High-pressure research on optoelectronic materials: Insights from in situ characterization methods

The past few decades have witnessed remarkable advances in optoelectronic materials, which bridge the fields of photonics and electronics, driven by their critical role in applications such as solar energy conversion, information storage, and solid-state lighting.^1–3 However, understanding and optimizing their performance requires deep knowledge of how their structural, electronic, and optical properties respond to external stimuli, such as pressure, temperature, and electric fields. In this context, high-pressure research has proven to be a powerful tool for probing the fundamental properties of optoelectronic materials and uncovering new phenomena that are inaccessible under ambient conditions.^4–6 

On compression of the crystal lattices of optoelectronic materials, the distances between atoms and the overall bonding environments change.^4,7,8 This modifies the band structure, often causing shifts in optical absorption and emission energies, altering electrical conductivity, and sometimes inducing new structural phases.^9–12 Advances in high-pressure science have transformed this into a vital area of materials research, yielding breakthroughs such as pressure-induced emission,^13,14 quantitative structure–property relationships,^15–18 enhanced structural polarizations,^19,20 pressure-induced metallization,^21,22 and the discovery of novel order–disorder hybrid materials.^11,23,24 The success of high-pressure research on optoelectronic materials, however, is heavily dependent on the availability of in situ characterization techniques that can probe the evolution of materials under compression.

Advances in in situ characterization methods have revolutionized high-pressure research, enabling real-time monitoring of structural, electronic, and optical changes in materials under extreme conditions. In this review, we aim to provide a comprehensive overview of recent advances in high-pressure research on optoelectronic materials, with a particular focus on insights gained from in situ characterization techniques. We summarize key methodologies and focus on real-time characterization techniques to track the changes of properties, including X-ray diffraction (XRD), absorption spectroscopy, photoluminescence (PL) spectroscopy, time-resolved PL spectroscopy, transient absorption (TA) spectroscopy, Raman spectroscopy, second-harmonic generation (SHG), electrical transport measurements, and photocurrent measurements. Recent breakthroughs demonstrate how high-pressure experiments deepen understanding of optoelectronic behavior, establish structure–property relationships, and drive the development of next-generation optoelectronic materials. Ongoing advances in these techniques promise to uncover more exciting properties and applications in the realm of optoelectronics.

The diamond anvil cell (DAC) is the most widely used apparatus for generating static high pressure in scientific studies.^25,26 In a DAC, a small sample is placed between the tips of two diamonds, and pressure is applied by tightening screws or using hydraulic mechanisms. Various pressure transmitting media (PTM), such as silicone oil, 4:1 methanol–ethanol, neon, and helium, can be chosen.²⁷ This approach offers precise control over pressurization and enables continuous monitoring of structural and property changes as the pressure increases. The transparency of diamond facilitates a wide range of in situ spectroscopic techniques, including X-ray spectroscopy, Raman, infrared, UV–Vis absorption, and PL. Additionally, the insulating properties of diamond make it suitable for in situ electrical transport measurements. As a result, high-pressure techniques based on the DAC provide a powerful platform for investigating optoelectronic materials (Fig. 1).

XRD is a widely used analytical technique to study the structure of optoelectronic materials. It is based on the diffraction that occurs when X rays interact with the periodic arrangement of atoms in a crystal lattice [Fig. 2(a)]. XRD patterns provide detailed information about the atomic and molecular structure of a material, including its crystallographic structure, phase identification, lattice parameters, and crystallite size.

High-pressure XRD experiments, as shown in Fig. 2(b), require synchrotron facilities because of the small sample size (<200 μm) and limited opening angles (<30°).^28, In situ synchrotron high-pressure XRD is performed using the Laue method, with diffraction patterns collected by an area detector without sample rotation. The data are integrated using Dioptas software,²⁹ and refined with the Rietveld method in GSAS software.^30,31 Coupling XRD with DAC technology enables real-time monitoring of structural transformations in optoelectronic materials under pressure, providing critical insights into pressure-induced phase transitions, lattice parameter evolution, and the emergence of new crystallographic phases, all of which affect a material’s optical and electronic properties.

