Time-resolved Electroluminescence Measurements Research Articles

Probing the Operation of Quantum-Dot Light-Emitting Diodes Using Electrically Pumped Transient Absorption Spectroscopy.

The quantum-dot light-emitting diode (QLED) is a new generation light emission source that holds great promise for display and lighting applications. Understanding the dynamics of electrons and holes in QLEDs during their operation is crucial for future QLED optimization, but a time-resolved technology capable of characterizing electrons is still lacking. To tackle this challenge, we develop a unique electrically pumped transient absorption (E-TA) spectroscopy to probe the density of electrons in the QD layer with a nanosecond time resolution. The E-TA result provides a comprehensive understanding of the electron dynamics in QLEDs by quantifying the electron injection time after external voltage on, electron release time after external voltage off, and equilibrated electron density (Ne) in the QD layer during device operation. By combining E-TA technology with time-resolved electroluminescence and transient current measurements, we present a comprehensive overview of the dynamics of both electrons and holes in a QLED during operation.

Enhancing carrier transport and carrier capture with a good current spreading characteristic via graphene transparent conductive electrodes in InGaN/GaN multiple-quantum-well light emitting diodes

In this work, InGaN/GaN multiple-quantum-wells light-emitting diodes with and without graphene transparent conductive electrodes are studied with current-voltage, electroluminescence, and time-resolved electroluminescence (TREL) measurements. The results demonstrate that the applications of graphene electrodes on LED devices will spread injection carriers more uniformly into the active region and therefore result in a larger current density, broader luminescence area, and stronger EL intensity. In addition, the TREL data will be further analyzed by employing a 2-N theoretical model of carrier transport, capture, and escape processes. The combined experimental and theoretical results clearly indicate that those LEDs with graphene transparent conductive electrodes at p-junctions will have a shorter hole transport time along the lateral direction and thus a more efficient current spreading and a larger luminescence area. In addition, a shorter hole transport time will also expedite hole capture processes and result in a shorter capture time and better light emitting efficiency. Furthermore, as more carrier injected into the active regions of LEDs, thanks to graphene transparent conductive electrodes, excessive carriers need more time to proceed carrier recombination processes in QWs and result in a longer carrier recombination time. In short, the LED samples, with the help of graphene electrodes, are shown to have a better carrier transport efficiency, better carrier capture efficiency, and more electron-hole recombination. These research results provide important information for the carrier transport, carrier capture, and recombination processes in InGaN/GaN MQW LEDs with graphene transparent conductive electrodes.

Transient overshoot and storage of charge carriers on ligands in quantum dot LEDs

Quantum dot light emitting diodes (QLEDs) emitting at 410 nm were studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off was seen, which is attributed to the accumulation and storage of charge carriers at the ligand-quantum dot interface. Shorter ligands showed a faster rise time and prevented the formation of an overshoot, whereas longer ligands caused the storage of charges, responded slower, and showed an overshoot. When the external voltage was switched off, the electric field between the injected and stored electrons and holes led to the occurrence of the overshoot. Applying a dual voltage pulse avoided this overshoot and instead a delayed luminescence was observed. As the accumulated charges were immobile and trapped in shallow states, a reverse pulse was applied to fully deplete the emissive layer. Because the transient overshoot disappeared after the device had been turned on, this can be used as a measure of the degradation of QLEDs. The ligands constitute a major obstacle for an efficient and long lasting blue/purple quantum dot emitter.

Sputtered gold nanoparticles enhanced quantum dot light-emitting diodes**Project supported by the National Natural Science Foundation of China (Grant Nos. 21603012, 61735004, and 61722502).

Surface plasmonic effects of metallic particles have been known to be an effective method to improve the performances of light emitting didoes. In this work, we report the sputtered Au nanoparticles enhanced electroluminescence in inverted quantum dot light emitting diodes (ITO/Au NPs/ZnMgO/QDs/TFB/PEDOT:PSS/Al). By combining the time-resolved photoluminescence, transient electroluminescence, and ultraviolet photoelectron spectrometer measurements, the enhancement of the internal field enhanced exciton coupling to surface plasmons and the electron injection rate increasing with Au nanoparticles’ incorporation can be explained. Phenomenological numerical calculations indicate that the electron mobility of the electron transport layer increases from 1.39 × 10−5 cm2/V⋅s to 1.91 × 10−5 cm2/V⋅s for Au NPs modified device. As a result, the maximum device luminescence is enhanced by 1.41 fold (from 14600 cd/cm2 to 20720 cd/cm2) and maximum current efficiency is improved by 1.29 fold (from 3.12 cd/A to 4.02 cd/A).

