Hybrid LEDs Research Articles

Enhanced Electroluminescence of CdSe/ZnS Quantum Dot Light–emitting Diodes with Phosphorescent Donors

ABSTRACTWe demonstrated the enhancement of electroluminescence (EL) from green CdSe/ZnS QDs in hybrid QD/organic light-emitting diodes (QD-LEDs) by employing blue phosphorescent dyes Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) as efficient exciton harvesters and energy transfer donors. Precise control of the concentration of the FIrpic donors doped in a 4,4’-N, N’-dicarbazole-biphenyl (CBP) host and their distance from the QD layer led to complete triplet exciton energy transfer and EL enhancement by a factor of 2.5. The Förster distance between FIrpic molecules and green CdSe/ZnS QDs was determined to be ∼ 8 nm, which is in a good agreement with the value calculated using the Förster model. Our study shows that integrating colloidal QDs with phosphorescent organic dyes provides an effective means for improving the quantum efficiency of QD-based hybrid LEDs.

ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology

Reliable and reproducible p-type doping is the main challenge for fabricating highly efficient ZnO-based light-emitting diodes. During the last few years, the lack of reliable p-type conductivity in ZnO has initiated research concerning the combination of ZnO and other semiconductors, which can then be doped p-type. One of these concepts is the combination of ZnO with GaN heterostructures aiming at the fabrication of hybrid LEDs. We discuss the problems as well as potential benefits from a combination of ZnO and GaN hybrid heterostructures in a single device. We also present our recent results on ZnO-GaN hybrid LEDs using an inverted LED concept. The hybrid LEDs have an external quantum efficiency of more than 35%.

Hybrid LEDs based on ZnO‐nanowire arrays

AbstractWe report on room temperature electroluminescence from an inorganic/organic heterojunction light emitting diode based on ZnO nanowire arrays. The heterojunction is fabricated by depositing the p‐type polymer poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on n‐type ZnO nanowires grown by electrodeposition on transparent conducting glass (ITO) substrates. The nanowires are encapsulated in a thin polystyrene film deposited from high‐molecular‐weight solution. Electron injection occurs through the transparent ITO layer while hole injection is mediated by the PEDOT layer and gold contacts. The current–voltage (I–V) characteristics of the device shows a diode‐like behavior. A broad emission spectrum covering most of the visible range and reaching to the near ultraviolet is observed at voltages between 6 and 8 V under ambient condition. The device operation is stable for more than 5 h.

A water soluble inorganic molecular oxide as a novel efficient electron injection layer for hybrid light-emitting diodes (HyLEDs)

We demonstrate that electron injection in single-layer polyfluorene based polymer light-emitting diodes (PLEDs) can be significantly enhanced by inserting a thin (<10 nm) inorganic polyoxometalate (POM) molecular oxide layer between the polymer layer and an aluminum cathode. Hydrophilic POM was spin-cast from methanol, an orthogonal solvent with regard to the hydrophobic polymer layer underneath, to form the thin cathode interfacial/electron injection layer. A lower turn-on and operating voltage and a higher luminance and current density was obtained in the POM-modified hybrid LEDs (HyLEDs) which are associated with the electron injection barrier reduction in the modified polymer/Al interface, evidenced by the increased open circuit voltage from photovoltaic measurements. These results demonstrate the potential of polyoxometalates as novel, stable cathode interfacial layers for efficient electron injection/transport in high performance HyLEDs.

In vitro evaluation of dental bleaching effectiveness using hybrid lights activation

PURPOSE: The aim of this in vitro study was to evaluate whether using different types of light activation during the in-office bleaching agent technique increases the effectiveness of the dental bleaching procedure by measuring the color change in teeth. METHODS: From 60 extracted bovine incisor teeth, specimen sizes were standardized and dentin was conditioned by immersion in 38% phosphoric acid for 15 s. Following this procedure, the samples were placed in a water bath with ultrasound for 20 min and then immersed in a 25% coffee solution for a two-week period. The dentin was polished with a felt disc and aluminum oxide paste and finally sealed with colorless nail polish. Specimens were randomly divided into five groups, and their original color was measured using the EasyShade (VITA) spectrophotometer. All specimens received three applications of the bleaching agent Opalescence Xtra-Boost (Ultradent) during a period of 15 min each. Group 1 did not receive light activation, Group 2 was activated with a Halogen light (15 min), Group 3 was activated with a blue hybrid LEDs and a laser (15 min), Group 4 was activated with light from a green LED and a laser (15 min) and Group 5 was activated with a red laser (15 min). Tooth color was measured again after performing the various bleaching techniques on all specimens. The resulting data on color variations "E, "a*, "b* and "L*, and the color variations referenced on the Vita Classical scale were subjected to ANOVA, Tukey and Dunn tests (α=5%). RESULTS: The bleaching performance of 38% hydrogen peroxide was affected by the light source used; color reduction was observed for the blue LED and halogen lights, and quantitative assessments of tooth color measured by spectrophotometer and matched to the Vita Classical color scale arrived at similar results in all experimental groups. CONCLUSIONS: The type of hybrid lights used interferes in the efficiency of the bleaching agent.

