Black silicon could boost efficiency of traditional solar cells
October 3, 2012
Researchers at the Fraunhofer Institute for Telecommunications have developed a system that allows solar cells to effectively harvest energy from the infrared spectrum, tapping into a source of energy that in the past has mostly been out of reach. The new technology, which promises to work well with commercially available solar cells, has the potential of becoming a standard in the solar panels of tomorrow.
When photons hit the surface of a solar cell, the energy they carry can be absorbed by a semiconductor. If the energy absorbed is higher than a set threshold – which is known as the energy gap, and depends on the semiconductor used – electrons are freed from the semiconductor and can be used to generate an electric current.
The energy carried by a photon is proportional to its frequency. In silicon solar cells, photons in the infrared often don't carry enough energy to produce electricity, and IR light simply passes through the cell, unused. Red photons are carrying just enough energy to knock an electron loose and photons in the blue spectrum or beyond (UV light) are carrying too much energy – so part of it is used to free a single electron, but the rest is wasted as heat. The inability to fully extract the energy carried by a photon is the main reason why solar cells are so inefficient.
Over the past few years, researchers have produced solar cells that can absorb infrared and ultraviolet light more effectively. Now, researchers at Fraunhofer Institute have come up with a straightforward way to capture more energy from the infrared spectrum, developing something that could very well become standard technology in the solar cell of the near future.
The research is based on absorbing infrared light using what's known as black silicon. This material is made by using precision lasers to "zap" sulfur atoms into the silicon lattice in well-defined patterns.
The sulfur lowers the energy gap, and therefore allows much lower-energy photons to free electrons from the semiconductor. In theory, this should boost the solar cell's efficiency; unfortunately, though, the smaller energy gap also makes it easier for electrons to "travel" in the opposite direction, causing electricity to be lost once again.
The researchers at Fraunhofer set out to address this issue and came up with a conceptually simple yet ingenious solution. They chose to change the patterns of laser pulses that drive sulfur atoms into the silicon lattice, altering their conformation to maximize the number of electrons that can climb "up" the energy gap and become conductive, while minimizing the number of electrons that can go back "down" it.
Prototypes of the cells have shown that the mechanism can double the efficiency of black silicon, but the researchers are still looking to identify the configuration of sulfur atoms that can result in the best performance.
The next step will be to embed the cells with already existing commercial technology, and the good news is that the two are very much compatible, and seem to complement each other. By simply removing the back cover of a traditional solar cell and incorporating a layer of black silicon, the team has found that they can increase efficiency of such panels by about one percent.
The researchers are now planning to market a laser system that manufacturers can use to produce the black silicon themselves and include it in their products as standard.
Source: Fraunhofer HHI