Low-dimensional perovskite materials and their optoelectronics

Three-dimensional (3D) organic – inorganic metal halide perovskite materials possess great potential applications for approaching efficient optoelectronics due to the unique optoelectronic properties of perovskite materials and cost-effective manufacturing possibilities of optoelectronics. However, the scientific and technical challenges of 3D perovskite materials were their inferior long-term stability, which hampered their practical applications. The low-dimensional perovskite materials composed of alternating organic and inorganic layers are one of the most credible paths toward stable perovskite photovoltaics and optoelectronics. In this short review, we first present a discussion of the crystal structure and nontrivial optoelectronic properties of the low-dimensional halide perovskites. The synthetic methods for the preparation of the low-dimensional halide perovskites are reviewed. After that, we focus on the recent development of perovskite photovoltaics, light-emitting diodes, and lasers by the low-dimensional halide perovskites. Finally, we out-line the challenges of the low-dimensional halide perovskites and their applications.

For alternating cations in the interlayer space (ACI) phase 2D perovskite with a formula, A 2 A 0 n B n X 3n + 1 , the shorter organic cation (MA + ) fills in both corner-sharing [BX 6 ] metal halide network and interlayer (Scheme 1(C)).
Different from shorter organic cations (e.g., CH 3 NH 3 + (MA + )) in 3D perovskite materials, longer organic cations (e.g., C 8 H 9 NH 3 + (PEA + )) with hydrophobic properties in 2D perovskite materials could slow down the degradation process in (2) step described below: 45 PbI 2 þ MAI⟺MAPbI 3 ð1Þ In addition, the electrically insulating nature of a long OS and perovskite conductor layers could lead to the natural multiple-quantum-well structures, as shown in Scheme 2(A,B).
The OS layers serve as the potential "wall" and the perovskite layers serve as the potential "well". In this case, the excitons are formed in the low-dimensional perovskite materials instead of free charge carriers due to S C H E M E 1 Schematic of (A) Ruddlesden-Popper (RP) phase, (B) Dion-Jacobson (DJ) phase, and (C) alternating cations in the interlayer space (ACI) phase two-dimensional (2D) perovskite, where Gua is guanidinium increased binding energy induced by quantum confinement and the generated electroluminescence in the low bandgap regions, which was confined effectively by higher energy gap regions, resulting in an efficient radiative charge carrier recombination. 46,47 These unique properties could make the low-dimensional perovskite materials to be great candidates for LEDs and laser applications. 48,49 As the organic layer thickness is increased, the quantum confinement effects are prominent, consequently resulting in blue-shifted absorption and emission. 39 The large bandgap of pure 2D perovskite materials (n = 1) certainly renders themselves less desirable for solar cell applications. Therefore, the lowdimensional perovskites with a larger n (n > 3) are proposed to be candidate materials for approaching efficient PSCs and PPDs.
In this short review, we firstly give a discussion of the crystal structures and optoelectronic properties of the low-dimensional perovskites. Second, the synthetic methods for the preparation of the low-dimensional halide perovskites are summarized. After that, the overview of the applications of solar cells, PDs, LEDs, and lasers are discussed. Finally, summary and key challenges toward high-performance optoelectronic applications of the low-dimensional perovskites are outlined.

| Multiple quantum well structures in 2D perovskites
The low-dimensional perovskite materials are composed of a corner-sharing [BX 6 ] metal halide network sandwiched between the organic barrier planes of A. 50,51 The variable n is the stacking number of the [BX 6 ] network layers between two organic barrier layers. In the case of n = 1, the thickness of the [BX 6 ] network layers decreases to the scale of de Broglie wavelength, leading to a strong quantum confinement effect on the charge carrier's behaviors. 46 As a result, the charge carriers transport in 2D perovskites with n = 1 is expected to be highly confined within the multiple quanta well (MQWs) structures. It is prevailingly agreed that the electrons under this condition are freely movable along the inplane direction but are restricted in the out-of-plane direction. Theoretically, 2D perovskites with n = 1 are ideal materials for LEDs application as it has natural the out-of-plane self-termination and is expected to exhibit 100% PLQY. Unfortunately, recent studies indicated that a low quantum yield observed from the low-dimensional perovskites suggested that the quantum losses were taken place within these materials. 40 So far, this is still an open question because of the lack of fundamental knowledge, such as exciton reduced mass and the spatial extension of the charge carriers wavefunctions. 52 Various macroscopic techniques have been applied for the investigation of the MQWs structures of 2D perovskites. [53][54][55] Mohite et al. demonstrated the importance of the Coulombic interactions in the low-dimensional perovskites. 54 They proposed a generic formulation of the scaling of the exciton binding energy with the thickness of the [BX 6 ] layer, which is described by: where n = 3-γe À L W 2a 0 , a 0 (4.6 nm) is the Bohr radius of 3D perovskites and the exciton ground state binding energy, E 0 (16 meV) is 3D Rydberg energy, L W is the physical width of quantum wells (QWs) for an infinite QW potential barrier (i.e., 0.641 for (BA) 2 Pb 1 I 4 , 1.255 for S C H E M E 2 (A) Schematic of multiple quantum wells structure and (B) correlated electronic band structure (BA) 2 (MA)Pb 2 I 7 ), where BA is butylammonium). In their model, the exciton is considered isotropic in an ndimensional space (1 < n < 3) and the γ is an empirical correction factor (i.e., 1.76) for the deviations from the pure quantum confinement regime. The factor γ contains the electron and hole densities and dielectric confinement effects. Based on this model, the decrease in the value of n would lead to a compression of the exciton wavefunction in the QW due to the dielectric confinement, which results in enhanced exciton binding energy. In the case of large n values, they further predicted that the exciton binding energy in the low-dimensional perovskites is higher than the thermal fluctuation (k B T = 25.7 meV) at room temperature as n values are larger than 20 ([BX 6 ] layers of~12.6 nm). Certainly, this work provided a fundamental step toward the design of novel low-dimensional perovskite materials for optoelectronic applications, such as solar cells, PDs, LEDs, and lasers.