Two-dimensional (2D) XRD images of MASnI₃ (MA = methylammonium) during two compression and decompression cycles are shown in Fig. 2(c).³² Above 3 GPa, some Bragg peaks disappear, and a broad diffuse background emerges, indicating partial amorphization. At 12.5 GPa, all Bragg peaks have vanished, replaced by broad peaks corresponding to an amorphous phase. During decompression to 2.0 GPa, the amorphous structure recrystallizes into a perovskite phase, with smoother diffraction rings indicating more uniform crystalline grains. Notably, in the second compression, no amorphization has occurred up to 31 GPa, and the diffraction peaks of the recovered MASnI₃ are broader than that after the first compression. Thus, the structural stability has improved after pressure treatment.

Bu et al.²³ used in situ single-crystal XRD to study the structural evolution of Cu₁₂Sb₄S₁₃ under pressure. Above 12 GPa, diffraction spots broaden, and diffuse halos appear, indicating partial disordering. Most diffraction spots have vanished at 16.5 GPa, leaving a few single-crystal spots on an amorphous scattering background, signifying a crystalline–amorphous hybrid state [Fig. 2(d)]. The remaining Bragg spots and peaks correspond to the Cu₁ framework, which stays crystalline above 16 GPa, while the Sb[Cu₂S₃]Sb subunit becomes disordered, creating a nested order–disorder structure with an ordered matrix and an amorphous-like interior.

In situ XRD measurements have been conducted to study the pressure dependence of structural behavior for Cs₂PbI₂Cl₂ [Fig. 3(a)].¹⁷ Owing to lattice contraction, all diffraction peaks shift to higher angles with increasing pressure [Fig. 3(b)]. Above 2.6 GPa, certain peaks show an anomalous left shift, indicating a structural phase transition. Guo et al. used in situ single-crystal XRD to determine the high-pressure crystal structure. Rietveld refinement profiles at various pressures are shown in Fig. 3(c), while Fig. 3(d) illustrates cell volume changes under pressure. Fitting the unit-cell volume as a function of pressure shows that phase II is less compressible. Figure 3(e) shows the axis-specific compressibility, revealing a negative linear compressibility in the high-pressure phase, where the lattice expands along the b axis despite an overall reduced volume. This behavior is due to anisotropic octahedral distortion, with a tiny increase in Pb–Cl bond length and a sharp decrease in the Cl–Pb–Cl bond angle, causing contraction along the sliding direction and expansion perpendicular to it [Fig. 3(f)]. Ratté et al.³³ explored the high-pressure structural evolution of CMA₂PbI₄, finding an unprecedented negative linear compressibility along the b axis and highlighting the important role of soft spacer cations in the pressure-tuned optoelectronic properties.

Complementing the structural information obtained from XRD, in situ optical spectroscopic techniques offer a direct probe of the optical responses of optoelectronic materials under high pressures. Techniques such as absorption, PL, and Raman spectroscopy can be readily integrated with DAC setups, enabling the simultaneous measurement of bandgaps, carrier dynamics, and vibrational modes of optoelectronic materials as functions of applied pressure. Adapting commercial optical equipment for high-pressure research is straightforward, requiring only a long-working-distance objective lens.

UV–Vis absorption is a widely used technique to study how ultraviolet and visible light interact with matter based on molecules absorbing light at specific wavelengths corresponding to electronic transitions. As illustrated in Fig. 4(a), when white light passes through the sample, certain wavelengths are absorbed, as electrons are excited from lower to higher energy states. The resulting absorption spectrum plots absorbance in terms of optical density against wavelength. The high bandgap of diamond allows in situ high-pressure absorption spectra to be measured from 0 to 5 eV. A clear single crystal is ideal for collecting spectra, and powdered samples should be tightly pressed before loading. When loading the sample, sufficient space must be left in the chamber to measure the reference signal. Analyzing the absorption spectrum provides insights into the bandgap, excitonic features, and electronic structure of a sample.^34,35

Mao et al.³⁶ used in situ UV–Vis absorption spectra to study the band structure evolution of MHyPbBr₃ (MHy = methylhydrazinium) under high pressure. As shown in Fig. 4(b), the absorption edge gradually redshifts with increasing pressure before abruptly blueshifting at 2.2 GPa. The bandgap of MHyPbBr₃ as a function of pressure is shown in Fig. 4(c), together with the energy variation ΔE_g of APbBr₃ perovskites. The band structure of lead bromide perovskites is influenced by the Pb–Br bond length and the distortion and tilting of Pb–Br octahedra. A shorter bond length reduces the bandgap, while octahedral distortion and tilting increase it. Under compression, the bandgap of APbBr₃ initially decreases owing to bond shortening, then increases owing to phase transition-induced octahedral tilting. Additionally, absorption measurements confirm the metallization of CuP₂Se.³⁷ Above 20 GPa, the absorption edge redshifts and disappears [Fig. 4(d)], indicating a semiconductor-to-metal transition in CuP₂Se.

