Halide Perovskites’ Multifunctional Properties: Coordination Engineering, Coordination Chemistry, Electronic Interactions and Energy Applications beyond Photovoltaics

Abstract

Halide perovskite materials have gained enormous attention for their semiconducting properties, higher power conversion efficiency and potential applications in a wide range of fields of study, along with their two key limitations: stability and toxicity. Despite great progress made on halide perovskites and many promising research developments, the issues of stability and toxicity have not been fully resolved. Therefore, the coordination engineering of a new framework to obtain alternative new halide perovskite materials and a fundamental understanding of the coordination chemistry and electronic interactions forming the structure of these newly engineered halide perovskite materials are possible ways to overcome the issues related to both stability and toxicity. In this review, we comprehensively review the current development of halide perovskite families, both lead halide perovskites and lead-free halide perovskites, followed by the coordination engineering of the new frameworks to engineer new halide perovskite materials. All concerns regarding the fundamental ideas of coordination chemistry and electronic interactions are vital in forming halide perovskite structures and thus form the main aim of this review. We also discuss recent potential energy applications beyond photovoltaics and thus answer an essential and open question, ‘what could happen in the future of halide perovskites?’ in order to excite commercial enterprises and research institutions again as well as to motivate new predictions on the future continuity of this field.

1. Background

Photovoltaic devices based on hybrid halide perovskites, specifically (CH₃NH₃)PbI₃, are the subject of current studies because of their higher efficiencies and simplicity of production [1,2]. From the time when they were pioneered in 2009 [3], CH₃NH₃PbI₃ perovskite materials have fascinated many owing to their potential applications in economic power conversion photovoltaics. Currently, solar cell devices based on hybrid organic–inorganic halide perovskites have achieved a power conversion efficiency of 29.15% (www.nrel.gov) [4], and this is currently a quick-growing photovoltaic field [5]. This is a greater than nine-fold step-up in cell efficiency within seven years [6], and the device has a theoretical maximum beyond the 30% efficiency record [7]. Its hypothetical limit is superior to 30%, and this could surpass the competence of silicon, the hypothetical limit of which is bounded to 27% and reasonably and nearly bounded to ~25% [8]. Researchers have stated that perovskite solar cells can reach an efficiency of 45% (https://www.ossila.com/pages/perovskite-solar-cells-vs-silicon-solar-cells#:~:text=Researcers%20Break%20the%2030%25%20Efficiency%20Barrier&text=According%20to%20Stefaan%20De%20Wolf,reach%20efficiencies%20of%20around%2045%25, accessed on 27 May 2024). Regardless of this promising efficiency, there are two key challenges for the real applications of these types of solar cells: long-term stability [9] and toxicity [10,11] during large-scale production. Different research groups have been investigating the reasons for stability challenges such as the degradation of the organic part, i.e., sensitivity of the methylamine group and/or its derivative to the ambient environment [9]. In addition to this, it is important to study if the bonding and coordination chemistry of the constituent atoms and coordination engineering of the structure contribute to high power conversion efficiency and to many applications [12,13,14,15,16,17]. It is highly essential to know whether the electrostatic energy is essentially liable for the stability of halide perovskites or if the electron–electron interactions contribute to the stability and to the electrical properties of such compounds.

The great halide perovskite material with suitable electronic and optical properties for various applications in photovoltaics and beyond photovoltaics is CH₃NH₃PbI₃ perovskite [18,19,20,21]. But the toxicity of the lead atom and its environmental impact during the mass production of the metal and waste disposal become challenging. The difficulty in the removal of the toxic lead atom is due to the unsuccessful replacement of the toxic lead metal atom using other environmentally friendly metal atoms. Furthermore, why the lead atom cannot be replaced and why the produced lead-free perovskite materials cannot achieve the required properties and performance like the lead halide perovskite materials become ambiguous questions. Is there any special behavior of this lead metal atom over the other post-transition metal atoms such as Sn, In, Ge, Sb Bi and others? On the other hand, does the lead atom have special characteristics of coordination engineering in forming the structure of the CH₃NH₃PbX₃ so that lead halide perovskites achieve suitable electronic and optical properties in order to realize their current efficiency? How are the special bonding characteristics of the halide perovskite complex holding the structure, electronegativity properties of the constituent atoms and electronic interactions of the constituent atoms as well as their contribution expressed?

Although some reviews have been reported [12,22,23,24,25], nowadays, the coordination chemistry and coordination engineering of halide perovskites (both the hybrid organic–inorganic halide perovskites and fully inorganic halide perovskites) have become more essential to understand. Similarly, understanding the electronic interactions forming the structure of halide perovskites should receive more focus. Unless solutions are found to these issues, there will be no guarantee of realizing the currently confirmed and promising application and the future of these halide perovskite materials. For this reason, the aspiration of this article is broadly organizing the recent information about the families of halide perovskites; the coordination engineering; and the coordination chemistry and electronic interactions forming the structure of halide perovskites. Subsequently, the energy applications of halide perovskites beyond photovoltaics such as lasers, light-emitting devices, photodetectors and efficient nonlinear emission sources, CO₂ reduction and photocatalysis processes such as solar water splitting and HX splitting are reviewed thoroughly. Also, this review shares the current status of research, focusing on what will be the future of halide perovskite materials with a wide range of applications. Furthermore, its scope is to review the current issues on the coordination engineering and coordination chemistry of halide perovskites and electronic interactions that help form halide perovskite structures followed by their energy applications beyond photovoltaics in order to assist industries and the scientific community. The future direction of halide perovskite materials and a concluding remark are developed to address the major issues in the scientific community and to help readers improve their understanding of the field of perovskites.

Finally, this review concludes with an open question to the scientific community and commercial enterprises: what will be the future of these materials? Their instability and toxicity create strong doubt that leads to the open question: will their commercialization and mass production be successful in the future with the expected performance or not? Based on this essential information, a critical conclusion is well developed.

2. Halide Perovskites

Halide perovskite materials have been revolutionizing the area of photovoltaics with remarkable efficiency, and by this time, their diversity, with many perovskite derivative species, has become a common topic for researchers. In brief, these currently reported families and a list of halide perovskite derivatives, as well as the research development and expansion are summarized in Scheme 1 and

Table 1. Currently reported families of halide perovskites.

No.	Hybrid Organic–Inorganic Halide Perovskite	Inorganic Halide Perovskites
1	MAPbX3 [26,27]	Rb3Sb2I9 [28]
2	FAPbX3 [29,30]	Cs3Sb2I9 [31]
3	MASnX3 [32]	Rb6Pb5Br16 [33]
4	FASnX3 [34,35]	Rb6Pb5I16 [33]
5	MAxFA1−xPbI3 [36]	CsRbPb2I6
6	FA1−xCsxPbI3 [37]	Cs2SnI6 [38]
7	MA1–xFAxGeI3 [39]	Cs2TiIxBr6−x [40]
8	MAGeX3 [41]	Cs2InBiCl6 [42]
9	FAGeI3 [43]	Cs2InSbCl6 [42]
10	C(NH2)3GeI3 [43]	Cs2TiX6 [44]
11	MAPbxSn1-xBr3	Rb2CuInCl6 [45]
12	MAPb1−xInxI3Clx [46]	Rb2AgInBr6 [47]
13	FA0.8Cs0.2SnI3 [48]	Cs2BiAgCl6 [49]
14	(PEA)2(FA)8Sn9I28 [50]	Cs2AgBiBr6 [51,52]
15	(BA)2(MA)3Sn4I13 [53]	Cs2AgInBr6 [54]
16	(FA)x(MA)1−xSnX3 [55]	In2TiX6 [56]
17	(CH3)3NHGeI3 [43]	K2TiX6 [57]
18	CH3C(NH2)2GeI3 [43]	Cs3Bi2I9 [58]
19	C5H6NBiI4 [59]	CsPbX3 [60]
20	(H3NC6H12NH3)BiI5 [61]	RbPbX3 [62]
21	MA3Sb2I9 [63]	Cs1−xRbxPbX3 [64]
22	(FA)2BiCuI6 [65]	CsSnX3 [66]
23	(NH4)3Sb2I9 [48]	H3SPbX3 [67]
24	HC(NH2)2PbI3 [68]	CsGeI3 [69]
25	(CH3NH3)1−x(HC(NH2)2)xPbI3 [70]	Tl2TiX6 [71]
26	(HC(NH2)2)0.9Cs0.1PbI3 [72]	CuPbX3 [73]
27	[HC(NH2)2]x[CH3NH3]1−xPbI3 [74]	AgPbX3 [73]

.

