PV – This form of perovskite is ideal for cheap and efficient solar cells

Perovskite materials are a cheaper alternative to silicon for the production of optoelectronic devices such as solar cells and LEDs. There are many different types of perovskite, as a result of different combinations of elements, but one of the most promising crystals that have emerged in recent years is the FAPbI3 crystal, based on formamdenium (FA).

This compound is thermally stable and the “band gap” (the minimum energy required to excite an electron, and thus the property most closely related to the device’s energy production), is very close to being ideal for photovoltaic applications. For these reasons, efforts are being made to develop commercially available FAPbI3 perovskite solar cells. However, the compound can exist in two slightly different phases; While one phase results in excellent photovoltaic efficiency, the other produces very little power.

“The big problem with FAPbI3 is that the required phase is only stable at temperatures above 150°C,” explains Tiaran Doherty of the Cambridge Cavendish Laboratory, first author of the research paper. “At room temperature, it transitions to the other phase, which is really bad for PV.”

Newer solutions to keep the material in its desired phase at lower temperatures have involved adding various positive and negative ions to the compound. “This has been successful and has led to a record number of PV devices, but energy losses still occur,” Doherty says. “There are areas in the film that are not at the right stage,” he explains.

So far, little was known about why additions of these ions improve the overall stability of the compound, or even about the structure of the resulting perovskite. Now, the Cambridge researchers have shown that by adding these ions, the resulting structures undergo very subtle structural deformation, giving them stability at room temperature.

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The distortion was so small that it had not been detected before, until Doherty and colleagues used sensitive structural measurement techniques not widely used in perovskite materials. Specifically, they used scanning electron diffraction, nanoscale X-ray diffraction, and nuclear magnetic resonance to see, for the first time, the appearance of the structure at this stable stage.

“Once we realized that it was the slight structural distortion that gave this stability, we looked for a way to achieve this in the film preparation without adding any other elements to the mix.” Research co-author Satyawan Nagane used an organic molecule called ethylenediaminetraacetic acid (EDTA) as an additive in the perovskite solution, which acts as a template that guides the perovskite to the desired phase as it forms. EDTA binds to the FAPbI3 surface to give a structure guiding effect, but is not incorporated into the FAPbI3 structure itself.

“This way, we can achieve the desired bandgap because we don’t add anything extra to the material, it’s just a template to guide the formation of a film with a deformed structure, and the resulting film is very stable,” says Nagani.

Adds another researcher, Dominic Kubicki, from the Cavendish lab at the University of Warwick.

The authors of the discovery hope that this study will help improve the stability and performance of perovskites and the manufacturing processes of these materials, and thus help achieve ideal perovskite photocells.

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