Chemical Behavior and Local Structure of the Ruddlesden-Popper and Dion-Jacobson Alloyed Pb/Sn Bromide 2D Perovskites

The alloyed lead/tin (Pb/Sn) halide perovskites have gained significant attention in the development of tandem solar cells and other optoelectronic devices due to their widely tunable absorption edge. To gain a better understanding of the intriguing properties of Pb/Sn perovskites, such as their anomalous bandgap's dependence on stoichiometry, it is important to deepen the understanding of their chemical behavior and local structure. Herein, we investigate a series of two-dimensional Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phase alloyed Pb/Sn bromide perovskites using butylammonium (BA) and 3-(aminomethyl)pyridinium (3AMPY) as the spacer cations: (BA)₂(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) and (3AMPY)(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) through a solution-based approach. Our results show that the ratio and site preference of Pb/Sn atoms are influenced by the layer thickness (n) and spacer cations (A'), as determined by single-crystal X-ray diffraction. Solid-state ¹H, ¹¹⁹Sn, and ²⁰⁷Pb NMR spectroscopy analysis shows that the Pb atoms prefer the outer layers in n = 3 members: (BA)₂(MA)Pb_xSn_n-xBr₁₀ and (3AMPY)(MA)Pb_xSn_n-xBr₁₀. Layered 2D DJ alloyed Pb/Sn bromide perovskites (3AMPY)(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) demonstrate much narrower optical band gaps, lower energy PL emission peaks, and longer carrier lifetimes compared to those of RP analogs. Density functional theory calculations suggest that Pb-rich alloys (Pb:Sn ∼4:1) for n = 1 compounds are thermodynamically favored over 50:50 (Pb:Sn ∼1:1) compositions. From grazing-incidence wide-angle X-ray scattering (GIWAXS), we see that films in the RP phase orient parallel to the substrate, whereas for DJ cases, random orientations are observed relative to the substrate.

Concerted Rattling in CsAg5 Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S(2) σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p-type thermoelectric material, CsAg5 Te3 , is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm(-1)  K(-1) ) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.

Air-stable molecular semiconducting iodosalts for solar cell applications: Cs2SnI6 as a hole conductor

We introduce a new class of molecular iodosalt compounds for application in next-generation solar cells. Unlike tin-based perovskite compounds CsSnI3 and CH3NH3SnI3, which have Sn in the 2+ oxidation state and must be handled in an inert atmosphere when fabricating solar cells, the Sn in the molecular iodosalt compounds is in the 4+ oxidation state, making them stable in air and moisture. As an example, we demonstrate that, using Cs2SnI6 as a hole transporter, we can successfully fabricate in air a solid-state dye-sensitized solar cell (DSSC) with a mesoporous TiO2 film. Doping Cs2SnI6 with additives helps to reduce the internal device resistance, improving cell efficiency. In this way, a Z907 DSSC delivers 4.7% of energy conversion efficiency. By using a more efficient mixture of porphyrin dyes, an efficiency near 8% with photon confinement has been achieved. This represents a significant step toward the realization of low-cost, stable, lead-free, and environmentally benign next-generation solid-state solar cells.

Local Symmetry Breaking in the High Temperature Regime of SnTe

The term emphanisis [1] has been coined to define the appearance of local off-centering displacements of ions from a high-symmetry ground state on warming, as recently discovered in PbTe [2]. Such a phenomenon is unusual because, in the canonical view of structural transformations, a low-symmetry ground state evolves into a higher symmetry state on warming. Although it is not uncommon for remnants of a low-symmetry phase to appear as spatial fluctuations at high temperature, the emergence of a locally broken symmetry state from a high symmetry ground state is quite rare. Emphanisis may be behind some long-known, but poorly understood anomalies seen in the lead chalcogenides. However, the origin and nature of emphanisis are still the subject of controversy. Several explanations for emphanisis have been suggested, including a simple response to an underlying anharmonic potential [3], a dynamic ferroelectric-like off-centering [2], and a temperature-dependent competition between ionicity and covalency [1], but an understanding remains elusive. In this talk I will report on atomic pair distribution function (PDF) measurements of the lead-free compound SnTe, which is isostructural to PbTe at high T but with a ferroelectric phase below Tc ~ 100K. Our data show that SnTe also exhibits an emphanitic response, but with an onset temperature well above Tc and a symmetry that is distinct from that of the ferroelectric phase. Taken together these results suggests that the emphanitic and ferroelectric responses are quite distinct.

Three-dimensional structure of nanocomposites from atomic pair distribution function analysis: study of polyaniline and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O

The three-dimensional structures of emeraldine base polyaniline (PANI) and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O have been determined by total X-ray scattering experiments. Atomic pair distribution functions (PDF) were measured to obtain experimental observables against which structural models were tested and refined. The PDF approach is necessary because of the limited structural coherence in these nanostructured materials. Polyaniline possesses a well-defined local atomic arrangement that can be described in terms of an 84-atom orthorhombic unit cell. The nanocomposite (PANI)(0.5)V(2)O(5) x 1.0 H(2)O too is locally well ordered and may be described in terms of a small number of structure-sensible parameters. The PDF approach allows the construction of structure models of PANI and (PANI)(0.5)V(2)O(5) x 1.0 H(2)O on the basis of which important materials' properties can be explained predicted and possibly improved.

Square nets of tellurium: rare-earth dependent variation in the charge-density wave of RETe3 (RE = rare-earth element)

The distortions in the square tellurium nets long known to exist in the structures of the charge-density wave materials, RETe3, have been elucidated. The (Te2)1- nets contain polytelluride oligomers which propagate in a fashion incommensurate from the adjacent (RETe)+ sublattice. The new information sets the stage for a much deeper understanding of these systems.

Mesostructured Selenides with Cubic MCM-48 Type Symmetry: Large Framework Elasticity and Uncommon Resiliency to Strong Acids

The metal mesostructured Pt/Sn/Se chalcogenides with cubic MCM-48 type pore symmetry are found to be surprisingly stable in concentrated oxidizing acids. Their metal chalcogenide framework exhibits high flexibility during reversible proton exchange as it expands and contracts in an apparent breathing-like action.

Two- and three-parameter equations for representation of retention data in reversed-phase liquid chromatography

Two-parameter equations that describe the dependence of ln kappa upon psi, where kappa is the retention factor and psi the volume fraction of the organic modifier in the mobile phase, are examined in what concerns the underlying approximations and their performance to fit experimental data obtained from reversed-phase liquid chromatography. Using 293 experimental systems, it was found that the performance of these equations to describe ln kappa versus psi data is rather low, since the percentage of the systems that can be described satisfactorily ranges from 40 to 60% depending on the fitting equation. This percentage may be raised to 75%, if the discreteness effect is properly taken into account. A further improvement to 90% of the systems studied can be achieved only by the use of three-parameter equations, which may arise by refinements of the rough approximations of the two-parameter equations. Although the refinements do not lead always to better equations, we developed a new three-parameter expression of In kappa that works more satisfactorily, since it combines simplicity, linearity of its adjustable parameters and the highest applicability.