| Edge states in 2D perovskites
In addition to considering the bulk physics in the lowdimensional perovskite materials, the layer edge states (ES) with nontrivial conducting phenomena in the lowdimensional perovskites have also been studied. 56 The ES in organic MQWs have untrivial properties. Fu et al. indicated that the ES states in HgTe QWs enable the dissipationless 1D electrical conduction. 57 Blancon et al. reported that the free charge carriers at the ES within 2D perovskite [(C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n À 1 PbI 3n + 1 ] can accelerate the exciton dissociation, which is favorable for solar cells application. 58 Recently, Priya et al. 59 provided the direct observations of the distinct conductive layer ES in the low-dimensional perovskite single crystals (C 4 H 9 NH 3 ) 2 PbI 4 (n = 1) by conducting atomic force microscopy mapping techniques. As shown in Figure 1 (A), it was acceptable that the out-of-plane current in the low-dimensional perovskite materials would behave as an insulating feature due to MQWs. It was found that the out-of-plane current in the bulk terrace region was pretty low, but an unexpected current along the contour of the layer edges was observed. As shown in Figure 1(B), the current in the bulk terrace region was negligible but it became sharp at the layer edges. The huge difference between the bulk and the layer edge indicated that there were a number of free charge carriers presented at the ES layer and the bulk terrace region behavior as an insulating characteristic due to the insulating organic layers. The total collected mobile charges per area across the layer edges are calculated by: where F is the force applied by the tip, E is the Young's modulus, v is the Posson's ratio of the samples, δ is nanoindentation from the contact geometry, R is the radius of tip curvature, I is the detected current, dQ is the differential of free charge carriers, and u is the local scan rate, respectively. Thus, the contact area determined by the nanoindentation δ and the total free charge carriers is estimated from Equation (7). The charge carrier density at the ES was calculated to be 1.1 Â 10 21 cm À3 , which is close to that of metals (10 21 cm À3 ). Noted that the conducting nature at the layer edges would not change with the scan rates. For the insulating behavior of the bulk terrace region in the low-dimensional perovskite single crystals (C 4 H 9 NH 3 ) 2 PbI 4 (n = 1), the electrons and holes are heavily bonded with a binding energy of 470 meV, leading to the formation of the exciton or further recombined through either photogeneration or F I G U R E 1 (A) Schematic of layout for conducting atomic force microscopy (c-AFM) measurement in contact mode and (B) current map of two-dimensional (2D) perovskite samples. Reproduced with permission from Reference 59. Copyright 2019 American Association for the Advancement of Science injection from the electrodes. Such an investigation on the ES layer of the low-dimensional perovskite materials offers great opportunities for designing novel photovoltaic materials and their applications.

| Crystal structure and bandgap in 2D perovskites
The crystal growth in the low-dimensional halide perovskite materials was also a significant difference compared to 3D halide perovskite materials. The latter one generally possesses multiple crystal planes, including the <100>, <110>, and <111> planes. 11 However, in the case of pure 2D perovskite materials (n = 1), there are only the <100> groups that favor the continuous growth, giving rise to a poor charge carrier transport in the vertical direction. 60 As n is increased to over 1, the <110> and <111> planes start to grow, which are favorable for charge carrier transport in the vertical direction since the oriented crystallization is parallel to the substrates, assisting in the growth of the charge-transport channels.
Compared to 3D perovskite materials, the lowdimensional halide perovskite materials show significantly different optical properties, which are dependent on dimensionality and size. 61 The bandgap (E g ) of a semiconductor refers to the minimum energy required for an electron to be excited from the ground state in the valence band (VB) to the conduction band (CB). Owing to the quantum confinement effect, the E g of the lowdimensional halide perovskites can be varied by the values of n (the thickness of [BX 6 ]). For example, increasing the number of [PbI 6 ] layer affords a narrowing of the E g from 2.24 eV for n = 1 to 1.60 eV for n = 4, in the case of BA 2 MA n À 1 Pb n I 3n + 1 perovskite. 62

| Influence of OSs on 2D perovskites
The long-chain OSs, as the crucial constituent, are essential to the optical and electronic properties of 2D perovskites. The commonly used OS cations are listed in Scheme 3.
It was reported that the methyl groups anchoring on the ammonium N atom have a significant influence on the device performance of PSCs by 2D perovskites. 63 For example, Li et al. 63 reported 2D perovskites based on phenylammonium (PA, primary ammonium), Nphenylmethylammonium (PMA, secondary ammonium), N,N-dimethylphenylammounium (tertiary ammonium), and phenyltrimethylammonium (PTA, quaternary ammonium) as OSs, and then fabricated PSCs by these 2D perovskites. They found that PSCs by 2D perovskites based on PTA OS with three methyl groups exhibited the highest PCEs, which was possibly due to the altered rigidity, size, and dielectric constant of the organic interlayer spacer. 63 In addition, it was found that the larger and more hydrophobic cations were beneficial for improving perovskite stability against moisture, although larger cations can adversely influence the device performance. 68 It was further found that the organic cations with flexible aliphatic hydrocarbons exhibited a better stereochemical configuration than rigid aromatic hydrocarbons. 79 Studies have also demonstrated that controlling the QW width distribution was crucial to overcoming theperformance-stability compromise in devices by 2D perovskites. 74 Sargent et al. 74 investigated how the OS influences QW width distribution. They investigated 2D perovskites based on allylammonium (ALA) (OS (OS) 16), PEA (OS 4), and BA (OS 17) organic cations and found that ALA + could induce the formation of higher-n with monodisperse QWs in 2D perovskite film, which was ascribed to its single C═C bond could promote the intermolecular interactions between QWs. However, PEA + and BA + have higher formation energies due to the van der Waals and π-stacking interactions between cations, favoring the initial formation of low-n and polydisperse distribution QWs. Noted that polydisperse distribution QWs in 2D perovskites may act as ultrafast shallow traps for holes in materials, resulting in stronger recombination. 74

