Diffusion-Recombination Determines Collected Current and Voltage in Polymer:Fullerene Solar Cells

The basic operating mechanisms of organic bulk-heterojunction cells comprising poly(3-hexylthiophene (P3HT): [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) blends are examined based on the selection of diverse materials as cathode contact. The use of both highly and poorly efficient cathodes in combination with impedance spectroscopy allows for a clear-cut separation of the physical mechanisms occurring within the active layer bulk from those taking place at the organic blend-contact interface. The system under study uses either Ca or CaO as the efficient and blocking contact, respectively. The impedance analysis demonstrates that charge carriers (electrons) move by diffusion within extended quasi-neutral regions, so that the cell operation is governed by the kinetic competition between charge transport toward the outer contacts and loss by recombination. It is also observed that contacts contribute with an additional, non-negligible, series resistance that accounts for the electron extraction process. The comparison between active layer thickness and the carrier diffusion length allows exclusion of bulk transport losses as a highly detrimental factor for this class of solar cells.