Rice University engineers created microscopic seeds to grow remarkably uniform 2D perovskite crystals that are both stable and highly efficient at harvesting electricity from sunlight.
Halide perovskites are organic materials made from abundant and inexpensive ingredients, and Rice’s seed growth method addresses both performance and production issues that have held back halide perovskite photovoltaic technology.
In a study published online in Advanced materials, chemical engineers at Rice’s Brown School of Engineering describe how to make the seeds and use them to grow homogeneous thin films, highly sought-after materials made up of uniformly thick layers. In laboratory tests, photovoltaic devices made from the films have been shown to be both efficient and reliable, a combination previously problematic for devices made from 3D or 2D perovskites.
âWe’ve come up with a method where you can really tailor the properties of macroscopic films by first tailoring what you put into solution,â said study co-author Aditya Mohite, associate professor of chemical engineering and biomolecular and materials science. and nanoengineering at Rice. âYou can come up with something very homogeneous in size and properties, which leads to higher efficiency. We got an almost state-of-the-art device efficiency for the 2D case of 17%, and that was without optimization. We think there are a number of ways we can improve this. “
Mohite said that making homogeneous 2D perovskite films has been a huge challenge in the halide perovskite photovoltaic research community, which has grown significantly over the past decade.
âHomogeneous films are expected to lead to optoelectronic devices with both high efficiency and technologically relevant stability,â he said.
The high-efficiency photovoltaic films grown from Rice seeds have been shown to be quite stable, maintaining over 97% of their peak efficiency after 800 hours of lighting without any thermal management. In previous research, 3D halide perovskite photovoltaic devices were very efficient but subject to rapid degradation, and 2D devices lacked efficiency but were very stable.
The Rice study also details the process of seeded growth – a method within the reach of many labs, said study co-author Amanda Marciel, chair of the board of directors of William Marsh Rice and assistant professor of engineering. chemical and biomolecular in Rice.
âI think people are going to take this paper and say, ‘Oh. I’m going to start doing this,’â Marciel said. “It’s a really good processing paper that goes deep in a way that hasn’t really been done before.”
The name perovskite refers both to a specific mineral discovered in Russia in 1839 and to any compound having the crystal structure of that mineral. For example, halide perovskites can be made by mixing lead, tin, and other metals with bromide or iodide salts. Research interest in halide perovskites skyrocketed after their potential for high-efficiency photovoltaics was demonstrated in 2012.
Mohite, who joined Rice in 2018, has studied halide perovskite-based photovoltaics for more than five years, particularly 2D perovskites – flat, almost atomically thin forms of the material that are more stable than their larger cousins. thick due to inherent resistance to moisture.
Mohite credited the study’s lead author Siraj Sidhik with a doctorate. studying in his lab, with the idea of ââpursuing seeded growth.
âThe idea that a memory or a story – some kind of genetic seed – can dictate material properties is a powerful concept in materials science,â Mohite said. “A lot of models work like this. If you want to grow a single crystal of diamond or silicon, for example, you need a seed of a single crystal that can serve as a model.”
While seeded growth has often been demonstrated for inorganic crystals and other processes, Mohite said this is the first time it has been demonstrated in organic 2D perovskites.
The process of growing 2D perovskite films from seeds is identical in many respects to the conventional process of growing such films. In the traditional method, precursor chemicals are measured like the ingredients of a kitchen – X parts of ingredient A, Y parts of ingredient B, etc. – and these are dissolved in a liquid solvent. The resulting solution is spread on a flat surface via spin coating, a widely used technique that relies on centrifugal force to evenly distribute liquids on a rapidly rotating disc. As the solvent dissolves, the mixed ingredients crystallize into a thin film.
Mohite’s group have been making 2D perovskite films this way for years, and although the films appear perfectly flat to the naked eye, they are uneven at the nanoscale. In some places the film can be a single crystal thick, and in other places several crystals thick.
âYou end up getting something that is completely polydisperse, and when the size changes, the energy landscape changes as well,â Mohite said. “What that means for a PV device is inefficiency, because you lose energy through scattering when the charges meet a barrier before you can reach an electrical contact.”
In the seeded growth method, seeds are made by slowly growing a uniform 2D crystal and grinding it into a powder, which is dissolved in a solvent instead of the individual precursors. The seeds contain the same ratio of ingredients as the classic recipe, and the resulting solution is applied by centrifugation on discs exactly as it would be in the original method. The evaporation and crystallization steps are also identical. But the seeded solution gives films with a homogeneous and uniform surface, much like that of the material from which the seeds were crushed.
When Sidhik first succeeded with the approach, it wasn’t immediately clear why he was producing better movies. Fortunately, Mohite’s lab adjoins Marciel’s, and although she and her student, lead co-author Mohammad Samani, had never worked with perovskites, they had the perfect tool to find and study all of them. pieces of undissolved seeds that could serve as a model for homogeneous films. .
âWe could follow this nucleation and growth using light scattering techniques in my group that we typically use to measure the size of polymers in solution,â Marciel said. “That’s how the collaboration was born. We’re neighbors in the lab, and we were talking about it, and I was like, ‘Hey, I have this piece of equipment. Let’s see how big these seeds are and if we can track them over time, using the same tools we use in polymer science. ‘”
The tool was dynamic light scattering, a basic technique in Marciel’s group. He found that the solutions reached a state of equilibrium under certain conditions, allowing a portion of certain seeds to remain undissolved in the solution.
Research showed that these seed pieces retained the “memory” of the perfectly uniform slow-growing crystal from which they were ground, and Samani and Marciel found that they could follow the nucleation process that would eventually allow the seeds to produce. homogeneous thin films.
Mohite said the collaboration produced something that is often attempted and rarely achieved in nanomaterial research – a self-assembly method for making macroscopic materials that hold the promise of the individual nanoparticles that make them up.
âIt really is the bane of nanomaterial technology,â Mohite said. âAt the level of an individual element, you have wonderful properties which are orders of magnitude better than anything else, but when you try to put them together into something macroscopic and useful, like a film, those properties disappear. sort of because you can’t do something homogeneous with just the properties you want.
“We haven’t done any experiments on other systems yet, but the success of perovskites raises the question of whether this kind of seeded approach might work in other systems as well,” he said. .
The research was supported by the Office of Energy Efficiency and Renewable Energies of the Ministry of Energy (DOE), the University Institute of France and the Office of Naval Research (N00014-20-1-2725 ) and used the DOE facilities at the Argonne National Laboratory. and Brookhaven National Laboratory.