Pressure responses of halide perovskites with various compositions, dimensionalities, and morphologies

Metal halide perovskites (HPVs) have been greatly developed over the last decade, with various compositions, dimensionalities, and morphologies, leading to an emergence of high-performance photovoltaic and optoelectronic applications. Despite the tremendous progress made, challenges remain, which calls for a better understanding of the fundamental mechanisms. Pressure, a thermodynamic variable, provides a powerful tool to tune materials’ structures and properties. In combination with in situ characterization methods, high-pressure research could provide a better fundamental understanding. In this review, we summarize the recent studies of the dramatic, pressure-induced changes that occur in HPVs, particularly the enhanced and emergent properties induced under high pressure and their structure-property relationships. We first introduce the characteristics of HPVs and the basic knowledge of high-pressure techniques, as well as in situ characterization methods. We then discuss the effects of pressure on HPVs with different compositions, dimensionalities, and morphologies, and underline their common features and anomalous behaviors. In the last section, we highlight the main challenges and provide suggestions for possible future research on high-pressure HPVs.


I. INTRODUCTION
In the past several years, photovoltaic devices based on halide perovskites (HPVs) have made impressive progress in their development, attaining energy conversion efficiencies of over 25%, surpassing the already commercialized polysilicon, cadmium telluride, and copper indium gallium selenide photovoltaic devices. [1][2][3][4] Furthermore, HPVs have also shown great potential in optoelectronics, including in lightemitting diodes, 5-8 photodetectors, 9 and lasers. 10,11 Their outstanding photovoltaic and optoelectronic functionalities have been attributed to their superior electronic and optical properties, such as long charge diffusion distances, small carrier effective masses, and high optical absorption coefficients. [12][13][14][15] These excellent characteristics, along with low fabrication cost, motivate further intense study for both theoretical and technological reasons. 16 Moreover, the rapid development of HPVs for energy-related applications does not only stem from their outstanding functionalities but also from the substantial adjustability of their crystalline structures and dimensionalities.
Despite their unique properties and impressive achievements, inherent challenges remain in HPVs that need to be addressed, calling for a better understanding of the mechanisms that dominate their outstanding functionalities. The soft lattices of HPVs render them sensitive to mechanical compression, which can effectively adjust their atomic and electronic structures, as well as their physical properties, without changing their chemical composition. In combination with various in situ characterization probes, high-pressure research could further our fundamental understanding of the structure-property relationship. As a fundamental parameter, pressure can efficiently adjust the structures of materials, leading to exceptional enhancements from external dimensional tuning. In this review, we summarize the recent progress made in the research of pressure-induced changes in the structures and properties of HPVs. We begin by introducing the characteristics of HPVs and the fundamental knowledge of highpressure techniques together with in situ characterizations. We then discuss the effects of pressure on HPVs with different compositions, dimensionalities, and morphologies, focusing on the enhanced and emergent properties induced by high-pressure treatment, as well as the structure-property relationship. The final section highlights the main challenges and outlook of high-pressure research on HPVs.

A. Crystal structures and physical properties of HPVs
Perovskite is a class of materials that has the general formula ABX 3 . 17 As shown in Fig. 1(a), cation "B" has six nearest-neighbored anions "X," and cation "A" occupies a vacancy formed by eight cornersharing [BX 6 ] 4− octahedrons. 18,19 In the case of HPVs, typically an "A" site can be occupied by either an inorganic ion (e.g., Cs + ) or an organic ion [e.g., CH 3 NH 3 + (MA), NH 2 CH NH 2 + (FA)], thereby generating different types of HPVs; "B" is a divalent metal cation (e.g., Pb 2+ , Sn 2+ , or Ge 2+ ) and "X" is a halide anion (e.g., Cl − , Br − , or I − ). The crystal symmetry and phase stability of the perovskite structure can be determined using the Goldschmidt's tolerance factor t (R A + R B )/ 2 √ (R B + R X ) and an octahedral factor m R B /R X . The tolerance factor (t) is defined as the ratio of the distances between A-X and B-X in the idealized rigid-body ball model, where RA, RB, and RX are the ionic radii of A, B, and X ions, respectively. 