Silicon nanocones and solar cells

How do you get more electrical yield out of silicon-organic hybrid solar cells? With silicon nanocones, tuned to the solar spectrum, that mimic moth eyes!

Three researchers at Nanyang Technological University in Singapore have discovered a technique for getting more electrical yield out of solar cells using highly ordered silicon nanocones fabricated by a cost-effective and scalable method, nanosphere lithography. It’s an interesting approach to the perennial challenge of increasing solar cell efficiency to get more power out of the same surface area without using tedious processing steps, to help make solar cells more viable as an energy source.

What Hao Wang, J. X. Wang, and Rusli discovered was that it is possible to make a hybrid organic-silicon solar cell (made with organic compounds ethylene dioxythiophene and poly styrenesulfate) treated with a particular nanostructure, in this case, nanocones distributed at regular intervals. They fabricated the nanocones in regular arrays with controlled

("Solar spectrum en" by Nick84 - http://commons.wikimedia.org/wiki/File:Solar_spectrum_ita.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg#/media/File:Solar_spectrum_en.svg)
(“Solar spectrum en” by Nick84 – http://commons.wikimedia.org/wiki/File:Solar_spectrum_ita.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg#/media/File:Solar_spectrum_en.svg)

dimensions via the nanosphere lithography technique. They found that these cones, which present a more gradual change in the effective refractive index and thus possess better antireflective property (mimicking moth eyes) while keeping manufacturing costs down, become more efficient depending on how far apart they are. They tested distances from 800 nm down to 400 nm,

and found that 400 nm increased power conversion efficiency by more than 7%. The cool thing about that? That tracks pretty closely with the sun’s peak output, in the blue-green range, as you can see by this chart

(via Wikipedia http://en.wikipedia.org/wiki/File:Solar_spectrum_en.svg). Which actually makes a lot of sense when you think about it.

Nanocone construction

Wang, Wang, and Rusli constructed the nonacones using nanosphere lithography. They write:

PS nanospheres were first deposited as a monolayer on an n-type (100) Si substrate as shown in Figure 1a. Briefly, the PS nanosphere solution was first mixed with ethanol in a volume ratio of 1:1 and then dropped on the water surface in a Petri dish. The nanospheres assembled into a monolayer and floated on the water and were subsequently transferred to the Si substrate by the Langmuir-Blodgett assembly method. To obtain SiNC arrays with different periodicities, PS nanospheres with different diameters of 400, 500, 600, and 800 nm were used. Note that the diameter determines the periodicity of the nanocones formed ultimately. The PS nanospheres were first etched using O2plasma to reduce their diameters under the conditions of 30-W RF power, 20-sccm O2 gas flow rate, and 200-mTorr pressure. Following that, chlorine (Cl2) plasma was used to etch the Si substrate masked by the PS nanospheres to form SiNC arrays (Figure 1c). The Cl2 plasma etching was conducted for 5 min with a Cl2 flow rate of 50 sccm under a RF power of 200 W and pressure of 160 mTorr. Note that the nanospheres were also etched in the process of the Si etching. After the etching process, the nanospheres were removed by the organic solvent toluene. For the fabrication of the hybrid cells, PEDOT:PSS (PH1000 from Clevios) solution mixed with 5 wt % dimethyl sulfoxide and 1 wt % Triton 100 was deposited on the SiNCs by spin coating at 3,000 rpm for 50 s. The thickness of the PEDOT:PSS film coated is about 50 nm. Silver grid electrode was then deposited on top of the PEDOT:PSS layer and Ti/Pd/Ag electrode (50/50/1,000 nm) on the rear surface of the Si substrate to complete the hybrid solar cell structure.

Their conclusion is that “The highest PCE of 7.1% has been achieved for the hybrid cell with a periodicity of 400 nm, due to the strong light trapping near the peak of the solar spectrum and good current collected efficiency.”

You’ll find the whole paper in Nanoscale Research Letters, here.

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