The exciton absorption peak, a distinct feature in the absorption spectrum, arises from excitons (bound electron–hole pairs) formed when photons with energy slightly below the bandgap are absorbed.³⁸ These excitons have discrete energy levels, producing sharp absorption peaks at specific wavelengths. The positions and intensities of these peak provide insights into a material’s electronic structure, exciton binding energy E_b, and quantum confinement effects.^39,40 Guo et al.¹⁸ obtained in situ absorption spectra of exfoliated (BA)₂(GA)Pb₂I₇ (BA = butylammonium, GA = guanidinium) thin flakes to study the evolution of E_b under high pressure. Under compression, the excitonic absorption peak redshifts, weakens, and totally vanishes at 4.8 GPa, indicating a decrease in E_b [Fig. 4(e)]. Huang et al.⁴¹ used absorption spectroscopy to study the evolution of the electronic structures of few-layer black phosphorus under high pressure. They found that the pressure-induced bandgap shift of black phosphorus depends strongly on the layer number [Fig. 4(f)]. Unlike bulk black phosphorus, which transitions from a semiconductor to a semimetal under high pressure, the bandgap of two-layer black phosphorus increases with pressure.

PL spectra represent the emission of a material excited by photons with sufficient energy, providing insights into its electronic and optical properties, such as bandgap, defect states, impurity levels, and recombination mechanisms. PL spectroscopy is a nondestructive, contactless tool widely used in materials science, semiconductor research, and nanotechnology. In a DAC, PL measurements enable direct observation of emissive states such as exciton or defect recombination.^35,42–44 Pressure-induced changes in PL peak energy, intensity, and linewidth reveal how lattice compression affects electron–hole interactions. A monochromatic laser is reflected by a dichroic beam splitter, while the optical signal passes through the splitter and is collected by a spectrometer to produce the spectrum [Fig. 5(a)]. For weak optical signals such as PL and Raman, type IIa ultralow fluorescence diamonds are recommended. Both powder and single crystals can be used, even without a pressure transfer medium.

Pressure-induced emission is a promising research area with applications in pressure sensors, optoelectronic devices, and material behavior under extreme conditions.^14,45 Shi et al.⁴⁶ performed PL spectroscopy on C₄N₂H₁₄SnBr₄ under high pressure, observing no PL response below 2.06 GPa. Above this pressure, a broadband emission appeared [Fig. 5(b)], with PL intensity increasing steadily until peaking at 8 GPa. For (C₂H₁₀N₂)₈[Pb₄Br₁₈]∙6Br, the self-trapped exciton (STE) emission initially increased up to 0.3 GPa, but then decreased with further compression [Fig. 5(c)].⁴⁷ Simultaneously, a newly formed free exciton (FE) emission around 400 nm intensified and dominated the PL spectrum at 4.0 GPa, indicating a transition from STEs to FEs.

Yin et al.⁴⁸ observed phase transitions in the PL spectra of (BA)₂PbI₄ [Fig. 5(d)]. At ambient pressure, the initial PL peak is at 524 nm, which redshifts slightly to 527 nm, with the appearance of a second peak at 503 nm around 0.14 GPa, indicating a second phase. Then, the second peak redshifts until a third peak emerges at 540 nm at 1.4 GPa, marking a third phase. With further compression, the third peak dominates and redshifts consistently, narrowing the bandgap to 611 nm at 5.3 GPa. Guo et al.⁴⁹ conducted high-pressure PL measurements at 78 K on (4Tm)₂PbI₄ to study the evolution of the electronic configuration of 2D perovskites with pristine type II band alignment [Fig. 5(e)]. At ambient pressure, no emission is detected, owing to efficient charge transfer at the perovskite/ligand interface. At ∼3.7 GPa, an excitonic emission peak emerges, indicating suppressed charge separation and stable exciton formation in the perovskite layers. Compression reconfigures the band alignment from type II at ambient conditions to type I under high pressure by altering the energy levels of the organic ligands and perovskite layers.

