Effect of CdSe/ZnS quantum dots on color purity and electrical properties of polyfluorene based hybrid light emitting diode

We report enhanced color purity of hybrid organic-inorganic light emitting diode based on polyfluorene-CdSe/ZnS quanum dot (QD) blend as emissive layer. Effect on structural, optical and electrical properties of different doping concentration (0–100 wt.%) of QD in polyfluorene (PFO) was studied. Photoluminescence and electroluminescence spectra confirm the β-formation of PFO by incorporation of CdSe/ZnS QD. Photoluminescence (PL) of blend film was also compared with another method based on one dimensional photonic band gap (1D-PBG) structure that has been used for color purity. In both the cases, that is, QD doped device and 1D-PBG based structures the narrowing of PL spectra was observed. But the fabrication of QD-doped device for color purity is easier than fabricating 1D-PBG structure using multilayer dielectric coating. The present study might find application for QD based color displays, where color purity is an important requirement.     

Suppressing molecular aggregation in solution processed small molecule organic light emitting diodes

Solution processing of low-molecular weight organic materials for optoelectronic devices is a challenging task due to often strong molecular aggregation. We present a facile and universal route for suppressing the aggregation of molecules during wet-deposition of emission layers for organic light emitting diodes by incorporating electronically inactive polymers. Moderate polymer concentrations of about 10 wt.% lead to only minor changes of the electrical performance while at the same time improving the film formation and consequently the device luminance significantly. The device performance matches the performance of vacuum processed devices with the same device architecture.     

Low-temperature cross-linkable hole transporting materials through chemical doping for solution-processed green PHOLEDs

Since the inter-layer mutual solubility is an obstacle to the development of solution-processed OLED, cross-linking is considered to be the best method to obtain solvent resistance. Vinyl is the most widely reported crosslinking group, but a problem raised that crosslinking usually need a high temperature. Here, two vinyl-crosslinked hole transporting materials, 3,3'-(1,3,4-oxadiazole-2,5-diyl)bis (N-phenyl-N-(4-vinylphenyl)aniline) (OXZ-VPAN), 3,3'-(4-phenyl-4H-1,2,4-triazole-3,5-diyl)bis (N-phenyl-N-(4-vinylphenyl)aniline) (TRZ-VPAN) were designed and synthesized. The introduction of pentaerythritol tetra(3-mercaptopropionate) (PETMP) and vinyl groups by thiol-ene reaction to reduce the crosslinking temperature. As a result, crosslinking can be achieved at 120 °C with the solvent resistance higher than 99%. The surface morphology of the films before and after crosslinking were characterized by atomic force microscope, and it was found that the roughness of the film was improved after dopped with PETMP. The solution-processed green phosphorescent OLEDs devices based on the obtained HTM exhibit excellent performance. Maximum current efficiency of 57.1 cd A⁻¹ and external quantum efficiency of 16.0% (Ir (mppy)₃) are obtained when OXZ-VPAN served as HTL. This low temperature feasible cross-linking process to prepare HTLs promotes the development solution-processed OLEDs.     

Solution-processable carbazole-based host materials for phosphorescent organic light-emitting devices

In this study, solution-processable carbazole-type host materials, 1,3-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)benzene (BCzPPh) and 4,6-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)pyrimidine (BCzPPm), were synthesized for use in phosphorescent organic light-emitting devices (OLEDs). Both host materials possess a high solubility in common organic solvents and high triplet energy to confine excitons to the phosphorescent emitter. The two nitrogen atoms in the central pyrimidine ring of BCzPPm have a profound effect on the photoluminescence properties and the electron-accepting capability. When doped with the green phosphorescent emitter tris(2-(4-tolyl)phenylpyridine)iridium (III), BCzPPh exhibited power efficiencies and external quantum efficiencies above 30 lm/W and 13%, respectively, in a simple bilayer OLED.     

