Perovskite | EPFL Graph Search

Perovskite (pronunciation: pə'rɒvskaɪt) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known as the perovskite structure. Many different cations can be embedded in this structure, allowing the development of diverse engineered materials. History The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski (1792–1856). Perovskite's notable crystal structure was first described by Victor Goldschmidt in 1926 in his work on tolerance factors. The crystal structure was later published in 1945 from X-ray diffraction data on barium titanate by Helen Dick Megaw. Occurrence Found in the Earth's mantle, perovskite's occurrence at Khibina Massif is restricted to the silica under-saturated ultramafic rocks and foidolites, d

Formate as Anti-Oxidation Additives for Pb-Free FASnI(3) Perovskite Solar Cells

Jaeki Jeong, Jinhyun Kim, Jiyoun Seo

Pb-free Sn-based perovskite solar cells (PSCs) have significant potential for application in photovoltaic devices because oftheir suitable bandgap, low exciton binding energy, high carrier mobility, and long diffusion length. However, their performance is hampered by several issues. Sn-based perovskites are highly susceptible to oxidation, which induces a high concentration of defects and degrades the chemical stability of the perovskite crystals. Herein, the anion formate (HCOO-) can effectively suppress the oxidation of Sn in the pure formamidinium tin triiodide (FASnI(3)) perovskite without using A-site cationic additives. Moreover, the presence of formate results in a uniform pinhole-free perovskite film with a low trap density, reduced charge carrier recombination, and improved charge extraction. Density functional theory calculations show that formate-treated FASnI(3) has improved stability against oxidative Sn degradation. The formate-treated PSC achieves a power conversion efficiency of 12.11% at a high open-circuit voltage of 0.71 V. The device exhibits improved stability in ambient air in which it maintained over 80% of its initial power conversion efficiency after 180 min because oxidation is inhibited owing to the strong interaction between Sn and formate.

WILEY-V C H VERLAG GMBH2022

Origins of the macroscopic symmetry breaking in centrosymmetric phases of perovskite oxides

Sina Hashemi Zadeh

Many perovskite materials experience a temperature-driven phase transition at the Curie temperature from a non-centrosymmetric polar ferroelectric phase to a paraelectric phase, where polarization is lost. The paraelectric phase is usually centrosymmetric and therefore non-piezoelectric. However, ferroelectrics still show some forms of electro-mechanical coupling in their paraelectric phase like flexoelectricity, which is not symmetry limited and is practically strong in ferroelectrics. It has been shown that the flexoelectric coefficient of perovskite (Ba0.67,Sr0.33)TiO3 in its centrosymmetric paraelectric state at room temperature is up to three to four orders of magnitude larger than its theoretically predicted values. Such enormous discrepancy demonstrates that either the theory is unable to fully explain the flexoelectric response, or that another electro-mechanical mechanism is playing a role in the apparently large flexoelectric response of the material. It was found that Ba0.60Sr0.40TiO3 (BST6040) samples with apparently large flexoelectric coefficient exhibit symmetry breaking and consequently exhibit forbidden piezoelectric- and pyroelectric-like behavior in its paraelectric phase. In this thesis work, our interest lies in describing and interpreting the conditions where breaking of the symmetry is not associated with the external field nor with the flexoelectricity, but is proper to the material. Origins and mechanism of symmetry breaking were investigated in two different cases: local breaking of centrosymmetry; and macroscopic symmetry breaking. The local breaking of the centrosymmetry may refer to the existence of microscopic polar entities and/or disordered displacement of ions in the unit cell. The global symmetry breaking could be associated with a non-uniform distribution of charged defects and/or the coupling of polar nanoâregions with the strain gradient in the sample. We compared dielectric, elastic, and pyroelectric properties of the Ba1-xSrxTiO3 (BST) family, pure BaTiO3 in perovskite (p-BTO) and hexagonal (h-BTO) forms, and SrTiO3 to study the governing mechanisms of symmetry breaking. In addition, the atomic structure of BST6040 was studied by high resolution scanning transmission electron microscopy for direct evidence of polar regions and to understand their exact nature and origin. According to experimental data, the paraelectric phase of p-BTO shows local and global breaking of nominal centrosymmetricity and possesses microscopic and macroscopic polarizations, which are associated with precursors of the ferroelectric phase ordered by the strain gradient in the sample. On the other hand, h-BTO does not exhibit macroscopic pyroelectric response because the precursors are paraelectric-ferroelastic at room temperature, and therefore non-polar. Studying the material response to dynamic electric field showed that the Rayleigh-like relation, which describes dynamics of domain walls in a random pinning environment, cannot describe the dynamics of polar entities in these materials. Results of our high-resolution atomic studies showed distinct static polar clusters with the average size of few nanometers where A-site, B-site and O ions are displaced from their ideal cubic positions. We believe that all presented experimental data can be consistently explained by the presence of microscopic precursors of the ferroelectric phase and their interaction with the strain gradient in the material.

EPFL2017

Investigation in Crystal Growth/Morphology and Interface Engineering of Perovskite Solar Cells by Different Deposition Methods

Nadja Isabelle Desiree Klipfel

A new and promising technology has emerged to compete with traditional silicon photovoltaic semiconductors. The low material and processing costs and high-power conversion efficiencies of organic-inorganic metal halide perovskites make these a promising technology for the future. Despite record efficiencies for small-area devices, stability remains challenging. Due to rapid progress in the field, avenues for large-area commercial production are being explored. One approach is via thermal evaporation, which is presented in this work. The focus is on the vacuum-deposition method itself, the interface engineering of perovskite layers, and the study of a family of low-cost hole-transporting materials (HTM). To improve reproducibility among research groups in vacuum-deposition processes, a better understanding and control of the crystal formation is essential. Of the experimental parameters, two were investigated that were not yet considered in-depth or combined: deposition speed and underlayer material. It was shown that perovskite growth rate changes affect preferred crystal orientation. In addition, it was found that the final perovskite composition is influenced by its underlayer chemistry. Multi-source vacuum deposition of perovskite layers is complex due to the laborious calibration process necessary for proper stoichiometry. Single-source evaporation of pre-synthesized perovskite powders can reduce effort and time. The feasibility of obtaining pure phase films was demonstrated and confirmed by multiple analysis methods.Interfaces in contact with the perovskite can be a source of defects with a deteriorating effect on stability and efficiency. The charge transport processes at the interface of electron transport material (ETM) and perovskite were investigated. The results showed a synergetic effect between cTiO2 and C60 and the importance of C60 as a charge extraction layer. The perovskite hole transporting material (HTM) interface was also systematically studied. Monitoring of dopants showed, for the first time in literature, TFSI anion migration through grain boundaries to the perovskite surface. It was further shown that non-radiative recombination pathways are reduced, indi-cating self-passivating of crystal defects during migration processes.The state-of-the-art HTM 2,2',7,7'-tetrakis (N,N-di-p-methoxyphenylamine)-9,9'spirofluorene (spiro-OMeTAD) is widely employed in spin-coated PSCs. Devel-oping a less costly manufacturing alternative to this material that retains similar properties is a key challenge. A new class of Zn (II) and Cu (II)-based phthalocyanine HTMs functionalized with butyl- and ethylhexyl- groups in the periphery was investigated. Devices fabricated with the Zn (II)-based HTM (n-butyl functionalization) demonstrated the best performance. The device architecture with the highest stability maintained 80% of its initial power conversion efficiency (PCE) over 20h of illumination. At 20.2%, the perovskite architecture with the highest PCE also was the least stable, indicating a trade-off between these two characteristics for this HTM class.

EPFL2022