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

摘要

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 C_60 spheres and CP chinas, as due to the orientational self-trapping barrier caused by modulation of pi-pi overlapping. Upon external CT provided by alkali-metal doping, the superconducting (SC) phase is found in poly (3-alkylthiophene)-C_60-K composite: PAT-(C_60)_y-K_x at y>0.005. The SC transition temperature T_c ranges from 12 to 17 K depending on y and x. Exceptionally strong low field microwave absorption (LFMA) shows phase reversal just below T_c in SC composites with large C_60 content (y>0.025), indicating the granular nature of the SC phase in which K_3C_60 clusters are weakly coupled through K_x-PAT intergranular barriers. The anomalous phase of LFMA and the paramagnetic Meissner effect, which are found at TT_c for certain values of y, suggest the existence of Josephson intergrain pi-junctions. Spin-carrying polarons P~+ in chains of PAT separatng K_3C_60 granules are suggested as the possible origin of such pi-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 C_60 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.