Phase Transitions in Low-Dimensional Layered Double Perovskites: The Role of the Organic Moieties

Direction Dependent Ferroelectricity and Conductivity in a Single Crystal 2D Halide Double Perovskite

Halide ferroelectric materials have garnered a lot of interest because of their distinctive electrical and structural characteristics. In this study, the design and development of a new non-centrosymmetric 2D layered halide double perovskite material, Cl1.14Br2.86PA4AgInBr8 (CPAIn) is reported. This material shows ferroelectric properties above room temperature, with a Curie temperature of 190 °C. This behavior is achieved through the substitution of the halogenated A-site organic linker, 3-chloropropylammonium. CPAIn exhibits anisotropic ferroelectric behavior with higher spontaneous polarization of 6.25 µC cm-2 along the perpendicular direction to the octahedral layers, whereas the value decreases to 0.174 µC cm-2 between sheets. While using bottom contact to study the nature of polarity within a sheet, the P-E loop displays capacitive loop. The nature and value of polarization is highly direction dependent, and to further understand the mechanism of conduction, a combination of temperature-dependent impedance studies and poling dependent conductivity techniques are employed. These directional dependent properties hold immense potential in memory devices, sensors and photovoltaics, piezoelectric devices and energy storage.

Two-Dimensional Halide Pb-Perovskite-Double Perovskite Epitaxial Heterostructures

Epitaxial heterostructures of two-dimensional (2D) halide perovskites offer a new platform for studying intriguing structural, optical, and electronic properties. However, difficulties with the stability of Pb- and Sn-based heterostructures have repeatedly slowed the progress. Recently, Pb-free halide double perovskites are gaining a lot of attention due to their superior stability and greater chemical diversity, but they have not been successfully incorporated into epitaxial heterostructures for further investigation. Here, we report epitaxial core-shell heterostructures via growing Pb-free double perovskites (involving combinations of Ag(I)-Bi(III), Ag-Sb, Ag-In, Na-Bi, Na-Sb, and Na-In) around Pb perovskite 2D crystals. Distinct from Pb-Pb and Pb-Sn perovskite heterostructures, growths of the Pb-free shell at 45° on the (100) surface of the lead perovskite core are observed in all Pb-free cases. The in-depth structural analysis carried out with electron diffraction unequivocally demonstrates the growth of the Pb-free shell along the [110] direction of the Pb perovskite, which is likely due to the relatively lower surface energy of the (110) surface. Furthermore, an investigation of anionic interdiffusion across heterostructure interfaces under the influence of heat was carried out. Interestingly, halide anion diffusion in the Pb-free 2D perovskites is found to be significantly suppressed as compared to Pb-based 2D perovskites. The great structural tunability and excellent stability of Pb-free perovskite heterostructures may find uses in electronic and optoelectronic devices in the near future.

Polarized Raman Study of Internal Vibrations of the Organic Cation in 3-Cyanopyridinium Lead Tribromide Post-perovskite

In this work, we apply polarized Raman spectroscopy for study of internal vibrations of the 3-cyanopyridinium cation in the halide post-perovskite (3cp)PbBr₃ (3cp = 3-CN-C₅H₅NH⁺). For a single cation, the vibrational frequencies and intensities of the Raman signal were calculated using the density functional theory. Selection rules were established for vibrations of cations in the crystal. These rules together with modeling results were used to identify the internal vibrations of the cation in the Raman spectrum of the crystal. Narrow and isolated internal vibrations of cations could be used as spectators of the crystalline environment.

Tailoring Photoluminescence by Strain-Engineering in Layered Perovskite Flakes

Strain is an effective strategy to modulate the optoelectronic properties of 2D materials, but it has been almost unexplored in layered hybrid organic-inorganic metal halide perovskites (HOIPs) due to their complex band structure and mechanical properties. Here, we investigate the temperature-dependent microphotoluminescence (PL) of 2D (C₆H₅CH₂CH₂NH₃)₂Cs₃Pb₄Br₁₃ HOIP subject to biaxial strain induced by a SiO₂ ring platform on which flakes are placed by viscoelastic stamping. At 80 K, we found that a strain of <1% can change the PL emission from a single peak (unstrained) to three well-resolved peaks. Supported by micro-Raman spectroscopy, we show that the thermomechanically generated strain modulates the bandgap due to changes in the octahedral tilting and lattice expansion. Mechanical simulations demonstrate the coexistence of tensile and compressive strain along the flake. The observed PL peaks add an interesting feature to the rich phenomenology of photoluminescence in 2D HOIPs, which can be exploited in tailored sensing and optoelectronic devices.

