Linkoping University scientists have made possible an organic transistor that is driven by temperature changes instead of by an electrical signal. Made of a thermoelectric material, the transistor brings about an appreciable current modulation for just a single degree rise or fall in temperature.

Professor Xavier Crispin, based at the Laboratory of Organic Electronics of the university, states that heat driven transistor is the first logic circuit to be developed that makes use of thermoelectricity.

Wide Range of Applications

The scientists foresee diverse uses for the new transistor. Since the device can record very small temperature changes, healthcare professionals can use it to fabricate therapeutic dressings that monitor the healing process along with treating the patient.

The scientists say it would be possible to build circuits that would respond to the heat contained in infrared radiations, too. This could be of particular use in developing heat cameras and similar devices.

The organic transistor is highly susceptible to minute heat changes. Compared to conventional thermoelectric devices, it is 100 times more sensitive to a drop or rise in temperature. This high level of heat sensitivity implies that just one electrical connector from a heat sensor electrolyte is adequate for sending a signal to the transistor. The researchers explain that a pair of a thermoelectric transistor and a sensor connector would be sufficient to make up a “smart pixel” for the camera.

A set of these smart pixels could make up a matrix, which may serve as a detector. This could be used in place of the numerous sensors used for detecting infrared rays in existing heat cameras. The researchers hope to add in more developments so that even a device as small as a mobile phone can include a heat camera. Since the materials needed for fabrication are non-toxic, inexpensive, and easily available, the feature could be had at a low cost.

Sunlight Charged Supercapacitor

The researchers built the heat-powered transistor by exploring a technique that allowed charging a supercapacitor by sunlight. The capacitor, developed a year ago, captures the light photons falling on it to convert to electricity, which is stored within it for further use. Crispin explains that it was crucial to establish the working of the heat driven supercapacitor before looking into possible electrolytes and the range of possibilities.

The university team researchers looked through a wide range of conducting polymers to turn out a liquid electrolyte that can produce a potential difference from a temperature gradient a hundred times more than that most electrolytes generate. While the positive ions of the electrolyte are small and move quickly through the liquid, the polymer molecules are negatively charged and massive, and move slowly. When a part of the electrolyte is heated, the lighter positive ions move to the colder regions rapidly. The separation of the positive ions from the negatively charged polymer molecules generates a potential difference or a voltage, which is adequate for transistor applications.

Team members Simone Fabiano, a lecturer, and Dan Zhao, a researcher engineer, have worked extensively with the electrolyte to show that heat signals can be used to make electronics controlled by heat signals.