Organic Light-emitting Diode Structure Research Articles

Influences of submonolayer proteins on organic light-emitting diodes

The authors sandwiched protein molecules into the layered structure of organic light-emitting diodes (OLEDs). Light emission from the OLEDs was suppressed on the area with the protein submonolayers of 4–7nm thick. They found that this suppression depended on the density of the proteins and light emission efficiency reflected molecular properties. A bovine serum albumin layer decreased both the current and light emission, keeping the efficiency almost unchanged. In the case of cytochrome C, the emission was suppressed while current increased, resulting in low efficiency.

Top-emitting OLED pixel employing cathode-contact structure with a-Si:H thin-film transistors

An active-matrix organic light emitting diode (OLED) display employing a hydrogenated amorphous silicon thin-film transistor prefers a cathode-contact structure to an anode-contact structure. A new normal top-emitting OLED employing cathode-contact structure is proposed. To implement normal OLED structure, the cathode layer on top is separated as sub-pixels by a negative photoresist separator. The current of the cathode-contact structure is maintained 20% higher after 20 h than that of the anode-contact structure during the accelerated life test.

Simulation for Double Ultrathin Separately Doped Red Organic Light-Emitting Diode

In this study, we employed an ultrathin separately doped organic light-emitting diode (OLED) structure to achieve the lowest turn-on voltage, highest luminance efficiency, and highest electroluminescence. In the simulation part, the spectrum intensity of the main emitting layer (EML) tris(8-hydroxy-quinoline) aluminum (Alq3) photoluminescence (PL) and the full width at half maximum (FWHM) of its Gaussian distribution were modulated to obtain the luminescence spectrum of an ultrathin separately doped device. It is found that as doping concentration increases, the extent of intensity modulation in simulation must be decreased in order to simulate the Alq3 peak drop. This result confirms that when the concentration of a red dopant is high, the red luminance is purer, but the brightness is weaker than that at a lower doping concentration; conversely, at a higher intensity a lower concentration of a red dopant results in more intense luminance, but an orange light, instead of a pure red light is emitted. This study aims to clarify how Alq3 intensity changes in, correspondence to different doping concentrations. For simulation results to coincide with the experimental data, it is deduced that the PL spectrum of 4-(dicuanomethylene)-2-methyl-6-(1,1,7,7-tetramethyljulol-idyl-9-enyl)-4H-pyran, DCJT (red dye), exhibits a red shift as doping concentration increases. Finally, we also adjust the thickness of organic layers (hole transport layer/EML) to more accurately approximate simulation results with respect to experimental data.

Magnetoresistance and efficiency measurements ofAlq3-based OLEDs

Magnetoresistance and efficiency measurements of indium tin oxide/$N,{N}^{\ensuremath{'}}$-diphenyl-$N,{N}^{\ensuremath{'}}$ bis(3-methylphenyl)-($1,{1}^{\ensuremath{'}}$-biphenyl)-$4,{4}^{\ensuremath{'}}$ diamine/aluminum tris(8-hydroxyquinoline)/cathode organic light-emitting diode structures have been made as a function of magnetic field and cathode type. It has been found that magnetoresistance occurs only when there is light emission from the devices, which suggests that the magnetoresistance is related to exciton formation. Comparison of the effects of applied field on device efficiency and magnetoresistance shows that the magnetoresistance cannot be due to the recombination current. We suggest that the effect may be due to trapping of charge carriers at triplet excitons within the device.

Coherent electrically excited organic semiconductors: visibility of interferograms and emission linewidth

Recently, a tandem organic light-emitting diode structure, excited electrically in the pulsed domain and confined within a double interferometric configuration, was observed to emit a low-divergence beam (approximately 1.1 times the diffraction limit) with a near-Gaussian spatial distribution. The emission originates from the laser dye Coumarin 545 T used as dopant. It has since been determined that the visibility of the interferograms, from the spatially coherent emission, is V approximately 0.90. This result is compared with the visibility obtained from a known narrow linewidth laser source (V approximately 0.95) and with various published values from the relevant literature. The significance of this result is discussed in addition to an interferometric estimate of the emission linewidth that yields Delta lambda approximately 11 nm. The present interferometric analysis indicates that the spectral component of the spatially coherent radiation is comparable with the spectral characteristics of well-known broadband dye lasers.

