Formamidinium lead iodide is a very good material for photovoltaic cells, but getting the correct and stable crystal structure is a challenge. The techniques developed so far have given rather poor results. However, scientists from the University of Groningen, led by professor of photophysics and optoelectronics Maria Antonietta Act, have cracked it – using a blade and dipping solution. The results were published in the journal Nanoscale March 15, 2019.
The lead iodide formamidinium (FAPbI3) is a perovskite, a crystal of distinctive structure. Perovskites are named after a mineral whose chemical formula is ABX3. In an idealized cubic unit cell, the X position is occupied by anions forming an octahedron with a central cation in position B, while the angles of the cube are occupied by the cations of position A (see illustration).
"This formamidinium lead iodide material has very good characteristics, but the formamidinium ion in position A causes instability of the structure," explains Loi. 3D films made from this material are most often found to be a mixture of a photoactive phase and a photoactive phase, the latter being detrimental to the final application. Loi has therefore engaged its doctoral student, Sampson Adjokatse, to seek a solution.
After trying different strategies, he found one that worked well. "And most importantly, an evolutionary model that could be used for industrial production," explains Loi. After all, solar cells must be produced in large panels and it is very important to find an efficient and economical technique. Adjokatse started with a different perovskite, in which formamidinium was replaced by a larger molecule of 2 phenylethylammonium, and thus formed a 2D perovskite. This material was deposited as a thin film using the "scrape" technique, related to techniques widely used in industrial processes such as printing.
"Basically, you spread the material on a substrate with the help of a blade," Adjokatse explains. The blade can be adjusted to produce a film of about 500 nanometers in thickness, creating the 2D perovskite layer. "The important thing is that these films are very smooth and that they have large crystal domains up to 15 micrometers," Adjokatse explains. Smooth 2D films based on 2-phenylethylammonium lead iodide were used as a matrix to produce 3D films based on lead iodide formamidinium.
This was done by dipping the 2D film into a solution containing formamidinium iodide. This resulted in the growth of a 3D film by "cation exchange", where formamidinium took the place of 2 phenylethylammonium. "These films have a much higher photoluminescence than 3D reference films with formamidinium iodide and exhibit increased stability when exposed to light or moisture," explains Loi. "This means that we now have a method for producing high quality films for perovskite solar cells using a scalable technique on an industrial scale.
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