabrication process.

We realized an efficient spectra-stable tandem white organic light-emitting device (WOLED) with pleasurable warm white emission and an extremely high color-rendering index (CRI) simultaneously by connecting a fluorescent blue unit and a phosphorescent dual-color unit via an easy-fabrication charge generation unit (CGU). Over a wide range of driving current density, the tandem warm white OLED exhibited general CRI (Ra) value exceeding 85 and special CRI (R9) value close to 57. In addition, Duvs are within the tolerance for CRI measurements, ensuring validity and logicality of the calculated results concerning Ra and R9. The obtained tandem OLED showed the maximum efficiencies of 38cd/A and 22.9lm/W, and still maintained high efficiencies of 35.5cd/A and 17.2lm/W at the luminance of 1000 cd/m2. Moreover, we studied the origins of both extreme color stability in dual-color OLED and extremely efficient electron injection within the CGU. To the best of our knowledge, this is the first report regarding quantitative values of both R9 and the Duv for a high-Ra tandem WOLED.

Photoluminescence properties and energy transfer in a novel Sr8ZnY(PO4)7:Tb3+,Eu3+ phosphor with high thermal stability and its great potential for application in warm white light emitting diodes

Energy transfer from Tb3+ to Eu3+ was observed and verified in a warm white light emission phosphor SZYP:Tb3+,Eu3+ with high thermal stability.

A red-emitting Sr3La(1-x)Eu x (AlO)3(BO3)4 phosphor with high thermal stability and color purity for near-UV-excited wLEDs.

A high efficiency red-emitting Eu3+ doped Sr3La(AlO)3(BO3)4 phosphor has been calcined successfully through a traditional solid-state method, and the photoluminescence and cathode-luminescence properties of samples are investigated in detail. The result of density functional theory (DFT) calculation shows that the Sr3La(AlO)3(BO3)4 host lattice has an indirect band gap. The broad excitation ranges from 200 nm to 500 nm, especially the strongest excitation located in the near ultraviolet (near-UV) region, suggests that the phosphor can match well with not only blue LED chips but also near-UV LED chips. Besides, the phosphor can emit bright red light peaking at 596, 618, 655 and 706 nm under either near-UV or blue light excitation and the internal quantum efficiency can reach 40%. The temperature-dependent PL properties and calculation of color purity show that the phosphor has an excellent thermal stability and outstanding color stability. Ultimately, by employing Sr3La(AlO)3(BO3)4:Eu3+ phosphor as a red light component, a stable warm white light emission is obtained with Ra = 80, CIE = (0.3443, 0.2903) and CCT = 4680 K. These results indicate that red Sr3La(AlO)3(BO3)4:Eu3+ phosphor may have a great potential for wLEDs with fantastic properties.

An adhesive bonding approach by hydrogen silsesquioxane for silicon carbide-based LED applications

We report an adhesive bonding approach using hydrogen silsesquioxane (HSQ) for silicon carbide (SiC) samples. A hybrid light-emitting diode (LED) was successfully fabricated through bonding a near-ultraviolet (NUV) LED grown on a commercial 4H-SiC substrate to a free-standing boron-nitrogen co-doped fluorescent-SiC epi-layer. The bonding quality and the electrical performance of the hybrid LED device were characterized. Neither voids nor defects were observed which indicates a good bonding quality of the proposed HSQ approach. A strong warm white emission was successfully obtained from the hybrid LED through an electric current injection of 30 mA.

