Based On Long-wave Emission Of Electroluminescent Light-emitting Devices And Their Associated Performance

Organic light emitting diode (OLED) has potentailly to be the candidate for next-generation display along with the current liquid crystal displays (LCDs) and plasma display panel (PDP) due to its high luminance and efficiency, full solid self-emission, wide view-angle and temperature range, ultrathin thickness, fast response and the possibility to realize flexible display. As a novel energy-efficient lighting method, white organic light emitting diode (WOLED) also attracts wide attention because of the global energy crisis. However, there are still several problems for the application of the OLED. During the fabrication of blue OLED, low energy emission located at green or red area was always detected in the electroluminescence (EL) spectra, which affects the color purity of the device seriously. So study on the low energy emission in OLED has great significance to control and enhance the color purity of the devices. Consequently, this work is dedicated to study the following issues.(1) Aiming at the long wave emission from the EL spectra of poly-(N-vinylcarbazole) (PVK), the influence of ultraviolet (UV) light on the photoluminescence (PL) spectra of PVK in different solvent, especially on the PL spectra and absorption spectra of PVK in chloroform and the following casted film, was investigated, and the EL spectra of PVK spin-coated from chloroform were also characterized. It is found that the influence on the PVK in chloroform and the following casted film is much stonger than other solvent. As the UV light irradiation time increase the PL spectra, especially in the PVK chloroform solution, redshift and the red long wave emission in EL spectra were enhanced. Additively, using BCP and TPBi as the electron transporting layer (ETL), respectively, double-layer devices with PVK as emission layer (EML) were fabricated, and the optical and electrical properties were compared, which indicated that the devices using TPBi as ETL have a better performance.(2) The influence of doping TPD on the long wave emission from PVK was analyzed, double-layer devices based on PVK:TPD doping system using BCP and TPBi as the electron transporting layer (ETL), respectively, were fabricated. And after comparing the performance of the two devices, it is found that stable blue and red emission with equal intensity could be obtained from the device, whose structure is ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Mg:Ag. Based on this double-layer devices, WOLED with structure of ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Alq3 (20 nm)/Mg:Ag were designed. By changing the doping rate of PVK and TPD, the EL spectra were regulated. Stable white light emission could be obtained from 11 V to 15 V when PVK and TPD were blended at the rate of 1:4. The influence of doping rate on the electrical properties indicated that the performance of the WOLED was improved as the proportion of TPD increased.(3) Small molecular fluorene derivative BFLBBFLYQ was investigated in this work. The absorption and PL spectra of the BFLBBFLYQ:TPD doping system in solution and film were analyzed, and the EL spectra of the OLED base of the doping system were also investigated. Long wave emission existed in the PL spectra of the doping system film and the EL spectra of the devices, the reason of which was studied in this work too. Additively, the influence of doping TPD on the current density and luminance of the devices was also discussed. Double-layer device with the structure of ITO/BFLBBFLYQ:TPD (50 nm)/Alq3 (40 nm)/Mg:Ag was fabricated. As the doping system and Alq3 has the same emission spectra, the carrier combination zone was studied by doping red fluorescent dye DCJTB into different position of the devices. Single-layer and double-layer devices base on BFLBBFLYQ were studied too, and a long wave emission at 590 nm was detected in the EL spectra of double-layer device but not existed in the single-layer device. It is found that the long wave emission may be the result of electroplex between BFLBBFLYQ and BCP, and it could be removed by doping PVK into BFLBBFLYQ.In summary, this work focused on the systematic study on the optoelectronic properties of vovel materials based carbazole and fluorene functional groups, which paved the way on the fabrication of color stable high performance blue OLED.