Photoelectric Properties of Functional Materials under High Pressure

The past decade has seen enormous interest in emerging photodetectors owing to their diverse applications in daily life and military fields. Tremendous progress has been made in the exploration and optimization of photoelectric materials. In particular, high pressure has been successfully adopted as a significant means to tune photoelectric properties. The remarkable enhancement in photocurrent by several orders of magnitude, induces inverse photoconductivity, and even extends the spectral response range, which was achieved by using the pressure strategy in several functional materials. In this prospective article, recent advances in pressure‐regulated photoelectric properties of various functional materials are briefly elaborated on. In addition, the current challenges and possible research directions are discussed. It is sincerely hoped to inspire more new endeavors to promote further investigation of the photoelectric properties of functional materials under high pressure and provide more insights into exploring and optimizing novel photoelectric materials.


Introduction
As the crucial component in the modern photoelectric industry, photodetectors are devices that convert incident light into electrical signals, which can be extensively applied in machine vision, optical communication, imaging, security checking, and many other fields. [1][2][3][4] Despite tremendous progress having been achieved in exploring and further optimizing novel photoelectric materials over the past several decades, photodetectors with higher responsivity and detectivity and a wider spectral response range still face fundamental challenges for the everincreasing demands for advanced applications. [2] Substitutions for conventional commercial photoelectric materials, such as Si and Ge, must be identified because of the complexity and DOI: 10.1002/apxr.202200102 expensive manufacturing technology required. [5,6] Emerging and promising candidates, including graphene, [7,8] black phosphorus, [9,10] MXenes, [11,12] transition metal dichalcogenides (TMDs), [13,14] and perovskite materials [15,16] have attracted tremendous attention for future photoelectric applications. However, practical applications require sufficient effort and insights. Recently, the construction of heterojunctions with different materials has shown great promise for the development of novel photoelectric devices with highly desirable performances. [17,18] However, assembling wafer-scale heterojunctions is still required for practical applications.
As a fundamental thermodynamic variable and significant external stimulus, high pressure directly changes the interatomic distances within crystalline structures while simultaneously regulating the macroscopic physical and chemical properties of materials without changing their chemical compositions. High pressure has been extensively employed to explore and further optimize novel structures and properties of materials otherwise unobtainable. [19][20][21][22] Encouragingly, significant progress has been made in enhancing the structural stability, structural phase transition, optical and luminescence properties, metallization, and superconductivity. [21,[23][24][25][26][27] Photoelectric properties are critical characteristics of semiconductors for practical applications and are closely related to their optical properties and bandgap. Intriguingly, significant results of pressure-regulated photoelectric properties of functional materials have sprung up in recent years. Including significant enhancements in photocurrent, induces inverse photoconductivity (IPC) and extends the spectrum response range to the nearinfrared waveband. [28][29][30][31][32] Some investigations have even demonstrated the feasibility of pressure-regulated photoelectric properties in functional materials for practical applications. [33][34][35][36] Such a 2.8-nm-wide edge-closed graphene nanoribbon was obtained by irreversibly squashing carbon nanotubes via high-pressure and thermal treatment, and superb field-effect properties were obtained. [33] These findings reveal that pressure possesses great potential for novel photoelectric material synthesis, optimization, and further practical applications. Here, we briefly elaborate on recent advances in pressure engineering for regulating the photoelectric properties of typical functional materials and exploring and developing novel photoelectric functional materials with broadband photoresponse and IPC characteristics. Typical materials include TMDs, perovskite materials, layered bismuth oxychalcogenides, and ferroelectric (FE) materials, etc., as illustrated in Figure 1. We sincerely look forward to inspiring additional endeavors to apply high pressure strategy to further explore and optimize novel photoelectric materials.

The Photoelectric Device within DAC Setup
The photoelectric device used for the high pressure in situ experimental investigation was constructed in the diamond anvil cell (DAC) setup, as shown in Figure 1. This is a metalsemiconductor-metal (MSM) structure photodetector. Generally, Pt or Au is preferentially considered as electrodes in highpressure in situ measurements owing to their excellent ductility and stability. A T301 stainless steel gasket or Re gasket was preindented to 40-60 μm in thickness, and then drilled a hole with appropriate diameter as the sample chamber. In order to ensure the electrical insulation between the electrodes and metal gasket, the mixture of epoxy and boron nitride (c-BN) powder was loaded into the DAC chamber and then firmly compressed. Usually, we do not use extra pressure transfer medium, and the pressure can be calibrated through the ruby.

