Investigation of the structural stability, electronic properties, and thermoelectric performance of graphene enhanced (BA)₂SnI₄ two-dimensional halide perovskite for advanced thermoelectric applications

Two-dimensional (2D) halide perovskites have demonstrated impressive long-term stability and superior device performance as compared to their three-dimensional (3D) counterparts. The potential of 2D halide perovskites for advanced photovoltaic applications can be enhanced by an understanding of how external factors like strain could be used to tune their optoelectronic properties. This study explores the effects of biaxial strain on the structure and electronic transport properties of 2D halide perovskites, focusing on the lowest energy (001) surfaces of (Cs2BCl4 and CsB2Cl5, B = Pb or Sn) with CsCl and BCl2 terminations. Using first-principles calculations, we find that the lower energy CsCl terminated surface, resulting in Cs2BCl4, couples strongly with biaxial strain. This termination shows bandgap modulations from approximately 1.5 eV to 1.8 eV for Cs2PbCl4 and 1.2 eV to 1.5 eV for Cs2SnCl4 with biaxial strain. Within the acoustic deformation potential theory, we compute hole mobilities, and find substantial enhancements of approximately 80% for Pb-based and 50% for Sn-based systems, thereby emphasizing the potential of strain engineering to further optimize charge transport properties in 2D halide perovskites.

Quantum confinement effect on the electronic and optical properties of two-dimensional halide perovskites

Two-dimensional organic-inorganic hybrid halide perovskites have garnered much attention owing to their outstanding stability alongside unique quantum-well structures and anisotropic properties, leading to improved charge dynamics. Two-dimensional perovskites can be divided into three phases including Ruddlesden-Popper, Dion-Jacobson, and Alternating cations in the interlayer space phase. Each phase of these perovskites shows distinguished phase-dependent structural and optoelectrical properties. Tuning their properties by designing the materials can be a key strategy to enhance the device performance in optoelectrical applications. Configuration of spacer cations and the control of octahedral layer numbers (n ) can be important parameters in material design, enabling the tuning of dielectric properties, exciton binding energy, and bandgaps, as well as materials structures, thereby influencing stability and charge transport behaviors. In this point, two-dimensional perovskite single crystals can play essential roles in not only understanding phase-dependent intrinsic natures but also enhancing performance of optoelectronic applications, specifically owing to their long carrier diffusion length and enhanced stability with little grain boundaries and low trap density. This review will deliver the strategy of phase-dependent materials design with an understanding of their anisotropic properties and charge dynamics for optoelectronic applications, including photodetectors and X-ray detectors.

Two-dimensional (2D) halide perovskites are a versatile material class, exhibiting a layered crystal structure, consisting of inorganic metal–halide sheets separated by organic spacer cations. Unlike their 3D counterparts, 2D perovskites have less strict geometric requirements, allowing for a wider range of molecules to be incorporated. This potentially offers a way to engineer the properties of a 2D perovskite through adequate selection of the organic spacer cations. Our study systematically analyzes the effect of spacer cation length on the electronic and optical properties of Ruddlesden–Popper lead-iodide-based 2D perovskites, using alkylammonium cations of varying chain lengths. Intriguingly, no linear correlation between interlayer distance and the optical gap or valence band position is observed in our measurements. Rather it matters whether the spacer cation contains an odd or even number of carbon atoms in the chain. Notably, these odd-even effects manifest in variations of ionization energy, optical gap as well as charge carrier mobility. Density functional theory calculations reproduce the changes in optical properties, allowing us to identify the underlying mechanism: while even-numbered carbon chains pack efficiently within the organic spacer layer, the shorter odd-numbered chains increase distortions. These distortions lead to variations in the Pb–I–Pb bond angle within the inorganic sheets, resulting in the observed odd-even effect in the (opto-)electronic properties. This understanding will be helpful to make more informed choices regarding the incorporated spacer molecules which can potentially help to enhance performance when integrating such 2D perovskite interlayers into devices.

