Electroluminescence Mechanism Research Articles

Anomalously bright single-molecule upconversion electroluminescence

Efficient upconversion electroluminescence is highly desirable for a broad range of optoelectronic applications, yet to date, it has been reported only for ensemble systems, while the upconversion electroluminescence efficiency remains very low for single-molecule emitters. Here we report on the observation of anomalously bright single-molecule upconversion electroluminescence, with emission efficiencies improved by more than one order of magnitude over previous studies, and even stronger than normal-bias electroluminescence. Intuitively, the improvement is achieved via engineering the energy-level alignments at the molecule−substrate interface so as to activate an efficient spin-triplet mediated upconversion electroluminescence mechanism that only involves pure carrier injection steps. We further validate the intuitive picture with the construction of delicate electroluminescence diagrams for the excitation of single-molecule electroluminescence, allowing to readily identify the prerequisite conditions for producing efficient upconversion electroluminescence. These findings provide deep insights into the microscopic mechanism of single-molecule upconversion electroluminescence and organic electroluminescence in general.

First Observation of Neutral Bremsstrahlung Electroluminescence in Liquid Argon.

A recent discovery of additional mechanism of electroluminescence (EL) in noble gases due to the neutral bremsstrahlung (NBrS) effect led to a prediction that NBrS EL should be present in noble liquids as well. A theoretical model of NBrS EL in noble liquids was developed accordingly in the frameworks of Cohen-Lekner and Atrazhev. In this work, we confirm this prediction: For the first time, visible-range EL has been observed in liquid argon at electric fields reaching 90 kV/cm, using gas electron multiplier (GEM) and thick GEM structures. Absolute light yields of the EL were measured and found to be in excellent agreement with the theory, provided that the momentum-transfer cross section of electron scattering is used for calculation of the NBrS cross section (instead of the energy-transfer one).

Electrically driven light emission from single quantum dots using pulsed MOS capacitors

Robust, nanoscale light-emitting devices are attractive for emerging photonic and quantum engineering applications. However, conventional approaches suffer from fabrication challenges or lack the potential to address individual nanostructures, such as quantum dots. In this paper, we present a device that can produce electrically driven light emission from a single quantum dot using a single carbon nanotube as the charge injection contact. The device has a metal-oxide-semiconductor capacitor structure and operates based on an unconventional mechanism of electroluminescence, which relies on the use of bipolar voltage pulses. The proposed device can be fabricated in a simple manner using conventional lithographic processes, offering a scalable approach toward the development of optoelectronic devices at the single dot level.

Neutral Bremsstrahlung Electroluminescence in Noble Liquids Revisited

Recent discovery of neutral bremsstrahlung (NBrS) mechanism of electroluminescence (EL) in noble gases in two-phase detectors for dark matter searches has led to a prediction that NBrS EL should be present in noble liquids as well. A rigorous theory of NBrS EL in noble liquids was developed accordingly in the framework of Cohen–Leckner and Atrazhev formalism. It has been recently followed by the first experimental observation of NBrS EL in liquid argon, which however deviates significantly from the previous theory. Given these results, we revise previous theoretical calculations of EL NBrS in noble liquids to be consistent with experiment. In particular, NBrS EL yield and spectra were calculated in this work for argon, krypton, and xenon with momentum-transfer cross section for electron scattering (instead of energy-transfer one) being used for calculation of NBrS cross section. The results for light noble liquids, helium and neon, are also reexamined.

Spiro[fluorene-9,9ʹ-xanthene] based deep-blue emitter and the hidden electroplex for solution-processed non-doped white organic light-emitting devices

