Solar dots: Each of these 16 dots is a solar cell made up of nanoscopic particles called quantum dots.
Credit: Ted Sargent

Spray-on Solar Goes Double-decker

Quantum-dot cells designed with two layers open potential for higher efficiencies.

•  JULY  2011

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.

Conventional solar cells are tuned to convert light of only one wavelength into electricity; the rest of the solar spectrum either passes through or is converted inefficiently. 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. The researchers chose transparent oxides for this layer, allowing light to pass through them to the bottom cell.

The result, described this week in the journal Nature Photonics, is a tandem-junction cell that captures a wide range of the spectrum and has an efficiency of 4.2 percent. Sargent says that the approach can be used to make triple-layer and even quadruple-layer, which could be even better. The team's goal is to exceed 10 percent efficiency within five years and keep improving from there. Conventional solar panels are around 15 percent efficient, but quantum-dot cells of somewhat less efficiency could still have an edge in terms of overall costs for solar power if they prove dramatically less expensive to manufacture.

John Asbury, a professor of chemistry at Penn State University, says that by opening up the ability to make multilayer cells from quantum dots, the U of T team has boosted the theoretical efficiency of the technology from 30 percent to almost 50 percent. But getting anywhere near those kinds of efficiencies will require a lot of work to eliminate "trapped states"—places within the quantum-dot material where electrons can become stuck. "The problem with quantum dots is that electrons have a high probability of not making it to the electrodes where they can be collected, so that has limited their efficiency," he says. "To really have an impact means developing strategies to control those trapped states."