Potentiometry on pentacene OFETs:: Charge carrier mobilities and injection barriers in bottom and top contact configurations

In a combination of experimental techniques including electrical probes, potentiometry, and charge transient spectroscopy (QTS), we develop concepts how to quantify the potential drops at the contacts, the mobility in the channel region, and the density of states of deep traps in pentacene OFETs. For OFETs grown from unpurified pentacene on prepatterned An bottom contacts a comparison between potentiometry and two-dimensional device simulations determines an injection barrier of 0.73 eV at the source contact and a hole mobility of 0.014 cm(2) V-1 s(-1) in the pentacene channel. Temperature-dependent QTS data reveal a trap level at about 125 meV from the hole transport band, indicating a relatively high density of unintentional dopants and therefore a high background density of majority charge carriers. In OFETs grown from purified pentacene onto a SiO2 gate dielectric and Au top contacts evaporated onto the pentacene channel without breaking the vacuum, potentiometry reveals a nearly perfect alignment of the metal work function with the hole transport level in the organic layer. The much lower density of deep traps in these samples raises the hole mobility to the range 0.1-0.2 cm(2) V-1 s(-1). A further improvement of the hole mobility and the resulting device performance can be achieved by a chemical treatment of the gate oxide with n-octadecytrichlorosilane (OTS). (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.