Structurally Flexible 2D Spacer for Suppressing the Electron-Phonon Coupling Induced Non-Radiative Decay in Perovskite Solar Cells.

This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron-phonon coupling of perovskite in perovskite solar cells (PSCs). Via A-site cation engineering, a weaker electron-phonon coupling in perovskite has been achieved by introducing the structurally soft cyclohexane methylamine (CMA+) cation, which could serve as a damper to alleviate the mechanical stress caused by lattice oscillations, compared to the rigid phenethyl methylamine (PEA+) analog. It demonstrates a significantly lower non-radiative recombination rate, even though the two types of bulky cations have similar chemical passivation effects on perovskite, which might be explained by the suppressed carrier capture process and improved lattice geometry relaxation. The resulting PSCs achieve an exceptional power conversion efficiency (PCE) of 25.5% with a record-high open-circuit voltage (VOC) of 1.20 V for narrow bandgap perovskite (FAPbI3). The established correlations between electron-phonon coupling and non-radiative decay provide design and screening criteria for more effective passivators for highly efficient PSCs approaching the Shockley-Queisser limit.

Two-dimensional lead halide perovskite lateral homojunctions enabled by phase pinning.

Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.

Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures.

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Broadband lens and graphene/silicon photodiode for wide spectral imaging.

We designed a broadband lens along with a graphene/silicon photodiode for wide spectral imaging ranging from ultraviolet to near-infrared wavelengths. By using five spherical glass lenses, the broadband lens, with the modulation transfer function of 0.38 at 100 lp/mm, corrects aberrations ranging from 340 to 1700 nm. Our design also includes a broadband graphene/silicon Schottky photodiode with the highest responsivity of 0.63 A/W ranging from ultraviolet to near-infrared. By using the proposed broadband lens and the broadband graphene/silicon photodiode, several single-pixel imaging designs in ultraviolet, visible, and near-infrared wavelengths are demonstrated. Experimental results show the advantages of integrating the lens with the photodiode and the potential to realize broadband imaging with a single set of lens and a detector.

Photophysical Behavior of Triethylmethylammonium Tetrabromoferrate(III) under High Pressure.

The pressure-induced properties of hybrid organic-inorganic ferroelectrics (HOIFs) with tunable structures and selectable organic and inorganic components are important for device fabrication. However, given the structural complexity of polycrystalline HOIFs and the limited resolution of pressure data, resolving the structure-property puzzle has so far been the exception rather than the rule. With this in mind, we present a collection of in situ high-pressure data measured for triethylmethylammonium tetrabromoferrate(III), ([N(C2H5)3CH3][FeBr4]) (EMAFB) by unraveling its flexible physical and photophysical behavior up to 80 GPa. Pressure-driven X-ray diffraction and Raman spectroscopy disclose its soft and reversible structural distortion, creating room for delicate band gap modulation. During compression, orange turns dark red at ∼2 GPa, and further compression results in piezochromism, leading to opaque black, while decompressed EMAFB appears in an orange hue. Assuming that the mechanical softness of EMAFB is the basis for reversible piezochromic control, we present alternations in the electronic landscape leading to a 1.22 eV band narrowing at 20.3 GPa while maintaining the semiconducting character at 72 GPa. EMAFB exhibits an emission enhancement, manifested by an increase of photoluminescence up to 17.3 GPa, correlating with the onsets of structural distortion and amorphization. The stimuli-responsive behavior of EMAFB, exhibiting stress-activated modification of the electronic structure, can enrich the physical library of HOIFs suitable for pressure-sensing technologies.

0D Pyramid-intercalated 2D Bimetallic Halides with Tunable Electronic Structures and Enhanced Emission under Pressure.

