Perovskites: Can this technology revive photovoltaics?

It is an ancient antiphon of photovoltaic industry : the imminent arrival of a technology capable of surpassing the performance of silicon-based panels. Or, at least, propose a more flexible alternative, which can be integrated in places (buildings, furniture) inaccessible to this same silicon, which rests on thick and rigid plates. Repeatedly postponed, this dream may finally be about to materialize thanks to perovskite-based cells. “In ten years of research, this technology has gone from 3% efficiency to 25%, that is, the performance of the latest generation monocrystalline silicon cells, which have been developed for more fifty years old “. explains Frédéric Sauvage, research director of the laboratory of reactivity and chemistry of solids (CNRS). Better: perovskites recover much better than silicon the blue part of the light spectrum, they can be associated with it within silicon cells. These so-called tandem cells show unprecedented theoretical yields of up to 33%.

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This material is called a very abundant crystalline structure in the Earth’s mantle. But it wasn’t until 2012 that Oxford Snaith’s team at Oxford University discovered the extraordinary properties of an artificial perovskite made of lead, iodine and an organic molecule. British researchers have shown in particular their photoelectric qualities, ie their ability to “convert” light particles (photons) into electrical charges (electrons), but also to effectively transmit these charges. “Perovskite retains these qualities even if there are defects in its structure, unlike silicon whose crystals must be extremely pure.” says Solenn Berson, head of technology development at the National Solar Energy Institute. In terms of manufacturing, perovskites can be formed at low temperatures (between 100 and 150 ° C), where the silicon must be melted at 1,400 ° C, then subjected to a vacuum-controlled cooling. “which allows the cells to be printed not only on rigid supports such as panels, but also flexible ones, such as plastic films, to facilitate integration into the frame”, continues the researcher. With the key to saving energy: “A classic panel takes two years to generate the energy needed to make it, compared to two months for perovskite.” exposed Frédéric Sauvage.

PEROVSKITES: FROM THE LABORATORY TO THE FACTORY

However, despite these promises, perovskites still suffer from two major flaws. The first is that current technology contains lead, a toxic heavy metal that is likely to be dragged by rain and released into the environment if the panel breaks. The second drawback lies in the short service life of current cells, whose performance drops after a few months to a few years, while a commercial silicon panel is certified for 20 to 30 years. This instability is mainly explained by an excessive affinity for moisture. This infiltrates the cell and ends up producing products that are detrimental to the normal functioning of the cell. “or generates free radicals, highly reactive species that attack photoactive molecules,” specifies Frédéric Sauvage. More disability: Cells also tend to decompose above 80 ° C, a temperature that is easily reached in summer by a black panel exposed to direct sunlight.

Will perovskites then end up among the eternal competitors of silicon, whose mass industrialization has resulted in unbeatable costs and progressively optimized yields? The history of photovoltaics is rich in this. Thin film cells, known as the second generation, have been limited by their 20% efficiency and recycling difficulty, while flexible organic photovoltaics, known as the third generation, still have a yields 5 to 8% and suffers from durability issues. . But this time, the story may be different: “After focusing on yields, which have proven to be exceptional, research is also focusing on encapsulation techniques to ensure cellular protection against moisture and the risk of lead diffusion.” emphasizes Solenn Berson. Other research explores alternative formulations that are completely free of lead, or additives that reduce the harmful action of UV and temperature.

© © DORLING KINDERSLEY / UIG / SCIENCE PHOTO LIBRARY – OXFORD PV

Tandem cells, developed by Oxford PV, mix a silicon base and a layer of perovskite. They are already recording a yield of 29.52%.

In addition, start-ups such as Oxford PV in the UK and Saule Technologies in Poland are already experiencing the transition from laboratory to factory through the construction of pilot production lines. “Its demonstrators will test perovskites in real conditions and provide valuable lessons on system improvement.” analyzes Frédéric Sauvage, who predicts it “Efficient, large-scale panels could see the light of day in ten years.” Because the media is there: Asian silicon panel manufacturers are closely following this opportunity to increase their yields, while the European Union sees perovskites as an opportunity to rebuild a strong photovoltaic industry. Researchers see even further: “The properties of perovskites are also very promising for lasers, transistors, photodetectors for imaging … in short, also for reinventing a new sector, that of the semiconductors of tomorrow.” predicted Frédéric Sauvage.

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