2D Hybrid Organic-inorganic Perovskites Research Articles

2D Molecular Ferroelectric with Large Out‐of‐plane Polarization for In‐Memory Computing

Abstract2D ferroelectric materials with out‐of‐plane polarization are crucial for future nanoscale logic devices due to the increasing demand for energy‐efficient architectures in artificial intelligence. However, only a few 2D out‐of‐plane ferroelectrics are confirmed experimentally. As an important branch of ferroelectrics, organic–inorganic hybrid perovskite ferroelectrics show flexible structures, making them eligible for constructing multifunctional materials. Here, a 2D organic–inorganic hybrid perovskite ferroelectric (6‐BHA)2CdBr4 (6‐BHA is 6‐bromohexylamine) is designed, which crystallizes in polar point group Cc. It experiences the reversal phase transition at 317.8 K and possesses multiaxial ferroelectric properties. More interestingly, it exhibits a large spontaneous polarization value of 3.26 µC cm−2 in out‐of‐plane direction of the film compared with typical 2D ferroelectrics. Moreover, an inverter based on (6‐BHA)2CdBr4 is fabricated, which serves as a proof of concept for the feasibility for logic‐in‐memory devices. This work not only enriches the family of molecular ferroelectrics but also shows the potential to create the next generation of in‐memory computing devices, nanoelectronics devices, and ultra‐high‐density memories.

Carrier Transport in 2D Hybrid Organic-Inorganic Perovskites: The Role of Spacer Molecules.

Two-dimensional organic-inorganic hybrid perovskites (2D HOIPs) have been widely used for various optoelectronics applications owing to their excellent photoelectric properties. However, the selection of organic spacer cations is mostly qualitative without quantitative guidance. Meanwhile, the fundamental mechanism of the carrier transport across the organic spacer layer is still unclear. Here, by using the first-principles nonadiabatic molecular dynamics (NAMD) method, we have studied the transport process of excited carriers between 2D HOIPs separated by a spacer cation layer in real time at atomic levels. We find that the excited electrons and holes can transfer from single-inorganic-layer 2D HOIP to bi-inorganic-layer 2D HOIP on a subpicosecond to picosecond scale. Moreover, we have developed a new method to capture the electron-hole interaction in the frame of NAMD. This work provides a promising direction to design new materials toward high-performance optoelectronics.

Topological Nodal-Point Superconductivity in Two-Dimensional Ferroelectric Hybrid Perovskites.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) with enhanced stability, high tunability, and strong spin-orbit coupling have shown great potential in vast applications. Here, we extend the already rich functionality of 2D HOIPs to a new territory, realizing topological superconductivity and Majorana modes for fault-tolerant quantum computation. Especially, we predict that room-temperature ferroelectric BA2PbCl4 (BA for benzylammonium) exhibits topological nodal-point superconductivity (NSC) and gapless Majorana modes on selected edges and ferroelectric domain walls when proximity-coupled to an s-wave superconductor and an in-plane Zeeman field, attractive for experimental verification and application. Since NSC is protected by spatial symmetry of 2D HOIPs, we envision more exotic topological superconducting states to be found in this class of materials due to their diverse noncentrosymmetric space groups, which may open a new avenue in the fields of HOIPs and topological superconductivity.

Giant intrinsic piezoelectricity in 2D hybrid organic-inorganic perovskites [C6H11NH3]2MX4 (M = Ge, Sn, Pb; X = Cl, Br, I).

2D-piezoelectric materials are attractive for micro-electromechanical systems (MEMS), medical implants and wearable devices because of their numerous exceptional properties. 2D-hybrid organic-inorganic perovskites (HOIPs) have attracted extensive research interest due to their merits of structural diversity, good mechanical flexibility, and ease of fabrication. The electronic energy band, charge density and the elastic properties of 2D-HOIP-[C6H11NH3]2MX4 (M = Ge, Sn, Pb; X = Cl, Br, I) were investigated using first-principles calculations. The excellent piezoelectricity of 2D-HOIP-[C6H11NH3]2MX4 has been analyzed in detail. More importantly, 2D-[C6H11NH3]2MX4 have giant intrinsic positive and negative out-of-plane piezoelectric coefficients under the effect of van der Waals interaction. The d31 and d32 of [C6H11NH3]2SnBr4 are 82.720 pm V-1 and -36.139 pm V-1, respectively, which are among the largest piezoelectric coefficients among all kinds of atomic-thick 2D materials reported. The high flexibility together with the giant out-of-plane piezoelectricity would endow these 2D-HOIP-[C6H11NH3]2MX4 with potential applications in ultrathin piezoelectric cantilever and diaphragm devices.