Hybrid metal halides are emerging semiconductors as promising candidates for optoelectronics. The pursuit of hybridizing various dimensions of metal halides remains a desirable yet highly complex endeavor. By utilizing dimension engineering, a diverse array of new materials with intrinsically different electronic and optical properties has been developed. Here, we report a new family of 2D-0D hybrid bimetallic halides, (C6 N2 H14 )2 SbCdCl9 ⋅ 2H2 O (SbCd) and (C6 N2 H14 )2 SbCuCl9 ⋅ 2H2 O (SbCu). These compounds adopt a new layered structure, consisting of alternating 0D square pyramidal [SbCl5 ] and 2D inorganic layers sandwiched by organic layers. SbCd and SbCu have optical band gaps of 3.3 and 2.3 eV, respectively. These compounds exhibit weak photoluminescence (PL) at room temperature, and the PL gradually enhances with decreasing temperature. Density functional theory (DFT) calculations reveal that SbCd and SbCu are direct gap semiconductors, where first-principles band gaps follow the experimental trend. Moreover, given the different pressure responses of 0D and 2D components, these materials exhibit highly tunable electronic structures during compression, where a remarkable 11 times enhancement in PL emission is observed for SbCd at 19 GPa. This work opens new avenues for designing new layered bimetallic halides and further manipulating their structures and optoelectronic properties via pressure.

Pressure-Modulated Anomalous Organic-Inorganic Interactions Enhance Structural Distortion and Second-Harmonic Generation in MHyPbBr3 Perovskite.

Organic-inorganic halide perovskites possess unique electronic configurations and high structural tunability, rendering them promising for photovoltaic and optoelectronic applications. Despite significant progress in optimizing the structural characteristics of the organic cations and inorganic framework, the role of organic-inorganic interactions in determining the structural and optical properties has long been underappreciated and remains unclear. Here, by employing pressure tuning, we realize continuous regulation of organic-inorganic interactions in a lead halide perovskite, MHyPbBr3 (MHy+ = methylhydrazinium, CH3NH2NH2+). Compression enhances the organic-inorganic interactions by strengthening the Pb-N coordinate bonding and N-H···Br hydrogen bonding, which results in a higher structural distortion in the inorganic framework. Consequently, the second-harmonic-generation (SHG) intensity experiences an 18-fold increase at 1.5 GPa, and the order-disorder phase transition temperature of MHyPbBr3 increases from 408 K under ambient pressure to 454 K at the industrially achievable level of 0.5 GPa. Further compression triggers a sudden non-centrosymmetric to centrosymmetric phase transition, accompanied by an anomalous bandgap increase by 0.44 eV, which stands as the largest boost in all known halide perovskites. Our findings shed light on the intricate correlations among organic-inorganic interactions, octahedral distortion, and SHG properties and, more broadly, provide valuable insights into structural design and property optimization through cation engineering of halide perovskites.

Anomalous Charge Transfer from Organic Ligands to Metal Halides in Zero-Dimensional [(C6 H5 )4 P]2 SbCl5 Enabled by Pressure-Induced Lone Pair-π Interaction.

Low-dimensional (low-D) organic metal halide hybrids (OMHHs) have emerged as fascinating candidates for optoelectronics due to their integrated properties from both organic and inorganic components. However, for most of low-D OMHHs, especially the zero-D (0D) compounds, the inferior electronic coupling between organic ligands and inorganic metal halides prevents efficient charge transfer at the hybrid interfaces and thus limits their further tunability of optical and electronic properties. Here, using pressure to regulate the interfacial interactions, efficient charge transfer from organic ligands to metal halides is achieved, which leads to a near-unity photoluminescence quantum yield (PLQY) at around 6.0 GPa in a 0D OMHH, [(C6 H5 )4 P]2 SbCl5 . In situ experimental characterizations and theoretical simulations reveal that the pressure-induced electronic coupling between the lone-pair electrons of Sb3+ and the π electrons of benzene ring (lp-π interaction) serves as an unexpected "bridge" for the charge transfer. Our work opens a versatile strategy for the new materials design by manipulating the lp-π interactions in organic-inorganic hybrid systems.

Manipulating Peierls Distortion in van der Waals NbOX2 Maximizes Second-Harmonic Generation.

