Making plastic laser diodes a reality

Organic semiconductors have made their way into many applications in recent years, including computer displays, lighting and solar panels. They have several advantages over their conventional electronic counterparts, including low cost and comparatively simple manufacture. Their optical and electrical properties make them suitable for transistors and light emitting diodes, but it has not been possible to develop electrically-driven laser diodes with them. Researchers at Imperial College, London, have been working with a class of material that may well make such devices a reality.

In a laser diode, an electric current flows through the junction between two different types of semiconductor, producing charge carriers in that region. When a pair of charge carriers (the negative electron and the positive hole where an electron is missing) recombine, a photon is emitted. Such events occur spontaneously and will induce further photon emission by stimulating other pairs of charged particles to recombine. The light emission amplifies as photons at specific wavelengths reflect back and forth inside the active region, thereby producing the laser output.

Up until now, no organic semiconductor has demonstrated the necessary optical and electrical properties for it to work as a laser diode. Tailoring the semiconductor material for its electrical properties had an adverse effect on its optical properties, and optimising it for the optical properties meant that it could not carry sufficient current. The Imperial College researchers have been experimenting with the properties of organic semiconductor materials and believe they are closer to finding one that will.

The researchers' report, published in Nature Materials¹, states that they have produced an organic semiconductor with both high charge mobility and excellent light-emitting characteristics, and they demonstrated its use as the semiconductor in a distributed feedback laser, a type of laser commonly used in optical communications. The device was optically pumped, meaning that a separate light source was used to excite the charge carriers responsible for the lasing action, however other experiments on the same material demonstrated that it also had high electrical mobility, suggesting that an electrically-pumped organic semiconductor laser may indeed be possible.

References:
1) "Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films", Boon Kar Yap, Ruidong Xia, Mariano Campoy-Quiles, Paul N. Stavrinou & Donal D. C. Bradley, Nature Materials 7, 376 - 380 (2008) Published online: 13 April 2008 | doi:10.1038/nmat2165