Novel Optical Properties of Fullerene Doped Conducting Polymers: Scenario of Photo Process, Persistent Photoconductivity and Enhanced Electroluminescence Quenching

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Abstract
Earlier we found that photoluminescence (PL) in conducting polymer with non-degenerated ground state (NDGS CP) is quenched remarkably upon doping by small amount of fullerenes such as C60 and C70. Here we show that electroluminescence is quenched even much more stronger indicating that each C60 molecule acts as a multiple trapping and/or recombination center for carriers. On the other hand photoconductivity (PC) in NDGS CP and also in conducting polymer with degenerated ground state (DGS CP) such as polyacetylene derivatives is enhanced significantly upon fullerene doping at the excitation of (a) the wavelengths of the inter-band transition in conducting polymer and (b) allowed and (c) forbidden optical transitions in fullerene. Remarkable polarity effect is observed in the spectral response of enhanced photoconductivity, proving that positively charged mobile carriers in polymeric chains contribute to PC. The effect of persistent photoconductivity at room temperature is found in C60/CP (conducting polymer) nanocomposites and we present first results on its dependence on C60 concentration, light intensity and temperature. These unique characteristics are discussed in frames of the general scenario of photogeneration enhancement in NDGS and DGS polymers due to photo-induced (or injection controlled) charge transfer between conducting polymers and fullerenes, emphasizing the role of self-trapped excitations: exciton-polarons, polarons and charged solitons in polymer main chain and string type polarons and bipolarons in C60 spherical molecules. It is suggested that bipolarons of C60′ BP2- c, play important role in persistent photoconductivity and EL quenching due to their long lifetime.