Spiny Photovoltaics

April 18, 2012

Aside from optimizing the photovoltaic material, itself, it's important that photovoltaic cells take advantage of all the insolation provided to them. In the case of flat panels, this often involves a motorized stage to track the sun. Even then, the refractive index difference between air and the material will cause some of the light to be lost through reflection.

One method of mitigating reflection is through the use of an antireflection coating. Putting a layer of an intermediate refractive index between your material (n = 3.96 for silicon at visible wavelengths) and air (n_o = 1.00) will help. The optimum refractive index of this coating would be √(n ⋅ n_o), which is about 2.00 for the silicon/air interface; but this trick will work for just a narrow range of wavelengths, and sunlight is broadband. That's why there have been many schemes to "trap" light in topographic structures.

In a recent article (Solar Whispers, February 22, 2012), I reported on a light-trapping approach developed by a team of Stanford University engineers. This involved hollow microspheres of nanocrystalline-silicon on the surface of photovoltaic cells. These nanoshells support the optical whispering gallery resonant modes that trap light to increase efficiency. Other approaches involve regular arrays of silicon nanowires on flat substrates that increase the path length of incident light by as much as seventy-three times.[1]

A team of materials scientists from the Swiss Federal Laboratories for Materials Science and Technology (EMPA) and the Institut Européen des Membranes at the Université Montpellier (France) has just published details of a light-trapping architecture for zinc oxide (ZnO) nanowire solar cells that's based on the spiny structure of the sea urchin.[2]

The green sea urchin (Strongheart droebachiensis). (NOAA image).

This is a follow-up to these authors' previous research in this area. Their previously published results on ZnO nanowires, electrodeposited onto surface-activated polymer spheres, showed a twofold improvement of light absorption compared to simple nanowire arrays; but these nanostructures had limited mechanical stability.[2] In the present work, they hollowed-out the spheres and produced a photovoltaic structure by a low-cost and scalable process that allows a fine tuning of the optical absorptive properties.

Ordered arrays of polystyrene spheres with ZnO nanowires. The scale bars are 2 μm. (Fig. 2(b) of ref. 3, via arXiv Preprint Server).[3]

The process starts with an array of polystyrene spheres self-organized into an array on a fluorine-doped tin oxide (FTO) coated glass substrate. These spheres are reduced in size by reactive ion etching, coated with ZnO using atomic layer deposition (ALD), and then overlaid with a forest of ZnO nanowires, as seen in the above figure. The detailed process steps are as follow:[3]

a) An ordered monolayer of polystyrene microspheres is deposited on an FTO covered glass substrate by dip coating.
b) The size of the spheres is reduced using reactive ion etching with an oxygen plasma. The final size of the spheres is controlled by the etching time.
c) A conformal layer of ZnO, about 20 nm, is deposited on the spheres using ALD.
d) n-type ZnO nanowires of controlled length and diameter are electrodeposited on the thin ZnO layer.
e) The polystyrene spheres are dissolved in toluene, leaving hollow ZnO spheres studded with nanowires.
f) The nanowires are coated by electrodeposition with an absorber film of cadmium selenide (CdSe).
g) The assemblage is covered with p-type CuSCN by chemical impregnation.
h) A final gold electrode is formed by physical vapor deposition.

Hollowed polystyrene spheres with ZnO nanowires. These spherical shells were uprooted from the substrate by scratching to reveal the underside. The scale bar is 2 μm in the leftmost micrograph.(Figs. 2(d) and 2(e) of ref. 3, via arXiv Preprint Server).[3]

These sea urchin inspired photovoltaics had a uniform absorption of up to 90% in the wavelength range of 400-800 nm. Solar conversion efficiencies of about 1.33% were achieved, which is less than that for published results for standard process ZnO/CdSe/CuSCN solar-cells (about 2%). The authors note that the process has the potential for optimization and improvement, and it might be applied to other materials.

Support for this research was from the Swiss Federal Office for Energy, Solar Cell project, No. 103296.

References:

1. Erik Garnett and Peidong Yang, "Light Trapping in Silicon Nanowire Solar Cells," Nano Letters, vol. 10, no. 3 (March 10, 2010), pp. 1082-1087; available as a PDF file, here.
2. Jamil Elias, Claude Lévy-Clément, Mikhael Bechelany, Johann Michler, Guillaume-Yangshu Wang, Zhao Wang and Laetitia Philippe, "Hollow Urchin-like ZnO thin Films by Electrochemical Deposition," Adv. Mater. vol. 22, no. 14 (April 12, 2010), pp. 1607-1612.
3. Jamil Elias, Mikhael Bechelany, Ivo Utke, Rolf Erni, Davood Hosseini, Johann Michler and Laetitia Philippe, "Urchin-inspired zinc oxide as building blocks for nanostructured solar cells," ArXiv Preprint Server, April 12, 2012).