Efficiency analysis of a—SiC：H thin film light emitting diodes

The hole injection ,the radiative recombination and the device lumi-nescent efficiencies of amorphous silicon carbide thin film p-I-n junction light emit-ting diodes are quantitatively calculated,and the effect of the carrier (especially the hole)injection and recombination processes on the device luminescent characteris-tics are revealed.Without considering the device junction temperature ,it is round that the device luminescent efficiency mainly depends on the hole injection efficiency at low field and the hole radiative recombination efficiency at high field respective-ly.The theoretical analyses are in well agreement with the experimental results.     

Explaining the different efficiency behaviors of PHOLEDs with/without a hole injection barrier at the hole transport layer/emitter layer interface

In this work we investigate the different efficiency behaviors of the devices with and without hole injection barrier, utilizing in our investigation the archetypical 4,4′-bis(carbazol-9-yl)biphenyl:Tris(2-phenylpyridine)iridium(III) host–guest PHOLEDs system. The results show that the recombination of electrons and holes on the host material generally leads to higher device efficiency in comparison to the case where recombination happens on the guest material. The results also show that in devices where a hole injection barrier between the HTL and the host material in the EML exists, the emission mechanism gradually changes from one based on host e–h recombination to one based on guest e–h recombination as the guest concentration is increased. When host e–h recombination is dominant, although it tends to produce higher device efficiency, host e–h recombination is generally also associated with significant efficiency roll-off; the latter arises from quenching of the host triplet excitons primarily due to host–host TTA. As the concentration of the guest molecules increases and the creation of host triplet excitons subsides (since most e–h recombination occurs on the guest) host–host TTA decreases, hence also the efficiency roll-off. In such case, quenching is mostly caused by polarons residing on guest sites. At optimum guest concentrations (∼8% Vol.), a balance between host e–h recombination and guest e–h recombination is reached, and thus also minimal TTA and Triplet-Polaron Quenching. On the other hand, in devices where hole injection barrier between the HTL and the host in the EML is insignificant, emission mechanism is always based on host e–h recombination irrespective of the guest concentration, and therefore have higher efficiency and the efficiency does not depend on guest concentration. The absence of the injection barrier in these devices results in a wider recombination zone, and hence a lower exciton concentration in general, which in turn reduces host–host TTA and thus lowers efficiency roll-off. In contrast, guest–guest TTA is not found to play a significant role in device efficiency behavior.     

Advantage of InGaN-based light-emitting diodes with trapezoidal electron blocking layer

In this study, a trapezoidal-shaped electron blocking layer is proposed to improve efficiency droop of InGaN/GaN multiple quantum well light-emitting diodes. The energy band diagram, carrier distribution profile, electrostatic field, and electron current leakage are systematically investigated between two light-emitting diodes with different electron blocking layer structures. The simulation results show that, when traditional AlGaN electron blocking layer is replaced by trapezoidal-shaped electron blocking layer, the electron current leakage is dramatically reduced and the hole injection efficiency in markedly enhanced due to the better polarization match, the quantum-confined Stark effect is mitigated and the radiative recombination rate is increased in the active region subsequently, which are responsible for the alleviation of efficiency droop. The optical performance of light-emitting diodes with trapezoidal-shaped electron blocking layer is significantly improved when compared with its counterpart with traditional AlGaN electron blocking layer.     

Fundamental Tradeoff between Emission Intensity and Efficiency in Light‐Emitting Electrochemical Cells

The characteristic doping process in polymer light‐emitting electrochemical cells (LECs) causes a tradeoff between luminescence intensity and efficiency. Experiments and numerical modeling on thin film polymer LECs show that, on the one hand, carrier injection and transport benefit from electrochemical doping, leading to increased electron‐hole recombination. On the other hand, the radiative recombination efficiency is reduced by exciton quenching by polarons involved in the doping. Consequently, the quasi‐steady‐state luminescent efficiency decreases with increasing ion concentration. The transient of the luminescent efficiency shows a characteristic roll‐off while the current continuously increases, attributed to ongoing electrochemical doping and the associated exciton quenching. Both effects can be modeled by exciton polaron‐quenching via diffusion‐assisted Förster resonance energy transfer. These results indicate that the tradeoff between efficiency and intensity is fundamental, suggesting that the application realm of future LECs should be sought in high‐brightness, low‐production cost devices, rather than in high‐efficiency devices.     

