Electroluminescent Diodes Research Articles

Tuning the bandgaps of polyazomethines containing triphenylamine by different linkage sites of dialdhyde monomers

Two series of conjugated aromatic polyazomethines (PAMs) have been designed and obtained by solution polycondensation of N,N′-bis(4-aminophenyl)-N,N′-di-2-naphthalenyl-4-benzene diamine, N,N′-bis(4-aminophenyl)-N,N′-di-1-naphthalenyl-(1,1′-biphenhyl)-4,4′-diamine with different aromatic dialdehydes, namely terephthalaldehyde, 1,3-isophthalaldehyde, phthaldialdehyde, 4,4′-diformyltriphenylamine, 2,5-thiophenedialdehyde, respectively. The structures of polymers were characterized by means of FTIR, 1H NMR spectroscopy, which showed an agreement with the proposed structure. All the polymers were amorphous determined by XRD technique with good solubility in many organic solvents, such as CHCl3, THF, N-methyl-2-pyrrolidinone (NMP) and N,N′-dimethylacetamide (DMAc), and could be cast into tough and flexible polymer films. The morphologies of PAMs have been detected by atom force microscopy (AFM) which showed more planar surface.All polymers displayed outstanding thermal stability, i.e. 20wt% loss in excess of 515°C under nitrogen atmosphere. Hybrid density functional theory (DFT) method was used to calculate the optimized geometry and electronic structure of PAMs. The calculated HOMO and LUMO energy levels of these polymers by theory method are in the range of −4.879 to −5.494 and −2.344 to −2.927eV vs. the vacuum level, respectively, which are similar to the experiment results that indicated the PAMs would be used in organic layer electroluminescent diode (OLED) and photovoltagic cell as hole transporting materials or functional dyes. All obtained PAMs revealed excellent stability of electrochromic characteristics, changing color from original yellowish to red or blue. And multiple reversible colors states were observed upon I2, HCl, FeCl3, UV irradiation doping and dedoping. The properties prove that the PAMs are multifunctional materials which will be subject of hole-transporting, electrochromic, chemical sensor application in near future.

Kinetically driven self-assembly of a binary solute mixture with controlled phase separation via electro-hydrodynamic flow of corona discharge

This feature article describes a new and facile process to fabricate a variety of thin films of non-volatile binary solute mixtures suitable for high performance organic electronic devices via electro-hydrodynamic flow of conventional corona discharge. Both Corona Discharge Coating (CDC) and a modified version of CDC, Scanning Corona Discharge Coating (SCDC), are based on utilizing directional electric flow, known as corona wind, of the charged uni-polar particles generated by corona discharge between a metallic needle and a bottom plate under a high electric field (5-10 kV cm(-1)). The electric flow rapidly spreads out the binary mixture solution on the bottom plate and subsequently forms a smooth and flat thin film in a large area within a few seconds. In the case of SCDC, the static movement of the bottom electrode on which a binary mixture solution is placed provides further control of thin film formation, giving rise to a film highly uniform over a large area. Interesting phase separation behaviors were observed including nanometer scale phase separation of a polymer-polymer binary mixture and vertical phase separation of a polymer-organic semiconductor mixture. Core-shell type phase separation of either polymer-polymer or polymer-colloidal nanoparticle binary mixtures was also developed with a periodically patterned microstructure when the relative location of the corona wind was controlled to a binary solution droplet on a substrate. We also demonstrate potential applications of thin functional films with controlled microstructures by corona coating to various organic electronic devices such as electroluminescent diodes, field effect transistors and non-volatile polymer memories.

Dicyanomethylene-4H-pyran chromophores for OLED emitters, logic gates and optical chemosensors

Dicyanomethylene-4H-pyran (DCM) chromophores are typical donor-π-acceptor (D-π-A) type chromophores with a broad absorption band resulting from an ultra-fast internal charge-transfer (ICT) process. In 1989, Tang et al. firstly introduced a DCM derivative as a highly fluorescent dopant in organic electroluminescent diodes (OLEDs). Integration of ICT chromophore-receptor systems based on DCM chromophores with ion-induced shifts in absorption or emission is a convenient method to perform the logic expression for molecular logic gates. In recent years, various DCM-type derivatives have been explored due to their excellent optical-electronic properties and diverse structural modification. This feature article provides an insight into how the structural modification of DCM chromophores can be utilized for OLED emitters, logic gates and optical chemosensors. In addition, the aggregation-induced-emission (AIE) of DCM derivatives for further optical applications was also introduced.

