Organic semiconductor spintronics: utilizing triplet excitons in organic electronics.

Organic electronics now supports a rapidly growing industry, including organic light-emitting diode (OLED) displays as used in cell phone displays and upmarket TVs. This has been enabled both by successful engineering of materials and devices, and also through the design of new device architectures that allow control of electron spin. This is a rapidly moving field, and some recent advances in the basic semiconductor science are likely to enable new applications. This issue will focus on the role of the spin state of the bound electron-hole pairs (excitons) that provide light emission in LEDs or separate to give free charge in solar cells. The spins of the two electrons involved in these excitons can be arranged as zero-spin ‘singlet’ states or spin-1 ‘triplet’ states, and for most organic semiconductors the spin exchange energy raises the singlet state substantially above the triplet, typically by 0.5 eV. For simple OLEDs, only 25% of the electron-hole recombination events can form spin singlet excitons that can then emit photons, with the remaining 75% forming non-emissive triplet excitons. This is a severe limitation to LED efficiency and a number of approaches are developed to avoid this limitation. First, it turns out that collisions between triplet excitons can result in their ‘fusion’ to form an emissive spin singlet exciton, and under some conditions this can be the dominant decay channel for triplet excitons. How much this can raise efficiency remains an active research question. Second, direct emission from the triplet exciton (phosphorescence) can be achieved if strong spin–orbit coupling can be introduced. Organometallic compounds containing iridium, platinum and osmium have been found effective, particularly for red and green emission. Third, there has been very recent progress in the design of molecular semiconductors with very