Luminescent Behavior of Sb³⁺-Activated Luminescent Metal Halide

Abstract

Metal halide perovskites have unparalleled optoelectronic properties and broad application potential and are expected to become the next epoch-making optoelectronic semiconductors. Although remarkable achievements have been achieved with lead halide perovskites, the toxicity of lead inhibits the development of such materials. Recently, Sb³⁺-activated luminescent metal halide perovskite materials with low toxicity, high efficiency, broadband, large Stokes shift, and emission wavelengths covering the entire visible and near-infrared regions have been considered one of the most likely luminescent materials to replace lead halide perovskites. This review reviews the synthesis, luminescence mechanism, structure, and luminescence properties of the compounds. The basic luminescence properties of Sb³⁺-activated luminescent metal halide perovskites and their applications in WLED, electroluminescence LED, temperature sensing, optical anti-counterfeiting, and X-ray scintillators are introduced. Finally, the development prospects and challenges of Sb³⁺-activated luminescent metal halide perovskites are discussed.

1. Introduction

Metal halide perovskites (MHPs) have attracted wide attention due to their excellent optoelectronic properties and solution processability [1,2,3]. The general formula for MHPs is ABX₃, where A is a monovalent organic or inorganic cation (e.g., Cs⁺, Rb⁺, or formamidinium (FA⁺)), B is a divalent metal cation (e.g., Pb²⁺, Cd²⁺, or Sn²⁺), and X is a halide anion (Cl⁻, Br⁻, I⁻) [4]. The history of MHPs dates back to 1893 [5], but it was not until 2009 that researchers became interested in them. Miyasaka et al. successfully applied MHPs to optoelectronic devices for the first time [6]. MHPs can self-assemble univalent cations of different properties and inorganic metal halide anions to form a new material, which can obtain a variety of derivatives by replacing organic molecules, ions, and ionic groups. Interestingly, MHPs are good at separating charge to generate electricity and aggregating charge to emit light [7,8,9,10]. In 2014, Tan et al. demonstrated for the first time at room temperature high-luminance LEDs based on solution-treated organometallic halide perovskites [11]. MHPs have made great progress in light-emitting diodes, solar cells, photodetectors, lasers, displays, and other fields over the past decade [12,13,14,15,16,17,18]. This is unmatched by other materials currently available for commercial optoelectronic devices, such as III-V semiconductors, organic materials, and conventional quantum dots (CdSe, CdS, etc.) [19,20]. MHPs have become an independent research field.

The most high-profile previous MHPs have relied on toxic lead, such as CsPbX₃ [21,22,23], (C₉H₂₂N₂)PbBr₄ [24], and CH₃NH₃PbI₃ [25]. This kind of MHP has great potential in light-emitting devices due to its tunable luminescence color [25,26], high photoluminescence quantum yield [27], and simple processing technology [28,29]. However, the toxicity of lead is restricted by consumer electronics regulations [30]. In addition, the instability of Pb²⁺ in light, humidity, heat, and other environments also hinders the further commercial application of MHPs [31,32]. Therefore, the development of efficient, stable, and non-toxic MHP is an urgent goal in this field. It is possible to replace the lead element with other non-toxic metal ions [33]. Previously, it was confirmed that the ⁶s₂p₀ electronic configuration of Pb²⁺ plays an important role in the excellent photoelectric properties of lead MHPs [34,35]. In general, metal ions with an ns² electron configuration have broad emission bands and large Stokes shifts, which can effectively avoid self-absorption and are ideal luminescent ions [36,37]. Therefore, metal ions with ns² electronic configurations such as Sb³⁺, Te⁴⁺, Bi³⁺, and Sn²⁺, which have similar electronic structures to Pb²⁺, have been proven to be a viable alternative. For example, Cs₃Sb₂I₉ [38], Cs₂TiCl₆ [39,40], Cs₂AgBiBr₆ [41,42,43,44], and Cs₂SnX₄ [45,46]. Sb³⁺-activated luminescent MHPs (Sb-MHPs) have been widely explored due to their emission bandwidth covering the whole visible light region, low toxicity, high PLQY, and high stability [47,48,49]. The unique singlet and triplet exciton emission mechanism can realize tunable emission in a large range of visible-near infrared [50,51,52,53], and this excellent optical performance is not possessed by most other metal ions. Currently, Sb³⁺ is considered to be one of the most likely materials to replace lead halides. For example, Cs₃Sb₂X₉ (X = Cl, Br, and I) is formed by the substitution of Sb³⁺ for 2/3 of the sites occupied in CsPbX₃. Another typical Sb³⁺-based metal halide, Cs₂AgSbCl₆, has a similar cubic structure to CsPbX₃ [37]. Their unique singlet and triplet exciton emissions have been discussed in many studies using a combination of experimental and theoretical methods.

In this review, the synthesis methods and luminescence properties of Sb³⁺-activated luminescent MHPs (Sb-MHPs) in recent years are reviewed, as well as the optical properties and luminescence mechanisms of different types and different emission colors of Sb³⁺-activated luminescent MHPs. Then, we discuss the latest situation and advantages of Sb³⁺-activated luminescent MHPs as emission layers in practical applications, such as the unique singlet and triplet emission of Sb³⁺ in a white light-emitting diode (WLED), anti-counterfeiting, temperature sensing, and other fields. Finally, we offer some suggestions on the challenges faced by Sb-MHPs and discuss their development prospects.

2. Synthesis

Since Miyasaka et al. prepared optoelectronic devices in 2009 [6], innovative breakthroughs have been made in MHP synthesis methods. Previously, most of the reported MHPs were prepared by the liquid growth method [54,55,56,57], and a few materials were prepared by vapor deposition [58,59]. This paper mainly discusses the most widely used liquid growth method in the preparation of Sb-MHPs. For example, the hot injection method and the ligand-assisted reprecipitation (LARP) method are used to prepare quantum dots (QDs) and nanocrystals (NCs). The crystal was prepared by the anti-solvent method, the hydrothermal method, and the solvent slow evaporation method. A thin film was prepared by the spinning coating method.

2.1. Quantum Dots (QDs) and Nanocrystals (NCs)

The most common method for the synthesis of Sb-MHPs, QDs, or NCs is the hot injection method. The hot injection method was first proposed by Murray et al. in 1993 for the synthesis of CdS QDs. In 2015, Protesescu et al. synthesized inorganic lead cesium halide perovskite QDs by the hot injection method for the first time [60]. In this method, the precursor solution is rapidly injected into the reaction solvent with a high temperature and boiling point, immediately forming nanocrystals [61,62]. When nanocrystals are synthesized by this method, the morphology, size, stability, and optical properties of nanocrystals can be controlled by changing the reaction temperature, crystal growth time, and ligand type [18,63,64]. In 2017, Pal et al. synthesized two Cs₃Sb₂I₉ NCs with different morphologies by hot injection [38], as shown in Figure 1a. They placed SbI₃ and ODE (1-Octadecene) in a three-necked round-bottom flask, degassed, and stirred at 80 °C for 60 min. OnA (octanoic acid) and OAm (Oleylamine) were added to the mixture at 80 °C under an N₂ atmosphere. Then the reaction temperature was raised to 140 °C to dissolve SbI₃. Afterward, pre-synthesized Cs-oleate was dissolved in the ODE at 100 °C and then rapidly injected into the reaction mixture at 180 °C and 230 °C. Within a few seconds, the reaction mixture turned cloudy red, and after 1 min, the reaction flask was immersed in an ice bath to obtain Cs₃Sb₂I₉ NCs. By changing the reaction temperature, they can adjust the morphology of Cs₃Sb₂I₉ NCs nanoplates (NPLs) and nanorods (NRs). They also obtained Rb₃Sb₂I₉ nanocrystals using a similar method. In 2021, Zhang et al. also used hot injection to obtain Rb₇Sb₃Cl₁₆ and Sb-doped Rb₃InCl₆ NCs [65]. In 2020, Cai et al. synthesized Cs₄CuSb₂Cl₁₂ double perovskite-type NCs by hot injection for the first time [66]. By gradually adjusting the material composition and feed ratio (Cs₄Cu_xAg_2−2xSb₂Cl₁₂, 0 ≤ x ≤ 1), the crystal structure and the corresponding electronic band structure band gap are realized.