20 In organicinorganic HPVs, a stable structure can exist with 0.81 < t < 1.11 and 0.44 < m < 0.90. If the tolerance factor is in the range of 0.9-1.0, a cubic structure is expected, while different values give less-symmetric crystal structures. Figure 1(b) shows t of different HPVs. 21 The earliest research conducted on organic-inorganic lead HPVs (e.g., MAPbX 3 , X Cl, Br, and I) mainly focused on their outstanding photovoltaic performance. All inorganic compounds (e.g., CsPbX 3 ) then became attractive because of their high stability and tunable functionalities. Both theoretical and experimental evidence has shown that the three elements of A, B, and X in the ABX 3 crystalline structure can be chemically substituted, resulting in a series of mixed compounds. 22,23 In addition, two divalent B 2+ cations can be replaced by a trivalent cation and a monovalent cation to form a quaternary halide with a dual perovskite structure. 10 This strategy has been used to explore novel lead-free double perovskites, for example, using Ag + and Bi 3+ cations to replace two Pb 2+ cations. 24 More recently, the reduced-dimensional HPVs, especially twodimensional (2D) layered perovskite materials, have given rise to a very diverse semiconductor subgroup with high tunability and eminently adjustable photophysical properties. 8,25 Ruddlesden-Popper (RP) HPVs can be imagined as molecular sheets inserted into a three-dimensional (3D) structure in one direction, which can be described as L 2 A n-1 B n X 3n+1 , where A represents cations in the voids among the octahedra (e.g., CH 3 NH 3+ ), L represents organic cations between different layers, and n is the number of metal halide octahedral layers between insulating L organic layers. 26 The 2D structure consists of a single-layer or a multilayer [BX 6 ] 4− octahedral sheet [A n-1 B n X 3n+1 ] 2− separated by L cations, and thus the 3D perovskite framework is reduced to a 2D structure due to steric hindrance [ Fig. 1(c)]. 27 The obtained 2D compounds possess a structure of alternating organic and inorganic layers, with adjacent organic layers interacting through weak van der Waals interactions. Figure 1(d) shows examples of RP-type HPVs. Due to their unique structure, 2D HPVs exhibit better stability and higher adjustability.
HPVs are typically direct bandgap materials with high optical absorption coefficients. The bandgap of a semiconductor determines the absorption width of the solar spectrum, and HPVs exhibit a bandgap ranging from about 1 to 3 eV. For 3D HPVs, most of their bandgap edges consist of orbitals from metal B and halogen X. 29 Their valence band maximum (VBM) usually consists of both np 6 orbitals from X (n is the principal quantum number, n 5, 4, and 3 for I, Br, and Cl, respectively) and ns 2 orbitals from B (n 6, 5, and 4 for Pb, Sn, and Ge, respectively). The conduction band minimum (CBM) mostly consists of the empty np 0 orbital from B. 20 The orbitals of A in HPVs structure are far from the band edges, so their direct contribution to the bandgap is negligible. 30 However, A cations affect the lattice constants, which indirectly influence band structure. In addition, A cations can affect inorganic octahedral distortion, dielectric constants, and hydrogen bonding. 31 Qualitatively, the band structure of the perovskites is strongly dependent on crystallographic symmetry. 32 Different from the 3D HPV analogs, 2D HPVs have less size constrict for the L-sites (A-sites comply with the same rules as 3D HPVs). Organic cations can not only act as an insulating barrier that confines the charge carriers in the 2D plane, but also as dielectric regulators that determine the electrostatic forces applied to the carriers.
The interaction among metal-halide units in 3D HPVs results in the formation of electron bands that have large bandwidths. In addition to possessing a long distance for carrier transport, HPVs absorb a wide range of wavelengths, possessing a high absorption coefficient, producing an outstanding photovoltaic performance. 33 Different from the 3D compounds, the special arrangement of alternating organicinorganic layers of 2D HPVs produces crystal-ordered 2D quantum wells that exhibit strong structural distortion and significant quantum confinement effects, giving rise to the formation of free excitons and trapped excited states during photoexcitation. [33][34][35] The bandgaps of the materials can be optimized by different methods, influencing optoelectronic performance. One possible way to adjust the bandgap of the material is to change the chemical composition or doping of the material, such as by changing the proportion of constituent halides. The mixed-cation HPVs with lead and tin at the B sites can also reduce the bandgap, but this shortens their carrier lifetime and compromises their stability. 10,[36][37][38] However, mechanical compression provides another way to effectively adjust the structures and properties. The relatively soft lattices of HPVs render them suitable for high-pressure investigations, which can not only enhance our fundamental understanding of them but also allow us to extract from them emergent and enhanced properties. The application of external pressure can cause dramatic changes in both the structural and physical properties of HPVs, including the shrinkage and titling of [BX 6 ] 4− octahedra, lattice disordering, bandgap closing, and carrier lifetime modification. 39 Furthermore, as ions A, B, and X usually have different compressibilities, the tolerant factors t change as a function of applied pressure, which leads to improved structure and electronic tunability.

