Low‐Dimensional Metal Halide Perovskite Crystal Materials: Structure Strategies and Luminescence Applications

Abstract Replacing methylammonium (MA+), formamidine (FA+), and/or cesium (Cs+) in 3D metal halide perovskites by larger organic cations have built a series of low‐dimensional metal halide perovskites (LDMHPs) in which the inorganic metal halide octahedra arranging in the forms of 2D layers, 1D chains, and 0D points. These LDMHPs exhibit significantly different optoelectronic properties from 3D metal halide perovskites (MHPs) due to their unique quantum confinement effects and large exciton binding energies. In particular, LDMHPs often have excellent broadband luminescence from self‐trapped excitons. Chemical composition, hydrogen bonding, and external factors (temperature and pressure etc.) determine structures and influence photoelectric properties of LDMHPs greatly, and especially it seems that there is no definite regulation to predict the structure and photoelectric properties when a random cation, metal, and halide is chosen to design a LDMHP. Therefore, this review discusses the construction strategies of the recent reported LDMHPs and their application progress in the luminescence field for a better understanding of these factors and a prospect for LDMHPs’ development in the future.


Introduction
As promising photoelectric materials, metal halide perovskites (MHPs) have been widely studied and applied to photovoltaic devices, photodetectors, light emitting diodes (LEDs), etc. [1] In particular, MHPs have unique structure tunability, the FA + , MA + , or Cs + in 3D MHPs replaced by larger sized organic cations DOI: 10.1002/advs.202004805 can build various low-dimensional MHPs (LDMHPs) at the molecular level, such as 2D, 1D, and 0D metal halide crystal materials. [2] 3D MHPs (ABX 3, A stands for monovalent cation, such as FA + , MA + , Cs + ; M represents metal ions, such as Pb 2+ , Sn 2+ , Ge 2+ etc., and X stands for halogen ion Cl − , Br − , I − ) [3] with the advantages of high absorption coefficient, adjustable optical bandgap, low exciton binding energy, and long carrier diffusion distance have been widely used in the field of solar cells with a certified power conversion efficiency (PCE) of world recorded over 25%. [4] In general, 3D perovskites have small exciton binding energies (≈20-50 meV), [5] which lead to their low radiation recombination efficiency and photoluminescent quantum efficiency (PLQE), and limit their application in luminescence. [6] On the contrary, LDMHPs commonly exhibit excellent luminescence due to their quantum confined effect and large exciton binding energies. [7] Different from nanosheets, nanowires, and quantum dots at the structural level, LDMHPs discussed in this work refer to the low-dimensional crystals at the molecular level in particular.
The selection of cations, metals, and halogens play a key role in determining chemical composition and crystal structures of LDMHPs. Crystals with different dimensions were constructed by selecting the size of organic cations, and regulating metal cations could also construct LDMHPs with different dimensions. [8] In addition to single metal, double perovskite, and quadruple perovskites could be formed by mixing metal cations. [9] The change of halogen could regulate the configuration and the distortion degree of LDMHPs, which is the key to realize the broadband white luminescence. [10] Generally, controlling growth of all-inorganic Cs-Pb-X perovskites with different dimensions could be easily realized by regulating the stoichiometric ratio of the precursor solution during the crystal growth progress. [11] However, for organic-inorganic hybrid perovskites, a kind organic cation usually only constructed a structure even the stoichiometric ratio of every elements in the precursor solution was changed unless it formed different valence states or interacted with other molecules. On the other hand, manipulating the hydrogen bonding interactions between the inorganic skeletons and organic cations could significantly affect the orientation and conformation of inorganic skeleton. [12] Furthermore, external pressure and temperature will provide thermodynamic energies to change the structures and photoelectric properties of LDMHPs. [13,14] Figure 1. a) Typical single crystal structures of 0D-3D organic-inorganic hybrid perovskites. 3D. Reproduced with permission. [15] Copyright 2018, Wiley-VCH. b) 2D. Reproduced with permission. [17] Copyright 2014, American Chemical Society. c) 1D. Reproduced with permission. [19] Copyright 2017, Springer Nature. d) 0D. Reproduced with permission. [12] Copyright 2019, Springer Nature.
Recently, it has spawned the "diamond fever" on the family of LDMHPs, and they have been intensely studied and applied as promising new photoelectric functional materials due to excellent luminescence properties and good stabilities. [15] In particular, LDMHPs exhibit typical luminescent properties of large stokes shift and broadband emission, [16] which could achieve tunable luminescence covering the whole visible light region and single phase white light emission. [17] Compared to 3D perovskites, the summarization on the structural design strategy and its relationship with photoelectric properties of LDMHPs are still very rare. In this article, the specific discussion on the construction strategies related to alteration of chemical constituents including cations, metal ions, halide anions, stoichiometric ratio, hydrogen bonding, temperature, and pressure will be mainly reviewed. In addition, LDMHPs' broadband emissions and their application in LED devices and phosphors in solid lighting are discussed here to have a systematic and prospective knowledge of promotion in luminescence properties when different structures of LDMHPs are selected.

