So-called colloidal quantum dots could be used as the light-absorbing component in cheap, highly efficient solar cells. This is because single, high-energy photons hitting such a photovoltaic material can produce excited electrons or holes that have energies at least equal to or greater than the band gap of the quantum dot. Electrons with an energy exceeding twice the band gap can transfer their excess energy to one or more valence electrons and excite them across the quantum dot's band gap, which leads to several excitons (electron-hole pairs) being produced for every photon absorbed. This process could help increase the power conversion of solar cells to up to 42%.

Such "multiple exciton generation" (MEG) is particularly interesting in group IV–VI quantum dots, like those made of lead selenide, because the band gap in these materials can be tuned through the near-infrared region. This means that light from a wide range of the solar spectrum can be harvested.

Studying MEG in core/shell quantum dots
Now, Laurens Siebbeles of the Delft University of Technology in the Netherlands and colleagues at the Technion Institute in Hafia, Israel, and the National Renewable Energy Lab in Colorado, US, studied MEG in core/shell quantum dots. The researchers say that when a lead selenide (PbSe) quantum dot is made larger by growing a lead sulphide (PbS) shell around an existing dot "core", the band gap of the dot decreases.

Siebbeles and colleagues obtained their results by making ultrafast time-resolved laser spectroscopic measurements on the quantum dots. They found that the optical absorption moves to longer wavelengths as PbS shells are grown around a PbSe core, and red-shift even more as the shell becomes thicker. According to electronic structure calculations, this phenomenon occurs thanks to delocalization of charge into the shell.

The team will now investigate how these charges can be extracted from multiple excitons in a quantum dot. "We hope to do this by bringing the dots into contact with electron-accepting materials, such as titanium dioxide," Siebbeles told nanotechweb.org.

The results were published in Nano Letters.