Study On Performance Optimization And Influential Factors Of Organic Electroluminescence Device

Compared with other display techniques, organic light-emitting diode (OLED) possess many advantages, such as active emission, fast response, wide view-angle, and can be anticipated to play a major role in the next generation of flat panel display (FPD), the investigation of essential mechanism and characterization in organic electroluminescence (EL) field is imperative for fulfilling commercialization requirements. Consequently, this work is dedicated to the systematic study of following issues.1. Indium tin oxide (ITO) thin film was widely used as the anode of OLED due to its high electric conductivity and transparency in visual range. Numerous study showed that surface treatment via chemical or physical means of ITO films can result in significant improvement of OLED's performance like brightness, lifetime and reliability, among which oxygen plasma treatment was considered to be the most efficient way. In this work, ITO was treated by sulfuric acid as well as NaOH, oxygen plasma and ethanol. Atomic force microscope (AFM), X-ray photoelectrons spectra (XPS), surface contact-angle analyzer, and X-ray diffraction (XRD) were employed to characterize the treated ITO films, sheet resistance and transmissivity of all treated films were also measured. Results showed that sulfuric acid- and oxygen plasma- treated ITO film has a significant lower roughness and carbon contamination than ethanol-treated sample while crystalline orientation, sheet resistance and transmissivity of all the treated samples underwent neglect variation. Performance measurements of OLED devices based on all treated ITO substrates demonstrated that the sulfuric acid-treated device was superior to the oxygen plasma-treated one.2. Several models describing current mechanism have been reported in literatures, including space charge limited current (SCLC), trapped charge limited current (TCLC), Fowler-Nordheim tunneling (FNTC) and thermal emission current (TEC). Among them, TCLC theory relatively well interpreted results of I-V relations observed in OLED, which predicts the current increases according to a high-power law with the increase of applied voltage (i.e., I(?)= V^m (m>2)) on the assumption that there are exponentially distributed traps in the organic layer. To identify the reliability of TCLC theory, a series of single- (SL) and double-layer (DL) Alq3-devices with different thicknesses were fabricated and device current-voltage characteristics were measured. Linear fitting of measured I-V curves in logarithmic graph was in good agreement with the prediction of TCLC theory. In addition, device performance with different film thicknesses was compared and it was found that only the device with suitably thick organic layer(s) had an optimum performance due to enough numbers of electron- and hole-carriers participate recombination, as well as the best balance between them. Furthermore, luminance efficiency (η_lm) measurements of OLED indicated thatη_lm reached its maximum value at low applied voltage and then decreased monotonously with increasing voltage, which originates from carrier-recombination mechanism in OLED.3. Device performance using different electrodes and under various temperatures as well were studied. Measurements of I-V curves of devices further proved the generality of TCLC theory in OLED based on Alq₃. Meanwhile, results demonstrated that injection of carriers was not solely determined by the work function of electrodes since the fabrication procedure of metal electrodes may alter the intrinsic feature of organic layer at the metal/organic interface or even damage the organic layer itself, and consequently influenced carrier injection. In addition, the performance of OLED showed that SL devices with metallic anodes or A1-, Cu-cathode were greatly deteriorated due to the electrode's quenching effect of excitons while DL devices displayed higher performance as the introduced NPB hole transport layer (HTL) avoided the quenching of electrodes effectively. Moreover, microcavity effect in OLED with metallic electrodes shifted and narrowed the EL spectra of devices and optimization of device structure may further improve device performance. Finally, current of OLED increased butη_lm decreased with the increased temperatures, the latter case was considered to be due to the degradation of organic layer at high temperature.4. Influences of commonly used DMQA, DCJTB and Rubrene fluorescence dyes on doped OLED were studied. The absorption and PL spectra of the three dopants were studied and indicated that Forster energy transferring (ET) did occur between all of the dopants and Alq₃ host. Meanwhile, EL spectra of doped OLED showed that doping site and the dopant itself have a significant impact on EL spectra of devices, which can be only interpreted by direct carrier trapping (DCT) processes. Furthermore, NPB as the matrix and DCJTB as the guest were adopted to investigate light emission mechanism in OLED with different host. Results showed that ET between NPB and DCJTB is far less effective than between Akfc and DCJTB, which indicates that light emission is dominated by DCT process in NPB-host OLED.5. NPB, conventionally used as HTL, was employed to fabricate a color-tunable device with the structure of ITO/NPB (30 nm)/BAlq (10 nm)/NPB (x nm)/Alq₃ (10 nm)/MgAg, where BAlq acted as the first emission layer (EL), the NPB layer at BAlq/Alq3 interface as a color tuning layer (CTL) and Alq₃ as the second EL and also the electron transport layer (ETL). The emission color of device can be easily controlled by changing the thickness of NPB CTL. In addition, blue light-emission OLEDs based on NPB were fabricated. By optimizing device structures, a high-performance blue light-emission device, with the structure of ITO/NPB (35 nm)/Alq₃ (3 nm)/NPB (15 nm)/BCP (3 nm)/Alq₃(25 nm)/MgAg was achieved, where NPB as HTL and EL, the 3 nm Alq₃ as an emission assisting layer (EAL), BCP as hole blocking layer (HBL) and 25 nm Alq₃ as ETL, respectively. The results showed that EL spectrum of the device peaked at 440 nm and device brightness reached to 3328 cd/m² andη_lm at 100 cd/m² was 0.17 lm/W.In summary, this dissertation concentrated on the above several subjects, and it should be beneficial to further work in OLED field.