Highly Efficient Organic Light-emitting Diodes Based On Electrically N-Doped Transport Layer

Organic light-emitting diodes (OLEDs) have attracted considerable attention due totheir possible application for self luminescence, large view angle, quick response,simple production process, low cost, flexible in flat-panel displays and as solid-statelighting sources. However, some problems, such as stability and lifetime, still need toresolve. In this paper, structure and manufacture craft of OLED are discussed, expectingto increase the performances of display.Green organic light-emitting devices with different structures were fabricated.Based on these devices, relevant mechanisms were studied. The organic films, thedoped layers and aluminum cathode layers are fabricated by vacuum vaporizationmethod deposited. Keithley2400-PR655test system which is controlled by computer isused to measure the luminance-current-voltage (L-I-V) curves of the device.First, the effect of ITO anode surface on the performance of OLED is explored.Thus, various surface treatments of ITO have been attempted to change the workfunction of ITO in order to reduce the hole injection barrier height. Then the effect ofthe hole transporting layer NPB and emitting layer Alq3’ thicknesses on the performanceof OLED are discussed. Observing and comparing the luminance and luminescentefficiency of OLED confirm the optimized thicknesses of different organic materials.Because the hole mobility was larger than the electron mobility in the organicsemiconductor, which would made the number of electrons and holes injected into theemitting layer be not balanced. Considering that the carrier density mobility in organicelectron transporting semiconductor is very low compared with the inorganicsemiconductor, which affects the efficiency and luminance of organic light-emittingdevices. To improve the performance of OLED, the electron injecting and transportingability should be enhanced, and n-type electrically doping can enhance the electrontransporting ability of the organic materials. ZnO was used as n-type dopant andAlq3:ZnO doped layer was used as the electron transporting layer for OLED. Theoptimal doping concentration and the thickness were7.5wt%and10nm. The maximum luminance of the device is4134cd/m2and the maximum current efficiencyof the device is3.35cd/A.Single-carrier devices were prepared in order to discuss the effect of n-type dopanton the electron mobility. The structure of the devices: device A: Al (100nm)/Alq3(65nm)/Al (100nm); device B: Al (100nm)/Alq3: ZnO (65nm,7.5wt%)/Al (100nm).By comparing the I-V curves of single-carrier devices, we know that the conductivitiescan be raised many orders of magnitude above the conductivity of nominally undopedmaterials.