Ordinarily, halide perovskite sensitizers depend on their 3D structure with the universal recipe AMX₃, where X = Cl⁻, Br⁻, I⁻; A is CH₃NH₃⁺ (MA) or HC(NH₂)₂⁺ (FA); and M is Sn²⁺ or Pb²⁺. The 3D organization is a progression of corner-sharing MX₆ octahedral involving the cubohoctahedral cavities, keeping up the electroneutrality of the framework. Along these lines, because of their tunability properties and the probability of different substitutions, different halide perovskites have been built, which is the focal point of the accompanying subsections. Following the announced performance of 3.8% (2009) [3], perovskite solar cells have risen to achieve performance beyond 22.1% in 2016 [75] and then 29.15% obtained from a monolithic tandem perovskite solar cell [4]. With such witnessed performance accomplished by straightforward construction forms, these devices are likewise extremely encouraging for supplementing silicon solar cells in a couple of arrangements.

2.1. HC(NH₂)₂PbI₃ and Its Derivatives

Among halide perovskites, CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃ are right now the hero hybrid organic–inorganic halide perovskite materials with ~22.1% record efficiencies. The larger size of HC(NH₂)₂⁺ results in a smaller band gap compared to the smaller size of CH₃NH₃⁺ with unknown reason. Normally, when the R-group (carbon chain) increases, the band gap becomes increased, and discourages achieving higher efficiency but encourages achieving better stability. It has been posited that the utilization of HC(NH₂)₂⁺ expands proficiency and predominant photostability [76,77,78,79,80,81,82] but brings down the dampness stability [83] of halide perovskites contrasted with CH₃NH₃⁺. The flimsiness of dark perovskite, HC(NH₂)₂PbI₃, is expected to be caused by the precariousness of the polar formamidinium itself near moisture. Notwithstanding its dampness insecurity, HC(NH₂)₂PbI₃ displays sudden concealment of photovoltaic impact as the framework experiences cubic-to-hexagonal progress after cooling.

2.2. (CH₃NH₃)x(HC(NH₂)₂)_1−xPbI₃ Perovskite

The paired cation perovskite of (CH₃NH₃)x(HC(NH₂)₂)_1−xPbI₃ was primarily accounted for by Grätzel [79]. To obtain a high-caliber and smooth halide perovskite film, the preparation of this blended perovskite (CH₃NH₃)_x(HC(NH₂)₂)_1−xPbI₃ was through a successive affirmation strategy by plunging PbI₂ in a CH₃NH₃I + HC(NH₂)₂I blended arrangement.

2.3. (HC(NH₂)₂)_1−xCs_xPbI₃ Perovskites

In order to improve photo- and dampness dependability, (HC(NH₂)₂)_0.9Cs_0.1PbI₃ has an elective light safeguard to CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃ [84]. (HC(NH₂)₂)_0.9Cs_0.1PbI₃ perovskite is shaped by the Lewis base adduct of PbI₂ [85]. The optoelectronic properties and photovoltaic execution of Cs-joined HC(NH₂)₂PbI₃ contrasted those of flawless HC(NH₂)₂PbI₃. An efficiency of 19.0% was estimated by the invert sweep and normal efficiency of 16.5% of the forward output exhibited by (HC(NH₂)₂)_0.9Cs_0.1PbI₃ film in a planar structure [84]. More critically, the steadiness of (HC(NH₂)₂)_0.9Cs_0.1PbI₃ film against the light and humidity was enhanced and contrasted with HC(NH₂)₂PbI₃ film. The improved photo and dampness security of hybridized Cs with HC(NH₂)₂PbI₃ started from the contracting of the cubo-octahedral volume and increment in the concoction association among HC(NH₂)₂⁺ and I [84]. Li et al. detailed that blending Cs with HC(NH₂)₂⁺ generously brought down the change in temperature from 165 °C to room temperature. By alloying the CsPbI₃, the resistance factor of the perovskite was tuned and a diminishing in stage progress temperature resulted from the change in the perovskite structure [86].

2.4. The Difficulty of Replacing Lead Atom by Other Metals

The essential inquiry regarding lead-free materials is the reason lead is so difficult to supplant, to which there is no simple answer. The properties for a B-Site particle are:

(a)

Ionic radius: This is best outlined by the tolerance factor. Replacing the toxic Pb in perovskite crystals needs an atom of similar size. The excellent performance of Pb-based perovskites is mainly because of high structural symmetry and strong anti-bonding coupling between Pb and I.

(b)

High polarizability: The lead (II) atom is considered to be a softer or borderline hard/soft cation that has polarizable outer electrons, large size, low electronegative and should interact most strongly with donor types. Typically, a soft cation will covalently bond with a soft donor atom, which has low electronegativity, highly polarizable low-lying empty orbitals and is easily oxidized, and a hard cation will form an ionic bond with a donor atom, which has high electronegativity, low polarizability, and high energy empty orbitals and is hard to oxidize.

(c)

Valence: A B site atom has in a perfect world a 2+ valence. Different configurations are conceivable, and they require remuneration to accomplish charge neutrality. Lead (II) has a stable oxidation state of +2 with a coordination number of 6. All six PbII-X bonds of the halogen ligands, the holo-directed structures in which the ligand atoms are connected to each other, are clearly ionic, but the ionic character of the bonds decreases as the atomic number of the halogen ligand increases and greater transfer of electron density from the ligands to the lead occurs as the electronegativity of the ligand decreases and the bond become covalent bond. If the arrangement is holo-directed geometry, the PbII-ligand bonds are all similar.

(d)

Lone pairs: Ideally, the B-site displays a 6s2 lone pair. When considering every one of these elements, of each of the 120+ elements, just lead has this alluring mix of properties.

3. Coordination Engineering of Halide Perovskite Crystals

Energy materials having superior characteristics encompass the perspective to stimulate upcoming technological development. Hence, finding other new perovskites becomes important in order to overcome two vital challenges: stability and toxicity. This section provides important concepts of the coordination engineering framework for engineering new perovskite materials with a special aim to improve stability and avoid toxicity as well as provide an insight into the future directions and new research horizons of this field. To improve device efficiency, the crystallization process is vital. During crystallization, there are two key factors that have to be considered: phase distribution and crystal orientation. Thus, phase control mechanisms are required to develop high-quality crystals required for a high-performance device. These phases have been synthesized as colloidal phases to achieve proper phase distribution and crystal orientation, as shown in Scheme 2a–d. Size distribution, absorption spectra and phase transformation are quite different in different solvents, such as dimethyl acetamide (DMAc), DMAc: dimethylsulfoxide (DMSO), DMAc: toluene (TOL), and dimethyl acetamide toluene (HI). To understand this variation in size distribution and absorption spectra, applying coordination engineering is useful. As shown in Scheme 2, poorly coordinating solvents give proper size distribution and crystal orientation. Such properties are important for high device efficiency. This property is obtained by producing a colloidal precursor solution, which is a useful procedure for producing quality crystals required for proper size distribution and crystal orientation. This situation is useful to achieve proper charge transport and a thermodynamically stable solution [17]. Thus, coordination engineering is useful to obtain a colloidal phase with high crystal orientation.

