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We implement the “time-of-flight” (TOF) method for charge carrier mobility (μ) 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, VTOF, is reduced from VS by a constant voltage that coincides with the OFET’s threshold voltage, VT: VTOF=VS−VT. Under this assumption, TOFs scale as expected from theory with both VS and channel length, L, and the extracted dynamically acquired μ agrees excellently between different samples, and experimental protocols (variation of VS / variation of L). However, “dynamic” μ is higher than the “static” μ 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.

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