Reasons for polytypism of crystals of the type MX2 II. Clasification of faults and structural series of polytypes; Conditions of polytypic growth of CdI2, PbI2, CdBr2, SnS2, SnSe2 and Ti1.2S2

Effects of the PbBr2:PbI2 Molar Ratio on the Formation of Lead Halide Thin Films, and the Ratio's Application for High Performance and Wide Bandgap Solar Cells

We investigate the effects of the molar ratio (x) of PbBr₂ on the phases, microstructure, surface morphology, optical properties, and structural defects of mixed lead halides PbI_2(1−x)Br_2x for use in solar cell devices. Results indicate that as x increased to 0.3, the surface morphology continued to improve, accompanied by the growth of PbI₂ grains. This resulted in lead halide films with a very smooth and continuous morphology, including large grains when the film was formed at x = 0.3. In addition, the microstructure changed from (001)-oriented pure PbI₂ to a highly (001)-oriented β (PbI₂-rich) phase. The plausible mechanism for the enhanced morphology of the lead halide films by the addition of PbBr₂ is proposed based on the growth of a Br-saturated lead iodide solid solution. Furthermore, iodine vacancies, identified by X-ray photoelectron spectroscopy, decreased as the ratio of PbBr₂ increased. Finally, an electrical analysis of the solar cells was performed by using a PN heterojunction model, revealing that structural defects, such as iodine vacancies and grain boundaries, are the main contributors to the degradation of the performance of pure PbI₂-based solar cells (including high leakage, low stability, and high hysteresis), which was significantly improved by the addition of PbBr₂. The solar cell fabricated at x = 0.3 in air showed excellent stability and performance. The device lost merely 20% of the initial efficiency of 4.11% after 1500 h without encapsulation. This may be due to the dense microstructure and the reduced structural defects of lead halides formed at x = 0.3.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Polytypism of vapor grown PbI2 crystals

Pure single crystals of lead iodide have been grown from vapor and characterised by X-ray diffraction for their polytypism and related phenomena. The theoretical estimation of the Jackson's α-factor and the surface micrographs of the crystals, coupled with the results of X-ray diffraction, indicate crystal growth by two-dimensional nucleation. Comparison of the experimental results with the earlier ones on gel-grown and melt-grown crystals reveals an obvious influence of the impurities on polytype formation, besides confirming the high purity of the vapor-grown crystals.