Nonlinear excited state phenomena and electro-luminescence in fullerene crystals

The characteristics of photo and electroluminescent emission from C60 crystals are reported. The photoluminescence emission is seen to increase nonlinearly with the third power of the input intensity above a threshold intensity. Associated with this nonlinear increase is the emergence of a long lifetime emission component and a redshifting of the emission spectrum. Furthermore, above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response increases with the cube of the input power. At high excitation densities, the photoconductive response becomes relatively temperature independent compared to the thermally activated behaviour observed at low intensities. The characteristics of the temperature dependence are associated with a high electron mobility phase in the highly excited state and therefore an optically driven insulator to metal transition is proposed as a description of the observed phenomena. Using gold and aluminium electrodes as contacts, a broad band electroluminescent emission, extending from 400 nm to 1100 nm is observed. The spectrum has a primary maximum at 920 nm and a weaker feature centred on 420 nm. The spectral characteristics are independent of the applied field and the longer wavelength region is identical to that measured in the high excitation density photoluminescence spectrum. In addition, the electroluminescent output intensity increases with the cube of the injection current.