Cations
As shown in Figure 1a, the common structure of the 3D perovskite is AMX 3 , in which the metal atoms are in the center and the halogen atoms are in the vertexes of the unit octahedrons extending in a 3D mode. The organic or inorganic monovalent cations are located in the cavities constructed by [MX 6 4− ] octahedrons, which is conforming to the theoretical formula of where R A , R M , and R X represent the radius of the A, M, X ions and t is the tolerance factor, respectively. For example, result from the construction formula limit, when t = 1, R M and R X are the maximum values (R Pb = 1.19 Å, R I = 2.20 Å), the maximum R A approximately is 2.6 Å. Therefore, only small cations which maximum lengths are shorter than 2.6 Å can satisfy the condition to build 3D APbI 3 perovskites. [18] Correspondingly, replacing the FA + , MA + , or Cs + with larger size organic cations, abundant LDMHPs can be obtained, including layered 2D, linear 1D and points distributed 0D metal halide hybrids (Figure 1b-d).
yellowish-white emission, 1D Sn-based C 4 N 2 H 14 SnBr 6 is easier to transform into a 0D of (C 4 N 2 H 14 Br) 4 SnBr 6 . [28] And the Snbased 0D (C 4 N 2 H 14 X) 4 SnX 6 (X = Br − , I − ) has a PLQE of nearly unity (Figure 3e). [29] Although the organic cations in 2D structure are not as small as the cations in 3D perovskites, they still need to fit into the frame of 2D inorganic layers, and the cross-sectional area of organic molecule must be suitable to the square range constructed by the four corner-sharing lead halide octahedrons, allowing the organic molecule to tilt and interlace, where the side length of the square is greater than or equal to twice the average bond length of the Pb-X. The space cannot accommodate the too large adjacent organic molecules, because the larger cations more possibly construct lower dimensional struc-tures, such as 1D and 0D structures. For organic molecules with large cross-sectional area, it's more likely to break up the 2D inorganic layers to form a 1D or 0D structure. For example, the large circular organic molecules C 9 NH 20 + (organic cation 2 in Figure 4) constructed a variety of 0D MHPs, such as (C 9 NH 20 ) 7 (PbCl 4 )Pb 3 Cl 11 with a blue emission, [31] (C 9 NH 20 ) 2 SnBr 4 with a deep-red emission, [32] (C 9 NH 20 ) 2 SbCl 5 with a yellow emission, [29] and (bmpy) 9 [ZnCl 4 ] 2 [Pb 3 Cl 11 ] [12] with a green emission, etc. Organic cations with large cross-sectional area in 1D and 0D structures are summarized in Figure 4, and the optical properties of these LDMHPs constructed by corresponding molecules are shown in Table 1. In summary, the size of the organic cations plays a key role in constructing LDMHPs' structures. 1-0D [30] (Ph 4 P) 2 SbCl 5 N/A Antisolvent method 648 136 87 2-0D [29] (C 9 NH 20 ) 2 SbCl 5 N/A Antisolvent method 590 119 98± 2 2-0D [31] (C 9 NH 20 ) 7 (PbCl 4 )Pb 3 Cl 11 Face-sharing Antisolvent method 470 84 83 2-0D [27] (C 9 NH 20 ) 2 S n Br 4 N/A Solution method 695 146 46 2-0D [12] (bmpy) 9
The diversified options of metal ions can not only construct the double perovskites, [9] but also theoretically design the structure of quadruple perovskites. For example, Lin et al. propose a strategy to design quadruple perovskites by introducing heterovalent cation to form double perovskites. And these two stable quadruple perovskite halides of Cs 4 CdSb 2 Cl 12 and Cs 4 CdBi 2 Cl 12 with vacancy-ordered structures were successfully synthesized by solvent heat method and exhibited broadband emissions. [61] Figure 6 shows the design diagram of quadruple perovskite, the quadruple perovskites provide a promising design method for structure regulation of perovskites.

Halogens
Halogens have great influence on the structures of LDMHPs, too.  [17] with white emissions. Crystal structures of these materials are shown in Figure 7, in which the Pb-Cl is a 100oriented structure. When the lead halide octahedron is deformed due to the different coordination environment of Pb 2+ , the 2D perovskites of Pb-Br and Pb-I became 110-oriented structures. Kanatzidis et al. reported 2D hybrid perovskites of EA 4 Pb 3 X 10 (X = Cl − , Br − ), [10] and when the halogen is I − , it did not form an analogue with EA 4 Pb 3 X 10 (X = Cl − , Br − ), but a 1D face-sharing perovskite.
