Spatially resolved photoresponse measurements on pentacene thin-film transistors
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A confocal setup with a spatial resolution in the submicron regime is employed for investigating the response of pentacene transistors to local illumination. The transistors show enhanced and inhomogeneous photoresponse in the proximity of the hole-injecting contact. These inhomogeneities represent contact areas of varying injection efficiency. Thus, this technique allows imaging of contact efficiencies with submicron resolution over large areas up to hundreds of microns. Drift-diffusion simulations including a photogeneration/recombination process have been performed to model the photoresponse. The simulations illustrate that the potential drop along the channel is dramatically reduced in the illuminated area due to photoconductance (i.e. photoinjection of excitons and subsequent dissociation). Also, the injection barrier for holes is reduced if the illumination is close to the hole-injecting electrode. The rapid decay of the photoresponse with increasing distance to the positively biased electrode is caused by the limited electron mean free path in our devices.