Time-resolved Electroluminescence 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.

Photoluminescence and Electroluminescence Confocal Imaging of an OLED

In recent years organic light-emitting diodes (OLEDs) have become one of the leading technologies for full-colour display panels in high-end smartphones and televisions. This rapid growth in use has occurred because OLEDs offer an all-around superior performance to liquid crystal displays (LCDs). For example, they are thinner, lighter, more flexible, less power consumptive, and brighter.When new OLEDs are developed, the optoelectronic properties of individual components and the complete device can be characterised using photoluminescence (PL) and electroluminescence (EL) spectroscopy. In this poster presentation, we use a confocal Raman microscope to characterise and spatially resolve the optoelectronic properties of a fabricated OLED device with four imaging modalities: PL, EL, time-resolved PL (TRPL), and time-resolved EL (TREL). Using a confocal microscope to characterise an OLED's spectral and time-resolved properties provides much greater detail than bulk measurements. Figure 1

Charge transport dynamics and emission response in quantum-dot light-emitting diodes for next-generation high-speed displays

Inorganic quantum-dot light-emitting diodes (QD-LEDs) have gained significant attention as optoelectronic devices for next-generation display systems due to their superior colour properties. A comprehensive understanding of the charge transport dynamics and transient emission responses of the QD-LED is crucial to achieving high motion picture quality next-generation QD-LED display systems. In this study, we investigated the transient emission response of QD-LED devices through an advanced charge transport simulation model tailored to the quantum-dots (QDs). The dynamic response of the QD-LED devices is evaluated using the time-resolved electroluminescence measurement method for both cadmium-based and cadmium-free red, green, and blue QD-LEDs. The QD-LED devices exhibit notable emission drops during pulse voltage application. The charge transport simulation quantitatively reveals that the on and off switching speeds and the emission drops are intricately influenced by the electron and hole injection balance and a combination of carrier recombination factors within the QD layer. The charge transport simulation also shows that space-charge accumulation, due to the combined effects of charge imbalance and Auger recombination, quantitatively explains a potential device degradation mechanism. Therefore, the QD-specified charge transport model provides a crucial approach in designing and optimizing QD-LED devices for next-generation high-speed QD-LED displays with ultimate colour quality and long lifetimes.

Identifying the origin of delayed electroluminescence in a polariton organic light-emitting diode.

Modifying the energy landscape of existing molecular emitters is an attractive challenge with favourable outcomes in chemistry and organic optoelectronic research. It has recently been explored through strong light-matter coupling studies where the organic emitters were placed in an optical cavity. Nonetheless, a debate revolves around whether the observed change in the material properties represents novel coupled system dynamics or the unmasking of pre-existing material properties induced by light-matter interactions. Here, for the first time, we examined the effect of strong coupling in polariton organic light-emitting diodes via time-resolved electroluminescence studies. We accompanied our experimental analysis with theoretical fits using a model of coupled rate equations accounting for all major mechanisms that can result in delayed electroluminescence in organic emitters. We found that in our devices the delayed electroluminescence was dominated by emission from trapped charges and this mechanism remained unmodified in the presence of strong coupling.

Light-emitting MOS junction for ultrahigh-resolution quantum dot displays

A high-resolution quantum-dot (QD) light-emitting device array is considered to be the key component in a high resolution near-eye micro-display. Although much research has been committed to the achievement of a high-resolution QD pattern, realizing a sub-10 micrometer or even a sub-micrometer device array is challenging because of the requirement for precise vertical multilayer alignment and the existence of electric-crosstalk effects. In this work, we propose a QD-based light-emitting metal/oxide/semiconductor junction (LE-MOSJ) with a super-simple structure of ITO/Al2O3/QDs/Ag with no injection or transfer layer. We measured the voltage-frequency-electroluminescence, spectrum-voltage, and spectrum-frequency characteristics, used voltage-dependent time-resolved electroluminescence to analyze the carrier transport behavior and the working mechanism, and attribute the electron source for the electroluminescence to free and surface defect-captured electrons. Finally, we successfully demonstrate an ultrahigh-resolution LE-MOSJ array with ∼4200 pixels per inch (PPI). We believe the proposed LE-MOSJ can provide an optional approach for realizing ultrahigh-resolution QD-based display technology.

