Pressure‐induced emission (PIE) in halide perovskites toward promising applications in scintillators and solid‐state lighting

High‐pressure chemistry has provided a huge boost to the development of scientific community. Pressure‐induced emission (PIE) in halide perovskites is gradually showing its unique charm in both pressure sensing and optoelectronic device applications. Moreover, the PIE retention of halide perovskites under ambient conditions is of great commercial value. Herein, we mainly focus on the potential applications of PIE and PIE retention in metal halide perovskites for scintillators and solid‐state lighting. Based on the performance requirements of scintillator and single‐component white light‐emitting diodes (WLEDs), the significance of PIE and PIE retention is critically clarified, aiming to design and synthesize materials used for high‐performance optoelectronic devices. This perspective not only demonstrates promising applications of PIE in the fields of scintillators and WLEDs, but also provides potential applications in display imaging and anti‐counterfeiting of PIE materials. Furthermore, solving the scientific disputes that exist under ambient conditions is also simply discussed as an outlook by introducing high‐pressure dimension to produce PIE.


INTRODUCTION
High-pressure chemistry is an interdisciplinary science which uses high-pressure experiments and theories to study the interactions, reactions, and transformations among atoms or molecules. [1]4][5][6][7][8][9][10] In 2009, halide perovskites as visible-light sensitizers for photovoltaics was reported, achieving a power conversion efficiency of 3.8%. [11][22][23][24][25] Nevertheless, there are still some limited factors such as low stability and low emission efficiency in the practical application of metal halide perovskites.As a novel research dimension and tuning knob, high pressure can effectively modify the structural and electronic phase transition and is thus expected to improve the optoelectronic properties of metal halide perovskites.
In this perspective, we mainly focus on the potential applications of pressure-induced emission (PIE) in metal halide perovskites for scintillators and solid-state lighting.Firstly, the proposal and development of PIE are introduced.Secondly, from the background and requirements of scintillators, we demonstrate the improved performance of scintillators through the retention of PIE by building steric hindrance effect.Thirdly, benefiting from the effective regulation of high pressure on the white light source evaluation index, the potential application in white light-emitting diodes (WLED) is elucidated.This perspective not only illustrates the role of PIE behavior in accelerating the application of scintillators and WLEDs, but also offers a new research idea for the design of other high-performance optoelectronic devices in the future.

CURRENT PROGRESS OF PIE
With the end of the incandescent lighting era, energy-saving bright WLED lighting has become a reality.Subsequently, halide perovskites with broadband white light emission have been developed as efficient single-component emitters.Self-trapped excitons (STEs) emission is the main origin of broadband white light in low-dimensional perovskites. [26]he transient distortion of photoexcited states driven by strong electron-phonon coupling is the main cause of STEs emission.However, some low-dimensional perovskites did not show any emission from STEs as expected, which greatly limited the development of optoelectronic devices.High pressure was recognized as one of the "Top-Ten Emerging Technologies in Chemistry" by the International Union of Pure and Applied Chemistry in 2020. [27,28][31] In 2018, Zou et al. investigated the high-pressure behaviors of lightless zero-dimensional (0D) perovskite Cs 4 PbBr 6 nanocrystals (NCs). [32]Exhilaratingly, the initially nonfluorescent Cs 4 PbBr 6 NCs exhibited a broadband emission at a pressure of 3.0 GPa (Figure 1A), accompanied by a phase transition from rhombohedral to monoclinic.This surprisingly emerged broadband emission from "0" to "1" under high pressure is mainly attributed to the following mechanism: The distortion of halide octahedra promoted wavefunction overlap between ground and excited states, enhancing the transition dipole moment and oscillator strength.Meanwhile, the electron-phonon coupling strength, relevant to the Huang-Rhys factor, was increased under compression, enhancing the STEs binding energy.The concept of pressure-induced emission (PIE) was thus proposed which refers to a novel phenomenon whereby a nonluminescent material exhibits emission upon compression. [1][35][36] In addition, compared to those low-dimensional halide perovskites, the relatively strong structural connectivity of 3D perovskites endows excellent charge-transport properties.Nevertheless, enabling conventional 3D perovskites to produce STEs and the corresponding competitive white emission is still challenging.Xiao et al. combined doping engineering with high pressure to realize the PIE of STEs emission in Mn 2+ doped typically 3D perovskite CsPbBr 3 NCs (Figure 1D). [37]Therein, the mismatch between dopant Mn 2+ and Pb 2+ is mainly used to introduce a large octahedral framework distortion, thus giving rise to the formation of localized STE.Note that the previously reported PIE was achieved through the necessary phase transition by pressure, which would greatly deteriorates the intrinsic structure.In this regard, the PIE was further discovered in allinorganic lead-free 2D vacancy-ordered halide perovskites Cs 3 Bi 2 Cl 9 NCs without any structural phase transition, which greatly promotes China's "double carbon" strategy in terms of stability and environmentally friendly purpose. [38]Actually, the development of PIE takes a great step forward in the fields of high-pressure science and materials science to "light up" the future.In this section, we mainly introduced in detail the novel phenomenon "PIE" of halide perovskites under high pressure and its current research progress.

