Visualization of thermally activated morphology evolution of N,N'-di(naphthalene-1-yl)-N,N'-diphthalbenzidine films on ITO/copper phthalocyanine underlying layer

The thermally activated morphological evolution of an N,N'-di(naphthalene-1-yl)-N,N'-diphthalbenzidine (NPB) thin film on an ITO-coated glass/copper phthalocyanine (CuPc) underlying layer has been studied in situ by exploiting variable-temperature tapping mode atomic force microscopy (VT-AFM) in detail. The morphological change, assumed as an apparent glass transition of the NPB thin film, was observed to initially occur at similar to 60 degrees C, and the transition finished at about 95 degrees C. Crystallization of the NPB thin film from the glassy state quickly appeared at about 135 degrees C in N-2. The NPB thin film gradually melted and disappeared with an increase in temperature, and simultaneously revealed the ITO/CuPc underlying layer. The morphological variations of the NPB thin film have been verified by micro-Raman spectra. Moreover, it has been found that heating history and moisture seriously affect the degradation evolution of the NPB layer. These observations should shed light on the thermally activated degradation pathways and the long-term operation stability and reliability of NPB-based organic light-emitting diodes.