Versatile,Benzimidazole/Amine‐Based Ambipolar Compounds for Electroluminescent Applications: Single‐Layer,Blue, Fluorescent OLEDs,Hosts for Single‐Layer,Phosphorescent OLEDs

A series of compounds containing arylamine and 1,2‐diphenyl‐1H‐benz[d]imidazole moieties are developed as ambipolar, blue‐emitting materials with tunable blue‐emitting wavelengths, tunable ambipolar carrier‐transport properties and tunable triplet energy gaps. These compounds possess several novel properties: (1) they emit in the blue region with high quantum yields; (2) they have high morphological stability and thermal stability; (3) they are capable of ambipolar carrier transport; (4) they possess tunable triplet energy gaps, suitable as hosts for yellow‐orange to green phosphors. The electron and hole mobilities of these compounds lie in the range of 0.68–144 × 10^?6 and 0.34–147 × 10^?6 cm² V^?1 s^?1, respectively. High‐performance, single‐layer, blue‐emitting, fluorescent organic light‐emitting diodes (OLEDs) are achieved with these ambipolar materials. High‐performance, single‐layer, phosphorescent OLEDs with yellow‐orange to green emission are also been demonstrated using these ambipolar materials, which have different triplet energy gaps as the host for yellow‐orange‐emitting to green‐emitting iridium complexes. When these ambipolar, blue‐emitting materials are lightly doped with a yellow‐orange‐emitting iridium complex, white organic light‐emitting diodes (WOLEDs) can be achieved, as well by the use of the incomplete energy transfer between the host and the dopant.     

Molecular design of large-bandgap host materials and their application to blue phosphorescent organic light-emitting diodes

New large-bandgap host materials with carbazole and carboline moieties were designed and synthesized for high-performance blue phosphorescent organic light-emitting diodes (PhOLEDs). The two kinds of host materials, 9-(4-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pP2CZCB) and 9-(3-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mP2CZCB), displayed promisingly high triplet energies of ∼2.92–2.93 eV for enhancing the exothermic energy transfer to bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III) (FIrpic) in PhOLED devices. It was found that the blue PhOLEDs bearing the new host materials and the FIrpic dopant exhibited markedly higher external quantum efficiencies (EQEs) than a device made with 1,3-bis(N-carbazolyl)benzene (mCP) as the host. In particular, the PhOLED device made with 3 wt% FIrpic as the dopant and mP2CZCB as the host material displayed a low driving voltage of 4.13 V and the high EQE of 25.3% at 1000 cd m⁻².     

利用ZnO薄膜界面层改善ZnO纳米棒/p-GaN发光二极管的电致发光性能

Heterostructure light-emitting diodes(LEDs)were fabricated by growing ZnO nanorods and undoped ZnO films on p-GaN templates.The heterojunction showed a diode-like I–V characteristic and emitted electroluminescence(EL)peaks at 383 nm,402 nm,438 nm,and 507 nm under forward bias.Since the electrons from ZnO nanorods and the holes from p-GaN could be injected into ZnO films with a relatively low carrier concentration and mobility,the radiative recombination was mainly confined in the ZnO film region.As a result,the ZnO nanorods/i-ZnO/p-GaN light emitting diode exhibits a stronger ultraviolet–violet emission peak.     

Magnetic field effects on electroluminescence in phosphorescence organic light emitting diodes

In this letter, we presented a method to study the MFEs on the triplets in phosphorescent OLEDs. The magnetic electroluminescence (MEL) was obtained by doping a red phosphorescent guest with low concentration into a fluorescent host, where the guest and host can simultaneously emit. Experimentally two different MEL shapes of Lorentz and linear were observed, depending on the used host materials. We presented two different mechanisms to explain their difference. The diffusion process of triplets from host to guest and prolonged lifetime of triplet by magnetic field were attributed to the formation of the Lorentz shape, and it is considered that the linear shape was caused by magnetic field increased Dexter energy transfer rate and determined by the triplet energy difference between guest and host. It can be seen that the competition of two processes lead to the formation of the two different MEL shapes in the phosphorescent OLEDs.     

Manipulating the LUMO distribution of quinoxaline-containing architectures to design electron transport materials: Efficient blue phosphorescent organic light-emitting diodes

A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials. The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the locations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All the compounds exhibit favorable electron affinity (2.73–2.88 eV) and good thermostability (glass transition temperatures in the range of 112–148 °C). Using these compounds as electron transport layers, the bis(4,6-(difluorophenyl)pyridinato-N,C^2′)picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency (η_c,max) of 30.2 cd A⁻¹ and a maximum external quantum efficiency (η_ext,max) of 14.2%. Moreover, these efficiencies reveal small roll-offs at high luminance.     

