Near-field spectroscopy of phase segregation in white-light-emitting blends based on low-mass molecules

We report on the direct observation of phase segregation occurring in thin-film blends of a thiophene monomer and an ammino compound, used in the fabrication of organic white-light-emitting diodes. In the homogeneous and uniform regions of the films, the interaction between the two molecular components gives rise to exciplex states responsible for a broad redshifted photoluminescence emission band, which disappears in the film zones where segregation occurs. This effect has been observed with submicrometer spatial resolution by means of local spectroscopic measurements performed in a scanning near-field optical microscope.