Connecting the dots

Layers of quantum dots can be used as solar cells. Better connections between the dots are needed however to boost the efficiency.

Ilustration: Elise Talgorn

They come in all colours. Nano-sized semiconductor crystals, endearingly called quantum dots or q-dots, manifest properties between those of molecular matter and bulk material. The reason is that an electron in a q-dot experiences quantum confinement. This results in discrete steps between energy states (bandgaps), which depend on the material as well as the size. Hence, by varying the size of the q-dots one can tune them to a specific colour, rendering them interesting candidates for use in solar cells.

In her PhD research, Elise Talgorn (MSc, Applied Sciences) has coated glass substrates with layers of quantum dots and tested their conductivity. To do this, she had to get rid of the insulating molecules surrounding the dots, because these kept the dots so far apart that electrons couldn’t move from one dot to the next. By chemically replacing these long molecules with shorter ones, Talgorn succeeded in increasing the charge carrier mobility by a factor 500 or so. Simply drying the layers (’thermal annealing’) increased the mobility even more (up to 10,000 times), but the outcome proved to be less controllable.

Recombination presents another challenge to using q-dots in solar cells. When a photon hits a q-dot, it creates a free electron and a ‘hole’; but wait a few nanoseconds and they simply recombine, as if nothing had happened. For an external current to be generated, one needs to apply an electric field to draw the electron-hole pair apart. Talgorn did this by combining cadmium selenium q-dots (which attract electrons) and cadmium tellurium q-dots that attract the holes. This could be the working principle for a new type of solar cell.

Elise Talgorn, Photoconductivity of quantum dot films, 30 November 2010, PhD supervisor Professor Laurens Siebbeles.