23 September 2020
The efficiency of harvesting light in tandem
Published online 6 March 2020
New double-layered solar cells could harvest more of the sun’s light energy.
An ongoing challenge for scientists is to improve solar cell efficiency to capture as much of the sun’s energy as possible whilst keeping costs down. Until recently, solar cell technology has been based on silicon, which efficiently absorbs only certain wavelengths of light, leaving part of the sun’s energy untapped.
Now, Stefaan De Wolf and colleagues at King Abdullah University of Science and Technology in Saudi Arabia, together with Edward Sargent and co-workers at the University of Toronto in Canada, have developed a highly efficient and stable tandem solar cell. Their design combines silicon with perovskite — an organic-inorganic metal halide with valuable properties for solar cell development.
Different semiconducting materials preferentially absorb different light wavelengths. This limits the maximum power that a solar cell can generate using only one semiconductor. Tandem solar cells use more than one semiconductor to increase their maximum output.
“In our design, textured silicon layered with a perovskite ‘ink’ provides the two semiconductors, improving the effectiveness of solar energy conversion,” says De Wolf. “Unlike silicon, perovskite crystals can be mixed with liquid to create an ink that can be printed onto a surface.”
Traditional silicon cells are textured with tiny, micrometre-sized pyramidal shapes. This textured pattern minimizes light reflection from the surface, increasing light capture.
“We wanted to retain this advantage,” says De Wolf. “While previous perovskite-silicon cell manufacturing processes needed a smooth silicon base, we were able to print a perovskite ink layer onto textured silicon.”
The perovskite crystals located in the ‘valleys’ between pyramids generated an electric field that helped separate the excited electrons in the silicon from those in the perovskite. This separation effect enhances the chances of the electrons flowing into the circuit, rather than migrating to other parts of the cell and being lost.
The tandem cell achieved an efficiency of 25.7% – one of the highest reported for this type of solar cell technology. It also remained stable at temperatures of up to 85 degrees Celsius for more than 400 hours.
“We will continue to improve the tandem devices, and have started to assess mass production of our solar cells,” says De Wolf.
Hou, Y., et al. Efficient tandem solar cells with solution-processed perovskite on textured crystalline silicon. Science 367, 6482 (2020).