Halide Perovskite Semiconductors E-Book

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Beschreibung

Halide Perovskite Semiconductors

Enables readers to acquire a systematic and in-depth understanding of various fundamental aspects of halide perovskite semiconductors

Halide Perovskite Semiconductors: Structures, Characterization, Properties, and Phenomena covers the most fundamental topics with regards to halide perovskites, including but not limited to crystal/defect theory, crystal chemistry, heterogeneity, grain boundaries, single-crystals/thin-films/nanocrystals synthesis, photophysics, solid-state ionics, spin physics, chemical (in)stability, carrier dynamics, hot carriers, surface and interfaces, lower-dimensional structures, and structural/functional characterizations.

Included discussions on the fundamentals of halide perovskites aim to expand the basic science fields of physics, chemistry, and materials science.

Edited by two highly qualified researchers, Halide Perovskite Semiconductors includes specific information on:

• Crystal/defect theory of halide perovskites, crystal chemistry of halide perovskites, and processing and microstructures of halide perovskites
• Single-crystals of halide perovskites, nanocrystals of halide perovskites, low-dimensional perovskite crystals, and nanoscale heterogeneity of halide perovskites
• Carrier mobilities and dynamics in halide perovskites, light emission of halide perovskites, photophysics and ultrafast spectroscopy of halide perovskites
• Hot carriers in halide perovskites, correlating photophysics with microstructures in halide perovskites, chemical stability of halide perovskites, and solid-state ionics of halide perovskites

Readers can find solutions to technological issues and challenges based on the fundamental knowledge gained from this book. As such, Halide Perovskite Semiconductors is an essential in-depth treatment of the subject, ideal for solid-state chemists, materials scientists, physical chemists, inorganic chemists, physicists, and semiconductor physicists.

Leseprobe

Halide Perovskite Semiconductors

Structures, Characterization, Properties, and Phenomena

Edited by Yuanyuan Zhou and Iván Mora-Seró

Editors

Prof. Yuanyuan ZhouThe Hong Kong University of Scienceand TechnologyDepartment of Chemical and BiologicalEngineeringClear Water Bay, Hong KongSAR 999077China

Prof. Iván Mora‐SeróUniversitat Jaume I (UJI)Institute of Advanced Materials (INAM)Avenida de Vicent Sos Baynats/n, 12071 Castelló de la PlanaSpain

Cover Image: © NANOCLUSTERING/SCIENCE PHOTO LIBRARY/Getty Images

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Preface

Despite that fact that metal halide perovskites have been known for several decades, they are receiving significant attention, especially in the last one, as a new type of semiconductor material family that has injected excitement in both material and device research. Structurally, halide perovskite semiconductors are like conventional inorganic semiconductors, exhibiting long‐range ordering at the atomic scale and crystalline grain characteristics at the microscopic scale. However, the predominantly ionic character of their bonds provides unusual ionic conductivity, which can limit the performance of some device configurations but promote other properties and even self‐healing processes. In addition, some halide perovskites also present benign defect physics. This favors the interactions of the materials themselves with photons, charge carriers, and phonons for excellent semiconducting properties even in polycrystalline films with numerous crystallographic defects. From the perspective of processing, perovskite semiconductors are similar to soft organic semiconductors, which can be fabricated using high‐throughput, low‐temperature solution printing at low cost. This unleashes the potential of flexible, massive device integration into our future intelligent world. In fact, intense investigations aimed at creating better and cheaper semiconductors have been performed for many decades, but the emergence of a semiconductor type that can combine all the above merits has been extremely rare. Therefore, perovskite semiconductors have triggered the interest of the science community to devote significant research efforts to probing the fundamental sciences that underpin their unprecedented materials and device behaviors. With the continuous blooming of perovskite research for more than a decade, as well as the substantially rising interest from the industry in scaling‐up perovskite technologies, a high level of new, overarching fundamental knowledge on perovskite semiconductors may be synthesized by the field based on the existing literature studies. We thus consider it urgent to summarize these new semiconductor sciences in a systematic manner. In this context, we have invited world‐reputed researchers to contribute chapters on near‐full range of fundamental topics covering from versatile crystal structures to characterization methods and from various properties to device implications. All the chapter authors have not only presented excellent summaries of reported findings but also incorporated their forward‐looking thinking to guide future research. We deeply appreciate their dedicated efforts that ensure the high quality and lasting impacts of this book. We sincerely hope that this book will create inspiration for the readers to discover more fascinating sciences and to invent more frontier technologies in future research and development of perovskite semiconductors.

