Electroluminescence Behavior Research Articles

Bias-polarity-dependent UV/visible transferable electroluminescence from ZnO nanorod array LED with graphene oxide electrode supporting layer

A simple top electrode preparation process, employing continuous graphene oxide films as electrode supporting layers, was adopted to fabricate a ZnO nanorod array/p-GaN heterojunction LED. The achieved LED demonstrated different electroluminescence behaviors under forward and reverse biases: a yellow-red emission band was observed under forward bias, whereas a blue-UV emission peak was obtained under reverse bias. Electroluminescence spectra under different currents and temperatures, as well as heterojunction energy-band alignments, reveal that the yellow-red emission under forward bias originates from recombinations related to heterointerface defects, whereas the blue-UV electroluminescence under reverse bias is ascribed to transitions from near-band-edge and Mg-acceptor levels in p-GaN.

Synthesis, characterization, and use as emissive layer of white organic light emitting diodes of the highly isotactic poly(N‐pentenyl‐carbazole)

The synthesis and the microstructural characterization of isotactic poly (N‐pentenyl‐carbazole) (i‐PPK) are reported. The achieved polymer is semicrystalline, the melting temperature is 138°C. i‐PPK Fourier transform infrared spectroscopy (FTIR) spectrum and X‐ray diffraction pattern are compared with those of polyvinylcarbazole. The observed differences are tentatively associated to higher flexibility of the i‐PPK chains due to the alkylene group connecting the carbazole group to the main chain. The presence of intrachain and interchain interactions between carbazole units, which bring these groups to stack each other, is assumed. i‐PPK optical properties are investigated and an electroluminescent behaviour of polymer is observed. Organic light emitting diodes based on i‐PPK layers are founded to emit white light. POLYM. COMPOS., 36:1110–1117, 2015. © 2015 Society of Plastics Engineers

Novel Br-DPQ blue light-emitting phosphors for OLED.

A new series of blue light-emitting 2,4-diphenylquinoline (DPQ) substituted blue light-emitting organic phosphors namely, 2-(4-methoxy-phenyl)-4-phenyl-quinoline (OMe-DPQ), 2-(4-methyl-phenyl)-4-phenylquinoline (M-DPQ), and 2-(4-bromo-phenyl)-4-phenylquinoline (Br-DPQ) were synthesized by substituting methoxy, methyl and bromine at the 2-para position of DPQ, respectively by Friedländer condensation of 2-aminobenzophenone and corresponding acetophenone. The synthesized phosphors were characterized by different techniques, e.g., Fourier transform infra-red (FTIR), differential scanning calorimeter (DSC), UV-visible absorption and photoluminescence spectra. FTIR spectra confirms the presence of chemical groups such as C=O, NH, or OH in all the three synthesized chromophores. DSC studies show that these complexes have good thermal stability. Although they are low-molecular-weight organic compounds, they have the potential to improve the stability and operating lifetime of a device made out of these complexes. The synthesized polymeric compounds demonstrate a bright emission in the blue region in the wavelength range of 405-450 nm in solid state. Thus the attachment of methyl, methoxy and bromine substituents to the diphenyl quinoline ring in these phosphors results in colour tuning of the phosphorescence. An electroluminescence (EL) cell of Br-DPQ phosphor was made and its EL behaviour was studied. A brightness-voltage characteristics curve of Br-DPQ cell revealed that EL begins at 400 V and then the brightness increases exponentially with applied AC voltage, while current-voltage (I-V) characteristics revealed that the turn on voltage of the fabricated EL cell was 11 V. Hence this phosphor can be used as a promising blue light material for electroluminescent devices.

