| Organic light-emitting device(OLED) which is likely to replace LCD to become the mainstream of next-generation display technology, has many advantages, such as being solid-state device, ultra-thin, high brightness, short response time, wide working temperature range, wide view-angle, energy-saving, environmental protection as well as flexible bending. OLED can be divided into different types in according to different standards. According to the driving mode, OLED can be divided into AMOLED and PMOLED. Although most of current OLED products are PMOLED, but AMOLED has advantages of faster response speed, more vibrant colors, higher contrast radio and larger size which PMOLED are still lacking. AMOLED will become the future development direction of OLED products.Although OLED has so many advantages, there are much technical bottleneck for OLED's development. Relative to the current mainstream LCD technology, OLED efficiency and lifetime are relatively low. OLED's efficiency is affected by many factors. That Electronic injection and transmission capacity is significantly weaker than the ability of hole is an important factor for low efficiency. Therefore, improving the electron injection and transmission capacity play a crucial role in attaining high efficiency and low drive-voltage OLED. A main way to improve the electron injection and transmission capacity is doping electronic transmission materials with n-type dopant. The conventional n-type dopants include alkali metals and the corresponding alkali metal salts, such as Li, Cs, and LiF and so on. The fatal drawback of alkali metal dopant is that it is unstable in air, so finding a stable n-type dopant is meaningful for the preparation of highly efficient OLED.Research found that Li3N as a stable substance in the air will decompose into lithium and nitrogen under specified conditions of pressure and temperature. When Li3N is used in OLED preparation, nitrogen will not affect the device performance and the alkali metal Li can be used as a good n-type dapant to improve the performance of the device .We doped Li3N in the electron transport material Alq3 in experiments and fabricated structures of Al/Li3N: Alq3/Alq3/NPB/m-MTDATA/ ITO. Then we studied the effect of doping concentration and thickness of Li3N: Alq3. The results show that when the doping concentration is 5% and doped layer thickness is 10nm the OLED presents the best brightness and efficiency: maximum brightness is 28640cd/m2 at 9V, the maximum current efficiency is 2.30cd/A at 8.5V and the maximum luminous efficiency is 1.19lm/W at 4V. Compared to the undoped device, the maximum brightness increased by 3.7 times, the maximum current efficiency is improved by 4.5 times and the maximum luminance efficiency is improved by 8.9 times. Furthermore, turn-on voltage decreases from 4.7V. To study the reason why the Li3N doping could improve the OLED performance, we prepared two single-carrier devices: ITO/Alq3/Al and ITO/Li3N:Alq3/Al. Through current density and voltage relationship measurements of two single-carrier devices, we found that the incorporation of Li3N and Alq3 effectively improves the electrical conductivity of Alq3, which means that the Li3N doping improves electronic transmission capability. The holes can be combined with more electrons, which could enhance the device brightness and efficiency.As AMOLED screen has to use amorphous silicon TFT (a-Si TFT) in the preparation of larger size screen and the a-Si only could be made into n-type TFT because of the limitations of a-Si, IBOLED may be a better choice to prepare AMOLED.We first fabricated the IBOLED structures: ITO/Li3N: Alq3/Alq3/NPB/MoO3/Al to investigate the effect of different thickness of Li3N: Alq3 layer. The results showed that when the doping concentration is 5% and doping thickness is 10nm, IBOLED exhibits the best performance. Compared to undoped IBOLED, IBOLED with the optimal doping and doping thickness improves the maximum luminance by 95 times, the maximum current efficiency by 24 times, and the maximum luminance efficiency by 26 times. Considering the Li3N doping greatly improved the brightness and efficiency of IBOLED, we prepared the conventional structures ITO/NPB/Alq3/Al and ITO/NPB/Alq3/LiF/Al to compare with the doped IBOLED. According to the results of experiments, the doped IBOLED has much better performance than that of OLED without LiF layer. Compared to OLED with LiF, the doped IBOLED showed comparable performance, especially in higher voltage, the brightness and efficiency of IBOLED are higher than the traditional OLED with LiF layer. These results will help expand the application of space.In summary, Li3N can be used as an effective n-type dopant to improve the device performance in traditional OLED or IBOLED. It's stable in the air and can be decomposed into alkali metal which could be a good n-type dopant. The fact provides new ideas for the improvement of OLED efficiency. |
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