Halogens affect structure distortions of LDMHPs significantly. As the ion radius of Cl − (1.67 Å) is less than that of Br − (1.82 Å), it does not accommodate the EA + cation well and so the EA 4 Pb 3 Cl 10 has highly distorted structure. [62] For EA 4 Pb 3 Br 10 , the intermediate layer of Pb-Br is of regular structure, while the outer layer Pb-Br has obvious structural distortion (Figure 8a). The distortion of intrinsic structure affects the luminescence characteristics of LDMHPs. That the effect of halogen replacement on structure distortion and luminescence properties have many examples and broadband emission is strongly associated with distorted degrees of lattices in LDMHPs. Mao et al. [10] found that different distortion levels in EA 4 Pb 3 Cl 10 (large distortion) versus EA 4 Pb 3 Br 10 (small distortion) and EA 4 Pb 3 Cl 10 has a broadband white-light emission, while EA 4 Pb 3 Br 10 has a narrow blue emission. By further tuning the ratio of chlorine/bromine for EA 4 Pb 3 Br 10−x Cl x (x = 0, 2, 4, 6, 8, 9.5, 10), they found that two of the Reproduced with permission. [50] Copyright 2018, Wiley-VCH. b) Structure of 0D perovskites (CH 3 NH 3 ) 3 Bi 2 I 9 . Reproduced with permission. [51] Copyright 2017, American Chemical Society. c) Structure of 0D perovskites ABiB 6 . Reproduced with permission. [52] Copyright 2018, American Chemical Society. d) Lead-free 0D double perovskite (C 8 NH 12 ) 4 Bi 0.57 Sb 0.43 Br 7 ·H 2 O. Reproduced with permission. [53] Copyright 2019, Wiley-VCH. intermediate compounds (x = 8 and 9.5) have more optimized white-light emissions than that of pure EA 4 Pb 3 Cl 10 ( Figure 8b). Yangui et al. [63] Figure 8c shows the PL variation of (C 6 H 11 NH 3 ) 2 [PbBr 4−x I x ] with mixed halogens. The full width at half-maximum (FWHM) of PL gradually widened with the decrease of I − content due to the difference of structure distortions. Furthermore, halogens can also tune structure distortion through hydrogen bonding formed between organic cations and halide anions. Here, different electronegative properties of halogens lead to different strength of hydrogen bonding force (where H-Cl > H-Br > H-I), determining the orientation of organic cation [18] in interlayer of metal halides and caused different distortion degrees of lead halide octahedron. Furthermore, the halogens can be replaced by other anions, such as the SCN − which has a similar radius with I − . The 100-oriented 2D perovskite (CH 3 NH 3 ) 2 Pb(SCN) 2 I 2 [64] can be obtained by partially replacing the I − with SCN − . Its structure is shown in Figure 8d, in which the lead coordination octahedron is constructed by four equatorial I − and two axial SCN − coordination. Due to the polarity of SCN − , the octahedron is distorted by Pb─S bonds. [65]
between negatively charged metal halide octahedrons and positively charged Cs + . For organic-inorganic hybrid perovskites, organic cations contain one or more terminal amines in which H atoms interacting with halogens on inorganic octahedrons by hydrogen bonding. For example, the 110-oritented 2D perovskites have been widely studied recently due to their excellent white emission properties, and their unique corrugated structures are stabilized by hydrogen bonding. For example, Kanatzidis et al. [45] reported the corrugated 2D perovskite (DMEN)PbBr 4 , as is shown in Figure 9a. Its 3 × 3 corrugated layer configuration is stabilized by hydrogen bonding. With H atoms on the primary and secondary amines of organic cations forming hydrogen bonding with Br atoms, the inorganic layer folds ≈90°, producing the strongly corrugated structure (Figure 9b). In addition, many other corrugated 2D structures, such as (N-MEDA)[PbBr 4 ] [17] which is shown in Figure 9c have been reported. It exhibits a white emission upon UV light with a high PLQE of 9% (Figure 9d). (Epz)PbBr 4 (Figure 9e) [66] is another example to demonstrate the role of hydrogen bonding in stabilizing the 110-oriented 2D perovskites (Figure 9f).
In fact, the formation of hydrogen bonding plays a decisive role in regulating the orientation and conformation of inorganic skeleton. Different amines, such as primary amines, secondary amines, tertiary amines all can construct metal halides 1D structures with different interlinkage modes (Figure 10a), [43] and the organic cations 1, 2, and 3 containing different N-H sites which are adjacent, interphase and pair sites constructed 2D, 1D, and 0D configurations, respectively (Figure 10b). [67] The structures of (GUA) 2 PtI 6 and (FA) 2 PtI 6 [68] are shown in Figure 10c, and here the discrete [PtI 6 ] 4− octahedrons are connected by 3D hydrogen  [17] Copyright 2014, American Chemical Society. e) Structure of (epz)PbBr 4 and f) hydrogen bonding diagram. e,f) Reproduced with permission. [66] Copyright 2018, American Chemical Society.