TMDs
In recent years, TMDs have been extensively investigated in the field of photoelectric detection because of their fascinating photoelectric properties. [13,[37][38][39] Nevertheless, practical applications of TMDs are severely limited by their low photoelectric response in the infrared waveband. Therefore, it is of essential to pursue appropriate methods for improving the performance of TMD-based photodetectors, particularly in the near-infrared waveband, for future applications pertaining to sensing, imaging, and communication.
In 2014, Akinwande et al. first investigated the photoconductivity of MoS 2 under high pressures. With increasing pressure, the photocurrent gain (defined as I on /I off ) was significantly diminished at a linear rate of −7.5% GPa −1 , ultimately converging to unity owing to metallization originating from the bandgap closure (Figure 2a,b). This was the first study to reveal that pressure can be used as an external stimulus to dynamically regulate the photoresponse or photoconductive gain of functional materials, providing new insights for designing novel photoelectric devices. [40] PtS 2 possesses a tunable bandgap from the bulk (0.25 eV) to the monolayer (1.6 eV), [41] even exhibiting superb photoelectric performance with a high responsivity of 1.56 × 10 3 A W −1 . [42] Intriguingly, effective improvement in photocurrent by almost 600% at 4 GPa compared to its initial value under ambient conditions was achieved via high pressure, underlying the indirect to quasidirect bandgap transition, which would be favorable to photoelectric applications. [43] Similar to PtS 2 , the photocurrent of ReS 2 exhibits a gradual increase upon successive compression, especially for the bulk ReS 2 sample, which is associated with an increase in the S-S interactions, leading to reductions in the bandgap and electric resistance. [44] As a typical TMD material, WS 2 exhibits broad prospects in the field of photoelectric devices owing to its fascinating physical properties, tunable bandgap, and high absorption coefficient. An individual WS 2 photodetector fabricated within a diamond anvil cell setup exhibited an ultrafast response speed of 35 μs and broadband detection of 1650 nm (Figure 2c,d). [45] With increasing pressure, the photoelectronic properties of WS 2 devices significantly improve, especially for the optical communication waveband, where the photocurrent is enhanced by almost three orders of magnitude. The significantly improved photoelectric activity originates from the increase in photogenerated carriers and bandgap reduction resulting from the pressure-induced enhancement of the S-S interlayer interaction. Such an effective enhancement of photoelectric properties has never been observed in single TMDs. [45]