POTENTIAL APPLICATION OF PIE: SCINTILLATORS
Nowadays, high-energy radiation detections such as Xrays detection have become an indispensable condition for medical diagnostic technologies, computed tomography, product quality testing, security checks, and many other applications. [39,40]Compared with conventional X-rays detectors (convert X-rays directly to electrons), scintillatorbased indirect X-rays detectors have attracted much attention because of their higher X-rays stopping power and faster response.Scintillator refers to a class of materials that can convert various high-energy radiation into low-energy visible photon emission, which is critical for applications in radiation exposure monitoring, security inspection and medical radiography. [41]Nonetheless, conventional scintillators are generally prepared at high temperature and their luminescence is beyond the visible spectrum.These limitations have forced researchers to search for new scintillator materials.In 2002, scintillation properties were observed in solution-prepared 2D organic-inorganic hybrid perovskites (C 6 H 13 NH 3 ) 2 PbI 4 , making lead-based halide perovskite a promising candidate for scintillators. [42]45][46] In order to better design excellent scintillator materials, the characteristics of the ideal scintillator should be understood (Figure 2A).The first key indicator is the light yield, defined as the number of photons emitted per 1 MeV of energy absorbed by the scintillator. [47]The light yield is closely related to the charge transport capacity and the photoluminescence quantum yield (PLQY) of materials.The second indicator is the decay time or response time, reflecting the rate at which the scintillator converts high-energy radiation into UV-visible light emission.The fast decay time facilitates applications such as dynamic X-ray imaging and medical diagnosis.The third parameter is the absorption coefficient.The absorption coefficient of scintillators mainly depends on the atomic number, which determines the X-rays absorption capacity. [48]The heavy atomic composition of the scintillators is beneficial to improve the X-rays absorption capacity.The fourth is the avoidance of self-absorption, which ensures the full utilization of excitation energy.
Although the PIE in halide perovskites has greatly alleviated the problem of low luminous efficiency to a certain extent, the high-pressure reversibility still largely limits the practical applications.[51][52][53][54] For most halide perovskites, the low phase-transition barrier prevents the pressure-derived excellent emission from retaining to environmental conditions, hindering further commercial applications.The steric hindrance effect has been proposed to increase the poten-tial barrier and thus preventing the metastable state at high pressure from recovering back to the initial stable state after releasing pressure (Figure 2B, left panel). [50,51,55]his concept has been further validated in 2D hybrid perovskite (NAPH) 2 PbCl 4 , where (NAPH) + represents 1-(2-naphthyl)methanamine with complex configuration. [52]herein, the steric effect index (SEI) was quantitatively determined to establish a correlation between PIE retention and steric hindrance.The high SEI of organic cations which possess complex aromatic configurations would increase the barrier for phase transition and stabilize the high-pressure metastable states (Figure 2B, right panel).It is of great benefit to harvest the useful materials and promote the future applications under ambient conditions.The pressure-quenched (NAPH) 2 PbCl 4 NCs exhibit enhanced emission intensity compared with the initial one (Figure 2C).Moreover, the quenched lifetime shows a decreased decay time of 1.73 ns (Figure 2D).Note that after the pressure is released, the Stokes shift is increased to 260.7 nm, much wider than its initial value of 124.8 nm (Figure 2E), which greatly facilitates the suppression of self-adsorption.Accordingly, the harvesting of emerging visible PIE at room temperature, large Stokes shift and short lifetime of quenched samples after pressure processing greatly satisfy the requirements of high light yield, fast response time and avoidance of self-absorption for the practical application of scintillators.Accordingly, the PIE enables a great step forward to the promising application of scintillators.In this section, we principally discussed the enhanced properties of halide perovskites after pressure treatment to contribute to the practical application of scintillators based on the key factors of scintillators.

