Apr 5, 2012
Heterostructures make better solar cells
Researchers in Israel and Hong Kong have made more efficient solar cells from nanocrystals that have different sizes and surface chemistry. Using different-sized crystals helps to increase the amount of light that can be absorbed while the use of mixed ligands reduces electron-hole pair (exciton) binding in the structures. This allows the electrons and holes to be more easily separated into free charges that can then generate electric current.
The team led by Nir Tessler from the Technion-Israel Institute of Technology and Andrey Rogach of the City University of Hong Kong looked at cadmium selenide and cadmium telluride nanocrystals in their work. These crystals - which are capped with different organic chemicals, like thioglycolic acid and mercaptopropionic acid and combined using a linking agent - have energy levels that are aligned in a “type-II” configuration. This arrangement allows for better exciton dissociation at the interface between the materials.
The researchers also changed the size of the CdSe nanocrystals so that the structures absorb sunlight over a wider range of wavelengths. For example, bigger crystals absorb over larger part of the Sun's spectrum.
This is not the first time that Tessler’s group has worked on improving the efficiency of solar cells in this way. "We previously showed that type II alignment between two identical nanocrystals can be achieved by changing the capping ligands on the nanocrystals and thus shift the nanocrystals' energy levels (relative to the vacuum level)," explained Tessler. "This shift mostly depends on the electric dipole created between the ligand's anchor group and the surface of the nanocrystals."
More recently, the scientists demonstrated that placing mixed capping ligands on the same nanocrystal creates a non-uniform surface dipole, or nanocrystal polarization. One important aspect of this polarization is the induced separation between electron and hole wavefunctions and namely a reduced exciton binding-energy, said Tessler.
"We hope that our study will stimulate theoretical studies that consider nanocrystals as hybrid inorganic/organic materials, where the role of the organic capping layer is accounted for," he told nanotechweb.org. "We believe that better device optimization combined with strategies like the one proposed here may lead to high-efficiency solar cells in the future."
The researchers now plan to control the morphology or the way different types of nanocrystal arrange themselves in space with the aid of polarizing ligands.
The present work is detailed in ACS Nano.
About the author
Belle Dumé is contributing editor at nanotechweb.org