B. Basic knowledge of high-pressure science and technology
As a thermodynamic parameter, pressure can be employed to tune the properties of materials by adjusting their interatomic distances, electronic orbitals, and bonding patterns. [40][41][42][43] The static compression realized by diamond anvil cells (DACs) has produced attainable pressures of up to hundreds of gigapascals (1 GPa 10 000 atm). 13,44 DACs [ Fig. 2(a)], with a pair of diamond anvils in opposing geometry, are used to compress samples immersed in a pressure transmitting media (PTM), like silicone oil, neon, and helium. At room temperature, helium solidifies at about 11 GPa, which provides the best hydrostatic-pressure conditions under high pressure. Diamond is a material with excellent transparency to a very wide range of electromagnetic radiation, making it attractive for in situ synchrotron, x-ray, and in-laboratory optical probes [Figs. 2(b) and 2(c)]. Furthermore, in situ electrical transport, photocurrent, and magnetic susceptibility measurements have also been developed within DACs.
In the last several years, the previously small field of highpressure science and technology has grown significantly, becoming increasingly attractive to the physics, chemistry, and materials sciences communities. 27,32,[45][46][47][48][49][50][51][52] Combing in-laboratory and synchrotron-based characterization methods, one may elucidate many interesting phenomena, as well as explore novel materials with enhanced and emergent properties, which are inaccessible at ambient pressures. Moreover, high-pressure synchrotron-based techniques have developed rapidly, greatly contributing to high-pressure research in physics, chemistry, and materials sciences 40,53,54 in various forms: x-ray diffraction (XRD) characterizes long-range crystal structure; x-ray emission spectroscopy provides information on electronic states; nuclear resonant x-ray spectroscopy examines phonon densities of state; x-ray Raman spectroscopy monitors changes in chemical bonding; the pair distribution function (PDF) reveals local bonding features at the atomic scale; and x-ray imaging examines internal strain, hierarchical structure, and dynamic processes. 32 By combining these synchrotron-based characterization tools with in-laboratory physical property measurement methods, including absorption spectroscopy, photoluminescence (PL), and photocurrent and electrical transport measurements, one can monitor the structural, optical, electrical, and optoelectronic properties in situ under high pressure. 55 By enabling new structures and properties, high pressure might not only offer new opportunities to explore novel materials, but might also introduce new ways to study structure-property relationships. Note that the high-pressure results reported by different research groups are sometimes inconsistent. 56 This discrepancy is possibly due to the different high-pressure methods used, as well as varying experimental conditions. A particularly important aspect is the pressure condition, including pressure anisotropy, strain levels, and gradients, which are determined by the pressure transmitting media and pressureloading dynamics. 57 Poor hydrostatic conditions result in higher deviatoric stresses that typically facilitate or even change pressureinduced transition processes.
The study of HPVs under high pressure can be traced back to the 1990s and focused on their P-T phase diagrams of MAPbX 3 . 58 Intense focus has been on HPVs since they delivered outstanding performances in photovoltaic applications. 2,10,[59][60][61][62][63][64][65][66][67] In recent years, high-pressure research in HPVs materials has produced increasing numbers of new discoveries and phenomena. 52,68-76 Several scientific review papers have focused on HPVs under high pressure, 15,27,39,47,48,[77][78][79][80] but a comprehensive review addressing the recent high-pressure discoveries regarding HPVs has remained rare. Here, we summarize the pressure effects on the lattice and the electronic, optical, and electrical properties of HPVs, with different compositions, dimensionalities, and morphologies. We focus on the enhanced and emergent properties induced by high-pressure treatment. High-pressure studies on the structural and physical properties of HPVs provide valuable clues for the further optimization of these materials, helping to extract enhanced performances and provide experimental baselines for theoretical modeling.

II. PRESSURE-TUNED AND PRESSURE-ENHANCED PROPERTIES
In general, pressure-induced shortening of the bond length narrows the bandgap while bending of the B-X-B angle widens it. 81 Effective tuning of the structures under high pressure leads to changes in their original nature and can produce emergent properties. In the past several years, research on the high-pressure behavior of HPVs has achieved unprecedented rapid development. 5,6,11,20,33,37,43,54,57,70,[82][83][84][85][86][87] As shown in Fig. 3, gratifying achievements in comprehensive analysis, including that related to pressure-induced variations in crystal structure, PL, electrical resistance, and photocurrent, have been increasingly reported (Fig. 3). In this section, we have summarized the pressure-induced structural evolution and property improvements of HPVs, with various compositions, dimensionalities, and morphologies.
