Temperature-dependent Electroluminescence Research Articles

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.

Abnormal temperature-dependent transient electroluminescence spikes induced by the leakage of hole carriers in organic light-emitting diodes

The amplitude of the emission spike at the transient electroluminescence (TEL) falling edge is an important benchmark for evaluating the quantities of trapped charges existed in organic light-emitting diodes and often shows a normal temperature dependence which increases with the decreasing temperature. Surprisingly, an unreported abnormal temperature-dependent TEL spike was observed in this work. A series of experimental results relevant to the electroluminescence spectrum and TEL measurements demonstrate that this abnormal temperature-dependent behavior is induced by the leakage of hole carriers from the emission layer (EML) to an electron transport layer (ETL). After the voltage pulse is turned off, these holes already leaked into the ETL drift back toward the EML, subsequently engaging in radiative recombination with trapped electrons on guest molecules to generate a spike at the TEL falling edge. However, the drift process is hindered by the reduced carrier mobility of the ETL material with the decrease in temperature. As a result, the spike intensity weakens as the temperature decreases, which contradicts the conclusions reported in previous literatures. Therefore, this study not only leads to the reconsideration for the judgment criteria of the number of trapped charges but also provides valuable insight into the TEL research field of organic optoelectronic devices.

Investigating the Thermo-Optic Properties of BCZT-Based Temperature Sensors.

Photoluminescent (PL) layers and electroluminescent (EL) systems have gained significant attention for their applications in constructing flat panels, screen monitors, and lighting systems. In this study, we present a groundbreaking approach to fabricating temperature sensors using barium-calcium zirconium titanate (BCZT) with thermo-optic properties, leading to the development of opto-thermal sensors for electric vehicle battery packs. We prepared zinc sulfide (ZnS) fluorescent films on BCZT ceramics, specifically two optimal compositions, BCZT0.85 (Ba0.85Ca0.15Zr0.1Ti0.9O3) and BCZT0.9 (Ba0.9Ca0.1Zr0.1Ti0.9O3), via the solid-state reaction method for the dielectric layer. The BCZT powders were calcined at varying temperatures (1200 and 1250 °C) and dwell times (2 and 4 h). The resulting phase formation and microstructure characteristics were analyzed using X-ray diffraction and scanning electron microscopy, respectively. Our investigation aimed to establish a correlation between the dielectric behavior and optical properties to determine the optimal composition and conditions for utilizing BCZT as thermal detectors in electric vehicle battery packs. All BCZT powders exhibited a tetragonal phase, as confirmed by JCPDS No. 01-079-2265. We observed an increase in the dielectric constant with higher calcining temperatures or longer dwell times. Remarkably, BCZT0.85 ceramic sintered at 1250 °C for 4 h displayed the highest dielectric constant of 15,342, establishing this condition as optimal for preparing the dielectric film with a maximum dielectric constant of 42. Furthermore, we investigated the temperature-dependent electroluminescence intensity of the samples, revealing a significant enhancement with increasing temperature, reaching its peak at 80 °C. Additionally, we observed a positive correlation between electroluminescence intensity and dielectric constant, indicating the potential for improved opto-thermal sensors. The findings from this study offer promising opportunities for the development of advanced opto-thermal sensors with potential applications in electric vehicle battery packs. Our work contributes to the expanding field of photoluminescent and electroluminescent systems by providing novel insights into the design and optimization of efficient and reliable sensors for thermal monitoring in electric vehicle technologies.

