White Organic Light-emitting Devices Based On A Bipolar Transport Layer And Phosphor-sensitized Layer

Organic light-emitting diodes (OLEDs) have been attracting more and more attention as a new flat panel display technology for the merits of active emitting, thin thickness, broad visual angle, low energy consume, fast response, broad operating temperature, availability for full color display and flexible display, etc. The research on this field gains rapid development especially since C.W.Tang firstly reported the high brightness OLED at low operating voltage in 1987, and then OLED has become a subject of advanced scientific research in the world and a focus of advanced technology competition between countries, which is a interdisciplinary, and calls for the cooperation of many subjects. In 2005, OLED, together with the Internet was listed as one of the human's most influential 25 innovative technologies by the CNN over the past 25 years.In the field of OLED, people have been paying much attention to white OLED since J. Kido reported the WOLED for the first time in 1994, because WOLED can be not only used for white display, but also used for full color display combined with mature technology of color filters, in addition, it can be used as backlights for LCD and even illumination light sources. In order to obtain better performance, various configurations have been proposed, such as using single emission layer, multiple emission layers, excimer or exciplex emission, micro-cavity structure, down-conversion system, tandem structure, etc.Among these structures, WOLED employing both phosphorescent dye and fluorescent dye is most effective, since it can not only avoid just using singlet excitons and wasting triplet excitons for fluorescent materials which reduce the efficiency, but also prevent the instability from some inherent flaws of the blue phosphorescent dyes. Based on above principle, the author demonstrates a series of white OLEDs with different structures which are based on blue fluorescent and green phosphorescent, and makes abundant researches on optical-electrical properties of these devices. OLEDs, which have the structures of ITO/m-MTDATA (30 nm)/NPB (20 nm)/DPVBi (15 nm)/CBP (X nm)/CBP: 5wt% Ir(ppy)3 (10 nm)/Bphen (40 nm)/LiF (0.8 nm)/Al, are firstly made. For the purpose of investigating energy transfer between phosphorescent dye Ir(ppy)3 and fluorescent dye DPVBi, a thin undoped CBP layer which has the character of bipolar transport is inserted between the two emission layers, and the thickness of this layer is adjusted to 0 nm, 2 nm, 5 nm corresponding to device A1, B1, C1 respectively. From the spectrogram of these devices, we can find that compared to device A1, the green intensity of device B1 is increasing, but the blue intensity has no change, which owns to the thin undoped CBP layer of 2 nm that hinders the triplet excitons'Dexter energy transfer from phosphorescent dye to fluorescent dye effectively, so it will avoid wasting this energy, and this conclusion can help highly effective WOLED using bipolar transport layer. In addition, compared to device B1, the green intensity of device C1 is enhanced, while the blue intensity falls off, which dues to more excitons forming in the interface between phosphorescent layer and hole blocking layer.In general, the emission spectra of organic materials is broader than that of inorganic materials, thus two complementary colors can produce white light emission, so the combination of green from phosphorescent dye Ir(ppy)3, yellow from fluorescent dye rubrene and blue from fluorescent dye DPVBi may obtain white OLED. The structures of these devices are ITO/NPB (40 nm)/DPVBi (30 nm)/CBP (Y nm)/CBP: 5% Ir(ppy)3: 0.5% rubrene (8 nm)/Bphen (40 nm)/LiF/Al, which contain bipolar transport layer and phosphor-sensitized fluorescence layer, and Y=0, 2, 5 corresponding to device A2, B2, C2 respectively. The phosphorescent dye Ir(ppy)3 and fluorescent dye rubrene are doped in the CBP at the same time, and undoped CBP layer is a bipolar transport layer. The author changes the thickness of bipolar transport layer, then analyzes the spectrogram of these devices, and find out that bipolar transfer layer blocks the triplet excitons'Dexter energy transfer from Ir(ppy)3 to DPVBi effectively, and the part of this energy is used by Ir(ppy)3 to improve the green intensity, while the other is transferred to fluorescence dye rubrene by FÇ'rster energy transfer to improve the orange intensity, so it may improve the current efficiency theoretically. In fact, the max current efficiency of device B2 is twice as much as the one of device A2.Although the current efficiency of the devices with bipolar transport layer and phosphor-sensitized fluorescence layer is higher than the one without that, the color purity of these devices is not good. In order to obtain high efficiency and good color purity of WOLED, the author designs a series of devices based on the previous work, and the structures of these devices are ITO/m-MTDATA (30 nm)/NPB (20 nm)/DPVBi (41-d nm)/CBP (2 nm)/CBP: 8%Ir(ppy)3: 0.5% rubrene (d nm)/Bphen (40 nm)/LiF/Al. For the good color purity, the thickness of the whole emission layers is invariability; however, each of the emission layers is variable, and the thickness (d) values is 4, 5, 6, 8 corresponding to the device A3, B3, C3, D3 respectively. By analyzing the character of the series of devices, we can find out that the max current efficiency of the device D3 is the highest of all, but the color emitted by this device is close to yellow green, so we select device C3 as the optimization device considering that max the current efficiency is highest except device D3's, and the color is still white, besides the highest current efficiency and the maximum brightness are 8.1 cd/A, 19 000 cd/m2 respectively.