Silicon Nanowire Solar Cells: Lowering the Cost of Solar Electricity

M.R. Black
Bandgap Engineering, US

Keywords: nanostructures

Summary:

Nanostructures have unique opto-electronic properties. The unique properties of silicon nanowire arrays, if used properly, can enhance the efficiency of silicon solar cells and are fabricated with a low cost process. The silicon nanowires are able to enhance the efficiency of crystalline silicon solar cells by reducing reflection and increasing absorption. Tapered silicon nanowire arrays have low optical reflection over a broad range of incident wavelengths and angles: less than 1% reflection for a weighted average over the solar spectra. In addition, the absorption of the silicon is enhanced by the nanostructured silicon. The path length is increased by an amount approaching the Lambertian limit for light trapping for much of the long wavelength portion of the solar spectra. The increased absorption increases solar cell efficiency for infrared light of wafer based silicon as well as significantly increasing the amount of light absorbed in thin crystalline silicon solar cells. The low reflection eliminates the need for the traditional anti-reflection coatings while the high absorption eliminates the need for traditional light trapping techniques. Eliminating these steps is especially important for thin crystalline silicon since they are either difficult or not effective with thin silicon. Furthermore, the nanowire process is independent of crystallographic orientation and therefore works on multi crystalline silicon. In addition to the high absorption of light in nanostructured silicon, nanowire solar cells eliminate the need of several processes that are cumbersome for thin silicon solar cells, and thus nanowires are especially well suited for thin crystalline silicon solar cells. Furthermore, nanowires unique properties nanowires allows for the use of a unique low cost contact process that has the potential to eliminate silver from the solar cell processing. In this talk, we present experimental results of the optical reflection and absorption of silicon nanowire arrays. In addition, we will show minority carrier lifetime measurements of nanowires on crystalline silicon as well as external quantum efficiency measurements for silicon nanowire solar cells, both demonstrating many of the concerns of nanostructured solar cells can be overcome with proper device and process engineering.