Electroluminescence Measurements Research Articles

InGaN/GaN Hybrid‐Nanostructure Light Emitting Diodes with Emission Wavelength Green and Beyond

Three sets of InGaN/GaN nanowire (NW) heterostructures are grown on Si(111) substrates under different growth conditions. A quasi‐two‐dimensional p‐GaN layer is grown on top of those structures using the epitaxial lateral overgrowth (ELOG) technique. Finally, the light‐emitting diodes (LED) are fabricated using these hybrid nanostructures following standard fabrication techniques. Electroluminescence (EL) measurement confirmed the emission wavelengths of 530.0 nm (green), 608.3 nm (orange), and 632.5 nm (red). The knee voltages of the devices are estimated to be in the range of 2.18–2.89 V, with higher knee voltages for samples emitting lower wavelengths. Further analysis of forward bias electrical characteristics suggests the dominance of tunneling current and an increase in the defect density in the heterostructures emitting higher wavelengths.

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.

Symmetry Molecular Design Strategy for Highly Efficient Blue Electroluminescence with Hot Exciton Mechanisms

AbstractEmitters with a hot exciton mechanism are regarded as one of the most promising candidates for organic light‐emitting diodes (OLEDs). In this study, a deep‐blue emitter with the hot exciton mechanism is reported, namely 2An‐PCz, by integrating a pair of carbazole groups with a 9,9′‐bi‐anthracene nucleus. Owing to the symmetric molecular architecture and intrinsic local excited state character, multiple high‐lying reverse intersystem cross (hRISC) channels and large overlaps of frontier molecular orbits (FMOs) can be formed, facilitating rapid hRISC processes as well as enhancement of radiative transition rates simultaneously. Combined with the strong luminescence properties brought by the unique X‐packing mode, a high photoluminescence quantum yield of 60.5% is achieved in the non‐doped state. Strikingly, non‐doped deep‐blue OLEDs exhibited a maximum external quantum efficiency (EQE) of 10.50% with minimal efficient roll‐off, which is one of the highest values for deep‐blue organic light‐emitting devices based on hot exciton emitters thus far. The magneto‐electroluminescence (MEL) experiment and transient electroluminescence measurements corroborated that both the high EQE and suppressed efficiency roll‐off are attributable to the rapid “hot exciton” channels.

Multi-Terminal GaInP/GaInAs/Ge Solar Cells for Subcells Characterization

Improvement of triple-junction (3J) III-V/Ge solar cells efficiency is hindered by the low current produced by the top and middle cells relative to the bottom cell (Ge). This can be explained by the difficulty of characterizing, on an individual basis, the subcells. We investigate the fabrication process of multi-terminal multi-junction solar cells (MTMJSC) and its potential as a promising architecture to independently characterize subcells of multi-junction solar cells. Here, we study monolithic triple-junction solar cells, with an InGaP top cell, an InGaAs middle cell and a Ge bottom cell interconnected by tunnel junctions. We demonstrate a fabrication process for MTMJSC on commercial wafers for characterization applications purposes. I-V measurements, under illumination, of two-terminals and MTMJSC were compared to validate that the MTMJSC fabrication process does not degrade the cells’ performance. The dark current of each subcell was also measured and an ideal-diode model used to determine the subcells electrical parameters. The results suggest a method to measure the relative absorption and the opto-electrical couplings between the subcells unambiguously, through EQE and electroluminescence measurements, based on basic micro-fabrication processes.

226 nm Far‐Ultraviolet‐C Light Emitting Diodes with an Emission Power over 2 mW

Far‐ultraviolet‐C (far‐UVC) light emitting diodes (LED) emitting at an emission wavelength of 226 nm with different n‐AlGaN contact layers, quantum well barriers, and quantum well numbers are compared regarding their emission power, operation voltage, and lifetime. Electroluminescence measurements show higher emission power but also an increased operation voltage with increasing Al mole fraction in the n‐AlGaN contact layer. Furthermore, it is found that both the mean emission power and the device lifetime decrease with increasing Al mole fraction (82–89%) of the quantum well barriers and therefore with increasing barrier height. Finally, 226 nm LEDs with 6 and 9 quantum wells are compared. It is observed that the sample with 9 quantum wells shows an around 30% lower mean emission power but, in contrast, the L70 lifetime of these LEDs is higher by a factor of around five. Based on these optimizations, 226 nm LEDs with a maximum external quantum efficiency of 0.28% (wall plug efficiency of 0.18%) as well as an emission power of 2.1 mW and an operation voltage of 9.6 V at 200 mA are realized.

