Study And Fabrication Of High-efficiency Organic Light-emitting Diodes (OLED)

Organic light-emitting diode (OLED), due to its advantage of high brightness, full color, low power consumption, and easy to fabricate, is considered as an important candidate for next-generation display and lighting technology. To fabricate OLED with high efficiency can make it work at low drive voltage and low current to increase lifetime and save energy, which is necessary to practical applications.In this work, electron injection behavior at cathode interface in OLED, new-type of organic electron-injecting materials, and simple but efficient light-outcoupling technique have been studied, and high-efficiency OLEDs have been fabricated with improved electron injection and improved light extraction.The electron injection behaviors of two materials, LiF and Liq, were compared in OLEDs. It is found that LiF as the commonly used electron injection material does not work well in some OLEDs. In stead, Liq exhibits better electron injection capability in each of the investigated OLEDs. The injection mechanism of the two materials has been investigated by using photovoltaic measurement to determine the injection barrier height at the cathode interfaces.A series of sodium quinoline derivatives were synthesized and utilized as electron injection materials in OLED to reduce barrier height, to balance carrier recombination, and to improve device efficiency. These materials were characterized by techniques of H1-NMR and UV-Vis and calculated by computer simulation to analyze the potential influence of molecular structure on electron injection.Furthermore, a simple sol-gel method to make light scattering layers has been investigated. The combination of light scattering layer with roughened surface by grinding is applied to increase light outcoupling. Based on the studies of the effect of film morphology, transmittance, particle size, and roughness on light extraction in the devices, a maximum enhancement of about 59% has been achieved with the TiO₂ size of 100 nm. In the aforementioned studies, the new electron injection materials, made of cheaper sodium metal instead of other expensive alkali metals (such as Li and Cs) and alkaline earth metals (such as Ba), could also have good electron injection capability. Moreover, the combination of scattering layer with roughened substrate surface not only greatly improves the light outcoupling, but also provides a cheaper and easier way than other methods like 2D photonic crystal and microlens array to make. Those low-cost ways to improve light-emitting efficiency is meaningful to the commercialization of OLEDs.