Dynamical characteristics of AC-driven hybrid WSe2 monolayer/AlGaInP quantum wells light-emitting device

The exploration of functional light-emitting devices and numerous optoelectronic applications can be accomplished on an elegant platform provided by rapidly developing transition metal dichalcogenides (TMDCs). However, TMDCs-based light emitting devices encounter certain serious difficulties, such as high resistance losses from ohmic contacts or the need for complex heterostructures, which restricts the device applications. Despite the fact that AC-driven light emitting devices have developed ways to overcome these challenges, there is still a significant demand for multiple wavelength emission from a single device, which is necessary for full color light emitting devices. Here, we developed a dual-color AC-driven light-emitting device by integrating the WSe2 monolayer and AlGaInP–GaInP multiple quantum well (MQW) structures in the form of capacitor structure using AlOx insulating layer between the two emitters. In order to comprehend the characteristics of the hybrid device under various driving circumstances, we investigate the frequency-dependent EL intensity of the hybrid device using an equivalent RC circuit model. The time-resolved electroluminescence (TREL) characteristics of the hybrid device were analyzed in details to elucidate the underlying physical mechanisms governing its performance under varying applied frequencies. This dual-color hybrid light-emitting device enables the use of 2-D TMDC-based light emitters in a wider range of applications, including broad-band LEDs, quantum display systems, and chip-scale optoelectronic integrated systems.Graphical

Stable electroluminescence in ambipolar dopant-free lateral p–n junctions

Dopant-free lateral p–n junctions in the GaAs/AlGaAs material system have attracted interest due to their potential use in quantum optoelectronics (e.g., optical quantum computers or quantum repeaters) and ease of integration with other components, such as single electron pumps and spin qubits. A major obstacle to integration has been the unwanted charge accumulation at the p–n junction gap that suppresses light emission, either due to enhanced non-radiative recombination or due to inhibition of p–n current. Typically, samples must frequently be warmed to room temperature to dissipate this built-up charge and restore light emission in a subsequent cooldown. Here, we introduce a practical gate voltage protocol that clears this parasitic charge accumulation, in situ at low temperature, enabling the indefinite cryogenic operation of devices. This reset protocol enabled the optical characterization of stable, bright, dopant-free lateral p–n junctions with electroluminescence linewidths among the narrowest (<1 meV; <0.5 nm) reported in this type of device. It also enabled the unambiguous identification of the ground state of neutral free excitons (heavy and light holes) as well as charged excitons (trions). The free exciton emission energies for both photoluminescence and electroluminescence are found to be nearly identical (within 0.2 meV or 0.1 nm). The binding and dissociation energies for free and charged excitons are reported. A free exciton lifetime of 237 ps was measured by time-resolved electroluminescence, compared to 419 ps with time-resolved photoluminescence.

Room-temperature electroluminescence and light detection from III-V unipolar microLEDs without p-type doping

The twentieth-century semiconductor revolution began with “man-made crystals,” or p-n junction-based heterostructures. This was the most significant step in the creation of light-emitting diodes (LEDs), lasers, and photodetectors. Nonetheless, advances where resistive p-type doping is completely avoided could pave the way for a new class of n-type optoelectronic emitters and detectors to mitigate the increase of contact resistance and optical losses in submicrometer devices, e.g., nanoLEDs and nanolasers. Here, we show that nanometric layers of AlAs/GaAs/AlAs forming a double-barrier quantum well (DBQW) arranged in an n-type unipolar micropillar LED can provide electroluminescence (EL) (emission at 806 nm from the active DBQW), photoresponse (responsivity of 0.56 A/W at 830 nm), and negative differential conductance (NDC) in a single device. Under the same forward bias, we show that enough holes are created in the DBQW to allow for radiative recombination without the need of p-type semiconductor-doped layers, as well as pronounced photocurrent generation due to the built-in electric field across the DBQW that separates the photogenerated charge carriers. Time-resolved EL reveals decay lifetimes of 4.9 ns, whereas photoresponse fall times of 250 ns are measured in the light-detecting process. The seamless integration of these multi-functions (EL, photoresponse, and NDC) in a single microdevice paves the way for compact, on-chip light-emitting and receiving circuits needed for imaging, sensing, signal processing, data communication, and neuromorphic computing applications.

