A University of Toronto team led by Professor Ted Sargent has reported the development of the first two-layer quantum dot solar cell, that can be a game changer in the field of solar energy. The team has used two different specification of colloidal quantum dots ( CQS) , one tuned to absorb the visible part of the solar energy, and the other to absorb the infrared and near-infrared part of it, increasing the theoretical junction efficiency from 31 percent to 42 percent.
The conventional bulk silicon solar cells can only absorb energy from the visible spectrum of sunlight. The rest of the energy is passed through unabsorbed on poorly absorbed.The PbS ( Lead Sulfide) colloidal quantum dots, used by the Toronto team, have the advantage of being capable of being tuned to absorb different parts of the solar spectrum.
Combining two layers into a “tandem junction” has been attempted earlier too, but the problem had always been the resistance at the junction that led to losses. The Toronto team has come up with a solution to this problem by using a combination of four metal oxides in a cascaded transition layer that has kept the resistance “nice and low” as Prof Sargent puts it. The team has also chosen transparent oxides to permit the sunlight to pass through the lower layer.
The team is reported to have applied the CQS in layers on a Tin Oxide coated glass plate in ambient air and temperature conditions. Since the process does not need specialised equipment, it could eventually be as easy to apply as spraying on. The technology also permits increasing the number of layers to 3 or 4 that could push the theoretical junction efficiency as high as 84 percent.
While the theoretical junction efficiency has been increased to 42 percent, the Toronto team’s first two layer cell has an efficiency of only a tenth of that at 4.2 percent. The Toronto team is confident that in the next 5 years, this efficiency will be increased to 10 percent. The lower efficiency of quantum dot cells will be offset by their lower costs both in manufacture and application.