Strong electroluminescence from SiO2-Tb2O3-Al2O3 mixed layers fabricated by atomic layer deposition

We report on the bright green electroluminescence (EL) with power efficiencies up to 0.15% of SiO2-Tb2O3-mixed layers fabricated by atomic layer deposition and partly co-doped with Al2O3. The electrical, EL, and breakdown behavior is investigated as a function of the Tb and the Al concentration. Special attention has been paid to the beneficial role of Al2O3 co-doping which improves important device parameters. In detail, it increases the maximum EL power efficiency and EL decay time, it nearly doubles the fraction of excitable Tb3+ ions, it shifts the region of high EL power efficiencies to higher injection currents, and it reduces the EL quenching over the device lifetime by an approximate factor of two. It is assumed that the presence of Al2O3 interferes the formation of Tb clusters and related defects. Therefore, the system SiO2-Tb2O3-Al2O3 represents a promising alternative for integrated, Si-based light emitters.

Substituted naphtholates of rare earth metals as emissive materials

The synthesis, characterization, photoluminescent (PL) and electroluminescent (EL) behavior of trivalent Sc, Nd, Gd, Er, Tm and Yb complexes with 3-(5-methylbenzoxazol-2-yl)naphthol (L-5Me) and 3-(6-methylbenzoxazol-2-yl)naphthol (L-6Me) ligands are reported.

Potential-induced Degradation at Interdigitated Back Contact Solar Cells

Potential-induced degradation (PID) is characterized by the power loss of solar modules under high voltage stress across the module layer stack between framing/glass surface and solar cells. Standard silicon solar cells with a front side emitter may suffer from PID through massive shunting (PID-s) under high voltage stress conditions. PID was also reported in the past for cell concepts with a local emitter at the back side. For this case the underlying physical mechanism is not fully understood.In this contribution the PID effect is investigated for interdigitated back contact solar cells (IBC cells). Parts of the front side of the cells are exposed to high-voltage stress using a recently developed cell test setup at variable temperature, voltage and polarity. Cells are investigated by means of electroluminescence (EL), IR-thermography, illuminated and dark I-V measurements before as well as after PID tests. Cell fragments are investigated after PID stressing using the electron beam induced current (EBIC) method in combination with scanning electron microscopy (SEM).PID tests with a positive voltage respect to the grounded cell cause a locally degraded EL signal in the region where the PID stress was applied. In contrast, PID tests with the opposite polarity do not affect the EL behavior at all. I-V curves and thermography images indicate that PID stress does not significantly increase Rshunt. The local decrease of the EL intensity indicates increased non-radiative recombination. SEM/EBIC reveals neither local shunts nor distinct local degradation of the EBIC signal that could be attributed to PID tests with positive voltage.The results indicate a degradation process related to a degradation of the front side passivation layer (PID-p), in contrast to the well-known PID-s effect. Based on the results a model concept for PID-p of IBC solar cells is proposed. Accordingly, the potential impact on the module power output under the influence of high voltage stress is assessed.

Energy Transfer at the Zeolite L Boundaries: Towards Photo- and Electroresponsive Materials.

A series of iridium(III) compounds have been used as stopper molecules at the pore openings of zeolite L and act as effective donor units for transferring excitation energy to dye molecules entrapped within the zeolite channels. The synthesis and photophysical characterization of the new iridium(III) complexes are described, along with Förster resonance energy-transfer experiments. Transfer efficiencies for the studied systems are discussed on the basis of the role played by the localization of the donor excited state and the acceptor distribution in the channels. Because iridium(III) complexes can also be electrically excited, the electroluminescent behavior of donor-acceptor zeolite systems can be explored, by embedding them into a polymeric active layer and constructing light-emitting devices (LEDs). Novel hybrid LEDs can be fabricated with emission from the dyes entrapped into the zeolites and sensitized by the electro-responsive iridium(III) complex.

Surexpression de l’IL-31 dans une forme sporadique de lichen amyloïde : un lien avec le prurit ?

