EL Spectra Research Articles

A New Approach for White Organic Light-Emitting Diodes of Single Emitting Layer Using Large Stokes Shift

DPPZ showed UV-Vis. and PL maximum values of 412 and 638 nm, meaning the large stokes shift. Blue host compound, TAT was synthesized and used for co-mixed white emission. TAT exhibited UV-Vis. and PL maximum values of 403 nm and 445 nm in film state. Thus, when two compounds are used as co-mixed emitter in OLED device, there is no energy transfer from blue emission of TAT to DPPZ due to large stokes shift of DPPZ. Based on the PL result, it is available to realize two-colored white in PL and EL spectra. As a result of this, two-mixed compounds showed vivid their own PL emission peaks of 449 and 631 nm in film state. Also, white OLED device using two-mixed compounds system was fabricated. EL spectrum shows 457 and 634 nm peaks and two separate EL peaks, respectively. As the operation voltage is increased from 7 to 11 V, EL spectrum does not change the peak shape and maximum wavelength values. EL performance of white device showed 0.29 cd/A, 0.14 lm/W, and CIE (0.325, 0.195) at 7 V.

Effects of Position of Exciton-Blocking Layer on Characteristics of Blue Phosphorescent Organic Light-Emitting Diodes

In this study, we systematically examined the effects of the position of the exciton- blocking layer (EBL) in blue phosphorescent organic light-emitting diodes. The EBL was located either in the front and/or the rear of the emission layer (EML), and its effects to the device performances and electroluminescence spectra were investigated. The width and location of the recombination zone related to the triplet exciton quenching occurred in the devices with/without a front- or rear-EBL resulted in an optical micro-cavity effect in the EL spectrum at approximately 500 nm. The EBLs provided the direct, extra path of charge carriers from the hole transport layer (HTL)/electron transport layer (ETL) to the ETL/HTL through EML, resulting that the device operating voltage did not increase. The device with both front- and rear-placed EBLs exhibited the highest device performance, as triplet exciton quenching did not occur in it at the interface between the HTL/ETL and the EML.

A new approach way for white organic light-emitting diodes based on single emitting layer and large stokes shift.

New red dopant, DPPZ based on porphyrin moiety was synthesized. DPPZ showed UV-Vis and PL maximum values of 412 and 638 nm, indicating the large stokes shift. New blue host compound, TATa was also synthesized and used for co-mixed white emission. TATa exhibited UV-Vis. and PL maximum values of 403 nm and 463 nm in film state. Thus, when two compounds are used as co-mixed emitter in OLED device, there is no energy transfer from blue emission of TATa to DPPZ due to large stokes shift of DPPZ. Based on the PL result, it is available to realize two-colored white in PL and EL spectra. As a result of this, two-mixed compounds showed vivid their own PL emission peaks of 466 and 638 nm in film state. Also, white OLED device using two-mixed compounds system was fabricated. EL spectrum shows 481 and 646 nm peaks and two separate EL peaks. As the operation voltage is increased from 8 to 11 V, EL spectrum does not change the peak shape and maximum wavelength values. EL performance of white device showed 0.041 cd/A, 0.018 Im/W, and CIE (0.457, 0.331) at 8 V.

Using an embedded nanocomposite layer to increase color-conversion efficiency of organic light-emitting diodes

Down-conversion white organic light-emitting diodes (WOLEDs) have a significant advantage in generating stabilized white-light emissions, but still have room for further improvement in terms of color-conversion efficiency. We demonstrated that TiO2 nanoparticles mixed with fluorescent dyes could be used to increase the absorption of dyes and thus boost the efficiency of color-conversion. WOLEDs with a nanocomposite color-conversion layer achieved high efficiencies of 12.3% (22.9cd/A and 22.5lm/W) and stable white-light emission. In addition, the EL spectra with different viewing angles are close to the ideal Lambertain curve. These outcomes indicate that the nanocomposite-based color-conversion possesses great potential for use in display and lighting applications.

