Australasian Science: Australia's authority on science since 1938

Solar Cells out of the Red

By Stephen Luntz

Low-energy photons of light have been combined to increase the efficiency of solar cells.

Low-energy photons of light have been combined to increase the efficiency of solar cells, marking a step towards panels with an efficiency ceiling of 43% in normal sunlight exceeding the previous theoretical maximum of 33%.

Long wavelength (red) light has less energy than shorter wavelengths, and all solar cells have a certain wavelength beyond which they cannot harvest. However, certain materials can combine two long wavelength photons into one shorter one, a process known as upconverting.

Some upconversion relies on the identical wavelengths produced in laser light, but in 2009 A/Prof Tim Schmidt of the University of Sydney demonstrated that upconversion can also be achieved using a low-intensity incoherent source such as the Sun (AS, Jan/Feb 2010, p.13).

Now Schmidt has put a backing on his trial cell so that it combines two red photons passing through the cell’s silicon into a single yellow one, which is then captured and turned into electricity.

Photons striking the upconverting material raise electrons to a higher energy state. In Schmidt’s material the spin of the electron is reversed in the process. Since two electrons with the same spin cannot occupy the same energy level, the electron is blocked from dropping back down by an electron of the same spin in the energy level below.

This keeps the electron in a raised state long enough for the atom to be struck by another photon, raising the electron’s energy still further. When it eventually gives that energy up, the photon released is more energetic than either of the original ones, and can be absorbed by the solar cell.

The current cell requires two photons to arrive within 10 ms of each other in a space about 200 nm across, ensuring that a tiny proportion of photons are upconverted. In a paper in Energy & Environmental Science, however, Schmidt sets out a path to improve this.

One of the major priorities is to increase the density of the converting material. If this can be achieved, Schmidt says it will be suitable “for all types of cells that let through red light, including amorphous crystalline solar and dye-sensitised cells”.

“Within 1–2 years we should have a good idea if this is really going to work,” Schmidt says. If it does he doubts that the extra efficiency will bring much cost.

“In lots of ways this is simpler than a solar cell. We just need to find the right materials. They should also degrade less than solar cells because they’ll only be subjected to low energy photons.”