Voltage modulated electro-luminescence spectroscopy to understand negative capacitance and the role of sub-bandgap states in light emitting devices

Voltage modulated electroluminescence spectra and low frequency (≤100 kHz) impedance characteristics of red electroluminescent diodes under forward bias are investigated. Light emission under periodic voltage modulation tracks the onset of observed negative capacitance for each modulation frequency. Active participation of sub-bandgap defects including the shallower states in minority carrier recombination dynamics is sought to explain the results. The phenomenon of negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the injected minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and with modulation frequencies support the dynamic influence of these sub-bandgap states in the radiative recombination process. Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. Effect of these states on the net density of minority carriers available for radiative recombination is discussed. These sub-bandgap states can even supress the external quantum efficiency of such devices under high frequency operation commonly used in optical communication.

Probing electric field in double-layer electroluminescence diode by optical second harmonic generation

Optical second harmonic generation (SHG) measurement has been employed for studying electric field distribution in organic double-layer electroluminescence diode. Taking advantage of the material dependence on the SHG, we measured the electric field induced SHG (EFISHG) from IZO/α-NPD/Alq3/Al diode. The generated SH signal wavelength was governed by the active organic layers, and two peaks were generated at 940nm from the α-NPD and at 1050nm from the Alq3 layers. The SHG intensity was dependent on applied d.c. voltages, but the voltage dependence was totally opposite between the two peaks. Interface carrier charging caused by the Maxwell–Wagner effect accounted for the results. We concluded that the EFISHG provides an effective way to probe the electric field distribution in double-layer electroluminescence diodes.

Micro Ceramic Cell Analyzer (MCCA) – Preliminary results

This paper describes the preliminary results of a detection module for a Micro Ceramic Cell Analyzer (MCCA) development and fabrication process. The detector with integrated light source (electroluminescence diode) and photodetector (light-to-voltage converter) is made using Low Temperature Co-fired Ceramics (LTCC) technology. The presented device is assigned for use in fluorescence activated cell detection. Its performance is examined experimentally for two test solutions, which consist of Escherichia coli and Saccharomyces cervisiae cells. Investigated biological material is marked using the DAPI (4′, 6-diamidino-2′-phenylindole, dihydrochloride) fluorescent stain. After exposure to a 370 nm excitation light, the cells bonded with DAPI fluorescent blue (460 nm). Using the photodetector, fluorescence intensity was found to be proportional to the number of the cells in the test solutions.

Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing

We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m2 and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer.

A novel red organic light-emitting diode with ultrathin DCJTB and Rubrene layers

A novel red organic electroluminescent diode based on an ultrathin 4-(dicyanomethylene)-2- t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) layer has been fabricated. The device with the ultrathin DCJTB layer inserted between N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1′-biph-enyl-4,4′- diamine (NPB) and tri-(8-hydroxyquinoline) aluminum (Alq 3) layers has an emission from both DCJTB and Alq 3. When utilizing the 5,6,11,12-tetraphenylnaphthacene (Rubrene) bilayer as the emitting-assistant layer in the fabrication of the device, the emission from Alq 3 disappears. We attribute the improvement of color purity to the exciton confinement and energy transfer from the Rubrene molecular. The device with 0.1 nm DCJTB layer has a luminance of 1130 cd/m 2 at 20 mA/cm 2 with an EL efficiency of 5.6 cd/A, and the CIE coordinates was [0.617, 0.379]. The maximum brightness of this device reaches 21,525 cd/m 2.

Electroluminescence of InGaAs/GaAs quantum-size heterostructures with (III, Mn)V and Ni ferromagnetic injectors

Electroluminescence characteristics of light-emitting diodes based on InGaAs/GaAs quantum well heterostructures with an injector layer made of ferromagnetic metal (Ni), semimetal compound (MnSb), or magnetic semiconductor (InMnAs) were comparatively studied. The general feature is electroluminescence quenching as the spacer layer thickness between a quantum well and a magnetic injector decreases. It was found that the temperature dependence of the electroluminescence in diodes with Ni and MnSb is caused by thermal ejection of carriers from the quantum well; in diodes with InMnAs, it is caused by the temperature dependence of the carrier concentration in magnetic semiconductor and thermal ejection of carriers from the quantum well in the high-temperature region.

Influence of delta〈Mn〉 doping parameters of the GaAs barrier on circularly polarized luminescence of GaAs/InGaAs heterostructures

The circular polarization of low-temperature electroluminescence of diodes based on heterostructures with an undoped quantum well InGaAs/GaAs and a delta〈Mn〉 layer in the GaAs barrier has been investigated. The possibility of changing the degree of circular polarization of the electroluminescence by varying the main structural parameters of diodes (spacer layer thickness, i.e., the distance between the delta〈Mn〉 layer and the quantum well, atomic concentration in the delta〈Mn〉 layer, and introduction of an additional acceptor delta layer) has been analyzed. It has been revealed that the variation in the spacer layer thickness is the most effective method for controlling the degree of circular polarization of the electroluminescence.