The Hong Kong University of Science and Technology, ChinaUniversitat Jaume I, Spain1 October 2023

Yuanyuan ZhouIván Mora‐Seró

1Introduction to Perovskite

Tianwei Duan1, Iván Mora-Seró2, and Yuanyuan Zhou1,3

1Hong Kong Baptist University, Department of Physics, Kowloon Tang, Hong Kong, SAR 999077, China

2Universitat Jaume I (UJI), Institute of Advanced Materials (INAM), Avenida de Vicent Sos Baynat, s/n, 12071 Castelló de la Plana, Spain

3The Hong Kong University of Science and Technology, Department of Chemical and Biological Engineering, Clear Water Bay, Hong Kong, SAR 999077, China

1.1 Evolution of Perovskite

Perovskite refers to a crystalline structure and extends to all the materials sharing this structure, despite the fact that it can present very different nature and properties. Initially, perovskites just denoted metal oxide minerals with a crystallography family of ABO3 stoichiometry. The beginning of perovskite dates back to the discovery of chlorite‐rich skarn at the Ural Mountains by the mineralogist German Gustav Rose in 1839. The component CaTiO3 was found in this mineral and named after the notable Count Lev A. Perovski (1792–1856), president of the Russian Geological Society. Thereafter, many metal oxides with perovskite structures, such as BaTiO3, PbTiO3 and SrTiO3, were widely studied. Many of the oxide perovskites were found to exhibit ferroelectric or piezoelectric properties [1–3].

More than 50 years after the discovery of oxide perovskite, a series of lead halide compounds with the general formula CsPbX3 (X = Cl, Br, I) were synthesized by Wells [4]. These metal halides were later proved to have a perovskite structure, ABX3, which is cubic at high temperatures and transforms from a tetragonally distorted structure at a lower temperature. The tunable photoconductivity of CsPbX3 has drawn much attention to the electronic property study, and also evolved the idea of organic molecules addition [5, 6]. Weber discovered that the organic cation methylammonium (CH3NH3+) substitutes for Cs+ form CH3NH3MX3 (M = Pb, Sn, X = I, Br) and published the first crystallographic study on organic lead halide perovskites [7, 8].

At the end of the twentieth century, abundant organic–inorganic halide perovskites were synthesized by Mitzi et al. [9–11]. Organic molecules, such as small and large organic cations, breathe new life into halide perovskite, embracing more diverse structures and physical properties in optoelectronic, photovoltaic, ferro‐ and antiferromagnetic, and non‐linear optical fields. In addition to flexible components and versatile functionality, the low‐forming energy makes halide perovskites facile to be fabricated into films, which makes them a promising material for commercialization in next‐generation semiconductors, and their interest in the development of light‐emitting diodes (LEDs) and transistors was demonstrated.

1.2 Structure of Perovskite

In a traditional view, perovskite represents a crystallographic family with the chemical formula ABX3, in which A and B are cations and X is an anion. The ideal perovskite is a cubic structure, having B cations as sixfold coordination surrounded by an octahedron of X anions, and A cations as 12fold cuboctahedral coordination, see Figure 1.1. Taking inorganic perovskite CsPbI3 as an example, the Cs+ cations are shown at the corners of the cube, and Pb2+ cations are in the center with I− anions in the face‐centered positions. In three‐dimensional (3D) perovskites, all six anions at the corners of the octahedra, with Pb at the center, are shared with the six nearest octahedra, see Figure 1.1. When large cations are included in the structure, not all the six halides can be shared with other octahedra, forming 2D, 1D, or 0D perovskite‐inspired materials. Many composition types of perovskites have been reported, involving lead halide perovskite, all‐inorganic cesium/rubidium lead halide perovskite, lead‐free or lead‐low halide perovskite, and halide double perovskite, as it will be extensively discussed in this book.