Two-dimensional (2D) van der Waals (vdW) materials, featuring relaxed phase-matching conditions and highly tunable optical nonlinearity, endow them with potential applications in nanoscale nonlinear optical (NLO) devices. Despite significant progress, fundamental questions in 2D NLO materials remain, such as how structural distortion affects second-order NLO properties, which call for advanced regulation and in situ diagnostic tools. Here, by applying pressure to continuously tune the displacement of Nb atoms in 2D vdW NbOI2, we effectively modulate the polarization and achieve a 3-fold boost of the second-harmonic generation (SHG) at 2.5 GPa. By introducing a Peierls distortion parameter, λ, we establish a quantitative relationship between λ and SHG intensity. Importantly, we further demonstrate that the SHG enhancement can be achieved under ambient conditions by anionic substitution to tune the distortion in NbO(I1-xBrx)2 (x = 0-1) compounds, where the chemical tailoring simulates the pressure effects on the structural optimization. Consequently, NbO(I0.60Br0.40)2 with λ = 0.17 exhibits a giant SHG of over 2 orders of magnitude higher than that in monolayer WSe2, reaching the record-high value among reported 2D vdW NLO materials. This work unambiguously demonstrates the correlation between Peierls distortion and SHG property and, more broadly, opens new paths for the development of advanced NLO materials by manipulating the structure distortions.

Correlating Photophysical Properties with Stereochemical Expression of 6s2 Lone Pairs in Two-dimensional Lead Halide Perovskites.

Two-dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light-emitting applications and excitonic devices. Fulfilling these promises demands an in-depth understanding on the relationships between the structural dynamics and exciton-phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out-of-plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb-I bond length, resulting in Pb2+ off-center displacement driven by stereochemical expression of the Pb2+ 6s2 lone pair electrons. Density functional theory calculations indicate that the Pb2+ cation is off-center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb2+ off-centering lead to a broad Raman central peak background and phonon softening, which increase the non-radiative recombination loss via exciton-phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs.

Chirality-Dependent Structural Transformation in Chiral 2D Perovskites under High Pressure.

Chiral perovskites have attracted considerable attention owing to their potential applications in spintronic- and polarization-based optoelectronic devices. However, the structural chirality/asymmetry transfer mechanism between chiral organic ammoniums and achiral inorganic frameworks is still equivocal, especially under extreme conditions, as the systematic structural differences between chiral and achiral perovskites have been rarely explored. Herein, we successfully synthesized a pair of new enantiomeric chiral perovskite (S/R-3PYEA)PbI4 (3PYEA2+ = C5NH5C2H4NH32+) and an achiral perovskite (rac-3PYEA)PbI4. Hydrostatic pressure was used, for the first time, to systematically investigate the differences in the structural evolution and optical behavior between (S/R-3PYEA)PbI4 and (rac-3PYEA)PbI4. At approximately 7.0 GPa, (S/R-3PYEA)PbI4 exhibits a chirality-dependent structural transformation with a bandgap "red jump" and dramatic piezochromism from translucent red to opaque black. Upon further compression, a previously unreported chirality-induced negative linear compressibility (NLC) is achieved in (S/R-3PYEA)PbI4. High-pressure structural characterizations and first-principles calculations demonstrate that pressure-driven homodirectional tilting of homochiral ammonium cations strengthens the interactions between S/R-3PYEA2+ and Pb-I frameworks, inducing the formation of new asymmetric hydrogen bonds N-H···I-Pb in (S/R-3PYEA)PbI4. The enhanced asymmetric H-bonding interactions further break the symmetry of (S/R-3PYEA)PbI4 and trigger a greater degree of in-plane and out-of-plane distortion of [PbI6]4- octahedra, which are responsible for chirality-dependent structural phase transition and NLC, respectively. Nevertheless, the balanced H-bonds incurred by equal proportions of S-3PYEA2+ and R-3PYEA2+ counteract the tilting force, leading to the absence of chirality-dependent structural transition, spectral "red jump", and NLC in (rac-3PYEA)PbI4.

