硅的替代品:为什么钙钛矿可以将太阳能电池的研发带到新高度?

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钙钛矿在创造太阳能电池板方面有很大的潜力,可以很容易地沉积在大多数表面上,包括柔性和纹理的表面。这些材料的生产成本也很低,重量轻,而且与今天主要是硅的领先光伏材料一样高效。鉴于它们的巨大潜力,它们是越来越多研究和投资的对象。然而,希望利用其潜力的公司必须在钙钛矿太阳能电池具有商业竞争力之前解决一些重大障碍。

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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。

钛酸钙(CaTiO3)是原始矿物钙钛矿,有一个独特的晶体构造。它有一个三部分结构,其组成部分已被标记为A、B和X,其中不同成分的晶格是交错的。钙钛矿家族由许多可能的元素或分子组合组成,这些元素或分子可以占据这三部分中的每一部分,并形成与原始钙钛矿本身类似的结构。

“你可以将原子和分子混合并匹配到结构中,但有一些限制。例如,如果你试图把一个太大的分子塞进结构中,你会使它变形。最终,你可能会导致三维晶体分离成二维分层结构,或完全失去有序的结构,”麻省理工学院机械工程教授兼光伏研究实验室主任Tonio Buonassisi说。“钙钛矿是高度可调控的,就像一种建造你自己的冒险类型的晶体结构。”

这种交错格子的结构由离子或带电分子组成,其中两个(A和B)带正电,另一个(X)带负电。通常情况下,A和B离子的大小相当不同,A离子更大。

在整个钙钛矿类别中,有许多类型,包括金属氧化物钙钛矿,它们已在催化和能源储存和转换中找到应用,如燃料电池和金属空气电池。但据Buonassisi说,十多年来,研究活动的一个主要焦点是卤化铅钙钛矿。

在这一类别中,仍有大量的可能性,世界各地的实验室正在进行繁琐的工作,试图找到在效率、成本和耐用性方面表现最佳的变体--迄今为止,这是最具有挑战性的三者。

许多团队还专注于消除铅的使用的变化,以避免其对环境的影响。然而,Buonassisi指出,“随着时间的推移,铅基设备的性能不断提高,而其他成分在电子性能方面都没有接近。”探索替代品的工作仍在继续,但就目前而言,没有一个能与卤化铅版本竞争。

Buonassisi说,钙钛矿提供的巨大优势之一是它们对结构中的缺陷有很大的容忍度。与硅不同的是,硅需要极高的纯度才能在电子设备中发挥良好的作用,而钙钛矿即使存在许多缺陷和杂质也能正常工作。

为钙钛矿寻找有前景的新候选成分有点像大海捞针,但最近研究人员想出了一个机器学习系统,可以大大简化这一过程。作为该研究的共同作者之一Buonassisi说,这种新方法可能会导致新替代品的开发速度大大加快。

虽然钙钛矿继续显示出巨大的前景,而且一些公司已经在准备开始一些商业生产,但耐久性仍然是它们面临的最大障碍。虽然硅太阳能电池板在25年后还能保持90%的电力输出,但钙钛矿的降解速度要快得多。已经取得了很大的进展--最初的样品只持续了几个小时,然后是几周或几个月,但较新的配方的可用寿命长达几年,适合于一些对寿命不重要的应用。

Buonassisi说,从研究的角度来看,钙钛矿的一个优点是它们在实验室里相对容易制造--化学成分很容易组装起来。但这也是它们的缺点:“这种材料在室温下很容易组合在一起。但它在室温下也很容易分离。来得容易,去得也容易!”

为了处理这个问题,大多数研究人员专注于使用各种保护材料来封装钙钛矿,保护它不暴露在空气和水分中。但其他研究人员正在研究导致这种降解的确切机制,希望能找到本质上更坚固的配方或处理方法。一个关键的发现是,一个被称为自催化的过程在很大程度上要归咎于这种分解。

在自催化过程中,一旦材料的一个部分开始降解,其反应产物就会作为催化剂开始降解结构的邻近部分,并开始进行失控反应。在对其他一些电子材料的早期研究中也存在类似的问题,如有机发光二极管(OLED),并最终通过对原材料增加额外的净化步骤而得到解决,所以在钙钛矿的情况下可能会找到类似的解决方案,Buonassisi建议。

Buonassisi和他的合作研究人员最近完成了一项研究,表明一旦钙钛矿达到至少十年的可用寿命,由于其较低的初始成本,将足以使其在经济上可行,成为大型公用事业规模太阳能农场的硅替代品。

他说,总体而言,钙钛矿的开发进展令人印象深刻,令人鼓舞。他说:“通过短短几年的工作,它已经实现了与碲化镉相当的效率。碲化镉存在的时间更长,但仍在努力实现这一水平。在这种新材料中达到这些更高的性能的容易程度几乎令人目瞪口呆。比较为实现1%的效率改进所花费的研究时间,钙钛矿的进展比碲化镉的进展快100到1000倍。这就是它如此令人兴奋的原因之一。”

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. "

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