Spectroscopic observation of triplet exciton dynamics during operation in polymer light emitting diodes

In typical fluorescent organic light emitting diodes (LEDs), the observation of triplet excitons is difficult because of their non-radiative features of decay. The generation process of triplet excitons was successfully monitored by employing spectroscopic techniques combined with LED operation. The generation dynamics of singlet excitons are then simultaneously examined by time-resolved electroluminescence measurements, enabling relative comparison of the generation processes of singlet and triplet excitons. As a result, the generation ratio of singlet excitons is shown to be enhanced by applied voltage. The enhancement is concluded to result from the increase of the generation rate in signlet excitons induced by electric field.

Towards the design of efficient quantum dot light-emitting diodes by controlling the exciton lifetime.

Time-resolved photoluminescence and electroluminescence measurements were used to explore the emission characteristics of excitons in quantum dot light-emitting diodes (QD-LEDs). It is found that the lifetime of excitons in the QDs can be varied by adjusting the distance between the excitons and metal Al mirror, which is due to the effect of local density of optical states (LDOS) on the exciton decay rate. QD-LEDs with different hole transport layer (HTL) thickness, i.e., different distance between QDs and Al reflective anode, have been fabricated and it is found that the HTL thickness affects the device efficiency performance greatly, and the optimal HTL thickness for the red QD-LED (emission peak is at 621 nm) is 80 nm. These results shed light on the factors affecting the efficiency and efficiency roll-off in QD-LEDs, thus may provide a clue for high performance QD-LEDs.

Efficient carrier relaxation and fast carrier recombination of N-polar InGaN/GaN light emitting diodes

Based on quantum efficiency and time-resolved electroluminescence measurements, the effects of carrier localization and quantum-confined Stark effect (QCSE) on carrier transport and recombination dynamics of Ga- and N-polar InGaN/GaN light-emitting diodes (LEDs) are reported. The N-polar LED exhibits shorter ns-scale response, rising, delay, and recombination times than the Ga-polar one does. Stronger carrier localization and the combined effects of suppressed QCSE and electric field and lower potential barrier acting upon the forward bias in an N-polar LED provide the advantages of more efficient carrier relaxation and faster carrier recombination. By optimizing growth conditions to enhance the radiative recombination, the advantages of more efficient carrier relaxation and faster carrier recombination in a competitive performance N-polar LED can be realized for applications of high-speed flash LEDs. The research results provide important information for carrier transport and recombination dynamics of an N-polar InGaN/GaN LED.

Highly efficient long-life blue fluorescent organic light-emitting diode exhibiting triplet–triplet annihilation effects enhanced by a novel hole-transporting material

A novel hole-transporting material with high singlet and triplet excitation energy levels was developed. Quantum efficiency of a fluorescent organic light-emitting diode (OLED) using this material as a hole-transporting layer can be increased because of facilitated triplet–triplet annihilation (TTA) due to exciton confinement in an emission layer. Furthermore, this material has a deep highest occupied molecular orbital level because of the absence of triarylamine structure. This feature also contributes to the increase in the quantum efficiency, owing to inhibition of a low-energy exciplex formed between the material and a host in the emission layer. Achieved consequently was a blue fluorescent OLED exhibiting a high external quantum efficiency of 11.9% and a long half-decay time of 8,000 h at 1,000 cd/m2. By the device analysis including time-resolved electroluminescence measurements, it was confirmed that TTA contributes to the high efficiency.

Degradation of AlGaN/GaN HEMT devices: Role of reverse-bias and hot electron stress

This paper describes a deep investigation of the degradation mechanisms induced by off-state and on-state stress in AlGaN/GaN HEMTs. Concerning reverse-bias degradation, results underline that the exposure to reverse-bias stress can induce (i) a recoverable change in the gate current due to the accumulation of negative charges under the gate, (ii) and a permanent degradation of gate characteristics due to the generation of vertical parasitic leakage paths through the AlGaN layer. Further analysis of the kinetics of this degradation mechanism, correlated with time-resolved Electroluminescence (EL) measurements, allowed to define a model that explains the time-dependence of the phenomenon and the role of gate voltage as accelerating factor, providing an interpretation for both recoverable and permanent modifications of the main device characteristics induced by reverse-bias stress. On the other hand, on devices that have shown an improved robustness against the reverse-bias gate degradation, we investigated the origin of the degradation under on-state stress. In this case, results obtained with a combined electrical and optical analysis, showed that on-state stress may induce a significant decrease in drain saturation current and Electroluminescence (EL) signal, with a degradation rate that strongly depends on the EL intensity measured before stress, which is representative of the presence of hot-electrons in the channel. On-state degradation can be ascribed to a decrease in the electric-field, due to the trapping of electrons within the barrier or at the surface induced by hot-electrons. Therefore, by using the EL signal as measure of the stress accelerating factor, it was possible to derive an acceleration law for hot-electron degradation on GaN HEMTs.

