Por qué las perovskitas podrían llevar las células solares a nuevas alturas

Esta familia de compuestos cristalinos está a la vanguardia de la investigación de alternativas al silicio.

Las perovskitas tienen un gran potencial para crear paneles solares que podrían depositarse fácilmente en la mayoría de las superficies, incluidas las superficies flexibles y texturizadas. Estos materiales también serían económicos de producir, livianos y tan eficientes como los principales materiales fotovoltaicos de la actualidad, que en su mayoría son silicio. Dado su enorme potencial, son objeto de una creciente investigación e inversión. Sin embargo, las empresas que buscan liberar su potencial deben superar algunos obstáculos importantes antes de que las células solares basadas en perovskita puedan ser comercialmente competitivas.

El telururo de silicio y cadmio, otros dos principales contendientes en el campo fotovoltaico, se refieren a materiales específicos. Por otro lado, el término perovskita hace referencia a toda una familia de compuestos. La familia de perovskita de materiales solares recibe su nombre por su similitud estructural con un mineral llamado perovskita, que fue descubierto en 1839 y lleva el nombre de LA Perovski, un mineralogista ruso.

Óxido de titanio y calcio (CaTiO₃), el mineral original perovskita, tiene una configuración cristalina distintiva. Tiene una estructura de tres partes, cuyos componentes se denominan A, B y X, en la que se entrelazan las redes de los diferentes componentes. La familia de las perovskitas consta de las muchas combinaciones posibles de elementos o moléculas que pueden ocupar cada uno de los tres componentes y formar una estructura similar a la de la propia perovskita original. (Algunos científicos incluso modifican un poco las reglas al nombrar “perovskitas” a otras estructuras cristalinas con elementos similares, aunque los cristalógrafos desaprueban esto).

“Puedes mezclar y combinar átomos y moléculas en la estructura, con ciertos límites. Por ejemplo, si intenta insertar una molécula demasiado grande en la estructura, la distorsionará. Eventualmente, podría hacer que el cristal 3D se separe en una estructura en capas 2D o perder la estructura ordenada por completo”, dice Tonio Buonassisi, profesor de ingeniería mecánica en[{” attribute=””>MIT and director of the Photovoltaics Research Laboratory. “Perovskites are highly tunable, like a build-your-own-adventure type of crystal structure,” he says.

That structure of interlaced lattices consists of ions or charged molecules, two of them (A and B) positively charged and the other one (X) negatively charged. Typically, the A and B ions are of quite different sizes, with the A being larger.

Within the overall category of perovskites, there are a number of types, including metal oxide perovskites, which have found applications in catalysis and in energy storage and conversion, such as in fuel cells and metal-air batteries. But a main focus of research activity for more than a decade has been on lead halide perovskites, according to Buonassisi says.

Within that category, there is still a legion of possibilities, and labs around the world are racing through the tedious work of trying to find the variations that show the best performance in efficiency, cost, and durability — which has so far been the most challenging of the three.

Many teams have also focused on variations that eliminate the use of lead, to avoid its environmental impact. Buonassisi notes, however, that “consistently over time, the lead-based devices continue to improve in their performance, and none of the other compositions got close in terms of electronic performance.” Work continues on exploring alternatives, but for now, none can compete with the lead halide versions.

One of the great advantages perovskites offer is their great tolerance of defects in the structure, according to Buonassisi. Unlike silicon, which requires extremely high purity to function well in electronic devices, perovskites can function well even with numerous imperfections and impurities.

Searching for promising new candidate compositions for perovskites is a bit like looking for a needle in a haystack, but recently researchers have come up with a machine-learning system that can greatly streamline this process. This new approach could lead to a much faster development of new alternatives, says Buonassisi, who was a co-author of that research.

While perovskites continue to show great promise, and several companies are already gearing up to begin some commercial production, durability remains the biggest obstacle they face. While silicon solar panels retain up to 90 percent of their power output after 25 years, perovskites degrade much faster. Great progress has been made — initial samples lasted only a few hours, then weeks or months, but newer formulations have usable lifetimes of up to a few years, suitable for some applications where longevity is not essential.

From a research perspective, Buonassisi says, one advantage of perovskites is that they are relatively easy to make in the lab — the chemical constituents assemble readily. But that’s also their downside: “The material goes together very easily at room temperature,” he says, “but it also comes apart very easily at room temperature. Easy come, easy go!”

To deal with that issue, most researchers are focused on using various kinds of protective materials to encapsulate the perovskite, protecting it from exposure to air and moisture. But others are studying the exact mechanisms that lead to that degradation, in hopes of finding formulations or treatments that are more inherently robust. A key finding is that a process called autocatalysis is largely to blame for the breakdown.

In autocatalysis, as soon as one part of the material starts to degrade, its reaction products act as catalysts to start degrading the neighboring parts of the structure, and a runaway reaction gets underway. A similar problem existed in the early research on some other electronic materials, such as organic light-emitting diodes (OLEDs), and was eventually solved by adding additional purification steps to the raw materials, so a similar solution may be found in the case of perovskites, Buonassisi suggests.

Buonassisi and his co-researchers recently completed a study showing that once perovskites reach a usable lifetime of at least a decade, thanks to their much lower initial cost that would be sufficient to make them economically viable as a substitute for silicon in large, utility-scale solar farms.

Overall, progress in the development of perovskites has been impressive and encouraging, he says. With just a few years of work, it has already achieved efficiencies comparable to levels that cadmium telluride (CdTe), “which has been around for much longer, is still struggling to achieve,” he says. “The ease with which these higher performances are reached in this new material are almost stupefying.” Comparing the amount of research time spent to achieve a 1 percent improvement in efficiency, he says, the progress on perovskites has been somewhere between 100 and 1000 times faster than that on CdTe. “That’s one of the reasons it’s so exciting,” he says.