Ligand-assisted reprecipitation (LARP) is also a common method used to prepare Sb-MHP QDs or NCs [31]. The simple process is to supersaturate and recrystallize the solution by changing the reaction conditions, but it requires certain ligands to precipitate the crystal, which can control the formation and growth of the crystal to the nanometer scale. The LARP method can be performed in the air using a simple device, as shown in Figure 1b. In 2017, Zhang et al. prepared Cs₃Sb₂Br₉ colloidal quantum dots by the LARP method [31]. They dissolved 0.3 mmol CsBr and 0.2 mmol SbBr₃ in 2 mL of DMF or DMSO, and then 10 μL of oleylamine or n-octylamine was added to the mixture to form a precursor solution. 1 mL of the precursor solution was vigorously stirred and quickly dropped into a mixture of 10 mL octane and 2 mL OA. After a few seconds of reaction, a strong blue emission of quantum dots can be observed. In 2019, Lv et al. synthesized Sb-based Cs₂AgSbX₆ (X = Cl, Br, or I) double perovskite quantum dots for the first time using LARP technology [67]. They rapidly dropped a mixture of DMSO, CsX, AgX, and SbX₃ into a mixture of oleic acid (OA) and ethyl acetate at room temperature, resulting in Cs₂AgSbX₆ quantum dots within seconds. The material has good air stability and strong blue emission, and the photoluminescence quantum yield is 31.33%. In 2022, Yang et al. synthesized highly efficient near-UV luminescence colloidal Cs₂NaBi_0.75Sb_0.25Cl₆ nanocrystals with a PLQY of 39.5% using LARP technology [68]. Different from the hot injection method, the synthesis temperature of this method is low, the reaction time is short, and it does not need inert gas protection. And it can easily mass-produce MHP NCs. However, it is difficult to obtain uniform-size products by this method.

Figure 1. (a) Synthesis of A₃SbI₉ (A = Cs, Rb) NCs by hot injection. Reprinted with permission from Ref. [38]. Copyright 2020 Wiley. (b) Synthesis of Cs₃SbX₉ NCs or QDs by thermal injection. Adapted with permission from Ref. [31]. Copyright 2017 American Chemical Society. (c) Synthesis of Cs₃Sb₂X₉ thin films by spinning coating. Reprinted with permission from Ref. [69]. Copyright 2019 American Chemical Society. (d) Synthesis of (TMA)₂SbCl₅ crystals by antisolvent method. Reprinted with permission from Ref. [48]. Copyright 2021 American Chemical Society.

Figure 1. (a) Synthesis of A₃SbI₉ (A = Cs, Rb) NCs by hot injection. Reprinted with permission from Ref. [38]. Copyright 2020 Wiley. (b) Synthesis of Cs₃SbX₉ NCs or QDs by thermal injection. Adapted with permission from Ref. [31]. Copyright 2017 American Chemical Society. (c) Synthesis of Cs₃Sb₂X₉ thin films by spinning coating. Reprinted with permission from Ref. [69]. Copyright 2019 American Chemical Society. (d) Synthesis of (TMA)₂SbCl₅ crystals by antisolvent method. Reprinted with permission from Ref. [48]. Copyright 2021 American Chemical Society.

2.2. Films and Single Crystals

The spin-coating method is commonly used to prepare Sb-MHP films by spinning the prepared precursor onto a clean substrate. In 2019, Singh et al. prepared the precursor solution by mixing SbI₃, CsI, and DMSO and then stirring continuously for 6 h at 70 °C [69]. The solution was dropped onto a substrate coated with poly (ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS), and the coating was then spun at 8000 rpm for 40 s to yield Cs₃Sb₂I₉ films. The films were preheated at 200 °C and annealed at 200 °C for 15 min, as shown in Figure 1c. Buonassisi et al. prepared three compounds, A₃Sb₂I₉, using a two-part spin coating method (where A is Cs, Rb, and K) [70]. Firstly, when AI and SbI₃ are dissolved in a mixed solution of DMF and DMFO, the molar ratio of AI and SbI₃ is fixed at 3:2. Sediment the Sb-based solution onto the pre-cleaned substrate through a two-step spin coating procedure (first step: 1000 rpm, 10 s; second step: 5000 rpm, 20 s). In the second spin coating step, chlorobenzene drops onto the rotating substrate within~5 s, and then the thin film is sintered on a hot plate in the glove box at 100 °C for 20 min.

With the rapid development of Sb-MHP single crystal preparation technology, anti-solvent methods, hydrothermal methods, solvent slow evaporation, and other methods have been widely used in the synthesis of various high-quality Sb-MHP single crystals. In the anti-solvent method, reactants are dissolved in a solvent to form precursors, and then an anti-solvent is added. Due to the combination of anti-solvent and solvent, the solubility of products to be produced in the solvent is reduced, and a supersaturated solution is produced, thus precipitating crystal products. In 2021, Wei et al. synthesized the (TMA)₂SbCl₅·DMF single crystal by the antisolvent method [48], as shown in Figure 1d. First, 0.5 mmol TMA·HCl and 0.25 mmol SbCl₃ (molar ratio 2:1) were dissolved in 2 mL and 1 mL DMF, respectively, and then mixed in a vial. Ethyl ether was placed in large vials and slowly diffused into 1 mL of mixed DMF solution at room temperature. After some time, colorless bulk crystals were obtained. In 2022, Peng et al. synthesized the (C₁₆H₂₈N)₂SbCl₅ single crystal by a similar method [71]. They completely dissolved SbCl₃ (0.2281 g) and C₁₆H₂₈NCl (0.5397 g) in DMF at room temperature (RT) to form a transparent solution. (C₁₆H₂₈N)₂SbCl₅ SCs were obtained by diffusing them into a DMF solution with diethyl ether at room temperature and then overnight. The products prepared by the anti-solvent method have high purity and few impurities, but more toxic solvents are used. In the hydrothermal method, reactants and solvents are added to the reaction kettle together, and the reactant raw materials can be completely dissolved under high temperature and pressure for some time. Slow cooling causes the solution to reach a highly supersaturated state and precipitate crystal nuclei, which begin to grow. The most important characteristics of the hydrothermal method are the high purity of products, fewer impurities, controllable crystal size, and low production cost. For example, Huang et al. synthesized a series of Sb-doped (Cs_1−xRb_x)₂InCl₅∙H₂O and (Cs_1−xRb_x)₃InCl₆ halide perovskites by hydrothermal method using concentrated hydrochloric acid (HCl) and methanol (MeOH) as reaction solvents [47]. They obtained products with different emission colors by changing the Cs/Rb feed ratio. In 2022, Chang et al. prepared a series of Sb-doped CD-based perovskites (Cs₇Cd₃Br₁₃, Cs₂CdCl₂Br₂, and Cs₃Cd₂Cl₇) with different phases by the hydrothermal method and effectively modulated PL emission by halogen substitution.