3.1. Cation and Anion Order Engineering of New Halide Perovskite

Other than the previously mentioned structural flexibility, this halide perovskite material has generous compositional flexibility, which is achieved through ion order engineering. Subsequently, from the viewpoint of particle arrangement designing, concoction substitutions happen onto every one of the three locales of the aristotype structure. Following this, the anion site can house a higher amount of vacancies and molecules exchange (e.g., halides) together while cation exchanges go up against either arbitrary or ordered orientations as depicted in Scheme 3, which affects the attributes of the designed materials.

In the giant mass of A₂BB’X₆ perovskites, the B and B’ cations go up against an organized model that resembles cation and anion positions in the crystal salt structure (Figure 1). In excess of 400 revealed points of reference of shake salt can be found in A₂BB’X₆ perovskites [87,88]. As a general rule when the oxidation states of B and B’ differ by less than two, a disordered arrangement is observed (e.g., La₂CrFeO₆) [89,90], while a variation larger than two almost always constructs an ordered arrangement (e.g., Sr₂NiWO₆) [91]. When the variation in oxidation states is just two, disordered (e.g., Sr₂FeRuO₆) [91], partially ordered (e.g., Sr₂AlTaO₆), or fully ordered (e.g., Sr₂YNbO₆) arrangements can result, depending on differences in size and/or bonding preference of the B and B’ cations [92,93,94]. There have been various broad reports of B-site cation arrangements in perovskites, and the forces that drive B-site cation ordering are normally understood and reported elsewhere [88,92,95].

3.2. ABX₃ Perovskite

The stability issues in halide perovskite materials become a default challenge for practicing. Thus, looking for other analogous materials with similar octahedral arrangement (i.e., 3D arrangement of corner-sharing octahedral BX₆ units) [97,98,99] or other materials that might fulfill the vision of the perovskite community (i.e., edge sharing octahedral arrangement) is a must. In this section, a representative cubic crystal structure was taken on by ABX₃ perovskite halides [100] as shown in Figure 2A, where A and B are twelve- and six-fold organized, and have plus one, plus two supposed charge states, correspondingly, whereas X [101] is a halide. Edge-sharing non-perovskite structures is a true universal in arrangements with ABX₃ stoichiometry (for example CsNiF₃ and CsCoCl₃ crystal structures) [100]. From the obtainable data on formability of ABX₃ structures, the possibility to create a model and forecast by means of an acceptable correctness whether a suggested structure through known option of cations with +1, +2 and halide with −1 charge ought to be a halide perovskite or a non-perovskite is shown in Figure 2B.

3.3. ABX₆ Perovskite

The breakthrough with a novel ABX₆ crystal showing superior performance leads to a broad occasion for lucid representation for sophisticated optoelectronic and solar cell function [103]. Whereas the photovoltaic characteristics of Sb(III) iodides were well explored previously, halide complexes of Sb(V) continue uncultivated. These halide complexes of Sb(V) materials are characterized as hued substances [104,105] and, subsequently, viewed as accommodating materials for photovoltaic cells. Substance arrangement and precious crystal structure of the items emphatically rely upon the idea of organic cations A, while no unmistakable relationships empowering an objective material plan have been built so far [106,107,108].

3.4. A₂BX₄ Perovskite

A₂BX₄ halide perovskites are two-dimensional materials with BX₆ octahedra [109], prompting adaptable mechanical properties and valuable light emission [110,111]. Furthermore, these halide perovskites can be differentiated by consolidating either divalent metal or an extended organic cation chain suggestive of numerous other imaginative bearings that could strengthen and enhance the usefulness of these materials.

3.5. A₂BX₆ Perovskite

The A₂BX₆ framework is another material that is expected to contribute. For instance, in recent times, Cs₂SnI₆ was introduced; its exceptional electronic and optical properties make it a capable applicant with novel efficiency [112]. Furthermore, this outline technique has likewise propelled the comprehension of the basic security for the perovskite sunlight based device, since two-dimensional [PEA]₂[MA]₂Pb₃I₁₀ has demonstrated superior protection from moisture [113] conceivably because of the hydrophobicity of the benzene ring.

3.6. A₃B₂X9 Perovskite-like 3D Framework

Most of the new attempts have spotlighted the examination of halide complexes of the post-transition-group-15 elements, for example, Bi and Sb. Alongside the spearheading writing about BiI [114] and A₃Bi₂I₉ (A = MA or Cs) [115,116,117,118], the scope of antimony (III) halides explored in photovoltaic cells is constrained to A₃Sb₂I₉ [28,31,119]. Both Cs₃Bi₂I₉ and MA₃Bi₂I₉ show different advantages over Pb- or Sn-based perovskites [120,121]. This is because of the upsides of non-danger, encompassing stability and low-temperature arrangement processability, which gives a promising answer for locating the poisonous quality and stability issues.

3.7. A₂BB’X₆ Double Perovskites

The journey for a total freedom from Pb continues in the halide perovskite photovoltaic. To understand 3D perovskite design, one must look for favorable circumstances for high efficiency. Twofold perovskite with 3D structure receives incredible consideration this time. As of late, Zhao et al. [122] found, through first-standards counts, a prosperous set of quaternary halide perovskites through A₂B⁺B³⁺X₆ in the transformation’s course of Pb²⁺ particles into one monovalent particle (B+) and one trivalent particle (B³⁺), as appears in Figure 3. The new perovskite viably kept away from harmful Pb²⁺ cations. All these have inborn thermodynamic solidness, appropriate band holes, few bearer-compelling masses, and low excitation-restricting energies. This suggests to us a potential strategy for taking out harmful Pb in PSCs. It might likewise be important to dope different molecules into the CH₃NH₃PbI₃ cross section, or to change to different materials, for example, Cs₂InSbCl₆, Cs₂InBiCl₆, Cs₂BiAgCl₆ and Cs₂AgBiBr₆, to enhance the conventional CH₃NH₃PbI₃ [49,122,123,124,125]. However, testing systems of perovskite photovoltaics with regard to lead still should rely on point-by-point examinations through first-principle computations.

3.8. AA’B₂X₆ Double Perovskite

Gulzhanat et al. performed ab initio calculation of CsRbPb₂I₆ halide perovskites with a ferroelectric AA’B₂X₆ perovskite framework [126]. The watched unconstrained polarization of AA’B₂X₆ perovskite materials is relied upon to be one of the critical properties that decide the efficiency of perovskite-based sunlight-based cells. The unit cell of perovskite contains 20 atoms. The space group of the [001] layered super cell is Pmc21 and [107] the rock salt super cell space assembly is Pna21; this is really a polar space bunch that appears in Figure 4.

3.9. AA’BB’X₆ Double Perovskite

Within twofold AA’BB’X₆ crystal structures through one:one proportions of components, layered, columnar and rock salt become unique potential outcomes for requesting on every A- and B-site cation sub-cross section [128]. On account of columnar requesting, cations of a similar kind are only ceaseless along one measurement and the shape sections of the interfacing Cl octahedral, while for the instance of layered requesting cations of every sort, frame-stacked 2D planes are relevant. Finally, rock salt requesting is relevant to the majority of symmetric one out of three conceivable outcomes, in light of the fact that the example of the A-site or B-site is comparable to the anion and cation arrangement observed from rock salt structures. For this situation, cations substitute in every one of the three symmetrical headings. The halfway replaced cations at A- and B-destinations result in an aggregate of nine unique potential outcomes for CsRbCaZnCl₆ (Scheme 4).

