Developing solar cells that are cheaper to produce and can harness the sun's energy more efficiently are both important factors in ensuring the widespread use of solar energy to provide a clean alternative to fossil fuels in the future. Stanford researchers have found that adding a single layer of organic molecules can achieve both these goals by increasing three-fold the efficiency of quantum dot solar cells, which are cheaper to produce than traditional solar cells.
Quantum dot solar cells
Quantum dot solar cells use tiny particles of semiconductors – the "quantum dots" – as the photovoltaic material instead of bulk materials such as silicon, copper indium gallium selenide or CdTe. While they are cheaper to produce as they can be made using simple chemical reactions, they have lagged well behind traditional solar cells in terms of efficiency. "I wondered if we could use our knowledge of chemistry to improve their efficiency," said Stacey Bent, a professor of chemical engineering at Stanford. She realized that is she was successful, the reduced cost of these solar cells could lead to mass adoption of the technology.
Bent says that, in principle, quantum dot solar cells should be able to reach much higher efficiency due to a fundamental limitation of traditional solar cells.
Solar cells work by using energy from the sun (photons) to excite electrons, which jump from a lower energy level to a higher one, leaving 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. It is this flow of electrons that leads to an electric current.
The amount of minimum energy required to fully separate the electron and the hole is specific to different materials and affects what color, or wavelength of light the material best absorbs. Because the energy required to excite its electrons corresponds closely to the wavelength of visible light, silicon is commonly used to make solar cells. Although higher efficiencies have been achieved with multi-junction solar cells, those made of a single material have a maximum efficiency of about 31 percent – a limitation of the fixed energy level they can absorb.
Size does matter
Because quantum dots don't share this limitation, they can theoretically be far more efficient. Instead, the energy levels of electrons in quantum dot semiconductors depends on their size – the smaller the quantum dot, the larger the amount of energy needed to excite electrons to the next level. This allows quantum dots to be tuned to absorb a certain of wavelength of light just by changing their size. By building more complex solar cells that have more than one size of quantum dot, the solar cells can absorb multiple wavelengths of light.
Organic molecule layer
In an attempt to take advantage of these properties, the Stanford researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. These self-assembling molecules packed together in an ordered way, with the quantum dots present at the interface of this organic layer and the semiconductor.The researchers tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells, but found that the exact molecule didn't matter. Just having a simple 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," said Bent.
New theory
But in hindsight, Bent said the result made sense, and the researchers came up with a new model with the length of the molecule, and not its exact nature that mattered. Molecules that are too long don't allow the quantum dots to interact well with the semiconductor. Bent's theory is that once the sun's energy creates an electron and a hole, the thin organic layer helps keep them apart, preventing them from recombining and being wasted. Although the group hasn't yet optimized the solar cells, currently achieving an efficiency of 0.4 percent at most, they can tune several aspects of the cell and believe, once they do, the three-fold increase caused by the organic layer would be even more significant.
Bent says the cadmium sulfide quantum dots she is currently using are not ideal for solar cells, and the group plans to try different materials. The group will also try other molecules for the organic layer, and could change the design of the solar cell to try and absorb more light and produce more electrical charge. Once the Stanford team has found a way to increase the efficiency of quantum dot solar cells, Bent hopes their lower cost will lead to wider acceptance of solar energy.