Efficient solution-processed hybrid polymer–nanocrystal near infrared light-emitting devices

We report solution-processed hybrid LEDs, emitting infrared light with enhanced efficiency, based on PbSe nanocrystal (NC) lumophores embedded a conjugated polymer forming the nanocomposite active layer. The effect of device geometry and the NC-polymer blend composition on device performance is studied in detail. Increasing the PbSe NC concentration leads to a substantial decrease in device current, and an increase in efficiency and turn-on voltage. Analysis of the device data indicates that the PbSe NCs act as carrier traps and radiative recombination centers. With optimized concentration of NC emitters and active layer thickness, the devices exhibit an external electroluminescence quantum efficiency of 0.83% at a peak emission wavelength of 1280 nm.

Green/Yellow Solid-State Lighting via Radiative and Nonradiative Energy Transfer Involving Colloidal Semiconductor Nanocrystals

LEDs made of InxGa1-xN and (AlxGa1-x)1-yInyP suffer from significantly reduced quantum efficiency and luminous efficiency in the green/yellow spectral ranges. To address these problems, we present the design, growth, fabrication, hybridization, and characterization of proof-of-concept green/yellow hybrid LEDs that utilize radiative and nonradiative [Forster resonance energy transfer (FRET)] energy transfers in their colloidal semiconductor nanocrystals (NCs) integrated on near-UV LEDs. In our first NC-LED, we realize a color-converted LED that incorporate green-emitting CdSe/ZnS core/shell NCs (lambdaPL = 548 nm) on near-UV InGaN/GaN LEDs (lambdaEL = 379 nm). In our second NC-LED, we implement a color-converted FRET-enhanced LED. For that, we hybridize a custom-design assembly of cyan- and green-emitting CdSe/ZnS core/shell NCs (lambdaPL = 490 and 548 nm) on near-UV LEDs. Using a proper mixture of differently sized NCs, we obtain a quantum efficiency enhancement of 9% by recycling trapped excitons via FRET. With FRET-NC-LEDs, we show that it is possible to obtain a luminous efficacy of 425 lm/W opt and a luminous efficiency of 94 lm/W, using near-UV LEDs with a 40% external quantum efficiency. Finally, we investigate FRET-converted light-emitting structures that use nonradiative energy transfer directly from epitaxial quantum wells to colloidal NCs. These proof-of-concept demonstrations show that FRET-based NC-LEDs hold promise for efficient solid-state lighting in green/yellow.

The Hybridization of CdSe/ZnS Quantum Dot on InGaN Light-Emitting Diodes for Color Conversion

We have demonstrated the fabrication and characterization of hybrid CdSe/ZnS quantum dot (QD)-InGaN blue LEDs. The chemically synthesized red light (lambda = 623 nm) QD solutions with different concentrations were dropped onto the blue InGaN LEDs with an emission peak of 453 nm and the turn-on voltage of 2.6 V. In this configuration, the CdSe/ZnS core/shell QDs played the role of a color-conversion center. It was clearly observed that the emission intensity from QDs was increased with increasing QD concentration. With a QD concentration of 10 mg/ml in toluene was incorporated, the ratio of emission intensity of QDs to that of InGaN quantum wells reached 0.17, whereas the Commission Internationale de lpsilaEclairage (CIE) chromaticity coordinates greatly shifted to (0.29, 0.14). From the spatial mapping of electroluminescence spectra, the decrease of the intensity of E QW seems to be faster than that of E QD, which suggests that the QD film thickness may be thicker in the edge of the surface of InGaN chip. There will, therefore, convert higher proportion of blue light to red light. Also, the resin-encapsulated hybrid LEDs have a divergence angle (the full angle at 1/e 2 intensity) of about 20deg as the device is operated at 10 mA. Furthermore, under the injection current of 20 mA and room temperature, this device can be operated for more than 1000 h without any obvious degradation. From our results, it can be proven that the synthesized QDs are promising nanophosphors for color-conversion applications of solid-state LEDs. However, to more efficiently convert the blue light to red light, a denser QD solution with higher quantum yield must be utilized.