Modulating Charge-Density Wave Order and Superconductivity from Two Alternative Stacked Monolayers in a Bulk 4Hb-TaSe2 Heterostructure via Pressure.

Two-dimensional (2D) van der Waals heterostructures (VDWHs) containing a charge-density wave (CDW) and superconductivity (SC) have revealed rich tunability in their properties, which provide a new route for optimizing their novel exotic states. The interaction between SC and CDW is critical to its properties; however, understanding this interaction within VDWHs is very limited. A comprehensive in situ study and theoretical calculation on bulk 4Hb-TaSe2 VDWHs consisting of alternately stacking 1T-TaSe2 and 1H-TaSe2 monolayers are investigated under high pressure. Surprisingly, the superconductivity competes with the intralayer and adjacent-layer CDW order in 4Hb-TaSe2, which results in substantially and continually boosted superconductivity under compression. Upon total suppression of the CDW, the superconductivity in the individual layers responds differently to the charge transfer. Our results provide an excellent method to efficiently tune the interplay between SC and CDW in VDWHs and a new avenue for designing materials with tailored properties.

Pressure driven rotational isomerism in 2D hybrid perovskites.

Multilayers consisting of alternating soft and hard layers offer enhanced toughness compared to all-hard structures. However, shear instability usually exists in physically sputtered multilayers because of deformation incompatibility among hard and soft layers. Here, we demonstrate that 2D hybrid organic-inorganic perovskites (HOIP) provide an interesting platform to study the stress-strain behavior of hard and soft layers undulating with molecular scale periodicity. We investigate the phonon vibrations and photoluminescence properties of Ruddlesden-Popper perovskites (RPPs) under compression using a diamond anvil cell. The organic spacer due to C4 alkyl chain in RPP buffers compressive stress by tilting (n = 1 RPP) or step-wise rotational isomerism (n = 2 RPP) during compression, where n is the number of inorganic layers. By examining the pressure threshold of the elastic recovery regime across n = 1-4 RPPs, we obtained molecular insights into the relationship between structure and deformation resistance in hybrid organic-inorganic perovskites.

Configuration-Induced Multichromism of Phenanthridine Derivatives: A Type of Versatile Fluorescent Probe for Microenvironmental Monitoring.

Fluorescent probes are attractive in diagnosis and sensing. However, most reported fluorophores can only detect one or few analytes/parameters, notably limiting their applications. Here we have designed three phenanthridine-based fluorophores (i.e., B1, F1, and T1 with 1D, 2D, and 3D molecular configuration, respectively) capable of monitoring various microenvironments. In rigidifying media, all fluorophores show bathochromic emissions but with different wavelength and intensity changes. Under compression, F1 shows a bathochromic emission of over 163 nm, which results in organic fluorophore-based full-color piezochromism. Moreover, both B1 and F1 exhibit an aggregation-caused quenching (ACQ) behavior, while T1 is an aggregation-induced emission (AIE) fluorophore. Further, F1 and T1 selectively concentrate in cell nucleus, whereas B1 mainly stains the cytoplasm in live cell imaging. This work provides a general design strategy of versatile fluorophores for microenvironmental monitoring.

Giant Tunability of Charge Transport in 2D Inorganic Molecular Crystals by Pressure Engineering.

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb2 O3 molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb2 O3 flake shows a sharp drop by 5 orders of magnitude in 0-3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

Reconfiguring band-edge states and charge distribution of organic semiconductor-incorporated 2D perovskites via pressure gating.