3. Optical Properties

3.1. Mechanism of Luminescence of Sb³⁺

The electronic configuration of the Sb³⁺ ion is 5s², corresponding to the electronic ground state ¹S₀, and the first excited state is nsnp, which gives rise to the four energy states of ¹P₁, ³P₀, ³P_1, and ³P₂ in the Russell–Saunders (RS) coupling, as shown in Figure 2a [72]. In this figure, E₀ represents the energy difference between the ground state and the excited state, H₀ contains the electron–nucleus interaction and Madelung potential, F is the electron-electron interaction resulting in Coulomb repulsion, and G is the exchange interaction. However, the RS coupling is an inappropriate approximation much larger than n, so the electron interaction (H_ee) + spin-orbit coupling (H_S·L) matrix is needed to find the energy level. The diagonal term in the spin-orbit coupling H_S·L (Russel–Saunders coupling) decompositions the ³P term into three components with J = 0, 1, 2, defines ³P₂, ³P_1, and ³P₀ as triplet states, and defines ¹P₁ as a singlet state [73]. The splitting of the energy levels is caused by the electron interaction and the spin-orbit coupling matrix, * indicates that ³P₁*, and ¹P₁* are not pure RS states but are formed from a mixture of off-diagonal matrix elements. The spin–orbit interaction separates the degenerate ³P level but also mixes the ³P₁ and ¹P₁ states. According to Hund’s rules, the energies of these excited states from ³P₀ < ³P₁ < ³P₂ < ¹P₁ start to increase. As a result, the transitions of ¹S₀–¹P₁ are parity-allowed, and the transitions of ¹S₀–³P₁ are spin-forbidden, but spin–orbit coupling relaxes this selection rule and is also parity-allowed (relaxation in the states of ³P₁ and ¹P₁). The ¹S₀–³P₁ transition is a lower-energy transition. The transition of ¹S₀–³P₀ or ¹S₀–³P₂ is strictly forbidden.

Under photoexcitation, the Sb³⁺ electrons move from the ground state ¹S₀ to the excited states ¹P₁ (triplet) and ³P₁ (triplet). ³P₁ and ¹P₁ can be coupled to different multiple phonons, resulting in singlet and triplet STEs [74,75,76]. The energy is transferred between singlet STEs and triplet STEs by intersystem crossing (ISC), which is generated by spin-orbit coupling. In general, S–T transformations have a temperature dependence, which is consistent with spin–orbit interactions in lattices, so singlet transitions are usually of low strength at room temperature (RT) [77,78]. Peng et al. proposed that (C₁₆H₂₈N)₂SbCl₅ is excited from the ¹S₀ excited state to the excited state (³P₁ and ¹P₁) under UV irradiation (such as 300 nm) [71]. Subsequently, excited electrons are rapidly transferred to triplet states via ISC and then returned to GS (ground state) to achieve radiative recombination of singlet and triplet STEs, as shown in Figure 2b. The transfer of power between singlet (S) and triplet (T) states also depends on temperature. Meng et al. proposed that excitation of Sb³⁺:RbCdCl₃ at 322 nm could produce singlet emission and triplet emission of Sb³⁺, and then presented singlet emission bands belonging to the ¹P₁–¹S₀ transition at low temperature (460 nm) and triplet emission bands belonging to ³P_n–¹S₀ at room temperature (596 nm) [79], as shown in Figure 2c. This fact reflects that the S-T transformation is strongly temperature-dependent. In addition, singlet and triplet excitons can exhibit two adjustable emission peaks at different excitation wavelengths. For example, Zhang et al. proposed the photoexcited radiation process of Cs₂ZrCl₆:XSb [80], as shown in Figure 2d. At 310 nm, the ground-state electrons are excited to the high-energy excited state, and the transitions of ¹S₀ → ¹P₁ and ¹S₀ → ³P₁ occur, accompanied by an energy transfer from ¹P₁ to ³P₁. At the same time, the electrons in the high-energy excited state can be captured by ¹STE and ³STE. Eventually, electrons recombine from ¹STE and ³STE to the ground state, resulting in two-band emission with a large Stokes shift. At low-energy photoexcitation (360 nm), the ³STEs exciton emission becomes stronger.

Figure 2. (a) Energy level diagram for an ion with the ns² ground-state configuration. Reprinted with permission from Ref. [72]. Copyright 1991, Elsevier. (b) Coordinate diagram of a photophysical process of (C₁₆H₂₈N)₂SbCl₅. Reprinted with permission from Ref. [71]. Copyright 2022, Springer. (c) Coordinate diagram of a photophysical process of Sb³⁺: RbCdCl₃. Reprinted with permission from Ref. [79]. Copyright 2022, American Chemical Society. (d) Coordinate diagram of a photophysical process of Sb³⁺: Cs₂ZrCl₆. Reprinted with permission from Ref. [80]. Copyright 2022, American Chemical Society.

Figure 2. (a) Energy level diagram for an ion with the ns² ground-state configuration. Reprinted with permission from Ref. [72]. Copyright 1991, Elsevier. (b) Coordinate diagram of a photophysical process of (C₁₆H₂₈N)₂SbCl₅. Reprinted with permission from Ref. [71]. Copyright 2022, Springer. (c) Coordinate diagram of a photophysical process of Sb³⁺: RbCdCl₃. Reprinted with permission from Ref. [79]. Copyright 2022, American Chemical Society. (d) Coordinate diagram of a photophysical process of Sb³⁺: Cs₂ZrCl₆. Reprinted with permission from Ref. [80]. Copyright 2022, American Chemical Society.

3.2. Structure and Luminescence of Sb-MHPs

3.2.1. Dimensions in Size

In addition to 3D perovskite, the Sb-MHPs system is consistent with other perovskite systems, and 0D structured quantum dots [65], 1D structured nanowires [38], and 2D structured nanosheets [81] can be prepared by a dimensional reduction in size, as shown in Figure 3a. Essentially, they are still composed of 3D metal halide frameworks; their properties are influenced by 3D perovskites and the quantum confinement effect; and they typically exhibit small Stokes shifts, narrow emission, and the quantum confinement effect [31]. Sb-MHP materials with different dimensions have excellent performance in optoelectronic properties and are often used to prepare various optoelectronic devices.

3.2.2. Dimensions at the Molecular Level

In the three-dimensional Sb-MHPs structure, when the cations located at the A site are replaced by large-sized organic cations, the complete three-dimensional perovskite structure will not be able to accommodate the large-sized organic cations due to the voids between the metal halide frameworks [82]. They are separated by layers of organic cations to form low-dimensional structures at the molecular level (isolated layer, isolated line, or isolated polyhedron), as shown in Figure 3b. In this case, the individual metal halide layers, long chains, or polyhedra inside the bulk crystal are isolated from each other by organic cations, enabling the bulk crystal to display the intrinsic properties of individual building blocks as well as quantum confinement effects at the molecular level. The transformation of 3D Sb-MHPs to lower dimensions allows great flexibility in the structure. The 2D Sb-MHPs are subject to quantum confinement and dielectric confinement effects in a specific direction, with a natural quantum well structure, large exciton binding energy (Eb), and excellent thermal stability [83]. The 1D Sb-MHPs behave at the molecular level as single, long chains separated by organic cations. The exciton-phonon interaction in the crystal is very strong, and the excitons are more likely to be self-trapped at this time, and they usually have large Stokes displacements [84]. The metal halide polyhedrons of 0D Sb-MHPs are separated by organic molecules, are independent of each other, exist alone, and form independent luminescent centers. It preserves the photophysical properties of individual metal halide polyhedra (e.g., strong Stokes shift, broadband emission, high PLQY, and long lifetime) [48]. Furthermore, a large number of broadband emissions with large Stokes shifts are observed in low-dimensional Sb-MHPs [50,85,86], generally believed to originate from excited-state STEs due to enhanced electron-phonon coupling. Electron-phonon coupling generally refers to the interaction between electrons and lattice vibrations, which significantly affects the electrical and optical properties of semiconductors. This is because organic cations have strong vibrational modes, and the incorporation of organic molecules can soften the lattice and generate deformation potential and local lattice distortion, which greatly enhances the electron-phonon coupling of the system. Therefore, in low-dimensional Sb-MHPs, the strength of electron-phonon coupling and the formation of STEs play crucial roles.

3.2.3. Coordination of Sb³⁺

In Sb-MHPs, the lone electron pairs of Sb³⁺ have a strong stereochemical effect on the coordination environment [87], and antimony halides can form coordination structures based on pyramidal MX₃ units [38], seesaw MX₄ units [88], square pyramid MX₅ units [48], and octahedral MX₆ units [88], as shown in Figure 3c. These coordination structures can exist in isolated units or in long-chain or planar forms through shared halogen atoms. Others exist in the form of polyaggregates. For example, M₂X₈, M₂X₁₀, M₂X₁₂, and M₃X₁₂ [89]. How these coordination structures affect the luminescence of Sb-MHPs is unclear, but the lone electron pairs of Sb³⁺ distort these coordination structures, resulting in lattice distortions that enhance the emission of STEs.