4. Coordination Chemistry of Halide Perovskite Structures

The investigation of lead halide structures (Scheme 5) is well established [129]. Metals with a s² electron setup (e.g., Ti⁴⁺, Sn²⁺, Pb²⁺, Sb³⁺) speedily encounter complexation with halide particles. Lead crystal structures, typically referred as “plumbates” (e.g., triiodoplumbate (PbI₃⁻)), fill in as forerunners in the arrangement. Right when separated in DMSO, PbI₂ is dreary, yet becomes a darker overwhelming supply of iodide particles. As planning dissolvable ligands superseded by the I- course of action of PbI₃⁻ and PbI₄²⁻crystals, the imperativeness of cutting down charge-trade retention groups is observed [129]. These new advances can quickly pursue and give an understanding of complexation events in the forerunner arrangement. The spectral study shows that CH₃NH₃PbI₃ has both a charge-separated band gap state (760 nm) and a charge transfer band (480 nm). Such bands are assigned to PbI₄²⁻ and PbI₃⁻, respectively, where this photoexcitation involves the existence of dual-nature excited states at longer and shorter absorption wavelengths [129]. This property is an unusual property of CH₃NH₃PbI_3, where the charge separated band gap state is responsible for light absorption and the charge transfer band at 480 nm is responsible for the low binding energy of electrons contributing to the high photocurrent and open-circuit voltage inducing high charge separation.

Figure 5 indicates retention spectra of PbI₂ recorded at various I⁻ focuses in DMF. The reliance of ingestion on the I⁻ fixation empowers estimation of complexation constants, as appear in Equations (1) and (2), respectively [130].

$$Pb{I}_2+I^{-}\leftrightarrow Pb{I}_3^{-}\left(K_1\right)$$

(1)

$$Pb{I}_3{}^{-}+I^{-}\leftrightarrow Pb{I_4}^{2^{-}}\left(K_2\right)$$

(2)

The K₁ and K₂ are in complexation equilibrium, where K₁ = 54 M⁻¹ and K₂ = 6 M⁻¹ [129]. Here, the environment of iodide anions does not affect the complex formation of Pb²⁺ with iodide anions. Close spooky highlights relating to lead halide structures were in similarly observed when CH₃NH₃I was supplanted with KI. An issue by and large examined in considering the coordination and stereochemistry of donor metals is that of the ‘stereochemical motion’ of valence shell lone electron sets [131,132,133,134]. There are two effects of the electrochemically active lone pair 6s² electrons, i.e., a gap in the coordination geometry caused by the occupation of coordination site and decreasing coordination number caused by the shortening of the Pb-X bond in the octahedral structure [131]. In addition to the stereochemistry of the 6s electron, there are other effects such as spin–orbit coupling, energetic stabilization and relativistic orbital contraction of s and p orbitals with the 6s electrons of the inert pair effect [132,133]. The stereochemically inactive 6s electron becomes stereochemically active owing to decreasing the size of the atom and increasing the covalence of Pb-X bonding [134].

4.1. Coordination Chemistry of Post-Transition-Metal Atoms

Of all p-block elements, lead(II) has a specific interest for coordination chemists [135], as it can receive a wide range of geometries in its complexes, permitting a tolerance for ligand arrangements that is not seen in, for instance, d-block components. The capacity to tie well to donor atoms characterized as soft and hard makes lead (II) an intriguing metal owing to its unique coordination chemistry. Lead(II) structures have also pulled in incredible intrigue in view of lead’s huge particle, changeable coordination number and the conceivable event of a stereochemically dynamic solitary combination of 6s² external electrons, in addition to novel system topologies [136]. Consistent with the hard–delicate corrosive base hypothesis, the moderate coordination capacity of Pb (II) implies that it can adaptably facilitate little nitrogen or oxygen atoms as well as huge sulfur atoms [137]. The examination of “stereo-synthetic action” of valence shell electron solitary matches in polymeric and supramolecular mixes might be additionally intriguing [138]. Lead (II) has an electronic structure [Xe]4f¹⁴5d¹⁰6s². The 6s orbital contracts and settles owing to relativistic effects. This settled 6s sets lessens its cooperation in the component’s science (turning into an “inert-pair”) and this clarifies why inorganic Pb creates lead organometal in a lower oxidation state than would be normal from its group number [139]. The clear hesitance of the 6s electrons to assume a role in the component’s science may likewise influence the stereochemistry of Pb (II) structures. This impact can be comprehended as long as basic valence shell electron-pairs or hybridization lead to arguments [140]. The 6s orbital, despite its change, can hybridize with the 6p orbitals to give a “stereochemically energetic” 6s electron combine possessing one position in the coordination circle of the metal. Since the pair is not specifically discernible, its manifestation is typically recognized by a void in the coordination’s conveyance bonds (asymmetrical coordination (hemi-directed), as seen in Scheme 6). On the off chance that hybridization does not happen and the combination has just a s orbital character, at that point, it is “stereochemically suppressed” and the complex does not show a hole or void in the security appropriation (symmetrical coordination (holo-directed) in Scheme 6) [139].

4.2. Proposed Ion Exchange and Ion Mixing Chemistry in Perovskites

Figure 6 shows the various ion exchanges, substitutions and ion mixing attempted for engineering new perovskite families. The figure provides a simple basis for a discussion and is indicative of many perovskite structures for many optoelectronic applications. If the proposed cation and anion exchange and substitution become successful, then there is improving the stability and overcoming the toxicity: the two most challenging issues for future commercialization of halide perovskites. These exchanges and substitutions and mixing of ions take place at the A site, B site and X site and the ion mixing can take place at all sites.

Figure 7 shows the total viewpoint of the strategy used for a halide substitute method of cesium lead halide quantum specks. Halide substitute is useful for obtaining the cubic CsPbI₃ that is normally gained at higher temperatures. Hoffman et al. [141] used a CsPbBr₃ quantum dots film and transformed it to a cubic CsPbI₃ arrangement by driving the CsPbBr₃ quantum dots layer (~75-nm thickness) into the iodide forerunner. In this way, the technique for the halide trade process in cesium lead halide quantum specks is direct and this strategy can swear off using the surface functionalization [142].

4.3. Coordination Chemistry of Single-Crystal Complex Formation

Grasping a crystal development system is extraordinarily significance for headway and manufactured strategies for additional functions. Typically, valuable precious crystal development in solution is segregated via three central forms: in situ change, dissolution and crystallization [144]. Furthermore, it has been established that the vitality of such valuable precious processes of crystal advancement decidedly relies upon the CH₃NH₃I concentration [145]. Then, lead iodide is completely organized with increasing iodine particles in order to frame an iodine prosperous condition (Equation (3)) [146]:

$${\mathrm{PbI}}_2{+\mathrm{xI}}^{-}\rightleftharpoons {\mathrm{PbI}}_{2+\mathrm{x}}^{\mathrm{x}-},(\mathrm{x}=1,2)$$

(3)

At that point, lead complexes continue to build units and recrystallize into a thermodynamically supported morphology within sight of ammonium cations (Equation (4)) [145].

$${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}{+\mathrm{PbI}}_{2+\mathrm{x}}^{{}^{\mathrm{x}-}}\rightleftharpoons {\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3+\left(\mathrm{x}-1\right){\mathrm{I}}^{-},\left(\mathrm{x}=1,2\right)$$

(4)

In this condition, the formation of CH₃NH₃PbI₃ from PbI₂ crystals depends on the amount of CH₃NH₃I concentration. Increasing the amount of CH₃NH₃I dissolves PbI₂ crystals, destabilizing its lattice framework and brings recrystallization of the solution to form CH₃NH₃PbI₃. Thus, the crystal growth kinetics of CH₃NH₃PbI₃ depend on the amount of CH₃NH₃I concentration. Similarly, in this in situ transformation of the crystallization process, the governing strategy is recommended to be the reactive crystallization process [147]. Such reactive crystallization, undergone in acetonitrile solvent, hence the formation of CH₃NH₃PbI₃, was fast with a stable powdered nanoparticle at micron size.