A. 3D organic-inorganic HPVs 3D organic-inorganic HPVs are the type studied most and they exhibit the best photovoltaic performance. In particular, the electronic structures and optical properties of MAPbX 3 under pressure have been experimentally and theoretically investigated. 55,[89][90][91][92] It has been reported that organic-inorganic HPVs exhibit cubic, tetragonal, and orthorhombic phases under different temperatures and pressures. The cubic structure typically possesses a wider electronic band, showing a smaller effective mass and higher mobility. The lower symmetry of tetragonal and orthogonal structures under different temperatures provides some references for the understanding of pressure-induced structural evolution. Wang et al. reported that MAPbCl 3 shows pressure-induced structural evolution, as in the following: Pm3m (1 atm) → Pm3m (0.8 GPa) → Pnma (2.4 GPa). The transformation from phase І to phase Ⅱ is attributed to the cubiccubic isostructural phase transition [ Fig. 4(a)]. 87 The cubic Pm3m MAPbBr 3 transforms to Im3 at 0.4 GPa, further to Pnma at 1.8 GPa, then starts amorphization at 2 GPa, reverting to the original crystalline structure upon decompression [ Fig. 4(a)]. 54 Capitani et al. reported that MAPbI 3 transforms from tetragonal I4/mcm at ambient pressure to orthorhombic Imm2 at 0.26 GPa, undergoing gradual amorphization at above 3 GPa [ Fig. 4(a)]. 90 Similar high-pressure behavior has been reported for the FA based compounds. In particular, the structural change in the FAPbBr 3 crystal exhibits a similar pressure-induced phase transition to MAPbBr 3 but requires higher pressures: Fig. 4(a)]. 86 The pressure points corresponding to the structural transitions of FAPbBr 3 are higher than those of MAPbBr 3 crystal, which means that the former is less compressible. It is reported that FAPbI 3 shows the following structural evolution under pressure Pm3m (1 atm  transitions of P4mm (1 atm) → Pnma (0.7 GPa) → amorphization (3 GPa). 37 Lee et al. reported that the phase transitions of FASnI 3 follow Pm3m → Im3 → I4/mmm → amorphization [ Fig. 4(a)]. 93 It is inevitable that pressure-induced structural transitions lead to variations in bandgaps. According to the so-called Shockley-Queisser theory, the optimum bandgap for solar cells is 1.34 eV, which offers a theoretical power conversion efficiency of 33%. However, the bandgaps of HPVs are normally greater than this ideal value. In situ high-pressure optical measurements, such as absorption spectroscopy and PL spectroscopy, could be used to monitor and understand the bandgap evolution of HPVs. During compression, the electron distribution and orbital interactions within the system can be changed by decreasing interatomic distances, thus adjusting the band structures. For the organic-inorganic HPVs with [BX 6 ] 4− octahedra, the pressure response of the bandgaps is between 17 and 100 meV/GPa. Kong et al. achieved a simultaneous evolution of bandgap narrowing (0.03 eV) and PL lifetime prolongation of single MAPbI 3 crystals under a pressure of 0.3 GPa. 82 Under ambient conditions, the bandgap of MAPbI 3 is 1.537 eV. As pressure increases, the bandgap gradually redshifts to 1.507 eV at 0.32 GPa, which is mainly caused by the upshift of VBM due to the shortening of bond length. In addition, a sudden blueshift occurs as pressure further increases, according to the phase transition from tetragonal I4/mcm to orthorhombic Imm2 [ Fig. 4(b)]. Time-resolved PL measurements were performed under pressure, which revealed a carrier lifetime prolongation from 425 ns at ambient pressure to 715 ns at 0.3 GPa [ Fig.  4(c)], enhanced by ∼70%. The bandgaps of various 3D organicinorganic HPVs and their relevant parameters are summarized in Fig.  4(d). The enhancement of carrier lifetimes in MAPbI 3 , MAPbBr 3 , and FAPbI 3 , are summarized in Fig. 4(e).
