Performance Of Electroluminescent Devices Research Articles

Investigating energy level alignments at organic–organic interfaces in practical devices

Energy level alignments are crucial for designing high-performance semiconductor devices. However, the reported energy levels, especially the lowest unoccupied energy levels (LUMOs), exhibit significant variability for a given molecular compound. This variability often leads to misunderstanding of device working mechanisms. In this study, single-carrier devices with organic/interlayer/organic structures are proposed to probe the energy level alignments at organic–organic heterojunctions. It is observed that carrier transport characteristics deviate significantly depending on charge scattering or trapping. Five organic molecules, including 1, 3, 5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPB), 4, 4′-bis(arbazole-9-y1)biphenyl (CBP), 2, 2′, 2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2), and tris(8-hydroxyquinolinato)aluminum (Alq3), are utilized to test the method. The deduced LUMO level order for these materials is found to deviate significantly from reported values. Furthermore, the effect of differences in the energy level arrangement on the performance of electroluminescent devices is investigated. This work suggests that determining LUMO energy alignments via single-carrier analysis is a valuable method for understanding device working mechanisms.

Multi‐Resonance Thermally Activated Delayed Fluorescence Molecules for Triplet‐Triplet Annihilation Upconversion

AbstractTriplet‐triplet annihilation upconversion (TTA‐UC) has made significant progress in recent years in several key applications, including solar energy harvesting, photocatalysis, stereoscopic 3D printing, and disease therapeutics. In TTA‐UC research, photosensitizers serve the vital function of harvesting low‐energy photons. The photophysical characteristics of photosensitizers, including absorbance, triplet state quantum yield, triplet state energy level, triplet state lifetime, etc., determine the performance of TTA‐UC. Thus, the study of photosensitizers has been a key aspect of TTA‐UC. In recent years, multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have received extensive attention due to their excellent photophysical properties and electroluminescent device performance. MR‐TADF molecules not only present a narrow energy gap between the singlet and triplet excited states, but also have stronger absorption and better wavelength regulation than conventional TADF molecules. Nowadays, the preliminary attempts in TTA‐UC using MR‐TADF molecules as photosensitizers have resulted in the development of green to ultraviolet, blue to ultraviolet, and even near‐infrared to blue emission. This concept will summarize the research progress of MR‐TADF molecules as photosensitizers in TTA‐UC, analyzing the challenges and giving possible solutions. Finally, we prospect the future development of MR‐TADF molecules as photosensitizers, including the molecular design as well as the possible application areas.

Detection of Dexter energy transfer process in interface-type OLED via utilizing the characteristic magneto-electroluminescence response of hot exciton reverse intersystem crossing

The maximum external quantum efficiency of the host-guest-type organic light-emitting diodes (OLEDs) with interface exciplex as the host has been over 36%. However, studies about the energy transfer processes occurring from the host to guest remain lacking. Herein, a strategy is proposed to probe the energy transfer processes in interface-type OLEDs by utilizing the characteristic magneto-electroluminescence (MEL) response from the hot exciton reverse intersystem crossing (T<sub>2,Rub</sub> → S<sub>1,Rub</sub>) of rubrene. Specifically, a donor/spacer/accepter (D/S/A)-type interface exciplex device and a D/spacer:<i>x</i>% Emitter/A (D/S:3% Rubrene/A)-type Rubrene-doped device are fabricated. The Förster resonance energy transfer (FRET) process occurring between the singlet state of the exciplex-host and the singlet state of Rubrene-guest is demonstrated by characterizing the photophysical properties of the donor, accepter, and guest materials. The Dexter energy transfer (DET, T<sub>1,Host</sub> → T<sub>2,Rub</sub>) process between the triplet state of the host and the triplet state of guest is visualized by the comparative studying of the current- and temperature-dependent MEL response curves of D/S/A and D/S:3% Rubrene/A devices, respectively. More importantly, the occurrence of the DET process greatly promotes the electroluminescence intensity of the D/S:3% Rubrene/A device. Furthermore, we also investigate the differences in the electroluminescence performance of devices at low temperature to demonstrate again the co-existence of FRET and DET process in the D/S:3% Rubrene/A system. Obviously, this work not only provides a promising strategy for probing the DET process in OLEDs, but also paves a new way for designing high-performance “hot exciton” type OLEDs.

