Studies On White Organic Light Emitting Devices

Organic light-emitting diodes (OLEDs) have been attracting considerable attention as an important flat panel display technology due to their ease of production, light weight, low cost, broad visual angle, fast response time, active emitting, high brightness, high efficiency, availability for full color display and compatibility to flexible substrates, etc. The research works in this field gained rapid development since C. W. Tang demonstrated the high brightness OLEDs at low operating voltage for the first time in 1987. In recent, OLED has become a project on the cutting-edge of scientific research that relates to many intercrossed branches of science and advanced technology. Owing to the research on new materials, optimization of device structure and improvement of fabrication processes, enormous progress has been made in the development and improvement of OLEDs for display applications. Intense research in both academia and industry has yielded OLEDs with remarkable color fidelity, device efficiencies and operational stability.Recently, White Organic light emitting devices (WOLEDs) with high brightness properties is expected, not only for a display device, but also for the backlights of a liquid crystal display and even illumination light sources. The research works in this field gained rapid development especially after 1994 when J. Kido reported the WOLEDs. In this thesis, we introduce some different structure WOLEDs with different materials, and study the characteristics of these devices.Firstly, we fabricated the WOLEDs with BPhen as hole blocking layer, DPVBi as blue emitting layer and an ultrathin DCM2 layer as yellow emitting layer, respectively. The structure of the device is ITO/m-MTDATA/NPB/DPVBi/DCM2/BPhen/Alq3/LiF/Al. The EL spectrum of the device can be adjusted by changing the thickness of DCM2. As a result, we obtained the WOLEDs whose maximum current efficiency, power efficiency and luminance are 7.64cd/A at 6 V, 4.80 lm/W at 5 V and 11 493 cd/m² at 15 V, respectively. The chromaticity coordinates, varying from (0.36, 0.31) to (0.28, 0.27) with increasing forward bias from 5 to 15V, are well within the white region, but the colour rendering index (CRI) is relatively low at some degree. In order to further improve the luminance, efficiency, CRI and chromaticity coordinates of the non-doped-type WOLEDs, we introduce the high-efficiency fluorescent dye QAD and DCJTB with dual ultrathin layers to the non-doped WOLEDs. The structure of the device is ITO / m-MTDATA/ NPB/ DPVBi/ DCJTB/ Alq₃/ QAD/ Alq₃ / BPhen/Alq₃/LiF/Al, in which 12nm DPVBi acts as blue emitting layer, when the thickness of QAD and DCJTB are 0.05nm, pure white emission with the CIE coordinates of the device change from (0.37, 0.36) at 4 V to (0.32, 0.33) at 15 V was obtained, and the CRI vary from 89 at 5V to 91 at 14V, respectively. Its maximum luminance was 27360 cd/ m² at 15 V and maximum power efficiency was 3.16 lm /W at 7 V. For further improve the efficiency of the devices, non-doped-type white organic electroluminescent devices based on DCM2 and C545T ultrathin layer have been fabricated. When the thickness of ultrathin C545T layer is 0.05 nm, the device has a maximum efficiency of 8.06 cd A^-1 at 6 V and a maximum power efficiency of 4.79 lmW^-1 at 5V, maximum luminance of 21770 cd m^-2 at 14V, respectively. White emission with CIE coordinates of (0.40, 0.38) at 5V and (0.32, 0.31) at 14 V was obtained, respectively. Moreover, the CRI was higher than 84 in all devices. The WOLED has better power efficiency, CIE coordinates and CRI in the non-doped type WOLEDs. The innovation of the device is to use the dual ultrathin layer in the device. As a result, the device has the simple structure,the higher efficiency,better CIE and CRI.We report on high luminous efficiency white light emitting device based on two-step energy transfer, WOLEDs with multiplayer structure have been fabricated. The structure of devices is ITO/NPB/NPB: QAD (1%): DCJTB (1%) / DPVBi / BCP/ Alq₃ /LiF/Al. In our devices, a red dye DCJTB and a green dye quinacridone (QAD) were co-doped into NPB act as yellow emitting layer and DPVBi acts as blue emitting layer. The device with 8nm BCP shows a maximum luminance of 12850cd/ m2 at 20V. The current efficiency and power efficiency reach 9.37cd/ A at 9V and 3.60lm/W at 8V, respectively. The thickness of the blocking layer permit the tuning of the device spectrum to achieve a balanced white emission with Commission International de'Eclairage (CIE) chromaticity coordinates of (0.33,0.33).Cheng et al reported a phosphorescent multiple emissive layer, in which a blue emissive layer is sandwiched between red and green ones, is employed in a white organic light-emitting device. In order to avoid using the blue phosphorescent material and the corresponding host materials that are very rare, we replace phosphorescent blue emissive layer to fluorescentc blue material DPVBi. Efficient white emission from the mixing of red emission from the bis(1-(phenyl)isoquinoline)iridium(III)acetylanetonate [Ir(piq)2(acac)], green emission from the fac tris (2-phenylpyridine) iridium [Ir(ppy)3], and blue emission from the NPB or DPVBi, is obtained. Ir(piq)2(acac) and Ir(ppy)3 are doped into CBP host,respectively. The device emission color is controlled by varying the thickness of blue emitting-layer. The maximum luminance, EL efficiency and power efficiency of the device with the thickness of 8nm DPVBi are 27 530 cd/m2, 17.6 cd/A and 13.7 lm/W, respectively. And Commission Internationale De L'Eclairage coordinates of the device vary from (0.54, 0.35) at 4 V to (0.30, 0.31) at 12V that are general within the white region. DPVBi can prevent the energy transfer between two phosphorescent emitting layers, and the CIE may tune by change the thickness of DPVBi. Synchronously, we also discuss the relationship between the color rendering index (CRI) and the electroluminescent (EL) spectra of some WOLEDs.Moreover, we also report on the fabrication of organic light-emitting devices (OLEDs) using BAlq/BPhen as hole-block layers on the electron transport layer and emitting layer. The current efficiency of the organic light emitting diode is improved by 43% to 9.16 cd/A as compared to the device with a single host of Alq₃ as the electron transport layer. The maximum power efficiency is 3.39 lm/W,which is 1.9 times than the single host Alq₃ device without block layer. The high performance of the device is the result of high probability of exciton recombination in the emitting layer.