| Organic electroluminescent devices (OELD) have been paid extensive attention due to their superior performances, such as high efficiency, energy saving, wide viewing angle, rapid response, shock-resistance and so on. Moreover, white OLED can not only combine with mature microelectronics etching color filter membrane technology to fabricate other kinds of OLED devices, but also be applied in the field of illumination and display. The blue OLED is the base to obtain white light. And the blue OLED is also significant to realize full-color display. Although the blue OLED has developed for more than 20 years, it can still not be used in commercial field or large-size display. There are still lots of vital problems that need to be addressed, including the immature materials of blue light OLED, the low level of luminance and efficiency, color changing of white light devices with fluorescent time and bias voltage, and the short life span etc... To solve the above problems, this paper mainly focus on increasing the blue OLED brightness and efficiency, improving the white OLED device color stability, improving the device lifetime as well as enhancing carrier injection efficiency of the device, so do the following aspect research:The main innovations of this paper are:Four groups of blue OLEDs with the structure of ITO(160 nm)/m-MTDATA(20 nm)/NPB(30 nm)/MADN:DPAVBi(x wt.%) (15 nm)/BCP(15 nm)/Alq3(30 nm)/Al(100 nm) were fabricated. The doping concentration of DPAVBi was 0,4,6,8 wt.% respectively. Comparing the four groups of devices we found that the doping concentration of DPAVBi affects not only the current density but also the luminescence characteristics of the device. The blue OLED device with the brightness of 57000 cd/m2, luminous efficiency of 47 cd/A was obtained when the doping concentration of DPAVBi was 6 wt.%. Then we changed the blue light-emitting layer thickness and fabricated four groups of bule OLED with the thickness of 15,20,25,30 nm respectively. We found that the electroluminescent peak intensity in 495nm increases with the increase of emitting layer thichness and this phenomeno results from the microcavity structure in device. The blue light OLEDs with double blue light emitting layer were prepared and the structure is ITO(160 nm)/m-MTDATA(20 nm)/NPB(30 nm)/MADN:DPAVBi(6 wt.%) (15 nm)/MADN:TBPe(5 wt.%) (15 nm)/BCP(15 nm)/Alq3(30 nm)/Al(100 nm). By changing the order of the light-emitting layer, contrast devices were prepared. We found that the most of carrier composites in the light emitting layer near the cathode, because the hole injection efficiency was far higher than the electron injection efficiency. The P type material F4-TCNQ doped into NPB as the hole transport layer and the bule OLEDs were fabricated, we found that the proper P type doping for NPB can effectively improve the luminescence of bule device.Moreover, the Rubrene doped in MADN as the emitting layer and white OLED with single emitting layer was prepared. The electroluminescence spectrum was analysed. We concluded that the Forster energy transfer occurred between the main material of MADN and doped material of Rubrene. The DPAVBi and Rubrene were doped simultaneously into the MADN as emitting layer and the double doped white OLED device with single emitting layer was fabricated. The electroluminescence spectrum was analysed and we found that the yellow component is far higher than the blue, because the charge will preferentially excite the Rubrene. The double-emitting layer blue OELD with the structure of ITO(160 nm)/m-MTDATA(20 nm)/NPB(30 nm)/MADN:Rubrene(5 wt.%) (5 nm) MADN:DPAVBi(6 wt.%) (15 nm)/BCP(15 nm)/Alq3(30 nm)/Al(100 nm) was prepared. Two groups of contrast devices were by changing the order of the light-emitting layer, the stability and lifetime of white OLED with emitting layer order of MADN:Rubrene/MADN:DPAVBi (from anode to cathode) is better than the device with emitting layer order of MADN:DPAVBi/ MADN:Rubrene. It may be attributed to the fact that Rubrene in the orange EML acts as a trapping site for holes, it can balance the carrier recombination area in devices. Then the thickness of yellow emitting layer was changed and the color coordinates of (0.3201,0.3459) the white OLED device was obtained.Another effective method to improve the luminous efficiency is adding a buffer layer between the electrode and the organic layer. The dissertation also preliminarily investigated on the influence of applying metal oxide semiconductor NiOx to the organic light emitting diode. Four groups of green devices were prepared for comparison according to the thickness of NiOx. The hole injection efficiencies of devices were significantly improved after adding NiOx buffer layer and the luminous efficiencies of devices were also improved.Flexible display is an important characteristic of OLEDs and it is also the trend of OLED future development. Blue OLED based on a flexible substrate PET was prepared; we found that the hole-blocking layer can increase the residence time of holes in the emitting layer. And thus it can increase the probability of holes and electrons compositing in the emitting layer.Finally, the mechanism of OLED degradation was discussed in the dissertation. The effect of organic layer surface roughness on device lifetime was studied. We found that the device with surface roughness of organic layer will generate a lot Joule heat because of the bump surface when the device operating voltage increase, which will lead to a serious degradation of devices and reduce their lifetime.The main innovations of this paper are:(1) Used MADN:DPAVBi for blue light-emitting layer, prepared blue OLED devices with the high brightness and efficiency, and studied the impact of the blue light-emitting layer thickness on device performance;(2) Prepared a double layer of blue light emitting device, changing the light emitting layer order to study the effects on blue OLED device carrier recombination and light emission efficiency;(3) Prepared a double layer of white OLED device, studied the influence of the double Effect emitting layer sequence and thickness on the stability and lifetime of the white OLED color device;(4) Electrochemically prepared NiOx anode buffer layer and a contrast device successfully prepared to study the influence of the NiOx buffer layer on green OLED device carrier injection efficiency. |
PDF Full Text Request