Charge-carrier injection characteristics at organic/organic heterojunction interfaces in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) having various guest molecules doped in an organic host matrix layer are fabricated [the OLED structure is anode/hole-transporting layer (HTL)/guest–host emitting layer/hole-blocking layer/electron-transporting layer/cathode], and the dependence of current density–voltage ( J– V) characteristics of the OLEDs on highest occupied molecular orbital (HOMO) levels of guest molecules are investigated. From the J– V characteristics of these OLEDs, we find two important results: (1) J– V characteristics of the OLEDs are controlled by the direct hole injection from the neighboring HTL to guest molecules, and (2) HOMO level alignment between the HTL and guest molecules provides efficient hole injection at this interface.

Measurement of Electron Mobility in ${\hbox{Alq}}_{3}$ From Optical Modulation Measurements in Multilayer Organic Light-Emitting Diodes

The dynamic characteristics of multilayer organic light-emitting diodes (OLEDs) determine the refresh rate in display applications, and are of great importance for practical organic displays. They also serve as an important tool in studying the transport mechanisms in organic conductors. Here, the modulation characteristics of several conventional small-molecule OLED structures [consisting of ITO/PEDOT:PSS(50 nm)/TPD(50 nm)/Alq3(various)/LiF(1 nm)/Al(90 nm)] are measured and analyzed in terms of mobility in and thickness of the Alq 3 layer. Their optical response was shown to be limited by electron transport across the Alq3. Extracted electron mobilities were about 2-4times10-6 cm2/Vmiddots (consistent with that reported in the literature) and near-identical values for mobility were obtained from devices of different thicknesses, suggesting that this method measures mobility independent of interface trap charging. This novel technique is a complement to large signal time of flight or delay time measurements (which can include interface and trap charging during the measurement) and can serve as a flexible method to study transport in actual devices

Direct observation of the evolution of occupied and unoccupied energy levels of two silole derivatives at their interfaces with magnesium

The electronic structures of 2,5-bis(6′-(2′,2″-bipyridyl))-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) and 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PPSPP) at their interfaces with Mg were investigated using ultraviolet, inverse, and x-ray photoemission spectroscopies. PyPySPyPy and PPSPP have been used as both electron injection/transport layers and emitters in high-efficiency organic light-emitting diodes (OLEDs). Deposition of either PyPySPyPy or PPSPP onto Mg results in the appearance of two energy levels within the energy gap of the organic. Upon deposition of Mg onto PyPySPyPy there is a shift of the occupied energy level structure to higher binding energy, away from the Fermi level, and appearance of two energy levels within the energy gap of PyPySPyPy. The lowest unoccupied molecular orbital is also shifted to higher binding energy. Upon deposition of Mg onto PPSPP there is also a rigid shift of the occupied energy level structure to higher binding energy, away from the Fermi level, but there are no apparent energy levels created within the energy gap of PPSPP. The different chemical reactivity of the two silole derivatives with magnesium is shown to have pronounced effects on the formation of cathode contacts in OLED structures.

Excitation of waveguide modes in organic light-emitting diode structures by classical dipole oscillators

Analytical techniques known in the literature are used to (i) identify all the planar waveguide modes in four top-emitting organic light-emitting diode (OLED) structures over the visible spectrum, and (ii) compute both TM and TE power spectra for classically radiating dipoles in the emissive layers of these OLED structures. Peaks in the computed power spectra are identified with the waveguide modes in the OLED devices, and areas associated with these peaks are used to estimate the excitation probability of the waveguide modes. In cases where ambiguities arise because of overlapping peaks, it is shown that computed power spectra can be approximated as sums of Lorentzian line shapes. It is found that for all four structures, the dipoles couple almost 80% of their radiant energy into TM modes with only about 20% going into TE modes. Furthermore, except for a narrow spectral band, the excited TM modes are primarily short-range surface plasmon polaritons. Excitations in the narrow spectral band correspond to TM and TE Fabry-Perot microcavity modes. Finally, the analysis shows that, in the absence of grating couplers, only light in the microcavity modes escapes into the air cover.

Using a low-index host layer to increase emission from organic light-emitting diode structures

The out-coupling efficiency of organic light-emitting diodes (OLEDs) may be significantly increased by use of a low-index host material for the organic emitters. We report on modelling undertaken for substrate-emitting and top-emitting OLED structures and show that up to a 2.5 fold increase in out-coupling efficiency, over that for structures containing a standard emissive layer, may be achieved in both cases. The relationship between radiation efficiency and refractive index varies for each type of structure and this is discussed, as is the nature of the electromagnetic (EM) modes supported.

