Perovskite is one of the most widely studied materials due to its many uses. A particularly promising field is photovoltaics, which are devices that efficiently convert light into electricity. The conversion efficiency of hybrid perovskite is now around 25.2%, exceeding that of commercial silicon-based solar cells.
An example of hybrid perovskites is methylammonium lead iodide (CH3NH3PbI3). It is called a hybrid because the three negative ions of the iodine atom (I-) are balanced against the inorganic positive ion of the lead atom (Pb2 +) and the organic positive ion of methylammonium (CH3NH3 +).
A study conducted at the Institute of Physics and Chemistry (DFQ-UNESP) at Sao Paulo State University in Ilha Solteira, Brazil, focusing precisely on methylammonium-lead iodide, has helped scientists understand the ferroelectric nature of perovskite and the origin of it excellent photovoltaic properties improved. An article about the results will be published in Acta Materialia.
The study was part of Fernando Brondani Minussi’s doctoral thesis, which was supervised by Eudes Borges de Araujo, professor at UNESP. It was supported by the Sao Paulo Research Foundation – FAPESP through the thematic project “Multiferroic and ferroelectric materials for energy converters: Synthesis, properties, phenomenology and applications”, whose main researcher is Jose Antonio Eiras, professor at the Federal University of Sao. is Carlos (UFSCar), also in the state of Sao Paulo. Araujo is a co-investigator on the project.
“In this study, we systematically examined the influence of temperature and an electrical direct current field on the electrical, dielectric and spectroscopic properties of methylammonium-lead iodide in the examined perovskite,” said Araujo.
Ferroelectric relaxors are materials whose structure is significantly changed in the presence of an electric field due to the displacement of ions in the crystal lattice. Positive and negative ions move in opposite directions. They are highly efficient in storing and converting energy because their dielectric constants far exceed those of ordinary ferroelectrics.
“Our results show the nature of the ferroelectric methylammonium lead iodide relaxor, but they also show the existence of a freezing temperature of the dipoles, which are responsible for the relaxor character of the material, and a diffuse phase transition at high temperatures, the relaxor property, is 270 K [-3.15 C], not far below the freezing point of water, “said Araujo.
Analysis of the results enabled the researchers to propose a novel phase diagram for the lead iodide-methylammonium system, indicating regions that delimit the existence of three different phases: ferroelectric, ergodic and non-ergodic.
“Ferroelectrics naturally have a spontaneous electrical polarization that can be reversed by applying an external electrical field. This ferroelectric order is destroyed at high temperatures and the material is not polarized by converting it into a paraelectric phase, ”said Araujo.
“Classic relaxors show a paraelectric phase at high temperatures, which in many ways resembles the paraelectric phase of a normal ferroelectric. When they are cooled below a certain level, called the Burns temperature, they begin to present polar nanoregions with randomly distributed dipole moments. Since these regions are highly dynamic and uncorrelated, we say that the relaxor behaves ergodically. The polar nanoregions interact and freeze below a certain temperature, which characterize a non-ergodic state. In a non-ergodic relaxor, however, a ferroelectric state can be induced by applying a sufficiently strong electric field. “
In mathematics, the term ergodicity refers to the idea that the average behavior of a system can be derived from the trajectory of a typical point, but in this case it has a different meaning, according to Araujo, who emphasized that while there is no that of expected behavior of a classical relaxor, the relaxor property of methylammonium lead iodide must be intrinsic and can be overshadowed by the ionic conductivity and the semiconducting properties of the material.
“In general, ferroelectric relaxors have a high dielectric permissivity and excellent electrocaloric and electromechanical properties. The hysteresis – the tendency of the system to retain its properties – is only suppressed at sufficiently high temperatures. From a technological point of view, these properties make them strong candidates for the manufacture of acoustic sensors, solid state coolers, transducers and actuators. From a scientific point of view, the fact that ferroelectric relaxors are still the least understood materials in condensed matter physics is a major stimulus for research, “said Araujo.
The original and truly significant results of the study could potentially help usher in a paradigm shift in understanding ferroelectricity and the origins of the extraordinary photovoltaic properties of halide perovskites. “I believe the study will greatly improve our understanding of the physics of this complex and fascinating system,” he said.
Research report: “Effects of frequency, temperature and DC electric field on the dielectric properties of methylammonium-lead iodide from the perspective of a relaxor-like ferroelectric”
Research Foundation Sao Paulo