Harvard University: Mazur's Si Black--this tech is definitely useful to enhance solar cell efficiency!

Irradiating a silicon surface with femtosecond laser pulses in the presence of a sulfur containing gas transforms the flat, mirror-like surface of a silicon wafer into a forest of microscopic spikes. The spikes are tens of micrometers tall and have tip sizes on the order of hundreds of nanometers. The surface morphology depends strongly on the characteristics of the laser pulses: they must be ultrashort and very intense. Also, the gas in which the silicon is placed during the irradiation is critical. Depending on the background gas the shape of microstructures can vary from sharp and tall to rounded and short. The importance of the surrounding gas suggests that chemical reactions are involved in the formation of the spikes.











silicon microstructures as viewed under a scanning electron microscope.












Black silicon surface viewed at high magnification

The structured surface is strongly light-absorbing; the surface of silicon, normally gray and shiny, turns deep black. In addition to near-unity absorption in the visible, the irradiated surface absorbs over 80 percent of infrared light for wavelengths as long as 2500 nm. Photodiodes with remarkable responsivity in both the visible and infrared can be made using this microstructuring process. We are investigating using the extended absorption range to make silicon solar cells that convert more of the sun's spectrum into usable electricity.Sulfur plays a critical role in the high absorption and unique optoelectonic properties. The laser irradiation leaves a thin, disordered surface layer that is highly sulfur doped that is crucial for the optical properties. Current work focuses on finding new optical and electronic applications for this unique surface, understanding the mechanism for light absorption and photocurrent generation in the infrared, and looking into other fruitful substrate/dopant combinations.

Plainly Speaking
Silicon is the material of computer chips and solar cells. It is ubiquitous throughout the high-tech world. Ordinarily, silicon absorbs a moderate amount of visible light, but a substantial amount of visible light is reflected as well, and infrared and ultraviolet light are transmitted through silicon or reflected from it with very little absorption.


Spiked silicon surfaces, in contrast, absorb nearly all light at wavelengths ranging from the ultraviolet to the infrared. This suggests it may be very useful in improving the performance of existing silicon devices, such as detectors and photovoltaics. Spiked silicon is made by shining a series of very short, very intense laser pulses at a silicon surface in a chamber filled with a gas such as sulfur hexafluoride or chlorine. In the presence of the laser light, the gas reacts with the silicon surface etches away some of it, leaving a pattern of conical spikes behind.