A laser-based method of building up films of photovoltaics allows exploitation of delicate metal-organic structures
Developed at Duke University in North Carolina, the technique builds upon research from the 1990s to allow engineers to construct thin-film samples of perovskites, materials of interest to photovoltaic developers because they have the potential to be as efficient as conventional crystalline-silicon solar cells but much cheaper.
The activity of perovskites derives from the properties of complexes of transition metals with organic compounds; these complexes tend to be complex in structure and somewhat unstable, making them difficult to synthesise. This has limited investigation of perovskites to simpler and more robust compounds, such as methylammonium lead iodide (MAPbI3), the most common perovskite used in research, which can match the efficiency of silicon with a film 100th of the thickness of a silicon crystal.
“Methylammonium lead iodide has a very simple organic component, yet is a very high-performing light absorber,” said Prof David Mitzi, of the department 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.”
Mitzi is working with a colleague, Dr Adrienne Stiff-Roberts, who has developed a laser ablation technique called RIR-MAPLE (Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation) a refinement of the MAPLE technology invented at Duke in 1999. MAPLE takes a frozen solution of the building blocks of the perovskite in a vacuum chamber and blasts it with a laser, vaporising a small portion and allowing the vapour to condense on a target suspended above the sample. Once enough of the sample has been deposited, the target is removed and baked to crystallise the perovskite and set the film in place.
Stiff-Roberts’ refinement is to tune the frequency of the laser to the molecular bonds of the frozen solvent. This means that the solvent absorbs most of the energy, preventing the delicate organic components of the perovskite from being decomposed by the energy. “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.”
In a paper in the journal ACS Energy Letters, Stiff-Roberts and Mitzi describe depositing films of MAPbI3 using RIR-MAPLE, and claim to have achieved photovoltaic power conversion efficiency of 12 per cent, which they say is high for pervoskite samples obtained through laser ablation. However, their ultimate goal does not lie with this material, but with developing new ones. “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.”