Electroluminescence in fullerene crystals

The observation of a broadband electroluminescent emission from fullerene crystals, with a spectral distribution comparable to that of the photoluminescence at high excitation densitiesis described. The emission intensity is highly nonlinearly dependent on the current. The response of the crystal to the application of an alternating current is investigated to determine the dynamic parameters of the emission process. In particular the frequency dependence of the emission intensity is described. Since the observed high frequency cut-off behaviour cannot be mimicked by a simple equivalent circuit, a rate equation model is used to describe the state of the system. Fits of the model to the observed behaviour provide rate constants which compare favourably to those reported for excited state decay in fullerenes. Observation of electroluminescence requires crystals of comparatively high conductivity and it is noted that the conductivities of different crystals from the same batch can vary by many orders of magnitude. Furthermore, when driven by high currents, the electroluminescent crystals undergo an irreversible increase in their conductance. In the region of the irreversibility, the electroluminescence output becomes unstable, and the current threshold for the onset of emission increases in a sample which has been driven to progressively higher currents increases. The process is discussed in terms of a current driven increase in the active volume of highly conducting crystalline pathways in the crystal.