Direct-Indirect Transition of Pressurized Two-Dimensional Halide Perovskite: Role of Benzene Ring Stack Ordering

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Spacer-Dependent and Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Halide Perovskites

Compared with their 3D counterparts, 2D hybrid organic-inorganic halide perovskites (HOIPs) exhibit enhanced chemical stabilities and superior optoelectronic properties, which can be further tuned by the application of external pressure. Here, we report the first high-pressure study on CMA₂PbI₄ (CMA = cylcohexanemethylammonium), a 2D HOIP with a soft organic spacer cation containing a flexible cyclohexyl ring, using UV-visible absorption, photoluminescence (PL) and vibrational spectroscopy, and synchrotron X-ray microdiffraction, all aided with density functional theory (DFT) calculations. Substantial anisotropic compression behavior is observed, as characterized by unprecedented negative linear compressibility along the b axis. Moreover, the pressure dependence of optoelectronic properties is found to be in strong contrast with those of 2D HOIPs with rigid spacer cations. DFT calculations help to understand the compression mechanisms that lead to pressure-induced bandgap narrowing. These findings highlight the important role of soft spacer cations in the pressure-tuned optoelectronic properties and provide guidance to the design of new 2D HOIPs.

Tunable 2D Hybrid Perovskites with Pd(II) Adsorption and Semiconducting Properties

Layered hybrid perovskites, due to their broad application potential in optical, electrical, and luminescence fields, are attracting increasing attention. Herein, we report two novel two-dimensional hybrid organic-inorganic perovskites [C₅H₁₀N₂S]PbBr₄ (1) and [C₅H₁₀N₂S]PbI₄ (2) ([C₅H₁₀N₂S]²⁺ is 3-ethylaminothiazolium), which possess phase transition and semiconductor properties. Intriguingly, 1 has semiconductor properties with an optical band gap of 2.70 eV and a photocurrent to dark current ratio of 50 for the photoresponse. By varying the halogen, the band gap of 2 is significantly reduced to 2.06 eV and the photocurrent to dark current ratio of the photoresponse reaches 10⁴. Moreover, compounds 1 and 2 are able to adsorb Pd(II) resulting from the presence of -SH groups in the cation. The adsorption of Pd(II) can turn a semiconductor into an insulator, weaken the fluorescence intensity, and cause the phase transition behavior to disappear. This work has remarkable implications for the continuous development of hybrid perovskites with phase transitions, metal adsorption, and semiconductor properties.

Chiral Perovskites for Next-Generation Photonics: From Chirality Transfer to Chiroptical Activity

Organic-inorganic hybrid halide perovskites (OIHPs) are commonly used as prototypical materials for various applications, including photovoltaics, photodetectors, and light-emitting devices. Since the chiroptical properties of OIHPs are deciphered in 2017, chiral OIHPs have been rediscovered as new hybrid systems comprising chiral organic molecules and achiral inorganic octahedral layers. Owing to their exceptional optoelectrical properties and structural flexibility, chiral OIHPs have received a considerable amount of attention in chiral photonics, chiroptoelectronics, spintronics, and ferroelectrics. Despite their intriguing chiral properties, the transfer mechanism from chiral molecules to achiral semiconductors has not been extensively investigated. Furthermore, an in-depth understanding of the origin of chiroptical activity is still elusive. In this review article, recent advances in the chiroptical activities of chiral OIHPs and polarization-based devices adopting chiral OIHPs are comprehensively discussed, and insight into the underlying chirality transfer mechanism based on theoretical considerations is provided. This comprehensive survey, with an emphasis on the chirality transfer mechanism, will help readers understand the chiroptical properties of OIHPs, which are crucial for the development of spin-based photonic and optoelectronic devices. Additionally, promising strategies to exploit the potential of chiral OIHPs are also discussed.

Pressure-induced bandgap engineering of lead-free halide double perovskite (NH4)2SnBr6

Lead-free halide double perovskites (HDPs) have recently been proposed as potential stable and environment-friendly alternatives to lead-based halide perovskites. Bandgap engineering plays a vital role in the optoelectronic applications of HDP materials. In this study, methods combining high-pressure techniques with density functional theory calculations were employed to implement the bandgap engineering of a classic HDP-based (NH₄)₂SnBr₆. Under high pressure, (NH₄)₂SnBr₆ exhibits a redshift of the bandgap with increasing pressure up to 6.3 GPa and a sudden blueshift up to 20.2 GPa, followed by a redshift at higher pressures, which is relevant to the cubic-tetragonal phase transition, direct-indirect transition, and amorphization, respectively. Our results enrich the understanding of the structural-optical properties of (NH₄)₂SnBr₆ and reveal the special role of NH₄⁺ cations in pressure-induced bandgap engineering, thus providing important information for application in optoelectronic devices and helping to design ideal materials with higher efficiency.

