Nanoscale dynamic inhomogeneities in electroluminescence of conjugated polymers

We report the observation and characterization of dynamic spatial heterogeneities in the electroluminescence (EL) of conjugated polymer organic light-emitting diodes via high-sensitivity fluorescence microscopy. The active layers of the single-layer devices are polymers of the poly(phenylene vinylene) family, i.e., poly2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene vinylene and a commercially available copolymer, Super Yellow. The devices are prepared directly on a microscope coverslip, making it possible to use high numerical aperture oil-immersion objective lenses with a diffraction-limited resolution of a few hundred nanometers for microscopic EL imaging. Detection via high-sensitivity CCD camera allows the measurement of EL dynamics with millisecond time resolution for a wide range of applied voltages. We found spatial heterogeneities in the form of high EL intensity sites in all devices studied. The EL from these sites is strongly fluctuating in time, and the dynamics is bias voltage dependent. At the same time, there is no difference in the local microscopic EL spectra between the high- and low-intensity sites. The results are interpreted in terms of a changing charge balance and local structural changes in the active film layer.