Rough Optics for Better Solar Cells

Thin-film solar cells made of crystalline silicon are inex­pensive and achieve effi­ciencies of a good 14 percent. However, they could do even better if their shiny surfaces reflected less light. A team led by Christiane Becker from the Helmholtz-Zentrum Berlin (HZB) has now patented a sophis­ticated new solution to this problem. “It is not enough simply to bring more light into the cell,” says Christiane Becker. Such surface structures can even ultimately reduce the efficiency by impairing the elec­tronic properties of the material.

The nanostructure for capturing light is imprinted on silicon oxide (blue) and then levelled with titanium oxide (green). The result is an optically rough but smooth layer on which crystalline silicon can be grown. (Source: HZB)

The idea that David Eisen­hauer worked out as part of his doc­torate in Becker’s team sounds quite simple, but it requires a completely new approach: to produce a structure that behaves opti­cally rough and scatters the light, but at the same time provides a smooth surface on which the silicon layer can grow with virtually no defects. The proce­dure consists of several steps: first, the researchers imprint an optimised nano­structure onto a still liquid silicon oxide precursor layer that is then cured with UV light and heat.

This creates tiny, regularly arranged cylin­drical ele­vations that are ideal for capturing light. However, the absorbing layer of crystal­line silicon cannot grow flaw­lessly on this rough surface, so these structures have an un­favorable effect on the quality of the solar cell. In order to resolve this conflict, a very thin layer of titanium oxide is spin coated on top of the nano­structure in order to produce a relatively smooth surface on which the actual absorber material can be deposited and crystal­lized. The coating has the descrip­tive name “SMART” for “smooth anti-reflective three-dimen­sional texture”. It reduces reflections and brings more light into the absorbing layer without impairing its electronic properties. The procedure is now patented. (Source: HZB)

Reference: D. Eisenhauer et al.: Smooth anti-reflective three-dimensional textures for liquid phase crystallized silicon thin-film solar cells on glass, Sci. Rep. 7, 2658 (2017); DOI: 10.1038/s41598-017-02874-y

Link: Young Investigator Group Nano-SIPPE, Helmholtz-Centre Berlin for Materials and Energy HZB, Berlin, Germany

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