Capturing wasted solar energy

Material chemists at the University of Texas at Austin and the University of Minnesota claim to be able to increase the efficiency of a solar cell by more than 60 percent, up from what was thought to be a limit of about 30 percent. They report their findings in the journal Science.

If a photon has about the same energy as the solar cell semiconductor’s band gap, it knocks an electron into the conduction band, where it can flow as current. However, some high-energy photons in sunlight exceed the band-gap energy of the cell and is wasted.

The ejected “hot” electron, quickly loses its excess energy, “cooling down” to the bottom of the conduction band within a picosecond. It has been impossible to retrieve the lost energy, according to the team which has discovered that the lost energy can be salvaged and transferred to an adjacent electron-conducting layer and this can be done in less than 50 femtoseconds (quadrillionths of a second).

Their experiment was carried out using a model system composed of colloidal lead-selenide nanocrystals, or quantum dots, coupled with an electron-conducting titanium dioxide layer.

Within the past 10 years, many studies have confirmed the promise held by quantum dots for slowing down the cooling of hot charge carriers so they keep their energy longer.

Essentially, the small size of the nanocrystal forces a high number of electron-electron interactions. This “quantum confinement effect” maintains electrons at a high level of excitement for up to a nanosecond, potentially enough time for their energy to be put to use.

So, we are moving in the right direction. It has been the conversion of the total solar energy which has been the issue. Looks like not for too long.