Improved materials are usually susceptible to concentration

Improved Efficiency Phosphorescent Light-Emitting Devices Abhishek Rai, Sumit Tyagi Electronics and Communication Engineering Department  National Institute of Technical Teachers Training and Research Chandigarh  Abstract-We report the fabrication of highefficiency organic light-emitting diodes employing a novel red phosphor by managing charge transport properties of every functional layer. Doped hole transport layer and stepwise carrier transport system were utilized to enlarge charge injection, improve transportation and reduce drive voltage. Keywords- Component; OLEDS; Phosphor; High Efficiency. I. INTRODUCTION Organic light-emitting diodes (OLEOs) have attracted much attention for the use in full-color flat panel displays and white OLEOs with wonderful saturation, for which three basic colors, red, green and blue (RGB) are needed. However, Red emissive molecular materials are usually susceptible to concentration quenching 1. An external quantum efficiency or current efficiency drastically decreases at currents, e.g., over 1 mAlcm2• A driving voltage of a red OLEO tends to be high, resulting in low power efficiency 2. Besides, efficiency of red OLEO is limited as wavelength of red light is beyond visible wavelength content of OLEO spectrum. As OLEO technology is rapidly building up momentum in the commercial marketplace, phosphorescent OLED (PHOLEO™) technology is proving to be a key component for a wide range of product applications 3. Among the various OLEO architectures, phosphorescent technology is most effective due to its demonstrated ability to achieve nearly 100% internal emission efficiency (IQE) for all primary colors necessary for display applications. Additionally, phosphorescent device lifetimes are rapidly increasing to be competitive with the best in the industry 4. In order to obtain more balanced charge injection from both the anode and cathode, multilayer OLEO devices are studied intensively to lower the device operating voltage and increase the luminance so as to raise output power efficiency. To achieve this end, one approach is to enhance electron injection at the cathode/ETM interface by: 1) using low work function metals and 2) introducing a thin layer of electron injection material with high electron affinity between the cathode and ETM to increase electron injection through stepwise injection from the cathode 5. A second approach is to increase the hole injection by using: 1) UV –ozone treated indium tin oxide (ITO); 2) high work function anodes;3) stepwise hole transporting system 6   II. HIGHLY EFFICIENT WHITE ORGANIC LIGHT EMITTING DIODES Ever since the first publications by Tang and VanSlyke in 1987 7, the performance of OLEDs has improved steadily. Due to their high efficiency, OLEDs are considered as a potential technology for future lighting sources. The design of our white OLED, which reaches the highest efficiency reported in the scientific literature. The device is based on the p-i-n-concept, i.e. it consists of electrically doped p and n layers enclosing an intrinsic emitting layer 8. We use ITO (on high index glass, nhigh = 1:78) as anode, MeO-TPD1 doped with NDP22 as p-doped hole transport layer, NPB3 as electron blocking layer, Ir(MDQ2(acac)4 doped into the matrix material TCTA5 as red emitter, FIrpic6 doped into the matrix TPBi7 as blue emitter, Ir(ppy)38 doped into TPBi as green emitter, TPBi as hole blocking layer, BPhen9 doped with Cs as electron transport layer, and finally Ag as cathode.  Doping leads to extremely low driving voltages. A brightness of 1000 cd=m2, which is relevant for lighting applications, is already reached. at a voltage of approximately 3 V. As the driving voltage is inversely proportional to the luminous efficacy, a low driving voltage directly relates to a high efficiency.  The luminous efficacy and several techniques to enhance the outcoupling efficiency are shown in the bottom part of Figure 2. Due to the high refractive index of the organic layer, the ITO and the glass, light can be reflected at the interface between glass and air, and the OLED forms an optical micro cavity. Without optimizing the optics of the device, approximately four out of five photons are trapped inside the cavity. The black line in Figure 2 denotes the efficiency measured without any out coupling enhancement, the green line is measured with a structure of small pyramids cut into a sheet of high index glass applied to the substrate and the red line is obtained by applying a half sphere to the substrate and coupling out all glass modes. The highest efficiency at 1000 cd=m2 is reached when a half-sphere is applied to the substrate. However, this outcoupling solution is not scalable to large areas and thus not practicable for later products. The pyramidal outcoupling structure is scalable and although the luminous efficacy drops to 90 lm/W, this value is still the highest efficiency reached so far for white OLEDs and surpasses the efficiency of fluorescent tubes.  Although several parameters such as the color point of these devices (0.41, 0.49) and the lifetime still need to be improved, these experiments clearly demonstrate the potential of white OLEDs. In the literature, several alternative approaches like the triplet-harvesting concept are discussed 9 that combine high efficiency with long lifetime and good colour quality.