Electrochemically intercalated MxC60 thin films in a solid state cell (M = Li,K): Optical and photoelectrochemical characterization

Solid-state electrochemical cells have been prepared with C₆₀ vacuum-evaporated thin films, a Li- or K-source counter electrode and a polymer PEO-LiClO₄ (PEO-KClO₄) electrolyte. The electrochemical intercalation in C₆₀ of Li⁺ (or K⁺) ions has been performed under constant current conditions up to a formal stoichiometry of the fulleride film equal to Li₁₂C₆₀ (K₅C₆₀). A complete charge-transfer pocess from the intercalated alkali to the alkali-metal compound has been assumed. Several quasi-equilibrium potential plateaux were observed during intercalation, that we associate with the coexistence of phases with different intercalant concentration. The electrochemical intercalation process is irreversible to a large extent. Optical and photoelectrochemical spectroscopy of the fulleride films was done in-situ at different moments of the intercalation reaction by illuminating the film electrodes through the transparent and conducting glass substrates. The photo electrochemical spectral response agrees well with the optical absorption spectra, both indicating a bandgap of 2.2 eV. The photoelectrochemical response shows a minimum forx = 3 (in the K_xC₆₀ compound) and a maximum atx = 4 (in both K_xC₆₀ andLi_xC₆₀ compounds), in agreement with previous conductivity results. NIR diffuse reflectance spectra of the solid-state cell show absorption bands in the fulleride films at a wavelength of 1100 nm. A band-energy diagram has been proposed for the M_xC₆₀/PEO-MClO₄/M electrochemical cell.On leave from Instituto de Fisica, Facultad de Ingenleria, C.C. 30, 11000 Montevideo, Uruguay