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

Synthesis, characterisation and electroluminescence behaviour of π-conjugated imidazole–isoquinoline derivatives

A series of highly fluorescent, isoquinoline π-conjugated imidazole derivatives have been synthesized and fully characterized. Their photophysical, electrochemical and thermal properties have also been discussed. The interplay of amorphous and crystalline nature of the compounds in thin film and fine powder have been analysed by using X-ray diffraction and atomic force microscopic (AFM) techniques. An organic light emitting diode consisting of 4-(4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenyl)isoquinoline as the emitting layer doped with 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CPB) showed ideal blue emission (CIE: 0.16, 0.08). “Pure” white light (CIE: 0.34, 0.32) comprising of a blue light from excimers and an orange light from electromers at a high voltage has been achieved by using the compounds 2-(4-(isoquinolin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole & 2-(4-(isoquinolin-4-yl)phenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole as single-emitting components. To elucidate the structural and optical properties, computational calculations have been performed. Computed results reveal all the electronic transitions are of π → π* type and energy of the S0 → S1 follows same variation trend with the band gap.

Highly Emissive Luminogens Based on Imidazo[1,2‐a]pyridine for Electroluminescent Applications

A search for novel organic luminogens led us to design and synthesize some N-fused imidazole derivatives based on imidazo[1,2-a]pyridine as the core and arylamine and imidazole as the peripheral groups. The fluorophores were synthesized through a multicomponent cascade reaction (A(3) coupling) of a heterocyclic azine with an aldehyde and alkyne, followed by Suzuki coupling and a multicomponent cyclization reaction. All of the compounds exhibited interesting photophysical responses, especially arylamine-containing derivatives, which displayed strong positive solvatochromism in the emission spectra that indicated a more polar excited state owing to an efficient charge migration from the donor arylamine to the imidazo[1,2-a]pyridine acceptor. The quantum yields ranged from 0.2 to 0.7 and depended on the substitution pattern, most notably that based on the donor group at the C2 position. Moreover, the influence of general and specific solvent effects on the photophysical properties of the fluorophores was discussed with four-parameter Catalán and Kamlet-Taft solvent scales. The excellent thermal, electrochemical, and morphological stability of the compounds was explored by cyclic voltammetry, thermogravimetric analysis, and AFM methods. Furthermore, to understand the structure, bonding, and band gap of the molecules, DFT calculations were performed. The performance of the electroluminescence behavior of the imidazo[1,2-a]pyridine derivative was investigated by fabricating a multilayer organic light-emitting diode with a configuration of ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al(100 nm) (ITO=indium tin oxide, EML=emissive layer, BCP=2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, Alq3 =tris(8-hydroxyquinolinato)aluminum), which exhibited white emission with a turn-on voltage of 8 V and a brightness of 22 cd m(-2).

Binary solvent mixture-induced crystallization enhancement for a white emissive polyfluorene copolymer toward improving its electroluminescence

The morphology of fluorescent polyfluorene (PF) derivatives has strong effects on their electroluminescent (EL) performance. Herein, the influence of processing solvents on the morphology of PF-based white emissive polymer and its correlation to the EL behavior is studied. It is found that the PF copolymer films prepared from chlorobenzene:toluene (CB:TOL) solvent mixtures show stronger tendency to form crystalline α phase PF upon thermal annealing than those prepared from pure TOL or CB solvents. The evaporation rate difference in solvent mixtures can assist in the formation of crystal nucleus and enhance the crystallinity of PF backbone after thermal annealing. The results also reveal that the solvent mixture-processed fluorescent PF-based white emissive polymer not only shows more efficient blue emission, but also more balanced electron and hole transport. Accordingly, both the white emission purity and the light-emitting efficiency of the light-emitting diodes based on PF-based white emissive polymer are greatly enhanced.

