Visualization of channel formation of organic field-effect transistors by scanning photocurrent microscopy

Abstract

Abstract Scanning modulated photocurrent microscopy by means of chopped red laser light for local excitation is employed in organic field-effect transistors having pentacene as active layer. The spatial modulated photoresponse presents a gradual increase from the source-to drain-side with increasing the negative gate-source voltage and undergoes enhancement under additional full device blue bias-bandgap illumination. The electric field-assisted exciton dissociation mechanism alone cannot explain the photoresponse enhancement under blue bias-light. This enhancement can be explained by considering that the electrons from the laser light-created triplet excitons have a large probability to be transferred to trapped holes accumulated at the pentacene-insulator interface creating mobile holes. In the framework of this interpretation, an increased local photoresponse reveals the channel region with upwards band bending and accumulated trapped holes from holes injected and extra holes created in case where an additional blue bias-light bandgap excitation is employed. Taking advantage of this property, the gradual increase of the spatial photoresponse along the channel, observed during turning-on the transistor with increasing the negative gate voltage, can be used to visualize the growing accumulation channel region from the source-to drain-side against the pinch-off depletion region. Moreover, the spatial photoresponse changes observed during transition from the linear to the saturation regime with increasing the negative drain voltage, can be used to monitor the suppression of the accumulation region and the increase of the electric field in the drain-side.