Application of rare-earth metals in organic electroluminescent device

Abstract

Organic light-emitting devices (OLEDs) with its fast response time, low driving voltage and self-emitting properties become next generation flat panel displays. Recently, many efforts have been done on the research on improving the luminescent efficiency, lifetime and development of new materials and etc. This thesis mainly focuses on the application of rare Earth metals such as Samarium (Sm) and Ytterbium (Yb) in OLEDs. (a) Contrast improvement of OLEDs with Sm:Ag cathode Sm:Ag, easily fabricated via simple thermal evaporation, was investigated as a single-layer light-absorbing cathode to increase the contrast ratio of organic light-emitting devices (OLEDs). The performance of OLEDs with Sm:Ag cathode was found comparable to that of the traditional OLEDs with Mg:Ag cathode in terms of brightness, electroluminescence (EL) efficiency, and turn-on voltage. The maximum EL efficiency of the device with Sm:Ag black cathode was 2.72 cd/A at 15 V, while the contrast ratio reached 390:1 at 10 V under 140 lx of ambient light, which was 8 times better than that of the traditional device. The lower EL efficiency and enhanced contrast are explained by the reduced optical reflectance of Sm:Ag black cathode, which was calculated to be about 0.15. (b) Transparent OLEDs with LiF/Yb:Ag cathode Highly transparent organic light-emitting devices (TOLEDs) based on LiF/Yb:Ag cathode are reported. The device with a structure of ITO/N,N′-bis-(1-naphthyl)- N,N′-diphenyl-1,1′-biph-enyl-4, 4′-diamine (NPB) (50nm)/tris (8-hydroxyquinoline) aluminum (Alq3) (50 nm)/LiF (0.5 nm)/Yb:Ag (15 nm, volume ratio 1:1) showed about the same luminance and spectra from both sides. At a current density of 20 mA/cm2, the luminance and EL efficiency of the top/bottom side were 165/167 cd/m2 and 0.825/0.835 cd/A, respectively. It is attributed to the characteristics to the high transmittance and low reflectivity of Yb:Ag cathode. The results suggest that LiF/Yb:Ag electrode can be used as an effective and stable cathode in TOLEDs or top-emitting OLEDs. (c) Transparent white OLEDs Transparent white organic light-emitting devices (TWOLEDs) with the structure of ITO/NPB (470 Å)/rubrene (1 Å)/NPB (30 Å)/2,2’,2’’-(1,3,5-phenylene) tris (1-phenyl-1H- benzimidazole) (TPBI) (300 Å)/Alq3 (200 Å)/LiF (5 Å)/cathode (150 Å) were fabricated. Two alloy cathodes of Mg:Ag (volume ratio 10:1) and Yb:Ag (volume ratio 1:1) were utilized, and the luminance, efficiencies, and electroluminescent (EL) spectra of the resulting TWOLEDs were investigated. Similar luminance and EL spectra were emitted from both sides of the TWOLED with the Yb:Ag cathode, while they were drastically different for the TWOLED with the Mg:Ag cathode. Analysis showed that the difference is due to the low-reflection and high transmission of the LiF/Yb:Ag electrode, which may serve as generic and effective cathode in white transparent or top-emitting organic light-emitting devices.

Date of Award	14 Jul 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)