One problem with silicon (Si) solar cells is that the large difference in refractive index at the Si-air interface reflects much of the incoming light. The problem can be partially resolved by adding an antireflective coating, such as SiN or porous Si, to break the discontinuity in the refractive index into two smaller steps.
Striemer and Fauchet have now created a continuous gradient in the refractive index by varying the porosity of a Si layer along its thickness. The authors modified the electrochemical etching step that creates porous Si by varying the current parabolically with time. The etched channels contained undulations at both the air and unetched Si interfaces that are likely to enhance the range of the effective porosity. Unlike standard antireflective coatings, which show exceptional antireflective properties only over a narrow range of wavelengths, the gradient porous Si showed good and uniform anti-reflectivity for the entire range of visible and infrared wavelengths found in the terrestrial solar spectrum. -- MSL
Appl. Phys. Lett. 81, 2981 (2002).
