Electroluminescence Threshold Voltage Research Articles

Determining the Origin of Half-bandgap-voltage Electroluminescence in Bifunctional Rubrene/C60 Devices

Lowering the driving voltage of organic light-emitting diodes (OLEDs) is an important approach to reduce their energy consumption. We have fabricated a series of bifunctional devices (OLEDs and photovoltaics) using rubrene and fullerene (C60) as the active layer, in which the electroluminescence threshold voltage(~1.1 V) was half the value of the bandgap of rubrene. Magneto-electroluminescence (MEL) response of planner heterojunction diodes exhibited a small increase in response to a low magnetic field strength (<20 mT); however, a very large decay was observed at a high magnetic field strength (>20 mT). When a hole-transport layer with a low mobility was included in these devices, the MEL response reversed in shape, and simultaneously, the EL threshold voltage became larger than the bandgap voltage. When bulk heterojunction device was examined, the amplitude of MEL curves presented an anomalous voltage-dependence. Following an analysis of the MEL responses of these devices, we proposed that the EL of half-bandgap-voltage device originated from bimolecular triplet-triplet annihilation in the rubrene film, rather than from singlet excitons that formed via an interface auger recombination. This work provides critical insight into the mechanisms of OLED emission and will help advance the applications of bifunctional devices.

Pyridine‐Containing Electron‐Transport Materials for Highly Efficient Blue Phosphorescent OLEDs with Ultralow Operating Voltage and Reduced Efficiency Roll‐Off

A series of pyridine‐containing electron‐transport materials are developed as an electron‐transport layer for the FIrpic‐based blue phosphorescent organic light‐emitting diodes. Their energy levels can be tuned by the introduction of pyridine rings in the framework and on the periphery of the molecules. Significantly reduced operating voltage is achieved without compromising external quantum efficiency by solely tuning the nitrogen atom orientations of those pyidine rings. Unprecedented low operating voltages of 2.61 and 3.03 V are realized at 1 and 100 cd m−2, giving ever highest power efficiency values of 65.8 and 59.7 lm W−1, respectively. In addition, the operating voltages at 100 cd m−2 can be further reduced to 2.70 V by using a host material with a small singlet‐triplet exchange energy, and the threshold voltage for electroluminescence can even be 0.2–0.3 V lower than the theoretical minimum value of the photon energy divided by electron charge. Aside from the reduced operating voltage, a further reduced roll‐off in efficiency is also achieved by the combination of an appropriate host material.

Optical microcavities and enhanced electroluminescence from electroformed Al-Al2O3-Ag diodes

Electroluminescence (EL) and electron emission into vacuum (EM) occur when a non-destructive dielectric breakdown of Al-Al2O3-Ag diodes, electroforming, results in the development of a filamentary region in which current-voltage (I-V) characteristics exhibit voltage-controlled negative resistance. The temperature dependence of I-V curves, EM, and, particularly, EL of Al-Al2O3-Ag diodes with anodic Al2O3 thicknesses between 12 nm and 30 nm, has been studied. Two filters, a long-pass (LP) filter with transmission of photons with energies less than 3.0 eV and a short-pass (SP) filter with photon transmission between 3.0 and 4.0 eV, have been used to characterize EL. The voltage threshold for EL with the LP filter, VLP, is ∼1.5 V. VLP is nearly independent of Al2O3 thickness and of temperature and is 0.3–0.6 V less than the threshold voltage for EL for the SP filter, VSP. EL intensity is primarily between 1.8 and 3.0 eV when the bias voltage, VS ≲ 7 V. EL in the thinnest diodes is enhanced compared to EL in thicker diodes. For increasing VS, for diodes with the smallest Al2O3 thicknesses, there is a maximum EL intensity, LMX, at a voltage, VLMX, followed by a decrease to a plateau. LMX and EL intensity at 4.0 V in the plateau region depend exponentially on Al2O3 thickness. The ratio of LMX at 295 K for a diode with 12 nm of Al2O3 to LMX for a diode with 25 nm of Al2O3 is ∼140. The ratio of EL intensity with the LP filter to EL intensity with the SP filter, LP/SP, varies between ∼3 and ∼35; it depends on Al2O3 thickness and VS. Enhanced EL is attributed to the increase of the spontaneous emission rate of a dipole in a non-resonant optical microcavity. EL photons interact with the Ag and Al films to create surface plasmon polaritons (SPPs) at the metal-Al2O3 interfaces. SPPs generate large electromagnetic fields in the filamentary region of the electroformed Al-Al2O3-Ag diode, which then acts as an optical microcavity. A model is proposed for electronic processes in electroformed Al-Al2O3-Ag diodes.

