Silicon-Perovskite-Tandem-Cell With Record Efficiency

Scanning-electron-microscope image of silicon’s pyramids covered with perovskite. (Source: EPFL)

In the field of photo­voltaic techno­logies, silicon-based solar cells make up 90% of the market. In terms of cost, stability and effi­ciency – 20-22% for a typical solar cell on the market –, they are well ahead of the compe­tition. However, after decades of research and investment, silicon-based solar cells are now close to their maximum theo­retical effi­ciency. As a result, new concepts are required to achieve a long-term reduction in solar elec­tricity prices and allow photo­voltaic techno­logy to become a more widely adopted 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 being widely researched in the scientific community, but are expensive to make. Now research teams in Neuchâtel from EPFL’s Photo­voltaics Labora­tory and the CSEM PV-center have developed an econo­mically compe­titive solution. They have inte­grated a perovskite cell directly on top of a standard silicon-based cell, obtaining a record effi­ciency of 25.2%. Their produc­tion method is promising, because it would add only a few extra steps to the current silicon-cell produc­tion process, and the cost would be reasonable.

Perovskite’s unique properties have prompted a great deal of research into its use in solar cells over the last few years. In the space of nine years, the effi­ciency of these cells has risen by a factor of six. Perovskite allows high conversion effi­ciency to be achieved at a poten­tially limited production cost. In tandem cells, perov­skite comple­ments silicon: it converts 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 Florent Sahli and Jérémie Werner.

However, creating an effec­tive tandem structure by super­posing the two materials is no easy task. “Silicon’s surface consists of a series of pyramids measuring around 5 microns, which trap light and prevent it from being reflected. However, the surface texture makes it hard to deposit a homo­geneous film of perovskite,” explains Quentin Jeangros. When the perovskite is deposited in liquid form, as it usually is, it accu­mulates in the valleys between the pyramids while leaving the peaks unco­vered, leading to short circuits.

The scientists have gotten around that problem by using evapo­ration methods to form an inorganic base layer that fully covers the pyramids. That layer is porous, enabling it to retain the liquid organic solution that is then added using spin-coating. The researchers subse­quently heat the substrate to a relatively low 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.

The new type of tandem cell is highly effi­cient and directly compatible with mono­crystalline silicon-based techno­logies, which benefit from long-standing industrial expertise and are already being produced profitably. “We are proposing to use equipment that is already in use, just adding a few specific stages. Manu­facturers won’t be adopting a whole new solar tech­nology, but simply updating the production lines they are already using for silicon-based cells,” explains Christophe Ballif, head of EPFL’s Photo­voltaics Labora­tory and CSEM’s PV-Center. At the moment, research is continuing in order to increase effi­ciency further and give the perovskite film more long-term stabi­lity. Although the team has made a break­through, there is still work to be done before their tech­nology can be adopted commer­cially. (Source: EPFL)

Reference: F. Sahli et al.: Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency, Nat. Mat., online 11 June 2018; DOI: 10.1038/s41563-018-0115-4

Link: Photovoltaics and thin film electronics laboratory PV-LAB, EPFL, Neuchâtel, Switzerland

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