In 2D halide perovskites, the exponential tails of PL peaks are attributed to trapped excitons caused by strong exciton–phonon interactions. With increasing pressure up to 1.6 GPa, the PL intensity of (HA)₂(GA)Pb₂I₇ (HA = n-hexylammonium) rises sharply, reaching 12 times its ambient value.⁵⁰ PL spectra fitting has revealed the contributions of free and trapped exciton emissions at different pressures. The trapped excitons significantly contribute to the weaker PL intensity at ambient pressure [Fig. 5(f)]. However, their contribution decreases and disappears entirely upon compression to 1.6 GPa, where the brightest PL is observed. Kong et al.⁴⁴ reported comprehensive high-pressure investigations on a series of 2D D-J perovskites, especially their irreversible PL intensity variation after decompression, which suggests that pressure engineering could viably improve optoelectronic material both in situ and ex situ.

Ruan et al.⁵¹ found that pressure treatment significantly enhances blue PL in phenylboric acid (PBA) dimers, increasing the photoluminescence quantum yield (PLQY) from 2.3% to 31.5%. This enhancement is achieved by reducing π–π stacking interactions and strengthening hydrogen bonds (O–H⋯O and C–H⋯π), which restrict benzene ring vibrations and minimize non-radiative transitions. BaLi₂Al₂Si₂N₆ (BLASN):Eu²⁺ phosphors exhibit tunable narrowband emission under varying hydrostatic pressures, with emission peaks shifting from 532 to 567 nm as the pressure increases to ∼20 GPa.³⁹ This shift is attributed to enhanced crystal field splitting, the nephelauxetic effect, and a Stokes shift caused by lattice contraction and distortion.

High pressure can be employed to address problems that arise under ambient pressure conditions. Ma et al.⁵² resolved the debate over whether the emission arises from intrinsic halogen vacancies or CsPbBr₃ impurities embedded in the Cs₄PbBr₆ matrix using high-pressure methods, showing similar PL trends between Cs₄PbBr₆ and CsPbBr₃ nanocrystals.

TRPL spectroscopy studies the dynamics of photogenerated charge carriers by analyzing the temporal evolution of PL after a short excitation pulse. Unlike steady-state PL spectroscopy, which examines the overall emission spectrum under continuous excitation, TRPL provides insights into lifetimes, recombination mechanisms, and energy transfer processes of excited states.

An α of 1 indicates excitons, 2 indicates free carriers, and values between 1 and 2 reflect the coexistence of excitons and free carriers.⁵⁵ At ambient pressure, α = 1.1 suggests that excitonic emission dominates in (BA)₂(GA)Pb₂I₇. With increasing pressure, α rises to 1.7 at 3.1 GPa, indicating the presence of more free carriers due to the decrease in E_b under high pressure, which affects carrier dissociation and recombination dynamics.

PL intensity, proportional to carrier density, can indicate carrier diffusion and recombination when detected away from the excitation site, allowing carrier transport kinetics to be extracted. Yin et al.⁵⁶ directly measured the carrier transport in MAPbI₃ using a time-resolved PL microscope. Figure 6(c) shows time-integrated PL images of MAPbI₃ under different pressures. Global fitting of the kinetics reveals that the carrier diffusion coefficient D increases from 1.82 cm² · s⁻¹ at 0 GPa to 2.73 cm² · s⁻¹ at 3.1 GPa [Fig. 6(d)].

Transient absorption (TA) spectroscopy, or pump–probe spectroscopy, is a time-resolved technique for studying excited-state dynamics and ultrafast processes in materials. Widely used in photochemistry, photophysics, and material science, it examines energy transfer, charge separation, recombination, and molecular relaxation. TA uses two laser pulses: a pump pulse excites the sample, creating a nonequilibrium state, and a delayed probe pulse monitors absorption changes over time. The difference in probe intensity before and after interaction provides the TA spectrum, revealing excited states, stimulated emission, and other transient phenomena. Advanced transient techniques such as X-ray TA have been applied in understanding the dynamic atomic coordination or crystal structure properties of optoelectronic materials.⁵⁷ 

