Study On High-performance Organic Light-emitting Devices

Organic electroluminescent (EL) devices of the visible region have attracted much attention because of their potential applications for multicolor emission with high quantum efficiency and for applications to full color flat-panel displays. Tang and VanSlyke (1987) developed a very efficient fluorescent material and demonstrated low-voltage-driven EL diodes using an emission layer and a hole-transport layer. Subsequently, the multilayer structures (Tang and VanSlyke 1987), doped emittinglayers (Shi and Tang 1997; Tang and VanSlyke 1989), doped hole-transport layers (Hamada et al. 1995), novel transport and luminescent materials including polymers (Tokito et al. 1996; Hosokawa et al.1995; Tasch et al. 1996; Huang et al. 1997) and efficient injection contacts (Broms et al. 1995; Hung et al. 1997; Li et al. 1997), etc., have been adopted to achieve the best device performance, such as the driving voltage, emission efficiency and durability. The design of EL structure for used in OLEDs is critical to device' s performance. Great strides have been made towards the development and improvement of small molecular OLEDs for display applications. Intense research in both academia and industry over the last 4-5 years has yielded OLEDs with remarkable full color, device efficiencies and operational stability. In this thesis, the reader will find a description of the novel structures in various colors organic EL research for their efficiency and brightness.Doping structures is commonly technology used in red OLEDs. By doping a red fluorescent dye of DCJTB in the hole-transporting layer and in the electron-transporting layer to form double emitting zone, improved devices can be obtained with efficiency declining more slowly than that of conventional doped devices (Alq3:DCJTB) in alarge scale of applied voltage. Meanwhile, good colour purity not only in low voltage region but also in high voltage region has been achieved. The optimal device, with a structure of ITO/CuPc/NPB/NPB:DCJTB/ Alq3:DCJTB/ Alq3 / LiF/Al, showed good chromaticity coordinates (x = 0.62, y = 0.36) at 8V. Uniquely, the current efficiency of the device was relatively independent of the drive voltage in a wide range from 8V to 20 V.A high bright yellow-organic-light emitting diode has also been fabricated with similar structure, which doped rubrene both in the hole-transporting layer (NPB) and the electron-transporting layer (Alq3) to form double emitting zone.We do a unprecedented study the effect of different thickness of NPB on the performance of blue OLEDs. DPVBi is a kind of excellent blue dye for OLEDs. We fabricated devices with structure of ITO/NPB/DPVBi/ Alq/LiF/Al, by adjusting the thickness of NPB layer, the performance of the blue OLEDs was improved. When the thicknesses of NPB,DPVBi, Alq, LiF, Al are 130 nm, 20 nm, 30 nm, 0. 5 nm, 100 nm, respectively. The maximum brightness and efficiency reached 6891cd/m2, 1.64cd/A, respectively. This phenomena attributed to the changing of transmissivity related withthe thickness of NPB layer.We have demonstrated a bright white emission organic light-emitting device based on an incomplete energy transfer, in which emission layer composes of a layer of blue-emitting material DPVBi, uniformly doped with orange emitting rubrene. organic white-light-emitting device has been demonstrated by mixing two colors from a single emissive layer. A stable white emission Commission Internationale de l'Eclairage (CIE) chromaticity coordinate ranging from (0.34, 0.36) to (0.32, 0.32) with bias voltage changing from 5 to 13V has been achieved. Its maximum luminance was 15840cd /m2 at 17v, and the maximum power and current efficiency were3.1 lm/W and 5.8 cd/A at 6 V, respectively.we describe the performance of an organic light emitting device ( OLEDs ) with ITO / NPB /copper phthalocyanine/ NPB / Bathocuproine / Alq3 / LiF/ Al structure, fac the CuPc layer inserted between the two layers of NPB as a hole-consuming layer, and the BCP as a hole-blocking layer. The EL spectrum peak is at 430nm, indicating that the carrier recombination is confined in the NPB layer (near to BCP), in additional light emission originates from NPB. Compared with the luminous efficiency of the conventional diode without CuPc layer,that of the diode with HCL has been sharply increased up to 2.43 cd /A. A blue OLED which has a structure like an SH-B type diode has been developed. The blue OLED consists of a hole-transporting emissive layer, a hole-blocking layer and an electron-injection layer (Alq3) formed on an ITO anode NPB was used for the hole-transporting emissive layer, which has an emission peak at around 430 nm. For the hole-blocking layer, we found that Bathocuproine is a very effective material. As bathocuproine has a good hole-blocking ability, the recombination area is in the NPB layer. When the thickness of bathocuproine is 8 nm, the blue OLED has color coordinate in CIE chromaticity (0.16, 0.17) and a brightness of 4989 cd/m2.