Professor of chemical engineering Stacey Bent said “I wondered if we could use our knowledge of chemistry to improve their efficiency”. She aims to reduce the cost of solar cells in order to make possible the mass adoption of solar energy technologies.
Produced using simple chemical reactions, these new kinds of solar cells have been around for a couple of years now. They use semiconductor particles called “quantum dots”, part of the ongoing revolution in the emerging field of Nanotechnology. Although cheaper to produce, the efficiency of the solar cells ha been limited.
However, thanks to the Bent Research Group recent discoveries, this limitation could be fixed, and even turned around. “Quantum dot cells can reach much higher efficiency”, Bent said, because of a fundamental limitation of traditional solar cells.
Traditional solar cells basically work as follows:
- The sun emits an electromagnetic energy that travels in the form of photons and hit the solar cell
- Photons energy breaks silicon electron-hole pairs
- The generated electric field on the junction pushes the electrons to opposite sides.
- Both sides are connected via metal contacts and electrodes to an external load which will transport the electrical current.
Subsequently, there is an electrical current generated by the flow of charges and an electrical voltage caused by the electrical field. There is power to sustain an electronic device.
However, it takes a certain minimum energy to fully separate the electron and the hole. The amount of energy required is specific to different materials and affects what colour, or wavelength, of light the material best absorbs. Silicon is commonly used to make solar cells because the energy required to excite its electrons corresponds closely to the wavelength of visible light. Solar cells made of a single material have a maximum efficiency of about 31 percent, a limitation of the fixed energy level they can absorb.
Because of these possibilities, the Bent Research Group has been investigating ways to improve the efficiency of quantum dot solar cells.
The researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. They found that the exact molecule kind didn’t matter – just having a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells. It’s the length of the molecule, and not its exact nature, that matters. Molecules that are too long don’t allow the quantum dots to interact well with the semiconductor.
Bent’s theory is that once the sun’s energy creates an electron and a hole, the thin organic layer helps keep them apart, preventing them from recombining and being wasted. The group has yet to optimise the solar cells, and they have currently achieved an efficiency of, at most, 0.4 percent. But the group can tune several aspects of the cell, and once they do, the three-fold increase caused by the organic layer would be even more significant. The improvements are just beginning.
We congratulate the Bent Research Group for their discoveries and hopes this soon leads to a less expensive and more efficient solar technology.