Tracking Structural Phase Transitions in Lead-Halide Perovskites by Means of Thermal Expansion

15. Mai 2019

Lead-halide perovskite materials have a realistic perspective to play an important role in future photovoltaic devices. In our study, we have demonstrated that thermal expansion is a powerful technique to precisely determine phase transition temperatures. Since any structural change has severe impact on the phononic and electronic structure, the information provided by this study can be employed strategically to predict changes in the optical and charge transport properties that are currently under intense investigation.

The extraordinary properties of lead-halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. Moreover they have potential applications in solar cells, lasers, light-emitting diodes, photodetectors, radiation detectors and magneto-optical data storage. The success of hybrid organic–inorganic perovskites (HOIP) in photovoltaic applications is based on their high power conversion efficiencies, up to 23.3%, in combination with low production costs.

It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature.

In our study at the High Field Magnet Laboratory in Nijmegen, we have demonstrated that linear thermal expansion is a powerful technique to probe the evolution of the crystal structure and to precisely determine phase transition temperatures. The linear thermal expansion of single crystals of APbX3 (A = methylammonium (MA), formamidinium (FA); X = I, Br) in the temperature range between 4.2 and 280 K is measured using a high-resolution capacitive dilatometer to determine the phase transition temperatures. For this reason, Robert Küchler from the Max Planck Institute for Chemical Physics of Solids in Dresden has developed first a compact and later one the world’s smallest high-resolution capacitive dilatometer. Meanwhile he is also marketing the precision instruments in a spin-off company http://www.dilatometer.info/. The new super compact design enables to measure thermal expansion from 0.3 K up to room temperature and magnetostriction up to 37.5 T in a bitter magnet at HFML in Nijmegen.

Solid evidence for structural phase transitions was uncovered for all compounds, though its overall shape depends strongly on the individual material. We observe that FA-based HOIPs exhibit a variety of interesting properties such as two regions of negative thermal expansion (NTE) (δ-FAPbI3) with a large NTE coefficient and a cascade of sharp transitions (FAPbBr3) with decreasing temperature. With temperature-dependent X-ray diffractometry, we have identified the corresponding phases for the FA-based compounds. For FAPbBr3, the symmetry changes identified by X-ray diffractometer data analysis are linked to the tilting of the PbBr6 octahedra. These unexpected observations underline the strength of thermal expansion experiments to probe the lattice parameters and to precisely determine the structural phase transition temperatures. Since any structural change has severe impact on the phononic and electronic structure, the information provided by this study can be employed strategically to predict changes in the optical and charge transport properties that are currently under intense investigation.

The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead-halide perovskite materials.

RK / CPfS

Abb. A super compact high-resolution capacitance dilatometer. The dilatometer is operated inside of a water-cooled, resistive Bitter magnet at the HFML in Nijmegen. The magnet can reach a static field of 37.5 T: A sample is clamped between the two plates of a capacitor whose capacitance changes when the sample expands or contracts. This change in capacitance can be measured very accurately. The dilatometer enables to measure changes in length of ΔL ≈ 10-11 - 10-12 m which are about hundred times smaller than the separations of the atoms in the crystal lattice. The scientific instrument can be operated from room temperature down to 10 mK.  

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