Perovskite Materials
Perovskite materials have attracted significant attention as nextgeneration photoelectric devices owing to their superb optical and electrical properties, low cost, and suitability for large-scale manufacturing technology. [16,[46][47][48] Thus far, a series of strategies have been applied to further optimize the properties of perovskites, particularly their structural stability and optoelectronic properties. [35,[49][50][51][52] Effective pressure-enhanced stability has been achieved for many perovskite materials. CH 3 NH 3 SnI 3 began amorphization at ≈3 GPa and subsequent recrystallization upon pressure release. Surprisingly, the pressure-treated CH 3 NH 3 SnI 3 persists in a crystalline state, and no amorphization occurs with recompression to at least 30 GPa because it possesses the higher crystallographic symmetry (cubic), which indicates an enhancement in structural stability. Intriguingly, compared to pristine CH 3 NH 3 SnI 3 , a significant increase in the photoresponse under visible-light illumination and a threefold enhancement in electrical conductivity were observed in the sample after highpressure treatment owing to a smaller unit-cell volume and larger orbital overlap in cubic structure (Figure 3a,b). With the second compression cycle, the unit-cell volume further decreases and better orbital overlap, resulting in further enhancement in photoresponse. [35] Regrettably, the photoresponse of perovskite materials is usually suppressed when they are transformed into an amorphous state. The photoresponse of CH 3 NH 3 PbI 3 nanorods under visible-light illumination persisted up to 3 GPa and was subsequently suppressed by pressure-induced amorphization. [54] Compared to CH 3 NH 3 PbI 3 , although the photoresponse of  [40] Copyright 2014, Springer Nature. c) Photocurrents of WS 2 under 1650 nm laser illumination at selected pressures. d) Photocurrent-pressure dependence of layered WS 2 with 1650 nm near-infrared laser illumination. Reproduced with permission. [45] Copyright 2022, Taylor & Francis Group. MPbBr 3 (where M = Cs, CH 3 NH 3 , or CH(NH 2 ) 2 ) was ultimately suppressed by amorphization, a remarkable enhancement in the photocurrent was achieved within the specific pressure range. [55][56][57] The photocurrent of CH(NH 2 ) 2 PbBr 3 exhibits a remarkable improvement with increasing pressure to 1.3 GPa and the maximum photocurrent is almost a factor of 10 higher than that in CH 3 NH 3 PbBr 3 and nearly a factor of 3 higher than that in CH 3 NH 3 PbI 3 . [57] All of these materials undergo a transition from mixed ionic-electronic conduction to pure electronic conduction at certain pressures, which would result in a change in the photoresponse characteristics. Under illumination, the photoresponse dominated by mixed conduction exhibited an initial sharp increase and then decreased slowly until the illumination was turned off, ultimately forming a needle-like peak. With increasing pressure, this feature disappears, as electronic conduction dominates the transport process. [55][56][57] Surprisingly, although the photocurrents of CH 3 NH 3 PbBr 3 are suppressed by pressure-induced amorphization starting at ≈2 GPa, CH 3 NH 3 PbBr 3 still exhibits a considerable photoresponse to visible-light illumination with increasing pressure to ≈30 GPa, while the anomalous bandgap increased and the resistance increased by almost five orders of magnitude (Figure 3c). [53] Strikingly, with increasing pressure, the photoresponse of Eu 3+ -doped cubic-phase CsPbCl 3 quantum dots was not suppressed and exhibited an obvious enhancement even when it started to transform to an amorphous state at ≈10.9 GPa. [34] The maximum responsivity value at 20.5 GPa is greater by a factor of 2 in comparison to the initial value. It can almost be preserved after decompression to ambient pressure, which would effectively promote further practical applications. The enhancement in the photoresponse originates from the reduced defect density and increased carrier mobility upon compression. [34] At ambient pressure, the 2D halide perovskites Cs 2 PbI 2 Cl 2 have a high exciton binding energy of 133 meV. With increasing pressure, the exciton binding energy decreases successively, dropping to 78 meV at the industrially achievable pressure of 2.1 GPa, which is comparable to the exciton binding energy of typical 3D perovskites (MAPbI 3 and MAPbBr 3 ). Therefore, the photogenerated excitons dissociated into free carriers and significantly enhanced the photocurrent by over three orders of magnitude (Figure 3d,e). [28] Among other 2D perovskites, the hybrid perovskite (C 6 H 5 CH 2 NH 3 ) 2 CuBr 4 consists of 2D CuBr 4 perovskites and C 6 H 5 CH 2 NH 3 groups within the layers of CuBr 4 . (PMA) 2 CuBr 4 possesses strong structural stability and no obvious volume collapse or amorphization occurs when the pressure is increased to 30 GPa. Notably, the photoresponse of (PMA) 2 CuBr 4 can be detected with increasing the pressure from 10 to 40 GPa. Upon further compression, the photoresponse exhibited successive increases, and a maximum photoconductivity value of 5 × 10 −3 S cm −1 occurred at 28 GPa. [49]

Layered Bismuth Oxychalcogenides
2D layered bismuth oxychalcogenides possessing fascinating chemical and physical properties, such as ultrahigh electron mobility and low thermal conductivity, have shown great promise for Figure 3. Photocurrents of CH 3 NH 3 SnI 3 before (first cycle) and after (second cycle) pressure treatments at a) a low pressure of 0.7 GPa and b) a high pressure of 25 GPa. Reproduced with permission. [35] Copyright 2016, Wiley-VCH. c) Photocurrent of CH(NH 2 ) 2 PbBr 3 as a function of pressure. d) Photocurrents of Cs 2 PbI 2 Cl 2 under high pressure. Reproduced with permission. [53] Copyright 2015, American Chemical Society. e) Pressure-induced evolution of photoconductivity, where the data at ambient pressure are derived from the literature. Reproduced with permission. [28] Copyright 2021, American Chemical Society.
thermoelectric and photoelectric applications. [58][59][60] Intriguingly, Bi 9 O 7.5 S 6 undergoes a structural phase transition from 2D layered to 3D networked structure without a crystallographic symmetry change upon compression to ≈58.1 GPa, which effectively contributes to increasing the electric conductivity by six orders of magnitude and enhancement in the photocurrent by four orders of magnitude (Figure 4a). [61] Upon compression, the Bi6s 2 lone-pair electrons in Bi 2 O 2 S are gradually suppressed, which contributes to the reduction of phonon anharmonicity and Bi 3+ centering, ultimately resulting in a structural transition from the orthorhombic phase to the tetragonal phase occurring at 6.4 GPa. These-transitions effectively improve the electrical and photoelectric properties of Bi 2 O 2 S, in particular, remarkably enhancing the photocurrent by almost three orders of magnitude (Figure 4b,c). [62]

