Branched Segments in Polymer Gate Dielectric as Intrinsic Charge Trap Sites in Organic Transistors

Abstract

Charge traps in polymer gate dielectrics determine the electrical stability of organic field-effect transistors (OFETs), and polar alkoxy groups are well-known extrinsic charge traps. However, the actual location of intrinsic charge traps in nonpolar polymer gate dielectrics has been poorly understood yet. Here, we demonstrate that the skeletal structure of polymer chain plays an important role in determining the electrical stability. To verify it, we prepared linear and branched polystyrene (<i>l</i>-PS and <i>b</i>-PS) and blended them, in which branched segments provide much larger free volume than the other segments. The current-insulating performance and field-effect mobility increased with decease of <i>b</i>-PS portion. In particular, the bias-stress stability was remarkably varied according to the change of <i>b</i>-PS portion even though all measurements excluded reactive components such as oxygen and water; the increase of <i>b</i>-PS resulted in time-dependent decay of mobility and threshold voltage under bias stress. This indicates that the branched segments in <i>b</i>-PS provide intrinsic and metastable charge trap sites. Our result suggests that the skeletal structure of polymeric chains in gate dielectric is one of the important factors affecting intrinsic long-term operational stability of OFET devices