One layer triples solar cell prowess

STANFORD (US) — Adding a single layer of organic molecules less than a nanometer thick improves the efficiency of solar cells threefold and could possibly lead to less expensive, more efficient solar panels.

Researchers have known that solar energy can provide a clean alternative to fossil fuels, but to date, high cost has been a major barrier to their widespread use.

Quantum dot solar cells are cheaper to produce that traditional ones because they can be made using simple chemical reactions. But despite their promise, they lagged well behind existing solar cells in efficiency.

“I wondered if we could use our knowledge of chemistry to improve (quantum dots’) efficiency,” says Stacey Bent, professor of chemical engineering at Stanford University.

Bent discussed her research, reported online in the journal  ACS Nano, at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.

In principle, quantum dot cells can reach much higher efficiency, Bent says, because of a fundamental limitation of traditional solar cells.

Solar cells work by using energy from the sun to excite electrons. The excited electrons jump from a lower energy level to a higher one, leaving behind a “hole” where the electron used to be.

Solar cells use a semiconductor to pull an electron in one direction, and another material to pull the hole in the other direction. This flow of electron and hole in different directions leads to an electric current.

But it takes a certain minimum energy to fully separate the electron and the hole. The amount of energy required is specific to different materials and affects what color, or wavelength, of light the material best absorbs.

Silicon is commonly used to make solar cells because the energy required to excite its electrons corresponds closely to the wavelength of visible light.

But solar cells made of a single material have a maximum efficiency of about 31 percent, a limitation of the fixed energy level they can absorb.

Quantum dot solar cells on the other hand don’t have this limitation and can in theory be far more efficient. The energy levels of electrons in quantum dot semiconductors depends on their size—the smaller the quantum dot, the larger the energy needed to excite electrons to the next level.

So quantum dots can be tuned to absorb a certain wavelength of light just by changing their size. And they can be used to build more complex solar cells that have more than one size of quantum dot, allowing them to absorb multiple wavelengths of light.

Because of these advantages, Bent has been investigating ways to improve the efficiency of quantum dot solar cells, with Associate Professor Michael McGehee, associate professor of materials science and engineering.

Quantum dots deposited on pieces of silicon wafer (black and silver samples).

The researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a thin single layer of organic molecules. These molecules were self-assembling, meaning that their interactions caused them to pack together in an ordered way.

The quantum dots were present at the interface of this organic layer and the semiconductor. Bent’s students tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells.

But the exact molecule was found to not matter—just having a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells.

“We were surprised. We thought it would be very sensitive to what we put down,” Bent says, but the result made sense in hindsight, and the researchers came up with a new model – it’s the length of the molecule, and not its exact nature, that matters.

Molecules that are too long don’t allow the quantum dots to interact well with the semiconductor.

Once the sun’s energy creates an electron and a hole, the thin organic layer helps keep them apart, Bent says, preventing them from recombining and being wasted.

The group has yet to optimize the solar cells, and they have currently achieved an efficiency of, at most, 0.4 percent. But several aspects of the cell can be fine-tuned, and once that happens, the threefold increase caused by the organic layer will be even more significant.

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