POTENTIAL APPLICATION OF PIE: SOLID-STATE LIGHTING
Currently, lighting accounts for about 22% of global electricity consumption. [56]Global warming, energy shortage and environmental pollution are constantly reminding people of the urgent need for low-carbon and energy-saving lighting technology. [57]LEDs stand out among many lighting technologies because of their high luminescence efficiency and long lifespan.When blue LED won the Nobel Prize in 2014, the award sentence was expressed as "incandescent light bulbs lit the 20th century, the 21st century will be lit by LED lights". [58]At present, WLEDs are widely applied in solid-state lighting, such as displays, architectural lighting, flashlights and vehicle headlamps. [59]Consequently, it is urgent to develop high-quality white light and produce lighting products with low cost, energy saving and excellent lighting effects to fully exploit the potential of WLEDs.
The development of lighting technology escorts the realization of high-performance, low-cost lighting sources to light up the world.The key indicators for evaluating white light sources are crucial (Figure 3A).In addition to the luminescence efficiency of white-light emission, the first indicator mentioned is Commission International de l Éclairage (CIE).CIE coordinates (x, y) can accurately judge the emission color perceived by the human eye. [60]Among them, the CIE coordinates (0.33, 0.33) mark the ideal white light source.The second indicator is the color rendering index (CRI), representing a range of 0-100, which determines the ability of the light source to reproduce the color of various objects compared with the ideal light source. [61]A higher CRI means that the light source is closer to natural light.Another indicator is the correlated color temperature (CCT).CCT is expressed in absolute temperature scale Kelvin (K), which is the standard for measuring the color temperature of the light source. [62]ommon WLED is generated by combining a blueemissive LED chip with yellow-emissive phosphors or by mixing two (blue and yellow)/three (red, green, and blue) phosphors to achieve a wide color space.However, the CRI and color stability are generally limited.In addition, commercial available YAG:Ce 3+ based WLEDs are subject to the expensive cost owing to the rare-earth elements, thus making consumers discouraged and limiting the large-scale commercial application.74] Pressure could precisely control the white light emission from metal halide perovskites without changing any chemical com-position.The important parameters of photoluminescence (PL) efficiency, CIE, CRI and CCT were reported to be well modulated in the process of PIE.
One of promising white-emitting 1D perovskites, C 4 N 2 H 14 PbBr 4 , possesses unique 1D quantum well configuration and is prone to STEs formation.However, C 4 N 2 H 14 PbBr 4 as cold white light is not very satisfactory with a CRI of 71 under ambient condition.Ma et al. exploited high pressure to study the relationship between crystal structure and broadband emission of 1D C 4 N 2 H 14 PbBr 4 NCs, aiming to help design high-quality white-light materials. [75]pon compression, the C 4 N 2 H 14 PbBr 4 NCs experienced an increase in CRI up to 86, accompanied by the high brightness white emission resulting from PIE (Figure 3C).The enhanced CRI after pressure processing can even persist upon decompression. [76]Likewise, the weak structure connectivity within low-dimensional halide perovskites severely reduces conductivity.Therefore, STEs emission in 3D halide perovskites is imperative and challenging.Shi et al achieved the PIE associated with the STE emission in a typical 3D perovskite Mn-doped CsPbBr 3 NCs, benefiting from the synergistic effect of high pressure and doping. [37]Note that under a pressure modulation of 7.6 GPa, the Mn-doped CsPbBr 3 NCs exhibited a high-quality white light emission with a near-standard CIE coordination of (0.330, 0.325) (Figure 3B).Furthermore, to meet the practical applications under ambient conditions, high-efficiency luminescent phase under high pressure was greatly longed to be quenched.Through building the steric hindrance, the PIE from 2D hybrid perovskite (PEA) 2 PbCl 4 NCs was retained at ambience which is of great benefit to the future application of high-quality WLEDs. [50]The cold-white light with CCT of 14295 K was preserved even after the pressure was completely released, transforming from the initial warm-white light with CCT of 4403 K (Figure 3D,E).This effective pressure regulation on the white-light emission was also identified in pressuretreated 2D hybrid perovskite (NAPH) 2 PbCl 4 and 0D hybrid perovskite (4AMP) 2 ZnBr 4 . [51,52]In a case of (NAPH) 2 PbCl 4 nanoplates, it shows a cold-white light with CIE coordinates of (0.28, 0.31) and CCT of 9453 K at environmental conditions.After pressure processing, the PL of (NAPH) 2 PbCl 4 nanoplates displayed twice more intensity of the warm-white light emission with CIE coordinates of (0.34, 0.39), CCT of 5189 K.In (4AMP) 2 ZnBr 4 microtubes, a nearly 10-fold enhanced cool daylight was preserved with CIE coordinates of (0.31, 0.36) and CCT of 6392 K from an initial blue-sky emission with CIE coordinates of (0.27, 0.30) and CCT of 9102 K.The PLQY of the quenched (4AMP) 2 ZnBr 4 microtubes also elevated from 11.58% to 88.52%.Moreover, the harvested PL intensity can still be stabilized up to three weeks (Figure 3F).In addition, the heating/annealing experiments indicated that the retention of PIE was stable even up to the temperature of above 165 • C. Therefore, the PIE paves a robust way of improving the application of solid-state lighting.In this section, we focused on presenting the prospect of PIE in halide perovskites for the high-efficiency application of single-component WLEDs according to the requirements of white light sources.