4.4. Coordination Chemistry Limits Crystallization of Halide Perovskites

The photovoltaic perovskite field is slowly shifting from highly defective and disordered perovskite solar cells [148,149] towards the generation of single-crystal perovskite devices [150,151], which can ultimately provide lower trap state densities, larger light absorption coefficients, lower energy losses and higher carrier motilities. Such preparation of single-crystal perovskite is successful via solvent engineering crystallizations [17,152,153]. During this crystallization, competition between iodides with solvent molecules to coordinate lead atoms will determine the species present during perovskite nucleation and growth [154]. Highly coordinating solvents such as DMSO will form partially covalent bonds with lead, Pb(DMSO)_xI₂, by bridging iodides and slowing crystallization kinetics, and its subsequent removal may be difficult. Poorly coordinating solvents like GBL cannot stabilize PbI₃⁻ moieties, enabling a fast reaction with the methyl ammonium cation. Importantly, the water present in environmental humidity is regarded as an additive that slows perovskite crystallization.

5. Electronic Interaction during Coordination Chemistry

In this section, we discuss bonding and complex bonding ideas, hydrogen bonding, electronegativity, electron localization, cation–anion orbital interaction and spin orbit coupling on the electronic structure as shown in Scheme 7. All these essential concepts result from electronic interactions and provide important electronic characteristics for much electronic functionality as discussed in next Section 5.

5.1. Bonding Idea in Lead Halide Perovskites

The lead halide perovskites are direct band gap materials. Despite the fact that this direct band gap nature is a substantial speculation, there are ongoing computations recommending special cases that happen in non-centrosymmetric halide perovskite on account of splitting [155,156,157,158,159,160]. CH₃NH₃PbI₃ in particular has unique properties different from the sp orbital semiconductors. The accurate explanation of band structure of this semiconductor is owing to its spin–orbit coupling and quasiparticle self-consistency [150]. Furthermore, electronic structure close to the band edge is primarily expressed as the fundamental BX₆ units [161,162,163,164,165,166]. Consequently, orbital outlines of segregated [BX₆]⁴⁻ bunches, similar to those in the zero dimension (CH₃NH₃)₄ [PbI₆]·2H₂O [167], give a base for thoughtful more multifaceted band structures. For [PbI₆]⁴⁻ units specifically, a Pb 6s−I 5p σ-anti-bonding orbital contains the most noteworthy involved sub-atomic orbital (HOMO), while Pb 6p−I 5p π-anti-bonding and Pb 6p−I 5s σ-anti-bonding orbitals include the least abandoned sub-atomic orbital (LUMO) (Figure 8a) [163].

The chemical bonds of the ions directly impact the perovskite structure; however, it can be challenging to directly link experimental results to theoretical calculations. Consequently, Brown [168] developed bond valence theory as a way of linking empirical bond lengths to theoretical bond valences. Bond valence theory also apparently predicts both bond length and stability (Equation (5)). The number resulting from Equation (4) is the total bond valence sum of that ion:

$$BV={\displaystyle \sum _N\exp \left[\frac{R_0-R_{A-XorB-X}}{b}\right]}$$

(5)

where BV is the valence of the A-X or B-X bond, R₀ and b are empirically determined parameters and R(A-X or B-X) is the experimentally determined A-X or B-X bond length.

In 2001, Lufaso and Woodward [169] used the bond-valence method to back-calculate ionic radii to calculate a so-called bond-valence tolerance factor. The bond valence tolerance factor (t_BV) may be a powerful method for predicting perovskite stability, but foreknowledge of the bond valence parameters is needed. It also fails to account for stoichiometric structural vacancies. In 2009, Ubic [170] derived a tolerance factor model as a function of the cubic/pseudocubic lattice constant, r_B and r_X (Equation (8)), which accounts for A-site point defects.

$$t_{BV}=\frac{R_{0\left(A-X\right)}-B\ln \left(V_A/N_A\right)}{\sqrt{2}{R}_{0\left(B-X\right)}-B\ln \left(V_B/N_B\right)}$$

(6)

where R_0(A-X) and R_0(B-X) are the unit valence bond lengths of the A-X and B-X bonds, V_A and V_B are the ideal valence states of the A and B cations, N_A and N_B are their coordination and B = 0.37. The ratios V_A/N_A and V_B/N_B are bond strengths for A-X and B-X, respectively [171].

$$t_1=\frac{a_c-0.05444}{0.66046\left(r_B+r_X\right)}-1.981012$$

(7)

where $a_c=0.05444+0.46701\left(r_A+r_X\right)+1.30838\left(r_B+r_X\right)$ and is cubic lattice constant.

The a_pc (average relative error = 0.60%) is given by Equation (8):

$$a_{pc}=0.06741+0.4905\left(r_{A\left(id\right)}^{VI}+r_{X\left(id\right)}^{VI}\right)+1.29212\left(r_B+r_x^{VI}\right)$$

(8)

where ${\mathrm{r}}_{\mathrm{A}\left(\mathrm{id}\right)}^{\mathrm{VI}}{,\mathrm{r}}_{\mathrm{B}}{\mathrm{and}\mathrm{r}}_{\mathrm{X}\left(\mathrm{id}\right)}^{\mathrm{VI}}$ effective ionic radii of A, B and X ions are in six-fold coordination. For the pseudocubic lattice, t₁ can be calculated as in Equation (9)

$$t_1=\frac{a_{pc}-0.011730139}{0.07209203\left(r_B+r_X\left(id\right)\right)}-1.760998$$

(9)

Therefore, these bonding parameters are helpful to understand how the bonding interaction, electronic interaction and coordination chemistry takes place, which give information on the distortion and tolerance factor of the structure formed. They also provide information on what bonding parameters are important during electronic interactions. This is primarily important in validating chemical structures.

5.2. Complex Bonding Idea in Lead Halide Perovskites

The leading complex bonding between the A site cation and BX₃⁻ complex anion is electrostatic. There is a strong electrostatic potential (~8 V) holding the cation at its lattice site. For instance, the positively charged ion, CH₃NH₃⁺, is within a negatively charged cage, PbI₃⁻. Further electrostatic role to the chemical bonding between the molecular dipole and the PbI₆ octahedral is the charge dipole interaction, which depends on the dipole orientation. Similarly, there is also the consequence of prime polarization. The expected induced dipole interaction is owing to the substantial polarizability of the iodide ions (ca. 7 × 10⁻²⁴ cm³). Because of these interactions, a molecular orientation correlation with octahedral deformation in molecular dynamic simulations is strongly expected [172] and more comprehensive investigations are continuing. Additionally, the van der Waals interactive forces together explain the intermolecular and Debye force interactions.

