Effect of light irradiation on the characteristics of organic field-effect transistors.

The effect of light irradiation on the characteristics of organic field-effect transistors containing sexithiophene (6-T) and pentacene was examined. Organic phototransistors (OPTs) in which 6-T and pentacene were incorporated were fabricated. Their response behaviors were investigated under conditions of irradiation by either modulated or continuous ultraviolet light with various intensities. Both devices showed two distinguishable responses, i.e., fast and slow responses from photoconductive and photovoltaic effects, respectively. The fast response is mainly the result of the generation of mobile carriers by the absorption of a photon energy higher than the band gap energy of the semiconductor and, subsequently, an increase in conductance via a greater flow of photogenerated mobile carriers into the channel layer. On the other hand, the slow response, which was confirmed by a light induced shift in the threshold voltage (V_th) or the switch-on voltage (V_O), is the result of a slow release of accumulated and trapped electrons in the semiconductor-gate dielectric interface. The V_O is defined as the flatband voltage of devices. Below the V_O, the channel current with the gate voltage is off current, and the channel current increases with the gate voltage above the V_O. The speed of release of the accumulated charge was dependent on the type of semiconductor used. Pentacene OPTs showed a particularly long retention time. Even after storage for ten days, the shifted V_O (or V_th) for the pentacene OPTs by light irradiation was not restored to the original value of the fresh devices. We conclude that this long sustained V_th shift renders them attractive for use in “light-addressable nonvolatile memory devices.” [ABSTRACT FROM AUTHOR]