Atomically Resolved Quantum-Confined Electronic Structures at Organic-Inorganic Interfaces of Two-Dimensional Ruddlesden-Popper Halide Perovskites

Two-dimensional lead halide perovskite lateral homojunctions enabled by phase pinning

Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.

Additive Engineering for Stable and Efficient Dion-Jacobson Phase Perovskite Solar Cells

Because of their better chemical stability and fascinating anisotropic characteristics, Dion-Jacobson (DJ)-layered halide perovskites, which owe crystallographic two-dimensional structures, have fascinated growing attention for solar devices. DJ-layered halide perovskites have special structural and photoelectronic features that allow the van der Waals gap to be eliminated or reduced. DJ-layered halide perovskites have improved photophysical characteristics, resulting in improved photovoltaic performance. Nevertheless, owing to the nature of the solution procedure and the fast crystal development of DJ perovskite thin layers, the precursor compositions and processing circumstances can cause a variety of defects to occur. The application of additives can impact DJ perovskite crystallization and film generation, trap passivation in the bulk and/or at the surface, interface structure, and energetic tuning. This study discusses recent developments in additive engineering for DJ multilayer halide perovskite film production. Several additive-assisted bulk and interface optimization methodologies are summarized. Lastly, an overview of research developments in additive engineering in the production of DJ-layered halide perovskite solar cells is offered.

Surface Characterization of the Solution-Processed Organic-Inorganic Hybrid Perovskite Thin Films

The surface properties of organic-inorganic hybrid perovskites can strongly affect the efficiency and stability of corresponding devices. Even though different surface passivation methods are developed, the microscopic structures of solution-processed perovskite film surfaces are not systematically studied. This study uses low-temperature scanning tunneling microscopy to study the organic-inorganic hybrid perovskite thin films, MA_0.4 FA_0.6 PbI₃ and MAPbI₃ , synthesized by the spin-coating method. Flat surface structures, atomic steps, and crystal grain boundaries are resolved at an atomic resolution. The surface imperfections are also characterized, as well as the dominant defects. Simulations on different types of iodine vacancy configurations are performed by density functional theory calculations. In addition, it is observed that the surface iodine lattice structure is unstable during scanning. Tip scanning can also cause the vertical migration of surface iodine ions. The measurements provide the direct visualizations of the surface imperfections of the solution-processed perovskite films. They are essential for understanding the surface-related optoelectronic effects and rationally designing more efficient surface passivation methods.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Origin and physical effects of edge states in two-dimensional Ruddlesden-Popper perovskites

The edge region of two-dimensional (2D) Ruddlesden-Popper (RP) perovskites exhibits anomalous properties from the bulk region, including low energy emission and superior capability of dissociating exciton, which is highly beneficial for the optoelectronic devices like solar cells and photodetectors, denoted as “edge states”. In this review, we introduce the recent progress on the edge states that have been focused on the origin and the optoelectronic properties of edge states in 2D RP perovskites. By providing theoretical frameworks and experimental observations, we elucidate the origin of the edge states from two aspects, intrinsic electronic properties and extrinsic structure distortions. Besides, we introduce the physical properties of the edge states and current debating on this topic. Finally, we present an outlook on future research about the edge states of 2D RP perovskites.