Technology Review - A research team at the University of Toronto has created the first two-layer solar cell made up of light-absorbing nanoparticles called quantum dots. Quantum dots, which can be tuned to absorb different parts of the solar spectrum by varying their size, have been seen as a promising route to low-cost solar cells because the particles can be sprayed onto surfaces much like paint. But cells based on this technology have been too inefficient to be practical. By discovering a way to combine two different types of quantum dots in a solar cell, the researchers could open the way to making such cells much more efficient.
Nature Photonics - Tandem colloidal quantum dot solar cells employing a graded recombination layer
To harness a greater percentage of the energy in sunlight, manufacturers sometimes stack materials designed to capture different parts of the spectrum. A two-layer cell, called a tandem-junction cell, can theoretically achieve 42 percent efficiency, compared with a maximum theoretical efficiency of 31 percent for cells with a single layer.
In the Toronto researchers' cell, one layer of quantum dots is tuned to capture visible light and the other to capture infrared light. The researchers also found a way to reduce electrical resistance between the layers, a problem that can limit the power output of a two-layer cell. They introduced a transition layer, made up of four films of different metal oxides, that keeps resistance "nice and low," says Ted Sargent, a professor of electrical and computing engineering who led the research at the University of Toronto.
Tuning of the electronic bandgap in colloidal quantum dots (CQDs) by changing their size enables the spectral response of CQD-based photodetectors and photovoltaic devices to be tailored. 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. Our tandem solar cell has an open-circuit voltage of 1.06 V, equal to the sum of the two constituent single-junction devices, and a solar power conversion efficiency of up to 4.2%.
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