White light-emitting quantum dot diodes and tuning of luminescence processes

A novel white light-emitting diode based on a large Stokes shift (~200 nm) and using pure green light-emitting CdSeS quantum dots (QDs) with an Ag/ZnSnO/QDs/spiro-TPD/ITO structure has been fabricated in which ZnSnO and spiro-TPD are served as the electron and hole transport layer, respectively. The large Stokes shift of the CdSeS QDs excludes potentially Forster resonance energy transfer process, which allows spiro-TPD to act as both an emitter and hole transport layer. The devices exhibit a wide EL spectrum consisting of three components: blue emission from spiro-TPD, green emission from QD band–band recombination, and red emission from QD surface-state recombination. We further found that as the intensity ratios among these three components vary with bias the color of the QD light-emitting diodes is tunable. The device displays a good white light-emitting characteristic with CIE coordinates of (0.281, 0.384) at an appropriate bias.

Current density impact on the emission behavior of GaN‐based blue emitting LEDs in the temperature range of 4.2–400 K

Blue emitting GaN‐based light emitting diodes (LEDs) show a distinct spectral behavior with respect to temperature and injection current density. Operating LEDs with short current pulses of 500 ns provides a steady state situation, which allows investigating the emission behavior of LEDs at a certain device temperature thereby maintaining thermal equilibrium. The LEDs were examined in a temperature range between 4.2 and 400 K and in a current density range between 2 and 50 A cm−2. Low temperature investigations showed a blue shift of the electroluminescence spectra (EL) with respect to junction temperature, which is assigned to the radiative recombination of localized excitons. In the elevated temperature region a distinct red shift due to energy gap shrinkage was observed. Further, we expect an exciton lifetime reduction at 4.2 K. Additionally, the influence of the driving parameters (pulse injection current or direct current (DC)) in the presence of piezoelectric fields is discussed and separated into band filling effect and occurrence of fields, which screen the quantum confined Stark effect. Low temperature investigations indicate that band filling is mainly responsible for the blue shift of the EL spectra with respect to the injection current.

A simple effective method to improve light out-coupling in organic light-emitting diodes by introducing pyramid-based texture structure

This paper showed a very simple method to increase the light extraction of organic light-emitting diodes (OLEDs) by attaching an inverted pyramid texture film to them. The texture film was fabricated by transferring the pyramid-based textured morphology of mono-crystalline silicon surface using a printing process. Compared to the reference device, our results showed a great improvement in the current efficiency (+37.0%) and external quantum efficiency (+36.6%) at a current density of 100mA/cm2. Furthermore, the EL spectrum and CIE coordinates of the OLEDs did hardly vary with the introduction of the texture film and variation of viewing angles, and the OLED with the texture film was proved to be a perfect Lambertian emitter, which suggested that the utilization of this method was a promising approach for the development of OLEDs and could be suggestive conduction to EL lightening and backlight.

Fabricating ZnO single microwire light-emitting diode with transparent conductive ITO film

In this paper, n-ZnO single microwire/p+-Si heterojunction LEDs are fabricated using the transparent conductive ITO film as an electrode. A distinct UV emission resulting from free exciton recombination in a ZnO single microwire is observed in the electroluminescence. Size difference of ZnO single microwire shows significant influence on emission efficiency. The EL spectra of n-ZnO single microwire/p-Si heterostructure exhibited relatively stronger UV emission which was compared with the EL spectra of n-ZnO single nanowire/p-Si heterostructure and n-ZnO film/p-Si heterostructure, respectively.

Novel violet emitting material synthesized by stepwise chemical reactions

In this paper, we report the design and synthesis of a novel bipolar violet emitting molecule CzPySiSF. The carbazole and pyridine moieties are employed to facilitate charge injection and balance carrier transport, whereas the spirofluorene is used as the violet emitter. The three functional groups, i.e., carbazole, pyridine and spirofluorene, are connected to tetraphenylsilane in a stepwise fashion using classical coupling reactions such as Suzuki and Ullmann. The resultant bipolar molecule CzPySiSF exhibits very stable thermal properties and a uniform amorphous morphology. The introduction of spirofluorene greatly enhances the fluorescence quantum efficiency of the molecule; moreover the PL spectrum of CzPySiSF in THF is mainly located in the violet region. The EL spectrum of CzPySiSF matches well with the PL spectrum with a maximum at 408 nm. The violet OLED of CzPySiSF exhibits the maximum external quantum efficiency of 0.59%.