| SYNTHETIC METHODS FOR THE PREPARATION OF THE LOW-DIMENSIONAL HALIDE PEROVSKITES
The first observation of quantum-confined 2D perovskite materials was reported by Tyagi et al. 80 They obtained a single-layer thick crystalline 2D methylammonium lead bromide (MAPbBr 3 ) perovskite by colloidal synthesis, where the octylammonium bromide was used as the long-chain ligand. The synthetic procedures involved adding lead bromide, methylammonium bromide, and octylammonium bromide into a stirring solution of oleic acid (OLA) and 1-octadecene at 80 C. The final 2D perovskite nanocrystals were obtained by further dilution, filtration, precipitation, and redispersion. The nanostructures with various morphologies of perovskites can be obtained by controlling the purification of 2D MAPbBr 3 nanocrystals. The as-synthesized 2D MAPbBr 3 exhibited an obvious blue-shifted (~0.5 eV) absorption compared to 3D bulk MAPbBr 3 , which is owing to the quantum confinements. 80

| Solution phase method
Yuan et al. 81 reported a one-pot synthetic method to prepare low-dimensional perovskite microdisks, (R 1 NH 3 ) 2 [(R 2 NH 3 ) 2 PbBr 4 ] (n À 1) PbBr 4 , where R 1 is a long octyl chain, R 2 is an aromatic alkyl group. The synthesis details are shown in Scheme 4. Hydrobromic acid was added into a dimethylformamide (DMF) solution containing aromatic methylamine (benzylamine or thiophenemethylamine), octylamine, and lead bromide to generate a yellow pale precursor solution, which was then injected into vigorous hexane and then stirred for 5 min at room temperature, followed by acetone quenching. These 2D perovskites were only a few micrometers in lateral size with a thickness of 100-150 nm. They found that the as-synthesized perovskites exhibited significantly improved photophysical properties over 3D bulk perovskites, such as narrow deep blue emissions peaked at 403-413 nm, high PLQY of 53%, and good stability.
Dou et al. further reported a direct growth of atomically thin 2D hybrid perovskite [(C 4 H 9 NH 3 ) 2 PbBr 4 ] from the solution. 82 In detail, a very dilute precursor solution (C 4 NH 9 NH 3 Br + PbBr 2 + DMF) was dropped on the precleaned surface of Si/SiO 2 substrate and dried under 75 C. After that, CB was selected as a co-solvent to reduce the solubility of (C 4 H 9 NH 3 ) 2 PbBr 4 in DMF, promoting crystallization. It was found that the crystallization process was uniform across the whole substrate due to the similar boiling point and evaporation rate of DMF and CB. This method overcame the limitations of the conventional exfoliation and chemical vapor deposition methods, which generally produced a thick perovskite plate. They found that the photoluminescence of the as-  83 The 2D NRs showed a shift to higher energies in the absorption and photoluminescence compared to 3D bulk perovskite. The formation mechanism of 2D NRs was analyzed by varying different ligands (from octylammonium to OLA). They further found that the bandgaps can be tuned from 1.90 to 2.26 eV by adjusting the halide from iodide to bromide.
Vybornyi et al. reported another synthetic method for the preparation of low-dimensional CH 3 NH 3 PbX 3 , which was without polar solvents (DMF). 84 The reaction between methylamine and PbX 2 was conducted in a nonpolar solvent such as 1-octadeecne ODE at an elevated temperature in the presence of oleylamine (OLM) and OLA as ligands in the precursor solution. The tetrahydrofuran solution containing methylamine and OLA was injected into the above precursor solution under vigorous stirring. The nanocrystals could be formed within seconds. They found that the low-dimensional perovskite nanocrystals exhibited either blue emitting or green luminescent by adjusting the amount of OLM. The platelets with n = 3 (approximately value) exhibited an absorption peaked at 450 nm and PL emission at 465 nm with a PLQY of 18%.

| Vapor-assisted method
The low-dimensional perovskite materials prepared by vapor-assisted methods generally have better crystallinity and fewer impurities. 85,86 Vapor-assisted methods have been demonstrated in 3D CH 3 NH 3 PbI 3 perovskites by Liu et al. 87 They used a dual-source thermal evaporation system to evaporate PbCl 2 and CH 3 NH 3 I to produce 3D CH 3 NH 3 PbI 3 , which was much uniform than solutionprocessed films. The first vapor-assisted method for preparing low-dimensional halide perovskite materials was reported by Guo et al. 86 They fabricated 2D/3D mixed hybrid perovskite thin films by low-pressure vaporassisted solution process. The solid-vapor reaction was conducted between spin-coated PEAI-doped PbI 2 thin film and MAI vapor under controlled pressure. Due to the weak van der Waals interaction between organic PEA + cation and inorganic octahedral [PbI 6 ], the vapor MA + cation can accessibly intercalate into the octahedral [PbI 6 ] to form 2D perovskites. They found that the PEAIdoped perovskite films by MAI vapor treatment exhibited a similar absorption with a cut-off at 780 nm, suggesting that the predominant structure of MAI-vapor-CH 3 NH 3 PbI 3 was 3D CH 3 NH 3 PbI 3 rather than 2D CH 3 NH 3 PbI 3 .