Temperature-dependent electroluminescence of red high-In-content MQWs of dual-wavelength micro-LED

Temperature-dependent electroluminescence (TDEL) measurements have been employed to investigate the carrier transport and recombination processes of InGaN red micro-LED based on dual-wavelength InGaN/GaN MQWs structure. EL peak energy and carrier transport of the red micro-LED both show temperature dependence, due to temperature-induced changes in defect activation. In addition, the current density at which the blue peak of the low-In-content appears in the EL spectrum varies with temperature. As the temperature increases, the blue peak of the low In component tends to appear at higher current densities, which may be attributed to the increase in thermally activated defects hindering the injection of holes into the low-In-content MQWs further away from p-GaN. Furthermore, the IQEs of the high-In-content MQWs are estimated from the TDEL method and then reveal the temperature-dependent efficiency droop. The IQE decreases as temperature increases, particularly above 50 K, where it drops sharply due to temperature-dependent nonradiative recombination. And the two different variation trends in IQE of MQWs with high and low In content reveal a competitive mechanism in carrier distribution, implying that more escaping holes from high-In-content MQWs will further reduce red emission efficiency but enhance carrier injection and blue emission in low-In-content MQWs.

From Performance Measurements to Molecular Level Characterization: Exploring the Differences between Ultraviolet and Damp Heat Weathering of Photovoltaics Modules

The degradation behaviors of the encapsulant and the imbedded additives significantly determine the reliability of solar modules. Nevertheless, a link between the degradation of the encapsulant, including the additive interactions, and the longevity of the overall module is rarely established until now. Herein, mini‐modules containing ethylene‐vinyl acetate copolymer (EVA) as encapsulant are subject to damp heat (DH) or ultraviolet (UV) weathering based on IEC 61215. Macroscopically, the degradation under both weathering types characterized by I–V measurements and electroluminescence (EL) measurements is diverging in dependence on the used stressor. Using electron paramagnetic resonance and orbitrap mass spectrometry, it is shown that deacetylation of the EVA occurs significantly for both types of weathering. In the case of DH, however, the mechanism of action of the UV stabilizer is hindered, so that strong encapsulant degradation is observed despite a lower energy input in comparison with UV. Furthermore, the produced acetic acid under DH weathering leads to the observed reduction in EL, an increase in series resistance, and, a reduction of the performance of the modules. The work carried out shows that the degradation of the solar modules is strongly dependent on the behavior of the UV stabilizer.

High Efficiency Triplet‐Triplet Fusion Blue Fluorescence OLEDs by introducing a “Hot Exciton” Efficiency Enhancement Layer

AbstractTriplet‐triplet fusion (TTF) can effectively transfer triplet excitons to singlet energy level and exhibit good stability, making it widely used in blue fluorescence organic light emitting diodes (OLEDs) for practical application. However, the efficiency of the resulting blue fluorescence OLEDs is still fundamentally limited. Herein, this work introduces a hot exciton efficiency enhancement layer (EEL) between TTF emissive layer and electron‐transporting layer for the first time. It can be found that the triplet excitons are greatly utilized by the efficient hot exciton EEL. As a result, a high efficiency blue fluorescence OLED is successfully developed, exhibiting a maximum external quantum efficiency of 15.2% at the luminance of 4068.0 cd m−2, achieving a breakthrough in the efficiency of blue TTF‐OLEDs. Transient electroluminescence (TrEL) measurements and detailed theoretical calculations confirm the key role of the effective hot exciton process from the high‐lying triplet energy level (Tn, n ≥ 2) to S1 via a reverse intersystem crossing (hRISC). Furthermore, the device lifetime is also greatly improved owing to the fast and efficient hRISC in the introduced hot exciton EEL. This work has important guiding significance for constructing high efficiency blue fluorescence OLEDs by introducing hot exciton molecules.

Efficiency improvement by using metal–insulator-semiconductor structure in InGaN/GaN micro-light-emitting diodes

InGaN/GaN micro-light-emitting diodes (micro-LEDs) with a metal–insulator-semiconductor (MIS) structure on the sidewall are proposed to improve efficiency. In this MIS structure, a sidewall electrode is deposited on the insulating layer-coated sidewall of the device mesa between a cathode on the bottom and an anode on the top. Electroluminescence (EL) measurements of fabricated devices with a mesa diameter of 10 μm show that the application of negative biases on the sidewall electrode can increase the device external quantum efficiency (EQE). In contrast, the application of positive biases can decrease the EQE. The band structure analysis reveals that the EQE is impacted because the application of sidewall electric fields manipulates the local surface electron density along the mesa sidewall and thus controls surface Shockley–Read–Hall (SRH) recombination. Two suggested strategies, reducing insulator layer thickness and exploring alternative materials, can be implemented to further improve the EQE of MIS micro-LEDs in future fabrication.Graphical

