Sub-micron texturing for photovoltaic antireflection and light-trapping

Abstract

The photovoltaic solar energy industry has experienced substantial growth over the last few years and this growth has led to manufacturing cost reductions that have brought solar energy to grid parity in many parts of the world. Grid parity will inevitably lead to further expansions of the industry as uptake of the technology will no longer be subsidy driven. As the industry continues to mature, scientific and technological innovations that reduce the $/Watt cost of solar energy will allow companies to gain a competitive edge and increasingly ensure that solar energy can become affordable for communities in the developing world. Development of antireflection and light trapping schemes that can help increase efficiency or else decrease material requirements of solar cells are one way that science and technology might contribute to the drive for reduced $/Watt.

Three novel types of sub-micron scale texturing are examined in this work. In each, advanced nanofabrication techniques, specialist characterisation apparatus and complex simulation software are used to gain insight into these new structures.

Sub-micron inverted pyramid structures formed in silicon by a combination of e-beam lithography and a Potassium hydroxide (KOH) etch are found to have a significant antireflective effect. The best structures are found to have a weighted reflectance of 10% when the periodicity is 700 nm, when the spacing between pyramids is as small as possible, and when a thin layer of SiO2 is added as a antireflection coating. Further reduction of reflectance is only possible by increasing feature size and there appear are no low-reflection sweet spots in sub-micron designs.

Mie Resonator structures were formed in silicon by a combination of e-beam lithography, lift-off and reactive ion etching through a metal hard mask. We have found that the best structures, consisting of arrays of silicon cylinders with diameters of 180nm, heights of 95 nm and periodicity of 500 nm, have weighted reflectance as low as 5.5% and, in fact, better than silicon moth-eye structures. Mie resonator structures seem to provide a very promising antireflective surface that also confer light-trapping effects. The structures be easier to fabricate that moth-eye structures while significantly reducing the silicon wastage associated with micron-scale inverted pyramid technologies that are conventionally used.

Finally, we have carried out an investigation of a self-forming silicon nanowire surfaces that can reduce weighted reflectance to values as low as 0.05% although these are impressive antireflective surfaces that might find application in some optoelectronic systems where scattered light should be reduced, we have concluded that the nanowire structures are unlikely to help solar cell design as increased surface area and surface contamination and damage is likely to greatly increase surface recombination and limit device efficiencies.

In conclusion, it is clear that Mie Resonator structures are amongst the most promising antireflective surfaces and further studies should focus on optimisation of light-trapping and device integration of these structures. The use of Mie resonator structures might eventually allow Crystalline Silicon (C-Si) wafer thickness to decrease to a few microns without reduction in device efficiency.

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