Spiked silicon surface absorbs more light than flat surfaces
Texturing silicon to produce a spiked surface results in one that absorbs much more incident light than flat silicon surfaces and could increase the efficiency of photovoltaics. Claudia Wu, Tsing-Hua Her, and Eric Mazur of Harvard University (Cambridge, MA) discussed the technique at CLEO `98 (San Francisco, CA). Texturing increases surface absorption by increasing the number of external and internal reflections--in this case, a forest of steep-sided 1-µm-wide spikes is formed by treating the surface with an ultrafast laser. The researchers used 450-µJ subpicosecond 800-nm pulses from a Ti:sapphire laser to irradiate the silicon under pressure in an atmosphere of sulfur hexafluoride and create the spikes over a large area. Depending on the laser fluence, spikes as tall as 40 µm form. Spikes are not formed for longer pulses.
The photocurrent from the spiked area averaged 60% more than that from untextured silicon at a high bias of 40 mV. The improvement is more dramatic at shorter wavelengths--near-infrared absorption is u¥by 20%, green is u¥by 62%, and blue absorption is doubled. There is a question about whether the improvement is caused by increased reflection or changed electrical characteristics of the material. Porous silicon also traps light well but does not absorb blue light quite as well as material textured with this technique, explained Wu.