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron-phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge-charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

Harvesting High-Quality White-Light Emitting and Remarkable Emission Enhancement in One-Dimensional Halide Perovskites Upon Compression

The pressure induced emission (PIE) behavior of halide perovskites has attracted extensive interest due to its potential application in pressure sensors and trademark security. However, the PIE phenomenon of white-light-emitting hybrid perovskites (WHPs) is rare, and that at pressures above 10.0 GPa has never been reported. Here, we effectively adjusted the perovskite to emit high-quality "cold" or "warm" white light and successfully realized pressure-induced emission (PIE) upon even higher pressure up to 35.1 GPa in one-dimensional halide perovskite C₄N₂H₁₄PbCl₄. We reveal that the degree of structural distortion and the rearrangement of the multiple self-trapped states position are consistent with the intriguing photoluminescence variation, which is further supported by in situ high-pressure synchrotron X-ray diffraction experiments and time-resolved photoluminescence decay dynamics data. The underlying relationship between octahedron behavior and emission plays a key role to obtain high-quality white emission perovskites. We anticipate that this work enhances our understanding of structure-dependent self-trapped exciton (STE) emission characteristics and stimulates the design of high-performance WHPs for next generation white LED lighting devices.

Photophysics of 2D Organic-Inorganic Hybrid Lead Halide Perovskites: Progress, Debates, and Challenges

2D organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs) have recently attracted increasing attention due to their excellent environmental stability, high degree of electronic tunability, and natural multiquantum-well structures. Although there is a rapid development of photoelectronic applications in solar cells, photodetectors, light emitting diodes (LEDs), and lasers based on 2D RPPs, the state-of-the-art performance is far inferior to that of the existing devices because of the limited understanding on fundamental physics, especially special photophysics in carrier dynamics, excitonic fine structures, excitonic quasiparticles, and spin-related effect. Thus, there is still plenty of room to improve the performances of photoelectronic devices based on 2D RPPs by enhancing knowledge on fundamental photophysics. This review highlights the special photophysics of 2D RPPs that is fundamentally different from the conventional 3D congeners. It also provides the most recent progress, debates, challenges, prospects, and in-depth understanding of photophysics in 2D perovskites, which is significant for not only boosting performance of solar cells, LEDs, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips.

Defect Origin of Emission in CsCu2I3 and Pressure-Induced Anomalous Enhancement

Lead-free metal halide perovskites CsCu₂X₃ (X = Cl, Br, I) with a high photoluminescence quantum yield are promising materials for optoelectronic devices. However, the origin of photoluminescence (PL) emission is still under debate, and the anomalous dependence of PL on pressure is unclear. Here, we systemically study the effects of high pressure on the structural, electronic, and optical properties of CsCu₂I₃ using a diamond anvil cell (DAC) and first-principles calculations. We argue that the ground state structure of CsCu₂I₃ belongs to the pnma phase rather than the cmcm phase under ambient conditions. There is a structural phase transition from the pnma to the cmcm phase for CsCu₂I₃ at ∼5 GPa. The optical band gap derivative from absorption spectra increases from 3.57 to 3.62 eV within a pressure range of 0 to 4.03 GPa, and it then decreases over 4.03 GPa. There are two major PL emissions peaks at 2.11 and 2.32 eV, which are attributed to the intrinsic defect related trap states in CsCu₂I₃. Interestingly, there is an anomalous dependence of both PL emissions on pressure, such that PL peaks show a blueshift and the PL intensity is enhanced from 0 to ∼4 GPa, with redshifting and decreasing at pressures above ∼4 GPa. The anomalous evolution of the two PL emissions also suggests a defect origin of emissions.

Pressure Effects on the Electronic and Optical Properties in Low-Dimensional Metal Halide Perovskites

Metal halide perovskites have shown enormous potential in perovskite solar cells and light-emitting diodes and made unprecedented progress in the past decade. Pressure engineering as an effective technique can systematically modify the electronic structures and physical properties of functional materials. Low-dimensional metal halide perovskites (0D, 1D, and 2D) with a variety of compositions have soft lattices that allow pressure to drastically modulate their structures and properties. High-pressure investigations have obtained a comprehensive understanding of their structure-property relationships. Simultaneously, discoveries of novel pressure-driven properties, such as metallization and partially retained band gap narrowing have contributed significantly to the further development of such materials. In this Perspective, we mainly highlight the effect of pressure on the properties and structures of low-dimensional metal halide perovskites, which is essential for designing new perovskite materials and advancing applications.