A deep-blue fluorescent emitter with an A−D−A-skeleton, named pTRASX, constructed from two diphenyltriazine acceptors and one spiro[fluorene-9,9ʹ-xanthene] (SFX) donor, was deliberately designed and synthesized. The blocking effect of O-atom in xanthene moiety of SFX and the rigid A−D−A-skeleton endow a deep-blue emission at 410 nm with a photoluminescence quantum yield of 43% in solid film. The solution-processed non-doped white organic light-emitting diode (OLED) was demonstrated by merging the emission of pTRASX and its electroplex. Based on the intermolecular through-space charge-transfer, the electroplex mechanism resulted in the non-doped white OLED with the wide spectral range from 400 to 800 nm, the color coordinate of (0.33, 0.27), the high color rendering index of 85, and the external quantum efficiency of 0.49%. The electro- and photo-luminance measurements clarify that the electroplex emission originates from the intermolecular interaction between pTRASX and the adjacent electron transporting layer, ranging from ca. 540 to 800 nm. This study illustrates that the rational molecular design cooperated with the innovative electroluminescent mechanism blazes a trail for simple solution-processed and metal-free blue and white OLED.

Luminescence and Degeneration Mechanism of Perovskite Light-Emitting Diodes and Strategies for Improving Device Performance.

Perovskite light-emitting diodes (PeLEDs) can be a promising technology for next-generation display and lighting applications due to their excellent optoelectronic properties. However, a systematical overview of luminescence and degradation mechanism of perovskite materials and PeLEDs is lacking. Therefore, it is crucial to fully understand these mechanisms and further improve device performances. In this work, the fundamental photophysical processes of perovskite materials, electroluminescence mechanism of PeLEDs including carrier kinetics and efficiency roll-off as well as device degradation mechanism are discussed in detail. In addition, the strategies to improve device performances are summarized, including optimization of photoluminescence quantum yield, charge injection and recombination, and light outcoupling efficiency. It is hoped that this work can provide guidance for future development of PeLEDs and ultimately realize industrial applications.

Study on the exciton dynamic processes of exciplex host–guest system by transient magnetic field effects

Utilizing exciplex as the host and fluorescence emitter with dopant materials has been proved successfully to fabricate highly-efficient organic light-emitting diodes. Exciton evolution and energy transfer in this exciplex host–guest system are complex. Gaining insight into the electroluminescence (EL) mechanisms in exciplex-based devices is key for further optimizing device configuration. Here, we have investigated exciton dynamics in devices with exciplex as host and 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as red fluorescence emitter. Two exciplexes, 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPY) doped 2,4,6-tris[3-(triphenylphosphine)phenyl]-1,3,5-triazine (POT2T), and 4,4′,4″-tris[3-methylphenyl(phenyl)amino]-triphenylamine (m-MTDATA) doped tris(8-hydroxyquinoline)aluminum(III) (Alq3), with different band energy are utilized as host materials. Combining the measurements of transient EL, transient photoluminescence and magnetic field effect (MFE), it is concluded that Dexter energy transfer, together with Förster resonance energy transfer, are confirmed in the pure fluorescence doped system. Meanwhile, it is found that DCJTB works with the hot excitons mechanism but not a traditional red fluorescence emitter as recognized previously. This work presents that the transient MFE is powerful for detecting excitonic dynamic processes in excipelx based host–guest EL systems.

Understanding the Electroluminescence Mechanism of CdSe/ZnS Quantum-Dot Light-Emitting Diodes With a Focus on Charge Carrier Behavior in Quantum-Dot Emissive Layers

We analyzed the electroluminescence (EL) mechanisms of quantum-dot light-emitting diodes (QLEDs) with a focus on charge carrier trapping/detrapping. Multilayer quantum-dot (QD) emissive layers (EMLs) exhibit low void density and many QD surface traps, which provide high efficiency in the low bias voltage region; the efficiency gradually decreases in the high bias voltage region. The roles played by traps and charge carrier trapping/detrapping during QLED EL were revealed by transient EL and impedance spectroscopy analyses. Charge carrier trapping/detrapping of QLEDs with a multilayer QD EML in the low bias voltage region promoted charge injection, rapid initiation of EL, and high current efficiency. In contrast to the conventional view, traps improved QLED performance.