Charge-Transfer Dynamics in OLEDs with Coexisting Electroplex and Exciton States

The reverse intersystem-crossing (RISC) channels from triplet to singlet charge-transfer states (such as exciplex or electroplex states) frequently occur in donor:acceptor heterojunction organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency. Although exciplexes have been extensively investigated, there are few reports on the physical microscopic processes for electroplex-based devices. Herein, three kinds of donor:acceptor heterojunction OLEDs are fabricated: two coexisting electroplex and exciton devices and one pure exciplex-based device. Amazingly, via the fingerprint magnetoelectroluminescence measurement at room temperature, we observe an abnormal current-dependent RISC, which is enhanced with increasing bias current (I) in the coexisting electroplex and exciton but electroplex-dominated device; the conversion from ISC to RISC in the coexisting electroplex and exciton but exciton-dominated device; and only the normal I-dependent ISC, which weakens with increasing I in the pure exciplex-based device. This is because low-energy-triplet electroplexes are well confined by the high triplet energies of electron donors and acceptors, promoting the occurrence of the RISC process in electroplexes, and Dexter energy transfer from triplet excitons to triplet electroplexes enhances the RISC process with increasing I, causing the abnormal I-dependent RISC and the conversion from ISC to RISC. Moreover, as the operational temperature rises from 10 to 300 K, these two coexisting electroplex and exciton devices present the conversion from ISC to RISC at different temperatures, but the pure exciplex-based device shows only the normal temperature-dependent ISC, which is enhanced with increasing temperature. This is because the RISC channel is an endothermic process and temperature-dependent electroluminescence measurements show that the electroplex emission weakens as the temperature is reduced, causing the weakened RISC process and even the presence of the ISC process at low temperature. These experimental results demonstrate that the electroplex emission plays an important role in the occurrence of strong RISC processes. Our work deepens our microscopic understanding of the charge-transfer states in donor:acceptor heterojunction devices but also paves the way for utilizing electroplex states to design highly efficient OLEDs.

Temperature-dependent electroluminescence of stressed and unstressed InAlGaN multi-quantum well UVB LEDs

The electroluminescence of UVB light-emitting diodes emitting at 310 nm before and after 1000 h of operation is studied in the temperature range from 20 to 340 K. Before operation, the external quantum efficiency (EQE) at 10 mA gradually increases with decreasing temperature from 0.8% at 340 K to 1.8% at 150 K and then levels off. This trend is attributed to a reduction of non-radiative recombination and finally the domination of radiative recombination at low temperatures. After 1000 h of operation, the EQE has dropped to 0.45% at 340 K with a maximum EQE of 1.4% at 80 K, followed by a drop for temperatures below 80 K. These findings suggest a stress-induced reduction of both the radiative recombination efficiency and the carrier injection efficiency.

Injection current and temperature dependence of the optoelectronic properties for electrical-stressed InGaN based micro-LEDs

InGaN-based micro-LEDs are promising for many applications, including visible light communication (VLC), micro-display, etc. However, to realize the above full potential, it is important to understand the degradation behaviors and physical mechanisms of micro-LEDs during operation. Here, the optoelectronic properties of InGaN-based blue micro-LEDs were investigated over a wide range of injection currents (1-100 mA) and temperatures (5-300 K) before and after stress. The results show that the optical power of the micro-LED degrades after stress, especially at lower current density, indicating that the Shockley-Read-Hall (SRH) nonradiative recombination increased for the stressed device. In addition, the slopes of log L-log I curves changes from 1.0 to 2.1 at low current density, and the ideality factor extracted from the I-V curves change from 1.9 to 3.4 after current stress, indicating there is an increase of the defects in the active layer after stress. The activation energy of defects evaluated from the temperature-dependent electroluminescence (EL) spectra is about 200 meV, which could be related to the N-vacancy related defects. Besides, the peak wavelength, peak energy and the full width at half maximum of the injection current- and temperature-dependent EL spectra were discussed. The electron-hole pair combines in the form of SRH nonradiative recombination, causing some carriers to redistribute and a state-filling effect in higher-energy states in multiple quantum wells (MQWs), resulting in the appearance of the shorter-wavelength luminescence in the EL spectra. These findings can help to further understand the degradation mechanisms of InGaN micro-LEDs operated under high current density.