Hydrofluoroethers as heat-transfer fluids for OLEDs: Operational range, stability, and efficiency improvement

In this work we introduce a simple yet very efficient method of heat dissipation by immersing OLED device into hydrofluoroether (HFE) fluids. It is shown that due to highly fluorous nature of this class of fluids, HFE do not damage organic semiconductors which are comprised in the OLED stack and therefore can be used as encapsulation media. HFE also have high thermal conductivity, low viscosity and can efficiently dissipate the heat by means of natural convection with laminar flow. By employing HFE we were able to significantly improve the OLED operating dynamic range. Lifetimes of OLEDs operating in HFE at high currents can be improved by about a factor of 8. Furthermore, HFE fluid significantly improves the light outcoupling by a factor of 70% due to higher than air refractive index (n = 1.3).     

LED在生物医学方面的应用和前景

本文综述了近年来发光二极管（light emitting diode,LED)在生物医学方面的研究与应用，内容包括：LED作用于动物细胞的研究；LED作用于动物模型的研究；LED在临床实验方面的研究；几种用于生物医学的LED新器件；LED在生物医学方面的前景展望。     

Tetrasubstituted-pyrene derivatives for electroluminescent application

Tetrasubstituted-pyrenes containing peripheral diarylamines (1–4) or fluorenes (5–6) have been synthesized. These compounds are highly fluorescent and possess high morphological stability and thermal stability. Compounds containing peripheral arylamines (1–3) can be used as the hole-transport and green-emitting materials for two-layered electroluminescent devices. Compounds with peripheral fluorenes (5–6) are efficient blue emitters and exhibit ambipolar carrier-transport characteristics with high electron mobilities (10⁻³–10⁻² cm²/V s) and high hole mobilities (>10⁻³ cm²/V s). Non-doped blue-emitting devices with promising electroluminescent performance (i.e., high efficiency and narrow/saturated emission) can be achieved using fluorene–substituted pyrenes as either the hole-transport/emitting layer or the electron-transport/emitting layer in the two-layered devices.     

A semi empirical approach for submicron GaN MESFET using an accurate velocity field relationship for high power applications

A semi empirical model has been proposed for sub-micron GaN MESFET's to calculate the I-V characteristics using an accurate velocity-field relationship obtained by fitting it with the Monte Carlo (MC) simulation. The results so obtained are compared with the experimental results to validate our model and are also compared with the results obtained from the simple saturation model to present the influence of electron drift velocity modeling on the device parameters. The model has been extended to predict the microwave parameters such as transconductance and output conductance of the device.     

Mercury cadmium telluride-based resonant cavity light emitting diode

A CdHgTe resonant cavity light emitting diode (RCLED) is proposed as a new infrared emitter. The device is prepared by molecular beam epitaxy on a CdZnTe substrate. A 10.5 periods Bragg mirror is first deposited. The cavity material is made of Cd_0.75Hg_0.25Te and contains a wide well (50 nm) designed to emit at 3.2 μm. The last three periods of the mirror are n-type doped while the cavity material is covered by a thin p-type CdZnTe layer. A gold layer closes the cavity, serving as the second mirror of a Fabry-Perot cavity tuned around 3.18 urn. It also provides an ohmic contact to the p-region. Under forward bias, the emission spectrum displays a narrow peak (8 meV full width at half maximum) corresponding to the cavity resonance. The position and linewidth of this line are independent of temperature. The directivity of the diode is also improved with respect to a conventional emitter, in agreement with theoretical expectations. Taking advantage of the spectral properties of the RCLED a new multispectral device has been fabricated.     

Using d-limonene as the non-aromatic and non-chlorinated solvent for the fabrications of high performance polymer light-emitting diodes and field-effect transistors

Aiming to environment protection, green solvents are crucial for commercialization of solution-processed optoelectronic devices. In this work, d-limonene, a natural product, was introduced as the non-aromatic and non-chlorinated solvent for processing of polymer light-emitting diodes (PLEDs) and organic field effect transistors (OFETs). It was found that d-limonene could be a good solvent for a blue-emitting polyfluorene-based random copolymer for PLEDs and an alternating copolymer FBT-Th₄(1,4) with high hole mobility (μ_h) for OFETs. In comparisons to routine solvent-casted films of the two conjugated polymers, the resulting d-limonene-deposited films could show comparable film qualities, based on UV–vis absorption spectra and observations by atomic force microscopy (AFM). With d-limonene as the processing solvent, efficient blue PLEDs with CIE coordinates of (0.16, 0.16), maximum external quantum efficiency of 3.57%, and luminous efficiency of 3.66 cd/A, and OFETs with outstanding μ_h of 1.06 cm² (V s)⁻¹ were demonstrated. Our results suggest that d-limonene would be a promising non-aromatic and non-chlorinated solvent for solution processing of conjugated polymers and molecules for optoelectronic device applications.     

