A great deal of solar energy is wasted in today's solar cells. Researchers at Chalmers, however, have an idea that may improve efficiency tremendously. They have already demonstrated that the principle works in the lab – now the research will move out on campus to continue under real conditions.
The sun is an infinitely rich and renewable source of energy. In about an hour, it radiates more energy to the earth than the world uses over the course of an entire year. It is for this reason that solar energy is such an interesting constituent in the transformation to a sustainable energy future – it is just a matter of converting the solar energy to electricity in an efficient manner.
The efficiency of current commercial solar cells is quite low – about 15 per cent, which is far from the theoretical limits. One important reason fot the low efficiency is that solar cells can only utilise a limited amount of the incoming sunlight.
Sunlight contains all colours, but only the photons in the energy-rich, bluer part of the sun's spectrum generate electrons, and hence electricity, in the solar cell. The photons' energy is too low in the redder part of the spectrum for the solar cells to be able to convert the photons to electrons. A large amount of the energy in sunlight is consequently wasted.
Solar cell researchers at Chalmers want to change this. The idea is to merge a pair of red photons with low energy to a blue photon with higher energy. Two previously unusable photons are thus converted into a usable photon.
"This enables the solar cell to utilise more of the light it encounters, thus increasing efficiency significantly," says Maria Abrahamsson, who is a researcher in physical chemistry and one of the project managers for Solar Initiative.
The objective is to create an photon conversion add-on layer that can be inserted into all existing types of solar cells. The efficiency of solar cells could then hopefully increase by serveral percentage points, maybe even more. This is a pronounced improvement compared to current solar cells. It would be a giant step forward if the add-on layer can also be produced inexpensively so that the cost for the solar cell only increases marginally – solar cells would become so competitive that they would be an obvious first choice for electricity production.
In their laboratory, the Chalmers researchers have already demonstrated that the principle works – using specially designed molecules and laser light, they have managed to create energy-rich photons from photons with lower energy.
"We have demonstrated that the principle works under specific, controlled conditions in the laboratory. Now it is time to move ahead and make things that work in real conditions as well," says Maria Abrahamsson.
Consequently, part of the experiment is being moved out on campus as part of the Chalmers Energy on Campus initiative, as a means to introduce authentic conditions for continued research. Sunlight is not near as orderly as laser light, and the temperature outdoors changes much more than in the laboratory. The researchers need to introduce aspects of this kind already at this stage in order to improve the chances of creating a practically useful add-on layer.
"Including aspects all the way down to molecular level when designing our systems for upconversion of photons entails a new way of thinking," explains Maria Abrahamsson.
The objective is a system where solvents are superfluous and where the entire photon conversion process takes place within one and the same molecule. In simple terms, the process is like this: the molecule's two ends absorb energy from two separate red photons. The absorbed energy is merged and added inside the molecule. The molecule then transmits the total energy in the form of one photon with higher energy.
The Solar Initiative project has received funding through the Energy on Campus initiative to employ a new postdoc to work on the outdoor experiment and to analyse new variants of photon conversion molecules in the lab. A research team in environmental systems analysis is also involved in the project to perform life-cycle assessments to ensure the add-on layer is financially and environmentally sustainable from the start.
"We really want this to be practically useful. The energy challenge is real, and it would be fantastic to be able to contribute to a solution," says Maria Abrahamsson.