Evolution of Frenkel defect pairs in β-Ga2O3 thin film with the introduction of oxygen and its application in GaN-based ultraviolet light-emitting diode

In this work, β-Ga2O3 thin films were deposited by radio frequency (RF) magnetron sputtering technique. The effect of oxygen-argon flow ratio (OAFR) on the formation of Frenkel defect pairs was systematically analyzed. The Frenkel defect pairs would become abundant when the OAFR was 25:25 sccm, resulting in a severe damage in crystalline quality and an increase in the density of disorder. With the further increase of OAFR, Frenkel defect pairs would dramatically decrease or even vanish due to the significant reduction of oxygen vacancies (V0). Immediately after that, on the basis of avoiding the influence of Frenkel defect pairs inside the β-Ga2O3 films to the devices, the p-NiO/i-Ga2O3/n-GaN diode was fabricated from the as-prepared high-quality β-Ga2O3 film after post-deposition annealing as the electron-blocking layer. Subsequently, its electrical characteristics were investigated in detail which exhibited excellent rectification characteristics under all test temperatures. Correspondingly, the device achieved electroluminescence (EL) with a dominant sharp emission peak in the ultraviolet range (∼375 nm). In the end, the energy band diagram was used for in-depth analysis of the EL mechanism.

Recent Advances in Triplet–Triplet Annihilation-Based Materials and Their Applications in Electroluminescence

Efficient and stable blue organic light-emitting devices (OLEDs) represent one of the most important components in future solid-state lighting and full color displays. Although red and green OLEDs have been successfully commercialized, it remains challenging to realize blue OLEDs showing high efficiency and high stability simultaneously, restricting their commercialization. Triplet–triplet annihilation (TTA)-based materials have been emerging as a new category of electroluminescent or host materials with promising potential for highly efficient and stable blue devices due to their appealing photophysical properties (e.g., ability to generate emissive singlet excited states through TTA process, high photoluminescence quantum yield, ease of molecular design, etc.). Recently, research on TTA-based materials toward high-performance blue OLEDs has received an increasing attention. In this review, recent research advances on the molecular design strategies, photophysical properties of TTA-based materials (both blue emitters and host materials), mechanism of TTA-based electroluminescence, as well as their OLED applications are presented comprehensively. Moreover, potential perspectives with future research opportunities in this field are also described. We believe that the emerging TTA-based materials will bring more opportunities for developing a large family of novel photofunctional materials showing appealing and tunable photophysical properties, thus providing new opportunities for high-efficiency and stable blue electroluminescence in the future.

Highly Efficient and Stable Blue OLEDs Based on B—N Bonds Embedded 6,12‐Diphenyl‐5,11‐dihydroindolo[3,2‐b]carbazole with Narrowband Emission and Extended Lifetime†

Comprehensive SummaryEmitters with narrowband spectra are of great importance nowadays because of the growing demand for ultra‐high‐definition displays in the fields of panel displays and solid‐state lighting. Though the reported multiple resonance (MR) emitters have been widely studied with extremely sharp spectra and color‐tunable emissions, the electrophilic borylation synthetic strategies and stable narrowband blue devices still face great challenges. Herein, by virtue of the advantages of both the MR effect of conventional indolocarbazole skeleton and the easy‐to‐access of B—N covalent bonds, the linear tert‐butyl benzene and carbazole groups modified 6,12‐diphenyl‐5,11‐dihydroindolo[3,2‐b]carbazole derivatives (tPh[BN] and Cz[BN]) containing two B—N covalent bonds are easily constructed through amine‐directed double borylation strategy. Both emitters behaved bright blue emission with full‐width‐at‐half‐maximum (FWHM) of 20—27 nm in solution, high fluorescence quantum efficiency over 90%, and excellent stability toward moisture. By employing triplet‐triplet annihilation electroluminescent mechanism, blue OLED devices based on them showed a small FWHM of 48 nm and maximum external quantum efficiency of ~7.1% with negligible efficiency roll‐off, and long operational lifetime (LT80) of 87 h at an initial brightness of 1000 cd·m−2, demonstrating the enormous application potential of B—N bonds embedded emitters in achieving ideal narrowband blue emitters.