Storage of charge carriers on emitter molecules in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) using the red phosphorescent emitter iridium(III)bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) [$\mathrm{Ir}{(\mathrm{MDQ})}_{2}(\mathrm{acac})$] are studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off is found, which is attributed to electron accumulation on $\mathrm{Ir}{(\mathrm{MDQ})}_{2}(\mathrm{acac})$ molecules. The mechanism is verified via impedance spectroscopy and by application of positive and negative off-voltages. We calculate the density of accumulated electrons and find that it scales linearly with the doping concentration of the emitter. Using thin quenching layers, we locate the position of the emission zone during normal OLED operation and after voltage turn-off. In addition, the transient overshoot is also observed in three-color white-emitting OLEDs. By time- and spectrally resolved measurements using a streak camera, we directly attribute the overshoot to electron accumulation on $\mathrm{Ir}{(\mathrm{MDQ})}_{2}(\mathrm{acac})$. We propose that similar processes are present in many state-of-the-art OLEDs and believe that the quantification of charge carrier storage will help to improve the efficiency of OLEDs.

Carrier Transport Study of TMIn-Treated InGaN LEDs by Using Quantum Efficiency and Time-Resolved Electro-Luminescence Measurements

The aim of this study is to determine the emission and carrier transport characteristics of Trimethylindium (TMIn)-treated InGaN green light emitting diodes (LEDs) by using quantum efficiency and time-resolved electro-luminescence measurements. As TMIn treatment time increased, a more homogeneous indium composition and low V-shaped defect density lead to slightly blue-shifted peak position, narrower spectrum width, and better luminescence efficiency. In addition, the ns-scale response time shows efficient carrier injection and carrier transport. The shorter response times of the longer-TMIn-treated LED suggest that a lower V-shaped density is beneficial to carrier injection into the quantum wells and that a slight carrier localization helps carrier recombination. Furthermore, a μs-scale decay time represents inefficient carrier recombination in the active region. The longer the TMIn treatment time, the shorter the response time, the faster the radiative decay rate, and the slower the nonradiative decay rate. With a forward applied voltage, lower V-shaped defect density, un-reduced polarization field, carrier delocalization, and weaker Auger recombination in the TMIn-treated samples lead to the inevitable efficiency droop. The resulting recombination dynamics are correlated with the device characteristics and performance of the TMIn-treated LEDs. The research results provide important information to solve the efficiency droop of LEDs.

Towards a deeper understanding of the reduced efficiency droop in low defect‐density GaInN wide‐well LEDs

AbstractA series of 400 nm emitting GaInN/GaN single‐well light‐emitting diodes, grown on ultra‐low dislocation density GaN templates with well widths varying between 3 and 18 nm, were investigated by pulsed and time resolved electroluminescence measurements using small signal modulation technique for the latter. A reduction of the efficiency droop at high current densities with increasing well width was observed. The highest overall external quantum efficiency was obtained for LEDs with 11 nm thick double‐heterostructure (or wide well) active region at current densities above 70 A/cm2. Furthermore, carrier lifetime and volume carrier density in the wells were determined. A model based on the interplay between defect assisted recombination, depending on dislocation density, and strongly carrier density dependent loss processes is presented explaining the well‐thickness dependence of the efficiency droop. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cavity-enhanced photonic crystal light-emitting diode at 1300 nm

We report direct evidence of enhanced spontaneous emission in a photonic crystal light-emitting diode (LED) at telecom wavelength ( λ ∼ 1300 nm). This result is crucial to obtain an electrically driven single photon source with high extraction efficiency. This kind of devices can be used for different types of applications in emerging fields like quantum key distribution, quantum information technology as well as in fundamental studies on quantum electrodynamics. In this work we present a device which combines a photonic crystal nanocavity with InAs quantum dots (QDs) and an LED, leading to cavity-enhanced emission at telecom wavelengths under electrical injection. The fabrication process, based on e-beam lithography and additive and subtractive processes, is described in detail. A well-isolated emission peak at about 1300 nm from the PhC mode electrically pumped is obtained ( Q ∼ 4000), and the enhancement of the spontaneous emission rate (∼1.5 fold) is clearly evidenced by time-resolved electroluminescence measurements.

