Research from a group in Taiwan has demonstrated the feasibility of using monolayers of nanospheres to improve the broadband optical absorption of silicon solar cells, greatly improving their conversion efficiency.

Silicon has been used in solar cells for decades, and currently comprises 70% of the global solar market. But despite its popularity, the energy conversion efficiencies of most commercial silicon solar cells fall well below that predicted by theory, or as demonstrated in the lab. One of the issues with these solar cells is that they are highly reflective – up to 40% of incident light is lost through surface reflection – and so, various research efforts have focused on the development of antireflective coatings.

Much of the literature suggests the use of silicon nanostructures to mitigate undesired surface reflectance, but most attempts have resulted in low energy conversion efficiencies. Other approaches have used nanowires or metal nanoparticles, but many of these coatings suffer from problems with low adhesion, or can only operate at limited wavelengths or angles of incident light. Finding a material with a similar refractive index to air would solve part of this issue, and a group of Taiwanese scientists believe they have the answer.

Nanospheres for omnidirectional solar cells

Using polystyrene nanospheres, the group have developed an antireflective coating that reduces broadband reflection, provides excellent coverage over a range of incident angles and improves the conversion efficiency of silicon solar cells by up to 21.6%. Early-stage simulations showed that a centre-to-centre distance of 450 nm would provide the optimal reduction in refection in the visible range, and so, a periodic hexagonal monolayer of polystyrene nanospheres was applied to silicon solar cells.

Analysis of the reflectance spectra in the range of 400 – 800 nm showed that the polystyrene-coated cells had significantly lower reflection than the bare silicon cells. Specular reflectance measurements also showed that the coating maintained its antireflective properties at incident angles of up to 85°, indicating that the nanospheres provide excellent light harvesting characteristics. It was also found that the nanospheres facilitated wave propagation into the device by Mie scattering. The resulting improvement in conversion efficiency approached 21.6%.

The group, led by Jr-Hau He, believe that their coting could improve the properties of a range of optoelectronic devices, and with the low cost of the process, they are confident that it will find its way to the market soon.

Nano Energy (2014) doi:10.1016/j.nanoen.2014.03.004

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