FE Photovoltaic Materials
Compared to the traditional photovoltaic effect, the FE photovoltaic effect can generate zero-bias photocurrent, which relies on the internal built-in electric field originating from the FE polarization, which holds great promise for further optimizing solar energy conversion. [63,64] KBiFe 2 O 5 is a room-temperature FE material with a bandgap of 1.6 eV and high optical absorption coefficient. With increasing pressure, the structural transition from the P2 1 cn to the Cmc2 1 phase occurred at 10.3 GPa. The highpressure Cmc2 1 phase is also an FE phase, which is stable up to 35 GPa. In particular, pressure-enhanced FE properties were observed for the Cmc2 1 phase. Notably, the zero-bias photocurrent of KBiFe 2 O 5 is successively enhanced with increasing pressure and reaches ≈160 pA (0.64 mA W −1 for responsivity) at 30.5  GPa, which is a factor of 4 greater than the initial value at 0.5 GPa (Figure 5a,b). [65] Using pressure to induce the inversion symmetry broken in SbSI results in a paraelectric (PE) to FE phase transition occurring at 2.9 GPa. Initially, the zero-bias photoresponse cannot be detected in the PE-phase SbSI. Subsequently, an obvious photocurrent under zero bias was observed at 2.6 owing to the pressure-induced PE-FE phase transition. Upon further compression, the photocurrent obviously increased and reached a maximum value at 14 GPa. [66] Therefore, pressure may provide an effective means to explore novel FE materials and further optimize photovoltaic performance.

Other Functional Materials
Effective regulation of the pressure on the photoelectric properties of materials also occurs in AgIn 5 S 8 , [67] PbBiO 2 X (where X = Cl, Br, or I), [68] CuInS 2 , [69] and g-C 3 N 4 films (Figure 6a,b). [70] Unquestionably, a remarkable enhancement in photoelectric properties was also achieved in PbBiO 2 X (where X = Cl, Br, or I), CuInS 2 , and g-C 3 N 4 films under high pressure. [68][69][70] In particular, PbBiO 2 Br exhibited partial irreversibility in both optical properties and crystal structure upon decompression to ambient pressure. Conversely, along with the structural phase transition from the Fd-3m phase to the Imma phase at ≈10 GPa, AgIn 5 S 8 exhibits significantly enhanced electrical conductivity but reduced photoresponse to visible light owing to a pressure-induced direct-to-indirect bandgap transition, which has rarely been found in previous studies (Figure 6c,d). [67] Harvesting high pressure state to ambient pressure is significant important for practical application. Chen et al. gained the sub-10-nm-wide edge-closed semiconducting graphene nanoribbon via squashing carbon nanotubes using the high-pressure. Intriguingly, it is irreversible and can be harvested to ambient condition via using the thermal treatment. and superb fieldeffect properties were obtained. Especially the field-effect transistor (FET) fabricated by an edge-closed 2.8-nm-wide graphene nanoribbon exhibits an I on -I off ratio as high as 10 4 , and high fieldeffect mobility of 2443 cm 2 V −1 s −1 .  [70] Copyright 2022, Elsevier. c) Photocurrent and d) photocurrent intensities of AgIn 5 S 8 as a function of pressure. Reproduced with permission. [67] Copyright 2019, American Chemical Society.

