Local Chemical Enhancement and Gating of Organic Coordinated Ionic-Electronic Transport

Abstract

Superior properties in organic mixed ionic-electronic conductors (OMIECs) over inorganic counterparts have inspired intense interest in biosensing, soft-robotics, neuromorphic computing, and smart medicine. However, slow ion transport relative to charge transport in these materials is a limiting factor. Here, it is demonstrated that hydrophilic molecules local to an interfacial OMIEC nanochannel can accelerate ion transport with ion mobilities surpassing electrophoretic transport by more than an order of magnitude. Furthermore, ion access to this interfacial channel can be gated through local surface energy. This mechanism is applied in a novel sensing device, which electronically detects and characterizes chemical reaction dynamics local to the buried channel. The ability to enhance ion transport at the nanoscale in OMIECs as well as govern ion transport through local chemical signaling enables new functionalities for printable, stretchable, and biocompatible mixed conduction devices.

Keywords: chemical sensing; interfacial transport; ion mobility and conductivity; organic electronics; organic mixed ionic‐electronic conductors (OMIEC).

Figure 1

Establishing an Ion Superhighway A) Chemical structure of the PEDOT:PSS. B) Two‐parameter VASE analysis of a PEDOT:PSS thin film to characterize the bilayer. Inset shows the measured thicknesses. Three other film thicknesses are consistent with this as shown in Section S2 (Supporting Information) C) Schematic of ECMF experiment setup. The camera view shows the image of a partially dedoped (blue) device. D) Gaussian smoothed CCD red‐channel intensity profiles (solid lines) and corresponding derivatives (dashed lines) acquired from video frames locate the dedoping front l. E) Dedoping transients for multiple trials on a single mixed conduction device at several driving voltages. Solid and broken lines represent the data and the linear fit, respectively (details in Section S4, Supporting Information). F) Ion mobility statistics from different devices, trials, and driving voltages with a PVA encapsulation layer. (n = 49) (G) Current versus $\frac{1}{\sqrt{\mathrm{t}}}$ and linear fits for a PVA mixed conduction device (analysis details in Section S5, Supporting Information). The time ranges used for the fits are identical to those used in E). H) Ion density, p, and I) ion conductivity, σ _ion , statistics from bounding scenarios of ion distribution in the hydrated channel. Explicit posted values are from the average for each scenario (n = 31).

Figure 2

Control Mechanisms of Ion Transport A) Ion mobility versus WCA of four different encapsulation layers. Line as a guide to the eye. SU‐8 is a negative photoresist; PMMA is polymethyl methacrylate; and PVC is poly(vinyl chloride). B) Na⁺ Ion mobility versus WCA of the PEDOT:PSS channel surface, which varies due to water sonication of the channel that increasingly removes the PSS‐rich top layer. Lines are guides to the eye. Devices with no PSS removal are designated as “Full”, PSS removal after PSS crosslinking “Half”, PSS removal before crosslinking “Removed”, and “ReAdded” means pure PSS is spin‐coated on top of a “Removed” layer (see “PEDOT:PSS Films” in Experimental Section for details). Devices with two different encapsulation layers are shown: hydrophilic (PVA) and hydrophobic (PVC). C) VASE analysis of channel swelling for the devices in Figure 2B as a function of ECMF dedoping cycle. Cycle 0 is the thickness in the initial dry state. Solid symbols are devices with the “Full” PSS channel while open symbols are for devices with the PSS channel “Removed”. Green squares are the PVA encapsulation layer thickness. D) pH litmus test on dedopoed devices without encapsulation using PSSH and NaCl electrolyte. E) C‐edge RSoXS profiles of PEDOT:PSS films at 270 eV (blue) and 285.1 eV (red, PEDOT carbon 1s → π* transition). The dark to light color indicates increasing in EG content of 0, 2, 5, 10, and 20 vol%. F) Ion mobility of PEDOT:PSS films with SU8 (red) and PMMA (blue) encapsulating layers versus EG cosolvent concentration. Note: PSS was “Half” removed for this series [e.g., blue symbols in (B)], reducing the mobility and interfacial effects compared to those in (A), similar to the processing of devices in previous reports using SU‐8 encapsulation.^{[ 7} , ^{12 ]} Insets depict enhanced gelation from EG. All mobility statistics from multiple trials and driving voltages on at least three identically prepared devices.

Figure 3

Sensing a Local Chemical Reaction A) Top surface WCA and thickness (VASE) of a PMMA encapsulation layer with accumulated UV ozone exposure time. B) Ion mobility (ECMF) as a function of accumulated UV exposure time in a PMMA‐coated device exposed to UV (red) and unexposed (blue). Figure S15 (Supporting Information) shows the experiment reproduced twice more. Schematic of “transistor” devices C) before and D) after UV ozone exposure. UV ozone reacts with the PMMA encapsulation layer to turn the PMMA ─OCH₃ group to ─OH, acting as a chemical gate signal. E) Schematic of device setup for electrical detection of a chemical change. An AC voltage (V _AC) oscillates ions in place (100 Hz) with the AC current sensing ion mobility. F) AC current transients for three sequential runs on one device. The green and violet lines indicate when V_AC was turned on and the UV exposure was activated, respectively. The peak in current (red line) represents maximum ion mobility when the chemical reaction reaches the buried channel with the red arrow representing the chemical reaction time. G) Chemical reaction time as a function of PMMA thickness (linear fit) under UV exposure.