Modern electronic devices rely on semiconductors such as silicon. But turning these inorganic materials into useful devices requires careful crystallization and painstaking etching processes that increase the cost of devices like solar panels and light-emitting diodes.

One alternative is to use carbon-based semiconductors, including certain polymers that can be dissolved in solution and then spread or sprayed onto a surface to leave a thin film. This fabrication method is potentially much cheaper and quicker than conventional semiconductor preparation. Most electronic devices, however, require several different layers of semiconductor materials to be sandwiched together to give functional heterostructures, which presents a big challenge for polymers: adding a second layer can often re-dissolve the first one, destroying its electrical properties.

A solution to this problem has now been developed by a team of collaborating researchers from the National University of Singapore, the University of Cambridge and Cambridge Display Technology in the UK.1

Fig. 1: A new photocrosslinking methodology promises to add functional layers to make polymer-based organic light-emitting panels and displays even more energy efficient.

Before applying the polymers to the substrate, the researchers added bis-fluorophenyl azide, developed in-house, to the polymer mixture. Exposing the azide to deep ultraviolet light then triggered a chemical reaction that induced the photocrosslinking of side chains, forming bonds among the polymers chains. This stopped the polymer re-dissolving when another layer was added over the top, without affecting the polymer’s semiconducting properties.

This relatively simple technique was used to make a variety of devices, including the light-harvesting part of a photovoltaic cell, a field-effect transistor (the basis of modern integrated circuits), and light-emitting diodes (LEDs).

The devices were just as efficient as similar polymer-based devices made without the azide additive. Furthermore, the scientists were able to fine-tune the electronic structure of the junctions between materials, and thus the performance of the devices, by precisely controlling the thickness of the polymer layer. In their LED, the addition of a 10 nm-thick layer of a polymer called TFB made the LED ten times more efficient than without the layer by confining charge carriers (electrons and ‘holes’) to the light-emitting film.

The team also used this technique to make adjacent donor-acceptor heterostructures in which the electron and hole conduction paths have built-in continuity, thereby increasing the photon-to-electron conversion efficiency of the device. The scientists say that their photocrosslinking technique can be applied to many different polymers, and should make it much simpler and quicker to build prototypes of a wide range of new electronic devices.