Pressure Engineering for Extending the Spectral Response Range
Constantly exploring novel photoelectric materials and strategies for modifying the photoelectric properties of materials for infrared detection is vital because of their extensive commercial and military applications. [71][72][73] However, the application fields of photodetectors are usually limited by the bandgaps of photoelectric materials. Consequently, extending the spectral response range would significantly enrich the applications and functionalities of photodetectors. Improving the photoresponsivity and spectral response range of photodetectors remains a significant challenge and critical issue. Notably, as a significant external stimulus, high pressure is a powerful means to modify the structure and bandgap of materials, enabling enhancement of the photocurrent and development of a tunable spectral response range of photoelectronic functional materials.
By applying the high-pressure strategy, the photoelectric properties of 2D layered semiconductor iodine were significantly enhanced, and the maximum photocurrent was four orders of mag-nitude greater than the initial value under visible-light illumination. Impressively, the spectral response range was successfully extended to the near-infrared waveband (1064 nm) by applying pressure, and the photoresponse properties were significantly enhanced with further compression (Figure 7a,c). The dramatically enhanced photoelectric activity is attributed to charge delocalization as well as the increased charge density in the region between the parallel molecules resulting from the pressure-induced charge transfer of iodine molecules. [29] Furthermore, direct-bandgap semiconductors are more beneficial for photoelectronic applications owing to their stronger absorption compared to that of indirect bandgap semiconductors. [74] Unquestionably, pressure-induced indirectdirect bandgap transitions would greatly enhance the photoelectric properties of semiconductor materials, despite their creation being a fundamental challenge and rarely achieved experimentally. Fortunately, a significant enhancement in the photoelectric properties was achieved via a pressure-induced indirect-direct bandgap transition in hypervalent CsI 3 . With increasing pressure, the photocurrents of CsI 3 rapidly increased Figure 7. a) Photocurrent of iodine during compression under 1064 nm laser illumination at 5 V bias. b) Photocurrent density (J ph ) and responsivity (R) of iodine as a function of pressure. c) External quantum efficiency (EQE) and specific detectivity (D*) of iodine as a function of pressure. Reproduced with permission. [29] Copyright 2021, Wiley-VCH. d) Photocurrent of CsI 3 at selected pressures under 1650 nm laser illumination of different wavelengths at a 15 V bias. e) Photocurrent density (J ph ) of CsI 3 as a function of pressure. Reproduced with permission. [30] Copyright 2022, Wiley-VCH. and ultimately enhanced the photocurrent by almost five orders of magnitude under visible-light illumination. Impressively, the spectral response range of CsI 3 was successively extended to 1650 nm with a significant enhancement in the photoelectric properties upon further compression (Figure 7c,d). This is the first time that an extended detection bandwidth from the visible light band to the optical communication waveband (1650 nm) has been achieved in photoelectric materials using a pressure strategy. Furthermore, high pressure was conducive to CsI 3 operation at an ultralower bias input, which has great significance for energy conservation and practical applications. Extensive spectroscopy analysis and theoretical calculations reveal that the pressure-induced indirect-direct bandgap transition and successive bandgap narrowing are fundamental reasons for the superb photoelectric properties of the P-3c1 phase CsI 3 , which originate from the enhancement in I-I interactions along the quasiendless linear chain directions upon compression. These findings provide new insights into the regulated photoelectronic properties of functional materials, particularly for achieving broadband photodetection. [30]  In addition to the significantly enhanced photocurrent by almost five orders of magnitude, extending the spectral response range to the optical communication waveband (1650 nm) was also observed in NiPS 3 . [36] Moreover, partial photocurrents can be retained after decompression to ambient conditions, and the photocurrent harvesting is two orders of magnitude greater than that before high-pressure treatment. [36]

Pressure Engineering for IPC
As a unique photoresponse characteristic that reduces electrical conductivity under light illumination, IPC is promising for devel-oping photoelectric devices and nonvolatile memories with low energy consumption. [75] However, compared to positive persistent photoconductivity (PrPPC), IPC is uncommon and exists in only a few photoelectric materials. Fortunately, it has been demonstrated that pressure can be an efficient means of designing IPC materials. An interesting pressure-induced IPC was observed in the WO 3 /CuO heterojunction system. This is mainly associated with changes in the crystalline structure, WO 6 distortion, bandgap, and complex carrier recombination process. [76] The pressure-regulated reversible p-n conduction-type transition is an important mechanism for achieving IPC. Although IPC was not observed, a pressure-regulated p-n transition has been achieved in many functional materials such as Si, Ge, and ZrSe 3 , etc. [77][78][79] With increasing pressure to ≈8 GPa, chalcopyrite CuFeS 2 undergoes a conduction-type transition from n-type to p-type accompanied by symmetry breakdown from space group I-42d to I-4, which results in IPC occurring at ≈8 GPa (Figure 8a,b). [31] A similar phenomenon was observed in Cr 1− Te (where = 0, 0.25, of 0.375). Compared with CrTe, which exhibits metallization at ≈24 GPa, Cr 3 Te 4 and Cr 5 Te 8 possess unexpected metal-semiconductor-metal transitions under high pressure. CrTe and Cr 5 Te 8 undergo the PrPPC-IPC transition at ≈9 and ≈3 GPa, respectively, while Cr 3 Te 4 retains IPC characteristics during the full compression process. This is the result of pressure-induced p-n switching. [80] The photothermal effect is another important mechanism for achieving IPC under high pressures. Generally, the resistance of metallic materials increases linearly with the light-induced increase in temperature, which contributes to IPC. Silicon is a conventional photoelectric material with excellent photoelectronic properties and a mature fabrication technology. To further improve the photoresponse of Si, particularly in the nearinfrared region, a high-pressure strategy was applied. As shown in Figure 8c,d, a significant improvement in the photoresponse with both visible and infrared illumination was achieved with increasing pressure from ambient pressure to 8 GPa. A unique PrPPC to IPC transition was observed at ≈10 GPa, along with a pressure-induced semiconductive-metallic phase transition. Effective pressure-driven PrPPC-IPC switching paves the way for the development of novel photoelectric devices. [81]