The excited state carrier dynamics of FAPbBr₃ (FA = formamidinium) have been studied under high pressures [Fig. 7(a)].⁵⁸ Two photon-induced absorption features (A₁ and A₂) and negative ground-state bleach signals (B₁ and B₂) are found [Fig. 7(b)]. As shown in Fig. 7(c), compression-dependent analysis using global fitting has identified three lifetimes, corresponding respectively to hot carrier relaxation (τ₁), Auger recombination (τ₂), and electron–hole recombination (τ₃). Increasing pressure slows thermal carrier relaxation, with a relaxation time of 230 fs at 0.61 GPa. Compression enhances interactions between organic cations and inorganic octahedra, inhibiting FA cation rotation and reducing electron–phonon coupling, thereby increasing thermal carrier relaxation and Auger recombination times.

Guo et al.⁴⁹ used femtosecond TA spectroscopy to study charge transfer and recombination dynamics of (4Tm)₂PbI₄ (4Tm = quaterthiophenylethylammonium). Near the excitonic energy, the positive and negative features correspond to excitonic absorption bleaching and photon-induced absorption, respectively [Fig. 7(d)]. Biexponential fitting reveals lifetimes of 9 and 53 ps, while derivative spectral features appearing after 100 ps, with a lifetime exceeding 1 ns, are attributed to charge separation at the ligand–perovskite hybrid interface. Increasing pressure weakens these long-lived features, indicating suppressed charge separation and pressure-induced exciton stabilization due to a band alignment shift from type II to type I.

Second-harmonic generation (SHG) is a second-order nonlinear optical process in which two photons of the same frequency combine in a nonlinear material to produce a photon with double frequency. It has diverse applications in laser physics, microscopy, telecommunications, materials science, quantum optics, and environmental sensing. It occurs only in materials lacking inversion symmetry, making it useful for studying structural polarization.

The strong organic–inorganic interactions in MHyPbBr₃ result in pronounced structural polarization and SHG. Upon compression, the SHG response of MHyPbBr₃ is significantly enhanced, peaking at 1.5 GPa with an 18-fold increase [Fig. 8(a)].³⁶ Beyond 2.2 GPa, the SHG response disappears, indicating a phase transition from noncentrosymmetric to centrosymmetric. Temperature-dependent SHG measurements show that the critical temperature increases from 408 K under ambient conditions to 454 K at 0.5 GPa [Fig. 8(b)]. This increased stability of the polarized structure under high pressure is due to stronger organic–inorganic interactions, requiring higher temperatures to overcome the energy of N–H⋯Br hydrogen bonding and Pb–N coordinate bonding.

Jiang et al.⁵⁹ conducted high-pressure SHG measurements on NH₄Cl to investigate its SHG switching behavior [Fig. 8(c)]. During compression, the SHG effect appeared at 1.1 GPa, rapidly increased, and stabilized at 2.1 GPa, demonstrating a rare SHG “off–on” switching. NH₄Cl maintained stable SHG intensity from 2.1 to 13.3 GPa, with an “on–off” switching due to a phase transition observed at 14.6 GPa. Additional high-pressure SHG experiments at 2.0 GPa with incident lasers ranging from 800 to 1500 nm showed SHG signals across the visible light spectrum [Fig. 8(d)].

The ability to retain the desirable characteristics of materials obtained under high pressure has long been a challenging goal, since pressure-induced changes are typically reversible. Luo et al.⁶⁰ introduce the concept of “pressure aging” (PA), which enables the permanent locking-in of high-pressure structures and their associated enhanced properties in functional materials. Specifically, through the application of PA at 3.3 GPa for 24 h, the 2D ferroelectric CuInP₂S₆ exhibits a 12-fold enhancement in SHG and an increase in Curie temperature T_c from 317 to 583 K.

Raman spectroscopy is a powerful technique for analyzing vibrational, rotational, and low-frequency modes of molecules. When light interacts with a molecule, most photons are elastically scattered (Rayleigh scattering), but a small fraction of photons undergo inelastic scattering, gaining or losing energy owing to molecular vibrations. This energy shift, called Raman shift, is measured in wavenumbers (cm⁻¹) and reflects the molecule’s vibrational modes. Since Raman spectra are unique to each molecule, depending on its chemical bonds and symmetry, the technique is invaluable for chemical identification and structural analysis.⁶¹ Unlike in situ XRD, which requires synchrotron facilities, Raman spectroscopy provides a convenient in-lab method to study the structural evolution of optoelectronic materials under high pressure. Type IIa ultralow-fluorescence diamonds, with a distinct second-order Raman peak, are crucial for detecting ultraweak optical signals such as Raman. NaCl and inert gases, which lack Raman peaks, are ideal pressure-transmitting media (PTM) for obtaining clean Raman signals.

