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A conventional solar cell, left, reflects light off its surface and loses light that penetrates the cell. New technology, right, develop by Princeton professor Stephen Chou and colleagues in electrical engineering, prevents both types of loss and is much thinner. Credit: Illustration by Dimitri Karetnikov Princeton researchers have found a simple and economic way to nearly triple the efficiency of organic solar cells, the cheap and flexible plastic devices that many scientists believe could be the future of solar power.
The researchers, led by electrical engineer Stephen Chou, were able to increase the efficiency 175 percent by using a nanostructured "sandwich" of metal and plastic that collects and traps light. Chou said the technology also should increase the efficiency of conventional inorganic solar collectors, such as standard silicon solar panels, although he cautioned that his team has not yet completed research with inorganic devices. Chou said the research team used nanotechnology to overcome two primary challenges that cause solar cells to lose energy: light reflecting from the cell, and the inability to fully capture light that enters the cell. With their new metallic sandwich, the researchers were able to address both problems. 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.
Princeton’s nanomesh nearly triples solar cell efficiency | ExtremeTechThere is huge potential in solar power. The sun is a giant ball of burning hydrogen in the sky, and it’s going to be sticking around for at least a few more billion years. For all intents and purposes, it’s a free source of energy. Sadly, humanity hasn’t been very good at harnessing its power directly. Our current methods of capturing the sun’s energy are very inefficient. For example, modern silicon and indium-tin-oxide-based solar cells are approaching the theoretical limit of 33.7% efficiency. Well, a research team at Princeton has used nanotechnology to create a mesh that increases efficiency over organic solar cells nearly three fold.
Led by Stephen Chou, the team has made two dramatic improvements: reducing reflectivity, and more effectively capturing the light that isn’t reflected. As you can see by the illustration below by Dimitri Karetnikov, Princeton’s new solar cell is much thinner and less reflective. By utilizing sandwiched plastic and metal with the nanomesh, this so-called “Plasmonic Cavity with Subwavelength Hole array” or “PlaCSH” substantially reduces the potential for losing the light itself. In fact, it only reflects about 4% of direct sunlight, leading to a 52% higher efficiency than conventional, organic solar cells.
PlaCSH is also capable of capturing a large amount of sunlight even when the sunlight is dispersed on cloudy days, which results in an amazing 81% increase in efficiency under indirect lighting conditions when compared to conventional organic solar cell technology. All told, PlaCSH is up to 175% more efficient than conventional solar cells. As you can see in the image to the right, the difference in reflectivity between conventional and PlaCSH solar cells is really quite dramatic.
The gold mesh that sits on top is incredibly small. It’s only 30 nanometers thick. The holes in the mesh are a mere 175nm in diameter. This replaces the much thicker traditional top layer made out of indium-tin-oxide (ITO). This is the most important part of the innovation. Because the mesh is actually smaller than the wavelength of the light it’s trying to collect, it exploits the bizarre way that light works in subwavelength structures. This unique physical property allows the researchers to effectively capture the light once it enters the holes in the mesh instead of letting much of it reflect away. The bottom layer of the cell remains the same, but this implementation allows the semiconducting layer of plastic in the middle of the cell to be much thinner.
The research team believes that the cells can be made cost effectively using a nanofabrication method that Chou himself invented over a decade ago. Most importantly, it replaces the costly ITO element from solar cells. This will be affordable, and much more flexible than the brittle ITO layer of traditional solar cells. While research is still being done using semiconducting materials other than plastic, this method should work for silicon and gallium arsenide solar cells as well, so it will be able to reduce the size and cost of them drastically while providing similar efficiency benefits.
