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Technique Could Offer Gentler Approach to Making Hybrid Photovoltaic Materials

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DURHAM, N.C., Jan. 5, 2018 — A thin-film deposition method for creating hybrid thin-film materials, applied to the fabrication of perovskite cells, could lead to a means to develop next-gen solar materials.

Researchers at Duke University have developed an approach to manufacturing perovskites using pulsed laser evaporation.  A solution containing the molecular building blocks for the perovskite is frozen, and the frozen block is blasted with a laser in a vacuum chamber. The laser vaporizes a tiny piece of the frozen target — about the size of a dimple on a golf ball — and the vapor travels upward in a plume that coats the bottom surface of any object hanging over the device. Once the surface of the object is fully coated, the thin-film deposition process stops, and the object is heated to crystallize the molecules and set the thin film in place.

The technique, called Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), is a gentle approach to thin-film deposition for delicate hybrid materials. It is adapted from MAPLE, a technology invented in 1999.

In RIR-MAPLE, 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 the organic material unscathed as it travels to the product’s surface.

RIR-MAPLE thin film deposition technique, Duke University.
This is an inside look at the RIR-MAPLE technique that has the ability to build new solar cell crystal technology. The white circle at the center of the table is a frozen solution containing the molecular building blocks for the solar cell material, which is blasted by lasers, vaporizing the solution which carries the materials to coat the bottom of the target above. Courtesy of E. Tomas Barraza.

According to the team, the composition, morphology and optical properties of the perovskite films produced by the RIR-MAPLE technique are comparable to those produced by more conventional methods such as spin coating. The device reached a stabilized power conversion efficiency of over 12 percent, a high value for perovskite solar cells deposited by a laser ablation process, demonstrating the ability of the new technique to produce device-quality films.

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

Methylammonium lead iodide (MAPbI3), one of the most common perovskites used in solar energy today, is also one of the few perovskites that can be created using standard industry production techniques. However, the mixture of organic and inorganic molecules in a complex crystalline structure can be difficult to manufacture. Organic elements are particularly delicate, but are critical to the hybrid material’s ability to absorb and emit light effectively.

RIR-MAPLE thin film deposition technique, Duke University.
This is a closer look at the target of frozen solution that contains the building blocks for the solar cell material. Courtesy of E. Tomas Barraza.

“Methylammonium lead iodide has a very simple organic component, yet is a very high-performing light absorber,” said professor David Mitzi. “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.”

While the materials made in the Duke study have higher solar cell efficiencies than those made with other laser-based technologies, they are not as efficient as those made with traditional solution-based processes. But the researchers’ focus was primarily to demonstrate an alternative manufacturing approach for solar materials.

RIR-Maple thin-film deposition technique, Duke University.
This is a view inside of the RIR-MAPLE chamber after the thin-film deposition process is over. None of the original frozen solution is left in the center, as it has all been vaporized to coat the bottom of the target hanging above. Courtesy of E. Tomas Barraza.

“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,” said Mitzi. “We also think these materials could be useful for other applications, such as light-emitting diodes, photodetectors and x-ray detectors.”

The research was published in ACS Energy Letters (doi: 10.1021/acsenergylett.7b01144).
Jan 2018
Research & TechnologyAmericaslaserspulsed lasersphotovoltaicssolarindustrialperovskitethin film depositionmaterialslight sources

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