Study Of Cathode Interface Layers And Their Applications In Solution Processed Organic Light-emitting Diodes

Organic light-emitting diode with its own advantages has attracted extensive attention in many research institutions because of the hope of applying OLED technology in flat panel displays and for lighting purposes. There are two types of OLED fabrication processing: vacuum evaporation and solution processing. Compared to vacuum evaporation techniques, solution processing is expected to reduce the processing cost and realize easily large-scale production. However, the solution processed devices still has some technical problems, such as the inferior efficiency, terrible stability and short lifetime. Based on existing materials, the study of the device structure and the physical mechanism is an effective way to improve the performance of the device. Organic light-emitting diodes(OLEDs) are “dual-injection” light emission devices whose performance strongly depends on the balanced charge densities inside the device achieved by the efficient bipolar carrier injection. Since most organic light-emitting materials have low LUMO(lowest unoccupied molecular orbital) level, the injection of electrons from the cathode is extremely critical to balance the charges. Reducing the electron injection barrier by modifying the organic/metal interface has been proved to be an effective approach. In our contribution, we have to improve the efficiency of the device by modifying cathode interface layer with commercial light emitting material P-PPV and simple device structure.Basing on high efficiency polymer light-emitting materials, low turn-on voltage and high efficient inkjet-printed polymer light-emitting diodes(PLEDs) are achieved by multifunctional buffer layer. The buffer layer is fabricated by mixing a novel crosslinkable ETL material poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene)](PF3N-OX) and curable epoxy adhesive. It offers the functions of solvent-proof, electron-injection, and affinity with the cathode ink. The turn on voltage of printed PLEDs was down to 3.25 V, and the maximum current efficiency(LEmax) reached 8.77 cd/A, compared to the devices of vacuum evaporation, the turn-on voltage reduced 2.25 V, and the maximum luminance of the device improved by 70%. More important, ink-jetting conducting nano-particles as cathode can provide high-resolution cathode patterns and excellent continuity along the fine cathode without any mechanical pressure on the organic layers.By novel solvent treatment on top of the organic active layer, significantly enhanced EL performance in organic light-emitting diodes(OLEDs) with aluminum(Al) electrode was achieved. Without any electron-injection-layer(EIL), the devices treated with ether solvents exhibited a maximal current efficiency of 19.54 cd A-1 at 5.25 V, which is more than two orders of magnitude higher than that of a similar OLEDs without the ether solvents treated. The enhanced efficiency resulted from the reduction of electron injection barrier height at the organic layer/cathode interface, which is confirmed from the photovoltaic measurements. The result from X-ray photoelectron spectroscopy(XPS) indicated the formation of a carbide-like layer after vacuum thermal depositing of Al in OLEDs with ether solvents treatment, and this is responsible for the injection of electrons through the Al cathode.