Some spirobiindane based 1 H-pyrazolo [3,4- b] quinoline chromophore as novel chromophore for light-emitting diodes

An influence of phenoxy groups for the luminescent and electroluminescent properties of spiro-compounds with pyrazolo [3,4- b] quinoline structure (fluorophore) chromophore has been studied. All the compounds exhibit strong fluorescence in solution and in solid state as well. The prepared compounds were used as dopant chromophore in PVK polymer matrices for electroluminescent (EL) and light-emitting diode (LED) devices with configuration ITO/PEDOT-PSS/PVK/PQ/Ca/Al. Role of the bathochromic shifts and solvent polarity in absorption and photoluminescent maxima is considered. Relation between the number of pyrazoloquinoline chromophore and presence of phenyl group on the fluorescence spectra is explored. Polarizability of the particular pyrazoloquinoline compounds on the solvatochromic effects is investigated. Possible ways of enhancement of the brightness in the light-emitting properties of the mentioned chromophore are discussed.

Structural, Magnetic and Magnetoresistive Properties of PTCTE Based Organic Spin Valves

Organic semiconductors are very promising materials in the field of spintronic devices like spin valves (SV) because of large spin relaxation time in ¿-conjugated molecules and the advantage of rather simple and low energy consuming device fabrication techniques. Most of the studies of spin injection in organic semiconductors focused on hole transport materials and one electron transport material: the tris(8-hydroxyquinoline) aluminum (Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ). In this article, we present our first results on SV based on a perylene derivative, PTCTE (PTCTE: tetraethyl perylene 3, 4, 9, 10-tetracarboxylate). Perylene molecules were traditionally used as electron transport materials in optoelectronic devices like OLED (organic electroluminescent diodes) and solar cells. NiFe/PTCTE/Co devices were prepared by dc sputtering for the electrodes (with direct or off axis sample configuration for the upper electrode), and vacuum sublimation for the organic layer. CPP (current perpendicular to plane) devices geometry was obtained with millimetric contact masks determining a CPP spin valve area of 1 mm × 1 mm. Up to now, our best samples have a spin valve magnetic behavior and exhibit up to 3 % MR response at low temperature for a 300 nm organic spacer layer.

Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode

A significant increase in electroluminescence was achieved through coupling with localized surface plasmons in a single layer of Au nanoparticles. We fabricated a thin-film organic electroluminescence diode, which consists of an indium tin oxide (ITO) anode, a Au nanoparticle array, a Cu phthalocyanine hole transport layer, a tris(8-hydroxylquinolianato) aluminum (III) electron transport layer, a LiF electron injection layer, and an Al cathode. The device structure, with size-controlled Au particles embedded on ITO, can be used to realize the optimum distance for exciton-plasmon interactions by simply adjusting the thickness of the hole transport layer. We observed a 20-fold increase in the molecular fluorescence compared with that of a conventional diode structure.

Probing of carrier behavior in organic electroluminescent diode using electric field induced optical second-harmonic generation measurement

By using the electric field induced optical second-harmonic generation (EFISHG) measurements, we probed the transient electric field in a double-layer indium zinc oxide (IZO)/N, N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4, 4′-diamine(α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al electroluminescent (EL) diode. Results evidently showed that EL was initiated by the injected hole transport across α-NPD layer, and holes accumulated at the α-NPD/Alq3 interface while EL was enhanced. Analysis based on the Maxwell–Wagner effect model well accounted for the hole accumulation. EFISHG measurement is useful as a tool for probing carrier behavior in organic EL devices.

Synthesis, Electrochemical and Photophysical Properties, and Electroluminescent Performance of the Octa‐ and Deca(aryl)[60]fullerene Derivatives

AbstractThree multifunctionalized organo[60]fullerene derivatives, C60Ph5(C6H4‐tBu‐4)5Me2 (cyclophenacene, 1), C60Ph5(C6H4‐tBu‐4)5Me2 (fused corannulene, 2), and C60Ph5(C6H4‐tBu‐4)3Me2 (phenylene‐bridged fused corannulene, 3) are synthesized by the reaction of C60Ph5Me with 4‐tert‐butylphenylcopper reagent in the presence of pyridine, followed by treatment with MeI. Compounds 1–3 undergo reduction in the range from −1.8 to −2.5 V versus Fc/Fc+ and exhibit photoluminescence behavior with fluorescent quantum yields of 18.5%, 2.5%, and 3.2% with fluorescent lifetimes of 67, 1.1, and 27 ns (1–3, respectively). Organic electroluminescent diode devices using 1–3 are fabricated with π‐conjugated polymers and show external electroluminescent efficiencies of 0.04%, 0.07%, and 0.03% emitting yellow, green, and red light, respectively. The device containing all three compounds emits white light. This result indicates that the bulky addends in 1–3 can effectively isolate the π‐conjugated systems of the molecules in the solid state and retard the intermolecular excited‐state quenching process.