In the case of organic–inorganic perovskite, at least one of the ions in ABX3 is organic, e.g. MAPbI3 and FAPbI3 (MA is methylammonium, CH3NH3+; FA is formamidinium, HC(NH2)2+). Recently, metal‐free perovskite has also been synthesized with the chemical formula ANH4X3, where A is a divalent organic cation, and X is halogen ions, e.g. MDABCO–NH4I3 (MDABCO is N‐methyl‐N′‐diazabicyclo[2.2.2]octonium).

Several conditions must be satisfied in order for perovskite structure to be formed. Generally, the valences of A and B cations must total to three times those of the X anion to preserve charge balance. Furthermore, the perovskite structure can only tolerate particular ion combinations because of the size restrictions between ions in order to preserve the anion‐corner‐sharing structure. This ionic size relationship is expressed in terms of the Goldschmidt tolerance factor τ, which is correlated to the ionic radii rA, rB, and rX[12]:

Figure 1.1 Crystal structure of 3D cubic perovskite. Cation A is located in the void between the BX6 octahedra. In a crystal unit cell, A is located in the corners, B is in a body‐centered position, and X is in a face‐centered position.

where τ is an empirical index to predict the different structures of ABX3. When 0.9 < τ < 1, the perfect cubic perovskite structure is formed; when 0.8 < τ < 0.9, the distorted perovskite structure with tilted octahedra is preferred; when τ < 0.8 or τ > 1, the structure is non‐perovskite [13]. Another factor is the octahedral factor,

which determines whether the B atoms will favor the octahedral coordination of X atoms over greater or lower coordination numbers; this criterion is met for values between 0.4 and 0.9 [14]. In addition to size and charge, the coordination preference of metal ions is also taken into consideration.

Nowadays, many structural variants of perovskite have been synthesized, and they are all derived from the original 3D perovskite structure based on the corner‐sharing BX6 structure. Although the ABX3 perovskite structure has rigid constraints, the low‐dimensional perovskite allows for broader structural and compositional tunability. When the 3D perovskite is conceptually excised into slices, the size restrictions for the A′, which is the interlayer cation, are lifted for low‐dimensional derivatives. According to the connectivity, the segregated component made of BX6 octahedra is usually present 2D, 1D, or even separately 0D types, see Figure 1.2. From the perspective of the dimensions of morphologies, perovskite materials can be categorized into 3D bulk, 2D nanoplatelets, 1D nanowires, and 0D nanocrystals.

In the case of 2D perovskite, such structures are made up of a cation monolayer or bilayers alternating with sheets of the corner‐sharing BX6 octahedra. The 2D perovskite features mono‐ or diammonium cations A′, showing the chemical formulas of A′2BX4 and A′BX4, which are frequently referred to as Dion–Jacobson (divalent A′) or Ruddlesden–Popper (monovalent A′) phases [15]. In A′2BX4 formed by monovalent cations, such as PEA+ (phenethylammonium, C6H5(CH2)2NH3+) and BA+ (butylammonium, C4H9NH3+), a van der Waals gap was generated by a bilayer of monovalent cations from two neighboring lead halide sheets. Instead, in the A′BX3 system, each pair of cations can be substituted by a single divalent cation with tethering groups at each end to attach to neighboring halide sheets. The other low‐dimensional perovskite derivatives feature much more separated BX6 links, including 1D “pillar”‐like BX6 octahedra connected chains and 0D isolated “dot”‐like octahedra, respectively; see Figure 1.2. Especially, the BX6 connectivity can also be separated by the different compositions, which form the mixed perovskites known as pseudo members.