| PSCs by low n quasi-2D perovskites (n < 5)
The low-dimensional halide perovskite materials have been intensively investigated in solar cells application over the last few years. 88 The first PSCs by 2D halide perovskites was reported by Smith et al. 44 In this study, the authors systematically investigated the origins of instability in traditional 3D CH 3 NH 3 PbI 3 and proposed a new concept of 2D layered perovskites, which was crafted by using PEAI molecules in place of MAI, as shown in Figure 2(A). The 2D layered perovskites with a structure of (PEA) 2 (MA) 2 Pb 3 I 10 (n = 3) exhibited a dramatically enhanced moisture stability compared to 3D S C H E M E 4 Schematic synthetic steps of two-dimensional (2D) perovskite microdisks. Reproduced with permission from Reference 68. Copyright 2016 Royal Society of Chemistry CH 3 NH 3 PbI 3 . However, as indicated in Figure 2(B,C), the PCEs were lower than 5% from PSCs with a device structure of FTO/compact TiO 2 /(PEA) 2 (MA) 2 Pb 3 I 10 (n = 3)/ spiro-OMeTAD/Au, due to low absorption coefficient and poor carrier transport of (PEA) 2 (MA) 2 Pb 3 I 10 (n = 3). Interestingly, Sargent et al. found that the 2D layered perovskite with (PEA) 2 (MA) 2 Pb n I 3n + 1 (n = 60) possessed superior long-term stability, and PSCs by this novel 2D perovskites exhibited decent PCEs. 41 PCEs observed from PSCs based on (PEA) 2 (MA) 2 Pb n I 3n + 1 (n = 60) were declined from initial 17.21 to 12.80% after 2 weeks, while PCEs from PSCs by 3D CH 3 NH 3 PbI 3 was reduced from initial 16.47 to 0.72% after 2 weeks in humidity air (RH 55%). However, these device performances were relatively poor since the insulating OS cations hindered charge transport. 46,47 Motivated by Smith's work, many researchers have recently focused on the development of alternative and effective spacer cations for balancing PCEs and stability of PSCs. 62,89 Kanatzidis et al. reported a highly oriented 2D perovskite thin films, where n-buthylammonium (n-BA) cation was substituted by PEA cation. 62 The 2D (n-BA) 2 (MA) 2 Pb n I 3n + 1 family of perovskite compounds (n = 1 to n = 4) was synthesized from a stoichiometric reaction between PbI 2 , MAI, and n-BA. As shown in Figure 3(A,B), (n-BA) 2 (MA) 2 Pb n I 3n + 1 perovskite thin film showed a highly remarkable orientation. In the case of n = 1, the (n-BA) 2 (MA) 0 Pb 1 I 4 has a preferential growth along the (110) direction, and thus revealed the (00l) reflection (crystallizing along the [hk0] plane, thereby showing only the [00l] reflection). As the n was larger (n > 1), the n-BA cations have an intention to confine the perovskite growth within the planar layer, but the MA cations have an intention to expand the perovskite growth outside the layer. For compound as the n = 2, the (0k0) reflections were split into the (111) and (202) reflections, which indicated that the vertical growth of the compound was pronounced. This effect was more serious for the compounds as n = 3 and 4. 62 Noted that the 2D (n-BA) 2 (MA) 2 Pb 3 I 10 thin films remained stable after 2 months as it was evident from the X-ray diffraction (XRD) spectra shown in Figure 3(C,D). Such enhanced stability was attributed to the hydrophobicity of the long BA cation chain, preventing direct contact of adventitious water within perovskite. PSCs by 2D (n-BA) 2 (MA) 2 Pb 3 I 10 with a device structure of FTO/TiO 2 (compact)/ TiO 2 (mesoporous)/(n-BA) 2 (MA) 2 Pb 3 I 10 /spiro-OMeTAD/ Au yielded a PCE of 4.02%. Such a low PCE was majorly attributed to poor charge carrier transport in the vertical transportation since the BA cation with a long chain serves as the potential barriers for charge carrier transporting. To circumvent the above issues, Liang et al. designed an alternative 2D perovskites with a short branched-chain, butylamine (iso-BA) spacer cations. 89 The 2D (iso-BA) 2 (MA) 3 Pb 4 I 13 (n = 4) were prepared from precursor solution by mixing PbI 2 , C 3 H 9 NH 2 , HI, and CH 3 NH 3 I at a stoichiometric ratio of 4:2:2:3 in DMF. As shown in Figure 4 from PSCs if the 2D (iso-BA) 2 (MA) 2 Pb 3 I 10 was cast from a hot solution. Such enhanced PCEs were ascribed to the out-of-plane orientation of 2D (iso-BA) 2 (MA) 2 Pb 3 I 10 thin films from the hot casting method, as indicated in Figure 4(B,C). It was reported that the low-dimensional perovskite materials tend to be growth in the direction parallel to the substrates, which hinders the charge transport in the vertical direction. The two-step method in preparation of the low-dimensional perovskite materials was proposed to address the above issues. Mhaisalkar et al. reported a sequential deposition method to fabricate lowdimensional perovskite thin films. 90 The fabrication method was presented in Figure 5(A). (IC 2 H 4 NH 3 ) 2 PbI 4 thin film was first prepared on the substrates by spincoating method, then the CH 3 NH 3 I solution was immersed on the surface of (IC 2 H 4 NH 3 ) 2 PbI 4 to form (IC 2 H 4 NH 3 ) 2 (CH 3 NH 3 ) n À 1 Pb n I 3n + 1 perovskite thin films. The 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to investigate the crystal orientation of (IC 2 H 4 NH 3 ) 2 (CH 3 NH 3 ) n À 1 Pb n I 3n + 1 thin films, concerning the dipping duration. As indicated in Figure 5 (B,C), the crystal orientation of perovskite thin films exhibited preferential crystal orientation in the out-ofplane direction as the dipping duration was last for 5 min. As a result, the highest PCE of 9.03% was observed from PSCs with a device structure of FTO/TiO 2 blocking layer/ mesoporous TiO 2 /(IC 2 H 4 NH 3 ) 2 (CH 3 NH 3 ) n À 1 Pb n I 3n + 1 / spiro-OMeTAD/Au.
The charge carrier transport along the out-of-plane direction is more difficult than that from the inorganic layer due to the insulating organic layer. It is accepted that additives could assistant the crystal growth of 2D perovskite films in the vertical direction. 91,92 The first additive used in the preparation of the low dimensional perovskites was demonstrated by Chen et al. 93 They added ammonium thiocyanate (NH 4 SCN) into precursor solution containing BAI, PbI 2 , and MAI, and then prepared perovskite thin films by a one-step method at room temperature. 93 The boundary-free with larger grains oriented in the vertical direction was found in the (BA) 2 (MA) 2 Pb 4 I 10 (n = 3) thin films processed with 1SCN additive (1 is the mole-ratio of NH 4 SCN to (BA) 2 (MA) 2 Pb 4 I 10 ). The perovskite thin films prepared by F I G U R E 5 (A) Schematic illustration of fabrication of (IC 2 H 4 NH 3 ) 2 (CH 3 NH 3 ) n À 1 Pb n I 3n + 1 perovskite. Two-dimensional (2D) grazingincidence wide-angle X-ray scattering (GIWAXS) data of (B) 1 min dipping time and (C) 5 min dipping time. Reproduced with permission from Reference 90. Copyright 2016. Wiley the above method exhibited enhanced charge carrier mobility. 93 As shown in Figure 6(A-D), they further fabricated PSCs based on (PEA) 2 (MA) 4 Pb 5 I 16 (n = 5) film processed by different concentrations of NH 4 SCN, and observed the optimal PCE of 11.01%. Such enhanced PCE was ascribed to the improved charge carrier mobility, vertically orientated 2D perovskite thin films. 94 The crystallization and charge carrier mobility were further improved by NH 4 SCN cooperated with NH 4 Cl. A PCE of 14.1% was observed from PSCs by (PEA) 2 (MA) 4 Pb 5 I 16 (n = 5). 91 It was also reported that solvent engineering was an effective method in controlling the morphology of quasi-2D perovskite films for achieving high crystallinity in the vertical direction. 95 Kanatzidis et al. first demonstrated that the mixed DMF and DMSO solvent had a significant influence on the crystallinity, crystal orientation, grain size, and film quality of quasi-2D perovskite films. 96 Gao et al. further explored its working mechanism. 97 It was found that for DMF only solvent, perovskite, and intermediate complex co-exist in the film after anti-solvent treatment, and therefore it has a tendency to grow in different directions after thermal annealing. However, by using DMF:DMSO mixed solvent, there was only one intermediate complex formed after anti-solvent treatment. As a result, the quasi-2D perovskites from DMF: DMSO mixed solvent exhibited highly oriented crystallization. In addition, alternative low polarity and suitable boiling point solvent dimethylacetamide also exhibited a remarkable effect on crystallization kinetics. 98 A mixture of DMF:DMSO: hydriodic acid (HI) has been reported to assist (3AMP)(MA 0.75 FA 0.25 ) 3 Pb 4 I 13 film (where AMP is 3-(aminomethyl)piperidinium) with much more preferred perpendicular orientation and better crystalline quality, yielding PSCs with a PCE of 12.04%. 99 The cooperation of different large spacer cations was further demonstrated by Chen et al. 100 Another spacer cation PEAI was added into BA 2 MA 4 Pb 5 I 16 (n = 5) perovskite precursor solution. 100 They found that PEAI in BA 2 MA 4 Pb 5 I 16 (n = 5) perovskite precursor solution can assist preferential nucleation and reduce the nucleation density, resulting in perovskite with large grains, and consequently PSCs with high PCEs (a PCE of 14.09%).
By substitution of the para position of PEA with fluorine, Zhang et al. introduced new OS F-PEA in 2D perovskites. 67 They found that F-PEA can assist perovskite sheets oriented well and enhance π orbital overlapping in the out-of-plane direction, leading to higher out-of-plane conductivity (Figure 7(A-D)). PSCs by F-PEA-based 2D perovskites with a device structure of FTO/c-TiO 2 /(F-PEA) 2 MA 4 Pb 5 I 16 /spiro-OMeTAD/Au exhibited a PCE of 13.64% (Figure 7(E)).
The development of efficient OSs and understanding the charge transport mechanism are very important to further boost PCEs of PSCs. Xu et al. 72 developed two multiple-ring spacer cations, 1-naphthalenemethylammonium (NpMA) and 9-anthracenemethylammonium (AnMA) for approaching high PCEs from PSCs. As shown in Figure 8  (A,B), the absorption and PL spectra of 2D perovskite thin films confirmed that 2D perovskite thin films based on either AnMA or NpMA OSs showed similar bandgaps and 3D-like phases. The slightly blue-shifted PL peak observed from 2D perovskite thin films based on NpMA OSs, as compared with that based on AnMA OS, indicated that the  (Figure 8(D)). However, it was found that PSCs fabricated by AnMA-based 2D perovskite thin films exhibited a much low PCE of 14.47%. Such poor PCE was attributed to the decreased charge carrier lifetime, enlarged trap density, and increased charge carrier nonradiative recombination loss in AnMA-based 2D perovskite thin films.
It was further reported that high-quality 2D perovskite thin films with oriented out-of-plane direction can be achieved through the incorporation of a suitable amount of FA + . 101 As shown in Figure 9 The ACI-type 2D perovskites exhibited a reduced bandgap compared to RP perovskites with the same n values, which was due to larger crystal symmetry and different stackings. 103 The first PSCs by ACI 2D perovskite (Gua)(MA) n Pb n I 3n + 1 (n = 1-3) was reported by Kanatzidis et al. (where Gua is guanidinium), which showed a PCE of 7.26%. 103 The film morphology of (Gua)(MA) 3 Pb 3 I 10 (n = 3) and QW's distribution were further tuned with the assistance of MACl additive. As a result, an impressive PCE (18.48%) was observed. 104 Recent advancement of PSCs by low n quasi-2D mixed perovskites (n < 5) is summarized in Table 1.