The impacts of sidewall passivation via atomic layer deposition on GaN-based flip-chip blue mini-LEDs

In order to investigate the photoelectric characteristics of 80 × 120 µm2 mini-light-emitting-diodes (mini-LEDs) with sidewall passivation by atomic layer deposition (ALD), this paper uses the techniques of spectrometer-based spectroradiometer and microscopic hyperspectral imaging (µ-HSI). The temperature-dependent electroluminescence is measured using a spectrometer-based spectroradiometer. By analyzing the rising parts of external quantum efficiency at room temperature with a two-level model, the difference of physical mechanisms between mini-LEDs with ALD and without ALD are determined. In addition, the thermal quenching indicates that the ALD sidewall passivation can enhance the temperature stability of the mini-LEDs. The ALD sidewall passivation also enhances the light extraction efficiency according to the theoretical calculation of transmittance. Moreover, the µ-HSI technique is used to evaluate different local areas of mini-LEDs. The obtained results reveal the optimization on lateral distribution of current density within the chip after sidewall passivation.

Temperature-Dependent Electroluminescence of Phosphor-Converted White LEDs With K₂SiF₆:Mn⁴⁺ Below Room Temperature

High-efficiency Mn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> -activated red phosphor of K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> :Mn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> synthesized at low temperature was utilized to fabricate phosphor-converted white light-emitting diodes (pc-WLEDs). Temperature-dependent electroluminescence (EL) measurements were undertaken at a constant forward voltage mode. A large negative-thermal-quenching (NTQ) effect is demonstrated for the luminescence intensity of this kind of red phosphor in the temperature range of 100–300 K. It is found that its enhancement rate is several times higher than those of the blue chip and the green component, such that the overall color of the pc-WLED shows a wide color gamut in the studied temperature range. By utilizing this outstanding NTQ property of the red phosphor, a color rendering index up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{R}_{\mathrm {a}}=93$ </tex-math></inline-formula> is obtained for the high-brightness pc-WLED with relatively low blue light from the LED chip.

Thermal assisted up-conversion electroluminescence in quantum dot light emitting diodes

Up-conversion electroluminescence, in which the energy of a emitted photon is higher than that of the excitation electron, is observed in quantum-dot light-emitting diodes. Here, we study its mechanism by investigating the effect of thermal energy on the charge injection dynamic. Based on the results of temperature-dependent electroluminescence and theoretical analysis, we reveal that at sub-bandgap voltage, holes can be successfully injected into quantum-dots via thermal-assisted thermionic-emission mechanism, thereby enabling the sub-bandgap turn-on and up-conversion electroluminescence of the devices. Further theoretical deduction and experimental results confirm that thermal-assisted hole-injection is the universal mechanism responsible for the up-conversion electroluminescence. This work uncovers the charge injection process and unlocks the sub-bandgap turn-on mechanism, which paves the road for the development of up-conversion devices with power conversion efficiency over 100%.

Temperature-dependent recombination dynamics and electroluminescence characteristics of colloidal CdSe/ZnS core/shell quantum dots

Exploring the temperature-dependent electroluminescence (EL) properties of quantum dots (QDs) is not only critical for the fundamental understanding of charge recombination processes in quantum dot light-emitting diodes (QLEDs) but also important to expand their particular applications at cryogenic temperatures. Herein, the temperature-dependent EL properties of typical CdSe/ZnS core/shell QDs were systematically studied for both the standard and inverted QLEDs in the temperature range of 100–300 K. It was found that EL intensity and efficiency were gradually enhanced and exhibited a pronounced blue shifting in EL spectra with the decrease in temperature. We demonstrated that temperature-dependent EL behaviors mainly originated from the inherent properties of QDs, while the different device structures could induce some fine temperature-dependent behaviors on this basis. Moreover, the effective Joule heat dissipation at low temperatures protects the charge transport and QD layers from thermal decomposition and damage, which prolongs the operational lifetime of devices.

R/G/B Micro-LEDs for In-Pixel Integrated Arrays and Temperature Sensing

In this paper, we demonstrate that nitride-based R/G/B micro-LEDs allow temperature sensing through a forward voltage response on both temperature and display devices. In temperature sensing, micro-LEDs operate in current constant mode, exhibiting an excellent performance with a sensitivity of 2.6/2.7/2.2 mV/K for blue/green InGaN micro-LEDs and red AlGaInP micro-LED under an injection current density of 1 mA/cm2, which is about two times lower sensitivity than the sensitivity limit obtained from the Shockley ideal diode equation. It is speculated that the lower sensitivity originated from parasitic resistances. It is also found that the band gaps of ternary InGaN and quaternary AlGaInP obtained from temperature-dependent electroluminescence spectra of red micro-LED agrees with the calculated values by using the semiempirical Varshni relationship. The EL of micro-LEDs is characterized by narrow emission widths of 18–41 nm, leading to wide color gamut covering up to 170/97% of the DCI P3/BT 2020, which are the standard color space of ultrahigh definition television system.