Electroluminescence as a Tool to Study the Polarization Characteristics and Generation Mechanism in Silicon PV Panels

Electroluminescence is a defect detection method commonly used in photovoltaic industry. However, the current research mainly focuses on qualitative analysis rather quantitative evaluation, since there exists some shortcomings, such as fuzzy edges, unclear texture, etc., in the obtained electroluminescence images. Electroluminescence polarization imagery is a new method for defect detection in photovoltaic modules, which can effectively make up for the aforementioned deficiencies. In this paper, the polarization characteristics and formation mechanism of silicon solar panels was investigated based on the principle of electroluminescence. Firstly, the polarization imaging mechanism of electroluminescence of photovoltaic modules was studied. Then, an electroluminescence polarization experimental platform was built, and the polarized electroluminescence images of photovoltaic panels were obtained and preprocessed with Gaussian filter. Finally, the influence of view angle, bias voltage and other factors on the polarization characteristics was discussed. The results show that the electroluminescence of photovoltaic modules has polarization characteristics, and the degree and angle of polarization are related to the view angle and bias voltage; the degree of polarization of photovoltaic panel electroluminescence increases with the angle of view, and first increases then rapidly decreases with bias voltage.

Exciton dynamics of an aggregation-induced delayed fluorescence emitter in non-doped OLEDs and its application as host for high-efficiency red phosphorescent OLEDs.

Aggregation-induced delayed fluorescence (AIDF) materials have great potential in non-doped OLEDs due to their high photoluminescence (PL) quantum efficiency in film, high exciton utilization in the aggregated state and negligible efficiency roll-off at high luminance. However, their efficient mechanism in OLEDs is not yet well understood. Here, the exciton dynamics are used to investigate the electroluminescence (EL) mechanism of an AIDF emitter (4-(10H-phenoxazin-10-yl)phenyl)-(9-phenyl-9H-carbazol-3-yl)methanone (CP-BP-PXZ) in detail. It can be seen that the high efficiency and negligible efficiency roll-off in non-doped OLEDs based on CP-BP-PXZ as the emitter are ascribed to the effective reverse intersystem crossing (RISC) from high level triplet T2 to singlet S1 in the aggregated state. Furthermore, CP-BP-PXZ also exhibits excellent properties as a phosphor host due to its good AIDF properties. Thus, high-efficiency red phosphorescent OLEDs with low roll-off efficiency are successfully fabricated based on CP-BP-PXZ as the host. The maximum external quantum efficiency (EQEmax) reaches 23% and is maintained at 21% at a luminance of 1000 cd m-2.

From aggregation-caused quenching to aggregation-induced delayed fluorescence: the impact of the effect of substituents

The electroluminescent mechanism of four imidazole-based luminogens are explored. Results show that introducing different substituents can change the molecules from aggregation-caused quenching to aggregation-induced delayed fluorescence.

Near-Infrared LEDs Based on Quantum Cutting-Activated Electroluminescence of Ytterbium Ions.

Cesium lead halide perovskite nanocrystals (PNCs) exhibit promising prospects for application in optoelectronic devices. However, electroactivated near-infrared (NIR) PNC light-emitting diodes (LEDs) with emission peaks over 800 nm have not been achieved. Herein, we demonstrate the electroactivated NIR PNC LEDs based on Yb3+-doped CsPb(Cl1-xBrx)3 PNCs with extraordinary high NIR photoluminescence quantum yields over 170%. The fabricated NIR LEDs possess an irradiance of 584.7 μW cm-2, an EQE of 1.2%, and a turn-on voltage of 3.1 V. The ultrafast quantum cutting process from the PNC host to Yb3+ has been revealed as the main mechanism of electroluminescence (EL)-activated Yb3+ for the first time via exploring how the trend between the EL intensity of PNC and Yb3+ varies with different voltages along with the tendency of temperature- and doping-concentration-dependent PL and EL spectra. This work will extend the application of PNCs to optical communication, night-vision devices, and biomedical imaging.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Annihilation luminescent Eu-MOF as a near-infrared electrochemiluminescence probe for trace detection of trenbolone