Near UV excitable warm white light emitting Zn doped γ-Ga2O3 nanoparticles for phosphor-converted white light emitting diode

Optical properties of Zn doped γ-Ga2O3 nanoparticles are investigated for the generation of white light, using near UV LED chip for phosphor-converted WLED (pc-WLED) application. Sol-gel combustion technique is adopted for the synthesis of Zn doped γ-Ga2O3 nanoparticles. The synthesized Ga2O3 nanoparticles are formed as metastable γ-phase with the face-centred cubic crystal structure. The structural stability is maintained up to 15 mol% doping of Zn ions. Formation of aggregated ultrafine particles of undoped and Zn doped γ-Ga2O3 with size less than 4 nm is found from TEM results. Doping of Zn ions has altered the optical band gap of γ-Ga2O3, and it varies from 4.13 to 3.97 eV. Broad visible emission in the wavelength range of 400–600 nm is observed and broadening of emission band increases after Zn doping. Formation of high defect concentration and smaller size of 10 mol% Zn doped γ-Ga2O3 nanoparticles resulted in higher emission intensity. Wide photoluminescence excitation band from 330 to 475 nm suggests that wide visible emission can be obtained from Zn doped γ-Ga2O3 nanoparticles with the excitation of both near UV and blue light. The bi-exponential behavior of PL decay curves of emission bands at the blue and yellow region indicated the complicate luminescence process of Zn doped γ-Ga2O3. The estimated lifetime of blue emission varies from 30 to 37 ns and of yellow emission varies from 54 to 62 ns. Under the excitation of 375 nm LED chip, warm white light emission with CIE color coordinates of (0.42, 0.33), color rendering index of ~ 87 and CCT of 2365 K is obtained for 10 mol% Zn doped γ-Ga2O3 nanoparticles which exemplifying its potential application in pc-WLED fabrication.

Tunable luminescence and energy transfer of Dy3+-activated Bi4Si3O12--Bi4Ge3O12 pseudo-system phosphor for warm white-emitting

Dy3+-activated Bi4Si3O12-Bi4Ge3O12 pseudo-system phosphors have been synthesized by a solid state reaction method and characterized using XRD, SEM and fluorescence spectrometry. Upon 279 nm excitation, the emission of Dy3+ ions is observed at 486 nm (blue), 574 nm (yellow) and 662 nm (red), corresponding to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 transitions, respectively. The different kinds of excitation for the luminescence intensity behaviors have been discussed through efficient Bi3+ → Dy3+ energy transfer. As Si4+ is substituted by Ge4+, the crystallization and the intensity of emission are improved; however, the value of correlated color temperature (CCT) is also increased. The values of CIE chromaticity coordinates and CCT endorse warm white light emission from the phosphor. The paper shows that phosphor could be a potential single-emitting-component warm white emitting for LED applications.

Wide range yellow emission Sr8MgLa(PO4)7: Eu2+, Mn2+, Tb3+ phosphors for near ultraviolet white LEDs

A series of single phased phosphors Sr8MgLa(PO4)7: Eu2+, Mn2+, Tb3+ were synthesized by solid state reaction methods for applications of white light emitting diodes(w-LEDs). The broad absorption band at the near ultraviolet (NUV) region of 250–450 nm was obtained, which is ascribed to the 4f7→ 4f65d1 electronic transition of the Eu2+ ions. Under NUV excitation, the broad yellow emission was obtained from tri-activated Sr8MgLa(PO4)7 phosphors via combining three emission bands centered at 500, 550, and 600 nm contributed by Eu2+, Tb3+ and Mn2+, respectively. The optimized concentrations of Mn2+ and Tb3+ were 6 mol%, respectively, which enhanced the total PL intensity up to 30% comparing to only Eu2+ doped phosphor. Moreover, the quantum efficiency of Sr8MgLa(PO4)7: Eu2+, Mn2+, Tb3+ phosphor had achieved up to 52%. The combination of a NUV chip with compound of Sr8MgLa(PO4)7: Eu2+, Mn2+, Tb3+ and BaMgAl10O17:Eu2+ achieved tunable w-LEDs with high CRI. The results suggest that this single phased Sr8MgLa(PO4)7: Eu2+, Mn2+, Tb3+ phosphor can be a key for the phosphor converted w-LEDs for warm white light emission.

Wide-Range Tuning and Enhancement of Organic Long-Persistent Luminescence Using Emitter Dopants.