Theory of a forward-biased diffused-junction P-L-N rectifier—Part I: Exact numerical solutions

Exact numerical solutions have been obtained for a forward-biased diffused-junction n⁺-p-p⁺silicon rectifier operating from low to high injection levels. The results indicate that under high-level injection conditions, there exists a quasi-neutral region (i.e., a region wherein the electron and hole concentrations are very nearly equal) in the device which is not confined to the lightly doped p-base region but stretches into the diffused n⁺region as the injection level increases. For the impurity profile and the Shockley-Read-Hall recombination model used, it is additionally found that the quasi-Fermi potentials for both electrons and holes are constant across a large portion of the diffused region adjoining the quasi-neutral or "effective base" region. The most significant finding, however, is that at the boundary between the effective base region and the diffused region, the hole current varies more or less directly as the carrier concentration, in contrast to the situation in a step-junction rectifier where the minority carrier current in the heavily doped regions varies as the square of the carrier concentration at the base edge. A simple analysis shows that this linear relationship can be related to the constant quasi-Fermi potentials across the diffused region. As a consequence of the aforementioned linear relationship, the injection efficiency of the diffused junction is roughly independent of the injection or current levels. It may be high or low depending on whether the concentration of recombination centers in the diffused region is small or large compared to that in the base region. This is unlike the situation in a step junction where the injection efficiency would be initially high and then degrade, as the current increases.     

Quantum-confined field-effect light emitters: device physics andexperiments

Extended experimental results on three-terminal quantum-confined field-effect light emitters with current injection and field control of luminescent characteristics in the quantum-well structure are reported. By incorporating superlattice buffer layers (SLBLs), the quantum efficiency of the device is dramatically improved and equivalently nonradiative recombination processes are sufficiently suppressed at room temperature. The red-shift of the emission spectra by the quantum-confined Stark effect assures that the electric field is effectively applied to the quantum well. The experimental data on the transient responses of emission intensity to input voltage pulses show fairly good correspondences with theoretical prediction and previous photoluminescence experiments. The authors discuss the ultimate capability of high-speed switching and point that an optical pulse with a duration as short as 30 ps and involving more than 100 photons can be generated by scaling down the size of the device with 1% external efficiency     

Optical Gain and Spontaneous Emission in GaAsSb–InGaAs Type-II “W” Laser Structures

The modal gain, modal loss and spontaneous emission of a GaAsSb-based type-II quantum-well (QW) laser structure emitting at 1.3 mum have been experimentally determined as a function of current injection and temperature. The system is able to provide a maximum of 900 cm^-1 of material gain from the n = 1 transition despite an electron-hole overlap of 32%, however, the gain from the n = 2 transition becomes dominant before this value can be achieved. The presence of the n = 2 transition has a detrimental effect on device performance, limiting the usable gain from the first transition and increasing the total radiative recombination current. Energy level calculations show that reducing the hole QW to 4 nm would increase the separation of the n = 1 and n = 2 transition by a further 45 meV, reducing the limiting effect of the transition. Carrier distribution spectra show the carriers are in thermal equilibrium for the temperatures and injection currents studied. A low radiative efficiency for this structure is measured due to a very large nonradiative current. We believe a combination of different mechanisms contribute to the nonradiative current.     

Recombination Efficiency and Width in Bilayer Organic Light-emitting Devices

A bilayer model with ohmic anode contact and injection limited cathode contact has been proposed to calculate the recombination efficiency and recombination zone width of the device. The effects of the thickness of hole transport layer and the barriers of organic/organic interface on the combination efficiency and recombination width have been discussed. It is found that： （1） When the electrons are blocked fully and the holes are not blocked significantly at the organic/organic interface, for a given Lh/L, the recombination efficiency increases with increasing the applied voltage, but at a higher applied voltage, the recombination efficiency decreases with increasing Lh/L; （2） The recombination efficiency increases with increasing applied voltage and Hh＇, and when applied voltage and Hh＇ exceed some value, the recombination efficiency appears as a plateau; （3） The recombination width decreases with increasing the applied voltage and Lh/L. This model might explain the relative experiment phenomena.     

Reverse bias light emission from GaAs1-xPx diodes

The reverse bias light emission originating at microplasmas was investigated in GaAs_1-xP_x diodes and compared with the forward bias emission. Both spectra were found to be almost identical and could be explained by the same radiative recombination processes. The presence of the strong electric field in the junction gave rise to the Franz-Keldysh effect manifested by the uniform shift (～3 meV) of the reverse bias emission towards longer wavelengths with the Stark effect broadening the free-exciton emission peak P₁. Measurement of the shift indicated that the electric field responsible for this, though high (～10³ V/cm), was considerably lower than that prevailing in the centre of the junction (～5×10⁵ V/cm). This pointed to recombination and the concomitant radiation occurring at the edge of the depletion region, the high field in the centre of the junction inhibiting the recombination of electron hole pairs.     

