“...OTFT-based chemical, biological, and pressure sensors may be extremely attractive for electronic skin application because these transistor sensors can be potentially printed over a large flexible substrate.”

I got my Ph.D. degree in Chemistry from the University of Chicago in 1995, where I developed new synthetic methodology to prepare solution-soluble conjugated polymers. After graduation, I joined Bell Labs, Lucent Technologies in Murray Hill, New Jersey, as a member of technical staff in the Polymer and Organic Materials Research Department.

At Bell Labs, my research focused on organic thin-film transistors (OTFTs) and their applications for displays, circuits, and sensors. In recognition of my research achievement, I became a Distinguished Member of Technical Staff in 2001. After spending eight stimulating years there, I relocated to Stanford University, Department of Chemical Engineering, as an Associate Professor in March 2004.

My current research interests are on rational design of organic semiconductors for various applications, including thin film transistors, chemical and biological sensors, nano-electronic devices, and solar cells.

I am interested in organic thin-film transistors because they hold great promise and potential for flexible printed electronics, a future generation of electronic devices. In addition, I am interested in the application of thin-film transistors as tools for characterizing charge transport properties of organic semiconductors.

We have two main research areas. First is on the rational design of organic semiconductors. The ability to accurately predict the charge carrier mobility of organic semiconductors is highly desirable. However, this is difficult because we are unable to precisely predict molecular packing, thin-film morphology, and solid-state electronic structures. As a result, the design of organic semiconductors is approached predominantly by trial and error. Additionally, we don’t know the nature of traps and their roles in charge transport. I believe if we are able to understand how molecular structures impact the various above-mentioned parameters, we will be able to ultimately realize much higher charge carrier mobility than what we can currently achieve.

The second focus of our research is to understand materials requirements to enable new applications for OTFTs, such as water-stable OTFTs for biological and chemical sensing and pressure sensors for tactile sensing.

When I first started working in this field in 1996, there were only a few reported organic semiconductor materials that work in a thin-film transistor. Now hundreds of new organic semiconductor materials have been evaluated for thin-film transistors, and some of them have already shown charge carrier mobility better than amorphous silicon TFTs. Some of the proposed applications, such as electronic paper, memory circuits, and sensors, have now been demonstrated.

I think there are a number of areas that OTFTs may have unique advantages. These are mainly applications that require the transistor devices to be integrated with low-cost plastic substrates where low-temperature processing is required. Lightweight ultrathin electronic paper and low-cost memory tags are among some of the proposed applications.

Additionally, OTFTs are suitable for applications requiring the devices to be distributed over a large area and maybe even on curved surfaces. For example, OTFT-based chemical, biological, and pressure sensors may be extremely attractive for electronic skin application because these transistor sensors can be potentially printed over a large flexible substrate.

I would like to find out what is the ultimate limit for charge carrier mobility for organic materials. Can we push it to a much higher value than the 20 cm²/Vs demonstrated for rubrene single crystals? Can we reach a mobility of 50 or even 100 cm²/Vs.

Zhenan Bao, Ph.D.
Department of Chemical Engineering
Stanford University
Stanford, CA, USA