Most long-persistent luminescent (LPL) materials, which slowly release energy absorbed from ambient light, are based on inorganic compounds. Organic long-persistent luminescent (OLPL) systems have advantages over inorganic LPL materials in terms of solubility, transparency, and flexibility. Here, the characteristics of OLPL emission are improved by doping emitter molecules into an OLPL matrix. Greenish-blue to red and even warm white emission are achieved by energy transfer from exciplex in the OLPL matrix to the emitter dopants. The dopants also improve brightness and emission duration through efficient radiative decay and the trapping of electrons, respectively. This technique will enable the development of a wide range of organic glow-in-the-dark paints.

Warm white light emission of apatite-type compound Ca4Y6O(SiO4)6 doped with Dy3+

A series of Ca4Y6O(SiO4)6: x%Dy3+ (x = 1, 3, 5, 7, 9) phosphors were synthesized successfully via a conventional solid-state method at high temperature. The X-ray diffraction results show that all obtained samples are the pure hexagonal crystal structure of the space group P63/m (No.176). The blue (4F9/2 → 6H15/2) and yellow (4F9/2 → 6H13/2) emissions of Ca4Y6O(SiO4)6:7% Dy3+ phosphors are of such a ratio to produce almost perfect white light with CIE coordinates of (0.330, 0.339). The correlated color temperature is about 5600 K, close to that of the midday sunlight (5500 K). In addition, both the emission intensity and the CIE coordinates exhibit an excellent thermal stability in the temperature range of RT ∼ 300 °C. Our study illustrates that Ca4Y6O(SiO4)6:7%Dy3+ may serve as potential warm white lighting phosphors in NUV-based solid state lighting and optical display applications.

Energy transfer process in white organic light-emitting devices based on carbazole/thioxanthene-S,S-dioxide host material

Phosphorescent white organic light-emitting devices (Ph-WOLEDs) fabricated utilizing carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) host material were investigated to enhance their efficiency by understanding the energy transfer process from the host to the dopant materials. Time-resolved photoluminescence (PL) spectra at room and low temperatures demonstrated that the energy transfer occurs between the EBCz-ThX host and the bis(2-benzo[b]thiophen-2-ylpyridine)(acetylacetonate)-iridium(III) (Ir(bt)2acac) orange dopant or the (bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate (FIrpic) blue dopant. The energy transfer process was inferred from the PL decay curves of Ir(bt)2acac doped EBCz-ThX with doping concentrations of 5, 10, and 20%. The quantum efficiency in the Ph-WOLEDs with 5% Ir(bt)2acac doped EBCz-ThX is the highest since its energy transfer process from EBCz-ThX to Ir(bt)2acac involves the charge transfer states and is most efficient. The quantum efficiency of the Ph-WOLEDs with 5% Ir(bt)2acac doped EBCz-ThX emission layers at 1,000 cd/m2 showed the highest value of 10.2%, and the commission internationale de l'Eclairage (CIE) coordinates were (0.45, 0.46), indicative of the warm white emission.

Lead borate glasses triply doped with Dy3+/Tb3+/Eu3+ ions for white emission

Fully amorphous lead borate glasses triply doped with Dy3+/Tb3+/Eu3+ were synthesized and next studied using optical spectroscopy. The characteristic luminescence bands originating to electronic transitions of Dy3+/Tb3+/Eu3+ ions were observed under direct excitation of Dy3+. Moreover, the influence of concentration of rare earth ions on spectroscopic properties of lead borate glasses was examined. The obtained experimental results indicate that energy transfer between these rare earth ions is possible. From the emission spectra, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x, y) are calculated in relation to potential application lead borate glasses for white light emitting devices. It has been proved that our glass systems exhibit warm white emission originating from the simultaneous generation of several bands of Dy3+, Tb3+, Eu3+ under the UV–visible light excitation.

