The “most efficient colloidal quantum dot (CQD) solar cell ever” has been created by researchers from the University of Toronto (U of T), King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University.

Quantum dots are nanoscale semiconductors that capture light and convert it into electrical energy. Because of their small scale, the dots can be sprayed onto flexible surfaces, including plastics. This allows for production of solar cells that are less expensive than existing silicon-based versions.

“We figured out how to shrink the wrappers that encapsulate quantum dots down to the smallest imaginable size — a mere layer of atoms,” said Professor Ted Sargent, holder of the Canada Research Chair in Nanotechnology at U of T.

Until now, quantum dots have been capped with organic molecules that separate the nanoparticles by a nanometer. On the nanoscale, that is a long distance for electrons to travel. To solve this problem, the researchers used inorganic ligands, which are sub-nanometer-sized atoms that bind to the surfaces of the quantum dots and take up less space. The combination of close packing and charge trap elimination enabled electrons to move rapidly and smoothly through the solar cells, thus providing record efficiency, with power conversion efficiency up to 6%.

“At KAUST, we used visualization methods with sub-nanometer resolution and accuracy to investigate the structure and composition of the passivated quantum dots,” said co-author Professor Aram Amassian of KAUST in Saudi Arabia. “We proved that the inorganic passivants were tightly correlated with the location of the quantum dots and that it was the chemical passivation, rather than nanocrystal ordering, that led to the remarkable colloidal quantum dot solar cell performance,” he adds.

The team’s quantum dots had the highest electrical currents and the highest overall power conversion efficiency ever seen in CQD solar cells. The performance results were certified by an external laboratory, Newport, which is accredited by the U.S. National Renewable Energy Laboratory.

Ref.: Jiang Tang, et al., Colloidal-quantum-dot photovoltaics using atomic-ligand passivation, Nature Materials, 2011; [DOI:10.1038/nmat3118]