| PSCs by 2D/3D mixed perovskites
PSCs based on low n quasi-2D perovskite (n < 5) exhibited superior long-term operational stability but possessed poor PCEs compared to pure 3D perovskites. Toward the ends, PSCs based on 2D/3D mixed perovskites were developed to enhance PCEs and boost longterm stability as well.
Zhou et al. reported a two-step method in fabrication of (PEI 2 PbI 4 ) x (MAPbI 3 ) 1-x perovskite thin film (where x is 2%). 112 which was prepared by spin coating a mixture solution of PbI 2 and PEIÁHI, and then followed with deposition of an MAI layer through spin-casting method. They found that a small amount of PEI 2 PbI 4 can adjust the film morphology and crystallization of perovskite thin film. A PCE of over 15% was demonstrated from (PEI 2 PbI 4 ) x (MAPbI 3 ) 1-x (x = 2%) perovskite thin film. The enhanced device performance was ascribed to the Later on, Huang et al. reported MAPbI 3 incorporated with diethylammonium iodide ((CH 3 CH 2 ) 2 NH 2 I) as an OS. 113 As shown in Figure 10(A), it was found that the DA mixed perovskite film exhibited stronger diffusion peaks at the (110) and (220) plans, with a smaller full width at half maximum as compared with those from MAPbI 3 , indicating that these novel perovskite thin films possessed better crystallinity. PSCs based on (DA 2 PbI 4 ) 0.05 MAPbI 3 perovskite thin film exhibited a PCE of 19.05%, which was much higher than that (15.73%) by MAPbI 3 thin film (15.73%). The results are shown in Figure 10(B). The authors believed that enhanced PCEs were mainly ascribed to the large grains and fewer grain boundaries.
To further enhance both PCEs and stability, Chen et al. reported mesoscopic PSCs based on highly stable 2D/3D mixed perovskite (PEA 2 PbI 4 ) x (MAPbI 3 ) (where x = 0.0017) thin film. 114 As shown in Figure 11(A), no shift in the 2θ values and no new diffraction peaks from the (PEA 2 PbI 4 ) x (MAPbI 3 ) thin film were observed, indicating that a tiny of 2D perovskite was hard to alter the crystal structure of MAPbI 3 thin film. Moreover, (PEA 2 PbI 4 ) x (MAPbI 3 ) thin-film exhibited blue-shifted absorption and PL spectra (Figure 11(B,C)) compared to that of pure MAPbI 3 thin film, which is due to the presence of 2D sheet in (PEA 2 PbI 4 ) x (MAPbI 3 ) thin film. As shown in Figure 11 Most of the work on 2D/3D mixed perovskites was focused on MA cation at the A site. Lately, such an approach was extended to FA as well as to Cs 1+ cation, and the final goal is to replace toxic lead as well as to maximize the device stability. Wu et al. reported Cs 0.1 (FA 0.83 MA 0.17 ) 0.9 Pb(I 0.83 Br 0.17 ) 3 incorporated Gua + to form CsGuaFAMA mixed cation perovskite. 115 As shown in Figure 12(A), all of the samples have a similar band edge at~770 nm, which indicated that a tiny amount of Gua + does not affect the bandgap of perovskite thin films. However, 10% Gua + in perovskite thinfilm showed the highest PL intensity and then PL intensities were gradually decreased as the concentration of Gua + was over 10%, which is due to the formation of 1D T A B L E 1 Recent advancement of solar cells based on low n quasi-2D mixed perovskite (n < 5) GuaPbI 3 , as illustrated in Figure 12(B). The time-resolved PL studies (Figure 12(C)) demonstrated that the bimolecular radiative recombination was gradually enhanced by embedding larger Gua + into perovskite thin film. In addition, by incorporating Gua + , the suppressed defect, elongated PL lifetime, reduced energy disorder in the band edge, enlarged charge recombination resistance, and suppressed trap state density were found in the

