Small molecule and polymeric semiconductors have been designed for solution processable transistor devices. Intermolecular overlap between electronic orbitals on neighbouring molecules, optimal for transport, can be guided by the subtle balance of electrostatic and van der Walls forces, and the steric size effects of carefully chosen side groups, leading to mobilities of greater than 1 cm2/Vs. In polymer semiconductors understanding the impact of electronic delocalisation, backbone conformation and arrangement of side chains, on the thermal properties and thin film microstructure, has led to more optimised processing regimes and higher electrical performance.
Currently, the most advanced application for organic transistors is in displays. These active matrix displays are comprised of pixels which give rise to an image when either an electrical field or current is applied across the pixel. Each pixel is switched by individual transistors that are driven electrically by conductive data lines, the so-called switching backplane. The now commercial electrophoretic display (e.g. ebook displays such as those used by the Amazon Kindle) is a reflective effect and each switching transistor can occupy almost the full area underneath the pixel. This means that the transistor width is maximised and can deliver more current per pixel, compensating for the lower mobility of the organic semiconductor. Another favourable aspect of the electrophoretic display effect is that once the pixel is charged no further power is required to retain the image. The load on each transistor is therefore minimised, and subsequently the devices last longer as their operational times are reduced.
In realising complex future applications, understanding processing-structure-property relationships is key to making robust organic transistors in a reliable and reproducible fashion.