Correlation between thin-film 3D growth modality and mobility in high performance n-type molecular water-gated OFETs

The Water-Gated Organic Field-Effect Transistor (WGOFET) is one of the most promising device architecture for stimulating and recording cell electrophysiological activity given the possibility of biofunctionalization of the organic/electrolyte interface [1]. Here we present for the first time the use of two n-type PDI derivatives, named PDIF-CN2 and PDI8-CN2 [2], as active materials in WGOFETs. The two materials have identical solid-state arrangement but show two different growth mechanisms: almost-2D layer by layer for PDIF-CN2 and 3D for PDI8-CN2.The electron mobility of PDI8-CN2 shows a saturation with increasing the semiconductor layer thickness (~10-4 cm2/Vs at 10-15 nm). Differently from other semiconductors used in WGOFETs, whose mobility saturates after 1-2 monolayers from the interface, the PDIF-CN2 mobility increases unexpectedly with the semiconductor film thickness up to 35 nm while preserving an almost-2D growth modality, thus reaching values comparable to state-of-the-art p-type semiconducors (~10-3 cm2/Vs). From the cross-correlation of experimental and theoretical evidences, we can suggest that charge mobility increase is likely correlated to a bulk conduction contribution to the field-effect charge transport.These insights may enable the definition of a new material paradigm for the realization of performing WGOFETs for use in biological signal transduction. [1]S. Toffanin et al., J.Mater.Chem.B 1 (2013) 3850-3859 [2] X. Zhanet al., Adv.Mater.23 (2011) 268-284