Electroluminescence from One-Dimensionally Self-Aligned Si-Based Quantum Dots with High Areal Dot Density

Self-aligned Si-based quantum dots (QDs) with an areal density as high as ∼1013 cm-2 have been successfully fabricated on GeH4-adsorbed ultrathin SiO2 by the process sequence consisting of Si-QDs formation by controlling low-pressure chemical vapor deposition (LPCVD) using pure Si2H6, selective Ge-LPCVD from 5% GeH4 diluted with He, thermal oxidation of the dots, thermal desorption of Ge oxide, and subsequent formation of the Si-QDs. In semitransparent Au-gate diodes with self-aligned dots so-prepared, when carriers were injected to the self-aligned Si-QDs from the n-Si(100) substrate for electrons and from the Au top electrode for holes, electroluminescence (EL) in the near-infrared region at room temperature becomes observable with an increase in current at positive biases over a threshold voltage as low as ∼1.2 V at the Au top electrode. Note that, in the case of an areal dot density of ∼1013 cm-2, the EL threshold voltage was reduced down to ∼60% of that of ∼1011 cm-2 and emission intensity was enhanced markedly by a factor of ∼425 in comparison with the case of ∼1011 cm-2 under the same current density. This is clear evidence of not only an increase in radiative recombination rate in the self-aligned structure but also an improvement of recombination efficiency due to a decrease in current leakage with increasing dot density.

Surface plasmon polariton-assisted electron emission and voltage-controlled negative resistance of Al–Al2O3–Au diodes

Electrons are ejected from an Au surface when their energy exceeds the work function of Au, ϕAu, which is 5.2 eV. The threshold voltage for the onset of electron emission from Al–Al2O3–Au diodes that develop voltage-controlled negative resistance in their current-voltage characteristics is VEM≅2.4 V. The threshold voltage for electroluminescence (EL) is VEL=1.6–2.0 V. The interaction of electrons with surface plasmon polaritons (SPPs) produced at the Al2O3–Au interface by EL photons provides a mechanism by which electrons gain enough energy to surmount the work function barrier at the Au-vacuum interface when the applied voltage is less than 5.2 V. The interaction of EL photons with surface plasmons in Au is a new mechanism for creating SPPs.

Equivalent circuits of polymer light-emitting diodes with hole-injection layer studied by impedance spectroscopy

Impedance spectroscopy (IS) of poly(9,9-dioctylfluorene) (F8) polymer light-emitting diodes (PLEDs) with hole-injection layers has been studied to determine their equivalent circuits. The equivalent circuits of the PLEDs below and above the threshold voltage for electroluminescence (EL) and their physical interpretation are shown. A significant difference in equivalent circuits is observed between below and above the threshold for EL. Above EL threshold, two R– L series branches, which are the origin of the negative capacitance effect, appear in the equivalent circuit.

