Spatially resolved extrinsic photoresponse under additional intrinsic bias photoexcitation of two terminal pentacene devices

Spatially resolved extrinsic photoresponse experiments in pentacene two terminal devices without or under additional intrinsic bias-light excitation are employed. These experiments are used to investigate the microscopic mechanism of the recently observed phenomenon of the photoresponse enhancement under additional bias-light intrinsic excitation and to preclude that this phenomenon arises from contact-related artifacts. It is found that the extrinsic photogeneration near the contacts via electric field-assisted exciton splitting and/or light-induced depletion width-reduction have a negligible contribution to the extrinsic photoresponse. Under additional bias-light intrinsic excitation, a uniform increase of the photogenerated hole density is found to take place across the whole conduction channel, without changes in the electric field distribution and in the interfacial properties of the contacts. The photoresponse enhancement by the blue bias-light becomes stronger upon increasing the red-light intensity. A nearly square root dependence of the photoresponse enhancement on the blue bias-light intensity is found. It is shown that the observed dependence of the photoresponse enhancement on the light intensities of the extrinsic and intrinsic excitation can be explained with the extrinsic photogeneration mechanism based on hole detrapping by triplet exciton dissociation.