Quantitative Structure-Property Relationships in Typical Functional Materials
Despite of the substantial progress in pressure-regulated properties of functional materials, fundamental challenges still remain to obtain a better insights into the quantitative structureproperty relationship. Distinguishingly, germanium halide perovskite (such as CH 3 NH 3 GeI 3 ) possess a polar space group of R3m. By employing advanced in situ high-pressure probes to regulate the off-centering distortion degree toward 0.2 results in the best photoluminescence (PL) performance in perovskites materials CH 3 NH 3 GeI 3 , HC(NH 2 ) 2 GeI 3 , and CsGeI 3 , which uncovers the quantitative relationship between structural distortion and PL property. [82] In another case, Huang-Rhys factor S is defined to quantitatively describe the exciton-phonon coupling in perovskite materials. Intriguingly, the 1D metal halide C 5 N 2 H 16 Pb 2 Br 6 exhibits highest photoluminescence quantum yield via pressure-regulated S factor toward 28. This rule is worth to be popularized in a wider scope, as evidenced by C 4 N 2 H 14 PbBr 4 , C 6 N 2 H 16 PbBr 4 , and (C 6 N 2 H 16 ) 3 Pb 2 Br 10 . [83]

Summary and Outlook
The fascinating modulation effect of the high-pressure strategy has motivated significant developments in the field of photoelectric devices. The contribution of pressure to the photoelectric properties can be divided into three aspects: significant enhancement in photocurrent, induction of inverse photoconductivity, and extension of the spectral response range (Figure 9). These achievements reveal that the high-pressure engineering strategy possesses great potential for developing novel photoelectric materials with high gain, broadband photoresponse, and low power consumption. Notably, to satisfy the demands for further practical applications, the harvesting of superior properties under high pressure to ambient conditions remains a critical challenge. The following possible strategies are worth considering. 1) Thermal treatment. Graphene nanoribbons are a typical example that can be referenced. [33] Specifically, samples with desirable properties are obtained under a certain pressure. Afterward, the DAC enclosed with the samples was heated to a determined temperature (e.g., 220°C) in a furnace and maintained for the suitable time (e.g., 40 min) for thermal treatment. Ultimately the high-pressure state can be retained after decompression to ambient condition. 2) Employing cations to afford steric hindrance for increasing the phase transition barrier, which maybe achieve the irreversible phase transition. As evidenced by Harvesting PL property in perovskite materials (C 6 H 5 CH 2 CH 2 NH 3 ) 2 PbCl 4 and (C 5 H 7 N 2 ) 2 ZnBr 4 . [84,85] 3) Another possible strategies, such as low temperature, different rates of rising and falling pressure and etc., are also worth trying.
Significantly, photodetection mechanisms include photoconductive effect, photovoltaic effect, photogating effect, photothermoelectric effect, bolometric effect, etc. Nevertheless, the current investigation on the photoelectric properties of functional materials under pressure regulation often defaults that the photocurrent originates from the photoconductive effect. It is still a mystery what the variation of the photoresponse generated by different mechanisms under high pressure. Therefore, it is urgent need to obtain in-depth insights into the quantitative structureproperty relationships and establish the universal law in photoelectric materials under high pressure. Which means that more material systems need to be explored, and more advanced highpressure probes, such as X-ray absorption spectroscopy, neutron scattering, etc., may be required. Meanwhile, combined with theoretical calculation in a high-throughput manner and machine learning approach is favorable to foresee and expedite the discovery and optimization of novel materials. Overall, high-pressure studies on the photoelectric properties of functional materials not only provide more insights into the structure and property relationships but also lay the foundation for further development of advanced photoelectric devices.