Influence of traps on transient electric field and mobility evaluation in organic field-effect transistors

A significant difference between the transient electric field profiles of the pentacene organic field-effect transistors (OFETs) with SiO₂ and poly(methyl-methacrylate) (PMMA) insulators was found by the time-resolved microscopic optical second-harmonic generation (TRM-SHG) experiment. The profile of former device was broad and changed smoothly, while the latter one had a sharp peak. Particularly, the peak of the transient electric field in SiO ₂ -insulated devices moved much faster than that in the PMMA-insulated one. Based on several experimental evidences and computational simulations, we proposed that these differences might arise from a higher trapped carrier density in the conductive channel on the PMMA insulator. Simple approaches were developed to evaluate the trap density and define dynamic carrier mobility in terms of the transient electric field measured by the TRM-SHG technique. This mobility quantitatively depicts that the transient hole transport in the OFET with the PMMA insulator is trap controlled.