Maximizing<tex>$hbox Alq_3$</tex>OLED Internal and External Efficiencies: Charge Balanced Device Structure and Color Conversion Outcoupling Lenses

In this paper, we report bright, efficient Alq3-based [tris-(8-hydroxyquinoline) aluminum] organic light-emitting diode (OLED) structures that incorporate hemispherical lenses for increased output power efficiency. The 6-layer hybrid (polymer/small molecule) OLED structure contains two spin-coated polymer layers and four thermally evaporated small molecule layers. This structure results in balanced charge injection, thus leading to a more efficient device. The use of index-matched transparent lenses resulted in luminous and external quantum efficiency of 7.5 lm/W and 8%, respectively. The size and shape of the lens was used to control the angular power distribution. Lenses incorporating color conversion media were used to achieve high OLED efficiency in various colors. Saturated yellow, orange, and red devices with external quantum efficiencies as high ~4% were obtained from this approach

Fullerene-organic nanocomposite: A flexible material platform for organic light-emitting diodes

Cu Pc : C 60 organic-nanocarbon composite coated metals (Au, Ag, and ITO) are found to form efficient hole injection anode structures for organic light-emitting diodes (OLEDs). A significant increase (∼ two times ) in current efficiency has been observed in OLEDs when the nanocomposite anode structures are used to replace the conventional CuPc/indium tin oxide hole injection structure. Moreover, the composite anode structures enable the use of simple metal electrodes for efficient and stable OLEDs. The composite provides, through a controlled variation in the C60 concentration, a flexible material platform in regulating the hole injection and transport through the various layers in an OLED.

Improving the Luminous Efficiency of Red Organic Light-Emitting Diodes Using a Carrier Balance Layer

This work proposes a carrier balance layer (CBL) to yield red organic light-emitting diodes (OLEDs) with high luminous efficiency. In the structure of OLEDs, the CBL (1,3,5-tris(-phenylbenzimidazol-2,yl)benzene, TPBi) is inserted into the emitting layer, 4-(dicyanomethylene)-2--butyl-6(1,1,7,7-tetra-methy-ljulolidyl-9-enyl)-4H-pyran (DCJTB)-doped tris(8-hydroxyquinolino)-aluminum . Experimental results demonstrated that the luminous efficiency of the OLED with a CBL was 30% higher than that of the typical OLED without a CBL. Inserting CBL into an emitting layer is suggested to rearrange and enlarge the recombination zone. The improvement in efficiency is attributed to the enlargement of the recombination zone in which more excitons were generated and recombined. Sufficient electron-hole recombination improved luminous efficiency.

Characterization of TFT-LCD and OLEDs Devices by Phase Modulated Spectroscopic Ellipsometry for Display Applications

ABSTRACTSpectroscopic ellipsometry is one of the most accurate and reliable optical technique to characterize polymers, liquid crystals (LCs) and organic light emitting diodes (OLEDs). Because these devices are formed by complex structure including optical anisotropy, absorbing and graded materials, the correct use of spectroscopic ellipsometer required a combination of the proper choice of hardware and the appropriate ellipsometric model. In this work, we presents ellipsometric results obtained by a commercially available phase modulated spectroscopic ellipsometer (PMSE) on a full TFT-LCD structure characterized from UV to NIR. As expected, strong anisotropy and inhomogeneous optical properties were found respectively on LCs and ITO materials. We also presents measurements of film thickness and optical constants of each layer constituing an OLED structure

Device optimization of tris-aluminum(Alq3) based bilayer organic light emitting diode structures

In this work we present a detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline)aluminum (Alq3) based bilayer organic light emitting diodes (OLEDs) as a function of the choice ofcathode, the thickness of the organic layers, and the position of the hole transportlayer/Alq3 interface. The calculations fully take into account dispersion in the glass substrate, theindium tin oxide anode, and in the organic layers, as well as the dispersion inthe metal cathode. The influence of the incoherent transparent substrate (1 mmglass substrate) is also fully accounted for. Four cathode structures have beenconsidered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer,N,N′-diphenyl–N,N′-(3-methylphenyl)–1,1′-biphenyl–4,4′-diamine (TPD) and N,N′-di(naphthalene-1-yl)–N,N′-diphenylbenzidine (NPB) were considered. As expected, the emitted radiation is stronglydependent on the position of the emissive layer inside the cavity and its distance from themetal cathode. Although our optical model for an OLED does not explicitly include excitonquenching in the vicinity of the metal cathode, designs placing the emissive layer near thecathode are excluded to avoid unrealistic results. Guidelines for designing devices withoptimum emission efficiency are presented. Finally, several different devices were fabricatedand characterized and experimental and calculated emission spectra were compared.

