High-Efficiency Solar Energy Collectors Made From Microscopic Seeds

Engineers can create seeds to grow near-perfect 2D Perovskite crystals.

Rice University engineers created tiny seeds to grow uniform, 2D perovskite crystals. These crystals are highly stable and efficient in harvesting sunlight’s electricity.

The organic materials of halide perovskites are made from inexpensive, abundant ingredients. Rice’s seeded growing method addresses production and performance issues that have hindered halide perovskite solar technology.

A study published online by¬†Advanced Materials¬†describes how chemical engineers at Rice’s Brown School of Engineering make seeds and grow thin films. These films are highly desired materials that have uniformly thick layers. Photovoltaic devices made of the movie were reliable and efficient in laboratory tests. This new combination was previously tricky for 3D or 2D perovskite devices.

Aditya Mohite (associate professor of chemical and biological engineering, materials science, and nanoengineering) said she had developed a method to tailor the properties of macroscopic films. You can achieve something very homogeneous in terms of its size and properties. This leads to greater efficiency. The 2D case was 17% efficient and almost state-of-the-art. This needed to be optimized. We have many ways to improve that.

Mohite stated that homogeneous films of 2D perovskites were a significant challenge for the halide-perovskite photovoltaic community. This has increased tremendously in the past decade.

He said homogeneous films could lead to highly efficient optoelectronics and technologically relevant durability.

High-efficiency, seed-grown photovoltaic films from Rice proved to be relatively stable. They retained more than 97% after 800 hours of illumination without thermal management. Previous research has shown that 3D halide perovskite photo solar cells are efficient but prone to rapid degrading, while 2D devices are less efficient but highly stable.

Rice also describes seeded growth, a process that can be used in many labs, according to Amanda Marciel (William Marsh Rice Trustee Chair, assistant professor of chemical, biomolecular, and engineering at Rice).

“I believe people will pick up the paper and say, “Oh. Marciel stated, “I’m going to get started doing this.” “It’s a very nice paper; it’s intense and has never been done before.”

Perovskite is a name that refers to a particular mineral found in Russia in 1839 and any compound with the same crystal structure. You can make halide perovskites by mixing tin, lead, and other metals with bromide, iodide, or bromide salts. After their potential for high-efficiency photovoltaics in 2012, research interest in halide-perovskites has skyrocketed.

Mohite joined Rice in 2018. He has been researching halide perovskite photo solar cells for over five years, particularly 2D perovskites, which are flat, almost atomically thin forms that are more stable than their thicker counterparts because of their inherent moisture resistance.

Mohite gave Siraj Sidhik, a Ph.D. student, the idea to pursue seeded growth.

Mohite stated that material properties could be determined by a person’s past or memory — such as a seed type or genetic variety — is a powerful concept within materials science. This is how a lot of templating works. A seed of one crystal can be used as a template to grow a single crystal, such as silicon or diamond.

Although seeded growth has been demonstrated inorganic crystals, Mohite stated that this is the first time it has been shown in organic 2D Perovskites.

Growing 2D perovskite film from seeds is similar to the traditional process in many respects. The conventional method uses precursor chemicals that are measured like ingredients in a kitchen. These are then dissolved in liquid solvents. Spin-coating is a popular technique that uses centrifugal force to spread liquids evenly across a spinning disk. The solvent will dissolve, and the mixture of ingredients will crystallize into a thin film.

Mohite’s team has been making 2D perovskite films this way for years. Although the movie looks perfectly flat to the naked eye, they are irregular at the nanometer scale. The film can be thinned in some areas, while others may have several crystals.

Mohite stated that you end up with something completely polydisperse. When the size changes, so do the energy landscape. Inefficiency results from energy loss due to scattering, which happens when charges meet a barrier and cannot reach an electrical contact.

Seeded growth is the slow-growing of a 2D crystal, then grinding it into a powder. The solvent replaces the individual precursors. The ratio of ingredients in the seeds is the same as for the classic recipe. Also, the solution is spun-coated onto disks precisely the way it was in the original method. Both the crystallization and evaporation steps are identical. The seeded solution produces films with a uniform, homogeneous surface very similar to the material from which they were made.

Although Sidhik initially succeeded with this approach, it was not immediately apparent why it produced better films. Mohite’s lab is adjacent to Marciel’s. While she and Mohammad Samani had never worked with perovskites before, they could use the ideal tool to find and study any undissolved seeds that could be templating homogeneous films.

Marciel stated that they could track the nucleation and growth of the polymers in solution using light scattering techniques that Marciel uses. That’s how we came up with the idea of collaboration. We are neighbors in the laboratory, and we were discussing this. I said, “Hey, I have this piece of equipment.” Let’s find out how large these seeds are and see if we can track their growth over time using the same tools we use in polymer sciences. ‘”

It was dynamic light scattering that Marciel used as an effective technique. It showed that solutions reach an equilibrium state in certain conditions. This allowed some seeds to remain intact in the solution.

Research revealed that these bits of seed retained the memory of the uniformly-grown slow-grown crystal from where they were taken. Samani and Marciel also discovered they could track the nucleation process, eventually allowing them to produce homogeneous thin films.

Mohite stated that the collaboration resulted in something rare and often unsuccessful in nanomaterials research: a self-assembly technique to create macroscopic materials that can live up to their nanoparticles.

Mohite stated that this is the biggest problem in nanomaterials technology. “At an individual level, you have unique properties greater than any other. But when you combine them into something valuable and macroscopic, such as a film or a computer program, those properties disappear because it is impossible to make something homogeneous with only those properties.

He said that although we have yet to conduct experiments on other systems, perovskites’ success raises the question of whether this seeded approach could work in other systems.

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