The sun provides more than enough energy for all our needs, if only we could harness it cheaply and efficiently.
Solar energy could provide a clean alternative to fossil fuels for power generation, but the high cost and low efficiency levels of solar cells have been a major barrier to their widespread use.
But US scientists at Stanford University have found that adding a single layer of organic molecules to a solar cell can increase its efficiency three-fold, and could lead to cheaper, more efficient solar panels.
Professor of chemical engineering Stacey Bent first became interested in a new kind of solar technology two years ago.
Solar cells use tiny particles of semiconductors called quantum dots. Quantum dot solar cells are cheaper to produce than traditional ones, as 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 their efficiency,” said Bent.
In principle, quantum dot cells can reach a much higher efficiency, Bent said, 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. They use a semiconductor to pull an electron in one direction, and another material to pull the hole in the other. This flow of electrons and their holes in different directions leads to an electric current.
But it takes a certain minimum energy to fully separate an electron and a hole. The amount of energy required is specific to different materials and affects what colour, 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%, a limitation of the fixed energy level they can absorb. Quantum dot solar cells, on the other hand, do not share this limitation and can, be far more efficient.
The researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very 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 she found that the exact molecule didn’t matter – just a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells.
She said 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.
Bent’s group has yet to optimise the solar cells.
Once she has found a way to increase the efficiency of quantum dot solar cells, she hopes their lower cost will lead to wider acceptance of solar energy.