Electrical conductivity and photocurrent are important characteristics for applications in optoelectronic devices. High-pressure studies on the electrical properties of HPVs are critical to revealing structure-property relationships. Wang et al. conducted electrical resistance measurements on MAPbBr 3 using a four-probe method within a DAC [ Fig. 5(a)]. 54 The measurements showed a rapid increase in the electrical resistance from 2 to 25 GPa, and the maximum value reached five orders of magnitude larger than the initial value, which may be attributed to pressure-induced amorphization. However, Jaffe et al. reported a distinct behavior of the pressure-induced conductivity  91 The conductivity increased during compression and reached a plateau at about 10 GPa, followed by a slight decrease up to 30 GPa. At higher pressures of up to 51 GPa, a significant increase in the conductivity by two orders of magnitude was observed [ Fig. 5(b)]. The authors ascribe the sharp increase in electron transport to pressure-induced metallization. It is difficult to explain the distinct behavior in these two studies from two different research groups, and more systematic studies are needed. Ou et al. reported that the photocurrent of FAPbBr 3 increased remarkably at 1.3 GPa, and the highest value was nearly ten times higher than that of MAPbBr 3 , and about three times higher than that of MAPbI 3 . 95 Lu et al. systematically investigated the differences in the structures and properties of lead-free HPV before and after high-pressure treatments. 37 MASnI 3 was treated by two sequential compressiondecompression cycles at up to 30 GPa. During the first compression process the phase transition sequence is P4mm (1 atm) → Pnma (0.7 GPa) → amorphization (3 GPa). Surprisingly, the crystalline nature of MASnI 3 can be maintained during the re-compression process up to 31 GPa [ Fig. 5(c)]. Pressure can change the crystal structures and properties of HPVs; however, whether the unique properties generated under high pressures can be retained in the sample upon decompression is a matter of great concern. From this perspective, Lu et al. systematically compared the structural stability, electrical conductivity, and photo responsiveness of lead-free MASnI 3 HPV before and after high-pressure treatment, by applying two sequential compression-decompression cycles at up to 30 GPa. 37 Remarkably, in situ XRD, Raman spectroscopy, transport, and photocurrent measurements show that the pressure-induced amorphous and recrystallized sample exhibits considerable improvements in structural stability, electron transport, and photo responsiveness [Figs. 5(c)-5(e)]. In the first cycle, the MASnI 3 experiences pressure-induced amorphization at about 3 GPa and recrystallizes to a crystalline phase upon pressure release, a typical pressure response in other HPVs. 48,84,90 Surprisingly, in the second compression process, no amorphization can be observed above 30 GPa. In situ resistance measurements show that the conductivity of MASnI 3 after high-pressure treatment is three-fold higher than the initial value [ Fig. 5(d)], which demonstrates higher electron mobility. Ambient pressure photocurrent measurements also disclose a significant enhancement in the photoresponsiveness of MASnI 3 after high-pressure treatment [ Fig. 5(e)]. Therefore, the optoelectronic properties of the pressure-treated perovskites are considerably enhanced, indicating the significant effects of pressure on HPVs. In addition, Yan et al. reported the electrical transport and photoresponse of MAPbBr 3 using in situ alternating-current (AC) impedance spectroscopy and photocurrent measurements. 70 Since the electrons and ions have very different responses to varied-frequency AC signals, in situ AC impedance spectroscopy provides an effective way to distinguish between electronic and ionic conduction. The mixed conduction from both ions and electrons was found in MAPbBr 3 and an ionic-electronic conduction transition occurred at 3.3 GPa. Combined with the results of structural variation, the conduction transition can be attributed to pressureinduced phase transitions. From the photocurrent measurements, the response to light of the high-pressure phase is obviously superior to that of the initial phases, signifying that pressure is effective in further enhancing the photovoltaic properties of the halide perovskite.
From these high-pressure investigations, one can optimize the performance of the 3D HPVs via introducing artificial stress and strain. Here, based on the pressure-induced changes in the crystal structure, lifetime, electrical resistance, and photocurrent of organic-inorganic HPVs, we underline some general conclusions and common features: (1) Pressure-induced structural transitions and further amorphization occur in 3D organic-inorganic HPVs, usually in the order of a few gigapascals, owing to their soft lattices and low bulk modulus; (2) The electronic bandgap usually narrows by pressure-induced lattice contraction and widens by increasing octahedral tilting angles or amorphization; (3) Electrical resistance can increase by two to five orders of magnitude during compression.