3.3. Sb³⁺-Doped and Un-Doped Luminescent Behaviors

3.3.1. Sb³⁺-Based

Fully inorganic Sb³⁺-based metal halides are usually present as A₃Sb₂X₉ (A = Cs, Rb; X = Cl, Br, I) crystals, which are formed by substitution of Sb³⁺ for sites in CsPbX₃ crystals. Cs₃Sb₂I₉ single crystals reported by Kyle M. McCall et al. have a broad asymmetric emission band at 560–770 nm, which is more typical of triplet-state STE emission [90]. In addition, the Rb₇Sb₃Cl₁₆ crystal powder was reported by Tang et al. [91]. The Rb₇Sb₃Cl₁₆ powder they synthesized has a broad STE yellow emission band at 560 nm with a PLQY value of 26%. For widely reported organic-inorganic hybrid Sb³⁺-based halide crystals, the luminescence is usually dominated by single- and triplet-state STE emission. Sb in Sb³⁺-based organic-inorganic hybrid metal halides usually exists in the form of Sb-X polyhedral clusters, which are separated by large organic molecules and form 0D structures at the molecular level. For example, we recently reported that (BTMAC)₂SbCl₅ has dual emission bands: the longer-lived triplet STE emission band (619 nm) and the shorter-lived singlet STE emission band (470 nm) [92]. Usually only the characteristic STE emission bands of Sb³⁺, i.e., triplet and singlet STE emission, are present in Sb³⁺-based metal halide crystals.

Currently, studies of Sb³⁺-based metal halide quantum dots usually focus on fully inorganic A₃Sb₂X₉ NCs. Such quantum dots usually exhibit narrow-band emission with band edges. Angshuman et al. reported Cs₃Sb₂I₉ nanorods and nanosheets with band-edge emission with half-peak widths of 32 nm and 30 nm, respectively [38]. Shan et al. realized an electrically driven violet LED at 408 nm by using Cs₃Sb₂Br₉ quantum dots as light emitters [93]. In addition, they synthesized Cs₃Sb₂X₉ quantum dots with different band gaps by adding different halogen ions, and their band-edge emission can be tuned in the range of 385~640 nm. This narrow band-edge emission of the quantum dots is clearly different from the STE broadband emission of the bulk.

3.3.2. Sb³⁺-Doped

In our previous work, we found that Sb³⁺ as a doping agent can introduce its characteristic STE emission into many metal halide crystals. We have observed STE emissions from Sb³⁺ in systems such as Sb³⁺-Doped RbCdCl₃ [94], Sb³⁺-Doped Cs₃InCl₆ [47], and Sb³⁺-Doped (C₁₃H₃₀N)₂SnCl₆ [95]. Compared to Sb³⁺-based halides, the STE emission efficiency generated by Sb³⁺ as a doping agent is particularly high. In addition, in some systems, doping Sb³⁺ can still retain the emission band of the host ion itself and tune different emission centers. For example, Li et al. synthesized a (BTPP) ₂MnCl₄:Sb with STE emission of Sb³⁺ and ⁴T₁–⁶A₁ transition of Mn²⁺ ions [96]. By changing the excitation wavelength, the emission of green, yellow, and orange can be adjusted. Li et al. also found a similar phenomenon in (TTPhP)₂MnCl₄:Sb³⁺ [97]. At the same time, they also found that STE emissions from Sb³⁺ and Mn²⁺ exhibit different thermal quenching effects, leading to the excellent temperature sensitivity of this material. As mentioned above, Sb³⁺ as a dopant can be introduced into multiple emission centers, enabling efficient, tunable emission.

Unlike the band-edge narrow-band emission of A₃Sb₂X₉ NCs, the emission of Sb³⁺ as a dopant in nanocrystals is usually derived from the broadband emission of STEs. Xu et al. observed triplet-state STE emission from Sb³⁺ in Sb-doped Cs₂NaInCl₆ NCs, showing blue emission with 84% high PLQY [26]. Hens et al. observed strong cyan emission in Cs₂CdCl₄:Sb³⁺ nanoplatelets, which also comes from the triplet state of Sb³⁺ [81].

3.4. Different Colors of Emission

3.4.1. Ultraviolet (UV) Emission and Blue Emission

At present, blue light exists in a large number of computer monitors, fluorescent lamps, mobile phones, digital products, LEDs, and other lights and has become an essential light source for daily life. A large number of reports have proved that Sb-MHP is a promising blue emission material without rare earth. Generally, when Sb-MHP is prepared into QDs, due to the strong quantum domain-limiting effect in the system, the emission peak usually shows a strong blue shift, and its emission wavelength may also be located in the ultraviolet or blue light region. In 2017, Zhang et al. reported a series of inorganic perovskite QDs (Cs₃Sb₂X₉) [31], in which Cs₃Sb₂Cl₉ QDs showed UV emission with an emission peak of 370 nm. At the same time, Cs₃Sb₂Br₉ QDs with blue (410 nm) emission were obtained by an anion exchange reaction, as shown in Figure 4a. Both can maintain high quantum yields (10~46%) and full width at half maximum (FWHM) of 40~60 nm. Compared with Cs₃Sb₂Br₉ SCs, the QD emission peak of Cs₃Sb₂Br₉ QDs showed a blue shift of 120 nm, showing a strong quantum confinement effect. This all-inorganic lead-free perovskite with high PLQY and stable emission quantum dots offers great potential for efficient emission candidates. Subsequently, Zhang et al. reported Cs₂AgSbX₆ QDs [67]. Figure 4b shows the absorption spectrum and photoluminescence (PL) spectrum of Cs₂AgSbCl₆ QDs, showing an exciton absorption peak at 325 nm and an emission peak at 409 nm. The inset shows a photograph of QDs illuminated with a 325 nm UV lamp, where bright blue emission can be observed. A blue shift of about 120 nm was similarly found compared to the previously reported bulk single crystals [98], also indicating a considerable quantum confinement effect of perovskite QDs. At the same time, they also mentioned blue-emitting Cs₂AgSbBr₆ QDs with an emission peak of 478 nm. Lead-free Sb-based blue emission perovskite quantum dots exhibit excellent optical properties, making them highly promising candidates for future optoelectronic applications.

Sb³⁺ in Sb-MHP transitions from the excited state ³P₁ to the ground state ¹S₀ under photo-excitation; this low-energy transition might also be in the blue region. In 2020, Noculak et al. reported Sb-doped Cs₂NaInCl₆ single crystals [99], which exhibited broadband blue PL emission with an emission peak at 445 nm (Figure 4c), and the Stokes shifts were 110 nm. In particular, when doped with 5%Sb, its PLQY reaches the highest value of 79%. While the undoped Cs₂NaInCl₆ compound showed no PL emission at RT. This blue emission can be attributed to the ³P₁–¹S₀ transition and belongs to the triplet emission of Sb³⁺. At the same time, Gray et al. also reported the intense blue emission of Cs₂NaInCl₆:Sb³⁺ SCs with 445 nm (Figure 4d) as the emission center and a PLQY of 79% [100]. They suggested that the reduced Stokes shift of Sb³⁺ triplet emission in Cs₂NaInCl₆ double perovskite relative to other types of perovskites might be due to the coordination number 6 of Sb³⁺ in Cs₂NaInCl₆. In addition, Cs₂NaInCl₆:Sb³⁺ exhibits excellent air and water stability, and these properties make it a promising blue phosphor for applications.

