Not all electronic devices are made from silicon and other inorganic materials. That’s fortunate because despite all their capabilities, inorganic materials are difficult to interface with biological systems. New technology based on organic materials that conduct both electrons and ions enable devices known as organic electrochemical transistors (OECTs) can allow the blending of electronics and biology in applications including flexible biosensors and other forms of wearable electronics. But OECTs still face some hurdles before becoming as versatile as their more traditional silicon counterparts, including comparatively sluggish performance, speed, and stability problems arising from the organic materials used to construct them.
A research group led by Northwestern University has developed a new type of complementary logic OECT that addresses these issues by using a unique vertical architecture. The work appeared in Nature.
In conventional OECTs, fabricated with a planar architecture similar to silicon-based field-effect transistors, current flows between a source and drain electrode through a channel made of an organic semiconductor as voltage is applied to a gate electrode in contact with an electrolyte, generating ions. This arrangement, however, requires extremely efficient semiconductor materials to achieve acceptable switching speeds and transconductance and despite use of sophisticated and expensive fabrication methods, results in semiconductor channel lengths of 1-10 micrometers, still too large to achieve the best performance.
The research team approached the situation from a different direction, literally, by designing a vertical OECT (vOECT) built in stacked layers like a sandwich rather than in a flat horizontal configuration. Using new semiconducting polymers, they constructed different types of vOECTs that are easier to manufacture, readily scalable, and demonstrate considerably higher speeds, transconductances, and stability than conventional OECTs. Along with scanning electron microscopy, the vOECTs were characterized by grazing-incidence small-angle scattering at the 8-ID-E beamline of the Advanced Photon Source.
The group used Cin-Cell, a redox-inert cinnimate-cellulose polymer that can be photopatterned, and blended it with redox-active semiconducting polymers to form the channel of the vertical OECTs, sandwiching it between gold source and drain electrodes. A phosphate buffer solution (PBS) over the structure serves as the electrolyte. Importantly, the thickness of deposited layers can be controlled much more precisely than their lateral dimensions, so the vOECT architecture routinely results in channel lengths of only 0.1 micrometers, greatly enhancing device performance.
Comprehensive testing procedures evaluated the morphology, microstructure, switching speeds, transconductance, transistor behavior, stability, and other parameters of the team's vOECT approach, demonstrating superior characteristics of both the n-type and p-type varieties compared to conventional OECTs. In particular, the n-type vOECT is not only greatly improved compared to previous n-type examples but even outperformed p-type OECTs in some respects. The experimenters also note that the vastly increased performance is partly due to their use of ion-impermeable contacts resulting in transistor behavior occurring throughout the entire semiconductor channel, rather than involving only a small volume near the contact points as in previous vertical OECT demonstrations.
The researchers proved the viability of their vOECTs by using them to construct vertical organic complementary logic devices. A vertically stacked complementary inverter (VSCI) combining both n-type and p-type vOECTs demonstrated excellent highly stable voltage and other operating characteristics that would enable the VSCI to be incorporated into various circuits. The team used the VSCI as the basis for a five-stage ring oscillator and a rectifier, and also used their vOECTs to fabricate NAND and NOR logic gates, the building blocks from which all microprocessors can be built. These examples provide further evidence of the versatility and practicality of the team's vOECT architecture and how it can be easily integrated into complementary electronic logic circuitry.
The experimental team expects that their new vOECT approach, as devised and demonstrated by this work, will open real possibilities for the development of truly useful applications for a wide range of flexible wearable and implantable bioelectronic devices, as well as a platform for the detailed study of organic semiconductor behavior and materials. The approach allows considerably less expensive fabrication with cheaper materials while offering markedly greater performance over both conventional OECTs and previous vOECT designs. These advantages are likely to be enhanced even further with continued modification and improvement of the fabrication techniques and materials, an effort that the research team continues to pursue. – Mark Wolverton
See: W. Huang1,2,, J. Chen1,3,4, Y. Yao1,5,6, D. Zheng1, Y. Ji1, L-W Feng1,7, D. Moore8, N. R. Glavin8, M. Xie2, Y. Chen1, R.M. Pankow1, A. Surendran1, Z. Wang1,9, Y. Xia10, L. Bai2, J. Rivnay1, J. Ping5,6, Y. Guo4, Y. Cheng2, T. Marks1, A. Facchetti1,10, “Vertical Organic Electrochemical Transistors for Complementary Circuits,” Nature 613, 496–502 (January 2023)
Author affiliations: 1Northwestern University; 2University of Electronic Science and Technology of China; 3Yunnan University; 4Southern University of Science and Technology; 5Zhejiang University; 6ZJU-Hangzhou Global Scientific and Technological Innovation Center; 7Sichuan University; 8Air Force Research Laboratory; 9North University of China; 10Flexterra Inc.
We gratefully acknowledge financial support from the AFOSR (grant nos. FA9550-18-1-0320 and FA9550-22-1-0423), the Northwestern University MRSEC (grant no. NSF DMR-1720139), the US Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design (CHiMaD) (award no. 70NANB19H005), the National Natural Science Foundation of China (grant nos. U1830207, 21774055 and 62273073), the National Key R&D Program of China (grant no. 2022YFE0134800), the Sichuan Science and Technology Program (grant no. 2022NSFSC0877) and Flexterra Corp. This work made use of the Northwestern University Micro/Nano Fabrication Facility (NUFAB), the EPIC facility, the Keck-II facility and the SPID facility of the NUANCE Center at Northwestern University, which is partially supported by the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633), the Materials Research Science and Engineering Center (DMR-1720139), the State of Illinois and Northwestern University. R.M.P acknowledges support from the Intelligence Community Postdoctoral Research Fellowship Program at Northwestern University administered by Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the U.S. Department of Energy and the Office of the Director of National Intelligence (ODNI). N.R.G. and D.M. acknowledge support from AFOSR under grant number FA9550-23RXCOR011. We thank J. Strzalka of the Argonne National Laboratory Advanced Photon Source for assistance with the GIWAXS measurements. Use of the Advanced Photon Source, an Office of Science User Facility operated for the US DOE Office of Science by Argonne National Laboratory, was supported by the US DOE under contract no. DE‐AC02‐06CH11357. We thank W. Yue and Y. Wang from Sun Yat-Sen University for providing the PIBET-AO polymer. We also thank Z. Ye and C. Wu from Zhejiang University for their discussion and help. W.H. thanks the UESTC Excellent Young Scholar Project for financial support.
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