The U of T device is a stack of two light-absorbing layers—one tuned to capture the sun’s visible rays, the other engineered to harvest the half of the sun’s power that lies in the infrared.

—lead author Dr. Xihua Wang

The team pioneered solar cells made using CQD, nanoscale materials that can readily be tuned to respond to specific wavelengths of the visible and invisible spectrum. By capturing such a broad range of light waves, tandem CQD solar cells can in principle reach up to 42% efficiencies. The best single-junction solar cells are constrained to a maximum of 31% efficiency. In reality, solar cells that are on the roofs of houses and in consumer products have 14 to 18% efficiency. The work expands the Toronto team’s world-leading 5.6% efficient colloidal quantum dot solar cells.

Multi-junction solar cells made from a combination of CQDs of differing sizes and thus bandgaps are a promising means by which to increase the energy harvested from the Sun’s broad spectrum. Here, we report the first CQD tandem solar cells using the size-effect tuning of a single CQD material, PbS. We use a graded recombination layer to provide a progression of work functions from the hole-accepting electrode in the bottom cell to the electron-accepting electrode in the top cell, allowing matched electron and hole currents to meet and recombine.

—Wang et al.

According to Edward Sargent, a Professor of Electrical and Computer Engineering at the University of Toronto and Canada Research Chair in Nanotechnology who led the project, the team engineered a cascade of nanometers-thick materials to shuttle electrons between the visible and infrared layers.

Sargent is hopeful that in five years solar cells using the graded recombination layer published in the Nature Photonics paper will be integrated into building materials, mobile devices, and automobile parts.

The publication was based in part on work supported by an award made by the King Abdullah University of Science and Technology (KAUST), by the Ontario Research Fund Research Excellence Program, and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. Equipment from Angstrom Engineering and Innovative Technology enabled the research.

Resources

• Xihua Wang, Ghada I. Koleilat, Jiang Tang, Huan Liu, Illan J. Kramer, Ratan Debnath, Lukasz Brzozowski, D. Aaron R. Barkhouse, Larissa Levina, Sjoerd Hoogland & Edward H. Sargent (2011) Tandem colloidal quantum dot solar cells employing a graded recombination layer. Nature Photonics, doi: 10.1038/nphoton.2011.123