Organic semiconductors show potential for building electronic circuits onto almost any material. As well as reducing the cost and increasing the performance of consumer electronic devices such as widescreen TVs, organic semiconductors are expected to lead to entirely new technologies, such as 'smart textiles' that can sense and respond to their environment.

Yet surprisingly little is known about how the electronic characteristics of these materials change when they are incorporated into working devices. To address this, Mitsumasa Iwamoto and colleagues from the Tokyo Institute of Technology in Japan1 have developed a direct observation technique for monitoring the movement of electronic charges inside organic field-effect transistors (OFETs) under normal operating conditions.

The number of different organic semiconductors that have been discovered is far greater than for inorganic semiconductors like silicon or gallium arsenide. This allows much greater flexibility in choosing an organic material with the most appropriate characteristics for a given task. But it also makes the job of determining the characteristics of so large a family of materials much more difficult.

The technique developed by Iwamoto and his colleagues eases this task by enabling the electronic behavior of an organic material to be studied optically. It works by focusing a laser onto a small spot within a material, and measuring the light that is emitted from the surface. Some of the light is emitted by a process known as second harmonic generation (SHG), which produces light at twice the frequency of the incident laser light and with an intensity and polarization that depends on the size, direction and strength of the electric field at the point where it is produced.

Fig. 1: Illustration of an optical technique for probing the electronic characteristic of a pentacene transistor.

By scanning the laser spot across the semiconducting pentacene channel of an OFET, the researchers were able to determine how the distribution and type of charge varies throughout the channel at different operating voltages (Fig. 1). And by capturing the emitted light on camera at different times after sending a pulse of electrical current through the device, the flow of charges could be captured. Such information is crucial to understanding, modeling and improving the performance of these and other organic devices and materials.