| PSCs by 2D/3D perovskites with a bilayer structure
It has been reported that the effective mitigation of defects in perovskite thin films is essential to further enhance the device performance of PSCs. Therefore, the formation of 2D/3D heterojunctions structure is a promising method to improve the perovskite absorber layer with lower defect densities and longer charge carrier lifetimes.
Zhao et al. explored a series of diammonium iodides, NH 3 I(CH 2 ) 4 NH 3 I (C4), NH 3 I(CH 2 ) 8 NH 3 I (C8), and NH 3 I (CH 2 ) 2 O(CH 2 ) 2 NH 3 I (EDBE) (Figure 13(A)) to passivate perovskite surface and grain boundaries. 121 It was found that the molecular structure of diammonium salts has a profound effect on the surface morphology and phase purity of perovskite thin films (Figure 13(B)). Also, C4and EDBE-caped perovskite thin films showed phase transformation during treatment, which is unfavorable for charge carrier transportation. The C8-caped perovskite thin-film could efficiently passivate perovskite thin film and thus, the C8 salt was used to dope the electron transport layer PCBM. As a result, PSCs by the C8-caped perovskites thin film exhibited a PCE of over 17.60%.
Chen et al. reported an innovative facile way to prepare PEA 2 PbI 4 capping layer on the top of 3D Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 3D perovskite thin film. 122 It was found that the PEA 2 PbI 4 layer upon 3D perovskite thin film can simultaneously improve the device performance and stability of PSCs by 2D/3D perovskites, as shown in Figure 14. PSCs by the 2D/3D perovskites treated by 1 mg ml -1 PEAI solution showed a PCE of 18.51%, which was~10% enhancement compared to that by 3D perovskites. The enhanced device performance was attributed to the reduced nonradiative recombination loss in the 2D/3D perovskites. Similar work was also reported by Jiang et al. 123 They prepared a PEA 2 PbI 4 layer on top of FA 1-X MA X PbI 3 thin film to suppress the surface defects of perovskite thin films for approaching efficient PSCs, as shown in Figure 15(A,B). It was found that a thin PEAI layer can not only stabilize the α phase of FAPbI 3 , but also can slow down the degradation of underneath 3D perovskite, which was confirmed by both XRD and XPS studies (Figure 15(C,D)). The PSCs by 2D/3D perovskites with a device structure of ITO/SnO 2 / Recent advancement of PSCs by 2D/3D perovskites with a bilayer structure is summarized in Table 3.

