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Carrier dynamics of composite and nanolayered films of zinc phthalocyanine and C-60 measured by time-resolved terahertz spectroscopy

Lane, Paul A.
Melinger, Joseph S.
Esentürk, Okan
Heilweil, Edwin J.
We present a study of charge transfer and carrier dynamics in films of zinc phthalocyanine (ZnPc) and buckmisnsterfullerene (C-60) investigated by time-resolved terahertz spectroscopy (TRTS). These films are model structures for charge generation layers in organic photovoltaics and their intrinsic properties are therefore of interest. We compare two classes of films: composite films of ZnPc and C-60 prepared by co-evaporation and layered ZnPc/C-60 films prepared by alternating deposition. We find evidence for a short-lived charge transfer state of C-60 that decays within several picoseconds of excitation. In contrast, both composite and multi-layered films have a long-lived THz absorption that depends on the composition and structure of the fims. The optimum composition for charge transfer within composite films is a 1:1 blend of ZnPc and C-60. Amongst the layered films, there is an increase in charge photogeneration with decreasing layer thickness with a sample having ultrathin (2 nm) exhibiting the strongest THz absorption. A much stronger THz absorption signal was obtained from the layered structure than for the best composite film, even both structures contain similar fractions of ZnPc and C-60.