Fabrication of WOLED by blue florescent host doped with red phosphorescent dyes

Abstract

A lot of efforts have been made to develop WOLED such as using single layer structure and multilayer structure in which different florescent and phosphorescent dyes are mixed which finally gives white light. As phosphorescent emitter system can harvest both singlet and triplet excitons, so WOLEDs based on these emitters have highest reported efficiencies but still the limited stability and life time of blue triplet emitter such as Firpic pose a drawback on efficiency and life time of WOLEDs. Therefore the high efficiency and color stability of WOLEDs depends on the use of blue florescent with green or red phosphorescent dyes. In the present work florescent Zn(hpb)2 is taken as a host. It emits greenish blue color with broad PL spectrum which covers region from 448 nm to 500 nm. In this host a yellow florescent i.e. rubrene and red phosphorescent i.e. [bis(2-2'-benzothienyl) pyridinato-N,C3) (acctylacetonate) iridium(III) dopents were mixed to give white color. The structure of the device is ITO/alpha-NPD (30 nm) / Zn(hbp)2: 0.1% Ir(acac): 0.01% rubrene (25 nm)/BCP (6nm)/Alq3 (28 nm)/LiF (lnm)/Al (100 nm)was fabricated and then characterized. Organic layers were deposited in high-vacuum (10-6 Torr) by thermal evaporation of the required organic material onto a cleaned indium tin oxide (ITO) coated glass substrate. Finally a LiF buffer layer and Al cathode were vapor deposited onto organic films. The EL spectrum of the device shows four peaks at 443 nm, 511 nm, 606 nm, and 660 nm. This indicates that EL of this device almost covers the visible region i.e. from 400 nm to 700 nm. The doubly doped emissive layer device exhibits white emission having CIE co-ordinates (0.27, 0.28) and maximum brightness 2280 cd/m2.