Fabrication And Investigation On Organic Light-Emitting Diodes With High Performance

Organic light-emitting devices (OLEDs) are initially commercialized in the field of plat panel display and solid state lighting. However, some problems are still existed in this field, especially for the urgent need to further improve the efficiency and lifetime of the white OLEDs, which are the hot spots in the field of optoelectronics. In this thesis, two ways including interface modification and new device structure are used to achieve high efficiency OLEDs:firstly, the monochrome (such as green, yellow, and red) OLEDs are fabricated to understand the related mechanism on the road to get an high efficient OLEDs and to suggest a way to realize a high efficient multicolor OLEDs (such as white OLEDs); secondly, a high efficient white OLEDs is achieved based on the pervious investigation on the monochrome OLEDs. Meanwhile, the electroluminescent (EL) properties of the devices as well as the physical processes in the devices are investigated. Several innovative results are achieved, which deepens the understanding of the physical processes in the OLEDs, and promotes the development of new type flat panel display and solid state lighting in the future. The main contents of the paper include the following four aspects:(1) A simple green OLED with high efficiency and long stability is fabricated and studied using indium trichloride (InCl3) to modify the indium tin oxide (ITO) anode, which suggests a way to fabricate a high efficient white OLEDs. It is found that the ITO anode thermally deposited by an ultrathin film of InCl3can further improve the hole injecting ability and get a green OLED with higher efficiency compared with the conventional processing method using UV ozone treatment. The maximum current efficiency of the device was82.2cd/A (2000cd/m2), and the efficiency was enhanced by36%compared with the conventional UV ozone treatment at a current density of20mA/cm2. More importantly, the lifetime measured at an initial luminance of1000cd/m2was improved more than threefold compared with the traditional processing method. Through the investigation of the ITO/InCl3interface by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and the bias-and temperature-dependent current density-voltage characteristics in the related hole-dominated devices, it is demonstrated that the improved device performance is mainly attributed to the enhanced hole injection resulting from the lowered hole injection barrier height in the InCl3-modified devices.(2) A yellow OLED with high efficiency and long stability is fabricated and studied by inducing a double-exciton blocking layer (d-EBL) between the hole transport layer (HTL) and the light-emitting layer (EML), which suggests a way to fabricate a high efficient white OLEDs. It is found that both the efficiency and lifetime of the device with a d-EBL inserted are improved by twice compared with the traditional device with single-exciton blocking layer (s-EBL). The maximum current efficiency and maximum power efficiency were58.5cd/A and50.9lm/W (117cd/m2), respectively. The investigation of the EL spectra of the doped and non-doped devices indicates that the insertion of the d-EBL confines the electrons and excitons better, thereby enhances the efficiency and lifetime of the devices.(3) A red OLED with high efficiency and long stability is fabricated and the related comparative study on the inorganic material (MoO3) and organic material (HAT-CN) as p-type dopants are conducted, which suggests a way to fabricate a high efficient white OLEDs. The investigation shows that the organic material HAT-CN doped in the hole-transporting material (NPB) is superior to the inorganic material MoO3doped one with further improvement in the efficiency and stability of the devices. The mechanism is investigated as follows. Combining with absorption spectral measurement, the effectiveness of a p-doped NPB in OLEDs does not solely rely on the generation of charge-transfer complexes in the doped NPB. Moreover, the detailed difference between MoO3and HAT-CN as p-dopants in NPB is further investigated by evaluating the hole injection efficiency and the hole barrier height. And it indicates that the hole injection efficiency is further enhanced and the hole injection barrier height is further reduced by the organic material (HAT-CN) doping compared to the inorganic material (MoO3) doping.(4) Based on the investigation of the monochrome OLEDs, a highly efficient white OLED has been demonstrated with a mixed interlayer between fluorescent blue and phosphorescent yellow emitting layers, and the relationship between the device structures and the EL properties of the OLEDs are investigated as well. It shows that the EL performance of the OLED can be modulated easily by adjusting the ratio of the CBP (a hole predominated transport material) to the TPBi (an electron predominated transport material) in the mixed interlayer. The enhanced EL performance is attributed to the enhanced charge carrier transporting, optimized excitons distribution and improved harvestings of singlet and triplet excitons between the fluorescent light-emitting layer and the phosphorescent light-emitting layer. It is found that the white OLED with a CBP:TPBi ratio equal to3:2exhibits a current efficiency of34cd/A and a power efficiency of29lm/W at1000cd/m2with warm white CIE(0.4273,0.4439).