Figure 1.2 Halide perovskite family tree. Schematic illustration of standard 3D perovskite and the low‐dimensional derivates, including Ruddlesden–Popper 2D, Dion–Jacobson 2D, “Pillar”‐perovskite 1D, “Dot”‐perovskite 0D, and the double perovskite pseudo‐0D perovskite.

1.3 Property and Application of Perovskite

Early research on perovskite oxides focused on the biaxial optical properties and ferroelectric properties. In contrast, halide perovskites open the door to studying the optoelectrical properties because of their unique electronic structures, including direct tunable bandgap, strong absorption, small and balanced electron‐hole effective masses, and defect resistance, thereby improving their photoluminescence quantum yield. Moreover, the unprecedented flexibility of perovskite composition can be brought about by organic or inorganic components with optical or electronic functionalities. The most important advantage of halide perovskites is their facile, accessible, high‐quality crystals and films, enabling structure–property correlation exploration and prototype device optimization. Thus, halide perovskite semiconductors will hold promise for a variety of fascinating applications, including photovoltaics (PVs), LEDs, photodetectors, memristors and lasers, see Figure 1.3, just to cite the ones that probably receive more attention. In addition to this versatility, it is important to highlight the enormous potential for the development of high‐performance devices on flexible substrates, extending the application range of high‐performance rigid photovoltaics.

Figure 1.3 Example of properties and applications of halide perovskite. Halide perovskites are promising semiconductors with excellent properties, including flexible composition, facile accessibility, tunable bandgap, strong absorption, long carrier lifetimes, and defects resistance. These merits enable halide perovskite to be competent in solar cells, LEDs, photodetectors, memristors, and lasers. Interestingly, halide perovskite devices can also be developed on flexible substrates due to the good performance of polycrystalline films and low‐temperature growth conditions.

The organic–inorganic halide perovskites have received wide attention, mostly due to their high efficiency and low cost in next‐generation PVs, which have made these materials start to compete with commercial thin‐film cells. Such materials harvest the energy of sunlight efferently because of their high absorption coefficients in both visible and near‐infrared light. The first report of the perovskite solar cell was in 2009 [16], and now the record laboratory‐scale power conversion efficiency of perovskite film is certified at 25.7% (https://www.nrel.gov/pv/cell-efficiency). Perovskite film‐based solar cells are easy to be fabricated at low temperature, more energy‐saving, and environmentally friendly than the conventional silicon wafer with a lower payback time [17], but with a lower contrasted long‐ term stability. Consequently, further commercialization process of perovskite‐based solar cells has been hindered by the stability problem, including degradation due to moisture, oxygen, heat, light, mechanical stress, and reverse bias. These failings do not detract from the overall excellence but focus the research effort on increasing device stability. Halide perovskite materials remain a cost‐effective solution to address vast electrical energy supplies.

While initial boost of halide perovskite research was the development of photovoltaic devices, the good performance of these solar cells is founded on low nonradiative recombination, which is also beneficial for other optoelectronic devices. Consequently, halide perovskites are also currently impacting the development of a broad range of optoelectronic devices and systems. One significant benefit of halide perovskites used as LEDs is their very high color purity, with full width at a half‐maximum of 20 nm for the blue or green–blue electroluminescence spectrum peaks [18]. Unlike traditional inorganic nanomaterials, the exceptional color purity of quantum‐well nanoparticles is maintained regardless of crystal size. As a result, halide perovskites have the potential to solve some of the drawbacks of existing LEDs, such as difficult synthesis challenges, high cost, poor color purity of organic LEDs, and high ionization energy of quantum dot LEDs. Since the first demonstration of perovskite LEDs in 2014, the external quantum efficiency (EQE) of these devices has rapidly increased from below 1% to 25.8% for red [19], 28.1% for green [20], and 14.8% for blue [21]. The new LED technology has seen a meteoric rise in device efficiencies, but many scientific and technical obstacles, such as the unsatisfied stability and efficiency of blue LEDs, still stand in the way of perovskite LEDs further advancement into real‐world applications.