Nanostructured colloidal quantum dots for efficient electroluminescence devices

The exceptional quality of light generated from colloidal quantum dots has attracted continued interest from the display and lighting industry, leading to the development of commercial quantum dot displays based on the photoluminescence down-conversion process. Beyond this technical level, quantum dots are being introduced as emissive materials in electroluminescence devices (or quantum dot-based light-emitting diodes), which boast high internal quantum efficiency of up to 100%, energy efficiency, thinness, and flexibility. In this review, we revisit various milestone studies regarding the core/shell heterostructures of colloidal quantum dots from the viewpoint of electroluminescence materials. Development of nanostructured colloidal quantum dots advanced from core/shell heterostructure, core/thick shell formulation, and delicate control of confinement potential shape has demonstrated close correlation of the photophysical properties of quantum dots with the performance of electroluminescence device, which provided useful guidelines on the heterostructured quantum dots for mitigating or eliminating efficiency limiting phenomena in quantum dot light emitting diodes. To enable practical and high performance quantum dot-based electroluminescence devices in the future, integration of design concepts on the heterostructures with environmentally benign systems will be crucial.

Feasible structure-modification strategy for inhibiting aggregation-caused quenching effect and constructing exciton conversion channels in acridone-based emitters.

Acridone (ADO) is an anthracene-based derivative that plays an important role in the construction of organic light-emitting diode emitters. However, ADO suffers from an aggregation-caused quenching (ACQ) effect because of its strong intermolecular stacking and tendency to form excimers. In this work, we appended some electron-donating moieties with different rotors and substitution patterns on ADO to prepare six ADO-based derivatives. In addition, a benzonitrile group was introduced onto the nitrogen atom of the ADO unit to fabricate a high-energy charge-transfer (CT) state that formed a reverse intersystem crossing (RISC) channel. Systematic spectral measurements revealed that the rotors effectively suppressed the ACQ effect. In addition, aggregation-enhanced emission (AEE) was observed for the ADO derivatives modified with triphenylamine (TPA) because of the existence of multiple rotors and propeller-like conformation in TPA block. Theoretical calculations and the performance of electroluminescent devices containing the derivatives confirmed that the exciton conversion channel was constructed at the high-energy level and activated during device operation. Although the performance of these ADO-based derivatives was not ideal in terms of efficiency, the results confirmed the feasibility of this structure modification strategy to simultaneously inhibit the ACQ effect and construct excitons conversion channels.

Enhancing thermally activated delayed fluorescence characteristics by intramolecular B-N coordination in a phenylpyridine-containing donor-acceptor π-system.

An efficient thermally activated delayed fluorescence emitter based on an N-heteroarene acceptor bearing an intramolecularly coordinated boryl group was developed. The impact of the B-N coordination on the optoelectronic properties and electroluminescence performance of devices was investigated.

Charge transfer through amino groups-small molecules interface improving the performance of electroluminescent devices

A carboxylic group functioned charge transporting was synthesized and self-assembled on an indium tin oxide (ITO) anode. A typical electroluminescent device [modified ITO/TPD (50nm)/Alq3 (60nm)/LiF (2nm)/(120nm)] was fabricated to investigate the effect of the amino groups-small molecules interface on the characteristics of the device. The increase in the surface work function of ITO is expected to facilitate the hole injection from the ITO anode to the Hole Transport Layer (HTL) in electroluminescence. The modified electroluminescent device could endure a higher current and showed a much higher luminance than the nonmodified one. For the produced electroluminescent devices, the I-V characteristics, optical characterization and quantum yields were performed. The external quantum efficiency of the modified electroluminescent device is improved as the result of the presence of the amino groups-small molecules interface.