Ferroelectricity and Thermochromism in a 2D Dion-Jacobson Organic-Inorganic Hybrid Perovskite.

2D organic-inorganic hybrid perovskites (OIHPs) have become one of the hottest research topics due to their excellent environmental stability and unique optoelectronic properties. Recently, the ferroelectricity and thermochromism of 2D OIHPs have attracted increasing interests. Integrating ferroelectricity and thermochromism into perovskites can significantly promote the development of multichannel intelligent devices. Here, a novel 2D Dion-Jacobson OIHP of the formula (3AMP)PbI4 (where 3AMP is 3-(aminomethyl)pyridinium) is reported, which has a remarkable spontaneous polarization value (Ps) of 15.6 µCcm-2 and interesting thermochromism. As far it is known, such a large Ps value is the highest for 2D OIHPs recorded so far. These findings will inspire further exploration and application of multifunctional perovskites.

(Invited) Atomic Imaging and Device Characterization of Molecularly Thin 2D Hybrid Perovskites

2D Organic-Inorganic Hybrid perovskites (OIHPs) possess alternating layers of organic molecules and inorganic quantum wells. By tuning the interaction between organic and inorganic layers, very interesting properties can be introduced. The successful isolation of hybrid perovskite monolayers with clean and flat surfaces are particularly suitable for atomic structure characterization and optoelectronic device fabrication. In this talk, I will present recent results on the excitonic properties and atomic structure of molecularly thin OIHPs1,2. We found that a reversible shift in excitonic energies can be induced upon laser irradiation in large-sized 2D hybrid perovskite monolayers which is attributed to the reversible structural reorientation of the surface BA cations in the easily deformable lattice3. The photodetection performance of monolayer, bilayer and thicker OIHPs were compared. The internal quantum efficiency was measured to be 34% for a monolayer (BA)2(MA)3Pb4I13 and 19% for the bulk crystal. In addition, 2D heterostructures of molecularly thin OIHPs/graphene were constructed which exhibited a lower barrier than gold for carrier injection, enabling applications in field effect transistors (FETs)4. Electron tunneling occurs across the interface of organic molecular layers on 2D perovskite and graphene, while photoinduced charge transfer occurs at femtosecond timescale (~50 fs).Resolving the atomic structure of 2D hybrid perovskite is a challenging task because it is easily damaged by electron beam. Here we use scanning tunneling microscopy (STM) & Qplus AFM to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs)5. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL) and promotes Rashba spin splitting for n > 1. Our studies demonstrate a protocol to investigate structure-property correlations in low dimensional 2D OIHPs, which will pave the way to better material design and device performance.References Leng, W. Fu, Y. Liu, M. Chhowalla & KP Loh* “From Bulk to Molecularly Thin Hybrid Perovskites” Nature Reviews Materials , 5, 482-500 (2020).Zhao, KP Loh, K. Leng* “Organic-inorganic hybrid perovskites and their heterostructures” Matter , 5, 4153-4169 (2022).Leng, I. Abdelwaha, I. Verzhbitskiy, M. Telychko, L. Chu, W. Fu, X. Chi, N. Guo, Z. Chen, Z. Chen, C. Zhang, Q. Xu, J. Lu, M. Chhowalla, G. Eda, KP Loh* “Molecularly thin two-dimensional hybrid perovskites with tunable optoelectronic properties due to reversible surface relaxation” Nature Materials , 17, 908-914 (2018).Leng, L. Wang, Y. Shao, I. Abdelwahab, G. Grinblat, I. Verzhbitskiy, Y. Cai, X. Chi, W. Fu, P. Song, G. Eda, S. A. Maier & KP Loh* “Electron Tunneling at the Molecularly Thin 2D Perovskite and Graphene Van der Waals Interface” Nature Communications , 11, 5483 (2020).Y Shao, W. Gao, H. Yan, R. Li, I. Abdelwahab, L. Zhuang, W. Fu, SP Lau, SF YU, Y. Cai, KP Loh, K. Leng* “Unlocking Surface Octahedral Tilt in Two-dimensional Ruddlesden-Popper Perovskites” Nature Communications , 13, 138 (2022). AcknowledgementK.L. would like to acknowledge the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU15305221 for GRF project funded in 2021/22 Exercise and Project No. PolyU25305222 for ECS project funded in 2022/23 Exercise).