B. 3D inorganic cesium lead HPVs
The substitution of organic cations (MA + and FA + ) by inorganic Cs + is a direct and effective way to enhance the stability of HPVs. 10,44,72,96 The structures and properties of CsPbX 3 show a sensitive pressure response, which suggests that pressure can effectively tune the lattice and electronic configurations in the inorganic HPV analogs. 38,84,96,97 In particular, nanostructured inorganic HPVs have attracted much attention due to their nano-size effects. 11,22,98,99 Generally, the multiple domains in bulk HPVs possess stacking faults and interfacial defects, which not only complicate structural assignment but also affect the kinetics of phase transitions at high pressure. For nanostructured inorganic HPVs with a uniform morphology, these problems can be avoided. Furthermore, nano-size effects can produce novel phenomena under high pressure, such as a unique morphology modulation and improved structural stability. In Table I, we summarize various crystal structures, phase transitions, amorphization, and pressures for the minimum bandgaps of nanostructured and bulk inorganic HPVs. In contrast with the case of organic-inorganic HPVs, the isostructural phase transition was discovered in bulk and nanostructured inorganic HPVs, caused by lattice distortion and PbCl 6 octahedral tilting. Different morphologies and sizes affected the structural phase transition and physical properties, which was likely due to the anisotropic topography and the geometric effects.
Nagaoka et al. investigated the pressure-induced behavior of CsPbBr 3 nanocube superlattices (NC-SLs). 98 Upon pressurization, individual CsPbBr 3 NCs were fused into 2D nanoplatelets (NPLs). Figure 6(a) shows a sequence of wide-angle x-ray scattering (WAXS) patterns collected for NC-SLs from ambient pressure to 17.5 GPa, where multiple phase transitions were observed. The CsPbBr 3 NCs were originally a mix of cubic and orthorhombic phases which transformed into a pure orthorhombic phase at 0.4 GPa [ Fig. 6(b)]. The orthorhombic phase remained stable at 5.1 GPa. As pressure further increased, the three adjacent peaks of the orthorhombic phase [i.e., (112), (020), and (200)] combined to a single but broad peak, representing the amorphous-like phase which remained stable at 17.5 GPa [ Fig. 6(c)]. The pressure-induced quasi-amorphous phase is an intermediate stage, revealing a short-range tetragonal ordering phase, as shown in Fig. 6(c). Upon decompression, the quasi-amorphous phase transformed into a cubic phase close to ambient pressure [  Fig. 6(d)]. Using small-angle x-ray scattering analysis (SAXS), WAXS, and TEM, the authors proposed pressure-induced nucleation and growth of the 2D HPV NPLs [ Fig. 6(g)]. Below 5.1 GPa, the SLs are in a hydrostatic state in which they maintain isotropy. When NC-SLs are compressed above 5.1 GPa, an anisotropic pressure gradient occurs on the SLs, causing deformation of the organic ligand; further increasing pressure leads to separation of the ligand from the NCs. After the separated ligands completely migrate out of the gap between the NCs, the NCs come in direct contact, facet-to-facet, and eventually fuse into 2D NPLs. Moreover, it is more noticeable that the PL peak of the CsPbBr 3 NC-SLs shifted from 528 nm to 525 nm at 0.1 GPa, and the PL intensity was increased by about six times [ Fig. 6(h)]. The authors suggest that the dramatic enhancement of emission may be caused by the reconstructed NC surfaces. When the pressure is totally released, a strong green emission reappears at 518 nm, whose intensity is 1.6 times higher than that of the initial emission.
Xiao et al. studied the pressure response of CsPbBr 3 with various morphologies (nanocrystals, nanowires, and bulk materials), where simultaneous carrier lifetime prolongation and bandgap narrowing were observed. 43 Morphology-dependent transition dynamics under high pressures were observed. The phase change interval of 0.58 GPa, ranging from 1.22 GPa to 1.80 GPa, of the nanowires is greater than that of the nanocrystals (0.26 GPa) but smaller than that of the bulk material (0.82 GPa). The bandgap redshift of 0.05 eV of CsPbBr 3 nanowires is smaller than that of nanocrystals (0.11 eV) but larger than that of their bulk counterparts (0.02 eV). In comparison with bulk CsPbBr 3 , the pressure-dependent PL spectra of the nanowires experienced a similar process. The unique geometrical morphology effects were responsible for the different values of the phase change interval and the bandgap redshift among different CsPbBr 3 materials.