Investigation on the Low Luminous Efficiency in a Polymer Light‐Emitting Diode with a High Work‐function Cathode by Soft Contact Lamination**

This article reports the main origin of the low luminescent efficiency in hole‐dominant polymer light‐emitting diodes by controlling the hole injection and by chemically modifying the cathode by molecular monolayers. Since molecular modification of the top electrode is impossible when one deposits the electrode using a vacuum deposition method, this study was performed using a soft contact lamination technique to form electrical contacts on top of the emissive layer. The top electrode was chemically modified with an alkane thiol self‐assembled monolayer (SAM) to act as an interfacial spacer layer between the emitting layer and the cathode. Herein, it is reported that, contrary to common belief, a high device quantum efficiency can be achieved from the dominantly hole‐transporting device with a high work‐function cathode (like Au) by facilitating more hole injection from the anode in the device with low population of exciton quenching channels near the cathode.     

Emitter injection efficiency in heterojunction bipolar transistors

Experimental approaches for measurement of the emitter injection efficiency in heterojunction bipolar transistors are discussed. The electron and hole currents crossing the base-emitter junction and the currents recombining within the quasi-neutral emitter and base region are also determined. The influence of the interface and space-charge region recombination is discussed qualitatively. New figures of merit for a bipolar transistor are introduced. Preliminary experimental results obtained on AlGaAs/GaAs transistors are presented.     

线性分布的In组分对紫色InGaN/GaN单量子阱发光特性的影响

采用数值模拟的方法研究了具有相同的平均In组分,但In组分的分布不同的3个紫色InGaN/GaN单量子阱样品的光谱特性.通过分析样品的电致发光谱、能带结构、波函数交叠以及载流子浓度分布等,发现沿生长方向阱内In组分线性增加的单量子阱样品的发光效率最高,而In组分线性减小的样品发光效率最低.这是因为In组分的线性增加能够减弱极化场对价带的影响,使阱内价带变得更加平缓.这不仅降低了空穴的注入势垒高度、增大了阱中的空穴浓度,还增强了阱内电子-空穴波函数的交叠积分,提高了辐射复合几率,从而使In组分线性增加的量子阱的发光效率显著提高.     

Origins of Low Quantum Efficiencies in Quantum Dot LEDs

The promise for next generation light‐emitting device (LED) technologies is a major driver for research on nanocrystal quantum dots (QDs). The low efficiencies of current QD‐LEDs are often attributed to luminescence quenching of charged QDs through Auger‐processes. Although new QD chemistries successfully suppress Auger recombination, high performance QD‐LEDs with these materials have yet to be demonstrated. Here, QD‐LED performance is shown to be significantly limited by the electric field. Experimental field‐dependent photoluminescence decay studies and tight‐binding simulations are used to show that independent of charging, the electric field can strongly quench the luminescence of QD solids by reducing the electron and hole wavefunction overlap, thereby lowering the radiative recombination rate. Quantifying this effect for a series of CdSe/CdS QD solids reveals a strong dependence on the QD band structure, which enables the outline of clear design strategies for QD materials and device architectures to improve QD‐LED performance.     

Interfacial dipole in organic p–n junction to realize write-once–read-many-times memory

A new approach is exploited to realize nonvolatile organic write-once–read-many-times (WORM) memory based on copper phthalocyanine (CuPc)/hexadecafluoro-copper-phthalocyanine (F₁₆CuPc) p–n junction. The as-fabricated device is found to be at its ON state and can be programmed irreversibly to the OFF state by applying a negative bias. The WORM device exhibits a high ON/OFF current ratio of up to 2.6 × 10⁴. An interfacial dipole layer is testified to be formed and destructed at the p–n junction interface for the ON and OFF states, respectively. The ON state at positive voltage region is attributed to the efficient hole and electron injection from the respective electrodes and then recombination at the CuPc/F₁₆CuPc interface, and the transition of the device to the OFF state results from the destruction of the interfacial dipole layer and formation of an insulating layer which restricts charge carrier recombination at the interface.     

Luminescent Efficiency in Single-layer Organic Electrophosphorescent Devices

Based on the charge injection and recombination processes and the triplet-triplet annihilation process, a model to calculate the electro.luminescent（EL） efficiency is presented. The influences of the applied electric field on the injection efficiency, recombination efficiency and electroluminescent efficiency are discussed. It is found that： （1） The injection efficiency is increasing while the recombination efficiency is decreasing with the applied electric field increasing. （2） The EL efficiency is enhanced at low electric field slowly but is decreasing at high electric field with the increase of applied voltage. （3） The EL efficiency is decreasing with the increase of the host-guest molecular distance （R）. So, it is concluded that the EL efficiency in single-layer organic electrophosphorescent devices is dominated by injection efficiency at lower electric field and recombination efficiency at higher electric field.     

