Robust organic functional materials by thermally doping with metal oxide

Abstract

We have investigated the failure mechanism of organic functional materials and organic light-emitting diodes (OLEDs) by annealing at high temperatures. We found that N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) doped molybdenum oxide and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene doped cesium carbonate can enhance the thermal stability significantly. The former composite film reveals the ions of NPB, as observed by X-ray photoelectron spectroscopy (XPS), the formation of which shows that NPB receives the electron that Mo loses. Meanwhile, it is stable for the binding energy of the element in the latter composite film from the XPS image. Through the research of carrier-only cells, the observation indicates that the thermal stability of the doped cell is better than that of the undoped cell at high temperatures. The current efficiency of the doped device is only reduced by 12% after annealing at 80°C; meanwhile the lifetime reaching 208 h is the longest among that of the devices. Simultaneously, the undoped device represents a larger decline even of about 30% with the lifetime reaching just 40 h. We assumed that the enhanced heat-resisting properties of organic materials by inorganic doping might be attributed to the decrease of energy barrier and the reduction of the interface charge accumulation phenomenon caused by high temperature. Inorganic doping paves an alternative way to substitute for synthesizing expensive functional materials with high glass transition temperature.