C. Low-dimensional HPVs
Low-dimensional HPVs, possessing intrinsic quantum confinement effects, are promising for advanced photovoltaic and optoelectronic applications, especially for 2D layered compounds. [101][102][103][104][105][106][107] In the 1990s, Mitzi et al. first synthesized a series of 2D organicinorganic HPVs. 108 In 2014, Dohner et al. reported corrugated-2D structures which exhibit white-light broadband emission. 109 Large structure distortion and strong exciton-lattice coupling with efficient exciton self-trapping have been discovered in this kind of material. More recently, by further lowering the dimensionality of HPVs to one-dimension (1D), at the molecular level, stronger quantum confinement and exciton-lattice interaction were revealed, exhibiting broader emission. 110 Furthermore, zero-dimensional (0D) compounds can be obtained by controlling the synthetic conditions where the octahedra or their clusters are completely isolated by organic molecules. 80 Compared with their 3D counterparts, low-dimensional perovskites show enhanced stability, stronger quantum-confinement properties, and wider tunability. [111][112][113][114] The PL of 3D MAPbX 3 quenches rapidly and disappears at 2-3 GPa, making the tunability of emission energy (or color) very limited (usually less than 80 meV). 15,47 Although the tunability of FAPbX 3 is 40 meV larger, 56,88,115 this level of tunability is still not good enough to optimize optoelectronic properties. The rapid quenching of PL can be attributed to the bending of chemical bonds and lattice distortion in the 3D structure. 26,107 Due to the unique multiple quantum well structures which possess out-of-plane quantum and dielectric confinements, the class of 2D HPVs is emerging as a novel platform for high-pressure research. 116 Recently, an increasing amount of highpressure research has been reported on 2D HPVs. 73,81,102,111 Liu et al. reported bandgap narrowing (633 meV) in an RP-type perovskite (BA) 2 (MA)Pb 2 I 7 under pressure. 112 The in situ highpressure PL shows an apparent redshift of the PL peak position, from 587 nm at 1 atm to 648 nm at 3.7 GPa, and then a blueshift to 631 nm at 4.7 GPa [ Fig. 7(a)]. As shown in the time-resolved PL spectra in [ Fig. 7(b)], the lifetime increases monotonically with increasing pressure to 5.3 GPa. Generally, 3D HPVs exhibit a blue jump after a redshift in the lower pressure range (0.4 GPa for MAPbI 3 and 2.0 GPa for FAPbI 3 ) due to lattice distortion. It is worth noting that a TABLE I. Pressure-driven evolution of structure and bandgap in bulk and nanostructured inorganic HPVs, including the pressure values of crystal structural transition Eg, the pressure values of phase transition ΔEg, the pressure at the minimum bandgap Eg min , and the pressure at initial amorphization and the bulk modulus (K 0 /B 0 ).    85 The PL intensity exhibits a continuous increase during compression until reaching the maximum value at 8 GPa [ Fig. 8(a)]. Furthermore, the emission color changes from dark yellow to bright yellow under pressure [ Fig. 8(b)] Pressure-dependent chromaticity coordinates of emission with increasing pressure from 2 GPa to 8 GPa are shown in Fig. 8(c). The authors claimed that C 4 N 2 H 14 SnBr 4 undergoes a structural transformation from monoclinic I2/m to triclinic P1. Figures 8(d) and 8(e) show crystal structures of C 4 N 2 H 14 SnBr 4 before and after the pressure-induced phase transition, and the corresponding Br-Sn-Br bond angle. Firstprinciples calculations indicate that the pressure-induced emission is likely due to the enhanced transition dipole moment and the increased binding energy of self-trapped excitons (STEs) under high pressure [ Fig. 8(f)].

Sample
In addition, Ma et al. reported that nonfluorescent 0D inorganic HPV Cs 4 PbBr 6 nanocrystals exhibit strong emission under high pressure. 11 PL appears at 3 GPa, the intensity increases with further compression and reaches a maximum at 6.2 GPa. The authors attributed the pressure-induced emission to the radiative recombination of the STEs associated with the large distortion of [PbBr 6 ] 4− octahedra after a phase transition. These high-pressure studies on low-dimensional HPVs demonstrate that pressure can not only be an effective tuner that modifies their optical properties, but also provides insights into understanding the relationship between structure and properties.

