Modeling bilayer polymer/fullerene photovoltaic devices

Abstract

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 (C60). Under monochromatic light of different wavelengths we measure the current-voltage (I–V) characteristic of diodes with variable thickness of the C60 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 C60 layer behaves like a photoconductor under illumination. By using the electrical conductivity of the C60 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 C60 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 C60 film to the diode photocurrent.