bonding network constructed by FA 2+ and the 2D network constructed by GUA 2+ cations, indicating that the hydrogen bonding force of organic cations affect the arrangement of inorganic [PtI 6 ] 4− octahedrons. Recently, Cui et al. reported the structure transition from the corrugated 1D structure to 0D by adjusting hydrogen bonding (Figure 10d,e). By employing a unique ureaamide cation containing H 4 N─C═O, which can form multiple hydrogen bonds between adjacent organic cations and inorganic skeleton. The corrugated 1D structure was stabilized by the hydrogen bonding of Pb─Br···H─N. When increase the concentration of amides in the growth process of crystals, the amides will form hydrogen bonds with H 2 O in precursor solution and adjacent cations to construct a large network (Figure 10f), which completely separated the [PbBr 6 ] 4− octahedrons to form a 0D perovskite. The 1D and 0D perovskites both exhibited stable white emission, and the dimension reduction from 1D to 0D increased the PLQE as much as five times, providing a new strategy to regulate the structure and improve the luminescence performance by regulating hydrogen bonding forces in MHPs. [12]

Stoichiometric Ratio of Precursor Solution
For all-inorganic Cs-Pb-X perovskite crystals, different structure dimensions can be easily regulated by changing the stoichiometric ratio of precursor solution. For example, a ternary phase diagram of 1D Cs 4 PbBr 6 , 2D CsPb 2 Br 5 , and 3D CsPbBr 3 is shown in Figure 11a, in which dimension regulation is achieved by stoichiometric ratio (CsBr:PbBr 2 ) control. As shown in Figure 11b, the traditional 3D structure of CsPbBr 3 has two phases, cubic phase (Pm3m) and orthorhombic phase (Pnma). When PbBr 2 is abundant, 2D structure CsPb 2 Br 5 is formed, [69] and Cs + ions appear between the inorganic layers. In contrast, when CsBr is excess, 0D structure Cs 4 PbBr 6 is formed, of which individual lead halide octahedrons are completely separated by Cs + . [70] The bandgaps of these Cs-Pb-Br perovskites increase with the dimension decreasing ( Figure 11c). Both CsPbBr 3 and Cs 4 PbBr 6 showed direct bandgap, while CsPb 2 Br 5 showed indirect bandgap. The Cs-Pb-I perovskites can also construct different dimensions by controlling the stoichiometric ratio of CsI versus PbI 2 .

Temperature Effect
Perovskite materials usually undergo phase transition when the temperature changes, which are mainly caused by changes in configurations of organic cations. For example, the organic cations were disorderly arranged as the temperature increased (> 300 K) and the disordered point was called the melting temperature. [75] The disordered cations led to sudden change of 2D interlayer distance and lattice parameters along the direction of organic cations. [76] Recently, Zeb et al. [77] synthesized a 1D perovskite crystal 1-methylpiperidinium triiodoplumbate(II) (MPIP). They found the MPIP had an obvious phase transforma-tion near the Tc = 202 K due to the ordered-disordered transition of 1-methylpiperidinium cation (Figure 13a). In addition, Liao's group [78] synthesized a 1D [C 6 H 11 NH 3 ] 2 CdCl 4 with high temperature dielectric response, due to it undergoes the phase transition at 367 K caused by the change of relative position of Cd atoms (Figure 13b).
Temperature not only affects LDMHPs' structures, but also causes difference in their photoelectric properties. For example, Eric et al. studied the exciton characteristics of 2D (C 4 H 9 NH 3 ) 2 PbI 4 in a temperature range of 10-300 K. [79] Under the drive of C 4 H 9 NH 3 + rearrangement, phase transition occurred at around 250 K, corresponding to the decrease of lattice spacing and the increase of exciton binding energy. In addition, Sieradzki et al. [14] synthesized a 2D (MHy 2 PbI 4 ) as shown in Figure 13c. With the temperature decreasing, MHy 2 PbI 4 underwent three structure phase transitions at 320, 298, and 262 K, respectively. The second phase transition at 298 K was related to the arrangement of MHy + , and this phase transition caused a significant change in the dielectric constant. As shown in Figure 13d, the interlayer spacing between the inorganic layers increased Figure 11. a) The ternary phase of Cs-Pb-Br, proportion of precursor solution (CsBr and PbBr 2 ). b) Conversion between (CsPbBr 3 , CsPb 2 Br, and Cs 4 PbBr 6 ). c) Electron band structure of CsPbBr 3 , CsPb 2 Br, and Cs 4 PbBr 6 . Reproduced with permission. [11] Copyright 2018, American Chemical Society.
with the temperature increasing. In addition, the emission of MHy 2 PbI 4 exhibited blueshift during cooling temperature from 300 to 80 K. It exhibited light yellow emission at 300 K and turned to yellow-green emission at 80 K (Figure 13e). The few-layer exfoliated 2D (C 4 H 9 NH 3 ) 2 PbI 4 underwent phase transition at ≈175 K, producing a new blueshift PL peak (Figure 14a). [80] It is worth mentioning that temperature has a great influence on the emission from relaxed states of LDMHPs. These materials usually have emission from relaxed states caused by lattice distortion or self-trapped excitons (STEs). The PL intensity of relaxed states is more efficient than the free exciton (FE) at low temperatures.