Simultaneously enhancement of quantum efficiency and color purity by molecular design in star-shaped solution-processed blue emitters

A series of fluorene-free bipolar star-shaped molecules, Sn-Cz-OXD (n = 1–5), with increasing conjugated length in branches were synthesized as high efficient blue emitters for OLEDs. With the extension of conjugated branches, the solid PL quantum efficiency and external quantum efficiency of Sn-Cz-OXD significantly increased with longer spacer, while the emission spectrum of these materials exhibited a blue-shift with enhanced color purity due to the unique molecular design. All materials maintained exceptionally high thermal stability after prolonged heat treatment at 150 °C in air. The photophysical, electrochemical, thermal properties of these emitters were studied in relation to the molecular structure. Nondoped device based on S4-Cz-OXD with structure ITO/PEDOT:PSS/EML/TPBI/LiF/Al emitted stable pure blue light with CIE coordinates of (0.157, 0.146). It exhibited high current efficiency and external quantum efficiency of 4.96 cd A⁻¹ and 4.20%, respectively. These values are among the best results for solution-processed non-doped blue device based on fluorene-free materials, indicating its potential for commercial applications.     

Intense-pulsed-light irradiation of Ag nanowire-based transparent electrodes for use in flexible organic light emitting diodes

Silver nanowire (AgNW) based transparent electrodes are inherently coarse and therefore typically are only ever weakly bonded to a substrate. A remarkable improvement in the characteristics of a AgNW network film has, however, been achieved through a simple and short process of irradiating it with intense pulsed light (IPL). This not only avoids any severe deterioration in the optical characteristics of the AgNW film, but also significantly improves its electrical conductivity, adhesion to a polymeric substrate, and ability to endure bending stress. Most important of all, however, is the finding that the surface roughness of AgNW networks can also be improved by radiation. In a series of measurements made of organic light emitting diodes fabricated using these treated electrodes, it was revealed that the leakage current can be notably reduced by IPL treatment.     

Design of high triplet energy electron transporting material for exciplex-type host: Efficient blue and white phosphorescent OLEDs based on solution processing

A high triplet energy electron transporting material 1,3,5-tris(diphenylphosphoryl)benzene (TPO) was successfully designed and synthesized to form an efficient exciplex with the commonly used hole transporting molecule tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The singlet-triplet energy difference in this exciplex was only 0.03 eV, which leads to the successive triplet up-conversion and delayed fluorescence. In addition, due to the high triplet energies of TPO and TCTA, the energy leakage from exciplex-state to the constituting molecule was eliminated. By employing this exciplex as host, solution-processed white phosphorescent OLEDs have been realized with a low turn-on voltage of 3 V and a high power efficiency of 20.5 lm W⁻¹. These results indicate that the well-designed exciplex can be used as efficient host material for low-cost solution-processed OLEDs.     

面向高工作温度应用的带间级联红外光电器件

高温工作探测器是第三代红外焦平面发展的重要方向之一。带间级联探测器结合了势垒结构与多级吸收区结构的特点,通过多量子阱弛豫和隧穿实现光生载流子单方向输运,可以有效降低来自PN结耗尽区的产生-复合暗电流;利用多级短吸收区结构,在扩散长度很短的情况下仍然可以有效地收集光生载流子,从而可以提高探测器在高工作温度下的探测性能。本文主要介绍了作者在带间级联红外光电器件方面的研究进展,包括高工作温度带间级联探测器、高带宽带间级联探测器以及带间级联发光器件等。     

OLED封装技术进展

有机电致发光器件(OLED)因具有较多的优点,在显示领域有着光明的前景,其最大的优越性在于能够实现柔性显示,制作成柔性有机电致发光二极管(FOLED).OLED对水蒸气和氧气非常敏感,渗透进入器件内部的水蒸气和氧气是影响OLED寿命的主要因素,因此,封装技术对器件非常重要.对现有的主要的FOLED衬底材料和封装方法进行...