Sensitivity of the hidden TADF to the linking topology of di-tert-butyl-carbazolyl and benzonitrile moieties in the molecules of emitters or hosts intended for efficient blue OLEDs

With the aim of detecting hidden thermally activated delayed fluorescence (TADF), six organic compounds were synthesized and characterized as emitters and hosts of blue organic light-emitting diodes (OLEDs). The effect of the linking topology of di-tert-butyl-carbazolyl and benzonitrile moieties in donor–acceptor-donor type molecules on device efficiencies was investigated. Just by altering the linking position of di-tert-butyl-carbazolyl moieties at benzonitrile fragment of the emitters and hosts of the blue TADF OLEDs, their maximum external quantum efficiencies (EQEs) can be changed from 1.13 to 5.17 % as well as from 13.8 to the record-high 22.8%. After the comprehensive investigations of the properties including the charge mobility measurements by time-of-flight technique, steady-state and time-resolved photoluminescence and electroluminescence studies, good correlation between EQE of OLEDs and the properties of the isomers with the differently linked donor and acceptor moieties were observed.

Quantitative Determination of Charge Accumulation and Recombination in Operational Quantum Dots Light Emitting Diodes via Time-Resolved Electroluminescence Spectroscopy.

In this work, we report the quantitative determination of charge accumulation and recombination in an operated QLED using time-resolved electroluminescence (TREL) spectroscopy. As a supplement technique, time-resolved current (TRC) measurement was introduced and simulated using equivalent circuit model with a series resistance, a parallel resistance, and a capacitance. By modeling the key processes in a typical TREL spectra, the stages of delay, rising, and decay can be correlated to the charge accumulations, charge injection and recombination, and charge release and recombination, respectively. In particular, the rising stage can be described using a modified Langevin recombination model. The electroluminescence recombination rate can be derived by fitting the rising stage curves in the TREL spectra, providing an intrinsic parameter of the emissive materials. In all, this work provides a methodology to quantitatively determine the charge accumulation and recombination of an operational QLED device.

Molecular glasses based on 1,8-naphthalimide and triphenylamine moieties as bipolar red fluorescent OLED emitters with conventional versus TADF hosting

Three new donor–acceptor molecular glasses were designed and synthesized linking 1,8-naphthalimide and triphenylamino groups though the different bridges. The comprehensive characterization of the compounds was carried out using theoretical and experimental approaches. The compounds showed efficient orange-red emission in solid state with photoluminescence intensity maxima in the range of 584–654 nm. The compounds showed extremely high thermal stability with 5 % weight loss temperatures up to 477 °C. They formed molecular glasses with glass-transition temperatures in the range of 161–186 °C. The fabricated organic light-emitting diodes (OLEDs) based on the developed emitters and conventional host showed maximum external quantum efficiency of 2.5 % in the best case. This value was increased up to 4.7 % by the usage of the host exhibiting thermally activated delayed fluorescence (TADF). OLED containing the TADF host displayed orange emission peaking at 589 nm with colour coordinates x of 0.53 and y of 0.45 combined with power efficiency of 6.7 lm·W−1 and current efficiency of 11.8 cd·A-1. Time-resolved electroluminescence technique was used to study the effect of the different guest–host systems on exciton utilization efficiency in devices based on the same emitter exhibiting prompt fluorescence and on the conventional or TADF hosts.

Benzimidazole-substituted bisanthracene: a highly efficient deep-blue triplet–triplet fusion OLED emitter at low dopant concentration

Two anthracene derivatives, MAnBz and DiAnBz, have been developed as emitters for deep-blue triplet–triplet fusion (TTF) fluorescence organic light emitting diodes (OLEDs). DiAnBz bearing two anthracene units at a closed distance, separated by one phenylene moiety, shows high photoluminescent quantum yields and deep-blue emission. The remarkable maximum external quantum efficiency (EQEmax) of 6.7% and 8.3% are obtained from DiAnBz-OLEDs with pristine prompt fluorescence and TTF fluorescence models, respectively. In particular, effectively diffusive TTF characteristics still can be detected at DiAnBz-OLED with an extreme low emitter concentration of 1 wt%, whose device performance exhibited a high EQEmax of 7.0% with deep blue emission at CIE (0.148, 0.078). The diffusive TTF characteristics with 20 times shorter delayed emission lifetime (τ2 of 3–4 μs), in comparison with that of MAnBz (τ2 of 60–70 μs) in the time-resolved electroluminescence experiments. This implies that pairing and TTF of the DiAnBz triplet excitons are more effective than that of MAnBz in the light emitting layer.

