Alkali-metal doping of fullerene-conducting polymer composite: evolution of conductivity and ESR

Doping effects of conducting polymer (CP)-C60 composites by n-type dopants, namely, by alkali metals, are investigated, emphasizing the electron spin resonance (ESR) spectroscopy together with conductivity changes upon K-doping of poly(3-alkylthiophene) (PAT)-(C60)y composites at various fullerene concentrations y. The ESR shows two types of lines: one from polarons on PAT chains, and the other from K3C60 clusters and monomolecular C60 radicals. The evolution of conductivity suggests that, at comparatively large C60 content y = 0.05, the composite can be viewed as a network of C60 clusters in the PAT host matrix at the initial K-doping stage. K3C60 metallic islands are formed, while the PAT matrix is still insulating so that increase of conductivity is slower than in the pure PAT case. These results are consistent with the granular nature of superconducting phases found by us in PAT(C60)yKx at y ≥ 0.005 by low-field microwave absorption (LFMA).