High-performance ultraviolet-blue light-emitting diodes based on an n-ZnO nanowall networks/p-GaN heterojunction

Vertically aligned ZnO nanowall networks were grown on p-GaN/sapphire substrates by metal-organic chemical vapor deposition, and further heterojunction light-emitting diodes based on n-ZnO/p-GaN were fabricated. Highly efficient ultraviolet-blue emission was observed from the diode under forward bias, and it operated continuously for 8.5 h with decay of only 8.08% under a continuous current of 12 mA. The diode exhibited low emission onset and good stability. In addition, temperature-dependent electroluminescence and current-voltage behaviors of the diode were investigated to examine the thermal effects on light output power, spectral line shift, and temperature sensitivity of the resulting voltage.

Saturated blue-violet electroluminescence from single ZnO micro/nanowire and p-GaN film hybrid light-emitting diodes

Saturated electroluminescence behavior was observed in single ZnO micro/nanowire and GaN film heterojunctions fabricated by transferring an individual ZnO micro/nanowire onto p-type GaN substrate. A strong blue emission of ∼460 nm was observed, as a result of interfacial radiative recombination of electrons from n-ZnO and holes from p-GaN. Light-output-current characteristic followed a power law of L ∼ Im, which revealed a superlinear dependence at low current (m = 1.16) and became sublinear (m = 0.72) at high current. According to theoretical analysis, the saturated electroluminescence at high current could be attributed to the saturation of nonradiative recombination and the limitation of electrical-to-optical conversion efficiency.

Radiative recombination of carriers in the GaxIn1−xP/GaAs double-junction tandem solar cells

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

Syntheses, photophysical, electroluminescence and computational studies of rhenium(i) diimine triarylamine-containing alkynyl complexes

A series of triarylamine-containing rhenium(I) diimine alkynyl complexes has been synthesized and some of their X-ray crystal structures have been determined. Low-energy transition bands at 402–444 nm were observed in the electronic absorption spectra and were tentatively assigned as an admixture of [dπ(Re) → π*(diimine)] metal-to-ligand charge-transfer (MLCT) and [π(CC−C6H4–R) → π* (diimine)] ligand-to-ligand charge-transfer (LLCT) transitions. They were found to give emission upon photo-excitation in the solid state and in fluid solution at room temperature and their emissive origins were assigned as derived predominantly from the triplet 3MLCT excited state, mixed with 3LLCT character as well as the 3IL state. Electrochemical studies revealed that the first oxidation process was mainly alkynyl ligand-based with mixing of the metal-centred contribution, whereas the first reduction was attributed to the ligand-centred reduction of the diimine ligand. Computational studies have been performed to provide further insight into the electrochemical and photophysical properties. The photochemical properties with triphenylamine were also studied by quenching experiments and time-resolved transient absorption spectroscopy. Some of these complexes were able to act as emitters in OLED devices and their electroluminescence (EL) behaviour was investigated.

Photo- and electroluminescence behavior of biphenyl derivative in blends with poly(vinylcarbazole)

Light emitting properties of poly[4-(5-(biphenyl- 4-yl)-4-hexylthiophen-2-yl)-2-dodecyl-7-(4-hexyl-thiophen-2-yl)-2H-benzo[d][1,2,3]-triazole] (PPhHTBT) and poly(9-vinylcarbazole) (PVK) blends were investigated with a configuration of ITO/PEDOT-PSS/BLEND/Ca/Al in ratios ranging from 5 to 100 wt\% in PPhHTBT. Thin film and solution spectra of all blends revealed that energy transfer was completely achieved. Device performance increased for all blends and reached 1.1 cd/A for 30 wt\% PPhHTBT.