Optoelectronic Characteristics of Direct-Current and Alternating-Current White Thin-Film Light-Emitting Diodes Based on Hydrogenated Amorphous Silicon Nitride Film

Direct-current and alternating-current white thin- film light-emitting diodes (DCW and ACW TFLEDs) have been fabricated and demonstrated with the intrinsic hydrogenated amorphous silicon nitride (i-a-SiN:H) film as the luminescent layer. The achievable brightness of the representative DCW and ACW TFLEDs were 200 and 170 cd/m2 at an injection-current density of 600 and 100 mA/cm2, respectively. The electroluminescence (EL) threshold voltage of the DCW TFLED was 10.9 V, and its peak wavelength and full-width at half-maximum (FWHM) of the EL spectrum were about 455 and 230 nm, respectively. For the ACW TFLED, the EL threshold voltage was 8.4 V, and its peak wavelength and FWHM of the EL spectrum were about 535 and 260 nm, respectively. In addition, their current-conduction mechanism was also investigated. Within the lower applied-voltage region, they showed an ohmic current, while for the higher applied-voltage region, the Frenkel-Poole emission was the main mechanism. It was also found that the H2 -plasma treatment of luminescent i-a-SiN:H layer of an ACW TFLED played an important role in improving device performances, such as decreased EL threshold voltage, increased brightness, and broadened and blue-shifted EL spectrum. The EL spectra of an ACW TFLED under either DC forward or reverse bias or the sinusoidal AC voltage were qualitatively very similar, with a peak wavelength at about 535 nm and a broad FWHM about 260 nm. Moreover, the EL intensity of an ACW TFLED increased with an AC frequency of up to 180 kHz and, then, decreased rapidly and became very weak as the frequency was up to about 500 kHz.

Hydrogenated Amorphous Silicon Carbide Alternating-Current Thin-Film Light-Emitting Diodes

Hydrogenated amorphous silicon carbide alternating-current thin-film light-emitting diodes ( ACTFLEDs) have been successfully fabricated on indium-tin-oxide-coated glass substrates and characterized. The electroluminescence (EL) spectra of an obtained ACTFLED under either a dc (positive or negative) bias or a sinusoidal ac voltage were qualitatively very similar, with a peak wavelength at about and a shoulder at about . The EL threshold voltage and brightness of the ACTFLEDs were sensitive to the contact behavior between the metal electrode employed and amorphous film. Moreover, the EL intensity of an ACTFLED increased with the frequency up to and then decreased rapidly and became very weak as the frequency increased to about . A model based on the equivalent circuit has also been proposed to explain this frequency-dependent EL behavior.

Effects of processing parameters on electroluminescence of rf magnetron sputter deposited ZnS:ErF3

Zn S : Er F 3 alternating current thin film electroluminescent (ACTFEL) devices are fabricated by rf plasma magnetron sputtering. In a full factorial design-of-experiment study, increasing deposition temperature, duty cycle on the doped target, and sputter gas pressure all result in increases in the 1.55μm near infrared (NIR) electroluminescence (EL) irradiance at 20V above threshold (B20). An increase in the EL threshold voltage (Vth) upon increasing the duty cycle of the undoped target is also reported. Postdeposition annealing of ACTFEL devices at 425°C for 1h improves the NIR EL irradiance by decreasing the F concentrations in the ZnS:Er films. The origins of these effects are discussed in terms of negative ion resputtering, surface mobility of sputtered species, crystallinity, and the effects of atomic concentration on the EL and radiative relaxation processes. A maximum irradiance, B20, of 147μW∕cm2 is measured for the 1.55μm NIR EL peak from ZnS:Er ACTFEL devices produced using a deposition temperature of 150°C, a duty cycle of 75%, an argon sputtering gas pressure of 24mTorr, and postdeposition annealing at 425°C for 1h in nitrogen.

Optoelectronic characteristics of MEH-PPV polymer LEDs with thin, doped composition-graded a-SiC:H carrier injection layers