New Ambipolar Blue Emitting Materials Based on Amino Coumarin Derivatives with High Efficiency for Organic Lightemitting Diodes

New blue emitting materials, 7-diphenylamino-4-methyl-coumarin (DPA-MC) and amino-tri(4-methylcoumarin) (T-MC) including coumarin moiety were synthesized by Ullman reaction. Optical and electronic properties were examined by UV-Vis. Absorption spectrum, PL spectrum, and cyclic voltammetry. UV-Vis. spectra of DPA-MC and T-MC in a film state showed maximum absorption wavelengths of 382 nm and 399 nm, respectively. PL spectra of DPA-MC and T-MC show maximum emission wavelengths of 463 nm and 481 nm respectively. Non-doped OLED devices were fabricated by using the synthesized materials as an emitting material layer. DPA-MC compound showed highly efficient luminescence properties. EL spectrum of DPA-MC exhibited a maximum value of 463 nm and DPA-MC device provided luminescence efficiency of 3.83 cd/A, power efficiency of 2.46 Im/W, external quantum efficiency of 3.71% and CIE coordinates of (0.154, 0.190) at a current density of 10 mA/cm2. In particular, Power efficiency increased by more than 1.6 times in DPA-MC (2.46 ImNW), which is higher than commercialized material, DPVBi (1.46 Im/W). High EL performance might come from ambipolar effects of a molecular structure.

Solution-processable tandem solid-state light-emitting electrochemical cells

Compared to organic light-emitting diodes (OLEDs), solid-state light-emitting electrochemical cells (LECs) exhibit simple single-layered structure and low operating voltages due to in situ electrochemical doped layers. However, device efficiencies of LECs are usually lower than those of sophisticatedly designed OLEDs. Furthermore, device efficiencies and lifetimes of LECs degrade significantly as brightness increases. In this work, we demonstrate tandem LECs to obtain nearly doubled light outputs (μWcm−2) in comparison with single-layered LECs under similar current densities. Since the output EL emission is modified by microcavity effect of the device structure, the EL spectra of tandem LECs exhibit EL emission peak at ca. 625nm while the EL spectra of single-layered LECs center at ca. 660nm. Better spectral overlap between the EL spectrum of tandem LECs and the luminosity function results in further enhanced candela values, rendering a tripled brightness (cdm−2). The device efficiencies can be optimized by adjusting the thickness of the connecting layer between the two emitting units of the tandem devices. The peak external quantum efficiency achieved in tandem LECs is up to 5.83%, which is higher than twice of that obtained in single-layered LECs due to improved carrier balance. When single-layered and tandem LECs are biased under higher voltages to reach similarly higher brightness, tandem LECs show higher device efficiencies and longer lifetimes simultaneously. These results indicate that device efficiencies and lifetimes of LECs can be improved by employing a tandem device structure.

Theoretical study of N-face InGaN light-emitting diodes with GaN–InGaN–GaN barrier near p-side and n-side

The GaN–InGaN–GaN barrier has been designed to replace the conventional GaN barriers near p-GaN and n-GaN side for N-face InGaN/GaN multiple quantum well light-emitting diodes (LEDs). The advantages of the N-face LED with GaN–InGaN–GaN barrier near n-GaN are numerically studied. The EL spectra, power–voltage performance curves, electron concentration, energy band diagrams and radiative recombination rate in the active region are systematically investigated. The simulated results show that the N-face LED with GaN–InGaN–GaN barrier near n-GaN has better performance over its conventional N-face LED and counterpart with GaN–InGaN–GaN barrier near p-GaN due to the appropriately modified energy band diagrams which are favorable for the injection of electrons and enhanced radiative recombination rate in the quantum wells.

