This shows the mechanism of crystallization from a DMF/DMSO solution during drying for the 2D/3D perovskite films, with crystals starting to grow at the air/solution interface. Image: G. Portale, University of Groningen.
This shows the mechanism of crystallization from a DMF/DMSO solution during drying for the 2D/3D perovskite films, with crystals starting to grow at the air/solution interface. Image: G. Portale, University of Groningen.

Lead-based perovskites are very promising materials for the production of solar panels. They efficiently turn light into electricity, but they also present some major drawbacks: the most efficient perovskite materials are not very stable, while lead is a toxic element. This has led scientists at the University of Groningen in the Netherlands to study alternatives to lead-based perovskites.

Two factors that significantly affect the efficiency of the materials in solar cells are their ability to form thin films and their structure. This makes it very important to investigate in situ how lead-free perovskite crystals form and how their crystal structure affects the functioning of the solar cells. This is what the scientists at the University of Groningen have now done, reporting their findings in a paper in Advanced Functional Materials.

Solar cells based on hybrid perovskites were first introduced in 2009 and rapidly became almost as efficient as standard silicon solar cells. These materials have a very distinctive crystal structure, known as the perovskite structure. In an idealized cubic unit cell, anions form an octahedron around a central cation, while the corners of the cube are occupied by other, larger cations. Different ions can be used to create different types of perovskites.

The best results in solar cells have been obtained using perovskites with lead as the central cation. As this metal is toxic, tin-based alternatives have been developed, including formamidinium tin iodide (FASnI3). This is a promising material, but it lacks the stability of some of the lead-based materials.

Attempts have been made to mix 3D FASnI3 crystals with layered materials containing the organic cation phenylethylammonium (PEA). “My colleague, Professor Maria Loi, and her research team showed that adding a small amount of this PEA produces a more stable and efficient material,” says Giuseppe Portale, an assistant professor at the University of Groningen. “However, adding a lot of it reduces the photovoltaic efficiency.”

Portale has developed an X-ray diffraction technique that allows him to study the rapid formation of thin films in real-time during spin-coating from solution. On a laboratory scale, perovskite films are generally made by spin coating, a process in which a precursor solution is delivered onto a fast-spinning substrate, with crystals growing as the solvent evaporates. At the beamline BM26B-DUBBLE at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, Portale investigated what happens during the formation of a tin-perovskite film.

“Our initial idea, which was based on ex situ investigations, was that the oriented crystals grow from the substrate surface upwards,” Portale explains. However, the in situ results showed the opposite: crystals start to grow at the air/solution interface.

In his latest experiments, Portale tried adding different amounts of 2D PEASnI4 to 3D FASnI3. In the pure 3D perovskite, crystals started to form at the surface but also in the bulk of the solution. Adding a small amount of the 2D material suppressed bulk crystallization and caused the crystals to grow only from the interface.

“PEA molecules play an active role in the precursor solution of the perovskites, stabilizing the growth of oriented 3D-like crystals through coordination at the crystal's edges. Moreover, PEA molecules prevent nucleation in the bulk phase, so crystal growth only takes place at the air/solvent interface,” Portale explains.

The resulting films are composed of aligned 3D-like perovskite crystals and a minimal amount of 2D-like perovskite, located at the bottom of the film. The addition of a low concentrations of the 2D material produces a stable and efficient photovoltaic material, while the efficiency drops dramatically at high concentrations of this 2D material.

The experiments conducted by Portale and Loi can explain this observation. “The 2D-like perovskite is located at the substrate/film interface,” says Portale. “Increasing the content of the 2D material to above a certain amount causes the formation of an extended 2D-like organic layer that acts as an insulator, with detrimental effect for the device's efficiency.”

He and Loi conclude that the formation of this insulating layer must be prevented to achieve a highly efficient and stable tin-based perovskite. “The next step is to realize this; for example, by playing with solvents, temperature or specific perovskite/substrate interactions that can break up the formation of this thick insulating layer.”

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.