The optoelectronic characteristics of poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) (MEH-PPV) polymer LEDs (PLEDs) have been improved by employing thin doped composition-graded (CG) hydrogenated amorphous silicon–carbide (a-SiC:H) films as carrier injection layers and O2-plasma treatment on indium–tin-oxide (ITO) transparent electrode, as compared with previously reported ones having doped constant-optical-gap a-SiC:H carrier injection layers. For PLEDs with an n-type a-SiC:H electron injection layer (EIL) only, the electroluminescence (EL) threshold voltage and brightness were improved from 7.3V, 3162cd/m2 to 6.3V, 5829cd/m2 (at a current density J=0.6A/cm2), respectively, by using the CG technique. The enhancement of EL performance of the CG technique was due to the increased electron injection efficiency resulting from a smoother barrier and reduced recombination of charge carriers at the EIL and MEH-PPV interface. Also, surface modification of the ITO transparent electrode by O2-plasma treatment was used to further improve the EL threshold voltage and brightness of this PLED to 5.1V, 6250cd/m2 (at J=0.6A/cm2). Furthermore, by employing the CG n[p]-a-SiC:H film as EIL [hole injection layer (HIL)] and O2-plasma treatment on the ITO electrode, the brightness of PLEDs could be enhanced to 9350cd/m2 (at a J=0.3A/cm2), as compared with the 6450cd/m2 obtained from a previously reported PLED with a constant-optical-gap n-a-SiCGe:H EIL and p-a-Si:H HIL.

Significant improvement on the electroluminescence characteristics of MEH-PPV by blending with PMMA

A significant improvement on the electroluminescence threshold voltage from 5.5 volts down to 2.9 volts of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) in the Ca/polymer/ITO diode is observed by incorporating a non-conjugated PMMA into MEH-PPV. The threshold voltage of the material decreases with the poly(methyl methacrylate) (PMMA) content. Near-field scanning absorption and photoluminescence micrographs show that the PMMA domains dispersed in the continuous MEH-PPV phase appear to be similar to sesame seeds embedded in the MEH-PPV pancakes. The resistivity of PMMA is two orders lower than that of MEH-PPV. The excellent electrical conduction of PMMA in Ca/PMMA/ITO glass devices is not due to a pin-hole defect mechanism but may result from the electron hopping from Ca through the carbonyl groups in the PMMA. The significant improvement in the threshold voltage may be due to the combination of the electron hopping in the thin PMMA sesame seeds and the thinner thickness effect of MEH-PPV in the light emitting layer.

Dependence of electroluminescent properties on cathode metalsin PVCz double-layered organic EL devices

We have investigated cathode metal dependence of electroluminescence (EL) characteristics of PVCz based EL devices by using Sr, Mg and Ag. The following has been clarified. First, we have obtained luminance as high as 1000 cd/m 2 for Poly(Nvinylcarbazole) (PVCz) EL devices without electron transport molecules but only emissive dye by using Mg and Sr metals cathode; while the device with Ag cathode showed a brightness about half as high. Secondly, threshold voltage for electroluminescence and barrier height for charge carrier injection both showed clear dependence on metal work function. Thirdly, chemical shift of core electron binding energy by X-ray photoelectron spectroscopy showed oxidation of Sr metal electrode near PVCz surface even if deposited under background pressure of 10 −10 Torr, while that of Mg showed negligible shift.

Selective light induced changes of the EL threshold voltage in poly(phenylene vinylene)-based light emitting diode

We present here a new method for lowering the electroluminescence (EL) threshold voltage in polymer-based LEDs. The method consists of spatially resolved illumination in the presence of electric field.

Optoelectronic characteristics of a -SiC:H-based pin thin film LEDshaving a thin Mo buffer layer in contact with p -type a -Si:H

To improve the electroluminescence (EL) properties of the a-SiC:H-based pin thin film light-emitting diodes (TFLEDs) with the promotion of hole injection efficiency, a thin Mo metal film was used as a buffer layer to prevent reactions between the ITO (indium-tin-oxide) electrode and the p+-a-Si:H layer. With the formation of semi-transparent Mo silicide after annealing, a lower EL threshold voltage and a significantly higher brightness for a finished TFLED were achieved. The brightness of the obtained device was 1300 cd/m2 at an injection current density of 600 mA/cm2, and its EL threshold voltage was 14 V.

