Photoleakage currents in organic thin-film transistors

We studied the light illumination effect on the performance of pentacene organic thin-film transistors (TFTs) exhibiting a field-effect mobility of 0.7 cm2 V s, threshold voltage of -7 V and on/off current ratio of 108. The transfer characteristics of the orgnaic TFT change remarkably by illumination in the subthreshold and off-state regions because of the photogenerated carriers in the TFT channel. The minimum off current increases by 105 under the illumination of 1350 lux. The drain current increases linearly with light intensity in the region where holes are majority carriers. On the other hand, the off-state currents increase superlinearly with light intensity when electrons are majority carriers. The photoleakage current increases with illumination time at the constant temperature. The results can be explained on the basis of the trapping of carriers at the grain boundaries and significant recombination of electrons and holes when electrons are majority carrier. The holes can be trapped at the grain boundaries during illumination, reducing the potential barriers at the boundaries.