Effect of heterostructure engineering on electronic structure and transport properties of two-dimensional halide perovskites

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites (HOIPs) with considerably hydrophobic phenyl ethylammonium (PEA) organic cations have been used in highly efficient solar cells and LEDs, which are stable and enjoy a long lifetime, even when exposed to moisture. Different from other 2D HOIPs with alkyl amine cations, a benzene ring is present in the PEA cation. Until recently, an understanding of the effects of PEA on the structural, electronic, and optical properties of 2D HOIPs under pressure has remained limited. We find that there is a direct-indirect band gap transition at around 5.8 GPa and that the direct band gap recovers when the pressure is released. The stacking order of the benzene rings in the PEA cation plays a critical role in the mechanical and electronic properties. Our present work demonstrates that 2D HOIPs with organic cations containing benzene rings prove highly attractive for use in flexible optoelectronics.

Monitoring of the spatially resolved exciton spin dynamics in two-dimensional semiconductors has revealed the formation of a spatial pattern and long-range transport of the spin-polarized excitons, which holds promise for exciton-based spin-optoelectronic applications. However, the spatial evolution has been restricted to cryogenic temperatures because of the short exciton spin relaxation times at room temperature. Here, we report that two-dimensional halide perovskites can overcome this limitation owing to their relatively long exciton spin relaxation times and substantial exciton-exciton interactions. We demonstrate the emergence of a halo-like spatial profile in spin-polarized exciton population and its ultrafast expansion at room temperature by performing time-resolved Faraday rotation imaging of spin-polarized excitons in two-dimensional perovskite (C4H9NH3)2(CH3NH3)3Pb4I13. Exciton-exciton exchange interactions induce density-dependent nonlinear relaxation and ultrafast transport of exciton spins and give rise to a rapidly expanding halo-like spatial pattern. The density-dependent spatial control suggests the potential of using two-dimensional halide perovskites for spin-optoelectronic applications.

The making of a reconfigurable semiconductor with a soft ionic lattice

Two-dimensional (2D) halide perovskites are receiving considerable attention for applications in photovoltaics, largely due to their versatile composition and superior environmental stability over three-dimensional (3D) perovskites, but show much lower power conversion efficiencies. Hence, further understanding of the structure-property relationships of these 2D materials is crucial for improving their photovoltaic performance. Here, we investigate by means of first-principles calculations the structural and electronic properties of 2D lead and tin Ruddlesden-Popper perovskites with general formula (BA)2An-1BnI3n+1, where BA is the butylammonium organic spacer, A is either methylammonium (MA) or formamidinium (FA) cations, B represents Sn or Pb atoms, and n is the number of layers (n = 1, 2, 3, and 4). We show that the band gap progressively increases as the number of layers decreases in both Sn- and Pb-based materials. Through substituting MA by FA cations, the band gap slightly opens in the Sn systems and narrows in the Pb systems. The electron and hole carriers show small effective masses, which are lower than those of the corresponding 3D perovskites, suggesting high carrier mobilities. The structural distortion associated with the orientation of the MA or FA cations in the inorganic layers is found to be the driving force for the induced Rashba spin-splitting bands in the systems with more than one layer. From band alignment diagrams, the transfer process of the charge carriers in the 2D perovskites is found to be from smaller to higher number of layers n for electrons and oppositely for holes, in excellent agreement with experimental studies. We also find that, when interfaced with 3D analogues, the 2D perovskites could function as hole transport materials.

In two-dimensional (2D) halide perovskites, four distinct types of intramolecular band alignment (Ia, Ib, IIa, and IIb) can be formed between the organic and inorganic components. Molecular design to achieve desirable band alignments is of crucial importance to the applications of 2D perovskites and their heterostructures. In this work, by means of first-principles calculations, we have developed molecular design strategies that lead to the discovery of 2D halide perovskites with favorable band alignments toward light-emitting and photovoltaic applications. The same design strategies can be extended to vertical and lateral heterostructures of 2D perovskites with selective light emissions from the organic and/or inorganic layer of constituent 2D perovskites. For each intramolecular band alignment, the charge density and binding energy of the lowest energy exciton are examined. The effect of spin-orbit coupling (SOC) on the band structures is assessed. While SOC significantly lowers the band gaps in type-Ia and type-IIa alignments, it has a negligible effect in type-Ib and type-IIb alignments.

