Influence Of Annealing Treatment On Organic Light-Emitting Devices Properties

Organic light-emitting device(OLED) have attracted much attention due to their advantages in simple preparation process, light and portable, abundant materials, wide viewing angle, flexible and easy bending, which make organic light emitting devices appealing candidate for display and illumination applications. Although it has been about thirty years since the first organic light emitting device was invented, the blue light-emitting devices are poor compared with the red and green ones in terms of efficiency, stability and life. Because the energy gap of blue materials is large, which is difficult to select suitable materials. In this thesis, controlling suit annealing temperature to achieve the emission spectrum of the green material Alq3 to blue, and analyze properties changes of devices from the interface barrier, carrier mobility, carrier recombination and other internal situation.In the first, the effect of ITO anode surface on the performance of OLED is explored. The standard devices ITO / NPB(50 nm) / Alq3(60 nm) / Al(100 nm) after ITO with different oxygen plasma power(0,40,50,60,70 W) and different processing time(0,5,10,15,20 min) are prepared,the voltage-current-luminance characteristics of the devices are measured by keithley 2400-PR655 computer-controlled test system to be find the best processing time and optimum power. After the oxygen plasma treatment, ITO anode surface not only ensure cleaning thoroughly, but also improve the work function of the ITO film. The processing reduce the hole injection barrier, improve the hole injection rate, the luminous efficiency and luminance of the device. Experiments show that the oxygen plasma treatment works best, when the vacuum is about 80 pa, and the power is 70 W, the time last 15 min.Then we explored influence of annealing treatment on the crystallization and the spectrum of Alq3. In order to study the crystal structure, photo-luminescence and transmission spectra, ITO / Alq3(60 nm) / NPB(50 nm) and ITO / NPB(50 nm) / Alq3(60 nm) samples are prepared in vacuum annealing at 90,120,130,145,160,180 ℃ treatment 90 min respectively. Experiments results prove Alq3 crystals which is β-Alq3 appeared in NPB(50 nm) / Alq3(60 nm) / ITO samples as the annealing temperature increases, especially above 130 ℃ temperature. Alq3 photo-luminescence spectra move from 539 nm those have never annealing treatment, to 503 nm, which shift about 30 nm. And the crystals lead the absorption peak blue-shifted,.Finally around the topic that influence of annealing treatment on the properties of OLEDs, standard devices with the same process are prepared in vacuum annealing at 90,120,130,145,160,180 ℃ treatment 90 min respectively to study the annealing treatment on their optical performance. Due to the annealing treatment make Alq3 occur crystallization, the carrier mobility of the organic layer reduce, the turn-voltage gradually increase and the brightness also decrease in the overall rendering rule as the annealing temperature increase. However, there is slightly changed when the annealing temperature increase from 130 ℃ to 145 ℃. Alq3 begin to crystallize, and the most become crystallization presumably. In this case, the organic layer interface makes the new opportunities emerge, which lead more migration of electrons and holes. In this process, when the annealing temperature is above 180 ℃, it occurs Al cathode off under low voltage conditions, thus the properties cant not be tested. Considering the lifetime and stability, as compared with unannealed devices, the ones after annealed don,t heat even burst obviously, when the voltage is increased. The maximum current efficiency of devises is 2.38 cd/A after 130 ℃ annealed. Meanwhile, the electro-luminescence spectra is blue-shift about 30 nm,and this is the same with the spectrum blue-shift phenomenon of β-Alq3 after the crystallization producing.