Alternating-current Light-emitting Diodes Research Articles

UV or blue light excited red persistent perovskite phosphor with millisecond lifetime for use in AC-LEDs.

Energy storage phosphors with millisecond period afterglow that compensate for the diming time of alternating current light-emitting diodes (AC-LEDs) have promising application. To obtain a persistent luminescence (PersL) white colour in AC-LEDs, we focussed on a red afterglow with short period phosphorescence. Ca4 Ti3 O10 forms a type of perovskite-related Ruddlesden-Popper phase structure. Doping Pr3+ ions into Ca4 Ti3 O10 , an ideal red PersL was obtained. X-ray diffraction and element analysis demonstrated that our target samples were crystallized well. Steady-state and afterglow luminescence properties were investigated in detail. Notably, the PersL intensity was dependent on various excitation wavelengths. By measuring three-dimensional thermoluminescence spectra, we found that the trap depths showed a continuous distribution and that the shallowest trap contributed to the millisecond afterglow. Two PersL mechanism models were used to elucidate the electron charging and de-trapping processes under UV or blue light activation.

Luminescence properties of alternating current light-emitting diodes (AC LEDs) through operating circuit and electrical characteristics

In this present work, luminescence properties of alternating current (AC) light-emitting diodes (LEDs) were discussed based on AC LEDs operating circuit and electrical characteristics. The luminous flux of AC LEDs almost has nothing to do with angular frequency ϖ, it first increases and then decreases with the increase of integral forward voltage (VF) and reaches maximum value when VF is around 140V under 220V/50Hz alternating current source. Luminous efficiency of AC LEDs which is nearly irrelevant to balance resistor and angular frequency ϖ exhibits a linear increase along with the increasing of VF and the limit luminous efficiency equals the efficiency of a single LED microchip. Experimental study of luminescence properties has been carried out through LED corn lamp simulating monolithic AC LEDs, and experimental results were similar to the theoretical properties. These achieved luminescence properties will provide some basis in investigating luminescence properties of AC LEDs through its electrical characteristics.

THE GREEN PHOSPHOR SrAl2O4:Eu2+, R3+ (R=Y, Dy) AND ITS APPLICATION IN ALTERNATING CURRENT LIGHT-EMITTING DIODES

The aim of the present investigation was to develop a phosphor to solve the flickering luminescence of alternating current (AC) light-emitting diodes (LED) by compensating the dark duration with appropriately persistent luminescence. The phosphor SrAl 2 O 4: Eu 2+ co-doped with Y 3+ or Dy 3+ was synthesized via solid-state reaction with H 3 BO 3 as flux. The crystal structure and morphology were characterized by using X-ray diffraction (XRD) and Scanning Electron Microscope (SEM), respectively. The photoluminescence spectra were collected with a fluorescence spectrometer. The results demonstrated that appropriate amount of Y 3+ or DY 3+ doped was beneficial to suppress the by-product of Sr 4 Al 14 O 25 which easily co-existed with the SrAl 2 O 4 phase brought by the flux of H 3 BO 3. However, too much Y 3+ or DY 3+ doped resulted in the formation of another impurity phase, i.e., the yttrium aluminum garnet of Y 3 Al 5 O 12 and Dy 3 Al 5 O 12. Comparatively, the doped DY 3+ was more helpful in prolonging the persistent luminescence, while Y 3+ was more efficient in enhancing luminescence intensity. To demonstrate the feasibility of the phosphor applied in AC LEDs, a nearly white AC LED was fabricated by coating the phosphor on a blue AC LED chip. The persistent luminescence was radiated from the AC LED device after turning power off. Moreover, the effect of the phosphor on compensating the AC LED dark duration through persistent luminescence was revealed by using the Keyence VW-9000 High-speed Microscope for the first time.

Analysis of three different junction temperature estimation methods for AC LEDs

Light-emitting diodes (LEDs) operating on alternating current (AC) are gaining popularity in lighting applications. The junction temperature of an LED significantly influences the performance and reliability of the product. Although there are many proven methods for estimating the junction temperature of direct current (DC) LEDs, only a few methods have been proposed for AC LEDs. Two recently proposed methods, the voltage drop method for AC LEDs and the rms current recovery method, indicated different thermal resistance values for the identical LED package. This discrepancy in thermal resistance values is due to the junction temperature rise in the first half cycle that is not accounted for in the rms current recovery method. The peak emission wavelength shift is another approach used in DC LEDs to estimate junction temperature. This method was used to estimate junction temperature for AC LEDs. The measured junction temperature results were compared to results obtained using the voltage drop method for AC LEDs. The junction temperature estimated using the two approaches agree.

