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LEDs Learn from Solar Cells

Photonics Spectra
Nov 2001
Paul Mortensen

A silicon LED with a conversion efficiency 100 times better than current devices promises to enable the integration of LEDs with microchips. The development could eliminate the need for wires for intra- and interchip communication and the electronic bottleneck of silicon-based devices.

The achievement emerged from work on solar cells, explained Martin A. Green of the Centre for Third Generation Photovoltaics at the University of New South Wales in Sydney, Australia, one of the researchers on the project. "Solar cells absorb light and convert it into electricity, the reverse process to light-emitting diodes. It turns out there's a relationship between the ability of silicon to absorb light well and to emit it well."

When light is absorbed by a solar cell, it may travel through the silicon until it hits part of the crystal, dislodging an electron and creating a "hole." A solar cell works by making the holes move in one direction and the freed electrons in the opposite, which generates an electric current.

The light-generating effect does the reverse. It encourages free electrons to fall into holes to a lower energy level and thus give up energy. Previously, this energy was wasted as heat, but the researchers have manipulated the conditions within the silicon crystal so that it emerges as light.

Specifically, to achieve a conversion efficiency of 1 percent, the researchers reduced parasitic internal absorption, such as by free carriers or by the metal corners of the substrate. A similar approach has produced silicon solar cells with an efficiency of 24.7 percent.

The team also increased the efficiency by manipulating the device's geometry. Both high-efficiency solar cells and the LED use inverted pyramids at the surface with a 5-µm base width.

Trapped light absorbed

In a solar cell, the pyramids increase light absorption by ensuring that the light reflected at the first point of incidence will travel downward and that it will be internally reflected in the cell. In the LED, the pyramids increase the efficiency by trapping weakly absorbed light and by maximizing the absorption at the proper subbandgap wavelengths.

The development could greatly influence the future of microelectronics. Rather than being confined to today's flatland of two-dimensional structures, engineers could create 3-D geometries, linked optically.

The group has taken out a patent on the work, and the university has funded a high-throughput testing laboratory. The researchers estimate that it will take five years to commercialize the LED.

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