Enhancement of light extraction of green top-emitting organic light-emitting diodes with refractive index gradually changed coupling layers

By means of refractive index gradually changed coupling layers, a highly efficient green top-emitting OLED (TOLED) with enhanced light coupling efficiency and stable colors over angles has been realized. The refractive index transition of the coupling layers including the doping layer smoothes light extraction from the semitransparent cathode metal to the air, which is the reason for the enhancement of light coupling efficiency. The doping layer in the coupling layers also acts as a microparticle diffuser to eliminate the shift in EL spectra with viewing angles. A universal simulation has also been carried out, and the result suggests that the light coupling efficiency will be enhanced further if the refractive index transition of the coupling layers is continuous.

Iridium(III) complexes with 5-substitute-8-quinolinolates and 2-phenylpyridine as ligands: Synthesis, characterization and photoluminescent as well as electroluminescent properties

The synthesis, structures and photophysics of a series of heteroleptic iridium(III) complexes with 5-substitute-8-quinolinolates and 2-phenylpyridine as the cyclometalating ligands are reported. The complexes have the general structure (C^N)₂IrQ, where C^N represents 2-phenylpyridine and Q is derivatives of 8-hydroxyquinoline. The crystal structure of the complex (ppy)₂IrQPhOMe (<bold>7</bold><bold>b</bold>) has been determined by X-ray crystallography. Assessment of the thermal stabilities of the complexes reveals greatly differences depending on the substituents on the quinolinolato ligand. At room temperature, the complexes with 5-phenyl-quinolinolato ligand have red phosphorescent emission with two structured emission peaks, which are almost equal intensity at 666 and 687 nm. It is found that the emission characteristics are dominated by the 5-phenyl-quinolinolato ligand, whereas substitute groups on phenyl have less impact on the emission characteristics. Furthermore, the complex with 5-amido-8- hydroxyquinoline as ligand, complex <bold>8</bold>, shows unstructured emission at 550 nm, which is similar to (ppy)₂Ir(acac) suggesting 2-phenylpyridine governed emission in this case. The polymer phosphorescent organic light-emitting device (OLED) was fabricated by doping complex <bold>7b</bold> in PFO + 30 wt% PBD, and gives red EL spectra with CIE coordinates of (0.61, 0.33) at 14 V and maximum luminance intensity of 350 cd/m² at 18 V.

P.144: Late‐News Poster: Synthesis and Electroluminescence Properties of Highly Efficient Blue Fluorescence Emitters Using Dual Core Chromophores

AbstractWe propose new dual core chromophore materials with blue emission. One of these derivatives, TP‐AP‐TP, exhibits a high luminance EQE value of 7.51% and twice the lifetime of a commercialized material, MADN. The dual core chromophore materials have narrower PL and EL spectra and better thermal properties than the single core chromophore materials.

The effect of absorption and coherent interference in the photoluminescence and electroluminescence spectra of SRO/SRN MIS capacitors

In this paper we present a technique that can be used to study the effect of absorption and coherent interference in the luminescence of multilayer structures. We apply the technique to the measured photoluminescence and electroluminescence spectra of MIS capacitors where the insulator is composed of a silicon rich oxide (SRO)/silicon rich nitride (SRN) bilayer structure. We remove the effect of the multilayer stack on the measured photoluminescence spectrum of the samples without the metal contact to find the intrinsic spectrum. Then we apply the effect of the MIS structure on the intrinsic spectrum in order to calculate the electroluminescence spectrum. Good agreement with the experimentally measured EL spectrum is found. We discuss which parameters affect the spectra most significantly.

Identifying the efficient inter-conversion between singlet and triplet charge-transfer states by magneto-electroluminescence study

Using the magneto-electroluminescence (MEL) as a tool, we demonstrated the efficient inter-conversion between singlet and triplet charge-transfer (CT) states in exciplex-based organic light-emitting diodes (OLEDs). Results show that the MEL of exciplex-based device is larger than that of exciton-based device by a factor of 3.2. The emission of exciplex-based devices comes from the direct intermolecular electron-hole pair recombination and their spin exchange energy is much smaller, which causes the efficient inter-conversion between singlet and triplet states. This argument was supported by the consistent evolutions of the MEL and EL spectra versus applied bias and donor concentrations. Finally, the bandgap effects on the MEL as well as the external quantum efficiency of exciplex-based devices were discussed. Our findings of MEL may offer a feasible way to unravel underlying mechanisms that limit the EL efficiency in the OLEDs.