Optimization of Metal Oxides Thickness and Related Organic Electroluminescent Device Performance

In this study, three commercial transition metal oxides such as molybdenum trioxide (MoO3), tungsten trioxide (WO3), and vanadium pent-oxide (V2O5) were employed as hole-injection layer to improve the electrical and optical performance of organic light-emitting diodes (OLEDs). The layer thickness of MoO3, WO3 and V2O5 were respectively varied by 1, 2, 5 and 10 nm inside blue OLED to characterize their effects on device performance. The optimal OLED with a 5 nm MoO3 hole-injection layer performs an enhancement of 12.9% in current efficiency, 17.5% in power efficiency and 9.3% in maximum external quantum efficiency, comparing to that of reference device without hole-injection layer.

Modeling study of mesh conductors and their electroluminescent devices

Numerical models were established to correlate with the experimentally measured properties of mesh conductors previously developed through a combined process of dip coating carbon nanotubes and inkjet printing poly 3,4-ethylenedioxythiophene: poly styrene sulfonate. The electroluminescent (EL) devices assembled with such mesh conductors as front electrodes were modeled by commercially available finite element method software COMSOL Multiphysics. The modeling results are in agreement with those from the experiments and suggest that an optimized fiber arrangement is the key for further improving the performance of EL devices based on mesh conductors.

The Influence on the Organic Electroluminescent Device Performance with Different DPAVBi Position

We studied the influence on the organic electroluminescent device performance with different DPAVBi position. When DPAVBi was the separate blue light emitting layer and its thickness was 20 nm, the performance of the device is better than others. The yellow light device performance with DPAVBi behind the Rubrene layer is better than the device with it in front of Rubrene layer. The device has a maximum luminous 23560 cd/m2 at 17V and maximum efficiency 6.63cd/A at 16 V. We have received the blue-green light device with the Rubrene doped to DPAVBi. The maximum efficiency is 5.37 cd/A at 9 v and the maximum luminance is 6377 cd/m2 at 16 V. The efficiency drops slowly when the voltage increases. So, all the devices have the current weak fluorescence quenching.

Synthesis of Polyfluorene Block Copolymers and Effect of Side Chain Group on Electroluminescent Device Performance

Abstract Blue-light-emitting block copolymers based on polyfluorene (PF) and poly(triphenylamine) (PTPA) were synthesized for polymeric electroluminescent (EL) materials. PF with diphenylamine terminals were prepared by the Suzuki coupling polymerization, and converted to PTPA-block-PF-block-PTPA triblock copolymers by C–N coupling polymerization. The block copolymers with different substituents on PTPA segment, hydrophilic trioxyethylene (TEO) group (PF–PTPA–TEO) or hydrophobic n-butyl group (PF–PTPA–C4), were prepared in order to investigate their effect on device performance. The block copolymers showed relatively high HOMO levels compared with PF. EL devices based on the block copolymers showed higher luminance and current efficiency than that of PF homopolymer because of the improvement of hole injection by the introduction of PTPA segment. Modification by TEO group provided higher device performance than n-butyl group, indicating that PTPA segment segregated at the interface with PEDOT:PSS by the hydrogen-bonding interaction.

New Conjugated Triazine Based Molecular Materials for Application in Optoelectronic Devices: Design, Synthesis, and Properties

Two novel triazine-based compounds with efficient hole-blocking ability were designed and synthesized, which promoted the performance of electroluminescent devices, especially for PLED even when Al was applied as the cathode. The results indicated that introducing electron-donating groups and extending π-conjugated were facile methodologies to improve the optoelectronic properties of the triazine derivatives.

Nanoscale Ordered Structure Distribution in Thin Solid Film of Conjugated Polymers: Its Significance in Charge Transport Across the Film and in Performance of Electroluminescent Device

We use ultrahigh-vacuum conducting atomic force microscopy to probe the local current distributions in spin-cast thin films from the solutions of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(9,9-di-n-octyl-2,7-fluorene) (PFO). We found that spatially homogeneous distribution of the ordered structures (well-packed chains and/or aggregates) in MEH-PPV can be controlled by the selection of solvent or mixed solvent, by which effects of spatial charge transport distribution in MEH-PPV thin films on the performance of polymer light-emitting diodes (PLEDs) are unambiguously clarified. For PFO thin film, after the treatment by immersing in the mixed nonsolvent composed of a solvent and nonsolvent, the ordered structures (beta-phase) are generated; its excess content can result in highly conducting regions. However, the device efficiency can be promoted significantly by optimizing the content of beta-phase.