D. Other novel HPVs
The halide-perovskite community continues to search for nontoxic, stable, and high-performance perovskite-like derivatives for photovoltaic and optoelectronic applications. Volonakis et al. reported the computational design and experimental synthesis of a new family of Pb-free inorganic halide double perovskites, such as Cs 2 BiAgCl 6 , materials that exhibit very promising optoelectronic properties, including tunable bandgaps in the visible range and low carrier effective masses. 24 Li et al. reported a significant narrowing bandgap in this inorganic double perovskite by high-pressure treatment, from ∼2.2 eV at ambient pressure to 1.7 eV at 15 GPa [Figs. 9(a)-9(c)]. 83 Interestingly, the narrowed bandgap of Cs 2 AgBiBr 6 can be partially retained after releasing pressure to ambient conditions due to incomplete recrystallization. Pressure-induced bandgap evolution is correlated with structural evolution. High-pressure Raman spectroscopy and angle-dispersive synchrotron XRD demonstrate that the Cs 2 AgBiBr 6 transforms from the ambient cubic Pm3m phase to the tetragonal I4/m phase at 4.5 GPa [Figs. 9(d)-9(f)]. As unit cell contraction increases, broadening and weakened diffraction peaks indicate gradual structural amorphization [ Fig. 9(d)].
Bounos et al. investigated the pressure-induced evolution of the structures and properties of defective perovskite Cs 2 SnX 6 (X Cl, Br, and I). 117 Upon compression, Cs 2 SnCl 6 and Cs 2 SnBr 6 maintain a face-centered cubic (fcc) structure up to 20 GPa [ Fig. 10(a)], while Cs 2 SnI 6 transforms initially from a cubic Pm3m phase to a more disordered structure at ∼3.3 GPa, and then to a low-symmetry monoclinic with an I2/m space group at about 8-10 GPa [ Fig. 10(b)], which involves tilting and elongation of [SnI 6 ] 4− octahedra [ Fig. 10(c)]. The structural changes of Cs 2 SnI 6 are reversible upon pressure release. Wu et al. reported pressure-induced bandgap closure and metallization in perovskite analog Cs 3 Sb 2 I 9 . 118 Cs 3 Sb 2 I 9 , with an initial bandgap of 2.34 eV reaching the Shockley-Queisser limit of 1.34 eV at 20.0 GPa [ Fig. 10(d)], accompanied by obvious piezochromism from orange-yellow to opaque black [ Fig. 10(e)]. These pressure-induced changes of the optical properties can be ascribed to atomic orbital overlap enhancement due to Sb-I bondlength contraction and Sb-I bond-angle diminution. Interestingly, Cs 3 Sb 2 I 9 undergoes a semiconductor-to-metal transition during compression and shows metallic conduction at 44.3 GPa [ Fig. 10(f)].
The structural manipulation and property enhancements achieved by high-pressure treatment provide motivation for the further exploration and modification of the optoelectronic properties of HPVs, with various compositions, dimensionalities, and morphologies. By integrating high-pressure techniques and in situ characterization methods, one can systematically investigate the pressure-induced variation of structures and properties. Furthermore, the conclusions obtained from high-pressure research offer a new perspective for a fundamental understanding of the relationships among the different compositions, structures, dimensionalities, morphologies, and optoelectronic properties of HPVs, providing the potential for optimization of these materials for future photovoltaics and optoelectronics.

III. SUMMARY AND OUTLOOK
HPVs have been extensively investigated for their unique properties and potential applications in photovoltaic and optoelectronic devices. Perovskite derivatives, with various compositions, dimensionalities, and morphologies, have been developed, which show superior properties, such as better stability and wider tunability. Impressive progress has recently been made in high-pressure research concerning this family of materials. In this paper, we have summarized this progress, where the pressure-induced variations of structural, optical, electrical, and optoelectronic properties are discussed. We pay particular attention to the enhanced and emergent properties induced by pressure and the structure-property relationship. Despite these achievements, high-pressure research on HPVs still faces a number of challenges: (1) A solid understanding of HPVs is still far from satisfactory, which is partly the result of limited in situ characterization methods, especially for real devices; (2) The small samples used (usually at the microscale) can introduce issues of nonuniformity and increased complexity, increasing uncertainty; (3) The intrinsic sensitivity of HPVs to light and moist air cause irreversible degradation and introduce difficulties in obtaining accurate data.
Future high-pressure research on HPVs should include: (1) Achieving a deeper understanding of the structure-property relationship in a more comprehensive manner with the help of more systematic characterizations. For example, revealing the nature of pressureinduced amorphous or disordered structures is particularly interesting for understanding emergent properties at high pressure. To this end, pair distribution function analyses of synchrotron x-ray and neutron total scattering data can be used to explore the characteristics of local structures. (2) Alternative routes can be explored to simulate the highpressure effects of DACs to realize unique structures and properties, e.g., by using a large-volume press, chemical tailoring, and interfacial engineering. (3) Developing theoretical simulations based on experimental results will provide mechanistic explanations and thus further our fundamental understanding.