One of the reasons that many consumers decide not to go solar is they feel the technology is still in its infancy, and needs to be further developed before they make a long-term commitment. Like any technology, progress is made steadily but slowly.
Pentacene Solar Cell Coatings Breaking Barriers
Up until now, a fundamental barrier referred to as the Shockley-Queisser limit has held that solar conversion of sunlight into energy cannot exceed an efficiency level of 33.7 percent. But a team of MIT researchers has finally disproven that theory. The team successfully demonstrated “singlet exciton fission,” where a solar cell coated with pentacene (an organic compound with semiconductor properties) was able to make a photon knock two electrons loose instead of one electron that a typical solar cell currently uses. The pentacene-covered solar cells double the number of electrons, and energy that can be harvested. The ability to knock loose two electrons instead of one would reduce the amount of sunlight wasted as heat instead of being converted into electricity—resulting in greater efficiency levels.
According to early research, this technique will allow solar cells to surpass the Shockley-Queisser limit. “We think it’s an exciting direction for solar to improve its efficiency” said Marc Baldo, engineer and co-author of the study.
The process of singlet fission has been around since the 1960s, but it has never before been harnessed in solar cells. The team at MIT confirmed that pentacene was in fact producing two electrons for every photon of light. Next, researchers coated the top of a silicon cell to see if the cell could harness more energy and discovered that the test solar cell did exactly that. It increased the amount of energy harvested from the blue and green light spectrum. The pentacene coating, only works on blue and green wavelengths, not the entire visible spectrum.
Nonetheless, the study is the first to demonstrate that pentacene can be used in a solar cell to make more energy. It represents a step towards creating solar cells that could become significantly more efficient than current silicon solar cells.
Solar PV Cells Achieve New Levels of Efficiency Using Nanoscale Structures
Researchers at the Fraunhofer Institute for Solar Energy Systems have discovered a new nanowire configuration that could lead to greater efficiency in photovoltaic cells. Led by physicist Magnus Borgström, the researchers found that when a solar cell that contained nanowires was exposed to sunlight, the cell was able to convert 14 percent of the incoming light to electricity—which is a new record. These results open up the possibility for cheaper and more effective solar power. Borgström believes the cost of these nanowire solar cells would come down if the process can be industrialized.
The nanowire solar cells use a novel semiconductor that is a combination of indium (a malleable metal) and phosphorous (a non-metallic mineral). The semiconductor is capable of absorbing much of the light from the sun (the common term for this is band gap). Currently, it absorbs 71 percent of the light above the band gap and Borgström believes that can be further increased.
The possibility is strong that these nanowire cells could even be built into mutijunction solar cells, which are compound devices that include several different types of semiconductor material that are arranged in layers so they can absorb as much of the sunlight’s energy as possible. Multijunction cells have converted up to 43 percent of the energy in sunlight into electricity, making them the most efficient photovoltaic devices in the world—and the most expensive. Their price may come down by combining low-cost lenses that concentrate the sunlight into smaller versions of these cells (nanowire cells), Borgström believes. It is an exciting development that holds great promise to increase solar cell efficiency at cheaper prices.
Both of these breakthroughs are proof that innovation will continue within solar efficiency and technology.