Temperature-dependent electroluminescence study on 265-nm AlGaN-based deep-ultraviolet light-emitting diodes grown on AlN substrates

Temperature-dependent electroluminescence measurements are performed for 265-nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) grown on AlN substrates. The external quantum efficiency (EQE) increases as the temperature decreases from 293 K to 6 K. Using two assumptions, the internal quantum efficiency (IQE) and current injection efficiency (CIE) are unity at the peak EQE at 6 K and the light extraction efficiency is independent of current and temperature, the current and temperature dependences of the product (IQE × CIE) are derived. The temperature dependence of the EQE cannot be simply explained by the Auger recombination processes. This observation enables the CIE and IQE to be separately extracted by rate equation analysis. The room-temperature EQE of the AlGaN-based DUV LEDs is limited by the CIE and not the IQE. We propose that the relatively low CIE may originate from the nonradiative recombination process outside quantum-well layers.

Effects of size on the electrical and optical properties of InGaN-based red light-emitting diodes

We investigated the effects of size on electrical and optical properties of InGaN-based red light-emitting diodes (LEDs) by designing rectangular chips with different mesa lengths. Larger chips exhibited lower forward voltages because of their lower series resistances. A larger chip helped to realize a longer emission wavelength, narrower full-width at half maximum, and higher external quantum efficiency. However, temperature-dependent electroluminescence measurements indicated that larger chips are detrimental to applications where high temperature tolerance is required. In contrast, a smaller red LED chip achieved a high characteristic temperature of 399 K and a small redshift tendency of 0.066 nm K−1, thus showing potential for temperature tolerant lighting applications.

Demonstration of low forward voltage InGaN-based red LEDs

Here we report InGaN-based red light-emitting diodes (LEDs) grown on () β-Ga2O3 substrates. AlN/AlGaN strain-compensating layers and hybrid multiple-quantum-well structures were employed to improve the crystalline-quality of the InGaN active region. A bare LED showed that peak wavelength, light output power, and external quantum efficiency were 665 nm, 0.07 mW, and 0.19% at 20 mA, respectively. As its forward voltage was 2.45 V at 20 mA, the wall-plug efficiency was 0.14%. The characteristic temperature of the LEDs was 222 K at 100 mA evaluated from the temperature dependence of electroluminescence.

Electroluminescence transients and correlation with steady-state solar output in solution-prepared CH3NH3PbI3 perovskite solar cells using different contact materials

Electroluminescence (EL) transients of solution-prepared CH3NH3PbI3 perovskite solar cells were recorded under different biasing voltage conditions. The EL transients are reversible and show a sharp increase and a peak in the range of 1 s to 10 s, while after the peak the signal decays in 30 s to 60 s. The possible origins of the different features are discussed, pointing to a shift in the region of dominating recombination during biasing, governing the EL increase, and the creation of ion migration-induced non-radiative recombination centers during the EL decrease. Moreover, when ramping up the polarization voltage, the EL transients shorten, suggesting an acceleration of the microscopic mechanism with increasing electric fields. Cells prepared with compact instead of mesoporous TiO2 electron contact show faster dynamics, highlighting the link between dynamics and interface properties. Furthermore, experiments using cells with different hole contacts show that the observed behavior and the duration of the transient is similar in cells using Spiro-OmeTAD and copper phatlocyanine (CuPc). When considering the steady-state EL, the open circuit voltage under solar operation correlates with EL across samples with different HTL materials. A non-monotonous behavior is also observed in temperature-dependent EL transients, where maxima in EL as well as in time to the peak are observed around 30 °C, which is close to the temperature of crystalline phase change from tetragonal to cubic phase known in CH3NH3PbI3 at 37 °C.

