As our culture makes the shift away from carbon-intensive energy sources, alternative sources like solar energy from photovoltaic cells are becoming an attractive option. However, while the cost of producing solar cells has been decreasing steadily in recent years, they still require a great deal of resources to make: the silicon crystals that solar cells use not only require hazardous solvents for their production, they also need to be baked at high temperatures — 1,000ºC (1,832ºF) — to attain the purity required for their use, an energy-intensive process that can increase the final product’s carbon footprint.
A new manufacturing process has been developed by a research team with the Faculty of Applied Science & Engineering at Canada’s University of Toronto, that produces a good-quality, low-cost solar cell. This new process utilizes a mineral called perovskite, a low-cost substitute for use in solar cells that would make production of the cells not only less resource-intensive, as they don’t require the same intensive purification that silicon does, but crystals of the mineral can also be made into an ink-like substance that can be printed directly onto the surface of an ESL (electron-selective layer), the component of the cell that gathers the electricity from the light-sensitive material.
However, using perovskite in solar cells as been hampered by the difficulty of manufacturing a good ESL layer to apply the perovskite crystals to. "The most effective materials for making ESLs start as a powder and have to be baked at high temperatures, above 500 degrees Celsius," explains Dr. Hairen Tan, head of the U of T development team. "You can’t put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell — it will just melt."
Tan’s team has come with an alternate method that uses a chemical reaction, rather than high heat, to produce the ESL layer, and while it still requires some heat to complete the process, the temperature remains below 150ºC (302ºF), well below the melting point of many types of plastic. The new process also creates a strong bond between the ESL layer and the perovskite, allowing for an extremely efficient solar cell, with Tan’s team reporting an efficiency of 20.1 percent.
"This is the best ever reported for low-temperature processing techniques," says Tan. Perovskite solar cells using the older, high-temperature method are only slightly more efficient at 22.1 per cent, and still quite close to the best that consumer-grade silicon solar cells can achieve, at 26.3 per cent. The new process also produces cells with a greater stability than their previous counterparts: where cells made using the older perovskite process would start to degrade after only a few hours, the new cells retained more than 90 percent of their efficiency after 500 hours of use.
Tan also forecasts potential benefits from hybridizing perovskite and silicon into an even more efficient solar cell. "With our low-temperature process, we could coat our perovskite cells directly on top of silicon without damaging the underlying material. If a hybrid perovskite-silicon cell can push the efficiency up to 30 per cent or higher, it makes solar power a much better economic proposition."