3.4.2. Cyan Emission and Green Emission

The Sb-MHPs of Cyan emissions have also been widely reported. The source of the cyan and green emissions is the triplet emission of Sb³⁺. Previously, we found that the emission tunability of MHPs can be achieved in Sb³⁺:(Cs_1−xRb_x)₃InCl₆ by controlling the Rb/Cs feed ratio. When the Rb/Cs feed is 2/1 or 2/5, ultra-high-efficiency cyan emission can be achieved, as shown in Figure 5a [47]. The PLQY of this highly efficient emission of cyan phosphor can be up to 90%. Alternatively, Locardi et al. also found cyan emission from the triplet of Sb³⁺ in two-dimensional Cs₂CdCl₄:Sb³⁺ nanoplatelets (Figure 5b) [81]. The nanoplatelets make it possible to use these compounds as optoelectronic materials. Recently, we found cyan emission from the triplet state of Sb³⁺ in Sb³⁺-doped Cs₂KInCl₆ double perovskite, as shown in Figure 5c [101]. Sb³⁺-doped Cs₂KInCl₆ double perovskite has the three-dimensional structure of a co-angular octahedron, good photoelectric performance, and high structural stability, which makes it a good candidate material for further photoelectric applications.

As one of the three primary colors of color light, green light is a sensitive color of the eyes and is widely used in lighting, transportation, medical treatment, and other fields. The Sb-MHPs of green emissions have attracted much attention due to their PLQY, low toxicity, low cost, and other advantages [94,102]. In 2020, Biswas et al. reported the green-emitting organic–inorganic hybrid Sb-based metal halide C₁₂H₅₂Cl₁₈N₈O₄Sb₃ [102]. The optical properties of this compound showed a strong and broad photoluminescence (PL) emission band centered at 517 nm with a FWHM of 110 nm. The long emission lifetime component they observed highlights that radiative emission (phosphorescence) originates from triplet excited states (³P₁). In the same year, Majher et al. reported green emissions in Sb³⁺-doped In-based MHPs (Rb₃InCl₆) [85], as shown in Figure 5d. The distorted crystal structure of Rb₃InCl₆:Sb³⁺ results in a larger Stokes shift (1.29 eV) and lower energy emission (λmax = 522 nm) compared to the blue-emission Cs₂NaInCl₆:Sb³⁺. Recently, researchers reported some green emissions with highly efficient and stable Sb³⁺-doped Cd-based MHPs, which has attracted extensive attention. Xia et al. doped Sb³⁺ into Rb₄CdCl₆ and achieved a bright green emission peak at 525 nm with a PLQY of 70.2%, as is shown in Figure 5e. We report green-emission perovskites such as Sb³⁺-doped Rb₃Cd₂Cl₇ [94] and Sb³⁺-doped Cs₃Cd₂Cl₇ [49] (Figure 5f), both of which are produced by triplet STEs of Sb³⁺. The emission peak of Sb³⁺-doped Rb₃Cd₂Cl₇ is 525 nm with a Stokes shift of 200 nm, and its PLQY is 57.47%. The emission peak of Sb³⁺-doped Cs₃Cd₂Cl₇ is 517 nm, and its PLQY is 66%. These high-efficiency and stable green-emitting phosphors have great application potential in lighting, transportation, medicine, and other fields.

3.4.3. Yellow Emission and Orange Emission

In our previous work, we found that Sb³⁺ exhibits yellow emission in low-dimensional structures [48,79]. Singlet and triplet emission bands can be observed in both (TMA)₂SbCl₅·DMF and Sb³⁺-doped RbCdCl₃ under high-energy optical excitation. The high-energy emission band belongs to the ¹P₁–¹S₀ transition, and the low-energy emission band belongs to the ³P_n–¹S₀ transition (where n = 0, 1, 2). In addition, there are numerous reports of similar phenomena. Zhang et al. fabricated novel 0D perovskite structures ([Emim]₈[SbCl₆]₂[SbCl₅]) by self-assembly of isolated [SbCl₅]²⁻ pyramidal structures and [SbCl₆]³⁻ octahedral structures with [Emim]⁺ cations [103]. [Emim]₈[SbCl₆]₂[SbCl₅]) has a broad yellow emission peak at 577 nm at an excitation wavelength of 354 nm, as shown in Figure 6a. Recently, Li et al. reported Sb-based MHPs (β-[DHEP]SbCl₅·2H₂O) with an emission peak of 552 nm (Figure 6b) and PLQY of 93.35% [78]. Its applications in information security, optical logic gates, and rewritable PL paper are proven. In addition, Shi et al. reported a highly emissive Sb³⁺-doped (C₆H₁₈N₂)InCl₅·H₂O with an emission peak of 565 nm (Figure 6c) and a PLQY of 74.6% [104]. Interestingly, (C₆H₁₈N₂)InCl₅·H₂O:Sb is a promising methanol detector. When it was exposed to methanol vapor, the emission peak shifted sharply from 565 nm to 663 nm, and after a few minutes in the air, the emission returned to 565 nm. These Sb-MHPs of yellow emission have shown great potential in the fields of solid-state lighting, anti-counterfeiting, and gas detection.

A large number of Sb-MHPs have been reported as a new orange emission source [105]. Xia et al. synthesized two Sb-based organic–inorganic hybrid MHPs ((C₆N₂H₁₆)SbCl₅ and (C₆N₂H₁₆)SbCl₅·H₂O) by a simple method [106]. The two compounds exhibit similar broadband orange emissions under UV excitation. (C₆N₂H₁₆)SbCl₅ exhibits an orange emission peak at 613 nm under UV excitation at 360 nm with a PLQY of 25.3%, as shown in Figure 6d. The emission peak of (C₆N₂H₁₆)SbCl₅·H₂O is red-shifted to 620 with a PLQY of 39.6%. They believe that the increase in PLQY is mainly due to the incorporation of water molecules, leading to more localized photoelectrons. Recently, we also reported orange emission of Sb-based MHPs ((C₁₆H₂₈N)₂SbCl₅ SCs) with a PLQY of 97%. Under UV excitation, (C₁₆H₂₈N)₂SbCl₅ exhibits a double emission band with an emission peak at 477 nm and 633 nm, which is generated by singlet and triplet STE emission bands, respectively, as shown in Figure 6e. In addition, orange emission was also reported in Sb³⁺-doped MHPs. A series of Sb³⁺-doped 2D [NH₃(CH₂)₄NH₃]CdBr₄ by Wu et al. Efficient energy transfer between [NH₃(CH₂)₄NH₃]CdBr₄ and Sb³⁺ dopant results in strong Sb³⁺ triplet emission and near-unity PLQY (Figure 6f). These orange emitters with excellent optical properties are of great significance for the study of MHPs.

Figure 6. (a) PLE and PL spectra of [Emim]₈[SbCl₆]₂[SbCl₅]. Reprinted with permission from Ref. [103]. Copyright 2021, Royal Society of Chemistry. (b) PLE and PL spectra of β-[DHEP]SbCl₅·2H₂O. Reprinted with permission from Ref. [78]. Copyright 2022, American Chemical Society. (c) ABS, PL, and PLE spectra of C₆H₁₈N₂)InCl₅·H₂O:Sb. Reprinted with permission from Ref. [104]. Copyright 2022, Springer. (d) ABS, PL, and PLE spectra of (C₆N₂H₁₆)SbCl₅. Reprinted with permission from Ref. [106]. Copyright 2020, Wiley. (e) PL and PLE spectra of (C₁₆H₂₈N)₂SbCl₅ SCs. Reprinted with permission from Ref. [71]. Copyright 2022, Springer. (f) PL spectra (λ_ex = 335 nm) of [NH₃(CH₂)₄NH₃]CdBr₄-X%Sb. Reprinted with permission from Ref. [86]. Copyright 2021, American Chemical Society.