CONCLUSION AND OUTLOOK
In summary, this perspective systematically introduced the current progress of PIE in halide perovskites and emphasized the potential applications of PIE in scintillators and solid-state lighting.Actually, the result of PIE is making the luminescence intensity enhanced through the pressure processing without changing any chemical composition.The realization of PIE in 0D perovskite Cs 4 PbBr 6 has brought the dawn for the improvement of the luminous efficiency of halide perovskites.The subsequent emergence of PIE in 1D, 2D, and 3D perovskites promoted the vigorous development of perovskites in optoelectronic applications.On the one hand, for scintillators applications, the irreversible PIE by introducing steric hindrance effects presents a novel design strategy that leads to shorter decay time, larger stokes shift as well as enhanced luminance efficiency.All of them are important factors for scintillators and greatly facilitate the rational application of high-performance scintillators.On the other hand, in the process of PIE, the CIE, CRI and CCT of halide perovskites can be also modulated to largely meet the demand of low-cost and high-quality WLED applications.This single-component white-light emitter of halide perovskites highly fill in gaps for the expensive price of commercial YAG:Ce 3+ -based WLED, as well as the limitation of low CRI and color instability which discourage consumers.Overall, the PIE indeed enables the improvements of applications in scintillators and solid-state lighting.In addition to its promising applications of PIE in scintillators and WLEDs, the potential applications of PIE are also anticipated in the fields of pressure sensing, anti-counterfeiting, information storage and security.Besides, from a future outlook, PIE can also solve the long-standing conventional scientific disputes under ambient conditions. [77]For instance, faced with the question of whether 0D perovskite Cs 4 PbBr 6 NCs is emitted at ambient conditions, PIE served as a high-pressure experimental evidence to identify the origin of green emission of Cs 4 PbBr 6 NCs. [78]The green emission of Cs 4 PbBr 6 NCs was attributed to CsPbBr 3 impurity, rather than intrinsic emission associated with halogen vacancy.Likewise, the PIE mechanism also provides solid evidence for the origin of the weak narrowband blue emission in the double perovskite Cs 2 AgInCl 6 . [79]herein, pressure was introduced as an efficient tool to rule out the possibility of the recombination of free excitons.The blue emission in double perovskite Cs 2 AgInCl 6 was confirmed to be related to the radiation recombination of singlet-state STEs.Apparently, PIE research is attracting more attention and exploration from global researchers.The above work also provides new ideas for the design of PIE materials.Low-dimensional halide perovskite materials constructed by introducing organic components with aromatic structure are expected to exhibit the intriguing PIE under the stimulus of external pressure.In general, the distortion behavior of these materials under pressure is easy to control owing to the soft lattice, and thus endowing the adjustment of electron-phonon coupling and the subsequent regulation of emission properties. [80,81]Moreover, the steric hindrance effect of organic cations with complex aromatic configurations is also expected to achieve the retention of PIE, promoting the practical applications at ambient pressure.Anyway, there are many things left we can do, and we are ongoing about the advances and underlying photophysical mechanism of PIE, as well as its potential applications.

A C K N O W L E D G M E N T S
This work is supported by the Jilin Provincial Science and Technology Development Program (grant number: 20220101002JC), the National Natural Science Foundation of China (grant number: 12174144), and the Fundamental Research Funds for the Central Universities.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare they have no conflict of interest.

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I G U R E 2 (A) Performance requirements for ideal scintillators.(B) Building steric hindrance diagram and calculations of steric effect index.Reprinted with permission from ref. [39], Copyright 2023, Cell Press.(C) Retention of enhanced warm-white emission in 2D hybrid perovskite (NAPH) 2 PbCl 4 after pressure release.Reprinted with permission from ref. [39], Copyright 2023, Cell Press.(D) Lifetime comparison of white-light emission in (NAPH) 2 PbCl 4 before and after pressure treatment.Reprinted with permission from ref. [39], Copyright 2023, Cell Press.(E) Stokes shift comparison of white-light emission in (NAPH) 2 PbCl 4 before and after pressure treatment.Reprinted with permission from ref. [39], Copyright 2023, Cell Press.