Conventional crystalline silicon solar panels are relatively efficient, but are expensive to produce. Although cheaper solar cells made from other materials are now available, they are inefficient in comparison. For example, organic solar cells have a maximum efficiency of just 8%.

Increasing efficiency

One way to increase the efficiency of solar cells is to use semiconductor nanoparticles, or quantum dots. In theory, the efficiency of cells made from these materials can reach 44%. This is thanks, in part, to the so-called avalanche effect whereby a single photon can excite several electrons at the same time in a sample. In conventional solar cells, an absorbed light particle usually only excites one electron (so creating an electron-hole pair).

To date, however, researchers had only succeeded in creating such photoexcited electron-hole pairs inside quantum dots. This is not very useful for real solar cells in which electron and holes need to be able to move freely throughout the entire sample. Only then can they create an electrical current that can be collected at an electrode.

Charges move freely

Arjan Houtepen and colleagues have now shown that electron-hole pairs can indeed move as free charges between dots when these semiconductor nanoparticles have been linked closely together using small spacer molecules. The arrangement means that the dots are very densely clustered together while remaining separate. “The nanoparticles are so close to each other that every single light particle that is absorbed by the solar cell results in the generation of mobile electrons and holes,” Houtepen told nanotechweb.org.

Since there are many of these electrons and holes, solar cells made of semiconductor nanoparticles could be very efficient, he added. “This is by no means trivial: in nanocrystals, photons usually create excitons (bound electron-hole pairs) that can not be extracted in a solar cell. The same applies for organic solar cells – absorbed photons do not normally produce free charges.”

Band-like transport

The team observed band-like charge transport in its samples, which means that the semiconductor nanoparticles are actually behaving like atomic crystals when the crystals are brought very close together. “This is exciting because it shows that the electronic properties of nanocrystals films can be (almost) as good as those of traditional semiconductors,” explained Houtepen. “What is more, the electronic properties can be controlled by controlling the distance between the nanocrystals.”

The researchers say that all the photoexcited electrons produced can move freely through the material and that they can be collected in a solar cell. “Alternatively, we can say that all absorbed photons result in mobile charge carriers and that no excitons are formed,” said Houtepen.

“Our work shows that no complicated tricks have to be applied in solar cells that are based on semiconductor nanocrystals when it comes to separating excitons and producing free charge carriers,” he added. “These solar cells can thus be made very simply and have a large photovoltage – since we do not waste any energy in trying to separate the excitons.”

The results are described in more detail in Nature Nanotechnology journal and Nano Lett..