Getting the Right Twist: Influence of Donor–Acceptor Dihedral Angle on Exciton Kinetics and Singlet–Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86{\deg}. Surprisingly, an experimentally obtained energy gap of 72+/-5 meV can only be explained by torsion angles in the range of 70{\deg}-75{\deg}. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host matrix shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency.

High-efficiency and thermally stable far-red emission of Mn4+ in double cubic perovskite Sr9Y2W4O24 for plant cultivation

A new Mn4+ activated Sr9Y2W4O24 far-red phosphor was synthesized by high temperature solid state reaction. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence spectra, temperature-dependent emission spectra (298–548 K) and electroluminescence spectra. Finally, a 365 nm ultraviolet (UV) light-emitting diode (LED) chip combined with Sr9Y2W4O24:0.005 Mn4+ far-red phosphor was used to fabricate the LED device. It is found that Sr9Y2W4O24:Mn4+ phosphor has a strong broad excitation band in the range of 300–550 nm. There is a wide emission band centered at 680 nm between 650 nm and 800 nm. The optimum doping concentration of Mn4+ is about 0.5 mol%. Importantly, Sr9Y2W4O24:0.005 Mn4+ has an IQE of up to 49.8%. The thermal activation energy Ea = 0.448 eV. In short, the emission spectra of Sr9Y2W4O24:0.005 Mn4+ phosphors are well matched with the absorption bands of PFR in plants excited by near ultraviolet light, so Sr9Y2W4O24:Mn4+ phosphors have potential applications in indoor plant cultivation.

Study on Carrier transportation in InGaN based green LEDs with V-pits structure in the active region

InGaN based LEDs with V-pits structure and consists of n-GaN, In0.1Ga0.9N/GaN superlattices (SLs), In0.15Ga0.85N blue quantum wells (BQWs), In0.25Ga0.75N green quantum wells (GQWs) and p-GaN were grown on Si substrate. Carrier transportation is studied by temperature-dependent electroluminescence (TDEL). With n-doping in the SLs and BQWs, the major emission peak is from the GQWs. Under low temperature and high injection current, side peaks emitted form SLs and BQWs can be observed. Reducing the number of GQWs will result in stronger side peaks. We propose that the disequilibrium in carrier concentration bring high drift velocity to the holes, partial holes fly across the sidewall of V-pits to n-side and recombined with the electrons in the BQWs and SLs.

Measuring the internal quantum efficiency of light-emitting diodes: towards accurate and reliable room-temperature characterization

Abstract For accurate and reliable measurement of the internal quantum efficiency (IQE) of light-emitting diodes (LEDs), the method should be theoretically solid and experimentally simple to use without any prior assumption of physical parameters or complicated equipment. In this paper, we critically review the conventional characterization techniques for measuring the IQE of LEDs, including the methods based on temperature-dependent electroluminescence and constant AB(C) models. After reviewing the limitations of the existing IQE measurement techniques, we present the recently proposed method based on the improved AB model, called room-temperature reference-point method (RTRM). The RTRM is then applied to various LED devices to show how the IQE measurement techniques can be utilized to analyze their optoelectronic performances quantitatively.

Measuring the Internal Quantum Efficiency of Light-Emitting Diodes at an Arbitrary Temperature

We present a method of measuring the internal quantum efficiency (IQE) of light-emitting diodes (LEDs) as a function of driving current at an arbitrary temperature only with experimentally measurable quantities of optical power and current. The measurement procedure starts to find an exact value of the IQE at the reference point where the injection efficiency is considered as 100% and the radiative efficiency is calculated from the infinitesimal change of the relative external quantum efficiency (EQE). Once the IQE at the reference point is exactly known, the IQE at any other point is calculated by the relative ratio of the EQE values to the reference one. We show the theoretical formalism of the proposed method and demonstrate its validity using a commercial InGaN blue LED as an example. The comparison with the IQE obtained by the conventional temperature-dependent electroluminescence is also given for various temperatures.