Two-dimensional (2D) semiconductor heterostructures are key building blocks for many electronic and optoelectronic devices. Reconfiguring the band-edge states and modulating their interplay with charge carriers at the interface in a continuous manner have long been sought yet are challenging. Here, using organic semiconductor-incorporated 2D halide perovskites as the model system, we realize the manipulation of band-edge states and charge distribution via mechanical-rather than chemical or thermal-regulation. Compression induces band-alignment switching and charge redistribution due to the different pressure responses of organic and inorganic building blocks, giving controllable emission properties of 2D perovskites. We propose and demonstrate a "pressure gating" strategy that enables the control of multiple emission states within a single material. We also reveal that band-alignment transition at the organic-inorganic interface is intrinsically not well resolved at room temperature owing to the thermally activated transfer and shuffling of band-edge carriers. This work provides important fundamental insights into the energetics and carrier dynamics of hybrid semiconductor heterostructures.

Nested order-disorder framework containing a crystalline matrix with self-filled amorphous-like innards.

Solids can be generally categorized by their structures into crystalline and amorphous states with different interactions among atoms dictating their properties. Crystalline-amorphous hybrid structures, combining the advantages of both ordered and disordered components, present a promising opportunity to design materials with emergent collective properties. Hybridization of crystalline and amorphous structures at the sublattice level with long-range periodicity has been rarely observed. Here, we report a nested order-disorder framework (NOF) constructed by a crystalline matrix with self-filled amorphous-like innards that is obtained by using pressure to regulate the bonding hierarchy of Cu12Sb4S13. Combined in situ experimental and computational methods demonstrate the formation of disordered Cu sublattice which is embedded in the retained crystalline Cu framework. Such a NOF structure gives a low thermal conductivity (~0.24 W·m-1·K-1) and a metallic electrical conductivity (8 × 10-6 Ω·m), realizing the collaborative improvement of two competing physical properties. These findings demonstrate a category of solid-state materials to link the crystalline and amorphous forms in the sublattice-scale, which will exhibit extraordinary properties.

New Lead-free Organic-Inorganic Hybrid Semiconductor Single Crystals for a UV-Vis-NIR Broadband Photodetector.

Organic-inorganic hybrid semiconducting (OIHS) materials, which can detect broader spectral regions, are highly desired in several applications including biomedical imaging, night vision, and optical communications. Although lead (Pb)-halide perovskites have reached a mature research stage, high toxicity of Pb hinders their large-scale viability. Tin (Sn)-based perovskites are the most common OIHS broadband light absorbers that replace toxic Pb; however, they are extremely unstable due to the notorious Sn2+ oxidation. Herein, a novel, non-toxic, and solution-processed millimeter-sized OIHS single crystal [Ga(C3H7NO)6](I3)3 has been grown at room temperature. Both the absorption measurement and density functional theory calculations have confirmed a narrow indirect band gap of 1.32 eV. The corresponding photodetector based on this single crystal demonstrated excellent performance including an ultraviolet-visible-near infrared (UV-vis-NIR) response between 325 and 1064 nm, fast response time (trise/tdecay = 3.8 ms/5.4 ms), and profound air storage stability (41 h), thus outperforming most common photodetectors based on Sn-based perovskites. This work not only provides a profound understanding of this novel organic-inorganic single-crystal material but also demonstrates its great potential to realize the high-performance UV-vis-NIR broadband photodetectors.

Understanding Electron-Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs.

The electron-phonon (e-ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e-ph interaction in LHPs. Herein, we advance a fundamental understanding of the e-ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure-property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e-ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e-ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.

Pressure-Enhanced Photocurrent in One-Dimensional SbSI via Lone-Pair Electron Reconfiguration.

Understanding the relationships between the local structures and physical properties of low-dimensional ferroelectrics is of both fundamental and practical importance. Here, pressure-induced enhancement in the photocurrent of SbSI is observed by using pressure to regulate the lone-pair electrons (LPEs). The reconfiguration of LPEs under pressure leads to the inversion symmetry broken in the crystal structure and an optimum bandgap according to the Shockley-Queisser limit. The increased polarization caused by the stereochemical expression of LPEs results in a significantly enhanced photocurrent at 14 GPa. Our research enriches the foundational understanding of structure-property relationships by regulating the stereochemical role of LPEs and offers a distinctive approach to the design of ferroelectric-photovoltaic materials.