Inorganic–organic marriage

Adv. Mater. 18, 2545–2548 (2006)

Organic light-emitting semiconductors show great promise for the optoelectronics industry, potentially opening the door to low-cost, flat and perhaps even flexible displays. Organic light emitters do, however, have a significant drawback: a short operation lifetime. The problem is especially serious for white-light organic LEDs (OLEDs) where several different organics are used and as each degrades at a different rate, the purity of the white light emitted changes.

Inspired by the knowledge that inorganic semiconductors suffer from no such limitation, a team at Lecce University in Italy has taken the step of making a hybrid organic–inorganic LED. The device consists of an OLED incorporating three inorganic components, emitting at 490 nm (blue), 540 nm (green) and 615 nm (red) to give a white-light output. Preliminary results suggest that the luminance of these initial LEDs already satisfies the requirements of many applications and that they could represent a cheap, efficient form of future lighting.

Photonic crystals boost output

Appl. Phys. Lett. 89, 173502 (2006)

By adding a spin-on-glass photonic crystal (PC) structure into an OLED, scientists from South Korea have succeeded in boosting the light output by up to 85%.

The approach used by Yoon-Chang Kim and co-workers relies on PCs that consist of a layer of high-index silicon nitride riddled with an array of nanoholes, and filled with a low-index material by spin coating. In order to reduce surface plasmon formation, which would degrade device performance, they then insert an additional high-index silicon nitride film between the PC and the indium tin oxide electrode of the OLED.

The researchers speculate that the high-index film collects photons in the adjacent organic layer and redirects the light to the front side through a microcavity effect. Experiments indicate that the PC-based OLEDs have a light-extraction efficiency that is enhanced by up to 85% at 500 nm, compared with conventional OLEDs.

Nanotubes drive OLEDs

Nano Lett. 6, 2472–2477 (2006)

Single-walled carbon nanotubes— cylindrical carbon molecules with nanoscale diameters — are becoming increasingly important thanks to their electrical, mechanical and optical properties. Now, Jianfeng Li and co-workers have demonstrated that a two-dimensional network of such nanotubes can create highly effective anodes that improve the operation of OLEDs.

The US-based researchers fabricate nanotube films from carbon nanotube powders and deposit the films onto a polyethylene terephthalate substrate. A polymer-blend hole-transporting layer is then spin-coated onto the nanotube films to produce the OLED structure. The nanotube-based OLEDs provide a maximum light output of 3,500 cd m−2 and a current efficiency of 1.6 cd A−1. The performance is attributed to the large surface area of the nanotube films, which provides a superior electron–hole recombination density and enhanced hole injection.

Inspired by their results, the team from Northwestern University and University of California, Los Angeles, is now striving to make nanotube OLEDs that are brighter and suit the needs of display applications.

Entering the mid-infrared

Appl. Phys. Lett. 89, 131110 (2006)

Convenient light sources that emit in the mid-infrared part of the spectrum (wavelengths between 3 and 10 μm) are desirable for a wide range of applications, including medical treatments and sensing greenhouse gases. To this end, there is a continuous drive to improve the efficiency of semiconductors that can generate light in this range.

A UK collaboration between the University of Bristol and QinetiQ has now made an efficient InSb emitter that can generate light with a peak emission between 4.5 and 5 μm. Although the device does operate at room temperature, its efficiency is superior at lower temperatures with 85% of injected electron–hole pairs being converted into photons at 15 K.

To optimize the LED efficiency, the team use thin layers of InSb sandwiched between AlInSb barriers; varying the thickness of the layer allows control over the peak emission wavelength.

Shape of things to come

Appl. Phys. Lett. 89, 171116 (2006)

Credit: © 2006 AIP

A fundamental limitation to the efficiency of LEDs is the high refractive index of most semiconductors, which severely restricts how much light can leave the device. Total internal reflection at a flat interface between the semiconductor and air means that only light emitted into a cone with a half angle of about 25° can be extracted from a device. Such is the case with GaN — a popular material for making blue LEDs and laser diodes.

Now, Akihiko Murai and colleagues at the University of California at Santa Barbara, USA, have improved the efficiency of GaN LEDs by a factor of four by bonding truncated pyramids of ZnO to the surface of the GaN. Texturing the surface of the LED in this way reduces the internal reflection, allowing more light to escape.

The resulting blue LED has an external efficiency of 22% at a drive current of 1 mA, compared with an efficiency of only 5% in a conventional device. The team says that the use of other common light-extraction strategies, such as encapsulation with a dielectric, could lead to still further improvements.