Enhanced spontaneous emission in a photonic-crystal light-emitting diode

We report direct evidence of enhanced spontaneous emission in a photonic-crystal (PhC) light-emitting diode. The device consists of p-i-n heterojunction embedded in a suspended membrane, comprising a layer of self-assembled quantum dots. Current is injected laterally from the periphery to the center of the PhC. A well-isolated emission peak at 1.3μm from the PhC cavity mode is observed, and the enhancement of the spontaneous emission rate is clearly evidenced by time-resolved electroluminescence measurements, showing that our diode switches off in a time shorter than the bulk radiative and nonradiative lifetimes.

Time‐resolved electro‐luminescence studies of InGaN blue LEDs with chip size variations

AbstractIn this study, we report carrier dynamics of three InGaN Blue LEDs with three chip sizes by time‐resolved electroluminescence (TREL) measurements. Experiments results show that the sample with the largest chip size has the smallest EL width and longest response time among the three samples. It implies that the current spreading for larger chip size need more time. It is partially attributed to the fact that carrier relaxation into the strongly localized states (high‐indium clusters) need more time. Furthermore, the long decay times are due to the carrier recombination from the deep trapped localization states and/or the high defect density inside the samples. Also, the decay times of the three samples are nearly independent of the chip size and applied voltage. This implies that the sum densities of localization states and defects of the three samples have nearly the same order of magnitude. By reducing the density of the defects, the quantum efficiency of the LEDs can be increased at least by a factor of 10. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time resolved electroluminescence measurements on GaAs and GaN devices

Time correlated single photon electroluminescence maps have been recorded on operating semiconductor devices with a spatial resolution approaching 1μm and a time resolution of 50 ps. The technique is noninvasive and observes signal wave forms optically, directly at the device. Spectral measurements in the near infrared and visible were also performed. Two examples of the application of this technique to semiconductor devices are presented. An AlGaAs∕GaAs heterojunction bipolar transistor typical of high speed bipolar devices used in rf circuitry, and an AlGaN∕GaN heterostructure field-effect transistor as an example of the application of optical techniques in characterizing the emerging technology.

Time-resolved transient electroluminescence measurements of emission from DCM-doped Alq 3 layers

Organic light-emitting devices (OLEDs) using NPB (α-naphthylphenylbiphenyl diamine) as the hole-transport layer and DCM [4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran] doped Alq 3[tris-(8-hydroxyquinoline)] as the emission layer were studied using a time-resolved transient electroluminescence method. Upon application of a pseudo-rectangular voltage pulse, the luminance increased and overshot to maxima. Using suitable spectrum filters the overshoot luminance peaks were identified to originate solely from the DCM emission. When two consecutive voltage pulses are applied, the overshoot vanished during the second pulse when time gap is shorter than 1 ms. It was considered to be related to carrier traps in the DCM molecules.

Time-resolved space charge and electroluminescence measurements in polyethylene under ac stress

Time-resolved space charge (SC) and electroluminescence (EL) measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic (PEA) technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 /spl mu/s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an unbalanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behavior of those materials under ac field.

Room-temperature silicon light-emitting diodes based on dislocation luminescence

We demonstrate electroluminescence (EL) with an external efficiency of more than 0.1% at room temperature from glide dislocations in silicon. The key to this achievement is a considerable reduction of nonradiative carrier recombination at dislocations due to impurities and core defects by impurity gettering and hydrogen passivation, respectively, which is shown by means of deep-level transient spectroscopy. Time-resolved EL measurements reveal a response time below 1.8 μs, which is much faster, compared to the band-to-band luminescence of bulk silicon.

Single quantum dot electroluminescence near [formula omitted

We present the first electroluminescence spectra to resolve individual single quantum dot emission lines to the telecommunications transmission window at 1.3 μm . The dot is embedded within the i-region of a p– i– n diode to enable electrical excitation. Detection is performed using a cooled InGaAs array detector. Photoluminescence spectra of single quantum dots in small mesas (∼1 μm) reveal the complex emission line structure of these dots for the first time. Initial time-resolved electroluminescence measurements at wavelengths above 1 μm indicate lifetimes similar to those commonly measured in dots emitting around 900 nm .