鈥淭hird-generation鈥� solar cells that could shatter the efficiency limit of conventional cells have come a step closer. A proof-of-concept device is the first to generate twice the standard current produced from the most energetic photons in sunlight.
A photon arriving at a solar cell needs a certain amount of energy to break an electron free from its atom and generate current. However, photons with energy above that threshold still generally release only a single electron, generating no additional current even if they arrive with more than enough energy to free two electrons.
In 1961, semiconductor pioneers and showed that the these factors limited single solar cells to converting no more than 31 per cent of incident solar energy to electrical energy. But around a decade ago of the University of New South Wales in Sydney, Australia, challenged that orthodoxy. He suggested the could break the 31 per cent barrier.
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Breaking the limit
Earlier this year, a team from the universities of Texas and Minnesota offered one way to break through the barrier, by capturing extra energy lost as heat after the electrons escape their atoms. Now and Justin Sambur at the University of Wyoming in Laramie, and Thomas Novet of Voxtel in Beaverton, Oregon, have taken the first steps along another route to super-efficient solar cells. Their approach involves harnessing particularly energetic photons 鈥� those with more than twice the energy needed to free an electron 鈥� and using them to free two electrons rather than one, potentially doubling the current generated.
The process had already been seen in macroscopic semiconductors, but it has been extremely inefficient. Seeking better results, Parkinson and colleagues turned to quantum dots, tiny chunks of semiconductor that confine current carriers in nanometre-sized volumes.
Parkinson鈥檚 team coated a smooth titanium dioxide electrode with a single layer of lead-sulphide quantum dots. The quantum dots chosen must absorb between 0.85聽and 1.39聽electronvolts to free up one electron.
The researchers illuminated the device with a variety of wavelengths of light. Using reddish light in which individual photons carry 1聽to 2聽electronvolts, 70 to 80聽per cent of the relatively small number of photons absorbed by the device freed up an electron. But then the team switched to light from the blue end of the spectrum, where individual photons carry over 2.4聽electronvolts, which is 2.7 times the threshold for freeing electrons. Now they collected an excess of electrons, up to about double the number of absorbed photons. This strongly suggests that many of the photons were generating two electrons.
Super-cheap cells
鈥淚t鈥檚 very good science,鈥� says of Boston College. The demonstration of collecting the electrons is an important step toward future solar cells, he says.
鈥淭his is a first step,鈥� Parkinson told New 杏吧原创. The single layer of quantum dots used in the experiment is so thin that it absorbed only a little of the incident light and total efficiency was very low. But he expects others to refine the technique and boost total efficiency.
The quantum-dot solar cell technology could replace dyes in dye-sensitised solar cells. These cells presently have a peak production efficiency of around 11 per cent, but it is 鈥渃ost per watt鈥� that is the most important metric, says Parkinson, and quantum-dot sensitised solar cells could be very cheap to make. 鈥淲hat we need is a 12 to 15 per cent efficient solar cell that you can manufacture for a cost not much more than newsprint.鈥�
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