Synthesis and luminescence properties of single‐component Ca5(PO4)3F:Dy3+, Eu3+ white‐emitting phosphors

AbstractA series of Ca5(PO4)3F:Dy3+, Eu3+ phosphors was synthesized by a solid‐state reaction method. The XRD results show that all as‐prepared Ca5(PO4)3F:Dy3+, Eu3+ samples match well with the standard Ca5(PO4)3F structure and the doped Dy3+ and Eu3+ ions have no effect on the crystal structure. Under near‐ultraviolet excitation, Dy3+ doped Ca5(PO4)3F phosphor shows blue (486 nm) and yellow (579 nm) emissions, which correspond to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions respectively. Eu3+ co‐doped Ca5(PO4)3F:Dy3+ phosphor shows the additional red emission of Eu3+ at 631 nm, and an improved color rendering index. The chromaticity coordinates of Ca5(PO4)3F:Dy3+, Eu3+ phosphors also indicate the excellent warm white emission characteristics and low correlated color temperature. Overall, these results suggest that the Ca5(PO4)3F:Dy3+, Eu3+ phosphors have potential applications in warm white light‐emitting diodes as single‐component phosphor.

Highly efficient tandem organic light-emitting devices employing an easily fabricated charge generation unit

We have realized highly efficient tandem organic light-emitting devices (OLEDs) employing an easily fabricated charge generation unit (CGU) combining 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile with ultrathin bilayers of CsN3 and Al. The charge generation and separation processes of the CGU have been demonstrated by studying the differences in the current density–voltage characteristics of external-carrier-excluding devices. At high luminances of 1000 and 10000 cd/m2, the current efficiencies of the phosphorescent tandem device are about 2.2- and 2.3-fold those of the corresponding single-unit device, respectively. Simultaneously, an efficient tandem white OLED exhibiting high color stability and warm white emission has also been fabricated.

Novel single component tri-rare-earth emitting MOF for warm white light LEDs.

Single-phase white phosphors could overcome many drawbacks of traditional phosphors, and warm white light phosphors have been considered suitable for indoor illumination applications. Thus, synthesizing new single-phase warm white phosphors is significant for the production of LEDs. Based on the above considerations, a novel single component warm white light phosphor SmxTbyDy0.2-x-y-metal organic frameworks (MOFs) has been prepared successfully in this study. The compound Sm0.1Tb0.04Dy0.06-MOF shows ideal warm white light emission with CIE coordinates (0.333, 0.3522), a color rendering index (CRI) value of 86.7, a low correlated color temperature (CCT) value of 4444 K, and good heating/cooling circulation. In addition, the LED devices fabricated with this novel warm white light phosphor have excellent warm white light quality even at high temperatures, which is necessary for its practical application.

Novel highly efficient single-component multi-peak emitting aluminosilicate phosphors co-activated with Ce3+, Tb3+ and Eu2+: luminescence properties, tunable color, and thermal properties.

A series of emission-tunable Ce3+/Tb3+/Eu2+ doped Ca2(Mg0.75Al0.25)(Si1.75Al0.25)O7 (denoted as CMAS) phosphors have been synthesized via a high temperature solid-state reaction method. The luminescence properties, color tuning, quantum yields (QYs), energy transfer of Ce3+ to Tb3+/Eu2+, thermal stability, performance of LED devices and ratiometric temperature sensing application have been systematically investigated, respectively. Importantly, through the study of thermal stability, we found that Ce3+ and Tb3+ co-doped samples were suitable for WLED applications, while Ce3+ and Eu2+ co-doped samples were suitable for temperature sensing applications. Due to the energy transfer, Ce3+/Tb3+ co-doped samples had high luminous efficiency and the quantum efficiency of more than 80% could be achieved. Their emission colors can modulate from blue to green. In addition, on the basis of the evaluation of the as-fabricated white LED lamps via selecting the corresponding phosphors, the CCT can reach 4275 K and the CRI can increase to 86.8, indicating that this series of phosphors can act as potential color-tunable phosphors for possible applications in ultraviolet light based white LEDs. Importantly, it is found that the fluorescence intensity ratio of CMAS : 5%Ce3+,0.5%Eu2+ displays linear correlation with temperature in a wide range of 253-373 K with a high sensitivity of 2.49% K-1, indicating that it could be a good candidate for ratiometric optical thermometry.

