Mar 9, 2011
Compound surface textures maximize solar-energy harvesting
As silicon photovoltaics evolve towards thin-wafer technologies, efficient optical absorption at near-infrared wavelengths has become particularly challenging. Current technology employs potassium hydroxide (KOH)-etched microgrooves with silicon nitrides (SiNx) to reduce losses due to reflectivity and the unsuccessful absorption of photons. Yet the technique does not work well for all wavelengths in the solar radiation spectrum. Now novel nanomaterials may offer a solution to this technology challenge.
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.
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.
About the author
The study was conducted by research teams of the Green Photonics Laboratory at National Chiao Tung University (NCTU), the Green Energy and Environment Research Labs at Industrial Technology Research Institute (ITRI) in Hsinchu, Taiwan, and the Research Center for Applied Sciences (RCAS), Academia Sinica in Taipei, Taiwan. C-H Chang, M-H Hsu and P-C Tseng are graduate students in electro-optical engineering at NCTU, who performed the nanowhisker growth, characterizations and analyses of devices. Prof. P Yu leads the Green Photonics Laboratory and guided the project. The group specializes in the synthesis and fabrication of nanomaterials for optoelectronic devices, particularly solar cells. Dr W-L Chang, Dr W-C Sun and Dr W-C Hsu at ITRI fabricated the silicon solar cells. Dr Y C Chang is head of the RCAS at Academia Sinica, who supported Dr S-H Hsu in measuring and analysing the optical properties of ITO nanowhiskers.