Weak intrinsic charge transfer complexes: A new route for developing wide spectrum organic photovoltaic cells

Solar emission extends in the near IR and one of the main issues in designing organic solar cells resides in extending the response into the near IR. Here we show that this may be achieved by making intimate interpenetrated networks Of C(60) and Zn-phthalocyanine (Zn-Pc) in the solid. Various spectroscopic investigations of co-sublimated thin films Of C(60) and Zn-phthalocyanine give indeed ample evidence of the existence of a weak charge transfer (CT) state at 1.4 eV, which quenches the photoluminescence of both molecules. The films produced by co-sublimation undergo to a spinodal decomposition producing domains prevalently constituted by Zn-Pc in contact with domain prevalently Of C(60). The domains size depends on the deposition conditions (rate, stoichiometry, and substrate temperature) forming a percolating 3D network. The separation in different domains is confirmed by the observation of two overlapping peaks, in the resonant Raman spectrum, that correspond to the A(g)(2) pinch mode (C-C double bond stretching) for pristine C(60) and for a partially (similar to 0.25e(-)) doped one. This indicates that only those donor molecules at the grain boundary, which are in contact with C(60), give rise to a renormalized new CT ground state. Photocurrent measurements of interpenetrated networks Of C(60) and Zn-Pc show a linear dependence with respect to the incident light as a consequence of direct absorption within the CT state. The CT state favors the charge separation between the two components, when it is inserted as inter-face in the organic photovoltaic p-n junction thus increasing the efficiency of the device. (C) 2002 American Institute of Physics.