John A. DeFranco

Photolithographic patterning of organic electronic materials

Hydrofluoroethers are shown to be benign solvents to a wide variety of organic electronic materials, even at extreme conditions such as boiling temperature. Coupled with fluorous functional photoresist-acidsensitive semi-perfluoroalkyl resorcinarene, they open new frontiers for photolithographical patterning for organic electronic systems. Summary of Research: Organic electronics is emerging as a promising technology to enable mechanically flexible devices through solution processing of organic materials [1]. As with traditional electronics, organic devices require active functional materials to be tailored into micropatterned and multi-layered device components. While the former relies on photolithographic patterning techniques, the latter is restricted from adopting such robust, high-resolution and high-throughput techniques because of the chemical compatibility issue between organic materials and patterning agents [3]. Namely, deterioration of materials’ performance occurs during the photoresist deposition and removal stages due to aggressive organic solvents, as well as in the pattern development steps by aqueous base solutions. In our search for universal, materials-friendly solvents, we have identified environmentally benign fluorous solvents combined with specifically tailored patterning materials as a possible solution to this complex problem. Fluorous solvents are poor solvents for non-fluorinated organic materials [2]. Among the variety of fluorous solvents, segregated hydrofluoroethers (HFEs) attracted our attention because of their nonflammability, zero ozone-depletion potential and low toxicity for humans [3]. We tested the impact of HFEs solvents on wellcharacterized and commercially available organic electronic materials. We demonstrated that HFE solvents do not damage or alter electronic and optoelectronic properties of wide class of organic electrnic materials, including: organic semiconductors (pentacene and poly-3-hexylthiophene (P3HT)), conducting polymer Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and organic light emitting polymers and small molecule compounds (polyfluorenes, and [Ru(bpy)3] (PF6 –)2 complex). To further demonstrate the orthogonality of HFEs to active organic material as well as to organic/metal interface we used aforementioned organic materials to make organic light emitting diodes (OLEDs) and thin film transistors (TFTs) which we characterized before and after exposing to HFE [4]. We found HFE did not significantly change the characteristic of tested devices even at elevated temperatures. For example, Figure 1 shows [Ru(bpy)3] (PF66 –)2 based electroluminescent device [5] in boiling HFE 7100 (61°C). We operated the device in the boiling HFE for one hour and did not observe any substantial change in its performance. This new dimension in solvent orthogonality which is enabled by the use of HFEs offers unique opportunities for the chemical processing of organic electronic materials. One example is in the area of photolithographic processing: One can use a photoresist that is properly fluorinated to be processable in HFEs [6]. We have successfully demonstrated this approach

Enzymatic sensing with organic electrochemical transistors

Since their development in the 1980s organic electrochemical transistors (OECTs) have attracted a great deal of interest for biosensor applications. Coupled with the current proliferation of organic semiconductor technologies, these devices have the potential to revolutionize healthcare by making point-of-care and home-based medical diagnostics widely available. Unfortunately, their mechanism of operation is poorly understood, and this hinders further development of this important technology. In this paper glucose sensors based on OECTs and the redox enzyme glucose oxidase are investigated. Through appropriate scaling of the transfer characteristics at various glucose concentrations, a universal curve describing device operation is shown to exist. This result elucidates the underlying device physics and establishes a connection between sensor response and analyte concentration. This improved understanding paves the way for rational optimization of enzymatic sensors based on organic electrochemical transistors.

Influence of Device Geometry on Sensor Characteristics of Planar Organic Electrochemical Transistors

The response of PEDOT:PSS planar electrochemical transistors to H2O2 can be tuned by varying the ratio between the areas of the channel and the gate electrode. Devices with small gates show lower background signal and higher sensitivity. The detection range, on the other hand, is found to be rather independent of the gate/channel area ratio.

All-Plastic Electrochemical Transistor for Glucose Sensing Using a Ferrocene Mediator

We demonstrate a glucose sensor based on an organic electrochemical transistor (OECT) in which the channel, source, drain, and gate electrodes are made from the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS). The OECT employs a ferrocene mediator to shuttle electrons between the enzyme glucose oxidase and a PEDOT:PSS gate electrode. The device can be fabricated using a one-layer patterning process and offers glucose detection down to the micromolar range, consistent with levels present in human saliva.

Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics.

An acid-sensitive semiperfluoroalkyl resorcinarene was synthesized, and its lithographic properties were evaluated. Its solubility in segregated hydrofluoroether solvents enables the patterning of delicate organic electronic materials.

Orthogonal processing: A new strategy for organic electronics

The concept of chemical orthogonality has long been practiced in the field of inorganic semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage organic materials, preventing the use of photolithography to pattern organic electronic devices. In this article we show that new photoresist materials that are orthogonal to organics allow the fabrication of complex devices, such as hybrid organic/inorganic circuitry and full-colour organic displays. The examples demonstrate that properly designed photoresists enable the fabrication of organic electronic devices using existing infrastructure.

Microfluidic gating of an organic electrochemical transistor

A microfluidic-based organic electrochemical transistor is reported. The integrated microfluidic channel not only confines and directs the flow of liquid electrolyte over the active layer of the transistor but also provides the gate electrode for the transistor. The active layer employed in this work is poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), which results in a transistor that is inherently “on” but that can be turned “off” through application of a positive gate voltage. The transistor behavior is understood in terms of an electrochemical mechanism and is shown to depend on the ionic strength of the electrolyte. The applicability of the device to microfluidic-based chemical and biological sensing is discussed.

Direct 120 V, 60 Hz operation of an organic light emitting device

We report on lighting panels based on ruthenium(II) tris-bipyridine complexes that can be sourced directly from a standard US outlet. With the aid of the ionic liquid 1-butyl-3-methylimidazolium, the conductivity of the light emitting layer was enhanced to achieve device operation at a 60Hz frequency. Lighting panels were prepared using a cascaded architecture of several electroluminescent devices. This architecture sustains high input voltages, provides fault tolerance, and facilitates the fabrication of large area solid-state lighting panels. Scalability of the drive voltage, radiant flux, and external quantum efficiency is demonstrated for panels with up to N=36 devices. Direct outlet operation is achieved for panels with N=16, 24, and 36 devices.

Dry photolithographic patterning process for organic electronic devices using supercritical carbon dioxide as a solvent

The particular challenge of micropatterning organic materials has stimulated numerous approaches for making effective and repeatable patterned structures with fine features. Among all the micropatterning techniques photolithography, being the preferred method for the inorganic semiconductor industry, did not create much impact due to its incompatibility with the majority of organic electronic materials. Here we introduce a novel, chemically benign approach to dry photolithographic patterning of organic materials using super-critical carbon dioxide (scCO2) as a green developing solvent. We illustrate the possible applications of the new technique by patterning conducting polymers and light emitting polymers for organic light emitting diodes.

Water stability and orthogonal patterning of flexible micro-electrochemical transistors on plastic

Water-stable, flexible and micro-scale organic electrochemical transistors (OECTs) based on poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) were fabricated on a plastic substrate using a new process based on a fluorinated photoresist. The PEDOT:PSS films, mixed solely with a biocompatible conductivity enhancer, show robust adhesion on plastic substrates, and exhibit unchanged electrical properties under extreme bending. This work simplifies the fabrication of high-performance OECTs and places them in a highly competitive position for flexible electronics and healthcare applications.