Halide perovskite‐based photodetectors exhibit comparable performance to commercially available photodetectors based on crystalline Si and III–V, offering significant potential for the technology of light‐signal detection. The outstanding intrinsic optoelectronic properties of halide perovskites, such as photoinduced polarization, high drift mobilities, and effective charge collection, have contributed to the current growth of cutting‐edge material studies in the field of light‐signal detection. Halide perovskite semiconductors feature effective light absorption, enabling the detection of a wide range of electromagnetic waves from ultraviolet and visible to near‐infrared and even radiations (X‐ray, γ‐ray, etc.), with low‐cost solution processability and high photon yield. This class of semiconductors may empower ground‐breaking photodetector technology in the areas of imaging, optical communications, and biomedical sensing; in this last case, further stability in polar solvent media, such as water, could increase enormously the range of applications of these systems.

Moreover, halide perovskites present a high ionic bonding character and ionic conductivity, causing the coexistence and coupling of ionic and electronic components of current and capacitance. This fact is at the base of nonconventional effects on optoelectronic systems, which could be a source of instabilities but can also be exploited. In this sense, halide perovskites exhibited good memristive properties supported by their electronic–ionic conductivity properties [22]. Memristor, that is memory resistor, is a leading candidate with robust capabilities in information storage and neuromorphic computing applications to address the growing challenge of approaching the end of Moore's law and the von Neumann bottleneck. The memristive property of halide perovskite is achieved through the synergistic coupling of photonic, electronic, and ionic processes, which enable perovskite to demonstrate novel functions such as optical‐erasing memory, optogenetically inspired synaptic functions, and light‐accelerated learning with multifunctionalization and novel photonic, logical, multilevel, and flexible functions.

Aside from the above‐mentioned, halide perovskites have much more extensive applications due to their outstanding attributes, such as lasers, X‐ray detectors, waveguides, scintillators, gas sensors, spintronics, and photocatalysis.

1.4 Summary and Outlook

The last two decades have seen the rapid development of halide perovskite materials, with researchers in particular pioneering systematic structural and property correlation studies based on halide perovskite composition and phases. The structure of perovskites has also been derived from ABX3 to various derivatives that are important for changing chemical properties, controlling energy bands, and granting new physical properties. Perovskite materials, as an excellent new generation of semiconductor materials, have been demonstrated to be useful in a wide range of application scenarios, including photovoltaics, displays, and sensing, storage.

However, the environmental and thermodynamic stability of halide perovskite‐based applications are two major challenges impeding their development, and some related phenomena and mechanisms should be thoroughly investigated to address perovskite device long‐term use problems. Scientists have begun to use multimodal characterization to study the structural changes of halide perovskites, to monitor structural changes related to physical processes using in situ technology, and to conduct large‐scale studies to establish correlations between components and properties using AI technology. The halide perovskite is like a treasure trove, and more interdisciplinary collaboration will lead to even more unexpected discoveries. This book will overview the intriguing properties of halide perovskites, making this system significantly different from other optoelectronic materials. Properties of materials and devices will be overviewed as well as the perspective on material and device development, always focusing on the fundamental properties.

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2Halide Perovskite Single Crystals

Clara Aranda-Alonso1,2 and Michael Saliba1,2

1Institute for Photovoltaics (IPV), University of Stuttgart, 70569 Stuttgart, Germany

2IEK5-Photovoltaics, Forschungszentrum Jülich, 52425 Jülich, Germany

2.1 Introduction

To further explore the potential of hybrid halide perovskite materials, including solar cells and beyond, their optoelectronic properties still need to be thoroughly investigated. In this respect, single crystals (SC), with fewer defects, are the ideal candidates to further analyze these properties without the detriment of the instability under atmospheric conditions that polycrystalline perovskites exhibit.

