We studied a mechanism of grain size increase (that is, island density decrease) in pentacene film prepared in hydrogen (H(2)) ambient. The island densities of pentacene films prepared in helium and deuterium were lower than those of vacuum-deposited films. This indicates that the decrease in the island density was not due to the chemical interaction between H(2) and pentacene or the substrate surface Furthermore the.. temperature dependence of the island density indicates that there is no difference in the surface diffusion energy in a vacuum and in H(2). We also improved mobility significantly in the pentacene thin film transistor fabricated on film grown in H(2) ambient on a chemically treated substrate. (C) 2009 Elsevier B.V. All rights reserved.