ELECTROCHEMICALLY INTERCALATED M(X)C(60) THIN-FILMS IN A SOLID-STATE CELL (M=LI,K) - OPTICAL AND PHOTOELECTROCHEMICAL CHARACTERIZATION

Solid-state electrochemical cells have been prepared with C-60 vacuum- evaporated thin films, a Li- or K-source counter electrode and a polym er PEO-LiClO4 (PEO-KClO4) electrolyte. The electrochemical intercalati on in C-60 Of Li+ (or K+) ions has been performed under constant curre nt conditions up to a formal stoichiometry of the fulleride film equal to Li12C60 (K5C60) A complete charge-transfer pocess from the interca lated alkali to the alkali-metal compound has been assumed. Several qu asiequilibrium potential plateaux were observed during intercalation, that we associate with the coexistence of phases with different interc alant concentration. The electrochemical intercalation process is irre versible to a large extent. Optical and photoelectrochemical spectrosc opy of the fulleride films was done ''in-situ'' at different moments o f the intercalation reaction by illuminating the film electrodes throu gh the transparent and conducting glass substrates. The photoelectroch emical spectral response agrees well with the optical absorption spect ra, both indicating a bandgap of 2.2 eV. The photoelectrochemical resp onse shows a minimum for x = 3 (in the KXC60 compound) and a maximum a t x = 4 (in both KXC60 and LiXC60 compounds), in agreement with previo us conductivity results. NIR diffuse reflectance spectra of the solid- state cell show absorption bands in the fulleride films at a wavelengt h of 1100 nm. A band-energy diagram has been proposed for the M(X)C(60 )/PEO-MClO(4)/M electrochemical cell.