Fullerene-conducting polymer composites: intrinsic charge transfer processes and doping effects

The processes of charge transfer (CT) between fullerene and conducting polymer (CP), both intrinsic photoinduced CT and extrinsic CT due to strong n-type (alkali-metal) dopants, are analyzed from a general point of view and illustrated by new experimental results. The origin of strongly suppressed backward recombinative CT is discussed in terms of polaronic effects on C60 spheres and CP chains, as due to the orientational self-trapping barrier caused by modulation of π-π overlapping. Upon external CT provided by alkali-metal doping, the superconducting (SC) phase is found in poly (3-alkylthiophene)-C60-K composite: PAT-(C60)y-Kx at y > 0.005. The SC transition temperature Tc ranges from 12 to 17 K depending on y and x. Exceptionally strong low field microwave absorption (LFMA) shows phase reversal just below Tc in SC composites with large C60 content (y > 0.025), indicating the granular nature of the SC phase in which K3C60 clusters are weakly coupled through Kx-PAT intergranular barriers. The anomalous phase of LFMA and the paramagnetic Meissner effect, which are found at T Tc for certain values of y, suggest the existence of Josephson intergrain π-junctions. Spin-carrying polarons P+ in chains of PAT separating K3C60 granules are suggested as the possible origin of such π-junctions. The possibility of a two-component SC phase in which electrons of CP chains are actively involved in SC pairing induced via hybridization with C60 molecules (which play a role of negative U centers) is analyzed and strategies for the search of such a two-component SC phase are discussed.