| PEROVSKITE PDS BY THE LOW-DIMENSIONAL HALIDE PEROVSKITES
PDs refer to light-responsive devices that convert optical signals into electric signals, which is central to modern science and technology because of their great applications, including imaging, vision, and digital display technology.
PDs could catalog as the two-terminal and three-terminal devices based on device architecture. The twoterminal devices are composed of photodiode and photoconductor, and the three-terminal devices refer to phototransistors with source, drain, and gate electrodes. PDs with a two-terminal device structure generally provide a low driving voltage and fast photoresponse owing to a narrow electrode spacing (~100 nm). Noted that the photodiodes-based PDs cannot exceed 100% external quantum efficiency (EQE) since no additional charge injection occurs under the reverse bias, but it happens in photoconductor-based PD. The photoconductive gain (G) is given by the ratio of trapped carriers (τ lifetime ) to the transit time of the transported carriers (τ transit ) through the device. The G is described as G ¼ τ lifetime τ transit . Generally, phototransistor-based PDs need a high driving voltage to obtain decent device performance due to their wide electrode space.
Currently, PDs market was mainly dominated by photodiodes based on crystalline inorganic semiconductors, such as silicon, Ge, and InGaAs. However, the preparation of these inorganic PDs was not a cost-effective process. Moreover, some of these inorganic-based PDs are required to be operated at extremely low temperatures, which substantially limit their applications. Thus, it is necessary to develop high-performance PDs with easy manufacturing and cost-effective techniques.
In recent years, the emerging of the low-dimensional halide perovskite single-crystal has attracted tremendous attention in the fabrication of PDs owing to their Abbreviations: 2D, two-dimensional; 3D, three-dimensional; PCE, power conversion efficiency.
remarkable optical and electronic properties, such as reduced defects and enhanced charge transfer, compared with the low-dimensional halide perovskite polycrystalline films. However, the challenges in reducing dark current and enhancing photocurrent along the direction of the charge transporting need to be addressed. Feng et al. developed a series of perovskite nanowire of (BA) 2 (MA) n À 1 Pb n I 3n + 1 with a pure (101) crystallographic orientation. 134 They found that (BA) (MA) n À 1 Pb n I 3n + 1 single-crystal exhibited fewer surface defects and grain boundaries, which allow efficient charge transport in the inorganic layer [PbI 6 ]. In addition, the BA + as an insulating organic barrier is responsible for suppressing the dark current and the exotic crystal edges in perovskites are a benefit for efficient exciton dissociation. Thus, (BA) 2 (MA) 3 Pb 4 I 13 single crystal-based PDs exhibited a high responsivity of 1.5 Â 10 4 A W À1 and a specific detectivity of over 7 Â 10 15 Jones (cm Hz 1/2 W À1 ).
Zhu et al. reported large-scale low-dimensional halide perovskite (C 4 H 9 NH 3 ) 2 PbBr 4 by the potassium ions assisted controllable crystal growth during the precipitation process. 135 They found that the (C 4 H 9 NH 3 ) 2 PbBr 4 nanobelts exhibited intense PL and good stability under ambient conditions. The PDs fabricated by dropping the (C 4 H 9 NH 3 ) 2 PbBr 4 nanobelts on the interdigitated Au electrodes exhibited a low dark current of 1.5 Â 10 À9 A, and photocurrents of 8.21 Â 10 À8 A and 6.76 Â 10 À8 A at light irradiation under 365 and 405 nm, respectively, and under an applied voltage of 5 V. They attributed such high photocurrent to the interconnected network enable better contact with the electrodes, resulting in a better carrier transfer, and the pores within the (C 4 H 9 NH 3 ) 2 PbBr 4 nanobelts could increase light scattering, leading to the light-harvesting capability for photodetection.
Wang et al. reported a high-performance PDs by freestanding (C 4 H 9 NH 3 ) n (CH 3 NH 3 ) n À 1 Pb n I 3n + 1 single crystals. 60 The (C 4 H 9 NH 3 ) n (CH 3 NH 3 ) n À 1 Pb n I 3n + 1 single crystals were prepared from the water-air interface. The crystallization process occurred through nucleation and where Λ depends on supersaturation, k B is the Boltzmann constant, and T is temperature. The energy barrier for nucleation is different in bulk solution and at the water-air interface.
where ξ is the cohesive energy of precursor molecules in the cluster, ξ A is the energy of precursor molecules, σ is the surface tension coefficient, and M j S and M A are the total molar concentrations of solvent and precursor molecules, χ is the increased energy of precursor molecules induced by extra tensile elastic stress at the waterair interface as indicated in Figure 16(C). Thus, a higher nucleation probability at the water-air surface is expected in contrast to that in the bulk solution. Therefore, in the process of crystallization, the self-assembly C 4 H 9 NH 3 + precursor cation at the water-air interface acts as a soft template helping the growth of nanostructures. The higher solvation energy of precursor molecules at the asymmetric water-air interface offers higher chemical potentials, leading to a low energy barrier and faster in-plane growth. Consequently, inch-size freestanding quasi-2D perovskite single crystals have been achieved at the water-air interface. 60 The PDs based on the quasi-2D perovskite single crystals with the smallest QW thickness (n = 1) exhibited a strikingly low dark current of $10 À13 A, higher on/off ratio of $10 4 , and faster rise time of $1.7 μs and drop time of 3.9 μs. 60 Similar work was reported by Liu et al. 137 They synthesized high-quality 36 mm sized 2D (PEA) 2 PbI 4 by inducing extra surface tension. As shown in Figure 17(B), the XRD patterns indicated that the top plane and side plane was well-defined with the (001) series and the (010) series of reflections, respectively. They found that PDs by the (001) plane of (PEA) 2 PbI 4 exhibited better photoresponse compared to that by the (010) plane, with a dramatically low dark current of 3.06 Â 10 À12 A under a bias of 5 V. Such low dark current was ascribed to boundary-free of (PEA) 2 PbI 4 single crystals. The (PEA) 2 PbI 4 -based PDs possessed the highest detectivity of 1.89 Â 10 15 Jones under a bias of 5 V, which was a record-high sensitivity for PPDs.
Recent advancement of PPDs by the low-dimensional perovskites is summarized in Table 4.
The iodide-based analog PEA 2 (MA) n -1 Pb n I 3n + 1 (n = 5) has been demonstrated by Sargent et al. to outperform 3D MAPbI 3 (n = ∞) for near-infrared emission, with an EQE of 8.8% and radiance of 80 W sr À1 m À2 . 145 They further ascribed the superior performance to a cascading energy transfer that funnels photoexcitation to the lowest-bandgap phase within mixed quasi-2D perovskite thin film.
Huang et al. demonstrated quasi-2D perovskite LEDs based on (NMA) 2 (FAPbI 3 ) n -1 Pb n I 3n + 1 , where NMA is naphthylmethyl ammonium, with a recorded EQE of 11.7% and radiance of 82 W sr À1 m À2 . 146 Similarly, they attributed the superior device performance to the funneling mechanism, which occurs within sub-ns timescales and outcompetes nonradiative exciton quenching and increases radiative recombination.

