第二腹反光金属电极定位发射器发光器件的光输出问题分析

Enhancing Light Outcoupling of Organic Light-Emitting Devices byLocating Emitters around the Second Antinode of the Reflective MetalElectrode 围绕第二腹反光金属电极定位发射器，提高有机发光器件的光输出

Abstract 摘要

Due to generally low conductivity and low carrier mobilitiesof organic materials, organic light-emitting devices (OLEDs) aretypically optimized for light outcoupling by locating emittersaround the first antinode of the metal electrode. 由于有机材料普遍较低的导电性和低载量的粒子迁移率的原因，有机发光器件（OLED）中的金属电极中的第一个波腹的位置附近的发射器的光输出耦合，通常是会被优化的。In this paper, byutilizing device structures containing conductive doping, weinvestigate theoretically and experimentally the influences of thelocation of emitters relative to the metal electrode on OLEDemission, and show that substantial enhancement in lightoutcoupling (1.2 times) or forward luminance (1.6 times) could beobtained by placing emitters around the second antinode insteadof the first antinode. Depending on the detailed condition, thesecond-antinode device may also give more directed emission asoften observed in strong-micrcavity devices yet without sufferingcolor shift with viewing angles.  http://www.ukassignment.org/ygkczy/

Introduction介绍

Organic light-emitting devices (OLEDs) have been the subjectof intensive investigation in recent years due to their applicationsin displays and lighting [1,2].近年来，由于到他们的应用程序在显示和照明中国的应用[1,2]有机发光器件（有机发光二极管）一直受到深入的调查。 The typical OLED structure usuallyconsists of a transparent substrate (e.g. glass and plastics), atransparent indium-tin-oxide (ITO) bottom electrode, a highlyreflective top metal electrode, and organic layers sandwichedbetween electrodes. In such structures, due to strong reflection ofthe metal electrode, directly out-going beams of the emissioninterfere with the beams reflected from the metal electrode,influencing outcoupled emission intensity [3,4]. To obtainconstructive interference and to optimize light extraction from thedevice, it roughly requires the locations of emitters to the metalelectrode be consistent with the antinode condition of majoremission wavelengths (i.e the emitter-to-metal round-trip phasechange equals multiple integers of 2 ) [3,4]. Due to generallylow conductivity and low carrier mobilities of organic materials,OLEDs are typically optimized by locating emitters around thefirst antinode of the metal electrode to minimize the layerthickness and device voltage. Furthermore, placing emitters at afarther antinode by using a thicker carrier-transport layer maysignificantly disturb and complicate the scenario of carrierrecombination (e.g the location and distribution etc.). Recentadvances in conductive doping of organic semiconductors andhigh-mobility materials [5-8], however, may remove suchconstraints. In this paper, by utilizing device structures containingconductive doping, we investigate theoretically andexperimentally the influences of the location of emitters relativeto the metal electrode on OLED emission, and show thatsubstantial enhancement in light outcoupling or forwardluminance of OLEDs could be obtained by placing emittersaround the second antinode instead of the first antinode 。在本文中，我们利用含有导电掺杂的器件结构，从理论和实验调查OLED发射到金属电极上的发射器对相对位置的影响，并表明，大幅提升光输出或远期OLED的亮度，可以得到发射器周围的第二个波腹，而不是第一个波腹。

Simulation and Analysis模拟与分析

The OLED structure investigated is: glass/ITO (120nm)/Bphen:Cs (5 wt.%, 20nm)/Bphen (20nm)/Alq3:C545T (1 wt.%,20 nm)/ -NPD (40 nm)/m-MTDATA:F4-TCNQ (1.5 wt.%, xnm)/Ag (100 nm), which adopts the inverted structure withconductive doping in carrier-transport layers for currentconduction and carrier injection. ITO and Ag serve as the bottomcathode and the top anode, respectively. Other layers in sequenceconsist of 4,7-diphenyl-1,10-phenanthroline (Bphen) doped with5 wt.% Cs as the n-doped electron-injection layer [6], undopedBphen as the electron-transport layer, tris-(8-hydroxyquinoline)aluminum (Alq3) doped with the fluorescent dye C545T as theemitting layer [2,3], -naphthylphenylbiphenyl diamine ( -NPD)as the hole-transport layer [9], 4,4’,4”-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) dopedwith 1.5 wt.% of tetrafluorotetracyano-quinodimethane (F4-TCNQ) as the p-doped hole-injection layer [5]. The thickness ofm-MTDATA:F4-TCNQ is varied to adjust the distance betweenemitters and the metal electrode. The inverted structure isadopted in this work purely because of the difficulty for ourdeposition system to deposit thick electron-transport layers withn-type conductive doping using the dispenser-type Cs or Lievaporation source. Nevertheless, according to our theoreticalanalysis, the results for both normal and inverted OLED structuresare similar.

然而，根据我们的理论分析，正常的和倒置的OLED结构的结果是十分相似的。

The optical model used for performing the analysis adopts aclassical approach based on the equivalence between the emissionof a photon due to an electrical dipole transition and the radiationfrom a classical electrical dipole antenna [10-13], which can takeinto account loss due to electrodes. With plane-wave expansionof the dipole field, the full-vectorial electromagnetic fieldsgenerated by a radiation dipole embedded in a layered structure iscalculated, from which the distribution of the radiation power intodifferent plane-wave modes and the far-field radiation related toemission characteristics of an OLED are obtained. To modelemission characteristics of an OLED, it is assumed that theemitting layer contains an ensemble of mutually incoherent dipoleradiators with distributions in dipole orientations (a randomisotropic distribution), locations (a decaying exponentialdistribution from the -NPD/Alq3 interface into Alq3 with anexciton diffusion length of 15 nm) [2], and frequencies (using thephotoluminescence (PL) spectrum of Alq3:C545T as the intrinsicspectral distribution of the dipole radiators). Radiationcharacteristics of OLEDs are then obtained by averagingcontributions over these distributions.了解有机发光二极管的辐射特性，然后获得通过这些分布来得到一些贡献的数据。