(Invited) Chirality-Induced Spin Selectivity Enabling Efficient Spin-Dependent Oxygen Evolution Reaction and High-Performance Photoelectrochemical Water Splitting

The sluggish and complex multi-step oxygen evolution reaction (OER) remains an obstacle to bias-free photoelectrochemical water-splitting systems. Several theoretical and experimental studies have suggested that OER efficiency can be significantly enhanced in chiral material-based photoanodes by utilizing the chirality-induced spin selectivity (CISS) phenomenon. The final product at the photoanode is the oxygen molecules, where the spin alignment of intermediate radicals is favorable to the generation of triplet 3O2 (most stable molecular oxygen species). Considering the energy difference between a singlet 1O2 and triplet 3O2, spin-dependent chemical reactions combined with the CISS phenomenon can play a crucial role in the OER process. Moreover, chiral material-based photoanodes are capable of paralleling spin distribution on the catalytic surface. The resulting spin-aligned intermediate radicals can promote the generation of spin-allowed triplet 3O2 over that of singlet oxygen, while hindering the formation of hydrogen peroxide, in turn leading to the enhanced STH efficiency of the overall PEC water-splitting system. In this regard, chiral materials (as a spin-filter layer) have been recognized as promising candidates for achieving a breakthrough in the solar-to-hydrogen efficiency of water-splitting devices. Herein, we report that the CISS phenomena can become an impressive approach by adopting chiral 2D organic-inorganic hybrid perovskites (OIHPs) as a spin-filtering layer on the 3D OIHP photoanode. Our chiral 2D OIHP-based water-splitting device achieved enhanced oxygen evolution performance with a reduced overpotential of 0.2 V, high fill factor, and increased photocurrent of 22 mA cm–2 at 1.2 VRHE compared to a device without a spin-filtering layer. For unbiased water splitting, a perovskite-based photocathode decorated with a spatially decoupled hydrogen evolution reaction catalyst is fabricated. This decoupled geometry enables the physical protection of the perovskite layer from the electrolyte, thus allowing excellent stability for over 100 h. To investigate the overall water splitting performance, this perovskite photocathode is connected in parallel with spin-filtering layer adopted photoanode based on chirality-induced spin selectivity. An unbiased solar water splitting was achieved using all the OIHP photoelectrodes, resulting in a remarkable unassisted solar-to-hydrogen efficiency of ~12% and a continuous 10 h stable operation.

Exciton–polaritons in a 2D hybrid organic–inorganic perovskite microcavity with the presence of optical Stark effect

This study investigates the properties of exciton–polaritons in a two-dimensional (2D) hybrid organic–inorganic perovskite microcavity in the presence of optical Stark effect. Through both steady and dynamic state analyses, strong coupling between excitons of perovskite and cavity photons is revealed, indicating the formation of polaritons in the perovskite microcavity. Besides, it is found that an external optical Stark pulse can induce energy shifts of excitons proportional to the pulse intensity, which modifies the dispersion characteristics of the polaritons.

Formation of multiple quantum wells m 2D/3D perovskite heterostructures invalidates phonon bottleneck effect

The importance of the phonon bottleneck effect on the optoelectronic properties of organic-inorganic hybrid perovskites (OIHPs) has been well documented. While randomly distributed multiple quantum wells (MQWs) are frequently generated in 2D/3D perovskite heterostructures, the influence of MQWs formation in 2D/3D perovskite heterostructures on the phonon bottleneck effect is poorly understood. In this study, we construct 2D/3D perovskite heterostructures using 2D OIHP (NH3(CH2)8NH3PbI4, OdAPbI4) to passivate 3D OIHP (FAPbI3) and investigate their excitons and carrier dynamics by employing photoluminescence (PL) spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy (fsOPTPS). We observe the formation of MQWs and establish a structure-dynamics-property relationship among the exciton formation time ( TEF), the hot carrier cooling rate (kcool) and the effective charge-carrier mobility(ϕμ). The formation of MQWs could significantly invalidate the phonon bottleneck effect and modulate the optoelectronic properties of 2D/3D perovskite heterostructures. This work provides valuable information on the importance of band alignment and the phonon bottleneck effect in the strategy of 2D/3D perovskite heterostructures.

Circularly Polarized Light Emission from Nonchiral Perovskites Incorporated into Nanoporous Cholesteric Polymer Templates.

Chiral perovskites have garnered significant attention, owing to their chiroptical properties and emerging applications. Current fabrication methods often involve complex chemical synthesis routes. Herein, an alternative approach for introducing chirality into nonchiral hybrid organic-inorganic perovskites (HOIPs) using nanotemplates composed of cholesteric polymeric networks is proposed. This method eliminates the need for additional molecular design. In this process, HOIP precursors are incorporated into a porous cholesteric polymer film, and two-dimensional (2D) HOIPs grow inside the nanopores. Circularly polarized light emission (CPLE) was observed even though the selective reflection band of the cholesteric polymer films containing a representative HOIP deviated from the emission wavelength of the 2D HOIP. This effect was confirmed by the induced circular dichroism (CD) observed in the absorbance band of the HOIP. The observed CPLE and CD are attributed to the chirality induced by the template in the originally nonchiral 2D HOIP. Additionally, the developed 2D HOIP exhibited a long exciton lifetime and good stability under harsh conditions. These findings provide valuable insights into the development and design of innovative optoelectronic materials.