5.3. Electronegativity and Electronic Bandgap Tuning

From the perspective of band structure, band gap tuning and structure factor, it is recommended to consider all electronic interactions, such as electronegativity of each atom, making the semiconductor materials. For instance, the band gap for CH₃NH₃SnI₃ ranges from 1.2 to 1.4 eV, while 1.5–1.6 eV are the band gap values for CH₃NH₃PbI₃ [173,174]. Thus, Pb (1.87) has a smaller Pauling electronegativity contrasted to Sn (1.96) [174], indicating that the band structure of Pb states have to be higher and thus larger band gap values. Hence, Sn is less metallic in contrast to Pb; therefore, Sn-I interactions have to be less ionic in contrast to Pb-I interactions, suggesting that a smaller band gap belongs to CH₃NH₃SnI₃. As confirmed from the conduction band edges of CH₃NH₃SnI₃ and CH₃NH₃PbI₃ at −4.17 eV and −3.90 eV, correspondingly, i.e., Pb states positioned higher in the band structure of CH₃NH₃PbI₃ [175]. Hence, considering the influence of electronegativity on the band structure is vital.

5.4. Cation–Anion Orbital Interaction

A cooperation between any two orbitals ϕ_i and ϕ_j, be it degenerate or nondegenerate, prompts two new vitality levels that look as though the collaboration repulses the associating levels from one another. As needs be, the orbital connection of an anion with a cation naturally balances out, since it includes the cooperation of an unfilled level ϕ_i of a cation with a filled level ϕ_j (often, a solitary combine orbital) of an anion (Figure 9).

This vitality change corresponds to the square of the cover indispensable S_ij = <f_i|f_j> and is contrarily relative to the vitality distinction Δe_ij = |e_i − e_j| between the two orbitals, S_ij²/Δe_ij [170]. When a cation encased in an enclosure of anions (e.g., the A cation of ABO₃ in a B₈ block and proportionately in a confine of 12O²⁻ anions), the aggregate change vitality ΔE_tot is related with the cation–anion cooperation obtained by summing up every individual commitment as in Equation (10), This vitality adjustment corresponds to the square of the cover indispensable S_ij = <ϕ_i|ϕ_j> and is contrarily relative to the vitality distinction Δe_ij = |e_i − e_j| between the two orbitals, S_ij²/Δe_ij [176]. When a cation is encased in an enclosure of anions (e.g., the A cation of ABO₃ in a B₈ block and proportionately in a confine of 12O²⁻ anions), the aggregate adjustment vitality ΔE_tot related with the cation–anion cooperation is obtained by summing up every individual commitment, as in Equation (10),

$$\Delta E_{tot}={\displaystyle \sum _j\Delta E_j}\alpha -{\displaystyle \sum _j\frac{S_{ij}^2}{\Delta e_{ij}}}$$

(10)

The extent of the cover vital S_ij diminishes exponentially as the interatomic remove r_ij increments. This prompts some stretched and contracted cation–anion bonds inside the enclosure. The cation–anion orbital interaction is an essential way to understand bonding in tin-based halide perovskites, including crystal structure (Figure 10a), Br⁻ concentration dependent band gap (Figure 10b), band edges of MAPbI₃, MA(Pb_0.75Sn_0.25)I₃, MA(Pb_0.25Sn_0.75)I₃, MASnI₃ (Figure 10c) and phase change from orthorhombic into the yellow phase as shown in Figure 10d [50,177,178,179,180]. Here, under various conditions, the main interacting players are coordination engineering, coordination chemistry and electronic interactions of the metal cage and the organic cage that everyone shall understand. In-depth understanding of these concepts is vital to understand material formation process and properties. Not only these parameters but also chemical species such as halide concentration, phase of the material formed, orbital interactions and the role played by each factor are important points that have to be considered.

The crystal structure stability of such metal halide perovskites can be determined by the Goldschmidt tolerance factor (t) = (r_A + r_x)/(2^1/2(r_B + r_x)), where r_A is the ionic radius of the A position, r_B is the ionic radius of the B position and r_X is the ionic radius of the x position in the crystal structure. As shown in Figure 10b, the concentration of Br- ion varies the band gap. An increasing concentration of Br⁻ ion increases the band gap. This indicates that the optimum concentration of halide anions validated for optimum band gap to enhance absorption capacity of the semiconducting material. The orbital interactions, such as M-X₆ octahedral interaction, determine the origin of the band gap and hence the efficiency in a semiconductor as shown in Figure 10c. In halide perovskites, such as lead and tin-based perovskites, the conduction band dominantly contains p orbitals of the metal (M) while the valence band contains dominantly p orbitals of the halide anions forming M-X, X = I⁻, Br⁻, Cl⁻. Thus, the electronic structure of halide perovskites depends on the orbital interactions and whether they are with a higher band gap or a small band gap [179,181,182].

6. Properties of Different Halide Perovskite Structures

What properties play a guiding role in the future development of halide perovskite materials? This question is a great question with the possibility of flourishing the future development of halide perovskites in various wide ranges of applications. Halide perovskite structures have promising properties that nurture future development in various applications. Because of a high dielectric constant [183], unique ambipolar charge transport [184,185], high quantum yield photoluminescence [186] and optoelectronic properties [187] such as high carrier mobility, long diffusion lengths and high optical absorption [188], halide perovskites have an extraordinary advantage of high efficiency [4], low cost and compatibility with roll-to-roll fabrication technologies [189,190,191,192,193] including screen or ink-jet printing [194] and screen printing (Scheme 8) [195,196,197,198,199] from which the renaissance of halide perovskites succeeds in revolutionizing a wide range of applications [200,201]. The metallization of the perovskite solar cell also attracts its own attention for the future development of the field [202]. Thus, the structure–property relations of perovskite materials make them highly attractive for the future developments in durable perovskite solar cells and other applications [203]. As shown in Scheme 8a–c, the roll-to-roll fabrication of tandem perovskite solar module is prepared for commercial purposes.

7. Energy Applications of Halide Perovskites beyond Photovoltaic

In recent times, the energy function of halide perovskite beyond photovoltaics has lengthened with excellent results to light-emitting devices [204], opportunity for innovative and cutting edges for perovskite-based lasers [205], light-emitting diodes (LEDs) [206], and field-effect light-emitting transistors (FETs) [207], photodetectors, nonlinear emission sources, efficient water, CO₂ and HX splitting, photocatalytic activities, active material in lithium and sodium ion batteries, halide reservoir in catalysis system and piezoelectric generators (Scheme 9), which are the focus of this section.

7.1. MAPbI₃ as a Photocatalytic Material for HI Splitting

The simultaneous oxidation response engaged with the HX part delivers esteemed included synthetic substances, for example, I₂/I₃⁻, Br₂/Br₃⁻ or Cl₂, which have an assortment of employments in the vitality and cleanliness industries [208,209,210,211,212,213]. Indeed, many spearheading works show fruitful HX splitting [211,212,213,214]. By using a Nafion-isolated silicon micro-wire cathode, the HI part is accomplished with a 0.6% proficiency and unadulterated products [215]. Park et al [216]. utilized MAPbI₃ as a photocatalytic material in a unique balance with fluid HI arrangement. The vibrant harmony between the MAPbI₃ and the soaked fluid arrangement speeds up the reaction via replacement of I with Br. MAPbI₃ experiences a stage change to hydrated stages or PbI₂ at various particle exercises in the watery arrangement and is steady just in particular fixation scopes of I⁻ and H⁺. It was discovered that the MAPbI₃ powder in the watery HI arrangement could adequately part HI into H₂ and I₃⁻ under obvious light illumination, the proficiency of which could expand via warm toughening in a polar dissolvable environment and by utilizing a Pt cocatalyst [216].