Prevention of potential‐induced degradation using a moisture barrier in crystalline silicon photovoltaic modules

AbstractAs photovoltaic (PV) modules are exposed to high temperatures and humidity over time, they generate leakage current, which leads to potential‐induced degradation (PID) and lower power output. In silicon, Cu(In,Ga)(Se,S)2 (CIGS) thin film and perovskite solar cells, PID has been shown to be driven by the presence of Na in the module glass. PID stability is crucial for the commercialization of such solar modules. This study aims to confirm the leaching phenomenon of Na in soda–lime module glass and study the use of polytetrafluoroethylene (PTFE) as a moisture barrier to prevent PID. By water immersion and exposure to different temperature and humidity conditions, we exhibited Na leaching in soda–lime glass. Moreover, we demonstrate the use of an anti‐PID moisture barrier made of PTFE, which was deposited using kinetic spraying between the cover glass and encapsulant in the solar module. The thickness of the moisture barrier was controlled by adjusting the deposition rate, and the PID characteristics were evaluated by manufacturing solar modules for different barrier thicknesses. Light current–voltage (LIV), dark current–voltage (DIV), and electroluminescence (EL) measurements confirmed that the PTFE moisture barrier effectively inhibits the degradation of solar cells. This study provides further insights into the Na leaching phenomenon and PID mechanism in PV modules and contributes to the design and development of more stable solar cells.

Study of pristine and degraded blue quantum dot light-emitting diodes by transient electroluminescence measurements

Limited stability of blue quantum dot light-emitting diodes (QLEDs) under current stress impedes commercialization. Multi-layer structures of the state-of-the-art blue QLEDs pose significant difficulty in the fundamental understanding of degradation mechanisms. Here, by applying transient electroluminescence measurements, we disentangle charge transport in both pristine and degraded blue QLEDs. By varying thicknesses of the charge transport layers and the emissive layer, respectively, we show that the charge transport in pristine QLEDs is primarily dominated by holes. Furthermore, the degradation of QLEDs under electrical stress is governed by the decrease of hole transport in the emissive quantum dot layer due to the formation of hole traps.

Size‐Dependent Characteristics of InGaN‐Based Blue and Green Micro‐Light‐Emitting Diodes

This study focuses on investigating the size‐dependent characteristics (chip diameter: 100–10 μm) of InGaN‐based blue and green micro‐light‐emitting diodes (μLEDs) with respect to their electrical and optical properties. The LED's epistructure is grown using the metal–organic chemical vapor deposition technique, preceded by simulation and optimization using SiLENSe LED simulator. The cross‐sectional transmission electron microscopy study confirms the epistructure. The Photoluminescence mapping of the epistructure reveals that the blue emission is at 461 nm (full width at half maximum [FWHM] ≈ 15.5 nm) and the green emission is at 510 nm (FWHM ≈ 17.5 nm). The study analyzes the size‐dependent external quantum efficiency (EQE), optical power, and peak wavelength shift between blue and green μLEDs. The electroluminescence (EL) study reveals that the redshift of the EL peak wavelength of μLEDs is attributed to the release of strain at reduced chip sizes. The size‐dependent EQE study shows a reduction of EQE with shrunken chip size, attributed to the increase of nonradiative Shockley–Read–Hall recombination. Finally, the study estimates the characteristics temperature of the μLEDs using temperature‐dependent EL measurement, revealing that blue μLEDs exhibit a significantly large value of characteristic temperature ≈1926 K.

Anthracene derivatives with strong spin-orbit coupling and efficient high-lying reverse intersystem crossing beyond the El-Sayed rule.

The attention to materials with hot exciton channel and triplet-triplet fusion (TTF) mediated high-lying reverse intersystem crossing (hRISC) has been raised for their ability to convert non-emissive 'dark' triplets into radiative singlet excitons. This spin conversion process results in high exciton utilization efficiency (EUE) that exceeds the theoretical limits. Notably, it is known that such spin conversion processes from the high-lying excited triplet to the singlet state are facilitated by the orthogonal orbital transition effect governed by the El-Sayed's rule. In this study, an anthracene derivative with indenoquinoline substituent 7,7-dimethyl-9-(10-(4-(naphthalen-1-yl)phenyl)anthracen-9-yl)-7H-indeno[1,2-f]quinoline (2MIQ-NPA) was synthesized and analyzed to investigate whether the hRISC process occurs in these molecules, even when the El-Sayed's rule is not followed. The hRISC channels of the emitter were fully unraveled through DFT calculations and experiments, which were quantitatively subdivided using transient electroluminescence measurements. The results showed that 2MIQ-NPA, which does not follow the El-Sayed's rule and has a relatively strong spin-orbit coupling matrix element of 0.116 cm-1 between the high-lying triplet state of T4 and the lowest singlet state of S1, effectively converted triplet excitons into singlet excitons with an EUE of 64.3%, contributed by a direct hot exciton channel of 19.2% and a TTF-mediated hot exciton channel of 15.1%. Despite the low outcoupling efficiency, the non-doped device with 2MIQ-NPA achieved an excellent device performance with an external quantum efficiency of 7.0%.