Fu et al.¹⁵ collected in situ Raman spectra to study the evolution of chemical bonding in NbOI₂ (Fig. 9). Under ambient conditions, the weak intensity of P₃ modes is due to the strong anharmonic vibration of Nb–I bonds, which weakens under compression. This leads to an abnormal Raman left shift and increased intensity. The P₁ mode splits into two peaks at 1.6 GPa, and the P₃ mode vibration strengthens, indicating more-asymmetric Nb–I vibrations and greater Nb off-centering displacement. The appearance of the P₄ mode corresponds to interlayer vibrations. These changes reflect increasingly distorted octahedra under high pressure, leading to stronger structural polarization and enhanced SHG.

Raman spectroscopy can also be used to analyze the vibrations and rotations of organic molecules under high pressure. As shown in Figs. 10(a) and 10(b), Yin et al.⁶² calculated the Raman spectra of the butylammonium (BA) molecule in tt, tg-kind1 (tg-kind2), and g⁺g⁻ conformations. Figure 10(c) shows the in situ high-pressure Raman spectra of BA⁺ in a 2D halide perovskite under compression. At 0.2 GPa, the δ_CCC and δ_CCN bands merge into a single band at ∼487.2 cm⁻¹, corresponding to the overlapping scissoring modes of BA in δ_tg and $δg+g−$ forms. The intensity of ρ_tt weakens, while the Raman band at ∼840 cm⁻¹ strengthens owing to the formation of Raman-active tg and g⁺g⁻ conformers. The scissoring and rocking modes of tg and g⁺g⁻ conformers exhibit a continuous blueshift from 0.6 to 3.0 GPa, indicating lamellar contraction. Above 3.0 GPa, Pb–I sublattice distortion lowers the lattice symmetry, splitting the degenerate scissoring and rocking bands, as seen in the Raman spectrum at 3.7 GPa.

Beyond structural and optical characterization, in situ transport measurements under high pressure offer crucial insights into the electronic properties of optoelectronic materials. Techniques such as electrical resistivity, Hall effect, and thermoelectric measurements can be conducted in DAC setups to study pressure-dependent charge carrier concentration, mobility, and thermoelectric performance. A cubic boron nitride (cBN) layer is used between the stainless-steel gasket and diamond culet to electrically insulate the leads from the metal gasket, with metal probes positioned to contact the sample in the chamber [Fig. 11(a)]. Artificial type IIa diamonds often contain metal catalyst residue, making them conductive and unsuitable for electrical transport measurements. Lithography techniques on diamonds are increasingly important for the electrical measurements of 2D materials and their heterostructures.

Ke et al.⁶³ investigated the pressure-dependent resistance of trilayer graphene. The resistance changes smoothly with pressure up to 30 GPa, but, starting from 33.0 GPa, it increases sharply [Fig. 11(b)]. Beyond this, the resistance rises by over three orders of magnitude up to 59.0 GPa, indicating bandgap opening. The resistance shows a weak temperature dependence (dR/dT ≈ 0), typical of semimetallic behavior up to 30 GPa [Fig. 11(c)]. The resistance becomes strongly temperature-dependent, increasing as the temperature decreases (dR/dT < 0) at 33.9 GPa, indicating semiconducting behavior, which intensifies at higher pressures up to 54.3 GPa [Fig. 11(d)].

Zhao et al.⁶⁴ measured the temperature-dependent resistivity of MoSe₂ [Figs. 12(a) and 12(b)]. Below 23.4 GPa, the resistivity–temperature curves show negative dρ/dT across all temperatures, indicating a semiconducting state. Between 27.0 and 37.0 GPa, dρ/dT is positive at high temperatures and negative at low temperatures. Above 41.0 GPa, positive dρ/dT is observed at all temperatures, signifying metallization of MoSe₂. The room-temperature resistivity decreases exponentially by more than 10⁶ from ambient conditions to 41.6 GPa [Fig. 12(c)]. Ke et al.⁶⁵ reported a pressure-induced quasi-one-dimensional metallicity in δ-CsSnI₃ above 40 GPa. The material transitions from an insulating state to a metallic state while maintaining its one-dimensional chain structure.