Synthesis and Characterization of Tb-doped AlBNO Films for Electroluminescence Devices

AbstractTb-doped AlBNO (AlBNO:Tb) films with various composition ratios are investigated for luminescence layers of inorganic electroluminescence(EL) devices. Luminescence layers with a wide bandgap and a low dielectric constant are required to realize high performance of EL devices. The ultraviolet-visible radiation absorption measurement and capacitance-voltage (C-V) measurement show that the AlBNO:Tb films have wider bandgap and lower dielectric constant than ZnS which is put to practical use as the host material of the luminescence layer. Photoluminescence (PL) measurement indicates that PL intensity increases with increasing B composition ratio in the range of 5 % - 10 %. Moreover, the suppression factor of the PL intensity can be understood through the annealing experiment. The PL intensity of the film with 800 °C annealing is about 10 times larger than that of the film without annealing. X-ray photoelectron spectroscopy (XPS) measurement suggests that Tb4+ ions decrease compared with Tb3+ ions after annealing treatment. O atoms in the AlBNO:Tb film are dissociated from Tb and bonded to B atoms by annealing treatment. This suggests that decrease of Tb4+ ions is related to increase of the PL intensity.

Red organic light-emitting diodes with high efficiency, low driving voltage and saturated red color realized via two step energy transfer based on ADN and Alq3 co-host system

Abstract We demonstrated efficient red organic light-emitting diodes based on a wide band gap material 9,10-bis(2-naphthyl)anthracene (ADN) doped with 4-(dicyano-methylene)-2- t -butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4 H -pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1 H ,5 H ,11 H -10(2-benzothiazolyl)quinolizine-[9,9 a ,1 gh ]coumarin (C545T) as an assistant dopant. The typical device structure was glass substrate/ITO/4,4′,4″-tris( N -3-methylphenyl- N -phenyl-amino)triphenylamine ( m -MTDATA)/ N , N ′-bis(naphthalene-1-yl)- N , N ′-diphenyl-benzidine (NPB)/[ADN:Alq 3 ]:DCJTB:C545T/Alq 3 /LiF/Al. It was found that C545T dopant did not by itself emit but did assist the energy transfer from the host (ADN) to the red emitting dopant via cascade energy transfer mechanism. The OLEDs realized by this approach significantly improved the EL efficiency. We achieved a significant improvement regarding saturated red color when a polar co-host emitter (Alq 3 ) was incorporated in the matrix of [ADN:Alq 3 ]. Since ADN possesses a considerable high electron mobility of 3.1 × 10 −4 cm 2 V −1 s −1 , co-host devices with high concentration of ADN (>70%) exhibited low driving voltage and high current efficiency as compared to the devices without ADN. We obtained a device with a current efficiency of 3.6 cd/A, Commission International d’Eclairage coordinates of [0.618, 0.373] and peak λ max = 620 nm at a current density of 20 mA/cm 2 . This is a promising way of utilizing wide band gap material as the host to make red OLEDs, which will be useful in improving the electroluminescent performance of devices and simplifying the process of fabricating full color OLEDs.

Enhancement of Electroluminescence Properties in OLEDs on Polyethylene Terephthalate with Ruthenium-Oxide-Coated Anode and Mg – Al Alloy Cathode

The electroluminescence performance of devices as enhanced by using ruthenium oxide as a hole injection layer (HIL) between indium-tin-oxide (ITO) anodes and 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl and using alloy as a cathode. Both the HIL and the alloy were applied to organic light emitting diodes (OLEDs) on both glass and flexible substrates. The operation voltage at a current density of decreased from 14.7 to when the interlayer and cathode were applied to OLEDs on glass substrate. Also, the operation voltage reduced from 17.1 to for OLED on polyethylene terephthalate (PET) substrate. The maximum luminance value increased from 1510 to in OLEDs on glass substrate using the interlayer and cathode. Synchrotron radiation photoelectron spectroscopy results showed that core level of the spectra shifted toward higher binding energy when was deposited on -coated ITO/PET substrate. Secondary cutoff spectra revealed that the work function increased by when the interlayer was deposited on ITO/PET substrate. Thus, the interlayer lowered the potential barrier for hole injection, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency.