Raman Spectra of Molecularly-Ordered 1-TNATA Thin Films and Organic Electroluminescence Device Properties

A molecularly-ordered thin film of vacuum deposited 1-TNATA(4,4′,4″-tris(N-(1-naphthyl)-N-phenylamino) triphenylamine) as a hole injection layer (HIL) is placed between indium tin oxide (ITO) electrode and a hole transporting layer (HTL) in multi-layered organic electroluminescence (EL) diodes. The molecular ordering of 1-TNATA thin film was obtained by thermal annealing and electromagnetic field after vacuum deposition and the film was investigated by the Raman spectra, XRD, and AFM. It was found that the current density-voltage (J-V) and luminance-voltage (L-V) characteristics of the multi-layered diodes were improved due to the ordering of 1-TNATA molecules, which could enhance the charge carrier mobility and luminance efficiency among the organic molecules in the molecularly-ordered film.

Optimized efficiency and angular emission characteristics of white top-emitting organic electroluminescent diodes

The concept of an additional capping layer, which is deposited onto the semitransparent top contact, is applied to minimize microcavity effects for white light emission from top-emitting organic light emitting devices (OLEDs). The influence on the optical properties of such devices with silver as top electrode material is discussed using an analytical method and numerical simulations. The results of the theoretical findings are experimentally verified for inverted top emitting devices on opaque substrates, showing broad spectral bandwidth and angle-independent color coordinates.

Electroluminescent devices based on amorphous SiN/Si quantum dots/amorphous SiN sandwiched structures

A single layer of dense Si quantum dots with average size of 4 nm sandwiched in amorphous SiN layers was prepared by laser crystallization of ultrathin amorphous Si film followed by subsequently thermal annealing. The electroluminescent diodes were fabricated by evaporating Al electrodes on back sides of p-Si substrates and the top surface of samples. Room temperature electroluminescence can be detected with applying the negative voltage around 10V on the top gate electrode and the luminescent intensity is increased with increasing the applied voltage. It was found that the integrated luminescent intensity is linearly proportional to the injection current which suggested the intensity depends on the concentrations of injected carriers after Fowler-Nordheim tunneling through amorphous SiN barriers. The influence of the amorphous SiN with different band gap on the device performance was also discussed briefly.

High Efficiency Composite Metal Oxide‐Polymer Electroluminescent Devices: A Morphological and Material Based Investigation

Composite metaloxide polymer based electroluminescent diodes are designed to obtain air stable operation along with sub-volt turn on and high brightness. We investigate different structures (mesoporous and compact) and different materials (TiO2 and ZnO) for electron injecting layers, and employ MoO3 for hole injecting layers. The highest efficiencies, close to 3 Cd A−1, are achieved using compact ZnO.

Cone mosaic imaging using an adaptive optics flood illumination camera system

Abstract Purpose The objective of this study was to develop an adaptive optics (AO) retinal imaging system and conduct in vivo pilot tests in a group of healthy eyes and in several pathological eyes. The experiments aimed at determining the device ability to image cone photoreceptors in presence of a variety of refractive errors. Methods We examined 20 healthy eyes and 3 eyes with inherited maculopathy using the instrument. The AO system was based on a 52‐actuator electromagnetic deformable mirror and a 1024‐lenslet Shack‐Hartmann wavefront sensor (respectively mirao52e and haso32e, both Imagine Eyes, France). The device was completed with a modified Badal arrangement in order to compensate for possibly large defocus errors. A super luminescent diode operating at 750nm was focused at the retina and the wavefront sensor. Additionally the retina was illuminated by a 3°x3°produced by an electroluminescent diode at 850nm. The center of this field was located at 3° off the fovea. The images were detected using a low‐noise CCD camera (Ropper Scientific, USA). Results Cone photoreceptor mosaics were imaged in the eyes with spherical refraction ranging between –7D and +4D and astigmatism up to 4.5D. The AO correction residual error ranged between 0.08 and 0.2µm in all tested eyes. The imaging resolution was estimated to be better than 5µm in all cases. The mosaic was visible over the entire 3°x3° field in several eyes. Conclusion It appeared that the retinal isoaplanetic field width was at least 3° experimentally in several tested eyes. This study demonstrated the feasibility of cellular retinal imaging in eyes with relatively large refractive errors using a compact AO instrument. Commercial interest