Impact of morphology on charge carrier mobility in top gate C₆₀ organic field effect transistors

Charge carrier mobility is a critical parameter in organic field effect transistors and it is strongly influenced by morphology and structure of the involved organic materials. In this work, we present a study on impact of grain size and surface roughness of the active layer on the mobility in top gate n-type C₆₀ organic field effect transistors. The morphology was varied by changing the substrate temperature during C₆₀ deposition from 100 °C to 200 °C. It is found that for the investigated top gate devices, the mobility does not strictly increase with increasing grain size, which is in disagreement with the trends reported for bottom gate OFETs. The observation is explained by the fact that the increasing grain size of C₆₀ leads to a concurrent increase in the surface roughness, which negatively impacts the charge carrier mobility in the active channel of the OFET. As a result an optimum of the mobility is reached at 150 °C of substrate temperature where grains are already quite big, but surface roughness is still not hindering the transport.