Ionized impurity induced photocarrier generation in organic energy conversion systems

Due to the high density of impurities, Schottky barriers in organic semiconductors usually have quite thin depletion regions, of the order of a few hundred angstroms. For typical built in potentials of few tenths of a volt, the average field in the barrier is of the order of 10 V/μm. In this paper, the influence of the localized nature of the ionized impurities on field dependent carrier generation will be investigated. Assuming the existence of exciton diffusion, it will be shown that carrier generation due to a strong local field of ionized impurities can far exceed the generation efficiency predicted on the basis of the average electric field in the barrier. The concept of exciton drift currents will be introduced, which will lead to a unique light collection mechanism by exciton funnelling to the ionized impurities, thus increasing further the probability of photocarrier generation. Experimental evidence will be presented which indicates that the ionized impurity induced carrier generation is the dominant mechanism of carrier production in x-metal-free phthalocyanine photovoltaic cells. The analogy between the proposed mechanism and photosynthesis will be discussed. It will be suggested that the assumption of a charged ’’special pair’’ could naturally explain the low probability for the back reaction in the chain of primary photosynthetic events.