Recently, RPP perovskite (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n−1 Pb n I 3n+1 (n = ≈1-4) has been reported, [81] and it undergoes structure distortion under laser irradiation, resulting in a relaxed redshifted emission (Figure 14b). The relaxation peak increased sharply with the temperature decreasing (Figure 14c). In addition, the STE emission intensity also increased as the temperature decreased. Some 2D 100-oriented perovskites can observe STE emissions at low temperature (Figure 14d). [82,48] When temperatures are high, the STEs are easy to detrap from STE states to FE states, and at low temperatures, the detrapping process is suppressed, [82] Therefore, STEs emission gradually appears and increases at lower temperature.

Pressure Effect
Exerting high pressure to the material is an effective way to regulate structures and photoelectric properties of LDMHPs. For example, Liu et al. [83] reported that 2D (BA) 2 (MA) n−1 Pb n I 3n+1 (n = 3) reduced the bandgap by 8.2% after increasing the pressure. This is because external pressure enhanced coupling between s orbitals of Pb atoms and p orbitals of I atoms. Yin et al. [84] studied the mechanically exfoliated 2D layers of (C 4 H 9 NH 3 ) 2 PbI 4 under pressure. As shown in Figure 15a, under initial compression from 1 atm to 0.5 GPa, the slight blueshifts of bandgap happened due to the decreased Pb─I─Pb bond angles. Perovskite lattices were compressed when pressure continued to increase, and the bond length of Pb─I─Pb inevitably decreased. This increased the overlap of electronic wave function of Pb and I atoms, and finally caused bandgap redshifts.
Shi's team [13] synthesized a unique 1D organic tin bromide perovskite C 4 N 2 H 14 SnBr 4 . The 1D inorganic chain deformed under the transition from monoclinal phase to triclinic phase (Figure 15b), and the exciton binding energy increased, resulting in the enhancement of PL intensity (Figure 15c). This work revealed a potential application of supercharging to improve the luminescence performance of LDMHPs. Zhang et al. [85] studied the PL properties of 0D Cs 3 Bi 2 I 9 under high pressure. As shown in Figure 15d, under high pressure, the bandgap of Cs 3 Bi 2 I 9 continued to become narrow, and finally reached the optimal value. In addition, at atmospheric pressure, the isolated [Bi 2 I 9 ] 3− octahedron exhibited only weak emission upon excitation. But it exhibited significant enhancement of PL intensity about ten times under relatively high pressure (< 1 GPa) due to the increased exciton binding energy.
It is worth mentioning that external pressure could distort the structure of the LDMHP to produce its STE emission. Recently, it has been reported that the pressure effect on regulating the structure distortion achieving the STE emission of (BA) 4 AgBiBr 8 . [86] With the increasing pressure, the emission of (BA) 4 AgBiBr 8 changed from a nonluminescent state under atmospheric pressure to a state with obvious fluorescence at 2.5-25.0 GPa (Figure 16a). The sample completely transformed to a  [73] Copyright 2020, Wiley-VCH. d) The structures of 1D (PI) 2 PbI 2 ·2DMSO and 0D (PDI) 2 PbI 2 . Reproduced with permission. [74] Copyright 2020, American Chemical Society. new structure at 2.1 GPa (Figure 16b). In addition, the new structure gradually distorted and its crystallinity reduced upon compression. Finally, a completely lower crystallinity phase formed at around 25.0 GPa, which led to the quenching of the luminescence. The luminescence mechanism is shown in Figure 16c. Under atmospheric conditions, the STEs were easy to detrap and return to FE states. When the pressure increases, the structure distortion of (BA) 4 AgBiBr 8 deepens its self-trapped states, which enhanced the energy barrier to avoid the detrap of excitons. And the bright emission from self-trapping realized.

