Highlight: Solvent-Free Synthesis of Advanced Materials

Perovskite powder
Orange powder made of a “perovskite” material usable for next-generation solar cells and other devices, created by a mechanical method known as milling. Photo credit: Z. Hong et al.

When thinking of chemists at work, we usually visualize them mixing liquids in vials and test tubes. This approach to chemistry, working with reagents dissolved in organic solvents such as ethanol or dichloromethane, is called solution synthesis. While common, it is not the only game in town; another set of techniques known as mechanochemistry, which involves directly mixing chemicals in their undiluted forms, is emerging as an environmentally friendly way to create state-of-the-art chemicals.

Researchers at Nanyang Technological University, Singapore (NTU Singapore) have recently developed new mechanochemical ​methods for making lead-free metal halide perovskites, an important class of materials that can be used for next-generation solar cells. Led by Han Sen Soo and Felipe García at NTU's School of Physical and Mathematical Sciences, and Nripan Mathews at NTU's School of Materials Science and Engineering, the team has shown that perovskites can be produced at high purities and in large quantities without using solvents. They have also demonstrated that the resulting materials can be used to fabricate devices such as photodetectors, again without solvents. These advances were reported in June 2019 in the journal iScience as a featured article.

Mechanochemistry involves the use of equipment, known as mills, to mix chemicals in solid form. The chemicals are typically powders made up of tiny solid particles. By repeatedly grinding particles of different chemicals together, the molecules in them can be made to react to form new chemical products.

The chief advantage of this approach is that it avoids the use of solvents, which are often toxic, flammable, and environmentally unfriendly. Moreover, it bypasses the need to isolate chemical products from solvents, an energy-intensive process that is commonly accompanied by the escape of toxic solvents into the environment.

The advantages of mechanochemistry dovetail with a worldwide surge of interest in developing energy-efficient and non-polluting chemical methods. This burgeoning scientific movement is known as green chemistry. Previously, researchers have used mechanochemistry to synthesize many different chemicals, including metal halide perovskites, the subject of the new NTU study. The NTU team, however, is the first to show that mechanochemical methods can be used to make metal halide perovskites that do not contain lead, a toxic element forbidden from most commercial products.

“Perovskites are some of the most promising materials for replacing silicon in solar cells, but the existence of lead in most of these materials has always been a major problem,” said Professor Han Sen Soo, one of the lead authors of the study. “Our work shows that lead-free perovskites can be manufactured at scale, and in a safe and sustainable way.”

To explore the extent to which the use of solvents can be avoided, the team took their work one step further by showing that the resulting perovskite powders can be directly formed into devices. They demonstrated that working photodetectors can be created by compressing the powders into pellets. This proved that the mechanochemical method yields products of sufficiently high quality for practical use.

Another exciting outcome of the project was that some of the perovskite materials made through mechanochemistry had never been isolated using conventional room-temperature solution synthesis. “Several technologically relevant metal halide perovskites are metastable, and transform easily into other undesired chemicals when one tries to prepare them by solution syntheses,” explains Professor Felipe García, another of the lead authors of the study. “It seems that by using mechanochemical methods, we are able to explore new chemical variants that were previously inaccessible. Some of these variants may, in the future, prove to be superior for various applications.”

Z. Hong, D. Tan, R. A. John, Y. K. E. Tay, Y. K. T. Ho, X. Zhao, T. C. Sum, N. Mathews, F. García, and H. S. Soo, Completely Solvent-free Protocols to Access Phase-Pure, Metastable Metal Halide Perovskites and Functional Photodetectors from the Precursor Salts, iScience 16, P312 (2019).