In this chapter, we address the main characteristics and applications of SC based on perovskite materials. The chapter is organized as follows: first, the three principal perovskite crystal structures are described, distinguishing between lead‐based, lead‐free, and all inorganic perovskite single crystals. Then, the different synthesis methods are described, highlighting the ones with significant impact. To further understand the potential of this material, the optoelectronic properties of the most commonly used SC are discussed in depth. Finally, the main applications of these materials in different technologies are described, including their abilities as photodetectors, scintillators, solar cells, light emitting diodes, and memristors.

2.2 Crystal Structure

In the general perovskite oxide structure, ABX3, A and B are cations of different sizes. A used to be larger than B, being B six‐coordinated by an X‐site anion to form a BX6 octahedron complex. To arrange a three‐dimensional (3D) system, the octahedron must share the corners, locating A‐cations in the cavities of the framework. The charge of each cation and anion must be such as to preserve electroneutrality. In the hybrid perovskite materials, the A‐cation is a monovalent organic amine, B is a divalent metal (Pb2+, Sn2+) and X is the halide element (I−, Br−, and Cl−), or an extended version associating molecular linkers (azides N3−, cyanides CN−, or even borohydrides BH4−). The divalent metal could also be replaced by mixed monovalent and trivalent metals, forming double perovskites A2BB′X6 structures. As well as for their polycrystalline counterparts, the capability of tuning the chemistry of perovskite single crystals is governed by the ionic ratio sizes of their components. The Goldschmidt tolerance factor (t) and the octahedral factor (μ) should be considered to design a stable and useful perovskite material. In Sections 2.2.1, 2.2.2, and 2.2.3, we will summarize the structure of the most commonly used perovskite formulations for SC growth, including lead‐based, lead‐free, and all inorganics.

2.2.1 Lead‐Based Perovskite Single Crystals

In Pb‐based halide perovskites, crystallization occurs in the ABX3 structure, which is isostructural to the initial perovskite oxide CaTiO3. The A and B cations coordinate with 12 and 6 X anions, leading to the 3D corner‐sharing cuboctahedral (12‐fold coordinated) and octahedral (sixfold coordinated) structures. In the MAPbX3 structure, the methylammonium position is disordered in the tetragonal phase from 160 K to room temperature, whereas it is ordered below 160 K in the orthorhombic phase (Figure 2.1) [1].

Lead‐based perovskite single crystals are the most studied to date. They correspond to the perovskite structures that have given record efficiency results in thin film solar cells. Thus, the information that can be extracted from the bulk of these materials will also be valuable for research on thin films. On the other hand, they can be synthesized by a wide range of methods, some of which are fully described in Section 2.3. Examples of different lead‐base perovskite single crystals already grown are shown in Figure 2.2[2].

It is well known that temperature influences the structural transitions of perovskite materials, having a direct impact on their optoelectronic properties. It is then essential to clarify the relationship between both phenomena in perovskite single crystals to understand their polycrystalline counterparts further as well. Ding and coworkers reported a depth analysis of the different crystal facets of MAPbI3 single crystal. They confirm the different atom densities for the different facets and how this influences ionic migration [3]. In Figure 2.3a, the X‐ray diffraction (XRD) pattern of facet (100) of both single crystal and powder diffraction is shown.

Figure 2.1 (a) Crystal structures for MAPbX3 single crystals. Cubic, tetragonal, and orthorhombic groups. (b) Tolerance factors for the different perovskite compositions, Pb and Sn containing.

Source: Reproduced with permission from Murali et al. [1]/American Chemical Society.

Figure 2.2 Photographs of Pb‐based perovskite SCs: MAPbCl3, MAPbBr3, MAPbI3, and FAPbI3

Source: Reproduced with permission from Liu et al. [2]/John Wiley & Sons.

Figure 2.3 X‐Ray diffraction patterns from the most commonly used lead‐based perovskite structures. (a) MAPbI3.

Source: Reproduced with permission Ding et al. [3]/American Chemical Society.

(b) MAPbBr3 single crystal.

Source: Reproduced from Wang et al. [4]/Springer Nature/CC BY 4.0.

(c) MAPbCl3 single crystal.

