Photogeneration and transport of charge carriers in a porphyrin p/n heterojunction

Using impedance spectroscopy, the formation of a depletion layer is demonstrated upon contacting two films of different types of porphyrins. This depletion layer can be described in the same way as in a conventional p/n heterojunction of inorganic semiconductors. From Mott-Schottky plots the doping concentration is found to be {≈}10¹⁷ cm^-3 for electropolymerized ZnTHOPP films and {≈}10¹⁹ cm^-3 for spin-coated H₂TMPyP films. Since the photocurrent action spectra are independent of the thickness of either of the layers, we conclude that charge separation and thus the photoactive part of the cell are confined to the interface of both layers. For charge collection, consecutive dissociation of the electron-hole pair competes with charge recombination. The internal field over the space charge layer causes the photoinduced charge carriers generated at the interface to drift through the bulk layer to the electrodes.