Reversible memory effects and acceptor states in pentacene-based organic thin-film transistors

Hysteresis is commonly seen in the current-voltage characteristics of organic thin-film transistors (OTFTs). In the absence of memory effects associated with the gate dielectric, the hysteresis in $p$ -channel pentacene-based OTFTs, as measured in air and under illumination, was found to be dominated by trapped electrons in the semiconductor. The responsible acceptor-type traps have very long lifetimes. The immobile, previously stored negative charge requires extra holes to balance it, resulting in early establishment of the channel and extra drain current. This model is unique in that it explains the memory effect in terms of the electrostatic effect on the majority carrier population of trapped charge, opposite in sign to the majority carriers. The model is supported by drain current transient decay data. By studying the presence or absence of this effect and repeatedly measuring same devices on one substrate under various ambient and illumination conditions, we found that the responsible acceptor states are introduced by air exposure and illumination and that their formation, as well as the resulting memory effect, is reversible. Efforts were taken in the data analysis to separate the bias stress effect from the memory effect under study. The impact of this memory effect on OTFT measurement and application is discussed.