Synthesis and electroluminescence properties of highly efficient blue fluorescence emitters using dual core chromophores

We describe two novel blue emission materials based on a new type of dual core concept. 1-Phenyl-6-(10-phenyl-anthracen-9-yl)-pyrene (Ph-AP-Ph) and 1-[1,1′;3′,1′′]terphenyl-5′-yl-6-(10-[1,1′;3′,1′′]terphenyl-5′-yl-anthracen-9-yl)-pyrene (TP-AP-TP) were synthesized through boronylation and Suzuki coupling reactions. The Tg values of Ph-AP-Ph and TP-AP-TP were 228 °C and 243 °C, respectively, compared to values of 135 °C and 139 °C for the single core materials 9-(3′,5′-diphenylphenyl)-10-(3′′′,5′′′-diphenylbiphenyl-4′′-yl)anthracene (MAM) and 1,6-bis-[1,1′;3′,1′′]terphenyl-5′-yl-pyrene (TP-P-TP). One of the dual core derivatives, TP-AP-TP, exhibited an ELmax value of 456 nm and a high luminance EQE of 7.51% when used in an EL device. The dual core chromophore materials had narrower PL and EL spectra and better thermal properties than the single core chromophore materials. A device based on TP-AP-TP showed twice the lifetime of a device based on the commercialized material, 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN).

Enhancement of electroluminescence in zirconium poly carboxylic acid-based light emitting diodes by bathophenanthroline ligand

The reactions of a zirconium salt with 1,2,4,5-benzenetetracarboxylate (btec), bathophenanthroline (Bphen) and thiocyanate ions were synthesized and studied by changing the mole ratio, the order of reactant and their pH. It is found that the coordination mode of btec acid depends on the control of reaction conditions. Monodentate, bidentate and bridging modes were investigated by FT-IR spectroscopy. The structures of Zr(btec) and Zr(btec)(Bphen) complexes were also characterized by UV-Vis, CHN, ICP-AES, (1)H NMR and cyclic voltammetry. The role of Bphen ligand in the photopysical properties of Zr(btec)(Bphen) complexes was investigated by DFT calculation. The photoluminescence (PL) emission of nine Zr(btec) complexes that have two peaks, a sharp and intense band for the first peak from 320 to 430 nm in comparison to the second peak with a less intensity and broadened in the regions of 650-780 nm. PL spectra of twelve Zr(btec)(Bphen) complexes also showed bands at 450, 550, 625 nm. LED devices with Zr complex as emitter layer and the structure ITO/PEDOT:PSS/PVK:PBD/zirconium complex/Al emitted a broad band centered at 550 and 650 originating from the Zr complexes. The EL spectra of Zr(btec) and Zr(btec)(Bphen) complexes indicated a long red shift rather than PVK:PBD blend. We believe that the electroplex occurring at PVK-Zr complexes interface is responsible for the green-red emission in the EL of the device. These observations suggest an important role for the Bphen ligand to improve EL performance.

Light extraction enhancement in organic light-emitting diodes based on localized surface plasmon and light scattering double-effect

We have demonstrated a new approach to realize the light extraction enhancement in organic light-emitting diodes (OLEDs) by using platinum–cobalt (Pt3Co) alloy nanoparticles (ANPs). The current efficiencies of 19.2 cd A−1 and 29.3 cd A−1 at a current density of 20 mA cm−2 are obtained for the device with Pt3Co ANPs unannealed and annealed, respectively, which correspond to a ∼46% and ∼123% enhancement compared to the control device without Pt3Co ANPs. A systematic study on the devices with and without unannealed Pt3Co ANPs shows that the significantly enhanced efficiency is mainly due to the resonance of localized surface plasmon (LSP). The analysis of surface topography, angular-dependent EL spectra and theoretical calculations of the devices with and without annealed Pt3Co ANPs reveals that the annealing treatment of Pt3Co ANPs could result in a further enhancement in light extraction due to the increased light scattering effect, thereby achieving a double-enhancement in light extraction by simultaneously realizing the resonance of LSP and the light scattering effect.