Organic Semiconductors in Tactile Sensor Technology

Ashley Seni
June 11, 2012

Submitted as coursework for PH250, Stanford University, Spring 2012

Computers and electronic devices are enhanced by the design of their user interface and the ease with which users are able to obtain information and perform a task. For instance, touch screens are widely used in a number of commercial devices and allow for direct interaction with a display. The extent to which a user can interact with a touch display is limited, however, by current tactile sensor technologies that rely on capacitive, resistive, ultrasonic, or optical effects to detect the presence and position of a single or multi-touch input on a display area. [1] Improving the sensitivity of tactile sensors is a major concern that can potentially be addressed by the application of organic transistors in tactile sensor technology. The use of organic transistors in electronic displays and pressure sensor networks has been demonstrated and show promise in the development of ultrasensitive tactile sensors for application in electronic devices. [2-4] This report will discuss the advantages of using organic semiconductors for tactile sensors, as well as review a case study in which organic field-effect transistors are used to develop flexible pressure sensors.

Background on Organic Semiconductors

Organic semiconductors are organic materials that exhibit semiconductor properties, such as pi-conjugated oligomers and polymers. The pi-bonding in organic semiconductors allows for interesting electrical and plastic-like mechanical properties due to the charge-transport mechanisms. [2] An organic field-effect transistor (OFET) is a device that uses an electric field to influence the modulation of the conducting channel in an organic semiconductor material and benefits from properties of organic semiconductors. OFETs can be manufactured on plastic films and are therefore mechanically flexible. In addition, they are easy and economical to fabricate and scalable. [2]

Gelinck et al. report fabrication methods for flexible active-matrix monochrome electrophoretic displays based on organic-based thin-film transistors. Displays were fabricated on 25 micron thick polyimide substrates and were bent to a radius of 1 centimeter without significantly interrupting device performance. [3] Furthermore, Someya et al. discuss the development of flexible, organic transistor-based sensor networks that exhibited high sensitivity to thermal and pressure changes. [4] Organic transistor-based pressure and thermal sensors were fabricated on a plastic film to form a flexible net-shaped structure, then attached to the surface of an egg to obtain pressure and thermal images. [4] This research suggests that OFETs are attractive candidates for tactile sensor applications as a result of their special mechanical and electrical properties.

Case Study: OFETs For Flexible Pressure Sensors

Mannsfeld et al. report the development of an "electronic skin" that is sensitive to very low pressure using OFET-based pressure sensors. [5] This sensitivity is a result of a thin rubber dielectric layer in which the rubber was structured into a grid of microstructured pyramids. Large arrays of pixel pressure sensors were fabricated onto a flexible substrate. They found that the high sensitivity of the films was tunable based on the microstructure of the dielectric layer and by modeling the layer as an ideal spring. [5] These structured sensors showed a sensitivity of 0.55 kPa-1 with rapid response speeds, which is approximately 1000x more sensitive than human skin. [5] The density of the rubber pyramid microstructures in the dielectric layer allow for the sensors to detect small subtleties in pressure. This research has application in touch-screen displays, as well as robotics, prosthetic limbs, and wearable electronics.

Implications for Future of Tactile Human-Computer Interactions

An electronic skin that exhibits remarkable sensitivity can potentially address the limitations of current tactile sensing technologies. The case study herein shows the opportunities for highly sensitive materials that can detect the slightest nuances in pressure and can perhaps be applied to touch screen technologies. This OFET-based technology suggests an exciting future for touch displays as this high-sensitivity may allow for a user interface that is more natural and intuitive and thus, more efficient for obtaining information and enjoyable to use.

© Ashley Seni. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] K. Weiss and H. Worn "The Working Principle of Resistive Tactile Sensor Cells," Mechatronics and Automation, 2005 IEEE Intl. Conf. 1, 471 (2005).

[2] Z. Bao and J. J. Locklin, eds., Organic Field-Effect Transistors, Vol. 1 (CRC Press, 2007).

[3] G. H. Gelinck et al., "Flexible Active-Matrix Displays and Shift Registers Based on Solution-Processed Organic Transistors," Nature Mater, 3, 106 (2004).

[4] T. Someya et al., "Conformable, Flexible, Large-Area Networks of Pressure and Thermal Sensors With Organic Transistor Active Matrixes," Proc. Natl. Acad. Sci. 102, 12321 (2005).

[5] S. C. B. Mannsfeld et al., "Highly Sensitive Flexible Pressure Sensors With Microstructured Rubber Dielectric Layers," Nature Mater. 9, 859 (2010).