It’s an old antiphon of photovoltaic industry : the upcoming advent of a technology capable of surpassing the performance of silicon-based panels. Or, at the very least, to propose a more flexible alternative, integrable 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%, i.e. the performance of state-of-the-art monocrystalline silicon cells, which have been developed for more than fifty years”, explains Frédéric Sauvage, research director at 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 display unprecedented theoretical yields, up to 33%.
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This material takes its name from a very abundant crystalline structure in the Earth’s mantle. But it wasn’t until 2012 that Henry Snaith’s team from Oxford University discovered the extraordinary properties of an artificial perovskite made from lead, iodine and an organic molecule. The British researchers have in particular shown its photoelectric qualities, that is to say its ability to “convert” particles of light (photons) into electrical charges (electrons), but also to effectively convey 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 temperature (between 100 and 150°C), where the silicon must be melted at 1,400°C, then undergo controlled cooling under vacuum, “which makes it possible to print the cells not only on rigid supports such as panels, but also flexible, 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 that was necessary for its manufacture, compared to two months for that of perovskite”, exhibits Frédéric Sauvage.
PEROVSKITES: FROM LABORATORY TO FACTORY
Yet, 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 washed away by rain and released into the environment if the panel breaks. The second downside lies in the short lifespan of current cells, whose performance drops after a few months to a few years, whereas a commercial silicon panel is certified for 20 to 30 years. This instability is primarily explained by an excessive affinity for humidity. This infiltrates the cell and ends up generating products that are harmful to the normal functioning of the cell, “or generates free radicals, highly reactive species that attack photoactive molecules”, specifies Frédéric Sauvage. More disabling: the cells also tend to decompose above 80°C, a temperature that is easily reached in summer by a black panel exposed to direct sunlight.
So, will perovskites end up among the eternal competitors of silicon, whose massive industrialization has resulted in unbeatable costs and progressively optimized yields? The history of photovoltaics is rich in this. Thin film cells, known as second generation, have found themselves limited by their 20% yield and their difficulty in recycling, while flexible organic photovoltaics, known as third generation, still have a ceiling at yields of 5 to 8% and suffers from durability issues. But this time, the story could be different: “After focusing on yields, which have proven to be exceptional, research is also focusing on encapsulation techniques to guarantee cell protection against humidity and the risk of lead dissemination”, emphasizes Solenn Berson. Other research explores alternative formulations totally devoid of lead, or additives reducing the harmful action of UV and temperature.
In addition, start-ups such as Oxford PV in Great Britain or Saule Technologies in Poland are already experimenting with the transition from the lab to the factory by building pilot production lines. “Their demonstrators will allow perovskites to be tested in real conditions and provide valuable lessons on improving systems,” analyzes Frédéric Sauvage, who predicts that “efficient and large-scale panels could see the light of day within ten years”. Because the means are there: the Asian manufacturers of silicon panels closely monitor this opportunity to boost their yields, while the European Union considers perovskites as an opportunity to rebuild a strong photovoltaic industry. The 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”, predicts Frédéric Sauvage.
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