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LOW COST ORGANIC GAS SENSORS ON PLASTIC FOR DISTRIBUTED ENVIRONMENTAL MONITORING
Impact/Purpose:
The overall objective of this research is to develop novel arrayed gas sensors on plastic that offer extremely high specificity and broad-range detection capability while maintaining low fabrication cost, making them viable for use in distributed environmental monitoring applications, in which cost is an important criterion.
Description:
This project focused on the development of low-cost arrayed organic sensors for environmental monitoring applications. All of the major goals of the original project have been achieved in the 3-year period of this grant.
In Year 1 of the project, we successfully developed a baseline sensor technology and used it to detect numerous common organic environmental contaminants, including various alcohols, ketones, and other solvents often found in industrial waste. The sensor technology made use of solution-processed organic semiconductors deposited on a common gate structure to form arrays of organic transistors. Importantly, in these devices, the channel layer of the transistor is exposed to the environment. By using appropriate channel materials, it is possible to demonstrate sensing functionality for various fluids. We demonstrated organic sensors using several different organic semiconductors, including several polythiophenes and pentacene derivatives. These were used to achieve sensing of several organic environmental contaminants. Studies were performed on cycle-life of these sensors, and qualitative models for sensor operation were identified. Simultaneously, to facilitate accurate sensor characterization, a robust sensor testing facility was constructed. This system uses multiple mass-flow-controllers and meters to inject controlled amounts of analyte and/or mixtures of analytes into a sensor test chamber. The chamber includes electrical input/output connections to enable real-time measurements on the sensor arrays. Sensing of liquid vapors is also possible using a bubbler. Using this methodology, we were able to study the transient response of organic transistor gas sensors for the first time.
In Year 2, we made substantial progress in improving the robustness, reliability, and repeatability of the technology and also increased the level of integration within the baseline sensor process. Studies were performed on cycle-life of these sensors, and quantitative models for sensor operation were identified. These studies lead to the development of a differential sensing technique that eliminates the drift and degradation issues identified as problems in the first year of this grant. Additionally, further integration of the sensor technology was achieved, with fully integrated arrays having been demonstrated along with associated differential sensing circuitry necessary for drift correction.
In Year 3 of the program, we completed our quantitative analysis of the drift mechanisms of our organic sensors. Briefly, we proposed a bulk-trapping mechanism, by which charges (primarily holes) are trapped within the entire thickness of the channel film of the sensor transistors. These charges in turn cause a threshold voltage shift, resulting in drift in the current-voltage characteristics of the sensor. The drift rate depends on a variety of factors, including net charge passing through the channel, sensor geometry, sensor channel materials, and so forth. An important conclusion of this analysis is that drift may be compensated adequately through use of the differential sensing technique proposed in Year 2 of the project. Specifically, the technique involves two components. First, every sensor element within the array consists of two sensing transistors. The first transistor actually is exposed to the analyte to be sensed, whereas the second transistor is protected using an encapsulant. Both transistors are biased at substantially the same current levels, and so their drift is similar and may be subtracted using a current bridge technique. Additionally, the transistors themselves are pulsed with low duty cycles to ensure the lowest possible drift rates. The combination of these two techniques ensures that drift is no longer a problem in our arrayed gas sensor architecture.
Using the new architecture, an integrated array sensor was demonstrated. The output of differentially organized arrayed sensor transistors was connected to a silicon-based analog–to-digital converter through a differential amplifier. The output of this converter was read through an alphanumeric display. The sensor was used to demonstrate sensing of a range of organic solvents including various alcohols and carboxylic acids. In this way, we were able to demonstrate a complete sensor system encompassing all of the major goals of this program, including sensing transistor design, development of sensor materials, establishment of sensing techniques, and integration of a functional sensor architecture. Overall, therefore, a methodology for sensor development has been established and a similar methodology may be used to develop such sensors for deployment in specific distributed environmental monitoring conditions.