Reduced Coulomb interaction in organic semiconductors: Tailoring the effective system permittivity
A major difficulty for organic photovoltaic (OPV) cells is the dissociation of excitons into free charge-carriers. This is caused by high binding energies, which are in part the result of a low organic semiconductor permittivity. Typically acceptor-donor systems in the form of a bulk-heterojunction are used to efficiently dissociate excitons. However, because of Coulomb attraction, the separated charges may remain trapped by geminate pair formation. Here we demonstrate that charge separation in organic semiconductors can be facilitated in the vicinity of high-k materials. Investigated are pentacene based devices with semiconductor layers of varying thickness on substrates with different permittivities. Silver is used as a top contact metallization. The electrical characterization of the samples was done using current-voltage measurements in the dark and under illumination. Using a first order model to exclude the morphology influence on the charge transport, we are able to derive an improvement of the exciton dissociation in pentacene with an increase in the substrate permittivity. This result is further substantiated by an investigation on substrates with chemically identical interfaces but varying permittivities. The realization of OPV comprising an organic matrix with integrated electrically insulating inorganic high-k nanowires to effectively enhance the systems permittivity and thereby reduce the Coulomb interaction to facilitate e.g. charge pair separation is discussed.
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