Coherence characteristics of electrically excited tandem organic light-emitting diodes

A tandem organic light-emitting diode structure, excited electrically in the pulsed domain and confined within a double spatial filter configuration, is observed to emit a low-divergence beam (deltatheta approximately 2.53 mrad, or approximately 1.1 times the diffraction limit) with a near-Gaussian spatial distribution. The emission originates from the laser dye Coumarin 545 T, which is used as a dopant. Spectral coherence was determined by use of a double-slit interferometer. The interferometric distribution from our device approximates the interferometric pattern obtained from well-known lasers emitting at lambda approximately 540 nm.

Lateral organic light-emitting diode with field-effect transistor characteristics

We succeeded in observing bright electroluminescence (EL) from 1wt%-rubrene doped tetraphenylpyrene (TPPy) as an active layer in a lateral organic light-emitting diode structure that allowed field-effect transistor operation. This device configuration provides an organic light-emitting diode structure where the anode (source) and cathode (drain) electrodes are laterally arranged, providing us a chance to control the EL intensity by changing the gate bias. We demonstrated that TPPy provides compatible transistor and EL characteristics. Further, not only rubrene doping into the TPPy host but also adjusting the source-drain channel length significantly improved the EL characteristics. We observed a maximum EL quantum efficiency (ηext) of ∼0.5% with a Cr∕Au source (S)-drain (D) electrode and a slightly higher ηext of ∼0.8% with S-D electrodes of MgAu∕Au, Al∕Au, Cr∕YAu∕Au, and MgAl∕Au multilayers, aiming for simultaneous hole and electron injection.

Separately Doped Structures for Red Organic Light-Emitting Diodes

This paper reports on the separately-doped structures of organic light-emitting diodes (OLED) in which red dye 4-(dicyanomethylene)-2-methyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJT) was doped into the host emitting layer of tris(8-hydroxyquinoline)aluminum (Alq3) by means of ultrathin separate doping (like quantum wells). The parameters in the ultrathin layer, including its width, position, number and spacing between separately doped layers, were changed to study their effects on electroluminescence (EL) characteristics. Thin doped layers increase the luminance efficiency. When the ultrathin separately doped layers were in the vicinity of the NPB/Alq3 interface, where N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) is used as a hole transport layer, luminance efficiency also increased. The highest luminance efficiency reaching 4 cd/A was achieved using a double ultrathin separately doped structure with optimum doping thickness and 2 nm of space between the two layers. The study also found that double ultrathin separately doped structures achieved better EL intensity than single or triple ultrathin separately doped structures.

Extremely-high-density carrier injection and transport over 12000A∕cm2 into organic thin films

We achieved extremely-high-density steady state carrier injection and transport at over ∼10000A∕cm2 into organic thin films using high thermally conductive substrates, which suppress the temperature rise inside the devices by transferring the joule heat into the substrates. Using a silicon substrate with a high thermal conductivity of 148W∕mK and a small size cathode with a radius of r=25μm, we achieved a maximum current density of Jmax=12222A∕cm2 and power density of Pmax∼105W∕cm2 in an ITO(110nm)∕copperphthalocyanine (CuPc) (25nm)∕MgAg(100nm)∕Ag(10nm) device during a fraction of a second under direct current sweep. Further, we also achieved Jmax=514A∕cm2 in a conventional organic light-emitting diode structure using the same techniques. In the CuPc based devices, we observed characteristic current density (J)–voltage (V) behavior, indicating that the J–V characteristics are controlled by the trap-free space-charge-limited currents in the high current region, and by the trapped-charge-limited current in the low current region.

Contrast-enhancement in organic light-emitting diodes

A high-contrast organic light-emitting diode (OLED) structure is presented. Because of poor contrast of conventional OLED resulting from high reflective metal cathode, the hybrid cathode structure was developed for low reflectivity. It consists the semitransparent cathode layers, passivation layers and a thick light-absorbing film. By optical reflectivity measurement and OLED electrical characterization tests for both OLED with the hybrid cathode and conventional OLED, it was found that the spectrum reflectance of OLED with hybrid cathode is among 8%-12%, about eight times lower than the conventional one when the two types of devices have similar turn-on voltages and current-voltage characteristics. The hybrid cathode for the high-contrast OLED is easily fabricated and its optical reflectance is slightly dependent on wavelength.