Electroluminescent SrS and BaS Thin Films Deposited by ALD Using Cyclopentadienyl Precursors

Electroluminescent (EL) SrS and BaS thin films were deposited by the atomic layer deposition (ALD) technique using cyclopentadienyl-based metal precursors. The films were doped either with Cu, Ce, Pb, Mn, or Eu. BaS:Mn and BaS:Eu thin films were deposited for the first time by the ALD. The usable range of deposition temperatures was wide, but results of this and earlier ALD studies indicate that high deposition temperature is needed for the high emission intensity. It is shown that emission intensities and colors of the EL devices are comparable with the earlier ALD and chemical vapor deposition studies. It is also suggested that the dopant precursors have stronger impact to the emission than the host metal precursors, and that the use of the cyclopentadienyl-based dopants may result in further improvements in the performance of EL devices. The double-peak nature of Mn emission is also reported in both SrS and BaS, which is suggested to originate from at different crystal sites. © 2004 The Electrochemical Society. All rights reserved.

Near‐Infrared Light‐Emitting Diodes (LEDs) Based on Poly(phenylene)/Yb‐tris(β‐Diketonate) Complexes

AbstractNear‐infrared‐emitting electroluminescent (EL) devices using blue‐light‐emitting polymers blended with the Yb complexes Yb(DBM)3phen (DBM = dibenzoylmethane), Yb(DNM)3phen (DNM = dinaphthoylmethane), and Yb(TPP)L(OEt) (L(OEt) = [(C5H5)Co{P(O)Et2}3]–) have been studied. EL devices composed of Yb(DNM)3phen blended with PPP‐OR11 showed enhanced near‐IR output at 977 nm when compared to those fabricated with Yb(DBM)3phen/PPP‐OR11 blends. The maximum near‐IR external efficiencies of the devices with Yb(DBM)3phen and Yb(DNM)3phen are, respectively, 7 × 10–5 (at 6 V and at 0.81 mA mm–2) and 4 × 10–4 (at 7 V, and 0.74 mA mm–2). The optimal blend composition for EL device performance consisted of PPP‐OR11 blended with 10–20 mol‐% Yb(DNM)3phen. A device fabricated using Yb‐(TPP)L(OEt)/PPP‐OR11 showed significantly enhanced near‐IR output efficiency, and future efforts will focus on devices fabricated using porphyrin‐based materials.

Effect of Hole-Transporting Film Thickness on the Performance of Electroluminescent Devices Using Polymer Langmuir−Blodgett Films Containing Carbazole

Electroluminescence (EL) of a film of poly(N-dodecylacrylamide-co-2-(9-carbazolyl)ethylacrylate) (DDA/CzEA copolymer) as a hole-transporting layer was investigated as a function of the mole fraction of CzEA and the film thickness. The EL devices have double-layer structures consisting of vacuum-deposited tris(8-quinolinolato)aluminum(III) complex (Alq3) as an electron-transporting and emitting layer, and the DDA/CzEA copolymer Langmuir−Blodgett (LB) films on anodic indium−tin-oxide (ITO) electrodes. The EL performance of the double-layer devices was compared with that of the single-layer device of Alq3. The DDA/CzEA copolymers with less than 0.31 mole fraction of CzEA formed stable LB films which acted as a good electron-blocking layer. The current density decreased with increasing film thickness of the DDA/CzEA copolymer LB films. The light conversion efficiency was improved and became almost constant with more than 11 layers because the excitons generated by the recombination of holes and electrons around the interface between the DDA/CzEA copolymer LB film layer and the Alq3 layer was not being quenched by the ITO anodes.

Novel Green Light-Emitting Carbazole Derivatives: Potential Electroluminescent Materials

Green light-emitting 3,6-bis(diarylamino)carbazoles (see Figure; R=H, Me, OMe) have been synthesized by palladium-catalyzed amination of 3,6-dibromocarbazole, it is reported. The performance of electroluminescent devices with these compounds as the hole-transport/emitting layer is shown to rival that of standard organic light-emitting devices.