Figure 6. (a) PLE and PL spectra of [Emim]₈[SbCl₆]₂[SbCl₅]. Reprinted with permission from Ref. [103]. Copyright 2021, Royal Society of Chemistry. (b) PLE and PL spectra of β-[DHEP]SbCl₅·2H₂O. Reprinted with permission from Ref. [78]. Copyright 2022, American Chemical Society. (c) ABS, PL, and PLE spectra of C₆H₁₈N₂)InCl₅·H₂O:Sb. Reprinted with permission from Ref. [104]. Copyright 2022, Springer. (d) ABS, PL, and PLE spectra of (C₆N₂H₁₆)SbCl₅. Reprinted with permission from Ref. [106]. Copyright 2020, Wiley. (e) PL and PLE spectra of (C₁₆H₂₈N)₂SbCl₅ SCs. Reprinted with permission from Ref. [71]. Copyright 2022, Springer. (f) PL spectra (λ_ex = 335 nm) of [NH₃(CH₂)₄NH₃]CdBr₄-X%Sb. Reprinted with permission from Ref. [86]. Copyright 2021, American Chemical Society.

3.4.4. Red Emission and Near-Infrared Emission

Red emission materials are widely used in display, transportation, lighting, medical treatment, and so on. Some studies have shown that Sb-MHPs have good application potential in the field of red emission materials [90]. For example, Ma et al. reported an efficient red-emitting (Ph₄P)₂SbCl₅ bulk single crystal with an efficient broadband red emission band at 648 nm with a PLQY of about 87% [107], as shown in Figure 7a. Recently, we synthesized highly efficient and stable Sb:Cs₂InClBr₄·H₂O crystals by anionic component engineering. The synthesized Sb:Cs₂InClBr₄·H₂O crystal has a wide emission band for self-trapping excitons at 680 nm (Figure 7b) [108], and its photoluminescence quantum yield is as high as 93%. These Sb-MHPs of highly efficient red-emission have greatly facilitated the development of low-toxicity metal halide perovskites and their applications in optoelectronic devices.

The near-infrared light source has been widely used in medical treatment, biosensing, health monitoring, spectroscopy, and other fields because of its low thermal effect, non-destructive nature, and good penetration [109,110]. The current near-infrared fluorescent materials all face the advantages of low efficiency, short lifetime, poor stability or complexity, etc. [111]. MHP with excellent performance has great application potential in the field of luminescence. In 2021, Xia et al. reported near-infrared emission of Cs₂ZnCl₄:Sb³⁺ with an emission peak at 745 nm (Figure 7c) and a PLQY as high as 69.9% [50]. The broadband NIR emission comes from triplet STEs emission from doped [SbCl₄]⁻ polyhedra (³P₁ → ¹S₀). Cs₂ZnCl₄:Sb³⁺ has good air/thermal stability. When Cl⁻ is replaced by Br⁻, the luminescence peak is red-shifted from 745 nm to 823 nm. Recently, Xia et al. again reported three Sb-based MHPs with NIR emission, as shown in Figure 7d [89]. Compounds 1 ((C₁₃H₂₂N)₂Sb₂Cl₈), 2 ((C₁₀H₁₆N)₂Sb₂Cl₈), and 3 ((C₁₆H₃₆P)Sb₂Cl₈) exhibit wideband near-infrared emission peaks at 865, 990, and 1070 nm under excitation at 350, 345, and 335 nm, respectively. These NIR emissions are all from the triplet emission of Sb³⁺. These environmentally friendly and efficient near-infrared emission Sb-MHPs greatly expand the potential applications of metal-halide perovskites.

3.4.5. White Emission

White emissive materials are considered to be an effective replacement for solid-state lighting, the next generation of lighting sources [112]. The general strategy is to display complementary light in the three primary colors (red, green, and blue). However, there are problems such as the non-uniformity of the luminescent properties of the compounds and different light attenuation and temperature-dependent properties [113]. Therefore, there is a need to find efficient single-composites of white emissive materials to generate white light. It was confirmed that Sb³⁺ with unique singlet and triplet excitons can exhibit blue and orange emission bands [114]. Therefore, singlet white light emission can be achieved by adjusting the singlet/triplet emission of Sb³⁺ ions. Shan et al. reported the dual emission band of Sb³⁺-doped Cs₂ZrCl₆ [80]. By changing the doping amount of Sb³⁺, the ratio of double emission bands can be adjusted to produce white light, whose PL spectrum is shown in Figure 8b. The best Cs₂ZrCl₆:1.5%Sb exhibited high-quality white light (Figure 8a) with a CRI of 96 and a PLQY of 52.48%. In addition, Lin et al. introduced %1.5Sb³⁺ into PA₆InCl₉ to obtain a white phosphor with nearly 100% PLQY and a color rendering index (CRI) close to 90 [115]. Recently, we reported a white luminescent material based on (C₁₃H₃₀N)₂SnCl₆:20%Sb [95], which generates strong white emission under 325 nm excitation and exhibits nearly 100% PLQY. These studies are bound to provide new insights into the preparation of single-component white phosphors for next-generation lighting technologies.

3.5. The Tunable Emission of Sb-Activated MHPs

The ability of halogen-substituted anion exchange reactions to modulate emission bands has been widely reported in CsPbX₃ (X = Cl, Br, and I) [23,60]. Such halogen ion substitution reaction-tuning emission bands have also been widely reported in Sb-MHPs. Song et al. obtained Cs₃Sb₂X₉ QDS by anion exchange reaction; the luminescence wavelength can be adjusted from 370 nm to 560 nm [31]. Zhao et al. reported all-inorganic double perovskite Cs₂AgSbX₆ quantum dots [67]. Changing the precursor’s halogen composition, the PL emission wavelength was tuned from 409 nm to 557 nm. Subsequently, Shan et al. synthesized Cs₃Sb₂X₉ quantum dots by the LARP method [93]. After in-depth research, they found that the halogen-rich surface of NCs has a quantum well structure, which is conducive to efficient radiative transitions. In addition, the emission spectrum of the product was adjusted by changing the composition of the halide, and the tunable wavelength range was 385–640 nm, as shown in Figure 9a. Tunability is significantly better than other lead-free perovskite quantum dots. Compositional variability enables further tunable optical properties of Sb-MHPs. Our previous work reported that the tunable emission of Sb-MHPs was achieved by modulating the composition. For example, Huang et al. achieved tunable green-yellow emission by controlling the structural transformation of Sb-doped indium halides A₃InCl₆ and A₂InCl₅∙H₂O (A = Cs, Rb) [47]. Recently, Chang explored a phase-selective solution synthesis route to obtain a series of different Cd-based perovskite derivatives doped with Sb³⁺ [49]. Combined with the halogen substitution method, a series of Cd-based perovskite derivatives with different crystal structures and emission spectra (517~625 nm) were obtained, which can achieve tunable emission from cyan to orange, as is shown in Figure 9b. In addition, by introducing Sb³⁺ ions into (NH₄) ₂SnC_l6 through a doping strategy, we achieved for the first time a change in excitation light from 360 nm to 390 nm at room temperature, making the emission dynamically tunable from yellow to near infrared (NIR) [116]. A hybrid STE with Sb and host is proposed for the first time to emit tuned luminescence.

3.6. Summarization of the Separated

3.6.1. Dimensionality

As mentioned above, Sb³⁺ activated luminescent metal halide bulk materials typically exhibit broadband STE emission. In metal halide materials, the formation of localized self-trapped excited states is closely related to their structural dimensions, and reducing the dimensions will make it easier for excitons to self-trap. The quantum confinement effect in the 0D structure is strong, and STE produces efficient luminescence with a particularly high PLQY. For example, the PLQY of 0D-(MePPh₃)₂SbCl₅ is 99.4% [117]. There are few reports on 1D-chain Sb-based metal halides. In 2021, Zhang et al. reported two 1D chain-shaped Sb-based metal halides, (2cepyH)SbCl₄ and (2cepyH)SbBr₄ [118]. No emission behavior was observed at room temperature for (2cepyH) SbBr₄. (2cepyH) SbCl₄ exhibits yellow emission at 570 nm at room temperature, with a FWHM of 115 nm and a lifetime of 6.85 μs. This is the first time broadband emissions have been observed in one-dimensional antimony-based hybrid materials. This emission behavior is basically consistent with the triplet STE emission behavior of Sb-based metal halides in 0D. But the PLQY of (2cepyH) SbCl₄ is very low, only 4.5%, which may be due to severe concentration quenching in one-dimensional chain structures. Currently, the reported 2D antimony-based metal halides are all inorganic, such as Rb₇Sb₃Cl₁₆, Rb₃Sb₂I₉, etc. [90,91]. These bulk materials exhibit broadband STE emission, with emission behavior consistent with 0D-Sb-based halides, but PLQY is relatively low (Rb₇Sb₃Cl₁₆ 26%).

