Study on post-deposition annealing influenced contribution of hole and electron trapping to threshold voltage stability in organic field effect transistors

Abstract

The effect of post-deposition annealing of copper phthalocyaine (CuPc) film acting as active layer, on the hysteresis and the threshold voltage stability of organic field effect transistors (OFETs) was studied. The morphology of annealed CuPc film, as verified by Atomic Force Microscopy (AFM), showed well connected grains, but also increase in surface roughness which could be due to desorption of certain number of CuPc molecules causing voids. Increase in hysteresis in the transfer characteristics and the threshold voltage shift were observed for devices with annealed films as compared to that for devices with as deposited films. Presence of both hole and electron trapping effects causing hysteresis were observed where hole trapping was found to be the dominant cause for hysteresis and threshold voltage shift in devices with annealed films. This could be attributed to increase in grain boundary density in CuPc films with annealing. Electron trapping effect was explained to be influenced by both increased silanol groups on dielectric surface caused by diffusion of oxygen and moisture from ambient through voids as well as by hole trapping effect itself.