7.2. Perovskite QD-GO Nanocomposite for Photocatalytic Reduction of CO₂

The reduction of CO₂ occurs via either electrochemical or photocatalytic reduction process. The look for a superior contender has not yet rested. Current and fast growths in halide perovskite materials have activated huge attention amongst investigators for optoelectronic functions, particularly solar cells [217,218,219]. Encouraged from the accomplishments of photovoltaic, these semiconductors are main contenders for performing proficient photosynthesis if the tremendous feature instability concerns of halide perovskites decided primarily [220,221]. Because of its improved stability, a CsPbBr₃ quantum dot/graphene oxide composite was created for the photocatalytic reduction of CO₂ to ethyl acetate (Figure 11) [222].

7.3. Halide Perovskite as Active Material for Battery

In parallel to photovoltaic uses, halide perovskites have been proposed as an active material for lithium-ion battery (LIB) anodes. Hybrid organic–inorganic halide perovskites such as methyl ammonium lead bromide (MAPbBr₃) exhibit reliable values of ≈200 mA h g⁻¹ with an outstanding rate potential [223]. These preliminary results are comparable to current commercial anode capacities. Xu et al. also demonstrated the utilization of perovskite-powered charging batteries of lithium amassed with a LiFePO₄ cathode and a Li₄Ti₅O₁₂ anode [224]. This device demonstrated a high electric change and capacity effectiveness of 7.80% and superb cycling dependability, which outflanks other revealed lithium-particle batteries. The newly introduced self-chargeable power units based on integrated halide perovskite solar cells and lithium-ion batteries hold promise for various functions.

Note that CH₃NH₃PbBr₃ accumulates two principal preferences: (i) it considers high inclusion fixations with x >> 1, and (ii) it displays little auxiliary mutilations. Critically, the rate ability does not show a huge decrease in charging flows between 1 °C and 0.25 °C, demonstrating great probability for functionalities in energy storage devices, such as battery.

7.4. Halide Reservoir in Catalysis Applications

From the perspectives of catalysis, it is important to find a material that reserve halides. In supporting this idea, in recent times, perovskite nanoparticles (P-NPs) have been prepared at the nanoscale through an extraordinary size and halide-tuned optical properties [60,225]. Of curiosity to the synthetic chemist is the apparent effortlessness with which P-NPs experience composition alteration by swapping using halides, as revealed at bulk [226] and at nano interfaces [143,227,228]. In addition, Doane et al. at that point found the capability of the P-NPs to continue as halide repositories for Finkelstein operation responses in halide perovskite, which give an uncommon colorimetric report of response kinetics [229]. It was hypothesized [229] that P-NPs might have the capacity to (1) fill in as a wellspring of high centralizations of halide synergist reservoirs [230], (2) screen-free halide spotlight alteration of amid halide disposal reactions [231] and (3) fill in as a quick subjective/quantitative colorimetric examine of free particles in arrangement. Figure 12 delineates these thoughts. As a halide supply (Figure 12a), the P-NP and its dynamic halide–ligand complex [232] supply halides in a nonpolar environment that can respond and operated with halide perovskites, whereas in the meantime displaying colorimetric input. As a measure (Figure 12b), tested response or question ions acquainted by means of P-NPs aliquots of recognized fixation whose shading change can give quantitative colorimetric reaction and merging, path a and b can give an immediate methodology toward colorimetric observing of compound responses continuously.

7.5. Piezoelectric Generators

It is very intriguing that a half and half piezoelectric nanogenerator in view of combinations of piezoelectric HC(NH₂)₂PbBr₃ nanoparticles and polydimethylsiloxane polymer was manufactured [233]. The HC(NH₂)₂PbBr₃ nanoparticles contain all around created ferroelectric properties with high piezoelectric charge coefficient (d₃₃) of 25 pmV⁻¹ [233]. The adaptable device showed superiority, with a greatest recordable piezoelectric yield voltage of 8.5 V and current thickness of 3.8 μAcm⁻² under occasionally vertical pressure and discharge activities. The exchanging vitality produced from nanogenerators utilized to charge a capacitor and light up a red light-transmitting diode through an extension rectifier. This outcome inventively grows the attainability of halide perovskites for function in a broad assortment of superior vitality collecting gadgets.

7.6. What Could Happen in the Future of Halide Perovskites?

The future of halide perovskites should answer three challenging issues: (1) Will perovskite solar cells achieve new breakthroughs beyond their current status of performance? (2) Will the future mass production and commercialization of these types of materials achieve the three ultimate goals of materials for optoelectronic applications: energy competent, low cost and environmental friendliness? (3) Will the currently proposed new framework of single and double perovskite materials achieve enough efficiency, like the efficiency of CH₃NH₃PbI₃ perovskites with improved stability and toxicity free new research horizon? These questions must be answered so that the commercialization and production processes become more economical, sustainable and environmentally friendly and their application may be realizable too. With these in mind, our question “what will be the future of these materials?” and “will their commercialization and mass production successful in the future with the expected performance or not?” are very important that everyone shall consider and dig for better development and application of the field. What we believe is the future development of halide perovskites is promising because of the high power conversion efficiency and wide range of applications raised from its structure property and simple synthesis and fabrication procedures for mass production. From all most challenges, both the toxicity of lead and moisture degradation of the organic cage are the most obstacles to practical applications. Because of these two reasons, there are doubts whether to eliminate halide perovskites or repair them to be commercialized well is still in progress.

The big doubt is owing to the release of lead toxic metal from industrial effluents as shown in Scheme 10. Furthermore, we hope that the commercialization of perovskite solar cell will be practical but (i) high stability and long lifetime, and (iv) low toxicity, (ii) controllable thin film deposition and growth, (iii) scalable and reproducible process must realize prior to commercialization [234,235]. There are strategies to eliminate and/or minimize the biological effect of lead metal released from a lead halide perovskite solar cell: (1) The chemical replacement strategy handles the development of lead-free halide perovskite solar cells [46,236,237,238]. (2) The lead detoxification strategy is the second strategy that can minimize the toxicity of halide perovskites [239,240,241]. (3) The chelation strategy [242] applies coordination bonding of organic molecules with lead metal to remove from industrial effluents during mass production of halide perovskites solar cells. (4) The lead uptake by plants strategy [243] applies plants to uptake lead metal released from the industrial effluents during manufacturing of lead halide perovskites. Plants are selected to store lead metal inside by absorbing it from the soil. This prevents the pollution of agricultural land from being polluted by lead released from industrial effluent. (5) The antioxidants strategy [244] is implemented by making reactive oxygen species interact with lead ion.

Scheme 10. Schematic diagram of the release of toxic lead from industrial effluents during commercialization. Reproduced with permission [245]. Copyright 2019, Royal Society of Chemistry.

Scheme 10. Schematic diagram of the release of toxic lead from industrial effluents during commercialization. Reproduced with permission [245]. Copyright 2019, Royal Society of Chemistry.

The most important application of halide perovskite materials is photovoltaic [246] and is superior to this application [247,248]. To realize this truth, the stability issue shall improve to the extent that it can give confidence to practicing. Moreover, perovskite-perovskite tandem solar cell has been reported as a promising cell for large-scale production [249]. Besides, MASnI_3–xBr_x based perovskite tandem solar cell with power conversion efficiency of 30.7% is reported, overcoming the toxicity problem of perovskite solar cells [250]. Many works also reported a path towards enhanced stability for commercialization by using strategies such as encapsulation [251,252,253,254,255,256], chemical replacement approach [256,257,258,259] and protective layers [260,261,262,263,264,265,266] such as molybdenum oxide, vanadium oxide to protect exposure from oxygen, moisture, high temperature and sunlight exposure [267], as well as fabrication of the perovskite tandem solar module [268,269,270,271,272,273,274,275] while others stated the scalability of module efficiency, achieving durability and stability, and process control, yield, etc., of manufacturing are vital for practical validations [234] There are different strategic choices to enhance the efficiency of perovskite solar cells: monolithic single-junction solar cell [4], Tandem perovskite solar cell [274,276], quantum dot solar cell [277,278,279,280,281,282] and metal nanoparticles with plasmon effect deposited on the photoactive surface induce a metallized perovskite solar cell [201,202,283,284]. These possibilities developed to overcome the limit that the rapid growth of the perovskite solar cell lacks suitable charge-selective contacts [285]. Implementing a tandem perovskite solar cell aimed at enhancing power conversion efficiency compared to a single-junction perovskite solar cell though it retains low cost of manufacturing [286,287,288].