| PEROVSKITE LASERS BY THE LOW-DIMENSIONAL HALIDE PEROVSKITES
A laser refers to a device that can emit coherent light with strong intensity and perfect directionality. By taking into consideration of high absorption coefficient and low density of defects, low-dimensional halide perovskites are excellent gain materials for the development of highperformance lasing devices.
Li et al. demonstrated that FA-based 2D RR perovskites thin films of (NMA) 2 (FA) n À 1 Pb n X 3n + 1 showed superior optical gain properties. 155 Different from the stimulated emission mechanism of 3D perovskites with electron-hole plasma at room temperature, 2D RR perovskites exhibited the feature of strongly bound electron- hole pairs (excitons) and naturally form an energy cascade. It was observed that amplified spontaneous emission (ASE) from these perovskites with a low threshold (<20.0 ± 2 μJ cm À2 ), tunable wavelengths from visible to the near-infrared spectral range (530-810 nm), and good photostability with an operation duration exceeding 1.2 Â 10 8 laser pulses.
After that, Zhang et al. demonstrated roomtemperature ASE and lasing from mixed multiple QWs in 2D-RPPs of (BA) 2 (MA) n À 1 Pb n Br 3n + 1 . 156 They performed femtosecond transient absorption characterization to reveal an ultrafast population transfer along the energy cascade from small-n-QW to large-n-QW, concentrating photo excitations at the lowest bandgap QWs (n ≈ ∞), enabling the population inversion for stimulated emission. They found that the 2D-RPPs of (BA) 2 (MA) 5 Pb 6 Br 19 (n = 6) show an ASE threshold of 13.6 μJ cm À2 and a high gain coefficient (G) of 112 cm À1 . Their findings reveal that 2D RPPs can be potentially applied for electrically driven lasers in on-chip integration of photonics and electronic circuits.
Recent advancement of perovskite lasers based on the low-dimensional perovskites is summarized in Table 6.

| SUMMARY AND OUTLOOK
The emergence of low-dimensional perovskites as semiconductors has revolutionized next-generation optoelectronics owing to their extraordinary optical and electronic properties. In this short review, we discussed the optoelectronic properties of the low-dimensional  halide perovskites. From this point of view, we highlighted the properties of 2D perovskite MQWs structure. The decrease in the value of n could lead to a compression of exciton wavefunction in the QW due to dielectric confinement, which results in enhanced exciton binding energy within 2D perovskites. In addition, the observation of charge carrier density at the ES is close to that of metal, which implies that the low-dimensional perovskites offer great opportunities for designing novel optoelectronics. Subsequently, we reviewed synthetic methods for the preparation of the low-dimensional halide perovskites, including the one-pot synthetic method, direct growth method, polar solvents free method, and vaporassisted method. Despite the great progress in the synthetic methods for the preparation of low-dimensional halide perovskites, more work needs to be done in the future toward the understanding of the crystal nucleation and growth mechanism, aiming to achieve ultra-uniform low-dimensional perovskite thin films. The low-dimensional perovskites with nontrivial optical and electronic properties display tremendous potential applications in solar cells, PDs, LEDs, and lasers. To further explore the unique characteristics of the lowdimensional perovskites for all these applications, more research is demanded to focus on materials design and the fundamental physical and chemical properties of the low-dimensional perovskites.
In terms of PSCs by the low-dimensional perovskites, even though enormous efforts have been devoted to approaching the vertical orientation of perovskite thin films, aiming to accelerate the charge carrier transport in the vertical direction, the efficiencies were still hard to compete with those by 3D perovskites. To overcome it, we think that we should establish the judicious selection criteria for OS's design principles for 2D perovskites, for example, the effect of different spacer lengths, functional units, and the substituent groups. It is necessary to develop low-dimensional perovskite structures with pure phases, like adopting mechanical exfoliation process, to avoid the drawbacks of the anisotropic properties. The synthetic techniques and characterization are required to study the optical and electronic properties of 2D perovskites, and a deep understanding of structure-property relationships of different 2D perovskite phases needs to be explored. To substitute the insulating OSs with conjugated OSs would be a fascinating direction for boosting PCEs of PSCs, In addition, forming low bandgap organic donor (or acceptor)/2D perovskite bulk heterostructure would not only contribute to additional absorption of the 2D perovskite but also minimize the energy required for dissociating exciton into free charge carriers.
For PPDs by the low-dimensional perovskites, great efforts are still demanded to achieve high device performance. To minimize the dark current in PPDs by the low-dimensional perovskites, surface passivation between the low-dimensional perovskites and charge transport layers, utilization of Lewis acid and/or Lewis base to passivate PbX 3À and/or Pb 2+ anti-site defects, and synthesis of the low-dimensional perovskite single crystals (no grain boundary effect) are crucial importance to reduce trap states, which is also essential to achieve high response speed.
Although low-dimensional perovskites are being considered for making next-generation LEDs and lasers in the future, there are some significant challenges. For example, the stronger exciton-lattice coupling and randomly oriented polycrystalline in the low-dimensional perovskite thin films are responsible for fast nonradiative exciton quenching. Toward the ends, large-scale size single-crystalline perovskite films with higher carrier mobility, lower defect density, are expected to replace the conventional polycrystalline perovskite films. It has been reported that the additives can not only effectively passivate perovskite surface traps, but also tune the film morphology, which thus ensures reduced nonradiative recombination. 161 Another effort should be focused on increasing PLQY by metal ions doping and defect passivation with different ligands. Through these novel designs, we would expect to develop devices with high performance by the low-dimensional perovskites in the future.