A Eu-MOF (Eu2[Ru(dcbpy)3]3) with novel mechanism of annihilation electroluminescence (ECL) was designed via synthesizing a MOF coordinated with photoactive metalloligand and lanthanide metal ion. Firstly, the redox of [Ru(H2dcbpy)3]·Cl2 ligand produced Ru(dcbpy)33+ and Ru(dcbpy)32+, then Eu2[Ru(dcbpy)3]3•+ was produced under the catalysis of Ru(dcbpy)33+ and Ru(dcbpy)32+. Secondly, Eu2[Ru(dcbpy)3]3 gained electrons to produce Eu2[Ru(dcbpy)3]3•-, thus realizing annihilation luminescence without any extra coreactant. Thirdly, the antenna effect of [Ru(H2dcbpy)3]·Cl2 ligand on Eu3+ greatly improved the luminescence efficiency of Eu2[Ru(dcbpy)3]3 by strong energy transfer. In addition, the introduction of Eu3+ endowed the ECL sensor characteristics of second near-infrared (NIR-II) luminescence. These allowed the ECL sensor to work normally in harsh conditions, accomplished non-destructive testing of samples, and made the ECL sensor with high and stable signals. Hence, the Eu2[Ru(dcbpy)3]3 can be used as a NIR probe to achieve efficient and sensitive detection of trenbolone with low limit of detection (LOD) of 4.83 fg/mL. Therefore, it is a considerable design strategy to realize ultrasensitive detection of environmental pollutants.

Neutral bremsstrahlung electroluminescence in noble liquids

Proportional electroluminescence (EL) is the physical effect used in two-phase dark matter detectors, to optically record in the gas phase the ionization signal produced by particle scattering in the liquid phase. In our previous works the presence of a new EL mechanism in noble gases, namely that of neutral bremsstrahlung (NBrS), was demonstrated both theoretically and experimentally, in addition to the ordinary EL mechanism due to excimer emission. In this work we show that the similar theoretical approach can apply to noble liquids, namely to liquid helium, neon, argon, krypton and xenon. In particular, the photon yields and spectra for NBrS EL in noble liquids have been calculated for the first time, using the electron energy and transport parameters obtained in the framework of Cohen-Lekner and Atrazhev theory. The relevance of the results obtained to the development of noble liquid detectors for dark matter searches and neutrino experiments is discussed.

Molecular design of DBA-type five-membered heterocyclic rings to achieve 200% exciton utilization for electroluminescence.

Achieving high exciton utilization is a long-cherished goal in the development of organic light-emitting diode materials. Herein, a three-step mechanism is proposed to achieve 200% exciton utilization: (i) hot triplet exciton (T2) conversion to singlet S1; (ii) singlet fission from S1 into two T1; (iii) and then a Dexter energy transfer to phosphors. The requirement is that S1 should lie slightly lower than or close to T2 and twice as high as T1 in energy. For this, a scenario is put forward to design a series of donor-bridge-acceptor (DBA) type molecules with 2E(T1) ≤ E(S1) < E(T2), in which the Baird-type aromatic pyrazoline ring is used as a bridge owing to its stabilized T1 (1.30-1.74 eV) and different kinds of donors and acceptors are linked to the bridge for regulating S1 (2.35-3.87 eV) and T2 (2.44-3.96 eV). The ultrafast spectroscopy and sensitization measurements for one compound (TPA-DBPrz) fully confirm the theoretical predictions.

Online Junction Temperature and Current Simultaneous Extraction for SiC MOSFETs With Electroluminescence Effect

In this letter, a junction-temperature and current extraction method is presented based on the electroluminescence mechanism of the SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> body diode. Starting from the observation of two characteristic peaks in the emitted light spectrum, we proved that the junction temperature and the drain current can be simultaneously measured. This novel method consists of decoupling the relationship between the intensity of the electroluminescence peaks, the current, and the temperature. Through this optical method with inherent electrical isolation, the junction temperature and current in the SiC chip can be simultaneously measured with high precision. The total error of the junction temperature estimation is within ±3 °C, and the error of the current estimation is about ±0.2 A.