Study of the Luminescence Decay of a Semipolar Green Light-Emitting Diode for Visible Light Communications by Time-Resolved Electroluminescence.

Time-resolved photoluminescence (TRPL) is often used to study the excitonic dynamics of semiconductor optoelectronics such as the carrier recombination lifetime of III-nitride light-emitting diodes (LEDs). However, for any real-world application that requires LEDs under electrical injection, TRPL suffers an intrinsic limitation due to the absence of taking carrier transport effects into account. This becomes a severe issue for III-nitride LEDs used for visible light communication (VLC) since the modulation bandwidth for VLC is determined by the overall carrier lifetime of an LED, not just its carrier recombination lifetime. Time-resolved electroluminescence (TREL), which can characterize the luminescence decay of an LED under electrical injection to simulate real-world conditions when used in practical applications, is required. Both TRPL and TREL have been carried out on a semipolar LED and a standard c-plane LED (i.e., polar LED) both in the green spectral region for a comparison study. The (11-22) green semipolar LED exhibits much faster differential carrier lifetimes than the c-plane LED. In addition to a fast exponential component and a slow exponential component of 0.40 and 1.2 ns, respectively, which are similar to those obtained by TRPL, a third lifetime of 8.3 ns due to transport-related effects has been obtained from TREL, which has been confirmed by capacitance measurements. It has been found that the overall carrier lifetime of a c-plane LED is mainly limited by RC effects due to a junction capacitance, while it is not the case for a semipolar LED due to intrinsically reduced polarization, demonstrating the major advantages of using a semipolar LED for VLC.

Triphenylamino or 9-phenyl carbazolyl-substituted pyrimidine-5-carbonitriles as bipolar emitters and hosts with triplet harvesting abilities

To obtain highly efficient organic semiconductors exhibiting fast emission decays, triplet-harvesting abilities and good bipolar charge-transporting properties for optoelectronic applications, compounds containing triphenylamine or 9-phenylcarbazole as donor moieties and pyrimidine-5-carbonitrile as electron-withdrawing unit were synthesised. Toluene solutions of the compounds demonstrated high photoluminescence quantum yields reaching 98%. As required for electroluminescent device applications, compound containing triphenylamino moiety showed high mobilities of both electrons and holes, which reached 4.4 × 10−4 cm2/V × s and 7.3 × 10−3 cm2/V × s, respectively at electric field of 3.6 × 105 V/cm. This triplet-harvesting mechanism was confirmed by the theoretical and experimental studies including a femtosecond transient absorption pump−probe technique and time-resolved electroluminescence spectroscopy. Pure-blue and greenish-blue fluorescent organic light-emitting diodes (OLEDs) with external quantum efficiency (EQE) reaching 7% and 6%, corresposndingly, were obtained using the newly synthesised compounds as emitters. The operation time (T50) of ca. 650 h were observed for blue OLED and of ca. 3800 h for greenish-blue OLED until reaching the half initial brightness (100 cd/m2). EQE of more than 20% and T50 exceeding 20,000 h were observed for electroluminescent devices based on emitter characterised by triplet−triplet annihilation and thermally activated delayed fluorescence which was utilised to test hosting properties of the differently donor-substituted pyrimidine-5-carbonitriles.

High-Bandwidth Extended-SWIR GeSn Photodetectors on Silicon Achieving Ultrafast Broadband Spectroscopic Response

The availability of high-frequency pulsed emitters in the 2–2.5 μm wavelength range paved the way for a wealth of new applications in ultrafast spectroscopy, free-space and fiber-optical communications, surveillance and recognition, artificial intelligence, and medical imaging. However, developing these emerging technologies and their large-scale use depend on the availability of high-speed, low-noise, and cost-effective photodetectors. With this perspective, here we demonstrate GeSn photodiodes grown on silicon wafers featuring a high broadband operation covering the extended-SWIR range with a peak responsivity of 0.3 A/W at room temperature. These GeSn devices exhibit a high bandwidth reaching 7.5 GHz at 5 V bias with a 2.6 μm cutoff wavelength, and their integration in ultrafast time-resolved spectroscopy applications is demonstrated. In addition to enabling time-resolved electroluminescence at 2.3 μm, the high-speed operation of GeSn detectors was also exploited in the diagnostics of ultrashort pulses of a supercontinuum laser with a temporal resolution in the picosecond range at 2.5 μm. Establishing these capabilities highlights the potential of manufacturable GeSn photodiodes for silicon-integrated high-speed extended-SWIR applications.