Photocurrent is the electrical current generated in a material or device when it absorbs light, commonly used to study photoelectric properties and the performance of photodetectors and solar cells. Light with sufficient energy excites electrons, creating electron–hole pairs that are separated and driven by an electric field, producing a measurable current. As already mentioned, artificial type IIa diamonds often contain metal catalyst residue, which has a photoresponse, and this makes them unsuitable for electrical transport measurements.

Guo et al.¹⁷ conducted in situ photocurrent measurements on a Cs₂PbI₂Cl₂ single crystal under high pressure. The photocurrents exhibit fast on–off switching and increase significantly up to 2 GPa [Fig. 13(a)]. The corresponding photoconductivity rises by three orders of magnitude, from 1.3 × 10⁻⁸ S · m⁻¹ at ambient pressure to 1.4 × 10⁻⁵ S · m⁻¹ at 1.93 GPa [Fig. 13(b)]. Figure 13(c) illustrates the mechanism of photoresponse evolution. At ambient pressure, the high E_b of Cs₂PbI₂Cl₂ stabilizes excitons. However, at 2.1 GPa, the reduced E_b promotes exciton dissociation into free carriers, leading to a significantly enhanced photoresponse.

High-pressure research is a powerful tool for exploring and tuning the behavior of optoelectronic materials. By applying pressure, researchers can uncover new structural phases, adjust optical and electronic properties, and transform our understanding of structure–property relationships. These advances have paved the way for the rational design of next-generation optoelectronic devices with enhanced functionality and performance. As the field evolves, insights from high-pressure studies will not only deepen our fundamental understanding of optoelectronic materials, but also drive the discovery of novel materials with exceptional properties for a wide range of advanced technological applications.

In situ characterization stands at the heart of these efforts, allowing real-time correlation between structural changes and their immediate effects on optical and electrical phenomena. Looking ahead, continued advances in in situ analytical techniques, coupled with the development of novel high-pressure apparatus, will undoubtedly expand the scope and capabilities of high-pressure research on optoelectronic materials.

Although high-pressure research on optoelectronic materials shows great promise, several challenges remain:

1. Retaining new phases. Excellent optical properties of optoelectronic materials that have been obtained by compression usually cannot be retained under ambient conditions. Several new strategies have been proposed to retain high-pressure phases, such as pressure-aging, locking of hydrogen bonds, and the use of nanostructured diamond capsules.

2. DAC design. Creating custom high-pressure cells for specialized in situ measurements, such as neutron diffraction, which requires materials with low neutron scattering, can be difficult and time-consuming.

3. Emerging characterization methods. More high-pressure characterization techniques, such as nuclear magnetic resonance for studying light atoms like hydrogen, thermal conductivity measurements for thermoelectric materials, and magnetic measurements for superconductors, need to be developed.

4. Signal quality. Weak signals and sample scattering pose challenges in studies under high pressures, owing to the small sample size, limited numerical aperture of the objective, and magnetic shielding of DACs.

5. Spatial resolution. The objective lens used for DACs requires a long working distance, resulting in low magnification, a small numerical aperture, and limited spatial resolution for optical measurements under high pressure. Designing special DACs, such as asymmetric DACs with a short working distance on one side, allows the use of objective lenses with a high numerical aperture and large magnification. This, in turn, enhances signal quality and improves spatial resolution.

This work was supported by the National Nature Science Foundation of China (NSFC) (Grant Nos. 22275004, 62274040, and 62304046), the Shanghai Science and Technology Committee (Grant No. 22JC1410300), the Shanghai Key Laboratory of Novel Extreme Condition Materials (Grant No. 22dz2260800), the National Key Research and Development Program of China (Grant No. 2022YFE0137400), and the Shanghai Science and Technology Innovationaction Plan (Grant No. 24DZ3001200).

The authors have no conflicts to disclose.

Songhao Guo: Writing – original draft (equal); Writing – review & editing (equal). Yiqiang Zhan: Writing – review & editing (equal). Xujie Lü: Supervision (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.