Solar Cells and Nanotechnology

On April 25, 1954, Bell Labs unveiled the Bell Solar Battery – the world’s first device to successfully convert sunlight into electricity. Even after around 60 years, scientists are looking to overcome limitations of the solar photovoltaic cells. Solar cells are still not that efficient at converting the large amount of power of the sun into electricity. Though commercial solar cells today have two to four times the 6% efficiency of the early solar cells, it’s not sufficient to take place of fossil fuels completely. But now, renewable energy researchers think that nanotechnology- the engineering of functional systems at the molecular scale, can help to boost the solar energy.

Bell Labs' Solar Cell 1954

Imagine something small that it’s a hundred-thousandth the width of a human hair. Then consider the technology to manipulate something that small to solve the problems and create new products. That's the world of nanotechnology. It is a study and control of matter at the nanoscale. Materials can have different properties and can function in unique ways when structured at the nanoscale. Here are few examples from the tiny world of engineering that could help to boost solar energy.

Billions of Nano-Sized Holes (Black Silicon) 

Several factors affect a solar cell's conversion efficiency value, including its reflectance efficiency. The absorption coefficient of a material indicates how far light with a specific wavelength (or energy) can penetrate the material before being absorbed. A small absorption coefficient means that light is not readily absorbed by the material. The absorption coefficient of a solar cell depends on two factors: the material of the cell and the wavelength or energy of the light being absorbed. [1] To reduce the amount of sunlight that is reflected away from silicon solar cells and wasted, manufacturers usually add one or more layers of anti-reflective material, which significantly increases the cost. 

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) announced new use of nanotechnology to reduce the amount of energy reflected by the silicon cells. NREL tailored a nanostructured surface while ensuring that the light-generated electricity can still be collected efficiently from the solar cell. The researchers made nano-islands of silver on a silicon wafer and immersed it briefly in liquids to make billions of nano-sized holes in each square-inch of the silicon wafer surface. [2] The holes and silicon walls are smaller than the wavelengths of light hitting them, thus light is absorbed rather than reflected.

The "Nano Sandwich" (Organic Solar Cells)

An organic solar cell or plastic solar cell, a type of polymer solar cell, are cheaper and easier to make than current silicon-based solar cells. But they haven’t been as efficient. Princeton researchers have found a simple and economical way to nearly triple the efficiency of organic solar cells.

Nanoscale metal mesh that makes up the top layer of the sanwich: The spacing of the mesh, the thickness of the sandwich, the diameter of the holes — are all smaller than the wavelength of the light which helps to trap the light.
(Image Source: http://www.princeton.edu/engineering/news/archive/?id=9141)

The researchers, led by electrical engineer Stephen Chou, were able to increase the efficiency of the solar cells 175 percent by using a nano-structured "sandwich" of metal and plastic that collects and traps light. With their new metallic sandwich, the researchers were able to address both problems that cause solar cells to lose energy: light reflecting from the cell, and the inability to fully capture light that enters the cell. The sandwich — called a subwavelength plasmonic cavity — has an extraordinary ability to dampen reflection and trap light. The new technique allowed Chou's team to create a solar cell that only reflects about 4 percent of light and absorbs as much as 96 percent. It demonstrates 52 percent higher efficiency in converting light to electrical energy than a conventional solar cell. [3]

Nanocrystal Solar Cell (Solar-Collecting Paint)

Even after employing different technologies to improve efficiency of solar cells, you need a place to put the setup outside for sun exposure. But what if you need not to worry about finding the place to set up your solar devices. University of Southern California chemistry professor Richard L. Brutchey and postdoctoral researcher David H. Webber have developed a technology that could turn your building into a solar collector. 

They’ve created an electricity-conducting liquid filled with solar-collecting nanocrystals, which can be painted onto surfaces such as window glass or plastic roof panels. The nanocrystals, made of cadmium selenide (CdSe) are able to float in liquid solution. Cadmium is a toxic heavy metal and appropriate precautions should be taken when handling it and its compounds. Therefore, researchers are looking to build nanocrystals from materials other than cadmium. [4] 

The commercialization of these technologies may still take years, but we can see a clear path forward toward integrating this into the next generation of solar cell technologies.

References:

[1] http://energy.gov/energybasics/articles/photovoltaic-cell-materials [accessed (3rd Oct. 2013)]

[2] http://www.nrel.gov/news/press/2012/2009.html [accessed (3rd Oct. 2013)]
I wrote this article originally for Experimentation Online - the premier student science newspaper based in Ireland and the UK. Reproduced here because the original website is no longer active. 

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