Modeling bilayer polymer/fullerene photovoltaic devices

We investigate the transport properties of organic photovoltaic devices formed by a heterojunction of a semiconducting polymer poly {3-[4(')-(1('),4('),7(')-trioxaoctyl) phenyl] thiophene} and the fullerene (C-60). Under monochromatic light of different wavelengths we measure the current-voltage (I-V) characteristic of diodes with variable thickness of the C-60 layer. We propose an analytical model assuming that; (i) holes are created in the polymer by charge carrier generation at the heterojunction; and (ii) the C-60 layer behaves like a photoconductor under illumination. By using the electrical conductivity of the C-60 layer as fitting parameter we reproduce quite well the experimental data, including the I-V curves and the changes of the open-circuit voltage with the variation of the C-60 layer thickness. We show that the values of the conductivity are closely related to the fullerene optical absorption coefficient, implying a large contribution of the C-60 film to the diode photocurrent. (C) 2004 American Institute of Physics.