Tikalon Blog by Dev Gualtieri

Blu-Ray Solar Cells

December 12, 2014

Perhaps you're an aspiring artist with a brilliant idea. You decide to produce a canvas that's entirely black. Unfortunately, you find that this has already been done. Undeterred, you have the idea of a black canvas with your fingerprint, rendered in white paint, along an edge. Better yet, this will be a series in which black canvases will display fingerprints of different colors.

After the paint dries on the first canvas, you're disappointed to see that the black isn't completely black. It seems to be somewhat gray, so you curse the paint manufacturer and try again with a different brand of paint. Same thing! Disgusted, with your artistic aspirations completely ruined, you decide to follow in your father's footsteps and become a physicist.

In your study of physics, you find that the culprit in this case wasn't shoddy paint manufacture, it was a basic law of optics. That's the principle that there's always reflection of light at an interface between two different dielectric constants. In your case, the difference in dielectric constant between the layer of black paint and air. The surface of any material with a refractive index n different from air (n=1) will necessarily reflect light according to the reflectance equation,

where n₁ and n₂ are the refractive indices of air and your material (in any order). I wrote about black and white materials, and how reflection is important to the qualities of black and white, in several previous articles (Whiter Whites, September 5, 2014 , Thin and Black, August 2, 2013, Paint it Black, February 13, 2013, Very White and Very Black, November 23, 2011, and White Roofs, March 19, 2012).

Reflectance is a problem for solar cells, since reflected light is lost light. As I wrote in a previous article (Light Trap, January 7, 2013), one way to increase solar cell efficiency is to add a light-trapping layer to the surface. This was done in 2012 by a team of Electrical engineers at Princeton University who added a nanoscale aperture mesh of 175 nm diameter to an organic photovoltaic cell (see figure).[1-2]

A nanoscale cavity mesh

The holes are 175 nm in diameter, separated by 25 nm, in a 30 nm thick gold film.

(Princeton University Image).

In order to stretch research funding, scientists are always looking for inexpensive ways to accomplish laboratory tasks. One example is the use of adhesive tape to exfoliate graphene sheets from graphite, a simple operation that led to the 2010 Nobel Prize in Physics. One common commercial article containing nanoscale holes is the Blu-ray Disc, designed for optical media applications with 405 nm light.

The digital data on a Blu-ray Disc is encoded with 150 nm pits in a spiral pattern with 320 nm pitch. The data on the disc are compressed, and there's a principle that ideally compressed data is indistinguishable from random noise. Because of the data compression, the pits have a nearly random distribution on the Disc.

Materials scientists, electrical engineers, and mechanical engineers from Northwestern University (Evanston, Illinois) decided to use a Blu-ray Disc to make a light-trapping layer for a solar cell.[3-6] Says Jiaxing Huang, an associate professor of materials science and engineering in Northwestern's McCormick School of Engineering and Applied Science,

"We had a hunch that Blu-ray discs might work for improving solar cells, and, to our delight, we found the existing patterns are already very good... It's as if electrical engineers and computer scientists developing the Blu-ray technology have been subconsciously doing our jobs, too."[4]

The Northwestern University Blu-Ray solar team. From left to right, Dongning Guo, Cheng Sun, Chen Wang, Alexander Smith and Jiaxing Huang. (Northwestern University photo.)

To produce the light-trapping layer for their solar cell, the research team delaminated the Blu-Ray Disc to expose the bit pattern, and then created a negative mold. The mold was used to transfer the pattern to the polymer photoactive layer by pressing, then an electrode was applied by evaporation.[5-6] Originally experimenting on blank Blu-Ray Discs, which have a periodic pattern of nanoscale pits, it was found that light-trapping wasn't effective.[4-6] Blu-Ray Discs with video content, however, demonstrated increased efficiency.

Shaorong Liu, a database engineer at IBM, suggested to her spouse, Jiaxing Huang, that the data compression used in the digital encoding was the likely difference. At that point, Dongning Guo, an associate professor of electrical engineering and computer science at Northwestern, joined the team, and he confirmed that the compression results in a quasi-random array of pits and islands with feature sizes between 150 and 525 nanometers. This range works well for light-trapping applications over solar wavelengths.[2-3,5]

It's the pits!

The pits in a Blu-Ray Disc with compressed digital content are about 75 nm wide and range from 150-525 nm long.

(Illustration by the author, rendered using Inkscape.)

Using the Blu-Ray Disc pattern increased the solar absorption of their solar cell by 21.8%, as compared to a non-patterned cell.[4,6] Since the polymer photovoltaic used is not that efficient, the overall conversion efficiency was raised to just 12%, but the technique can be applied to other types of solar cells.[3-5] This research was funded by the National Science Foundation.[4]

Blu-Ray Solar Cells

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