Blue And White Organic Light-emitting Devices Based On DPV And Study On Efficiency Roll-off

Since 21st century, China, Japan, Korea and some Western countries are all doing the research of organic light-emitting diode (OLED), with the rapid development of the OLED-based displays, lighting industry and the industrial chain. In the flat panel display, because of its chemical DC drives, high efficiency, self-luminous, high brightness, low power consumption, wide angle, ultra-thin, long life time, large area displays, LED color is complete, the system stable temperature, and other prominent advantages, OLED is expected to be the next-generation flat panel display products and technologies after the liquid crystal display (LCD), plasma display panel (PDP).With the development of OLED technology, especially the white organic light-emitting device, as it can not only be used in full-color display,but also can be used in solid-state lighting, so in recent years, so the research gained rapid development, gradually the application area had wider to the commercial application stage, such as flexible substrates lighting, large area of surface light illumination and so on.Blue OLED not only is the major constituent for red-green-blue full color displays,but also the indispensable element for the white OLED, which has practical solid-state lighting applications. OLEDs employing phosphorescent materials are most effective because phosphorescent materials can harvest both singlet and triplet excitons which lead to the potential for achieving 100% internal quantum efficiency. However, due to the common problem of short operational lifetimes and the high material cost, electrophosphorescence-based OLEDs with acceptable blue color purity are relatively rare. So the combination of blue fluorescent and orange (or green, red) phosphorescent dyes may solve these problems and provide efficient and stable WOLEDs.2-Diphenylamino-7-(2,2"-diphenylvinyl)-9,9'-spirobifluorene (DPV) is a very promising nondoped blue fluorescent material for its color purity and high quantum yield. We make blue and white organic light emitting devices based on blue fluorescence material DPV with some different structure. We also have a deep research on the problem of OLED roll-off.Firstly, we delimit the OLEDs efficiency, then we analysis the factors that affect the efficiency, especially introduce the reason for the efficiency roll-off:carrier transmission imbalance and exitons diffusion and non-radiative exciton quenching. The mutual triplet-triplet or triplet-plaron annihilation, a process that becomes very efficient at high triplet concentration, is a general explanation for the efficiency roll-off under high current density. The field-assisted dissociation of charge pairs is another principal cause of the efficiency drop at high electric fields. On the basis of experimental and analytical results, we propose some methods to solve the problem of efficiency roll-off. To select molecular systems with strongly bound excitons, low exciton diffusivity and a wide recombination zone in addition to short lifetime of triplet excitons are thus expected to minimize the exciton quenching effect.We make a non-doped blue organic light-emitting devices with a structure of ITO/hole transporting layer (HTL)/2-diphenylamino-7-(2,2"-diphenylvinyl)-9,9'-spirobifluorene/electronic transporting layer (ETL)/LiF/Al are fabricated. The performances of the devices are dependent on the charge mobility, charge injection, and energy level characteristics of HTL and ETL. The device with 4,4',4"-tris(3-methylphenylphenylamino)triphenylamine and 4,7-diphenyl-1,10-phenanthroline as HTL and ETL shows high efficiency and brightness at low voltage (5.8 cd/A and 1000 cd/m2 at 4.6 V). Furthermore, the device shows a slight efficiency roll-off of 16% from the brightness at maximum current efficiency to 10 000 cd/m2. We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device.In order to make a outperformance white OLED based on DPV, we introduce a red fluorescent material DCV. For the purpose of acknowledge the character of DCV, we make a yellow OLED with a structure of:ITO/m-MTDATA(30nm)/ NPB(20nm)/DCV:Alq3(40nm)/BCP(5nm)/Alq3(30 nm)/LiF/Al. The maximus current efficiency is 4.8cd/A. As the concentration increasing, the spectrum shows a little red-shift and the efficiency declines.Moreover, we also make the red OLEDs based on DCV with the structure of ITO/m-MTDATA(30nm)/NPB(20nm)/DCV(xnm)/BCP(5nm)/Alq3(30 nm)/LiF/Al. The red emission layer with two thickness (40nm and 0.5nm). When the thickness is 40nm, the spectrum show the typical DCV ELcharacteristics with a peak at 640nm. When the thickness is 0.5nm, the spectrum shows a white light emission with two peak, which from DCV and NPB emission.We fabricat the white OLEDs using fluorescent donor-acceptor-substituted spirobifluorene compounds, red DCV and DPV with four different structures. The maximum current efficiency is 7.2 cd/A. Remarkably, the EL spectrum of the devices and the CIE coordinates remains almost the same when the brightness ranged from 1000 cd/m2 to 10000 cd/m2. Besides the high efficiency and the stable colour, the present device's efficiency roll-off was only 4.8% from the brightness at maximum current efficiency to 10000 cd/m2.Then we hope to make a high efficiency F/P WOLEDs using the ambipolar blue fluorescent emitter 2-diphenylamino-7-(2,2"-diphenylvinyl)-9,9'-spirobifluorene (DPV) and the phosperescent material (BT)2Ir(acac). Particularly, DPV has a relatively electric-field independent hole and electron mobilities. The effects of the triplet energies and charge transporting properties of the blue materials on the performance of the white device are discussed. By using such a blue emitter in the device, a broader charge recombination zone is formed and the energy loss is reduced. WOLEDs with a maximum current efficiency of 25.1 cd/A which shift to 19.5 cd/A at 10000 cd/m2 have been achieved. The power efficiency of the device can reach 14.1 lm/W at 1000 cd/m2. By attaching a microlens array on the backside of the substrate, the outcoupling of electroluminescence in the forward direction is enhanced, resulting in elevated power efficiency 18.6 lm/W at 1000 cd/m2.