Organic semiconductors have many attractive features: they are easy to make, they can emit visible light, and there is tremendous scope for tailoring their properties to specific applications by changing their chemical structure. Research groups and companies around the world have developed a wide range of organic-semiconductor devices, including transistors, light-emitting diodes (LEDs), solar cells and lasers.
Semiconducting polymers or plastics are an important class of organic semiconductor. They can be dissolved in solution and then printed to make devices such as transistors and LEDs, which allows flat and even flexible electronics and displays to be produced.
Light emission from semiconducting polymers is a particularly promising area of application, most notably in displays. Moreover, the light emitted by the polymer provides a fascinating window through which to view the physics of the materials.
A polymer LED consists of a thin polymer film sandwiched between two contacts. When a voltage is applied between the contacts, electrons are injected from one contact and positive "holes" from the other. These charges move through the device under the applied electric field and then meet up to form an excited state called an exciton. Some of these excitons emit light when they decay to the ground state.
The "spin" associated with electric charges such as electrons and holes has profound consequences across the whole of physics, and polymers are no exception. When an electron and a hole interact in a polymer, quantum mechanics tells us that their spins can combine in four different ways. Three of the possible outcomes are called "triplets" and the fourth is a "singlet" (see figure). Symmetry arguments prevent the triplets from emitting light when they decay, but the singlet has the opposite symmetry and can emit photons. Since there are three times as many triplets as singlets, the efficiency of the light emission has a theoretical maximum of 25%.
Overcoming this problem has been a vigorous area of research in recent years. Stephen Forrest of Princeton University and Mark Thompson of the University of Southern California have shown that triplets can emit light if a heavy-metal atom such as platinum or iridium is introduced into the organic semiconductor. The spin-orbit coupling caused by the heavy atoms means that light emission from triplet excited states is not completely forbidden.
Groups in Cambridge, Oxford and St Andrews have shown that this approach also works in other organic semiconductors such as polymers and snowflake-shaped molecules called dendrimers. However, in all the materials studied so far the concentration of the heavy metal has been very high approximately one metal atom per molecule or repeat unit.
Now John Lupton of the Max Planck Institute for Polymer Research in Mainz and co-workers have observed strong light emission from triplet excitons in a polymer that does not contain a metal in its structure (J M Lupton et al. 2002 Phys. Rev. Lett. 89 167401). The material investigated is a member of a family of polymers that has been extensively studied, which makes the finding all the more surprising.
In the December issue of Physics World, Ifor Samuel explains how small an amount of palladium in polymer based LEDs is needed to improve its light-emitting properties.