As shown in Figure 13A, the bifacial tandem solar cell absorbs both direct sunlight and albedo light. While the bifacial single junction perovskite solar cells work under front irradiation and only rear illuminations, the bifacial tandem solar cells work from illumination from both sides. If illuminated from only one side, there happens a large current mismatch among the subcells. Figure 13B shows the scanning electron microscope image of all-perovskite tandem solar cells with the embedded structure of the light trapping properties of the device architecture. Figure 13C indicates increased external quantum efficiency (EQE) owing to the light trapping structure obtained from front illumination, while Figure 13D indicates illumination from the rear side. Figure 13E shows the J-V curve obtained from the bifacial tandem solar cell and Pb-Sn cell. Figure 13F shows the stability of efficiency (29.3%) up to 100 s obtained from the tandem solar cell under illumination with 30% albedo light.

The bifacial tandem all-perovskite solar cells give an EQE curve under front illumination and rear-side albedo light as shown in Figure 13G. Equivalent efficiency and J-V curve are indicated by Figure 13H,I. All these reliable data are great indications of illumination from both sides induced performance of bifacial all-perovskite tandem solar cells with embedded light-trapping structure. From these results, the bifacial tandem solar cell is an excellent alternative for commercialization of perovskite solar cell. This bifacial tandem solar cell also requires encapsulation for long-term stability during practical application [274]. This tandem structure gives increased efficiency, as shown in Figure 13F.

Tandem perovskite solar cell to be comercialized, it requires confirming thermal, moisture, oxygen and light stability [289,290,291,292,293]. In order to solve all these concerns, coordination engineering and coordination chemistry of the various organic and inorganic halid materials structure forming both single and double perovskites with their related electronic interactions should be carefully studied. Moreover, the strategic coordination engineering frameworks stated in this review article (Section 3) should get especial attention. Another wondering thing in these halide perovskite materials that make them possible for future development is their wide range of new applications! It highly applies in photovoltaic devices to meet the aim of energy demand, electronic devices such as lasers, photodetectors, phototransistors, LED and nonlinear emission sources to meet the goal of optoelectronic engineering, such as efficient water, CO₂ and HX splitting to meet photocatalytic goals, energy storage devices such as battery, and in efficient catalysis in order to achieve the purpose of halide reservoirs.

After all, which field of study is not enjoying with the application of these highly essential materials? Since halide anions are excellent redox mediators, halide perovskites may also be important in fields of membrane and reaction engineering owing to their ability as halide reservoir. They can be useful in reducing global warming by reducing and splitting CO₂. If we raise applications in biological system and life science besides the physical and chemical sciences: Growth of microorganism which needs materials that can absorb light at infrared and near infrared regions, halide perovskites fulfills this criterion. But the lead atom is toxic and may affect the growth of microorganism. Other environmentally friendly metal atoms, which could be important to achieve this goal, should replace this toxic metal atom. On another account, being toxic should also be important for some reasons. For instance, it would also wonder that if halide perovskite materials apply for agricultural aspects such as pesticides for killing some insects and organisms since lead atom is toxic, showing that halide perovskites are not only used as halide reservoirs but also toxic metal such as lead metal reservoirs.

8. Concluding Remarks

The halide perovskite field is a fast-growing research area with improved device efficiency and the photophysics properties for a wide range of applications, but it is less stable. Consistently, imperative research themes that have not received enough attention are a fundamental understanding of the coordination chemistry and coordination engineering as well as electronic interactions forming the halide perovskite structures in addition to their photophysics properties. Generally, grasping these fundamental concepts is quite relevant in five main concerns of this field: (1) stability improvements, (2) toxicity reduction, (3) discovery of new materials with multifunctional properties, (4) remarkable semiconducting properties and performance improvements and finally, (5) realizing the existing and new potential applications of halide perovskite materials in practice. All these enhancements take place as a result of the modifications at either A, B or X sites and modifications at all A, B and X sites.

(1)

Stability improvement as a way for flexible practical applications: Currently, this is the first challenge that blocks practical applications of halide perovskite materials. This limitation is not only for devices, but also the material itself is easily prone to degrade. As a key parameter for any optoelectronic applications, materials’ environmental stability and durability determine the lifespan of the device. Hence, in-depth sympathetic chemistry and engineering of halide perovskites is helpful in enhancing stability in two ways: (a) Enhancing the hydrophobic character of halide perovskites to overcome the solubility and dissolution of these materials. This can be done by increasing the carbon chain in the organic tail to reduce its hydrophilic character or to increase the inorganic character of the halide perovskites by completely replacing CH₃NH₃⁺ with water-resistant metal atoms such as Cs, Rb, etc. Using stoichiometric composition engineering of the organic tail with smaller amount of the organic tail could also enhance the hydrophobicity of these materials. (b) The coordination engineering framework of the halide perovskite structure can overcome the stability issues in these materials.

(2)

Toxicity reduction in mass production of halide perovskites: Toxicity is the second most challenging issue that hinders the commercialization and mass production of halide perovskites. Understanding the chemistry and engineering of halide perovskites is highly relevant to partially or completely avoid toxicity in these materials. This can be completed by (a) complete removal of the lead atom and replacing it with environmentally friendly metal atoms such as Ti, Sb, Bi, etc., or (b) completely replacing the lead atom with at least less toxic metal atoms such as Sn and Ge, which do not affect the environment significantly. (c) If both mechanisms are unsuccessful, mixing metal ions can be the last alternative for optimizing toxicity in lead-based halide perovskites. (d) If all these modifications are unsuccessful, engineering and other perovskite materials with new framework and new stoichiometric composition and structure could be the last alternative to avoid toxicity.

(3)

Enhanced semiconducting properties, such as optical and electrical properties, as well as efficiency enhancement. This basic intention of coordination chemistry and coordination engineering of halide perovskite materials is to improve optical and electrical properties and to design new material with better semiconducting properties for better performance.

(4)

Another exciting behavior of halide perovskites is their wide range of potential applications owing to enhanced semiconducting properties that may benefit from and require the fundamental concepts of chemistry and engineering in addition to their photophysics properties: A line of inquiry is whether halide perovskites are applicable beyond photovoltaic applications, for instance, (a) many optoelectronic devices such as lasers, LED, photodetectors, transistors and nonlinear emission sources, (b) photocatalytic activities such as efficient water, CO₂ and HX splitting, (c) storage devices such as active materials for LIB and Na ion batteries as well as halide reservoirs for catalysis purposes. Moreover, the discovery of new perovskite materials with multifunctional properties and improved semiconducting properties: this point of view may be important to fabricate a new device that fulfills the ‘Triple E’ rule: efficient, economical and environmentally friendly solar cell device.

Finally, in order to provide direction in order to the future continuity of this field, we have drawn a great concern: ‘what will happen in the future of halide perovskites?’ Would the new approaches, coordination chemistry and coordination engineering of halide perovskites, bring new research horizon for the future or not? This concern indicates the future fighting among the challenges and promising opportunities of halide perovskite materials, i.e., will this field stop or realized in the commercial enterprises and industries so that the future energy applications become powered by these materials? Moreover, this concern may also be important to draw new predictions in this field.