NONLINEAR EXCITED-STATE PHENOMENA AND ELECTROLUMINESCENCE IN FULLERENE CRYSTALS

The characteristics of photo and electroluminescent emission from C60 crystals are reported. The photoluminescence emission is seen to incre ase nonlinearly with the third power of the input intensity above a th reshold intensity. Associated with this nonlinear increase is the emer gence of a long lifetime emission component and a redshifting of the e mission spectrum. Furthermore, above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response inc reases with the cube of the input power. At high excitation densities, the photoconductive response becomes relatively temperature independe nt compared to the thermally activated behaviour observed at low inten sities. The characteristics of the temperature dependence are associat ed with a high electron mobility phase in the highly excited state and therefore an optically driven insulator to metal transition is propos ed as a description of the observed phenomena. Using gold and aluminiu m electrodes as contacts, a broad band electroluminescent emission, ex tending 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 spectra l 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 electroluminesce nt output intensity increases with the cube of the injection current.