OLED显示与照明——从基础研究到未来的应用

有机发光二极管(OLEDs)已经成为当今最重要的显示和照明技术,不仅在产业上得到了应用,在学术上也得到了广泛研究,它不仅涉及化学、材料科学,也包含了物理学、光学、电子学、器件物理学、凝聚态物理学和半导体物理学等诸多学科内容。本文从OLED的工作原理和所涉及的材料出发,介绍了设计高效率OLED器件结构的物理基础和设计原则,最后对OLED在显示和照明领域的应用做了展望。     

基于DPV的高效率蓝光有机电致发光器件

制备了基于荧光染料2-diphenylamino-7-(2,2 diphenylvinyl)-9,9'-spriobifluoreme(DPV)的高效率蓝光有机电致发光器件(OLED).器件的结构为ITO/m-MTDATA(30 nm)/NPB(20 nm)/DPV(d nm)/BCP(10 nm)/Alq3(30 n...     

Sputter-patterned ITO-based organic light-emitting diodes with leakage current cut-off layers

We demonstrate the cost-effective fabrication of organic light-emitting diodes (OLEDs) using a sputter-patterned indium–tin-oxide (ITO). This scheme brings in a leakage current on the slope of the sputter-patterned ITO edges due to spike-like surface. To suppress it, we place thermally evaporated organic insulating molecules right on the ITO edges for preventing hole leakage, just below the aluminum (Al) cathode for blocking electron leakage, or both on the ITO edges and below the Al cathode. It is demonstrated that blocking off both hole- and electron-leak pathways (via the spikes) is highly desired to enhance the current efficiency and lifetime of the sputter-patterned ITO-based OLEDs.     

Straight-forward control of the degree of micro-cavity effects in organic light-emitting diodes based on a thin striped metal layer

We investigated the control of micro-cavity (MC) effects in organic light-emitting diodes (OLEDs) with the introduction of a striped thin metal layer between the indium tin oxide (ITO) layer and the hole transporting layer (HTL). With an enhanced MC effect obtained through the inserted metal layer, the forward emission of the OLED became stronger and the angular distribution became more forward-directed, leading to a current efficiency (CE) that was nearly 1.45 times higher than that of the reference device without the inserted metal layer. The net CE of the OLEDs with a striped metal layer was found to be determined by the area-weighted average of the CE’s of full-cavity-enhanced OLEDs and non-cavity OLEDs. It was also observed that the trade-off between resonance enhancement in efficiency and angle-dependent color stability, often found problematic in MC-based OLEDs, could be mitigated in a straight-forward manner by changing the relative portion of the metal-covered area.     

Optical and thermal properties of large-area OLED lightings with metallic grids

We investigate the effects of auxiliary metal electrodes on the optical and thermal properties of large-area (30 × 120 mm²) opaque and transparent white OLED lighting panels. Enlarging their emission area inevitably entails a non-uniform current distribution due to the limiting conductivity of transparent electrodes, causing local heat generation. To tackle it, we have used grid patterned Cr, Mo/Al/Mo, or Cu metal lines (0.15 mm in width) as auxiliary metal electrodes on an ITO anode. Among those, Cu metal grids exhibit the highest luminous efficacy with the least heat generation, and the most uniform light distribution by virtue of its lowest sheet resistance, followed by Mo/Al/Mo and then Cr metal grids. It is also found that local heat generation appears more seriously in large-area transparent OLED panels. With attempt to suppress it, we have also deposited Al metal lines (2 mm in width) on a semitransparent Al/Ag cathode by thermal evaporation, which brings in a highly uniform heat distribution. Furthermore, we study the effect of the shape of the light-emitting area on the luminance and heat distributions. A round-shaped OLED panel with a hexagonal metal grid exhibits highly homogeneous luminance and surface temperature distributions.     

Optical and barrier properties of thin-film encapsulations for transparent OLEDs

We investigate the optical and barrier properties of thin-film encapsulations (TFEs) for transparent organic light-emitting diodes (TOLEDs). To improve the barrier property of OLEDs, the number of dyads (Al₂O₃/polymer) and the thickness of polymer layer in the TFE structure are required to be increased. It is, however, demonstrated that a sharp dip appears in the transmittance of TFE films due to the interference of light caused by organic/inorganic multi-layered configuration, resulting in a dip in the top emission spectrum of TOLEDs. We have found that such a transmittance dip deepens when the number of dyads is large. What is worse, the number of transmittance dips and their sharpness are raised with increased thickness of the polymer layer. When the number of dyads is small, however, the effect of the polymer layer thickness on such a transmittance dip is weak. Therefore, we have addressed that the number of dyads needs to be reduced, but the thickness of the polymer layer should be increased to meet both optical and barrier properties of TOLEDs at the same time.     

Effects of surface treatment of ITO anode layer patterned with shadow mask technology on characteristics of organic light-emitting diodes