First-principles investigation on adsorption of anchors on two-dimensional halide perovskite material

Two-dimensional halide perovskites are attracting attention due to their structural diversity, improved stability, and enhanced quantum efficiency compared to their three-dimensional counterparts. In particular, Dion-Jacobson (DJ) phase perovskites exhibit superior structural stability compared to Ruddlesden-Popper phase perovskites. The inherent quantum well structure of layered perovskites leads to highly anisotropic charge transport and optical properties. Therefore, controlling the preferred crystal orientation (parallel or perpendicular) is crucial for optimizing device performance. This work presents a rational strategy to control parallel and perpendicular crystal growth in C6N2H16PbI4 (4AMPPbI4)-based DJ phase perovskite thin films. We demonstrate that crystal orientation depends on crystal growth rates, which can be controlled by varying the solvent composition, antisolvent, and annealing temperature. Direct and inverse photoelectron spectroscopy reveals that the electronic structure of 4AMPPbI4, including its work function, ionization energy, and electron affinity, is orientation-dependent. Different orientations significantly affect carrier transport as confirmed by single-carrier devices. This study highlights the critical role of crystal orientation in DJ phase perovskites for designing high-performance optoelectronic devices.Graphical

Effect of heteroatoms on structural, electronic and spectroscopic properties of polyfuran, polythiophene and polypyrrole: A hybrid DFT approach

Titanium carbide (Ti3C2), belonging to the MXene family, has become a focal point of research due to its exceptional electronic and structural properties. As a two-dimensional transition metal carbide, Ti3C2 exhibits promising potential in a variety of applications, ranging from electronic devices to energy storage. Among the key parameters influencing its electronic behavior, the work function plays a pivotal role. This study aims to elucidate the intricate relationship between concentration, surface termination, and the work function of Ti3C2X2 through first-principles density functional theory (DFT) calculations. First-principles DFT calculations were employed to investigate the electronic structure of Ti3C2X2, focusing on the influence of varying concentrations of surface functional groups. Hydroxyl (-OH), oxygen (-O), chlorine (-Cl), and fluoride (-F) terminations were specifically chosen as representative functional groups for their prevalence in experimental studies and potential technological applications. These calculations provide a quantum-level understanding of the electronic properties and behaviors of Ti3C2 under different surface conditions. Our results indicate a notable sensitivity of the work function of Ti3C2 to changes in both concentration and the type of surface termination. The presence of functional groups induces charge redistribution within the MXene layers, significantly impacting the overall electronic landscape. The variation in charge distribution directly correlates with changes in the work function, illustrating the dynamic nature of Ti3C2 in response to surface modifications. Furthermore, an in-depth analysis of the correlation between concentration, surface termination, and the energetics of charge transfer at the Ti3C2 surface provides valuable insights. The interplay between these factors reveals intricate mechanisms that govern the observed variations in the work function. The concentration-dependent charge transfer at the surface serves as a key determinant, emphasizing the importance of carefully tailoring the composition to achieve desired electronic properties. The understanding gained from this study has significant implications for the tailored design and utilization of Ti3C2 in various technological applications. By manipulating concentration and surface termination, researchers and engineers can fine-tune the work function to meet specific requirements in electronic devices, sensors, and energy storage systems. This level of control over electronic properties enhances the versatility and applicability of Ti3C2 in emerging technologies. In conclusion, our investigation highlights the critical roles of concentration and surface termination in modulating the work function of Ti3C2. The pronounced sensitivity of this parameter to surface modifications underscores the need for a comprehensive understanding of the underlying electronic mechanisms. By elucidating the interplay between concentration, surface termination, and charge transfer, this study provides valuable insights that pave the way for the strategic design and implementation of Ti3C2 in advanced technological applications. The ability to fine-tune electronic properties opens new avenues for exploiting the full potential of Ti3C2 in diverse fields, driving innovation and progress in materials science and technology.

Two-dimensional (2D) halide perovskites are emerging as promising candidates for nanoelectronics and optoelectronics. To realize their full potential, it is important to understand the role of those defects that can strongly impact material properties. In contrast to other popular 2D semiconductors (e.g., transition metal dichalcogenides MX2) for which defects typically induce harmful traps, we show that the electronic activities of defects in 2D perovskites are significantly tunable. For example, even with a fixed lattice orientation one can change the synthesis conditions to convert a line defect (edge or grain boundary) from electron acceptor to inactive site without deep gap states. We show that this difference originates from the enhanced ionic bonding in these perovskites compared with MX2. The donors tend to have high formation energies and the harmful defects are difficult to form at a low halide chemical potential. Thus, we unveil unique properties of defects in 2D perovskites and suggest practical routes to improve them.