Discrete and Process Automation

Laser Evaporation Could Create Better Solar Cells

03 January 2018

Materials scientists from Duke University have developed a new method to create hybrid thin-film materials that have been difficult or impossible to make. This technique could be the gateway to new generators of solar cells, light-emitting diodes and photodetectors.

This is a closer look at the target of frozen solution that contains the building blocks for the solar cell material. Source: E. Tomas BarrazaThis is a closer look at the target of frozen solution that contains the building blocks for the solar cell material. Source: E. Tomas Barraza

Perovskites are a class of materials that — when combined with the correct elements — have a crystalline structure that makes them well-suited for light-based applications. The ability to absorb light and transfer the energy efficiently makes them a common target for researchers who are developing new types of solar cells.

The most common perovskite used in solar energy today, called methylammonium lead iodide (MAPbl3), can convert light to energy just as well as today’s best commercially available solar panels. It can do so using a fraction of the material — silver 100 times thinner than a typical silicon-based solar cell.

Methylammonium lead iodide is one of the few perovskites that can be created using standard industry production techniques. But it still has issues with scalability and durability. In order to truly unlock the potential of perovskites, new manufacturing methods are needed because the mixture of organic and inorganic molecules in a complex crystalline structure that can be difficult to make. Organic elements are particularly delicate and they are critical to the hybrid material’s ability to absorb and emit light effectively.

"Methylammonium lead iodide has a very simple organic component, yet is a very high-performing light absorber," said David Mitzi, the Simon Family Professor of Mechanical Engineering and Materials Science at Duke. "If we can find a new manufacturing approach that can build more complex molecular combinations, it will open new realms of chemistry for multifunctional materials."

In the study, Mitzi teamed up with colleague Adrienne Stiff-Roberts, associate professor of electrical and computer engineering at Duke, to demonstrate this manufacturing approach. The technique is called Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation, or RIR-MAPLE for short, and was developed by Stiff-Roberts at Duke over the last 10 years.

Adapted from a technology invented in 1999 called MAPLE, the technique involves freezing a solution containing the molecular building blocks for the perovskite and blasting the frozen block with a laser in a vacuum chamber.

When a laser vaporizes a small piece of the frozen target about the size of a dimple on a golf ball, the vapor then travels upward in a plume that coats the bottom surface of an object hanging overhead, like a component in a solar cell. When enough of the material builds up, the process is stopped and the product is heated to crystallize the molecules and set the thin film in place.

In Stiff-Roberts’ version of this technology, the laser’s frequency is specifically tuned to the molecular bonds of the frozen solvent. This causes the solvent to absorb most of the energy, leaving delicate organics unscathed as they travel to the product surface.

"The RIR-MAPLE technology is extremely gentle on the organic components of the material, much more so than other laser-based techniques," said Stiff-Roberts. "That also makes it much more efficient, requiring only a small fraction of the organic materials to reach the same final product."

Even though no perovskite-based solar cells are yet available on the market, there are a few companies that are working to commercialize methylammonium lead iodide and other closely related materials. While the materials in the study have solar cell efficiencies better than those made with other laser-based technologies, they don’t reach those made with traditional solution-based processes, yet.

But Mitzi and Stiff-Roberts say that is not their goal.

"While solution-based techniques can also be gentle on organics and can make some great hybrid photovoltaic materials, they can't be used for more complex and poorly soluble organic molecules," said Stiff-Roberts.

"With this demonstration of the RIR-MAPLE technology, we hope to open a whole new world of materials to the solar cell industry," continued Mitzi. "We also think these materials could be useful for other applications, such as light-emitting diodes, photodetectors and X-ray detectors."

The paper on this research was published in ACS Energy Letters.

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