Sweet Electronics: Honey-Gated Complementary Organic Transistors and Circuits Operating in Air

Abstract

Sustainable harnessing of natural resources is key moving toward a new-generation electronics, which features a unique combination of electronic functionality, low cost, and absence of environmental and health hazards. Within this framework, edible electronics, of which transistors and circuits are a fundamental component, is an emerging field, exploiting edible materials that can be safely ingested, and subsequently digested after performing their function. Dielectrics are a critical functional element of transistors, often constituting their major volume. Yet, to date, there are only scarce examples of electrolytic food-based materials able to provide low-voltage operation of transistors at ambient conditions. In this context, a cost-effective and edible substance, honey, is proposed to be used as an electrolytic gate viscous dielectric in electrolyte-gated organic field-effect transistors (OFETs). Both n- and p-type honey-gated OFETs (HGOFETs) are demonstrated, with distinctive features such as low voltage (<1 V) operation, long-term shelf life and operation stability in air, and compatibility with large-area fabrication processes, such as inkjet printing on edible tattoo-paper. Such complementary devices enable robust honey-based integrated logic circuits, here exemplified by inverting logic gates and ring oscillators. A marked device responsivity to humidity provides promising opportunities for sensing applications, specifically, for moisture control of dried or dehydrated food.

Keywords: edible electronics; electrolyte-gated transistors; honey; organic electronics; printed electronics.

Figure 1

HGOFETs structure, fabrication, and electrical characterization. a) A schematic cross‐section of the p‐type (left) and n‐type (right) HGOFETs with the representation of a simplistic charge carriers distribution and molecular structures of the exploited semiconductors: P3HT and P(NDI2OD‐T2). b) Illustration (top) and a digital image (bottom) of the HGOFET with gold source and drain electrodes on glass, spin‐coated semiconductor, and drop‐cast honey (pink color corresponds to the spin‐coated P3HT thin film). c) Transfer (left) and output (right) characteristic curves for p‐type HGOFETs in linear (V _ds = −0.01 V; −0.05 V) and saturation (V _ds = −1 V) regimes; V _gate sweep rate was 5 mV s⁻¹. d) Transfer (left) and output (right) characteristic curves for n‐type HGOFETs in linear (V _ds = 0.01 V; 0.05 V) and saturation (V _ds =1 V) regimes; V _gate sweep rate was 2 mV s⁻¹; OFETs geometrical parameters: L =10 µm; W = 20 000 µm. e,f) The reversible electrical response of HGOFETs to different humidity conditions (RH in air ≈50%; in inert nitrogen environment ≈0%). The humidity‐dependent behavior of V _th for the p‐type (e) and n‐type (f) HGOFET operating in both linear (V _ds = ± 0.05 V) and saturation (V _ds = ±1 V) regimes. The rectangular regions correspond to the hydration of the dehydrated devices right after exposing them to air.

Figure 2

HGOFETs‐based logic circuits and their electrical characterization. a) Schematic representation of the inverting logic gate device realized with P3HT and P(NDI2OD‐T2). b) VTCs, and corresponding derivative curves to extract gain, of a complementary HGOFETs‐based inverter as a function of input voltages; the arrows indicate the sweep (rate of 25 mV s⁻¹) direction; the inverter configuration is reported as an inset. c) Schematic representation (left) and the circuit configuration (right) of the three‐stage HGOFETs‐based ring oscillator. d) Output waveforms of the HGOFETs‐based oscillator operating in air at different supply voltages V _DD. e) Output oscillation frequency (black line) and propagation signal delay per stage (red line) of the HGOFETs‐based oscillator operating in air as a function of V _DD. f) Output waveforms of the HGOFETs‐based oscillator at different humidity conditions (in air; in inert environment) at V _DD = 1 V. g) Output oscillation frequency (black line) and stage delay (red line) of the HGOFETs‐based oscillator as a function of the humidity condition. The rectangular region schematically corresponds to the hydration of the dehydrated device right after exposing it to air.

Figure 3

HGOFETs on flexible edible tattoo‐paper substrate: electrical characterization and devices conformability. a) Transfer in saturation (V _ds = −0.7 V) regime and b) output characteristic curves for p‐type HGOFET after the transfer onto the apple surface. c) Digital photograph of HGOFETs (in the middle), HGOFETs transferred onto different food items: an apple (top) and tomato (bottom).