Source: Reproduced from Lee et al. [5]/MDPI/CC BY 4.0.

(d) Mixed MA and FA lead iodide single crystal stabilized.

Source: Reproduced with permission from Li et al. [6]/Royal Society of Chemistry.

In the case of MAPbBr3, the crystal belongs to the cubic Pmm space group at room temperature, as seen in Figure 2.3b from both single crystal and powder diffraction methods. However, the crystal experiences several structural transitions with lowering temperatures, from cubic to tetragonal, tetragonal to orthorhombic I, and orthorhombic I to orthorhombic II [4]. But this composition is not the only one suffering from this temperature instability; MAPbCl3 (Figure 2.3c) also shows two structural phase transitions: (i) from the cubic‐to‐tetragonal phase at around −95 °C and (ii) from the tetragonal to the orthorhombic when lowering to −116 °C [5].

The room‐temperature instability of the FA derivatives is well‐known in polycrystalline devices, and their monocrystalline partners are not spared either. FAPbI3 perovskite transforms from cubic phase to non‐perovskite phase, making its practical application difficult. Kuang and coworkers stabilized the black phase (cubic) by adding MA to the precursor solution. The XRD pattern of the mixed MA0.45FA0.55PbI3 perovskite single crystal, stable for 14 days, is shown in Figure 2.3d [6].

2.2.2 Lead‐Free Perovskite Single Crystals

Pb‐free SCs presume a variety of crystal structures, which are not necessarily limited to the typical ABX3 architecture. The replacement of Pb2+ ions leads to a nanoscale deformation and a depth change in the optoelectronic properties due to the differences in chemical valence and ionic size. In this case, the crystal structure depends mainly on substitutions, including group‐14 elements (Sn and Ge), adjacent elements such as Sb or Bi, and double elements (i.e. Bi combined with Ag) [7]. These substitutions extend from orthorhombic to trigonal lattice systems, generating quaternary structure A2B+B3+X6. A vacancy‐ordered double perovskite can also be synthesized by removing part of the B atoms from the octahedron center. Isolated clusters can be obtained using transition or post‐transition elements to form two face‐sharing [M2X9]3− octahedra. Below is a summary of the possible substitutes for lead to develop lead‐free perovskite single crystals (Figure 2.4).

Figure 2.4 Diagram of the main routes to replace Pb in perovskite crystals.

Source: Reproduced with permission from Zhang et al. [7]/John Wiley & Sons.

2.2.3 All‐Inorganic Perovskite Single Crystals

If perovskite does not contain any organic component, it can be categorized as all‐inorganic perovskite. The main advantage of this inorganic variant is the absence of organic cations due to their associated intrinsic instability. Therefore, the all‐inorganic perovskites show impressive thermal and environmental stability, opening new ways to further applications with limited restrictions.

Regarding the structure, all‐inorganic perovskite single crystals can be divided into three main categories: ABX3 type, special structures, and doped perovskites (Figure 2.5) [8].

Regarding the traditional ABX3 type, the principal protagonist lacking lead is the CsPbBr3 formulation. This inorganic single crystal is grown by the Bridgman method (described in Section 2.3) and the solution‐grown method.

He et al. optimized the growth conditions using the Bridgman method, considering the relation between the crystal's defects and an excessive temperature gradient [9]. After thoughtful optimization, CsPbBr3 with high purity was obtained, performing as a great gamma‐ray detector. Their work reported that CsPbBr3 exhibits two nondestructive phase transitions at low temperatures. The transition first occurs around 130 °C, from a cubic to tetragonal system. This is followed by a second‐order transition at around 88 °C to the orthorhombic phase, which is stable at room temperature.

Figure 2.5 Summary of all‐inorganic PSC: ABIB IIIX6 type, low‐dimensional type, mixed type, and ABX3 traditional type PSC.

Source: Reproduced from Wu et al. [8]/MDPI/CC BY 4.0.

Another structure that shows high stability in air and excellent optoelectronic properties is TlPbI3