Halide perovskite single crystals: growth, characterization, and stability for optoelectronic applications

Single Crystal Ruddlesden-Popper and Dion-Jacobson Metal Halide Perovskites for Visible Light Photodetectors: Present Status and Future Perspectives

2D metal halide perovskites (MHPs), mainly the studied Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases, have gained enormous popularity as optoelectronic materials owing to their self-assembled multiple quantum well structures, tunable semiconducting properties, and improved structural stability compared to their bulk 3D counterparts. The performance of polycrystalline thin film devices is limited due to the formation of defects and trap states. However, as studied so far, single crystal-based devices can provide a better platform to improve device performance and investigate their fundamental properties more reliably. This Review provides the first comprehensive report on the emerging field of RP and DJ perovskite single crystals and their use in visible light photodetectors of varied device configurations. This Review structurally summarizes the 2D MHP single crystal growth methods and the parameters that control the crystal growth process. In addition, the characterization techniques used to investigate their crystal properties are discussed. The review further provides detailed insights into the working mechanisms as well as the operational performance of 2D MHP single crystal photodetector devices. In the end, to outline the present status and future directions, this Review provides a forward-looking perspective concerning the technical challenges and bottlenecks associated with the developing field of RP and DJ perovskite single crystals. Therefore, this timely review will provide a detailed overview of the fast-growing field of 2D MHP single crystal-based photodetectors as well as ignite new concepts for a wide range of applications including solar cells, photocatalysts, solar H2 production, neuromorphic bioelectronics, memory devices, etc.

Organic ammonium salt assisted crystallization and defect passivation of a quasi-two-dimensional pure blue perovskite at the buried interface

Quasi-two-dimensional (quasi-2D) perovskites exhibit excellent performance in light-emitting diodes (LEDs). However, the quality of perovskite films prepared via the solution method is significantly impacted by the enormous number of defects that unavoidably form at the grain boundaries and interfaces during the precursor to the crystal formation process. Here, we propose a strategy to assist perovskite crystallization and defect passivation at the buried interface through interfacial modification. The organic ammonium salt, ethylamine chloride (EACl), is added to the hole transport material and modifies the buried interface of the perovskite film. EACl introduces the nucleation sites for perovskite precursors, and promotes the crystallization process of the perovskite grains, contributing to the formation of high-quality perovskite films. At the same time, the presence of Lewis base (-NH2) groups in EACl and their lone electron pairs effectively inactivate unlocated Pb2+ ions at the buried interface, thereby reducing non-radiative recombination. In addition, chloride ions help to mitigate defects and to improve the morphology of perovskite films. Devices with this modification show a higher performance than control devices on all metrics. This work proposes a facile but efficient way for improving quasi-2D pure blue perovskite crystallization and growth.

Organic cations in halide perovskite solid solutions: exploring beyond size effects

Halide perovskites are a class of materials of consolidated optoelectronic and electrochemical applications, reaching efficiencies compared to established materials in respective fields. In this scenario, the design and understanding of composition-structure-property relations is imperative. In solid solutions containing mixed cations, some direct relations between the sizes of the substituents and the properties of perovskites are generally observed. However, in several cases, these relations are not observed, implying that other characteristics of these cations play a major role. Despite its importance, this understanding has not been comprehensively deepened. To address this issue, we synthesized and characterized the structure, electrical behavior, and stability of methylammonium lead iodide-based perovskites with equal amounts of the substituents guanidinium, ethylammonium, and acetamidinium. These three large organic cations have essentially equal sizes but other remarkably different characteristics, such as the number of N-H bonds, intrinsic dipole moment, and order of C-N bonds. Herein, we show that these cations have dramatically different effects over important fundamental and applied properties of resulting perovskites, including the orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions, microstructural development, ionic conductivity, I-V hysteresis, electronic carrier mobility, and stability against light-induced degradation. These effects are correlated with the characteristics of the large substituent cations and help pave the way for a better rational chemical design of halide perovskites.

