Organic Light-emitting Diodes And Their Electroluminescent Characteristics

As a new research field, organic light emitting has been attracted much attention due to their potential applications in next-generation displays and lightings. The rapid development of information technology, the display technology with high performance ever-increasing demands. Organic light-emitting devices (OLEDs) have the most prominent promise of next generation of flat-panel displays due to their high luminous efficiency, low driving voltage, fast response, rich colors, visual angle, etc. In this thesis, based on the point of device physics, we predominantly focused on OLED fundamental researches such as OLED theory, performance improvement and electroluminescent characteristics.(1) For bottom OLED, the ITO surface has a certain roughness, which leads to injection barrier at the interface between the electrode and organic layer. Therefore, it is necessary to modify ITO surface and further investigate the mechanism of carriers'behavior. Firstly, composite hole injection layer with stepped energy level are applied to OLEDs. We have observed that the insertion of such composite layer leads to a striking improvement in hole injection and device performance. The luminous efficiency is 4.88 cd/A, which is about 34 % higher than other device. The result has been verified by using the J–V curves of'only'devices and further explained with the help of F-N tunneling theory. Then, P type doping as hole transporting layer are demonstrated. It is found that using this stratagem could effectively reduce hole transport and the device efficiency are greatly enhanced. The luminous efficiency is 5.22 cd/A, which is about 50 % higher than other device. The result has been explained with the help of energy level diagram. Lastly, the behaviors of carriers'in emission layer are investigated based on the principle of carrier balance. Using partial electron transport of [TBADN: DSA-ph: BPhen] to replace the light-emitting layer of [TBADN: DSA-ph], the device shows CIE color coordinates of (0.167, 0.220), maximum luminance of 6100 cd/m2 and maximum luminous efficiency to 6 cd/A. While using partial hole transport of [TBADN: DSA-ph: NPB], the device shows worse performances with CIE color coordinates of (0.184, 0.309), maximum luminance of 142.3 cd/m2 and maximum luminous efficiency to 0.9 cd/A. Our experiments show that TBADN may be a partial hole transporting light-emitting material. The improvement in the light-emitting system of [TBADN: DSA-ph: BPhen] is attributed that BPhen could benefit TBADN to capture carriers in emission layer. But NPB doping will lead to device quenching.(2) Blue OLED materials have some defects, including broad bandwidth and weak electron affinity. It results in some lags between the research and practical application in luminous efficiency and color purity for blue OLEDs. Firstly, DPVBi-based blue OLEDs with Ag2O//MoOx composite hole injection layer are investigated. Using this strategy, the device has obtained better color chromaticity, less color shift and a weak current-induced fluorescence quenching. The device shows deep blue emission with CIE color coordinates of (0.148, 0.101). Also, the power efficiency reaches 4.08 Lm/W, which is about 187.3 % higher than other device. Then, we fabricate blue OLEDs by using [DPVBi: BCzVB] as the emission layer and Alq3 as the electron transporting layer. The device shows blue emission from deep blue color [CIE (0.15, 0.12)] to sky-blue color [CIE (0.18, 0.30)]. It is found that the optimal performance of blue OLED is obtained in between the maximum luminous efficiency and the minimum color coordinate. Next, we investigate blue OLEDs based on double-dopant and double-EML structures. It is found that this stratagem could greatly enhance color chromaticity and device efficiency. At the current density of 20 mA/cm2, the device shows blue emission with CIE color coordinates of (0.155, 0.212). Beside, the luminous efficiency is 6.31 cd/A, which is about 1.36 times higher than other device. Our result shows that fine tune carrier transport with an appropriate stacking sequence promotes the balance of carriers in emission layer and thus improves device performance. Lastly, we propose blue OLEDs based on an adjustable chromaticity layer. The device has been constructed by sandwiching a thin [DPVBi: BCzVB] layer between hole-transport layer and primary emission layer of [TBADN: DSA-Ph]. Using this structure, the blue device shows a striking improvement in device perfomance with CIE color coordinates of (0.166, 0.201) and luminance efficiency of 8.43 cd/A.(3) For white OLEDs, it needs to solve some practical problems, including efficiency, color stability, quenching effect and cost. We have mainly focused on performance improvement and related theory for white OLEDs. Firstly, we present white OLEDs based on a novel double-EML consisting of blue and white emitters. Through the optimization of device structure, the device has obtained optimal performances. At the current density of 20 mA/cm2, the device shows emission with CIE color coordinates of (0.33, 0.37). Also, this device gives stable color coordinates, i.e., the maximum color shift is less than 0.02 units on CIE color coordinates at the current density range of 4 to 200 mA/cm2. The maximum luminance reaches 21,044 cd/m2 at a driving voltage of 17 V, and the maximum luminance efficiency achieves 9.12 cd/A at the luminance of 292 cd/m2. According to the theory of excitons generation and diffusion, we have set up an equation that relates EL spectra to the thickness of two emitters and to the exciton diffusion length. We make an analysis for electroluminescent characteristics of white OLED by using the above equation. Then, white OLEDs with double-graded structure have been investigated. This strategy can greatly improve color stability. The optimized device gives a negligible CIE color shift of⊿x, y=±[0.000, 0.001] from 4 to 200 mA/cm2. Next, white OLEDs with C545T doped emitting system have been investigated. The Forster's radius has been given to clarify actual energy transfer process. Our result suggests that the energy transfer from DPVBi to DCJTB via the intermediation of C545T seems to be more dominant. Finally, white OLED using Alq3 as green emission has been demonstrated. Our experiments indicate that Alq3 doping not only improves color coordinates and color stability, but also reduces current-induced fluorescence quenching. For comparison, white OLEDs with different components as the green emission and without the component of green emission are also fabricated and investigated. Our results show that the introduction of green emission with an appropriate component significantly improves device efficiency and color properties. This result has been further verified based on the theory of exciton generation and diffusion.