Time-of-flight mobility measurements in organic field-effect transistors

We implement the "time-of-flight" (TOF) method for charge carrier mobility (mu) measurements in organic field-effect transistors (OFETs) by applying voltage steps, VS, to OFETs. We use the electric scheme for OFET-TOF introduced by Dunn et al. [Appl. Phys. Lett. 88, 063507 (2006)]. Our investigation of a series of low-threshold poly(triaryl amine) OFETs with different channel lengths, L, suggests that in the OFET-TOF setup, the effective voltage driving carriers across the channel, V-TOF, is reduced from VS by a constant voltage that coincides with the OFET's threshold voltage, V-T: V-TOF= V-S-V-T. Under this assumption, TOFs scale as expected from theory with both V-S and channel length, L, and the extracted dynamically acquired mu agrees excellently between different samples, and experimental protocols (variation of V-S / variation of L). However, "dynamic" mu is higher than the "static" mu conventionally extracted from saturated transfer characteristics, which is also less consistent between samples. Also, we observe that the TOF in OFETs switching from OFF. ON is longer than the TOF in the same OFET switching ON. OFF under the same VS. We rationalize this difference by trap filling in the populated OFET channel. (C) 2008 American Institute of Physics. [DOI:10.1063/1.3006443]