Doping chloro boron subnaphthalocyanines and chloro boron subphthalocyanine in simple OLED architectures yields warm white incandescent-like emissions

We have incorporated chloro boron subphthalocyanine (Cl-BsubPc) and chloro boron subnapthalocyanines (Cl-ClnBsubNcs) into organic light emitting diodes (OLEDs) that enabled an overall warm white emission with CIE coordinates close to that of a 60 W incandescent lightbulb. More specifically, we have shown that Cl-BsubPc and Cl-ClnBsubNcs can be used as dopant emitters in a simple host-dopant architecture, and we have compared the use of NPB and Alq3 as potential hosts for these materials. When doped into Alq3, Cl-BsubPc shows a strong orange emission, and Cl-ClnBsubNcs shows a moderately strong red emission. We have further demonstrated that Cl-BsubPc and Cl-ClnBsubNcs can be co-doped into the same layer giving combined orange and red emission peaks. A “cascade” energy transfer mechanism of sequential absorption and re-emission is proposed. Device performance characteristics such as luminance, current efficiency, photoluminescence efficiency, and external quantum efficiency are tabulated. Additionally, in view of ongoing research into white emitting OLEDs for indoor lighting purposes, the Colour Rendering Index (CRI), R9 values, and CIE co-ordinates for these devices are also discussed. We conclude from this study that the BsubNc chromophore has potential application as a red dopant in OLEDs including for indoor lighting. Additionally, given the scope for axial and peripheral derivatization of the BsubNc motif, we believe that this chromophore has many unexplored molecular design handles that will affect its ultimate performance and application in OLEDs and other opto-electronic devices.

Highly-efficient tandem organic light-emitting device employing bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine doped with cesium azide in charge generation unit

Abstract In this research, we have investigated the charge generation and electron injection processes in charge generation unit (CGU) consisting of CsN3-doped Bis-4, 6-(3, 5-di-3-pyridylphenyl)-2-methylpyrimi-dine (CsN3:B3PYMPM)/Al/1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile (HAT-CN). By employing this CGU, a high-efficiency phosphorescent tandem device achieving best efficiencies of 164.3 cd/A, 46.51% and 109.8 lm/W has been realized. Particularly, in addition to the improvement in current efficiency by more than two times, the tandem device has better power efficiency compared to the corresponding single-unit device at the same current density. Simultaneously, an efficient tandem white OLED showing high color stability and warm white emission over a wide range of luminance has been also fabricated.

Luminescence and energy transfer in warm white-light-emitting K3Y(PO4)2:Tb,Eu phosphor for LEDs applications

To obtain warm white-emission in a new single-phase phosphor, a series of Tb–Eu doped K3Y(PO4)2 (KYP) samples were prepared by solid-state reaction method, and their photoluminescence properties were investigated. For Tb-single-doped KYP, the green emission mainly owing to the 5D4–7FJ (J = 3–6) transitions was obtained. For Eu-single-doped KYP, two kinds of emissions were observed, i.e., the line-peak and broad-band emissions, belonging to the Eu3+ 5D0–7FJ (J = 0–4) and Eu2+ d–f transitions, respectively. When Tb–Eu ions were co-doped into the KYP host, the Eu2+, Tb3+ and Eu3+ emissions were all found, which contribute to the blue, green and red components in the visible spectrum, respectively. By adjusting the relative concentrations of Tb and Eu, the emitting light color can be tuned and the warm white-emission was gained in the KYP:0.03 Tb,0.03Eu phosphor. The thermal luminescence stability of the KYP:0.03 Tb,0.03Eu sample was studied. The above investigations reveal that the present phosphors could have potential application in white LEDs.