Light-Emitting Diodes (LEDs)
Due to the size effect and significant differences in dielectric constants between the organic and inorganic parts, excitons in LDMHPs are spatially confined. This suppresses the separation of excitons and greatly improves the radiation recombination efficiency. Therefore, organic-inorganic LDMHPs are consid-ered to be good candidate materials for stable LEDs with highefficiency. Most 2D perovskite electroluminescent layers can be fabricated via the one or two-step fast crystallization spin-coating method using precursors. Table 2 shows a performance summary of LEDs based on recent 2D MHPs. However, 1D and 0D fail to be fabricated by solution method, which is the main reason for no 1D and 0D electroluminescent diodes. Therefore, unlike "structure-level" 1D nanowire/nanorod perovskites and 0D nanocrystal perovskites, 1D and 0D organic-inorganic MHPs at the molecular level have not been used in the electroluminescent layer. [87] In order to prepare color-adjustable LEDs, two main strategies have been tried so far. One is to rely on mixed halides, [88] and the other is to control quantum well structures. [89] However, ion migration and phase separation are prone to occur under light and electric fields in LED devices with mixed halide perovskites. [90] resulting in the changes in electroluminescent color during device operation. [91] Controlling the quantum well structure to obtain quasi-2D perovskite is another effective method to obtain the target color. The LEDs produced by this method have a very stable Figure 13. a) Phase transition and dielectric response of 1D MPIP perovskite structural phase transition induced by temperature. Reproduced with permission. [77] Copyright 2018, Royal Society of Chemistry. b) Structural phase transition of [C 6 H 11 NH 3 ] 2 CdCl 4 at 367 K. Reproduced with permission. [78] Copyright 2014, American Chemical Society. c) MHy 2 PbI 4 underwent three structural phase transitions and the tilt degree of octahedral PbI 6 4− at each phase. [14] d) The interval between lead iodide sheets varies with temperature. [14] Copyright 2019, American Chemical Society. e) CIE coordinates of MHy 2 PbI 4 at different temperatures. Reproduced with permission. [14] Copyright 2019, American Chemical Society. Figure 14. a) Changes of PL spectrum of stripped monolayer (C 4 H 9 NH 3 ) 2 PbI 4 with temperature. Reproduced with permission. [80] Copyright 2018, American Chemical Society. b) 2D perovskite (n = 2) under laser irradiation, PL redshift, and then blueshift reversible process after laser annealing. Reproduced with permission. [81] Copyright 2018, Springer Nature. c) Temperature-dependent changes in PL intensity, highlighting changes in the relative intensity of relaxed peak. Reproduced with permission. [81] Copyright 2018, Springer Nature. d) The broadband emission appears at low temperature in 100-oriented 2D perovskite. Reproduced with permission. [82] Copyright 2017, Royal Society of Chemistry.  [86] Copyright 2018, AmericanChemical Society. b) 1D structure distortion caused by pressure. c) The relationship between PL spectra of C 4 N 2 H 14 SnBr 4 and pressure, and luminescence photographs at different pressure points. b,c) Reproduced with permission. [13] Copyright 2019, American Chemical Society. d) Optical absorption spectrum of Cs 3 Bi 2 I 9 at high pressure. Reproduced with permission. [87] Copyright 2018, Wiley-VCH.  6 ] octahedrons, respectively). c) Schematic diagram of the pressureinduced luminescence mechanism of self-trapped exciton. Reprinted with permission. [86] Copyright 2019, Wiley-VCH. www.advancedsciencenews.com www.advancedscience.com Table 2. Performance summary of reported LDMHPs-based LEDs.
In addition, the conversion of 3D perovskites to quasi-2D perovskites will introduce more defects on the surface or grain boundaries due to the reduction of crystal size, resulting in nonradiative recombination rate, which will also reduce the electroluminescence efficiency 2D perovskite LEDs. [97] The intrinsic defects in the perovskite films can be passivated by introducing additives, or the interface modification methods can effectively reduce the defects in the perovskite films, thereby improving the radiation recombination rate in 2D perovskite LEDs. [98] For example, Zhao et al. [2] reported quasi-2D and 3D (2D/3D) perovskite-polymer (poly (2-hydroxyethyl methacrylate)) heterostructure LEDs. The introduction of the polymer effectively eliminated the nonradiative recombination pathway, and the external quantum efficiency of the LED devices was as high as 20.1% (current density is 0.1-1 mA cm −2 , Figure 17e). And Yang et al. [99] investigated LEDs with high efficiency based on quasi-2D PEA 2 (FAPbBr 3 ) n−1 PbBr 4 (n = 3) as the emitting layer. External quantum efficiency (EQE) of the champion device with Figure 17. a) EL spectra of LEDs operating under different voltage. Inset is a photograph of the device. Reproduced with permission. [92] Copyright 2018, Springer Nature. b) EL spectral stability under 3.5 V continuous voltage operation. Inset is a photograph of the device. Reproduced with permission. [93] Copyright 2018, Wiley-VCH. c) Current efficiency-voltage curve of the LED device. d) External quantum efficiency-voltage curve of the LED device. c,d) Reproduced with permission. [96] Copyright 2019, Wiley-VCH. e) EQE-current density curve of the LEDs (peak EQE = 20.1%). Inset is the peak EQE histogram of 320 devices. Reproduced with permission. [2] Copyright 2018, Springer Nature. f) EQE of the champion device of PEA 2 (FAPbBr 3 ) n−1 PbBr 4 (n = 3 composition) with TOPO as passivation layer. Inset is the EL image of the green LED. Reproduced with permission. [99] Copyright 2018, Springer Nature.

