New Technology Could Boost Solar Cell Efficiency By 30 Percent

 A decades-old discovery may be crucial to increasing solar panel efficiency.

 Scientists looking to boost the efficiency of solar panels are taking a fresh look at an exotic physics phenomenon first observed nearly 50 years ago in glowing crystals.

Called singlet fission, the process can enable a single photon of light to generate two electrons instead of just one. This one-to-two conversion, as the process is known, has the potential to boost solar cell efficiency by as much as 30 percent above current levels, according to a new review paper published in the Journal of Physical Chemistry Letters.

Singlet fission "was originally proposed to explain some weird results that were observed in fluorescent organic crystals," said the study's first author Christopher Bardeen, a chemist at the University of California, Riverside. "It received a lot of attention in the 1960s and 1970s, but then it was mostly forgotten."

But beginning around 2006, Bardeen and other scientists exploring new ways to boost the solar-energy conversion rates of photovoltaic panels began taking a renewed interest in singlet fission. In recent years, experiments conducted by Bardeen's group not only helped confirm that the phenomenon is real, but also that it can be highly efficient in a variety of materials. The hope is that singlet fission materials can be incorporated into solar panels to increase their energy conversion efficiency–the ratio of electrons produced to the amount of photons absorbed–beyond the current theoretical ceiling of approximately 32 percent, which is called the "Shockley-Queisser Limit."

"The efficiency of most commercial-grade PV panels, like the ones you would install on your house, are around 20 to 25 percent," Bardeen said.

Engineers have managed to overcome the Shockley-Queisser Limit through clever engineering to boost the efficiency of photovoltaic, or PV, panels up to 50 percent – for example, one technology, called multi-junction solar cells, involves combining two or more semiconductor panels. But such technologies are currently limited mostly to military and space applications due to their high costs.

"It may be possible to find a way to make [multi-junction cells] cheaply … Some companies are trying to do this, but without much impact so far," Bardeen said.

Many scientists believe the only way the next wave, or "third generation," of photovoltaic technology will surpass the Shockley-Queisser Limit while remaining inexpensive is if they make use of new physical processes such as singlet fission.

"First generation solar cells were based on silicon, and they were efficient but expensive. The second generation cost much less and was based on thin-film technology. The goal of the third generation is to keep cost down but get efficiency as high as possible," Bardeen said.

Currently, solar cells work by absorbing a photon and generating an exciton -- a bound electron with a negative charge and a positively charged "hole" -- which subsequently separates into an electron-hole pair. The electrons are then harnessed as electricity. In singlet fission, however, some photons -- those with higher energy -- get converted into two excitons, each of which can split to yield two electrons. Bardeen's team estimates that singlet fission can boost efficiency of solar cells by up to 30 percent, resulting in a maximum efficiency of above 40 percent instead of the current 32 percent.

Experts predict that it could be another 5 to 10 years before solar panels based on singlet fission technology are ready for commercial use. Before that can happen, scientists will need to gain a much better understanding of how singlet fission works, said Josef Michl, a photophysicist at the University of Colorado, Boulder, who helped revive interest in singlet fission several years ago. At the moment, the main challenge for researchers trying to create a singlet fission solar panel is "a thorough understanding of the underlying physics that should allow chemists to come up with more practical materials than the few that we now know to work well in the laboratory," said Michl, who was not involved in the study.

Michl called Bardeen's group a "key player" in the worldwide effort to develop the technology, and said that his team's experimental work has helped singlet fission shed its "reputation of an obscure and inefficient phenomenon."

The other primary hurdle toward a functional singlet fission solar panel will be one of engineering, Bardeen said. Once more materials that can undergo singlet fission are developed, they will still need to be incorporated into photovoltaic cells to convert solar energy into electricity. Researchers led by Marc Baldo at the Massachusetts Institute of Technology recently reported that they had proven that it was possible to create a solar panel that uses singlet fission, but the efficiency of their device was only 2 to 3 percent.

"Baldo's group showed that it could be done," Bardeen said, "but nobody's going to be putting those on rooftops tomorrow."A

Luminescent Solar Concentrators(LSC)

A solar concentrator does exactly what its name implies: It takes the sunlight that strikes a wide area and bunches it together. The coolest part of the system is that it doesn't just concentrate the sunlight; it also directs that sunlight to a very specific, smaller location.

Unlike a solar tracker, a solar concentrator is stationary. The main components in the traditional design are plastic, dye molecules and solar cells. A variety of dye molecules are sprayed onto a­ sheet of plastic. At the outer edges of the plastic are solar cells.

The combination of the plastic and dyes works as a waveguide. A waveguide is any device that traps light and then moves those light waves along a path to a particular destination. In this case, when light hits the plastic, the dyes absorb it. The sun's energy is thereby transferred to the dye, causing the electrons in those molecules to jump to a higher energy level. When the electrons fall back to a lower energy level, the dye molecules release that energy into the plastic sheet, where it gets stuck. In a process called total internal reflection, the light can escape the plastic. It just bounces around in the material, ultimately making its way to the outer surface. At the outer surface, the solar cells are waiting to absorb the light and generate electricity.

A solar concentrator doesn't require a cooling system, and there are no moving parts, making it less expensive than a solar tracker. There's a drawback to the traditional design, though. While the light energy bounces around in the plastic, it sometimes gets reabsorbed into the dye molecules and ends up emitted as heat. This energy, then, never makes it to the solar cells.

The researchers at Massachusetts Institute of Technology (MIT) made a few primary changes to the system to make it more efficient and even less expensive. They call it the luminescent solar concentrator (LSC). First, they traded in the plastic for glass. Glass is easier to manufacture, and it opens up some new possibilities in terms of applications. They also added a new element that eliminates the loss of energy to re-absorption.

The LSC is composed of a sheet of glass coated with particles of dye. It basically works like the old plastic version except for two additions. First, a type of aluminum called tris(8-hydroxyquinoline) is added to the mix of dye molecules. These aluminum molecules cause the dyes to emit light waves at a frequency the dyes can't absorb. In this way, no light is lost to re-absorption as it makes its way to the solar cells at the edges of the glass.

In this system, 10 times more of the sunlight that hits the panel is converted to electricity compared to a traditional solar panel [Source: Economist]. Each solar cell is exposed to much more sunlight, meaning fewer silicon cells are needed and the cost goes way down.

How far down? Researchers aren't putting a dollar amount on the product, but it's sure to be cheaper than solar trackers. With increased efficiency, it'll also be able to generate more electricity per dollar than the solar panels you see on people's roofs now. Retrofitted onto current solar-panel systems, luminescent solar concentrators could increase efficiency by 50 percent [Source: ScienceDaily]. By far the most amazing aspect of the product, though, comes from the use of glass: By making windows out of these LSCs, the glass that lets sunlight into our homes and offices could also generate the power we need to run those spaces.

We're not quite there yet, though. The biggest obstacle to getting these solar-power windows onto our homes is longevity. In fact, the LSC prototype only lasts about three months [Source: TreeHugger]. The MIT group is working to get the panel to maintain stability for the 10 or so years people expect these types of devices to last. But it's pretty close -- we can expect luminescent solar concentrators to be available for sale within three years [Source: MIT].