Researchers in Taiwan have discovered that by overlaying an indium-tin-oxide (ITO) nanowhisker material on top of conventional solar cells, the efficiency response is significantly improved in the visible and near-infrared parts of the electromagnetic spectrum. Such an improvement arises from the nanowhiskers' broadband antireflective properties (R<5% from 350 nm to 1200 nm). Cells that feature the compound surface textures appear nearly black to the human eye.

Optical buffer

As it turns out, optical reflection loss arises when light crosses the boundary between two different dielectrics, in other words, when it experiences a discontinuity in refractive indices. From their studies, the scientists discovered that ITO nanowhiskers possess a graded refractive-index profile due to varying air densities from top to bottom, which serves as an excellent buffer between air and silicon for a broad spectral range. Materials with such unique refractive indices are not readily available in nature.

The nanowhiskers are fabricated by electron-beam evaporation in an oxygen-deficient ambient. Compared with other nanofabrication techniques that involve dry or wet etching, the deposition of ITO nanowhiskers does not create additional surface states that degrade photocurrent flow, which is truly important for solar cells.

The experiment demonstrates that compound surface textures are an effective means to improve light harvesting in thin silicon solar cells. What's more, the result may open the door for the application of other nanomaterials in solar cells.

A full description of this research can be found in the journal Nanotechnology.