3D versus 2D Electrolyte–Semiconductor Interfaces in Rylenediimide-Based Electron-Transporting Water-Gated Organic Field-Effect Transistors

Water-gated organic field-effect transistors (WGOFETs) are relevant devices for use in the fields of biosensors and biosystems. However, real applications require very stringent performance in terms of electrochemical stability and charge mobility to the organic semiconductor in contact with an aqueous environment. Here, a comparative study of two small-molecule electron-transporting perylenediimide semiconductors, which differ only in the N-substituents named PDIF-CN₂ and PDI8-CN₂ is reported. The two materials present similar solid-state arrangements but, while the PDI8-CN₂ shows a more 3D growth modality and electron mobility independent of the semiconductor layer thickness (≈10⁻⁴ cm² V⁻¹ s⁻¹), the PDIF-CN₂ has an almost-2D growth modality and the mobility increases with the semiconductor film thickness, reaching a maximum value of ≈5 × 10⁻³ cm² V⁻¹ s⁻¹ at 30 nm. Above this thickness, the PDIF-CN₂ switches to a more 3D growth modality, and the mobility drops by one order of magnitude. XRR analysis indicates that a PDIF-CN₂ film can be modeled as a dense layered structure in which each layer is decoupled from the others due to the presence of fluorocarbon-chains. The availability of additional pathways for charge transport from buried layers and the 2D versus 3D growth can explain the mobility dependence on the film thickness.