Rylenediimide derivatives as a new molecular platform for n-type WG-OFETs: the role of thin-film growth modality in engineering the electrolyte-semiconductor interface

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 electron-transporting 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 layer thickness). Differently from other semiconductors used in WGOFETs, whose mobility saturates after 1-2 monolayers from the semiconductor/water interface, the PDIF-CN2 mobility increases unexpectedly with the semiconductor film thickness up to 35 nm while preserving an almost-2D growth modality. The achieved mobility values are comparable to state-of-the-art p-type semiconductors (~10-3 cm2/Vs). Cross-correlated experiments and theoretical evidences suggest that crystallinity and end-substituents (fluorinated-chain for PDIF-CN2 and alkyl-chain for PDI8-CN2) are not the major causes of the more performing electrical characteristics of PDIF-CN2-based WGOFET. Features of semiconductor/water interface are found to be dependent on the overall film thickness for PDI8-CN2 while no change is observed for PDIF-CN2 until the almost-2D growth modality is preserved. Upon these experimental results, we can suggest that the charge-mobility increase in PDIF-CN2 is likely correlated to a bulk-conduction contribution to the field-effect charge transport. Moreover, the electron-rich end-substituents of PDIF-CN2 organize into an ordered and dense interlayer at the semiconductor/electrolyte interface, which increases the surface hydrophilicity while avoiding water molecules percolation. 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. Zhan et al., Adv.Mater.23 (2011) 268-284