Phosphors in Solid Lighting
LDMHPs have larger exciton binding energies, which greatly increases the exciton recombination rate, resulting in LDMHPs with high photoluminescence quantum yield (PLQE). The Ma's group [115] has reported an efficient broadband yellowemitting phosphors composed of 0D tin mixed halide perovskites (C 4 N 2 H 14 Br) 4 SnBr x I 6−x (x = 3). The phosphors had a full width half maximum of 126 nm (FWHM) and a photoluminescence quantum efficiency (PLQE) of about 85% due to the structural reorganization of the excited state. By mixing the yellow phosphors and commercial Eu-doped barium magnesium aluminate blue phosphors (BaMgAl 10 O 17 : Eu 2 + ) with the weight ratio 1:4, a near-perfect white emission with CIE coordinates of (0.32, 0.32), the color rendering index (CRI) of 84 and the correlated color temperature (CCT) value of 6160 K can be fabricated (Figure 18a). UV-pumped white LEDs was as high as 85. Zhong's group [116] used the HBr-assisted slow cooling method (SCM) to grow centimeter-sized Cs 4 PbBr 6 crystals with embedded CsPbBr 3 nanocrystals, showing excellent green PL emis-sion. Great green light-emitting performance up to 97% of PLQE and excellent thermal stability made the materials an attractive candidate for white light-emitting diodes (WLEDs) manufacturing. They also fabricated WLED devices by using green-emitting crystals Cs 4 PbBr 6 with CsPbBr 3 nanocrystals, red-emitting phosphor K 2 SiF 6 :Mn 4+ (KSF), and blue-emitting GaN chips. The optimized devices had a luminous efficiency of 151 lm W −1 at 20 mA and a chromaticity coordinate value of (0.331, 0.331) (Figure 18b-d).
Actually, white-light-emitting from a single emitter layer is very important for solid-state lighting applications because it simplifies the device structure and avoids problems, such as selfabsorption and color instability in hybrid emitters and multiple emitters. [46] Recently, a series of LDMHPs with 2D, 1D, and 0D structures have been reported. There is a variety of lowdimensional single crystal growth strategies including but not limited to cooling crystallization method, antisolvent method and solution method. For 2D and 1D, adjacent octahedrons can be connected by corner-sharing, edge-sharing, and face-sharing. They all exhibit broadband white-light-emitting and are applied to a single white-emitting WLED phosphor. For example, The Ma's group [19] also reported an 1D organic halide perovskites C 4 N 2 H 14 PbBr 4 with the edge-sharing octahedral lead bromide chains [PbBr 4 2− ] that emitted bluish white-light with a PLQE Figure 18. a) Emission stability and luminous performance of 0D (C 4 N 2 H 14 Br) 4 SnBr x I 6−x LED. Reproduced with permission. [115] Copyright 2017, American Chemical Society. b) Schematic diagram of the configuration of the prototype device. c) A picture of surface mounted device (left) and the WLED operated at a forward bias current (right). d) EL spectrum of the prototype WLED device. b-d) Reproduced with permission. [116] Copyright 2018, Wiley-VCH. e) CIE coordinates of (EDBE)(PbBr 4 ) and sunlight at noon. Inset is a photograph of (EDBE)(PbBr 4 ) under 365 nm irradiation. Reproduced with permission. [17] Copyright 2014, American Chemical Society. f) CIE coordinates and emission spectrum of white-light emitters and sunlight a noon. Inset is a photograph of (N-MEDA)[PbBr 4 ] under 380 nm irradiation. Reproduced with permission. [46] Copyright 2014, American Chemical Society. g) CIE coordinate of (EDBE)PbBr 4 . Inset is an image of a UV-pumped WLED. Reproduced with permission. [124] Copyright 2016, Wiley-VCH. a) The figures highlighted in blue refers to the corresponding organic cations shown in Figure 4. Figure 19. a) Temperature dependence of the light yields. The inset shows details of the curves from 290 to 310 K. Reproduced with permission. [123] Copyright 2016, Spring Nature. b) Schematic diagram of X-ray imaging setup using 1:1 Li-(PEA) 2 PbBr 4 as scintillator. c) X-ray image of the safety pin using Li-(PEA) 2 PbBr 4 as scintillator. b,c) Reproduced with permission. [126] Copyright 2020, Spring Nature. d) Photographs of a capsule containing a spring inside, a circuit board, a crab and the corresponding X-ray images by X-ray imaging. Reproduced with permission. [127] Copyright 2020, American Chemical Society. e) The normalized total amount of photons comparison with the same X-ray excitation. f) X-ray fluorescence spectra under 50 kV Ag tube irradiation. Gray: NaI:TI commercial scintillator. Blue: Bmpip 2 PbBr 4 pellet. Red: Bmpip 2 SnBr 4 pellet. e,f) Reproduced with permission. [129] Copyright 2019, American Chemical Society. g) Schematic diagram of a simplified device for gas sensing application. h) Digital photos of (C 9 NH 20 ) 2 MnBr4 upon exposure to six kinds of organic vapors for 10 min under 365 nm UV excitation. g,h) Reproduced with permission. [131] Copyright 2019, American Chemical Society.

