Scanning force microscopy of organic thin-film amorphous hole transport materials

Abstract

Amorphous organic thin films of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)- 4,4′-diamine (TPD) and 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) which function as hole transport materials were fabricated on slide glass plates by vapor deposition. By heating the films in air, the change in photoluminescence (PL) spectra for TPD was little, while that for m-MTDATA was significant and a new broad peak appeared in the longer-wavelength range. The latter change was pronounced in an oxygen atmosphere, but was suppressed in a nitrogen atmosphere. To clarify the difference in the effect of the storage conditions on the PL characteristics between TPD and m-MTDATA, these two films were also studied by the scanning force microscopy techniques of atomic force microscopy (AFM) and friction force microscopy (FFM). It was found from the observation by AFM that the m-MTDATA films showed no change in topography, while the TPD films were crystallized easily at room temperature in the air. However, it was found by FFM that the m-MTDATA film stored at 80 °C above Tg showed crystal-like structure, which was difficult to detect by topographic AFM imaging due to little morphological change. It was finally concluded from the above results together with FFM after scratching the film surface with the AFM tip(i) that the crystal-like structure was not due to crystallization of m-MTDATA itself, but due to the oxidation product of m-MTDATA, because the crystal-like structure was observed only near the surface; (ii) that m-MTDATA is better than TPD in terms of difficulty in crystallization of their amorphous films by heating, but is inferior to TPD in terms of oxidation and the film requires protection from atmospheric oxygen; and (iii) that FFM can be used to observe the reaction products over the film surface on the nano scale. Interdiffusion for m-MTDATA by heating in two layered films with tris(8-hydroxyquinoline) aluminum was more suppressed than that for TPD due to higher Tg and lower thermal structural change of the m-MTDATA film.