Thickness dependence of performance parameters in organic thin film transistors with thin pentacene films fabricated with top contact structures

We investigated the dependences of the mobility, the threshold voltage, and the drain saturation voltage on the pentacene thin film thickness in organic thin film transistors(OTFTs) with top contact structures. The dependences were quite different from those for OTFTs with thick pentacene layers. The mobility exhibited a peak value at. a certain pentacene thickness. While the threshold voltage steadily decreased with increasing thickness, The origin of the mobility degradation below the peak value was found to be gaps at the grain boundaries which were filled by pentacene molecules as the thickness increased. The gaps also seemed to be the cause of the large threshold voltage in the thin pentacene case. The optimum pentacene thickness was found out to 500 angstrom evaporated at 0.052 angstrom/sec, producing a mobility of 0.42 cm(2)/V center dot sec.