Linked quantum dots could create cheap, efficient solar cells

September 30, 2011

Self-assembled binary superlattices of two different kinds of quantum dots. The controllable, ordered interface between the different semiconductor nanocrystals represents a promising system for use as efficient solar cells. (Credit: TU Delft)

Researchers at the Kavli Institute of Nanoscience at TU Delft in the Netherlands have demonstrated that electrons can move freely in layers of linked semiconductor nanoparticles under the influence of light  in a breakthrough that could lead to cheaper, more-efficient solar cells made from quantum dots.

Right now, crystalline silicon solar panels are expensive to produce, said the researchers, led by TU Delft Assistant Professor A.J. Houtepen. Cheaper solar cells are available but are inefficient. For example, an organic solar cell has a maximum efficiency of 8 percent, the researchers said.

More-efficient quantum dots

One way of increasing the efficiency of cheap solar cells is use of semiconductor nanoparticles called quantum dots. In theory, the efficiency of these cells can be increased to 44 percent due in part to an “avalanche effect” demonstrated by researchers from TU Delft and the FOM Foundation in the Netherlands in 2008.

In current solar cells, an absorbed light particle can only excite one electron, creating an electron-hole pair, while in a quantum dot solar cell, a light particle can excite several electrons. The more electrons that are excited, the greater the efficiency of the solar cell, the researchers said.

Up to now, the creation of electron-hole pairs under the influence of light was only demonstrated within the limits of a quantum dot. To be usable in solar cells, though, it is essential that electrons and holes are able to move and create an electrical current that can be collected at an electrode.

The researchers have now demonstrated that the electron-hole pairs can also move as free charges between the nanoparticles. They linked nanoparticles together, using very small molecules, so that they were very densely clustered while still remaining separate from each other. The nanoparticles are so close together that every single light particle that is absorbed by the solar cell actually causes electrons to move.

Ref.: Elise Talgorn, et al., Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids, Nature Nanotechnology, 2011; [DOI:10.1038/nnano.2011.159]