We investigated the effects of various surface treatments of indium tin oxide (ITO) on the electrical and optical characteristics of organic light-emitting diodes (OLEDs). A 150-nm-thick ITO anode layer was patterned directly with a shadow mask during the sputtering process without the use of a conventional photolithography patterning method. The sputtered ITO layer was subjected to thermal and oxygen plasma treatments to reduce the sheet resistance and improve surface roughness. The thermal treatment was performed for 1 h at temperatures of 250 and 380 °C, which were chosen so that the glass substrates would not deform from thermal damage. The measured sheet resistance decreased from 30.86 Ω/sq for the as-sputtered samples to 8.76 Ω/sq for the samples thermally treated at 380 °C for 1 h followed by oxygen plasma treatment. The root-mean-square surface roughness measured by atomic force microscopy considerably decreased to 3.88 nm with oxygen plasma treatment. The thermal treatment considerably decreased the sheet resistance of the ITO anode layer patterned with the shadow mask. The spike-like structures that are often formed and observed in shadow mask-patterned ITO anode layers were almost all removed by the oxygen plasma treatment. Therefore, a smooth surface for shadow mask-patterned ITO layers with low sheet resistance can be obtained by combining thermal and oxygen plasma treatments. A smooth surface and low sheet resistance improves the electrical and optical characteristics of OLEDs. The surface-treated ITO layer was used to fabricate and characterize green phosphorescent OLED devices. The typical characteristics of OLED devices based on surface-treated shadow mask-patterned ITO layers were compared with those fabricated on untreated and photolithography-patterned ITO layers to investigate the surface treatment effects. The OLED devices fabricated by thermal treatment at 380 °C for 1 h followed by oxygen plasma treatment for 180 s showed the highest luminance and current density. Furthermore, the leakage current that might be induced by the rough ITO surface was dramatically reduced to 0.112 mA/cm². Our study showed that the shadow mask-patterned ITO anode layer treated by heat and plasma and having a low sheet resistance and surface roughness yielded excellent electrical and optical properties for OLEDs compared to those based on an untreated ITO layer. The fabricated OLED devices using the surface-treated shadow mask-patterned ITO layer exhibited comparable characteristics to those obtained from a conventional photolithography-patterned ITO anode.     

GaAs基AlGaInP LED的研究和进展

探讨了GaAs基AlGaInP LED的最新研究与进展,介绍了AlGaInP红光LED外延材料的能带结构和基本的外延层结构以及限制外量子效率的问题,探讨了几种高效率的LED器件结构设计。着重介绍了目前一些外量子效率比较高的LED器件的制作方法以及一些提高外量子效率的AlGaInP LED器件结构,最后探讨了AlGaInP LED在作为固体光源发展过程中仍然需要面对的挑战。     

Single walled carbon nanotube anodes based high performance organic light-emitting diodes with enhanced contrast ratio

Commercially-available single walled carbon nanotubes (SWCNTs) were used to fabricate SWCNT sheets for anodes of organic light-emitting diodes (OLEDs) by spray-coating process without any use of surfactant or acid treatment. A layer of DMSO doped PEDOT:PSS was spray-coated on the SWCNT sheets to not only lessen the surface roughness to an acceptable level, but also improve the conductivity by more than three orders of magnitude. For our SWCNT-based OLEDs of tris-(8-hydroxquinoline) aluminum (Alq₃) emission layers, a maximum luminance 4224 cd/m² and current efficiency 3.12 cd/A were achieved, which is close to the efficiency of ITO-based OLEDs. We further found out that our OLEDs based on the PEDOT:PSS covered SWCNT anodes tripled the contrast ratio of the conventional indium tin oxide (ITO) based OLEDs.     

提高LED外量子效率

提高发光二极管的发光效率是当前的一个研究热点.简要介绍了从芯片技术角度提高发光二极管(IED)外量子效率的几种途径,生长分布布拉格反射层结构、制作透明衬底、衬底剥离技术、倒装芯片技术、表面粗化技术、异形芯片技术、采用光子晶体结构等.此外还介绍了发光材料、能带结构以及工艺对外量子效率的影响.     

Improved efficiency in green polymer light-emitting diodes with air-stable electrodes

By dispersing an electron transporting molecular dopant into the active semiconducting luminescent polymer, we have achieved improved efficiencies for green light-emitting diodes (LEDs). These green emitting LEDs were fabricated by adding an electron transporting molecular dopant, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD), into the semiconducting luminescent polymer as the emitting layer in the polymer LEDs. The devices used poly(2-cholestanoxy-5-thexyldimethylsilyl-l,4-phenylene vinylene) (CS-PPV), a new soluble green light emitter, as the semiconducting luminescent polymer and either aluminum or indium as the electron injection electrodes. Quantum efficiencies of LEDs with the electron transporting molecular additive in the luminescent polymer and an Al electrode are about 0.3% photons per electron, better by a factor of 18 than similar devices made without the addition of the electron transport molecular dopant; quantum efficiencies of similar LEDs fabricated with an In electrode are 0.23% photons per electron, better by a factor of 16 than devices without the electron transport molecular additive.     

白光有机电致发光显示器件的特性与制备方法的研究

有机电致发光显示器因其高亮度、低功耗、结构简单、响应快、视角宽等优点而愈来愈受到大家的青睐.利用白色OLED是一种实现全彩色显示的方法,因为白光加滤色膜的方式可以获得红、绿、蓝三基色.文章采用双发光层方法,即TBPe掺杂到ADN中作为蓝色发光层,DCJ TB掺杂到Alq3中作为红色发光层,从而实现白光显示.该制作工艺简单、容易控制、实验可重复性比较高,且色度比较稳定,随电压的变化幅度较小,最佳色度为(0.3345,0.333),几乎与标准白光色度重合.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Prism coupling method to evaluate the light extraction efficiency of outcoupling substrates

One of the key challenges in organic light-emitting diodes (OLEDs) for lighting applications is efficient light extraction from the planar, multi-layered OLED stack. Several different light extraction approaches are being explored currently by researchers, however characterizing light extraction films after fabricating OLEDs is not a viable approach when the outcoupling films have large surface roughness and is time consuming as well. Here we apply prism coupling method (PCM), a simple and elegant tool, to characterize outcoupling films. We show the effectiveness of PCM in estimating light extraction efficiency of outcoupling films. PCM can expedite selection and optimization of various light extraction approaches without the need to build OLEDs. The experimental results are corroborated by the optical simulations done using ray tracing method taking into account Mie scattering from wavelength sized spherical inclusions in an outcoupling film.