Comparative Study of Red/Green/Blue Quantum-Dot Light-Emitting Diodes by Time-Resolved Transient Electroluminescence.

To understand the electronic processes in quantum-dot light-emitting diodes (QLEDs), a comparative study was performed by time-resolved transient electroluminescence (TREL). We fabricated red, green, and blue (R-, G-, and B-) QLEDs with poly(9,9-dioctylfluorene-co-N-(4-sec-butylphenyl)diphenylamine) as the hole-transporting layer with conventional structures. The external quantum efficiency (EQE) and current efficiency were 19.2% and 22.7 cd A-1 for R-QLEDs, 21.1% and 93.3 cd A-1 for G-QLEDs, and 10.6% and 10.4 cd A-1 for B-QLEDs, respectively. The TREL results for B-QLEDs were remarkably different from those for R- and G-QLEDs because of the insufficient electron injection crossing the type II heterojunction between the emission layer and the electron-transporting layer. We further applied poly(N-vinylcarbazole) as the hole-transporting layer and obtained much better performance for B-QLEDs, with EQE and current efficiency of 15.9% and 15.4 cd A-1, respectively. Concomitant with the increase in EQE are an increase in the turn-on voltage from 2.3 to 3.7 V and a transient electroluminescence spike after voltage turn-off.

Self-organized micro-light-emitting diode structure for high-speed solar-blind optical wireless communications

The origin of the fast modulation characteristics of deep ultraviolet (DUV) AlGaN light-emitting diodes (LEDs) grown on AlN/sapphire templates with vicinal off-angles is reported by employing time-resolved electroluminescence (EL) and micro-imaging experiments. The LEDs have recently demonstrated Gbps-class optical wireless communication (OWC) under both room-lighting and direct-sun. The frequency response (f3dB) of the LED reached 184 MHz, which is far beyond expectations by considering the size of the LEDs. Since self-organized micro-LED structures with a low electric capacitance (C) are observed by the EL experiments, the compatibility of high efficiency and fast modulation nature of the AlGaN LEDs is explained. Our approach can overcome the dilemma, where micro-LEDs can be modulated fast but have low power, and therefore, the self-organized micro-LED structure is an ideal solution to realize practical DUV OWCs.

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.

Polymer Light Emitting Diodes with Doublet Emission.

Organic light-emitting radicals have developed rapidly due to their unique doublet emission and great potential in display technology. Although some organic light-emitting diodes (OLEDs) exploiting small-molecular radicals as the emitters have been reported, there is no report about the polymer-radical-based OLEDs until now. Herein, a kind of polymer radical, PS-CzTTM, is adopted as the emitter to fabricate solution-processed OLEDs. A maximum external quantum efficiency of 3.0% is achieved for a deep-red device with an emissive layer of PS-CzTTM lightly doped in 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi). Temperature-dependent time-resolved photoluminescent spectra and transient electroluminescence of radical emitters and devices are first measured. The results demonstrate that the emission channels for both thin films and devices are from the transition of doublet excitons, indicating that the unique doublet emission mechanism of radicals is maintained in PS-CzTTM films and PS-CzTTM-based OLEDs.

Time‐Resolved Electroluminescence Study for the Effect of Charge Traps on the Luminescence Properties of Organic Light‐Emitting Diodes

The electroluminescence characteristics of pristine and degraded organic light‐emitting diode (OLED) devices are studied using the time‐resolved electroluminescence (TREL) technique. It is found that materials degradation results in notable changes in the temporal profile of TREL curve, which involves a shorter onset time, a longer rise time, and a longer decay time of electroluminescence. It is found that these temporal characteristics are affected by trapped charges formed during the OLED operation. More detailed analysis reveals that there are two types of charge traps. The longer decay time is found to be due to the excitons produced by weakly bound charges trapped in the organic layer that are released by thermal energy even without applying the voltage pulse. In contrast, the shorter onset time of electroluminescence is due to the excitons from strongly bound charges that can be released only when a voltage pulse is applied. It is demonstrated that TREL can be used to investigate the underlying mechanisms of OLED degradation. The presence of different types of charge traps found in this study may prove useful for more elaborate design of OLED devices toward enhanced durability against degradation and higher duty cycle of device operation.