Controlling charge‐transfer at the gate interface of organic field‐effect transistors

Abstract

The performance and charge transport characteristics of pentacene transistors with oxide gates such as SiO2 and Al2O3 have been recently shown to largely depend on the density of residual carriers due to electron acceptor defects at the oxide surface. The threshold field in particular is strongly connected to the density of these surface states. We demonstrate that by vapor deposition of self‐assembled monolayers on the oxide gate prior to pentacene growth this charge transfer process is inhibited, the residual charge concentration is decreased by up to two orders of magnitude, and the field‐effect mobility increases by up to a factor of four. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pentacene - SiO<inf>2</inf> interface: Role of the environment prior to pentacene deposition and its impact on TFT DC characteristics

“…One point contradicting with [6] for our devices is that the I ON /I OFF ratio for the first group of devices is higher than for the reference transistors. According to [6], when the number of residual charges increases and the channel becomes conductive even for positive V G , the I ON /I OFF ratio decreases, whereas in our case we observe an increase.…”

“…In addition, the mobility is also affected having slightly higher values for the first group of devices (those with prolonged exposure to air). One can find very similar behavior in the literature [6] where SiO 2 surfaces are treated with oxygen plasma. In that case, one can observe big shifts of V T accompanied by an increase in the fieldeffect mobility.…”

“…This has as a result the creation of free carriers (holes) in pentacene even for positive values of V G , as it is the case for our samples. We think that the long exposure to air, and consequently to O 2 of the SiO 2 surface prior to pentacene deposition has the same effect as the oxygen plasma in [6]. According to [6], the field effect mobility becomes limited by the counter-ions from the surface recombination centers that are non-saturated.…”

“…We think that the long exposure to air, and consequently to O 2 of the SiO 2 surface prior to pentacene deposition has the same effect as the oxygen plasma in [6]. According to [6], the field effect mobility becomes limited by the counter-ions from the surface recombination centers that are non-saturated. When the concentration of residual charges at the SiO 2 surface increases over a certain value, the mobility increases too, as the density of the residual charges may be enough to screen those counterions.…”

“…Threshold voltage shifts are the most pronounced effects and are related to charge transfer from the pentacene molecules to traps located at the interface with the dielectric or in the dielectric itself. Some groups have also investigated the role of oxygen plasma treatment in the creation of residual charges at the SiO 2 surface [5][6]. In this work we have observed that prolonged exposure of the SiO 2 to ambient air can significantly change the behavior of the pentacene devices, causing threshold voltage shifts comparable to those detected after treatment by oxygen plasma of the SiO 2 surface prior to the deposition of the pentacene layer.…”