After decades of extensive research, silicon-based solar cells have hit their maximum theoretical efficiency. New concepts are required to achieve a long-term decrease in solar electricity costs and permit photovoltaic technology to become a more accepted way of generating power. One solution is to place two different types of solar cells on top of each other to maximize the conversion of light rays into electrical power. These "double-junction" cells are researched in the scientific community, but are expensive. Now research teams from EPFL's Photovoltaics Laboratory and the CSEM PV-center created an economical answer. The researchers added a perovskite cell on top of a standard silicon-based cell that has a record efficiency of 25.2%. The method is promising because the price is more reasonable to the current silicon-cell production process.
Perovskite's unique properties have prompted research into its use in solar cells. The efficiency of the cells has risen by a factor of six and has a high conversion efficiency at a limited production cost. In tandem cells, perovskite complements silicon. It translates blue and green light more efficiently, while silicon is better at converting red and infra-red light. "By combining the two materials, we can maximize the use of the solar spectrum and increase the amount of power generated. The calculations and work we have done show that a 30% efficiency should soon be possible," say the study's main authors Florent Sahli and Jérémie Werner. Despite its huge popularity, developing an effective tandem structure by superposing the two materials is difficult. Silicon's surface have several pyramids measuring around 5 microns that traps light and prevent it from being reflected. The surface’s texture makes it an issue to place a film of perovskite. When the perovskite is deposited in liquid form, it stores between the pyramids that result in short circuits.
Scientists have resolved the problem by applying evaporation methods to form an inorganic base, porous layer that covers the pyramids. This enables it to retain the liquid organic solution added using a thin-film deposition method called spin-coating. Its heated to a temperature of 150°C to crystallize a homogeneous film of perovskite on top of the silicon pyramids. "Until now, the standard approach for making a perovskite/silicon tandem cell was to level off the pyramids of the silicon cell, which decreased its optical properties and therefore its performance, before depositing the perovskite cell on top of it. It also added steps to the manufacturing process," says Florent Sahli. This solution has made silicon with perovskite successful.
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