Studies On White Organic Light-emitting Devices

Organic light-emitting diodes (OLEDs) have attracted more and more attention ever since C. W. Tang for the first time reported organic light-emitting devices (Alq3 as light emitting layer) with low driving voltage in 1987. OLEDs are expected to replace liquid crystal display (LCD) to be the next generation display with some excellent properties, such as light-weight, low-cost, thinness, low driving voltage, bright emission, et al. White organic light-emitting devices (WOLEDs) have attracted considerable interest owing to their potential in applications such as flat panel displays, lighting source and back light of a LCD. The author has made great effort to develop devices with new structure and highly efficient WOLEDs.Currently, bright and efficient red and green OLEDs with internal quantum efficiency approaching 100% have been achieved successfully by employing electrophosphorescent materials. However, for blue electrophosphorescent OLEDs, the instability and their demand for appropriate host materials, which should have higher triplet excited state energy than that of the guest to realize exothermic energy transfer, will hamper their application in the field of display or lighting. Hence, blue electrofluorescent OLEDs have been attracting much attention at present.The driving voltage is usually high for devices with conventional heterojunction (HJ), for the injection barrier is high at the HJ interface, which will lower the power efficiency. Mixed host (MH) OLEDs compared to a conventional HJ device, due to poor charge confinement of MH structure, has poor current efficiency. So, although the use of MH in a single emission layer (EML) helps to increase the carrier mobility of the EML and decrease the driving voltage, the power efficiency is still comparatively low. Here we for the first time came up with the idea of combining the MH structure and the HJ structure in one device in order to obtain high power efficiency. A high power efficiency blue fluorescent OLED was presented. The EML of OLED comprises two light-emitting parts:the first part, next to the hole transport layer (HTL), is a thin layer (about 8 nm) of 3% DSA-ph doped MADN; the second part, next to the electron transport layer (ETL), is 3% DSA-ph doped 50% MADN and 50% NPB mixed layer (22 nm). We found out that the power efficiency at a luminance of 1000 cd/m2 of the device is 4.2 Im/W, which is almost four times higher than 1.1 1m/W in MH control device and 35% higher than 3.1 1m/W in HJ control device. Furthermore, the device shows a maximum power efficiency of 5.0 1m/W.For the application in solid-state lighting sources, high efficiency and long operation lifetime are both essential. Phosphorescent WOLEDs generally are more efficient due to its demonstrated potential for achieving 100% internal emission efficiency by utilizing both the singlet and triplet excitons. However, in comparison with fluorescent WOLEDs, phosphorescent WOLEDs typically have shorter operational lifetime. Furthermore, the color stability of all-phosphor-doped WOLEDs is generally limited, as the blue electrophosphorescent devices have especially short operational lifetime. Therefore, the development of all-fluorescent WOLEDs with high efficiency and long lifetime is indispensable. In this study, WOLEDs based on DSA-ph doped in MADN as a blue emitting layer and a ultrathin rubrene layer as a yellow emitting layer were fabricated. We have investigated the influence of rubrene layer thickness on the device performance, and find out that device with 0.05 nm rubrene has the best performance. Bright white light over 10000 cd/m2 was successfully obtained at a low driving voltage of～8 V. The maximum EL efficiency and power efficiency are 8.69 cd/A at 7V and 5.5 lm/W at 4 V. Furthermore, we also investigated the reasons for the Commission International de L'Eclairage (CIE) coordinate change.Double-emitting-layer white organic light emitting devices have been fabricated: DSA-ph doped MADN as one emitting layer, Ir(ppy)3 and Ir(piq)2acac codoped into CBP as the other emitting layer. Besides, a thin layer of NPB with different thicknesses was inserted between these two emitting layers in order to adjust the EL spectra of the WOLEDs. With the thickness of NPB increasing, the emission of blue light increases, and that of red and green light decreases. Eventually, a WOLED with good CIE and high current efficiency was obtained. The current efficiency of the WOLED can reach 8.0 cd/A, the maximum luminance is 16890 cd/m2.Here we reported an efficient WOLED by introducing a bipolar transport CBP layer between fluorescent blue and phosphor-sensitized-fluorescent yellow emitting layers, where DPVBi and CBP codoped with rubrene and Ir(ppy)3 are used as blue and yellow EMLs, respectively. The presence of CBP may prevent the quenching between the fluorescent blue and the phosphor-sensitized-fluorescent yellow emitters. This quenching may occur through the nonradiative triplet energy level of the fluorescent blue emitter, which is positioned lower than the radiative triplet energy level of the phosphorescent green emitter. Performances of the WOLED can be improved by introducing a 2 nm undoped CBP layer between these two emitters. A white emission with a CIE of (0.22,0.33) was obtained. The maximum luminance and current efficiency of the device are 22360 cd/m2 and 10.7 cd/A, respectively.