Double Graded-Gap Hydrogenated Amorphous Silicon Carbide Thin-Film Light-Emitting Diode with Composition-Graded N Layer and Carbon-Increasing P Layer

An hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diode (TFLED), in which a composition-graded n layer and carbon-increasing p layer respectively replace the constant optical-gap p-a-SiC:H layer and n-a-SiC:H layer employed in the previously reported double graded-gap (DG) TFLED and which contains dopant-graded p-i and i-n junctions, was successfully fabricated to improve the electroluminescence (EL) of TFLED. This device had a brightness of 400 cd/m2 at an injection current density of 600 mA/cm2 and its EL threshold voltage (V th) was only 9.9 V. The device EL spectrum showed a peak at 586 nm wavelength and emitted yellowish-orange light. The proposed TFLED had good stability of EL intensity and the EL spectrum during normal operation. The optimum conditions of rapid thermal annealing (RTA), which improved the ohmic contact between the amorphous layer and external electrode of the device, were 5 min at 300°C in a 250 Torr H2 ambient.

Electroluminescence characteristics and current-conduction mechanism of a-SiC:H p-i-n thin-film light-emitting diodes with barrier layer inserted at p-i interface

In order to improve the electroluminescence (EL) characteristics of the hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diode (TFLED), a barrier layer (BL) was inserted at its p-i interface to enhance the hole injection efficiency under forward-bias operation. The a-SiC:H TFLED's with various optical gaps of BL had been fabricated and characterized. In addition, a composition-graded n/sup +/-layer was used to reduce its series and contact resistances to the Al electrode and hence the EL threshold voltage (V/sub th/) of an a-SiC:H BL TFLED. The highest obtainable brightness of an a-SiC:H BL TFLED was 342 cd/m/sup 2/ at an injection current density of 600 mA/cm/sup 2/ and the lowest EL V/sub th/ achievable was 6.0 V. The current-conduction mechanism of an a-SiC:H BL TFLED had also been investigated. Within the lower applied-bias region, it showed an ohmic current, while within the higher applied-bias region, a space-charge-limited current (SCLC) was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hydrogenated amorphous silicon-carbide thin-film light-emitting diode with quantum-well-injection structure

To improve the electroluminescence (EL) intensity of the hydrogenated amorphous silicon carbide (a-SiC : H) pin thin-film light-emitting diode (TFLED), a quantum-well-injection (QWI) structure has been incorporated into the i-layer of an a-SiC : H pin TFLED at the p-i interface. The obtained brightness of this QWI TFLED is 256cd/m2 at an injection current density of 800mA/cm2, which is about three orders of magnitude higher than the brightness of an a-SiC : H pin TFLED. Also, a comparatively lower EL threshold voltage of 6 V was observed for this a-SiC : H QWI TFLED.

Photoquenching of electroluminescence in ZnS:TbF3 thin films

The photoquenching effect of electroluminescence (EL) has been observed for the first time in thin-film ZnS:TbF3 EL devices. The luminescent intensity and the conductive charge are quenched, when the device is exposed to a visible light of energy between 1.75–2.75 eV. The maximum photoquenching rate of 45% is observed at a voltage slightly higher than the EL threshold voltage. The recovering time from the photoquenching is of the order of several seconds. This photoquenching effect is likely to be due to deep hole traps.

Electroluminescence in forward-biased zinc selenide Schottky diodes

Schottky diodes consisting of a metal contact on n-type ZnSe emit light when biased in the forward direction if there is a layer of insulating or semi-insulating material between the metal and the semiconductor. The electroluminescence spectrum is characteristic of luminescent centres in the ZnSe. There is a threshold voltage for electroluminescence which corresponds to the difference between the metal Fermi level and the top of the valence band of the semiconductor. For gold on ZnSe the threshold voltage is 1·3 V. Characteristics of the device differ from those of similar MIS systems because the oxide used here has appreciable conductivity.

The Effect of Electric Fields on Scintillations in Crystalline Zinc Sulphide†

ABSTRACT The effects of both static and alternating electric fields on scintillations produced by alpha particles in zinc sulphide have been examined. The results indicate that in electroluminescent crystals the brightness is reduced when the alpha particles are incident on the negative electrode. A change of barrier thickness with frequency was estimated from the results. The variation is interpreted as a change in dielectric constant with frequency. The results aeo in good agreement with earlier work and are supported by observations of the electroluminescence threshold voltage.