3.6.2. A-Position Cation

Previously, Buonassisi et al. reported on the effect of A-site cations on all-inorganic Sb-based metal halides and found that A-site cations dominate the structure and photoelectric properties of these compounds [70]. Cs₃Sb₂I₉ has a 0D structure and an indirect bandgap (Figure 10). Rb₃Sb₂I₉ has a 2D structure and a direct bandgap. K₃Sb₂I₉ has a 2D structure and an indirect bandgap. The ion radius of Cs⁺ is greater than Rb⁺ and K⁺, and Cs₃Sb₂I₉ tends to form a 0D structure. However, the PLQY of these compounds is very low, which may be due to the small size of A-site ions. Due to the large size of organic cations, most of the reported organic-inorganic hybrid Sb-based metal halides have a 0D structure [37]. Compared to all-inorganic Sb-based metal halides, the PLQY of organic-inorganic hybrid Sb-based metal halides is much higher, some even close to 100% [117].

3.6.3. Coordination

The coordination of Sb³⁺ ions in Sb³⁺-based hybrid metal halides can affect emission, and different coordination structures can be found in different compounds. The 4-coordinated seesaw (SbX₄), 5-coordinated pyramid (SbX₅), and 6-coordinated octahedron (SbX₆) are relatively common. Mao et al. conducted extensive experiments on 4-coordinated and 5-coordinated Sb-based metal halides [119]. They found that 4-coordinated Sb-based metal halides did not exhibit emission behavior at room temperature, while 5-coordinated Sb-based metal halides had a high PLQY (>90%). Kundu et al. successfully synthesized the SbX₆ compound C₁₂H₅₂Cl₁₈N₈O₄Sb₃ and the SbX₅ compound C₁₂H₅₀Cl₁₄N₈O₃Sb₂ by controlling the ratio [102]. The former exhibits green emission (517 nm), while the latter exhibits yellow emission (590 nm). V. Kovalenko et al. also reported a similar situation, where [C@Ba]₄[SbCl₆]₂[Sb₂Cl₈] with SbX₆ exhibited green emission (550 nm), while [C@Cs]₂CsSbBr₆ with SbX₅ exhibited yellow emission (600 nm) [120]. SbX₅ typically exhibits low energy emissions, while SbX₆ exhibits relatively high energy and green emissions.

3.7. Summary of Luminescence Performance of Sb-Activated MHPs

In summary, the optical properties of Sb³⁺ show great differences among Sb-MHPs with different structures.

Table 1. Comparison of Optical properties of Sb-based inorganic metal halide perovskites.

Chemical Formula	PLE/PL Peak (nm)	Color	FWHM (nm)	PLQY	Lifetime (ns)
Cs3Sb2Cl9 QDs [31]	/370	violet	52	11%	2.96
Cs2AgSbCl6 QDs [67]	325/409	blue	70	31.33%	10.09
Cs3Sb2Br9 QDs [31]	375/410	blue	41	46%	4.29
Cs2NaInCl6:5%Sb [99]	330/445	blue	~80	79%	1029
Cs2AgSbBr6 NCs [67]	440/465	blue	82	0.7%
14%Sb3+:Cs2KInCl6 [101]	290/495	cyan	~95	99.2%	2990
Cs2CdCl4:5%Sb3+ NPs [81]	280/510	cyan	103	20%	1625.9
Sb3+:(Cs1−xRbx)3InCl6 [47]	320/513	cyan	~105	91.8%	3090
C12H52Cl18N8O4Sb3 [102]	360/517	green	110	45%	4800
20%Sb3+:Rb3Cd2Cl7 [94]	325/525	green	115	57.47%	19,400
0.4%Sb3+:Rb4CdCl6 [77]	340/525	green	116	70%	1845
Cs3Sb2Br0.5I0.5 QDs [67]	/531	green		90%
Sb:Cs2CdCl2Br2 [49]	355/559	yellow	123	35.23%	2140
β-[DHEP]SbCl5·2H2O [78]	365/552	yellow	122	93.35	10,550
Rb7Sb3Cl16 NCs [65]	325/556	yellow	101	≤1%
[Emim]8[SbCl6]2[SbCl5] [103]	354/577	yellow	146	11.2%	2250
C24H88Cl25N16O4Sb3 [102]	360/590	yellow	140	43%	5200
Rb7Sb3Br16 SCs [105]	350/600	orange	≈95
(TPA)2SbCl5 [75]	375/610	orange	130	95.3%	5300
Sb:Cs7Cd3Br13 [49]	375/625	orange	147	57.42%	2170
Cs3Sb2I9 SCs [90]	473/632	red	94
Rb3Sb2I9 SCs [90]	473/646	red	117
C12H50Cl14N8O3Sb2 [102]	340/650	red	160	6%	5600
Sb:Cs2InClBr4·H2O [108]	380/680	red	180	93%	3310
Sb3+:Cs2ZnCl4 [50]	316/745	near-infrared	175	69.9%	12,800
(C13H22N)2Sb2Cl8 [89]	350/865	near-infrared	~230	5%	1870
(C10H16N)2Sb2Cl8 [89]	345/990	near-infrared	~260	3%	1580
(C16H36P)SbCl4 [89]	335/1070	near-infrared	~280	1%	1340
Cs2ZrCl6:1.5%Sb [80]	310/480,580	white		52.48%	5.62/61.64
PA6InCl9-Sb [115]	282/468,580	white		90.6%
(C13H30N)2SnCl6:20%Sb [95]	325/510,670	white		99.32%	23.48/7760

summarizes a large number of existing Sb-MHPs and their optical parameters. As mentioned above, the unique singlet and triplet exciton emission mechanisms of Sb³⁺ can realize tunable emission and single-color emission in a wide range of visible to near-infrared wavelengths. Sb-MHPs with excellent optical properties and environmental friendliness will have the opportunity to replace toxic lead halide perovskite materials and have great application potential in solid-state lighting, LED, anti-counterfeiting, temperature sensing, photodetection, and other fields. However, what factors affect the triplet and singlet states of Sb³⁺ and the relationship between them is still unclear.

4. Applications

4.1. Light Emitting Diode (LED)

4.1.1. White LED

At present, blue wafers (InGaN) coated with yellow phosphor (YAG:Ce³⁺) are widely used to produce commercial WLEDs [121]. Here, WLEDs with a high color rendering index using different Sb-MHPs as luminescent layers are reviewed. In our previous report, we fabricated WLEDs by mixing orange-emission Cs₂InCl₅·H₂O:Sb³⁺ with commercial blue (Ba₂SiO₄:Eu²⁺) and green (BaMgAl₁₀O₁₇:Eu²⁺) phosphors, as shown in Figure 11a [75]. The CIE coordinates of the prepared WLED are (3.48, 3.50) (Figure 11b), the color rendering index CRI is 93.2, and the CCT is 4900 K. The CIE coordinates located in the white light region and the ultra-high CRI demonstrate that (TPA)₂SbCl₅ is a promising down-conversion phosphor for WLED. However, this strategy of mixing several phosphors to obtain white light emission cannot achieve both color rendering and color stability. Recently reported as a one-component phosphor (Cs₂ZrCl₆:1.5%Sb) with very superior performance with broadband white emission [80], WLED was prepared by using a 310 nm excitation chip, as shown in Figure 11c,d. WLED exhibits bright white emission with CIE coordinates of (0.32, 0.33) and a record CRI of (96). In addition, T₅₀ is confirmed to be ~2003 h, which is higher than all previously reported unleaded perovskite WLEDs. However, finding reasonable strategies to improve the stability of Sb-MHP phosphors is of great significance for the commercialization of Sb-MPH-based WLED devices.