Photo- and Electroluminescence from 2-(Dibenzo[b,d]furan-4-yl)pyridine-Based Heteroleptic Cyclometalated Platinum(II) Complexes: Excimer Formation Drastically Facilitated by an Aromatic Diketonate Ancillary Ligand

To clarify the ancillary ligand effect on excimer formation of heteroleptic cyclometalated platinum(II) complexes, we investigated photo- and electroluminescence behavior for green-phosphorescent [(dibenzo[b,d]furan-4-yl)pyridinato-N,C3′]platinum(II) 1,3-diketonates Pt-1 and Pt-2 (1,3-diketonate ancillary ligands; 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (bdbp) and dipivaloylmethanate (dpm) for Pt-1 and Pt-2, respectively). The X-ray crystallographic study reveals that both Pt-1 and Pt-2 form dimeric pairs in the solid states, indicating that they are likely to form the excimers. In PMMA films doped with Pt-1, red-shifted photoluminescence at >600 nm, assignable to the excimer emission, increasingly emerges along with the original monomer emission at 516 and 552 nm as the doping level of Pt-1 is increased to 44 wt %. Pt-2-doped films give quite modest excimer-based photoluminescence even when heavily doped to the same extent as Pt-1-doped films. This clearly indicates that the aromatic ancillary lig...

ZnO-nanorod-array/p-GaN high-performance ultra-violet light emitting devices prepared by simple solution synthesis

Pure ultra-violet (UV) (378 nm) electroluminescence (EL) from zinc oxide (ZnO)-nanorod-array/p-gallium nitride (GaN) light emitting devices (LEDs) is demonstrated at low bias-voltages (∼4.3 V). Devices were prepared merely by solution-synthesis, without any involvement of sophisticated material growth techniques or preparation methods. Three different luminescence characterization techniques, i.e., photo-luminescence, cathodo-luminescence, and EL, provided insight into the nature of the UV emission mechanism in solution-synthesized LEDs. Bias dependent EL behaviour revealed blue-shift of EL peaks and increased peak sharpness, with increasing the operating voltage. Accelerated bias stress tests showed very stable and repeatable electrical and EL performance of the solution-synthesized nanorod LEDs.

Influence of the polymer concentration on the electroluminescence of ZnO nanorod/polymer hybrid light emitting diodes

The effects of the polymer concentration on the performance of hybrid light emitting diodes (LEDs) based on zinc oxide nanorods (ZnO NRs) and poly(9,9-dioctylfluorene) (PFO) were investigated. Various characterization techniques were applied to study the performance of the PFO/ZnO NR hybrid LEDs fabricated with various PFO concentrations. The fabricated hybrid LEDs demonstrated stable rectifying diode behavior, and it was observed that the turn-on voltage of the LEDs is concentration dependent. The measured room temperature electroluminescence (EL) showed that the PFO concentration plays a critical role in the emission spectra of the hybrid LEDs. At lower PFO concentrations of 2-6 mg/ml, the EL spectra are dominated by blue emission. However, by increasing the concentration to more than 8 mg/ml, the blue emission was completely suppressed while the green emission was dominant. This EL behavior was explained by a double trap system of excitons that were trapped in the β-phase and/or in the fluorenone defects in the PFO side. The effects of current injection on the hybrid LEDs and on the EL emission were also investigated. Under a high injection current, a new blue peak was observed in the EL spectrum, which was correlated to the creation of a new chemical species on the PFO chain. The green emission peak was also enhanced with increasing injection current because of the fluorenone defects. These results indicate that the emission spectra of the hybrid LEDs can be tuned by using different polymer concentrations and by varying the current injected into the device.

Comparative study of electroluminescence from annealed amorphous SiC single layer and amorphous Si/SiC multilayers

Si quantum dots (Si QDs) films were prepared by annealing amorphous SiC single layer and amorphous Si/SiC multilayers fabricated in plasma enhanced chemical vapor deposition system. The microstructures were characterized by Raman spectroscopy as well as Fourier transforms infrared spectroscopy for both samples. It was found that Si QDs with average size of 2.7nm were formed after annealing and the electroluminescence (EL) band centered at 650nm can be observed at room temperature. The EL intensity from the Si/SiC multilayers was obviously improved by one order of magnitude and the corresponding EL band width was reduced compared with that from SiC single layer film. The improved electroluminescence behavior can be attributed to the formation of the denser Si QDs, good size distribution and the strong confinement effect of carriers in multilayerd structures.