Peculiarities of radiative recombination in amorphous molecular semiconductors doped with polymethine dye

Processes of charge carrier photogeneration and recombination are investigated in films of poly-N-epoxypropylcarbazole doped with polymethine dye. Films with blocking contacts were illuminated with light from either the region of dye absorption or beyond this region. The kinetics of accumulation and relaxation of electron–hole pairs with lifetimes greater than tens or hundreds of seconds was studied. It is presumed that the reason for the growth of recombination luminescence intensity in an external electric field is connected with the increase in efficiency of radiative recombination stimulated by electrons captured from photogenerated excitons. © 1998 John Wiley & Sons, Ltd.     

Minority carrier injection characteristics of the diffused emitter junction

For the double-diffused transistor, a one-dimensional analysis is presented on the minority carrier injection properties of a diffused emitter junction. This junction is bounded on one side by a reverse biased collector and on the other by an ohmic contact of arbitrary recombination velocity. Furthermore, arbitrary magnitudes of minority carrier lifetime are assumed in both the emitter and base regions of this semiconductor device. Injection efficiency characteristics are graphically illustrated throughout a wide range of physical and geometrical parameters. Assuming, for example, variations in the emitter junction depth, injection properties are demonstrated for transistors exhibiting a fixed collector location and also for transistors exhibiting a fixed base width. A comparison is also shown between the calculated minority carrier injection from this analysis and from other, more approximate, methods.     

Light generation in diffused GaP light emitting diodes (LEDs)

The results of a programme of research on nitrogen doped green-emitting diffused VPE LEDs are reported. The purpose of this work was to establish whether the recombination processes which control final device efficiencies occur on the p or n side of the junction. To do this we have used a novel technique which involves fabricating diodes down a bevelled epitaxial “sandwich” structure consisting of a nitrogen doped region between non nitrogen doped ones. We find that the light output from the n side is approximately a factor 1.5 times the p side output. Hence both the p and n regions of these diffused structures yield significant light outputs, although the contribution from the n side usually dominates. The above result together with cathodoluminesence efficiency measurements on p- and n-type material suggests the device injection ratio (β), defined as the ratio of injected hole and electrom currents, is β = J_h/J_e ～ 3 ? 4.5. Attempts at calculating β for these LEDs have shown that the effects of high level injection, built-in field and breakdown of the depletion approximation must be taken into account if realistic device modelling is to be carried out. The results of this work are believed to have important practical implications because they suggest that significant improvements in device efficiencies can be obtained by modifying the doping profile to increase the electron injection efficiency into the higher luminescence efficiency p-type material.     

High current conduction with high mobility by non-radiative charge recombination interfaces in organic semiconductor devices

We report a unique non-radiative p-n-p junction structure to provide high current conduction with high mobility in organic semiconductor devices. The current conduction was improved by increasing p-n junctions made with intrinsic p-type hole transport layer and n-type electron transport layer. The excellent hole mobility of 5.3 × 10^?1 cm²/V s in this p-n-p device configuration is measured by the space charge limited current method with an electric field of 0.3 MV/cm. Enhanced current conduction of 248% at 4.0 V was observed in fluorescent blue organic light-emitting diodes with introduction of non-radiative p-n-p-n-p junction interfaces. Thereupon, the power efficiency at 1000 cd/m² was improved by 22% and the driving voltage also was reduced by 17%, compared to that of no interface device. Such high current conduction with high mobility is attributed to the carrier recombination at p-n-p interfaces through coulombic interaction. This non-radiative p-n-p junction structure suggested in this report can be very useful for many practical organic semiconductor device applications.     

Mechanism of light production in metal-insulator-semiconductor diodes; GaN:Mg violet light-emitting diodes

The mechanism of electroluminescence in MIN GaN:Mg violet light-emitting diodes was analyzed by considering observed structural features of the diodes as well as their electrical and optical characteristics. Comparison of the observed I–V characteristic, including dependence upon temperature and upon film thickness, with all possible mechanisms for conduction in insulators lead to a proposed conduction mechanism of quantum mechanical tunneling, with the I–V characteristic being well represented by the Fowler-Nordheim equation. The proposed mechanism of light production involved impact ionization of luminescent centers near the i-n junction, with subsequent radiative recombination. This proposed mechanism was supported by measurements of carrier multiplication in the device and a steep voltage gradient at the i-n junction. The impact ionization process occurs in discrete regions coincident with sub-grain boundaries in the GaN film.