Aalto University researchers have developed a photovoltaic device with an external quantum efficiency of 132 percent that could be a major breakthrough for solar cells and other photovoltaic devices
The researchers used nanostructured black silicon to achieve this efficiency of above 100 percent. With a 100 percent efficiency, a photovoltaic device can generate one electron every time a photon of light strikes the external surface and can be harvested as electricity.
At 132 percent, that means you get on average 1.32 electrons for every photon. The photovoltaic device was made of black silicon as the active material, and nanostructure shaped like cones and columns to absorb UV light.
The 132 percent efficiency means that one has a 32-percent chance of generating two electrons from a single photon. This might sound fanciful as it is impossible to create energy from nothing.
Black silicon can absorb photons and release electrons efficiently in a much better manner as it forms billions of small, nano-scale pits during the process. Additionally, the purpose behind using cone nanostructures is because they reduce electron recombination at the surface of the material. Combining all these features of the black silicon helped the photovoltaic device to achieve the external quantum efficiency of above 130% in the UV range without any external amplification.
This is how it works. The researchers used photons to strike the active silicon to displace an electron out of its atoms. But under certain circumstances, one high-energy photon has the capacity to move tow electrons, without violating any laws of physics.
This has immense possibilities for improving the design of solar cells. In generating solar energy as in most such phenomenon, a lot is wasted in the process. In the photovoltaic devices, the photons just reflect back or the electron generated falls back into the active material without activating the circuit.
“We can collect all multiplicated charge carriers without a need for separate external biasing as our nanostructured device is free of recombination and reflection losses,” explains Prof. Hele Savin, head of the Electron Physics research group at Aalto University.
Black silicon absorbs far more photons than other materials, and the cone and column nanostructures reduce electron recombination at the surface of the material, says the team of researchers. Black silicon can absorb photons and release electrons efficiently in a much better manner as it forms billions of small, nano-scale pits during the process. Combining all these features of the black silicon helped the photovoltaic device to achieve the external quantum efficiency of above 130% in the UV range without any external amplification.
The photovoltaic device with the 130 plus efficiency was tested and verified by the German National Metrology Institute, Physikalisch-Technische Bundesanstalt (PTB).
The new detectors are being manufactured for commercial use already, according to the research team. The 130 efficiency can improve the workings of any photodetector based devices like solar cells and light sensors.
“Our photodetectors are gaining a lot of attraction at the moment, especially in biotechnology and industrial process monitoring,” says Dr. Mikko Juntunen, CEO of Aalto University spin-off company, Elfys Inc.
The research has been published in the journal Physical Review Letters.
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