With help from everyday, standard-issue solar cells, an Australian solar energy research team has set a new record for the conversion of sunlight into electricity.
The conversion, to be announced this week at the Asia-Pacific Solar Research Conference, was demonstrated first at a Sydney laboratory and then again in the United States, where it could be confirmed by the National Renewable Energy Laboratory. This efficiency is a rough doubling of a 1989 record set by the same team, led by UNSW professor Martin Green, aka "the godfather of photovoltaics."
The technology is based on what are known as triple-junction solar cells, which are basically a sandwich of differently tuned semiconductors with each one able to capture a different wavelength of sunlight. In a single-junction cell, with just one semiconductor layer snagging photons within one frequency range, enough incoming photons go unused such that the material's maximum theoretical efficiency is held to 34 percent, compared to 86.8 percent in a triple-junction cell.
Sunlight is really a big package of light at different frequencies, some visible and some not, like UV and infrared light. So photons of all frequencies —though biased toward some—are colliding against a given solar cell. Those photons, when they smash into the cell, or atoms within it, are sometimes energetic enough to knock electrons lurking in the solar cell material loose. Cascades of these freed electrons moving across the cell are electrical current, which is captured and shuffled away.
This only works within the correct bandgap, however. If photons are too energetic for the material, they just collide with stuff and their added energy is lost as heat. If they aren't energetic enough, the photons won't be able to knock the electrons out of place and so a current won't be generated. Beating the bandgap is a lofty target.
So, a triple-junction cell or any multi-junction cell just serves to extend the gap to higher and lower energies. This is a commercial technology, but is usually only worth it in centralized schemes where sunlight is concentrated by mirrors.
A new efficiency record isn't going to suddenly put multijunction solar cells on neighborhood roofs, but it can at least make centralized solar schemes, in which mirrors are focused at a central tower, better. The improvement comes with the addition of not just top-to-bottom stacks of different-tuned semi-conductors, but materials that can be split up side-to-side. Sunlight hits a mirror and the mirror delivers different frequencies to different zones of the collector. Here:
A Google search will actually reveal a couple instances of higher sunlight conversion efficiencies, which can be seen in the graph above. Even just a few days ago, a French-German collaboration announced a 46 percent efficiency.
Green explained it to me as such: "Our 40.4 percent result is for the conversion of actual sunlight to electricity in an operating system of a reasonable size (287 cm2). The result includes all the optical losses associated with getting the sunlight to the cell." So: the difference between in vitro and in vivo then.
There is something of a solar arms race underway, however, and several new technologies recently put forth go beyond multijunction into schemes that recapture lost heat, either by feeding it back into a thermovoltaic cell or just into a regular old steam power plant that uses sunlight instead of nuclear fission reactions or fossil fuel combustion.
In any case: more power, less effort. That's always a good thing.