The mystery of the amorphous perovskite solved

The illustration shows that the more methylammonium acetate (MAAc) we add, the less order we see in our material. It ranges from completely crystalline, in green, to amorphous with crystalline, very bright inclusions, to completely amorphous. Credits: AMOLF

AMOLF researchers Eric Garnett, Susan Rigter and colleagues were the first to irrefutably show that amorphous perovskite exists. The material can significantly increase the efficiency of solar cells produced from perovskite. The research was published today in the journal Advanced functional materials.

Perovskite, a very promising new material for solar cells, is naturally crystalline; in other words, the atoms are packed into an ordered pattern. From traditional silicon solar cells, we know that the efficiency of cells can be increased if part of the material is amorphous, which means that the atoms are randomly packed.

Eric Garnett (AMOLF nanoscale solar cells) was the first to realize that amorphous perovskite can fulfill the same function. The next challenge was to produce the material and study its properties. Garnett explains why this was difficult: “Perovskites are made up of ions. By nature, they are easily organized into a crystal lattice, just like table salt, for example. We had to come up with a trick to prevent these crystals from forming and we managed to do just that. Using techniques such as X-ray diffraction, we subsequently also showed that the material is amorphous. This provided the first irrefutable proof that amorphous perovskite exists. “

Vinegar makes perovskite amorphous

The trick Garnett, the first author of the article Susan Rigter, and their colleagues used was to change the amount of methylammonium acetate, one of the components of perovskite. More acetate (a key ingredient in vinegar) results in more amorphous perovskite because it interferes with the crystallization process and accelerates the disappearance of the solvent. “We were actually surprised that we could form an amorphous perovskite, so we wanted to investigate the mechanism of formation,” Garnett says. “We have shown that a complex that interferes with crystallization is formed as an intermediate phase in the solution. When we subsequently heat the solution to evaporate the solvent, the complex decomposes so rapidly that there is no time for crystallization.”

The method devised by researchers to make amorphous perovskite is widely applicable. The most studied perovskite is methylammonium lead iodide, but the synthesis works both with other ammonium salts and with other halides, such as bromide, instead of iodide. Furthermore, it turned out that the variation of these components led to a shift in bandwidth, a property of the substance that indicates which color of light the solar cell most efficiently absorbs and converts into electricity. The ability to adjust the amorphous spread band allows many materials with different ranges to be combined, leading to more efficient solar cells.

Efficient solar cells

Analogous to silicon solar cells, the amorphous layer of perovskite can help improve efficiency by providing a so-called passivating layer, Garnett says. Electrons are released in the material as a result of light shining on the solar cell. These electrons move to the surface where they are removed via electronic contacts. This leads to electricity. In a crystal, electrons can be trapped at the crystal boundary. In record-breaking silicon solar cells, the amorphous passivating layer ensures that this does not happen, leading to higher solar cell power output. Amorphous perovskite could also fulfill this function, which would further increase the efficiency of solar perovskite cells. “We measure stronger and longer-lasting light emission when we use amorphous perovskite as a passivating layer, which is an indicator of better solar cell performance,” says Garnett.

Therefore, the next step in the research is the production of this type of solar cells, starting with a layer of crystalline perovskite covered with a layer of amorphous perovskite. This is more difficult than producing only amorphous perovskite, because the underlying crystal layer provides an ordered template, facilitating the packaging of atoms in an arranged manner. “I consider the silicon analogy to be the most exciting aspect of our research,” says Garnett. “I think this is a significant breakthrough for peers who have huge opportunities.”


A strategy to improve the efficiency and long-term stability of perovskite solar cells


More information:
Susan A. Rigter et al. Characteristics of passivation and mechanism of formation of thin films of amorphous halide perovskites. Advanced functional materials (2021).

Citation: The Solved Mystery of the Amorphous Perovskite (2021, February 17) retrieved February 17, 2021 from https://phys.org/news/2021-02-mystery-amorphous-perovskite.html

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