Strain control and its effect on the optical properties of InGaN/GaN multiple quantum wells

The strain modulation mechanism of green InGaN quantum wells (QWs) was investigated by designing and growing additional InGaN/GaN strain modulation layers with different Indium (In) content and structures. Electroluminescence (EL) measurements were employed to analyze the strain in InGaN QWs by checking the change of EL peak energy with injection current. The experimental results suggest that proper In content in the InGaN strain modulation layer is required to reduce the compression strain in multiple quantum wells (MQWs). Meanwhile, the material quality and flatness of the pre-grown modulation layer may significantly affect the homogeneity and the strain state of MQWs. Temperature-dependent photoluminescence (TDPL) and micro-luminescence were measured to characterize the optical properties of MQWs prepared with strain modulation layers. By carefully modulating the strain in QWs, the InGaN/GaN green multiple quantum wells (MQWs) exhibit higher In incorporation and uniform luminescence.

Experimental evidence of hole injection through V-defects in long wavelength GaN-based LEDs

Hole injection through V-defect sidewalls into all quantum wells (QWs) of long wavelength GaN light emitting diodes had previously been proposed as means to increase efficiency of these devices. In this work, we directly tested the viability of this injection mechanism by electroluminescence and time-resolved photoluminescence measurements on a device in which QW furthest away from the p-side of the structure was deeper, thus serving as an optical detector for presence of injected electron–hole pairs. Emission from the detector well confirmed that, indeed, the holes were injected into this QW, which could only take place through the 101¯1 V-defect sidewalls. Unlike direct interwell transport by thermionic emission, this transport mechanism allows populating all QWs of a multiple QW structure despite the high potential barriers in the long wavelength InGaN/GaN QWs.

PVK-based LEDs doped by cadmium sulphide nanocrystals coated by mixed thiophenol and 1-decanethiol

Nanoparticles capped with more than one ligand have great potential to improve the properties of optoelectronic devices. To discuss this possibility, we synthesized cadmium sulphide nanocrystals coated by two different ligands, 1-decanethiol and thiophenol. In addition, as a reference, cadmium sulphide nanocrystals coated with thiophenol were synthesized. Photoluminescence and UV–Vis absorption measurements indicated that the maximum emission peaks of both cadmium sulphide nanoparticles were located around 600 nm and the absorption edges showed the existence of two different types of crystalline structure, cubic zinc blende and wurzite-type hexagonal. But the nanoparticles with both ligands had higher emission intensity than nanocrystals coated only by thiophenol. Finally, hybrid light-emitting devices have been manufactured with the following structure: Indium Tin Oxide/Poly(3,4-ethylenedi-oxythiophene): poly(styrene sulfonate)/poly(9-vinylcarbazole): cadmium sulphide/Aluminum. These devices presented an active layer composed of poly(9-vinylcarbazole) doped by cadmium sulphide nanocrystals coated only by thiophenol in one case and with thiophenol and 1-decanethiol in the other case. The inclusion of nanocrystals in the active layer increased the current density of these devices. In addition, the solubility in chlorobenzene of nanoparticles with both ligands was improved, due to the presence of 1-decanethiol on their surface. Moreover, the presence of the second ligand intensified the influence of nanoparticles in electroluminescence measurements compared to cadmium sulfide nanocrystals without 1-decanethiol.

Piezo-phototronic enhanced dual-wavelength InGaN/GaN multiple quantum wells Micro-LED arrays

As a promising technology for next-generation displays due to their high brightness and low power consumption, InGaN-based micro light-emitting diodes (Micro-LEDs) have attracted great attention in recent years. In this work, we detailedly investigate the carrier recombination and transportation process in dual-wavelength InGaN/GaN multiple quantum wells (MQWs) Micro-LED arrays under piezo-phototronic effects using electroluminescence (EL) and time-resolution photoluminescence measurements. With the increase of external strain, the violet EL intensity of dual-wavelength Micro-LED arrays first increases obtaining a maximum enhancement of ∼12% and then decreases, whereas blue EL emission almost maintains constant. Additionally, as the size of Micro-LED decreases, the enhancement obtained via piezo-phototronic effect will reduce, which is attributed to their inherently weaker piezoelectric polarization effect. Combining with dynamic analysis of carriers in the blue quantum well (QW), it is concluded that strain-induced interface polarized charges promote the wave function overlap of electron–hole pair, but reduce the injection of hole carriers in blue QW. Superposition of the above two factors enables the blue EL intensity stable under piezoelectric coupling. These results present a promising potential of piezo-phototronic effects to improve the Micro-LEDs devices.