Uniaxial-Oriented Perovskite Films with Controllable Orientation

Perovskite films with large crystal size, preferred orientation, and facile fabrication process, combining advantages of single-crystal and polycrystalline films, have gained considerable attention recently. However, there is little research on the facet properties of perovskite films. Here, (111)- and (001)-oriented perovskite films with bandgaps ranging from 1.53 to 1.77 eV, and systematically investigated their orientation-dependent properties are achieved. The (111)-oriented films show electron-dominated traps and the (001)-oriented films show hole-dominated traps, which are related to their atomic arrangement at the surface. Compared with the (001)-oriented films, the (111)-oriented films exhibit lower work function and superior water/oxygen robustness. For the wide-bandgap films, the lattice of the (001)-oriented film provides an unobstructed passage for ion migration. Comparably, the (111)-oriented films exhibit suppressed ion migration and excellent phase stability. The optimized unencapsulated solar cells based on both (001) and (111) orientations show a similar high efficiency of ≈23%. The (111)-oriented solar cell exhibits excellent stability, maintaining 95% of its initial efficiency after 1500 h maximum power point (MPP) tracking test, and 97% initial efficiency after 3000 h aging in ambient conditions. This work paves the way for the rational design, controllable synthesis, and targeted optimization of uniaxial-oriented perovskite films for various electronic applications.

Towards Long-Term Stable Perovskite Solar Cells: Degradation Mechanisms and Stabilization Techniques

It is certain that perovskite materials must be a game-changer in the solar industry as long as their stability reaches a level comparable with the lifetime of a commercialized Si photovoltaic. However, the operational stability of perovskite solar cells and modules still remains unresolved, especially when devices operate in practical energy-harvesting modes represented by maximum power point tracking under 1 sun illumination at ambient conditions. This review article covers from fundamental aspects of perovskite instability including chemical decomposition pathways under light soaking and electrical bias, to recent advances and techniques that effectively prevent such degradation of perovskite solar cells and modules. In particular, fundamental causes for permanent degradation due to ion migration and trapped charges are overviewed and explain their interplay between ions and charges. Based on the degradation mechanism, recent advances on the strategies are discussed to slow down the degradation during operation for a practical use of perovskite-based solar devices.

Two-dimensional CsPbI3/CsPbBr3 vertical heterostructure: a potential photovoltaic absorber

First-principles methods have been employed here to calculate structural, electronic and optical properties of CsPbI3 and CsPbBr3, in monolayer and heterostructure (HS) (PbI2-CsBr (HS1), CsI-CsBr (HS2), CsI-PbBr2 (HS3) and PbI2-PbBr2 (HS4)) configurations. Imaginary frequencies are absent in phonon dispersion curves of CsPbI3 and CsPbBr3 monolayers which depicts their dynamical stability. Values of interfacial binding energies signifies stability of our simulated heterostructures. The CsPbI3 monolayer, CsPbBr3 monolayer, HS1, HS2, HS3 and HS4 possess direct bandgap of 2.19 eV, 2.73 eV, 2.41 eV, 2.11 eV, 1.88 eV and 2.07 eV, respectively. In the HS3, interface interactions between its constituent monolayers causes substantial decrease in its resultant bandgap which suggests its solar cell applications. Static dielectric constants of all simulated heterostructures are higher when compared to those of pristine monolayers which demonstrates that these heterostructures possess low charge carrier recombination rate. In optical absorption plots of materials, the plot of HS3 displayed a red shift and depicted absorption of a substantial part of visible spectrum. Later on, via Shockley-Queisser limit we have calculated solar cell parameters of all the reported structures. The calculations showed that HS2, HS3 and HS4 showcased enhanced power conversion efficiency compared to CsPbI3 and CsPbBr3 monolayers when utilized as an absorber layer in solar cells.

Influence of layer thickness on the power conversion efficiency of tin halide-based planar heterojunction solar cells

In recent years, perovskite-based solar cell technologies have sparked much interest. Organic-inorganic halide perovskites (OIHPs) are a type of perovskite that has shown promise in various optoelectronic applications. The power conversion efficiency (PCE) of organic-inorganic perovskite materials is improving with new materials development. However, inorganic materials studied in labs have limited PCE. Lead-free tin halide CH3NH3SnX3 shows good PCE. In this work, we study the electrical characteristics of tin halide as a perovskite absorbing layer in planar heterojunction solar cells with the help of GPVDM solar cell simulation software. A comparative study of power conversion efficiency has been done for tin halide of chloride, bromide, and iodide. The effect of electron and hole drift-diffusion, carrier continuity equations in position space to represent charge flow within the device, Poisson's equation, and charge carrier recombination has been studied in detail.