Scintillator and Transducer
It is generally believed that LDMHPs have higher PCEs, faster decay and larger stokes shifts compared with 3D ones. [122] These advantages demonstrate the potential of LDMHPs for large-area and low-cost scintillator devices in the field of medical imaging, nondestructive detection, space exploration, etc. For example, Birowosuto et al. [123] reported three X-ray scintillator characteristics of 3D MAPbI 3 and MAPbBr 3 and 2D (EDBE)PbCl 4 hybrid perovskite crystals. Comparing to 3D perovskites, 2D (EDBE)PbCl 4 has less thermal quenching due to the large exciton binding energy so that moderate light yield of 9000 photons MeV −1 can be obtained even at room temperature (Figure 19a). Moreover, Masanori et al. [124] researched 2D organicinorganic hybrid perovskite-type scintillator single crystal of 5 × 6 × 1 mm 3 of (PEA) 2 PbBr 4 (PEA + = C 6 H 5 (CH 2 ) 2 NH 3 + ). Under 662 keV gamma-ray excitation, the crystal shows a substantial light yield of 10 000 photons MeV −1 , with a main decay time component of 9.4 ns. Subsequently, they further optimized the crystal and investigated scintillation properties of (PEA) 2 PbBr 4 under gamma-ray and X-ray irradiations. They fabricated 17 × 23 × 4 mm 3 bulk sample, which shows a remarkably high light yield of 14 000 photons MeV −1 and very fast decay time of 11 ns under gamma-rays. [125] In addition, Xie et al. [126] demonstrated Li-dopant (PEA) 2 PbBr 4 crystal and explored the application of Li-(PEA) 2 PbBr 4 scintillator in alpha particle detection, X-ray imaging and discrimination between alpha particle and gamma-ray. It is worth mentioning that they obtained first satisfactory X-ray imaging pictures of a ubiquitous safety pin using Li-(PEA) 2 PbBr 4 film (Figure 19b,c). However, the toxicity of lead in these lead-based halide perovskites may restrict its potential commercial applications. Thereby, Cao and his colleagues [127] developed a lead-free 2D layered (C 8 H 17 NH 3 ) 2 SnBr 4 perovskite scintillators, which not only exhibit a high PCE of 98% and a large Stokes shift of 246 nm but also provide nontoxicity, good PL intensity, and stability under X-ray illumination. The results make the novel perovskite scintillators suitable for X-ray imaging applications (Figure 19d). And Hardhienata et al. [128] studied the optical and scintillation properties of manganese-based 2D organic-inorganic hybrid perovskite crystals X 2 MnCl 4 (X = PEA, PPA). Both of them show promising PL properties, reasonable decay time (about 3-4 µs) and small bandgaps (about 2 eV), making them applicable for scintillator in X-ray imaging application. Viktoriia et al. [129] reported isostructural 0D halide complexes of Bmpip 2 SnBr 4 and Bmpip 2 PbBr 4 (Bmpip = 1-butyl-1-methylpiperidinium cation) that can exhibit potent X-ray fluorophores being comparable to that of a commercial inorganic NaI:T1 X-ray scintillator (Figure 19e,f).
At the same time, LDMHPs have also shown potential in transducer field due to their adjustable structural phase transition and fluorescence emissions. For example, Zhou and his colleague [130] reported a novel 0D all-inorganic perovskite single crystal Cs 2 InBr 5 ·H 2 O for sensitive water detection. And Li et al. [131] have carefully researched the temperature-dependent phase transition and photoluminescence properties of 0D organic-inorganic hybrid metal halide (C 9 NH 20 ) 2 MnBr 4 . Based on this compound, they further developed a fluorescent sensor for Acetone. By simplified gas-sensitive detecting device with (C 9 NH 20 ) 2 MnBr 4 , rapid fluorescence quenching shows when the organic solvent is acetone, which cannot be found when applying other organic solvents (Figure 19g,h).

Prospects and Outlook
As a new perovskite-type photovoltaic material system, LDMHPs exhibit excellent stability to water, heat and light, and are expected to solve the instability problem of traditional 3D perovskites, which plays a crucial role for the final industrialization of perovskite materials. By adjusting the dimensions and crystal structures, LDMHPs are expected to provide more opportunity for various optoelectronic devices. However, the LDMHP are still a new type material which has not been fully developed yet, and there are still some problems to be solved a) How to decrease environmental pollution. Although the stability of organic-inorganic hybrid perovskites has been greatly improved, the potential toxicity problem of Pb 2+ has not been solved. At present, some people try to use other metal elements to replace or partially replace Pb to prepare leadfree or lead-less perovskite solar cells, however, the PCE is not ideal. Public health concerns about lead toxicity will promote the development of lead-free LDMHPs. b) Currently, LDMHPs with inherent white emission have many attractive advantages, but their PLQEs are still too low when used in WLEDs. Moreover, there is a lack of systematic researches and understanding of the design, synthesis, and luminescence process mechanism of these LDMHPs. In addition, 1D and 0D perovskite materials still need a lot of work to explore especially for the application to LED devices. c) Although more and more LDMHPs-based optoelectronic devices show multifunctional performance and long-term stability, the properties of intrinsic materials and the composite dynamic process need further research. At the same time, an in-depth understanding of the structure-performance relationship is very necessary for the continued development of LDMHPs.