Non‐Doped Blue AIEgen‐Based OLED with EQE Approaching 10.3 %

AbstractAggregation‐induced emission (AIE) luminogens (AIEgens) are attractive for the construction of non‐doped blue organic light‐emitting diodes (OLEDs) owning to their high emission efficiency in the film state. However, the large internal inversion rate (kIC(Tn)) between high‐lying triplet levels (Tn) and Tn‐1 causes a huge loss of triplet excitons, resulting in dissatisfied device performance of these AIEgens‐based non‐doped OLEDs. Herein, we designed and synthesized a blue luminogen of DPDPB‐AC by fusing an AIEgen of TPB‐AC and a DMPPP, which feature hot exciton and triplet‐triplet annihilation (TTA) up‐conversion process, respectively. DPDPB‐AC successfully inherits the AIE feature and excellent horizontal dipole orientation of TPB‐AC. Furthermore, it owes smaller kIC(Tn) than TPB‐AC. When DPDPB‐AC was applied in OLED as non‐doped emitting layer, an outstanding external quantum efficiency of 10.3 % and an exceptional brightness of 69311 cd m−2 were achieved. The transient electroluminescent measurements and steady‐state dynamic analysis confirm that both TTA and hot exciton processes contribute to such excellent device performance. This work provides a new insight into the design of efficient organic fluorophores by managing high‐lying triplet excitons.

Non-Doped Blue AIEgen-Based OLED with EQE Approaching 10.3 .

Aggregation-induced emission (AIE) luminogens (AIEgens) are attractive for the construction of non-doped blue organic light-emitting diodes (OLEDs) owning to their high emission efficiency in the film state. However, the large internal inversion rate (kIC (Tn) ) between high-lying triplet levels (Tn ) and Tn-1 causes a huge loss of triplet excitons, resulting in dissatisfied device performance of these AIEgens-based non-doped OLEDs. Herein, we designed and synthesized a blue luminogen of DPDPB-AC by fusing an AIEgen of TPB-AC and a DMPPP, which feature hot exciton and triplet-triplet annihilation (TTA) up-conversion process, respectively. DPDPB-AC successfully inherits the AIE feature and excellent horizontal dipole orientation of TPB-AC. Furthermore, it owes smaller kIC (Tn) than TPB-AC. When DPDPB-AC was applied in OLED as non-doped emitting layer, an outstanding external quantum efficiency of 10.3 % and an exceptional brightness of 69311 cd m-2 were achieved. The transient electroluminescent measurements and steady-state dynamic analysis confirm that both TTA and hot exciton processes contribute to such excellent device performance. This work provides a new insight into the design of efficient organic fluorophores by managing high-lying triplet excitons.

Fabrication and characterization of an AlGaN light emitting diode with Al-doped ZnO as a current spreading tunnel junction layer

We fabricated an AlGaN light emitting diode (LED) with a heavily Al-doped n-type ZnO layer on a p-type contact layer as a tunnel junction (TJ) to improve carrier injection into the LED. We characterized its electrical and optical properties and compared them to those of an AlGaN LED without ZnO. From the I–V characteristic of the LED with ZnO, we observed a threshold voltage of circa 2 V, which could be due to Zener breakdown of the type II heterostructure of n-ZnO/p-GaN as a TJ. From the electroluminescence measurement, we observed a similar emission peak in both AlGaN LEDs at ultraviolet (UV) wavelengths, but a broad emission band around 365 nm in the LED with ZnO. This emission could be originating from ZnO photoexcited by the UV LED emission. The dependence of these peak intensities on input currents shows that there is a monotonic increase in the light emission intensity for the UV LED emission, but a saturation behavior after the threshold voltage for the emission from the ZnO. This saturation behavior is attributed to an overflow of photoexcited electron–hole pairs into p-GaN, strongly suggesting that n-ZnO/p-GaN works as a TJ. Electroluminescence data also show that the presence of the ZnO film facilitates current spreading, which enables device operation at large input currents. Therefore, ZnO can work as a current spreading TJ layer and improve the performance of the AlGaN LED.