硅和碲化镉是光伏领域的另外两个主要“竞争者”，指的是特定的材料。另一方面，术语钙钛矿指的是整个化合物家族，此类氧化物最早被发现，是存在于钙钛矿石中的钛酸钙化合物。这种矿物于1839年被发现，并以俄罗斯地质学家列夫 · 佩罗夫斯基 ( Lev Perovski )的名字命名。
Silicon and cadmium telluride are two other major "competitors" in the photovoltaic field, referring to specific materials. On the other hand, the term perovskite refers to the entire family of compounds, and these oxides were first discovered as calcium titanate compounds present in perovskite. The mineral was discovered in 1839 and named after Russian geologist Lev Perovski.
Calcium titanate (CaTiO3) is a primitive mineral perovskite with a unique crystal structure. It has a three-part structure whose components have been marked A, B and X, in which the lattices of different components are staggered. The perovskite family consists of many possible elements or molecular combinations that can occupy each of these three parts and form a structure similar to that of the original perovskite itself.
"you can mix atoms and molecules and match them into the structure, but there are some limitations. For example, if you try to stuff a molecule that is too large into a structure, you will deform it. In the end, you may cause three-dimensional crystals to separate into two-dimensional hierarchical structures, or completely lose order, "said Tonio Buonassisi, a professor of mechanical engineering at MIT and director of the photovoltaic research lab. "perovskite is highly controllable, like a crystal structure of your own type of adventure."
The structure of this staggered lattice consists of ions or charged molecules, of which two (An and B) are positively charged and the other (X) is negatively charged. In general, the sizes of An and B ions are quite different, and An ions are larger.
In the entire perovskite category, there are many types, including metal oxide perovskite, which have found applications in catalysis and energy storage and conversion, such as fuel cells and metal-air cells. But according to Buonassisi, a major focus of research activities for more than a decade has been lead halide perovskite.
There are still plenty of possibilities in this category, and labs around the world are doing tedious work trying to find the best variants in terms of efficiency, cost and durability-by far the three most challenging.
Many teams are also focused on eliminating changes in the use of lead to avoid its impact on the environment. However, Buonassisi points out, "over time, the performance of lead-based devices continues to improve, while other components are not close in terms of electronic performance." The search for alternatives continues, but for now, none can compete with the lead halide version.
Buonassisi said that one of the great advantages that perovskite offers is that they have a high tolerance for defects in the structure. Unlike silicon, silicon requires a very high purity to play a good role in electronic equipment, while perovskite can work even if there are many defects and impurities.
Finding promising new candidates for perovskite is a bit like looking for a needle in a haystack, but recently researchers have come up with a machine learning system that could greatly simplify the process. Buonassisi, one of the study's co-authors, said the new approach could lead to much faster development of new alternatives.
Although perovskite continues to show great promise, and some companies are already preparing to start some commercial production, durability is still the biggest obstacle they face. Although silicon solar panels can maintain 90% of their power output after 25 years, perovskite degrades much faster. Great progress has been made-the initial sample lasts only a few hours, then weeks or months, but the newer formulations have a useful life of several years and are suitable for applications that are not important to life.
From a research perspective, one of the advantages of perovskite is that they are relatively easy to make in the laboratory-the chemical composition is easy to assemble, Buonassisi said. But this is also their disadvantage: "this material is easy to put together at room temperature." But it is also easy to separate at room temperature. Easy to come, easy to go! "
To deal with this problem, most researchers focus on using a variety of protective materials to encapsulate perovskite to protect it from exposure to air and water. But other researchers are studying the exact mechanism that leads to this degradation, hoping to find intrinsically stronger formulations or treatments. A key finding is that this decomposition is largely to blame for a process called autocatalysis.
In the process of autocatalysis, once a part of the material begins to degrade, the reaction product will act as the adjacent part of the degradation structure of the catalyst and begin the out-of-control reaction. Similar problems were found in early studies of other electronic materials, such as organic light-emitting diodes (OLED), which were eventually solved by adding additional purification steps to the raw materials, so similar solutions may be found in the case of perovskite, Buonassisi suggested.
Buonassisi and his co-researchers recently completed a study showing that once perovskite reaches at least a decade of useful life, it will be economically viable enough to become an alternative to silicon for large utility-scale solar farms because of its lower initial cost.
Overall, he said, the progress in the development of perovskite is impressive and encouraging. "through just a few years of work, it has achieved the same efficiency as cadmium telluride," he said. Cadmium telluride has been around for longer, but efforts are still being made to achieve this level. How easy it is to achieve these higher properties in this new material is almost stunning. Compared with the research time taken to achieve a 1% efficiency improvement, the progress of perovskite is 10 to 1000 times faster than that of cadmium telluride. That's one of the reasons it's so exciting. "