Pushing the efficiency limit of solar cells further

Some laboratories have achieved impressive efficiencies under concentrated sunlight. SunPower Corp. is much closer to the market. The California-based company recently produced a record-setting 24.2 percent efficiency large area silicon wafer in its Philippine factory. Heat loss from available silicon solar cells reduces the maximum efficiency to around 30 percent. Professor Xiaoyang Zhu at the University of Texas at Austin believes his new method, which could produce 66 percent efficiency, may solve this heat loss problem. He and his colleagues are working on semiconductor nanocrystals, or quantum dots, which are suitable for slowing down hot electron cooling. This is the first step needed to produce what Zhu calls “the ultimate solar cell.” The second step is to use the energy from these electrons. The team discovered that the electrons can be transferred from nanocystals to an electron conductor, but it is a long way from a hot electron transfer to a high-efficiency solar cell.

Between SunPower Corp’s near-market solar cell and Zhu’s quantum dot research laboratories have produced successful trials. One of them comes from Fraunhofer Institute for Solar Energy Systems in Germany. A team led by Frank Dimroth have managed to produce a triple-junction solar cell with 41.1 percent efficiency achieved under concentrated sunlight.

Many solar cells with record-breaking efficiencies will never reach the market place. Solar cell durability and high consumer cost, however, remain considerable challenges to successful, high efficiency solar cell use in Europe. Still, highly efficient solar cells are crucial if European companies want to compete against Asian manufacturers whose production costs are lower. It will probably be many years before solar cells exceeding 25 percent efficiency will be widely available.

(14 July 2010)

Annette Oestrand