Growth of Hybrid Perovskite Crystals from CH3NH3PbI3-xClx Solutions Subjected to Constant Solvent Evaporation Rates

In this work, we subjected hybrid lead-mixed halide perovskite (CH₃NH₃PbI_3-xCl_x) precursor inks to different solvent evaporation rates in order to facilitate the nucleation and growth of perovskite crystals. By controlling the temperature of perovskite solutions placed within open-air rings in precise volumes, we established control over the rate of solvent evaporation and, thus, over both the growth rate and the shape of perovskite crystals. Direct utilization of diluted lead-mixed halide perovskites solutions allowed us to control the nucleation and to favor the growth of only a low number of perovskite crystals. Such crystals exhibited a clear sixfold symmetry. While crystals formed at a lower range of temperatures (40-60 °C) exhibited a more compact dendritic shape, the crystals grown at a higher temperature range (80-110 °C) displayed a fractal dendritic morphology.

Bulk Perovskite Crystal Properties Determined by Heterogeneous Nucleation and Growth

In metal halide perovskites, charge transport in the bulk of the films is influenced by trapping and release and nonradiative recombination at ionic and crystal defects. Thus, mitigating the formation of defects during the synthesis process of perovskites from precursors is required for better device performance. An in-depth understanding of the nucleation and growth mechanisms of perovskite layers is crucial for the successful solution processing of organic-inorganic perovskite thin films for optoelectronic applications. In particular, heterogeneous nucleation, which occurs at the interface, must be understood in detail, as it has an effect on the bulk properties of perovskites. This review presents a detailed discussion on the controlled nucleation and growth kinetics of interfacial perovskite crystal growth. Heterogeneous nucleation kinetics can be controlled by modifying the perovskite solution and the interfacial properties of perovskites adjacent to the underlaying layer and to the air interface. As factors influencing the nucleation kinetics, the effects of surface energy, interfacial engineering, polymer additives, solution concentration, antisolvents, and temperature are discussed. The importance of the nucleation and crystal growth of single-crystal, nanocrystal, and quasi-two-dimensional perovskites is also discussed with respect to the crystallographic orientation.

Doping/Alloying Pathways to Lead-Free Halide Perovskites with Ultimate Photoluminescence Quantum Yields

Tailored modifications of halide lead-free perovskites (LFPs) via doping/alloying with metal cations have been recognized as a promising pathway to highly efficient inorganic phosphors with photoluminescence (PL) quantum yields of up to 100 %. Such materials typically display selective sensitivity to UV light, a broad PL range, and long PL lifetimes as well as a unique compositional variability and stability-an ideal combination for many light-harvesting applications. This Minireview presents the state-of-the-art in doped LFPs, focusing on the reports published mostly in the last two to three years. We discuss the factors determining the efficiency and spectral parameters of the broadband PL of doped LFPs depending on the dopant and host matrix, both in micro- and nanocrystalline states, address the most relevant challenges this rapidly developing research area is facing, and outline the most promising concepts for further progress in this field.

Hybrid Lead Bromide Perovskite Single Crystals Coupled with a Zinc(II) Complex for White Light Emission

Herein we report the fabrication of green emitting hybrid lead bromide perovskite single crystals (HLBPSCs), their anion exchange mediated tunable yellow luminescence and thereby their coupling ability with blue emitting inorganic complex leading to generation of a photostable white light emission, with properties close to bright day sunlight. The partial anion exchange reaction to green emitting HLBPSCs led to formation of yellow emitting anion exchanged HLBPSCs─which are termed as AE-HLBPSCs herein. Then, AE-HLBPSCs were chemically combined with blue emitting Zn-aspirin complex to produce white light with a photoluminescence quantum yield (PLQY) of 47.7%. The solid form of the white light emitting (WLE) composite (followed by coating with poly methyl methacrylate─PMMA) showed color coordinates of (0.34, 0.33), color rendering index of 76 and correlated color temperature of 5282 K. Furthermore, the PMMA coated inorganic complex coupled AE-HLBPSCs showed the preservation of their WLE nature and luminescence stability in their solid form.