A new green phosphor of SrAl2O4:Eu2+,Ce3+,Li+ for alternating current driven light-emitting diodes

New phosphors are desired to overcoming the flickering luminescence of alternating current light-emitting diodes (AC-LEDs) by compensating dark duration with appropriately persistent luminescence. Here, the phosphor of SrAl2O4:Eu2+,Ce3+,Li+ was developed for this application. The phosphor was synthesized with solid-state reaction method by using H3BO3 as flux. The XRD patterns show that the impurity phase of Sr4Al14O25 which always formed together with SrAl2O4 during reaction was suppressed by using Ce. The charger defects caused by the non-equivalent substitution of Sr2+ with Ce3+ were compensated with Li+. Thus, the luminescence intensity of SrAl2O4:Eu2+ was enhanced significantly by co-doping Ce3+ and Li+. The fluorescence lifetime of the phosphor is about 36.5ms. The energy dispersive spectra (EDS) assisted with scanning electron microscope (SEM) pictures reveal that Ce ions mainly distribute along grain boundary.

Effects of Cell Distance on the Performance of GaN High-Voltage Light Emitting Diodes

GaN light emitting diodes (LEDs) are recently attractive light source in the area of visible and near-ultraviolet light generation, with applications in roles such as large full-color displays, solid-state lighting and back lighting of liquid crystal displays. 1‐3 Generally, to obtain more LED output power, the LED chip size is enlarged to bear higher injection current operation. However, the “efficiency droop” of LEDs easily occurs at operating high injection current, which partially is attributed the occurrence of carrier overflow in quantum wells (QWs) 4 and low thermal dissipation of sapphire substrate. 5 Series-connection micro-cell GaN LEDs, defined as HV-LEDs, operating at high voltage (>20 volt) and low injection current (<100 mA) offers another approach to reduce the probability of “efficiency droop” and maintain high LEDs output. 6‐8 Moreover, the injection current can uniformly spread to the small and independent micro-cell LEDs and turn on all of them. In contrast with the similar configuration of LEDs, like alternating current (AC) LEDs, the total output power of HV-LEDs exhibits two times higher than one of AC LEDs in the same condition because only half of the active area was driven for AC LEDs. While the HV-LEDs show superior potential in improving “efficiency droop” and enhancing LED output power compared with the conventional LEDs and AC LEDs, the influence of spacing between cell LEDs still needs to be studied in detail. Therefore, in this letter, we designed four kinds of HV-LEDs with different spacing of 20, 40, 60 and 80 μm. By spacing changes, the HV-LEDs performance was thoroughlyinvestigatedthroughdiscussingtheresultsofoutputpower and wall-plug efficiency.

High frequency alternating current light-emitting diodes using Langmuir–Blodgett films

Alternating current (AC) light emitting diodes (LEDs) have been fabricated where Langmuir–Blodgett (LB) films of quinquethiophene (QT) or poly(3-hexylthiophene) (PHT) have been used as the active material sandwiched between insulating LB layers of poly(methylmethacrylate) (PMMA) or emeraldine base polyaniline (PANI), respectively. The frequency response of the devices has been studied, and as the frequency limit of operation we have used the −3 dB frequency. We have shown that high frequency AC LEDs can be fabricated with as few as 10–15 LB layers of the active material. Electroluminescence (EL) is observed almost equal in intensity in both biases for PMMA/QT/PMMA devices. The EL spectrum for these devices shows a broadening to the low-energy side as compared with the photoluminescence (PL) spectrum. The role of the interfaces for the frequency response is discussed.

Alternating-Current Light-Emitting Diodes and their Transient Characteristics: Response Time and Carrier Transport

AbstractWe report the fabrication and characterization of alternating current light-emitting diodes (LEDs) with quinquethiophene as the emitting material. We have obtained equal electroluminescence intensity in both bias sections. From the frequency response of the LEDs, we have estimated the device response times and compared them with the response times obtained from the transient response of dc LEDs. Langmuir-Blodgett film deposition technique has been employed to control the thickness of the emitting layer on the molecular scale. We have shown that the response times originate from the accumulation rather than the transit of charge carriers. We have compared the photo- and electroluminescence spectra of QT LEDs.