New materials: Ion substitution alters the stability of semiconductor crystals

19 May 2021

LMU physicists have shown how the intrinsic lattice vibrations of hybrid perovskites are altered by ion substitution. Their findings highlight halide-ion dependent stability of perovskites and may soon lead to better solar cells and LEDs.

Bromide perovskite crystals under UV light. | © Debnath / LMU

There is a great demand for novel opto-electronic materials for use in solar cells and light-emitting diodes (LEDs), as well as applications in medical and technical sensors. Researchers at the Nano-Institute Munich (Chair of Photonics and Optoelectronics, LMU led by Prof. Dr. Jochen Feldmann) are currently investigating the potential of synthetic nanocrystalline semiconductors based on the so-called perovskite lattice, which have already shown great potential as photovoltaic devices. LMU physicist Tushar Debnath is studying how the atoms in perovskite nanocrystals respond to electromagnetic excitation with laser light, using a method that makes it possible to determine how much energy is absorbed or emitted under these circumstances. Both parameters are important for the function of these structures in solar cells and LEDs, respectively.

The perovskite structure was first described for the natural mineral calcium titanate. The atoms in its crystal lattice are organized into octahedral unit cells, and this basic form can be chemically manipulated to create the materials that are currently of great interest for opto-electronic and photovoltaic applications. “These perovskites are synthetic structures, and they differ in fundamental ways from those that occur naturally in minerals,” says Debnath, who works with organometallic halide perovskites. As their name suggests, these are hybrid structures that contain both inorganic and organic constituents. Perovskites in general conform to the chemical formula ABX3. In the present context, A is a positively charged ion (in this case, formamidinium), B is a divalent metal (here, lead) and X is a negatively charged halide anion (either iodide or bromide). In a new paper, Debnath and colleagues demonstrate how these organic and inorganic constituents interact in atomic scale that determines their stability as a function of halide-ion in the perovskite crystal lattice.

The nature of the halide ion has an impact on lattice vibrations and stability

Following chemical synthesis of the desired compound as perovskite nanocrystals, Debnath uses ultrafast, spatially resolved, ‘pump-probe’ spectroscopic techniques to monitor the changes in differential absorption spectrum that has been used to reconstruct the spectrum of vibrations in these structures. In the experiments, an extremely short ‘pump’ laser pulse causes the crystal lattice of the sample to vibrate, and it is rapidly followed by a ‘probe’ pulse that reveals how these excitations behave at the atomic level. “We discovered that the nature of the halide ion has a strong influence on the interactions between the organic and inorganic components of the crystal,” says Debnath. This is because the lead and bromide (or iodide) ions together form octahedral cages, within which the organic anion formamidium is trapped. The experiments revealed that, while the organic molecule is effectively immobilized in the bromide cage, it is free to rotate within the cage formed by iodide ions. “Systems that are more tightly packed, as in the case of the Br structure, are more strongly influenced by their nearest neighbors,” Debnath explains. This in turn dictates that they vibrate harmonically. When the interacting partners are farther apart – as in the iodide-containing lattice – the organic molecules have more freedom of movement. Because they are less constrained within the lattice, this introduces a degree of ‘anharmonicity’ into the spectrum of vibrations in the system.”

These findings not only reveal the underlying reason why bromides are more stable than the iodide perovskites but also important for designing bromide perovskite-based solar cells and LEDs.
Nature Communications, 2021

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