Competition between Charge-Transfer Exciton Dissociation and Direct Photocarrier Generation in Molecular Donor-Acceptor Compounds

The interfacial charge-separation and photovoltaic characteristics of a molecular donor-acceptor charge-transfer compound were examined. Measurements of laser beam-induced currents on the single crystals allowed selective detection of hole and electron photocurrents through the metal-semiconductor interfaces. This method also reveals the exceptionally long diffusion length of 20 mu m in the crystal. The transition from charge-transfer exciton dissociation to direct photocarrier generation is discussed on the basis of the photon-energy-dependent diffusion length and photon-to-current conversion spectrum.