Electron trapping in pentacene based p- and n-type organic field-effect transistors

We report on electron trapping in the channel of pentacene field-effect transistors due to electron injection over large energy barriers. Using scanning Kelvin-probe microscopy, one observes temporary and permanent electron trapping in the transistor channel for both $n$ - and $p$ -type organic field-effect transistors driven in ambipolar mode, proving the ability of electrons to be injected from Au contacts into pentacene despite a large energy barrier. The trapped negative charge carriers are determined to be the origin of hysteresis in $n$ -type devices and appear to be the source of threshold voltage shifts for $p$ -type transistors.