Temperature-dependent electroluminescence spectra of organic light emitting diodes based on thermally evaporated bis-imido-phenylene vinylene derivative

This letter reports on the effect of temperature on the electroluminescence spectrum of organic light-emitting diodes based on a low molecular weight compound. Bis-imido-phenylene vinylene derivative has been chosen as green-emitting molecule because of its similarity with poly-phenylene-vinylene derivatives. The electroluminescence spectra are found softly dependent on the degree of crystallinity of the layer and strongly dependent on the temperature. Electroluminescence spectra are fitted with multiple Gaussian peaks corresponding to phonon replica. With increasing temperature a blueshift of the zero-phonon line is observed. This phenomenon, commonly observed with poly-phenylene-vinylene derivatives, is usually explained as a reduction of the effective conjugation length. Such a model cannot be used with small molecules due to a finite conjugation length. A model of thermally activated statistic occupation of excited states is used to fully describe the experimental data. The results definitely rule out any significant role of a reduction of the conjugation length by increasing the temperature. It is also found that increasing the degree of order of the molecular morphology induces a reduction of the width of density of excited states.