4.1.2. Electroluminescent LEDs

For Sb-MHPs-based electroluminescent LEDs (Sb-PLEDs), the highly emissive perovskite layer is only one component required and typically also has an anode, a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and a cathode. The device structure is similar to solution-processed organic LEDs (OLEDs) and conventional quantum dot-based LEDs (QLEDs) [2]. When a voltage is applied to the device, holes and electrons are injected from the anode and cathode and enter the EML through the HTL and ETL, where they form excitons, followed by efficient radiative recombination to emit photons [122]. Chu et al. reported a red Sb-LED based on Cs₃Sb₂I₉ [69], the structure of which is shown in Figure 11e. Under the current condition, the electroluminescence spectrum of LED becomes a broadband red spectrum (Figure 11f). When the voltage is 6v, the radiance reaches its highest, which is 0.012 W·sr⁻¹·m⁻² (Figure 11g). Recent advances in Sb-PLEDs show their bright future in displays, lighting, and optical communications. However, as a young technology, there is still a gap in the operational stability and efficiency of Sb-PLEDs, which is a challenge for their practical application.

4.2. Temperature Sensor

Previously, a large number of reports have shown that the emission color of Sb-MHPs changes significantly with temperature [48,71]. This is mainly because the relative intensities of singlet STEs and triplet STEs of Sb³⁺ change with temperature, and these emission color changes can be used for temperature sensing. In the Sb-based metal halide system, the emission of singlet and triplet STE is closely related to the strength of electron-phonon coupling. Interestingly, the sensitivity of these two STE emission bands to the strength of electron–phonon coupling is usually different. As the temperature changes, the strength of electron phonon coupling also changes, and the changes in STE emission behavior of singlet and triplet states are usually inconsistent [79]. Usually, singlet STEs exhibit strong emissions at low temperatures. As the temperature increases, the PL peak shifts blue and the intensity decreases due to the intensified acoustic phonon exciton interaction and thermal expansion [92]. Until now, luminescence thermometers have mostly been based on the relationship between temperature and the luminescence intensity of a single luminescence transition [123], while Sb³⁺ can determine the temperature by the intensity ratio of the two luminescence transitions. In 2019, Fei et al. reported a strong luminescent material based on Sb³⁺-based coordination polymer [124], and they verified the linear relationship between the correlated color temperature and the absolute temperature in the range of 157~457 K, thus proposing the first photoemission energy-based Sb³⁺ based solid-state luminescent thermometer. Recently, we reported a temperature-sensitive Sb:RbCdCl₃ material [79]. Figure 12a shows the pictures and color coordinates of 40% Sb:RbCdCl₃ at different temperatures under UV excitation. It can be seen that the emission color changes significantly with temperature, which shows its application potential in temperature detection. The emission intensity ratio from 464 to 600 nm decreased linearly with temperature in the range of 100–260 K (Figure 12b). Figure 12c shows the sensitivity versus stability curve, which shows a maximum of 6% at 260 K, illustrating the possibility of using it as a luminescence thermometer in this temperature range.

4.3. Optical Anti-Counterfeiting Technology

In addition, in the field of luminescence, Sb-MHPs also have great development potential in the field of optical anti-counterfeiting technology. Lei et al. fabricated a three-mode reversible PL switch anti-counterfeiting technique based on three Sb-based MHPs [78], including non-emitting α-[DHEP]SbCl₅ (1), yellow-emitting β-[DHEP]SbCl₅·2H₂O (2), and red-emitting β-[DHEP]SbCl₅ (3) three compounds, where the reversible transition between 1 and 2 is triggered by acetone or methanol, enabling reversible PL on-off switching (yellow). Recently, we have also done some original work in this direction. By inserting the organic ligand Ph₃SCl (triphenylsulfonium chloride) into the SbCl₃ lattice, we have obtained two different zero-dimensional (0D) organic antimony chlorides, [Ph3_S]₂SbCl₅·2C₂H₃N and [Ph₃S]₂SbCl₅ [125]. The dynamic transformation process between the compounds [Ph3_S]₂SbCl₅·2C₂H₃N and [Ph₃S]₂SbCl₅ can be completed by stripping/inserting acetonitrile (C₂H₃N) under different external stimuli. Continuous heating of [Ph₃S]₂SbCl₅ causes the emission color to change again from red to yellow. Based on this process, we build a variety of anti-counterfeiting and information encryption models, as shown in Figure 13. In addition, we also report a kind of Sb³⁺ doped hexagonal CsCdCl₃ anti-counterfeiting material with ultra-long afterglow emission [126]. Color-reconcilable and time-dependent ultra-long afterglow emission can be achieved by adjusting the doping concentration of Sb³⁺. In view of this unusual afterglow emission property, we built an optical anti-counterfeiting and information encryption system.

4.4. Scintillators

The scintillator is a material that luminesces after absorbing high-energy particles or rays. Sb-MHPs also make them an efficient X-ray scintillator due to the advantages of a simple synthesis process, low cost, and large Stokes shift. In 2021, Manna et al. reported scintillators based on 0.7% Sb-doped Cs₂NaInCl₆ and 0.9% Sb-doped Cs₂KInCl₆ NCs. The RL spectra are dominated by peaks at ~2.7 and ~2.4 eV, respectively, which are very similar to their respective PL emissions, demonstrating that the same excited states as UV excitations are generated under X-ray irradiation, indicating that these two materials are promising candidates for scintillation applications [127]. Recently, Zang et al. reported an X-ray scintillator based on [Na(DMSO)₂]₃SbBr₃Cl₃ [128]. To further verify the application of X-ray scintillators, a schematic diagram of the experimental setup as shown in Figure 14a was established. The spatial resolution of the [Na(DMSO)₂]₃SbBr₃Cl₃@PMMA film is 15.5 lp mm⁻¹ @ MTF = 0.2 (Figure 14b). Figure 14c shows an image of a spring pin, electronics, and snail as targets under white light and X-ray illumination, which nicely shows the fine details inside the sample. This work demonstrates the potential of Sb-MHPs for X-ray imaging.

5. Summary and Perspectives

In conclusion, Sb-MHPs can overcome the inherent toxicity of lead halide perovskites, and great progress has been made in various aspects. However, there are still some problems, and there are still many challenges in many aspects. In this paper, the structure and luminescence mechanism of Sb³⁺ are summarized, the synthesis methods and basic luminescence properties of various types of Sb-MHPs are reviewed, and the applications in WLED, electroluminescence LED, temperature sensing, optical anti-counterfeiting, and X-ray scintillation are discussed. We believe that further theoretical and experimental breakthroughs are needed to develop Sb-MHPs with high performance and stability. We present the following challenges:

(1)

At present, there is a lack of theoretical guidance in the development of novel Sb-MHPs, including whether we can use theoretical calculations to predict how various materials self-assemble to form novel Sb-MHPs. In addition, the influencing factors of the singlet and triplet double emission bands of Sb³⁺ are still unclear, and more theoretical research is needed to understand this optical property.

(2)

As ionic compounds, Sb-MHPs are physically and chemically unstable in environments such as heat, humidity, and light. Especially in lighting applications, the excitation threshold increases significantly with increasing temperature, which requires the rational design of calcium Sb-MHPs with temperature insensitivity. Further research is needed to further improve its stability and realize its industrial production and application as soon as possible.

(3)

It is necessary to develop a variety of synthetic routes to develop efficient, stable, and multifunctional Sb-MHPs and to introduce more application fields, such as fluorescence sensors, lasers, and other fields.

Although Sb-MHPs still have a long way to go on the road to commercialization, this material has demonstrated strong potential in optoelectronic applications and is very promising as a next-generation light source. We hope that this review can deepen the understanding of Sb-MHPs in the field.