Scanning probe microscopy studies of solid-state molecular electroluminescent devices based on tris(2,2′-bipyridine) ruthenium(II) complexes

The charge injection, transport, and light emission of a solid-state molecular electrochemiluminescent thin (100 nm) film of tris(2,2'-bipyridine) ruthenium(II) complex, Ru(bpy)(3)(ClO4)(2), was investigated with tuning-fork-based scanning probe microscopy (TFSPM) in combination with electroluminescence and current measurements as a function of potential. Spin-cast [Ru(bpy)(3)(ClO4)(2)] thin films are shown to contain rather uniformly distributed nanostructures. These nanostructures produce heterogeneity in the current and luminescence responses of the films. Both single and double charge carrier (unipolar and bipolar) injection can take place in these thin films, depending on the magnitude of the bias voltage. Potential-step transients allow the determination of the effective diffusion coefficients of charge carriers. The effective diffusion coefficients of electrons or holes depend strongly on the electric field strength (or the applied voltage). We have also used the TFSPM to penetrate the [Ru(bpy)(3)(ClO4)(2)] thin film to estimate the local thickness of the film.