Correlating Symmetries of Low-Frequency Vibrations and Self-Trapped Excitons in Layered Perovskites for Light Emission with Different Colors

The soft hybrid organic-inorganic structure of two-dimensional layered perovskites (2DLPs) enables broadband emission at room temperature from a single material, which makes 2DLPs promising sources for solid-state white lighting, yet with low efficiency. The underlying photophysics involves self-trapping of excitons favored by distortions of the inorganic lattice and coupling to phonons, where the mechanism is still under debate. 2DLPs with different organic moieties and emission ranging from self-trapped exciton (STE)-dominated white light to blue band-edge photoluminescence are investigated. Detailed insights into the directional symmetries of phonon modes are gained using angle-resolved polarized Raman spectroscopy and are correlated to the temperature-dependence of the STE emission. It is demonstrated that weak STE bands at low-temperature are linked to in-plane phonons, and efficient room-temperature STE emission to more complex coupling to several phonon modes with out-of-plane components. Thereby, a unique view is provided into the lattice deformations and recombination dynamics that are key to designing more efficient materials.

Temperature dependent phase transitions and their relation to isosbestic point formation. Case study of C(NH2)3PbI3

Besides the vast research regarding the hybrid organic-inorganic perovskite (HOIP) materials used in the solar cell production, their properties are still not fully uncovered. In this paper, detailed investigation on the phase transitions in guanidinium lead iodide (GAPbI₃) using vibrational spectroscopy techniques (IR and Raman) are presented. In addition to the well-known three phases of GAPbI₃ (denoted as I, II and III) another one existing in the temperature range from 48 °C to 160 °C is characterized. The thorough inspection of the vibrational spectra revealed some interesting changes occurring in the low temperature region (from -90 to -62 °C) that suggest presence of a new phase. Finally, a redefinition of the phase nomenclature according to the recommendations given by the IUCr is proposed.

Mixed Dimethylammonium/Methylammonium Lead Halide Perovskite Crystals for Improved Structural Stability and Enhanced Photodetection

The solvent acidolysis crystallization technique is utilized to grow mixed dimethylammonium/methylammonium lead tribromide (DMA/MAPbBr₃ ) crystals reaching the highest dimethylammonium incorporation of 44% while maintaining the 3D cubic perovskite phase. These mixed perovskite crystals show suppression of the orthorhombic phase and a lower tetragonal-to-cubic phase-transition temperature compared to MAPbBr₃ . A distinct behavior is observed in the temperature-dependent photoluminescence properties of MAPbBr₃ and mixed DMA/MAPbBr₃ crystals due to the different organic cation dynamics governing the phase transition(s). Furthermore, lateral photodetectors based on these crystals show that, at room temperature, the mixed crystals possess higher detectivity compared to MAPbBr₃ crystals caused by structural compression and reduced surface trap density. Remarkably, the mixed-crystal devices exhibit large enhancement in their detectivity below the phase-transition temperature (at 200 K), while for the MAPbBr₃ devices only insignificant changes are observed. The high detectivity of the mixed crystals makes them attractive for visible-light communication and for space applications. The results highlight the importance of the synthetic technique for compositional engineering of halide perovskites that governs their structural and optoelectronic properties.

Identifying and Passivating Killer Defects in Pb-Free Double Cs2AgBiBr6 Perovskite

Pb-free double perovskites, such as Cs₂AgBiBr₆, are alternatives to lead halide perovskites for photovoltaic applications due to superior stability, low toxicity, and promising optoelectronic properties. However, their performance is subpar. We combine nonadiabatic molecular dynamics and real-time time-dependent density-functional theory to show that the negatively charged Br vacancy in Cs₂AgBiBr₆ creates an extremely detrimental donor-yielded (DY) center, which is a typical defect in six-coordinated semiconductors. Ag⁺ and Bi³⁺ form a bond by attraction through the anisotropic vacancy charge, generating a midgap state that traps holes within tens of picoseconds. Substituting Ag with indium by doping produces a weak and long In-Bi bond, lifting the defect energy level to the conduction band. Hole trapping slows down by an order or magnitude, and trap-assisted charge recombination decreases 4-fold. The simulations bring atomistic insights into defects of Pb-free double perovskites and provide a defect mitigation strategy for rational design of high-performance optoelectronic devices.