Fluorination and Conjugation Engineering Synergistically Enhance the Optoelectronic Properties of Two-Dimensional Hybrid Organic-Inorganic Perovskites.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are expected to be a viable alternative to three-dimensional (3D) analogs in solar cells (SCs) and optoelectronic devices due to their high stability, diverse composition, and physical properties. However, unsuitable band alignment and large bandgaps limit the power conversion efficiency (PCE) improvement of SCs based on 2D HOIPs. Here, we report a molecular design strategy that combines fluorination and conjugation engineering to tune the electronic structure and optimize the PCE of 2D HOIPs. Our results show that type IIa band alignment and tunable bandgaps can be achieved in 2D Dion-Jacobson (DJ) HOIPs by H/F substitution of organic cations with different degrees of conjugation. In general, the bandgap of 2D DJ-HOIPs decreases monotonously with the increase of the number of F atoms, which is due to the gradual decrease of the lowest unoccupied molecular orbitals (LUMO) of organic cations. In addition, the enhanced interlayer charge transfer and higher dielectric constant suggest that the fluorination-induced dielectric limitations are weakened. The estimated PCE of 2D DJ-HOIPs is exponentially increased and positively correlated with the degree of conjugation and fluorination of organic cations, with a PCE approaching 29% under their synergistic effect. Our results not only provide promising candidates for photovoltaic device applications but also provide an effective method for PCE optimization.

First-principles study of electronic properties of two-dimensional metal mixed perovskites solar cell and water resistance performance

Two-Dimensional (2D) layered hybrid organic-inorganic perovskite (HOIP) materials have been considered as potential photoconductive materials for solar cells owing to extraordinary conversion efficiency and easy preparation process. In this study, we perform first-principles design on metal mixed 2D HOIPs (BA2(Cs1-x Rb x )2Sn1.5Pb1.5I10 with different proportions of composition (x = 0, 0.5 and 1)) to investigate their electronic properties and stability against moisture. We find that the band gap and effective mass of BA2(Cs1-x Rb x )2Sn1.5Pb1.5I10 increases as the concentrations of mixed Rb atoms increases. Moreover, the result of calculated adsorption energy indicates that the metal mixed 2D HOIPs with different composition ratios of Rb atom exhibit better capacity on the resistance of water than the pure one (BA2MA2Sn3I10). The metal mixed 2D HOIPs shows great potential to be a new generation of solar cell with low-cost, suitable band gap, high thermal stability, and desirable power conversion efficiency.

Chiral Cation Doping for Modulating Structural Symmetry of 2D Perovskites.

Cation mixing in two-dimensional (2D) hybrid organic-inorganic perovskite (HOIP) structures represents an important degree of freedom for modifying organic templating effects and tailoring inorganic structures. However, the limited number of known cation-mixed 2D HOIP systems generally employ a 1:1 cation ratio for stabilizing the 2D perovskite structure. Here, we demonstrate a chiral-chiral mixed-cation system wherein a controlled small amount (<10%) of chiral cation S-2-MeBA (S-2-MeBA = (S)-(-)-2-methylbutylammonium) can be doped into (S-BrMBA)2PbI4 (S-BrMBA = (S)-(-)-4-bromo-α-methylbenzylammonium), modulating the structural symmetry from a higher symmetry (C2) to the lowest symmetry state (P1). This structural change occurs when the concentration of S-2-MeBA, measured by solution nuclear magnetic resonance, exceeds a critical level─specifically, for 1.4 ± 0.6%, the structure remains as C2, whereas 3.9 ± 1.4% substitution induces the structure change to P1 (this structure is stable to ∼7% substitution). Atomic occupancy analysis suggests that one specific S-BrMBA cation site is preferentially substituted by S-2-MeBA in the unit cell. Density functional theory calculations indicate that the spin splitting along different k-paths can be modulated by cation doping. A true circular dichroism band at the exciton energy of the 3.9% doping phase shows polarity inversion and a ∼45 meV blue shift of the Cotton-effect-type line-shape relative to (S-BrMBA)2PbI4. A trend toward suppressed melting temperature with higher doping concentration is also noted. The chiral cation doping system and the associated doping-concentration-induced structural transition provide a material design strategy for modulating and enhancing those emergent properties that are sensitive to different types of symmetry breaking.

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.

Reversible and Irreversible Layer Edge Relaxation in Laser‐Radiation‐Hardened 2D Organic–Inorganic Perovskite Crystals

The layer edge states or low energy state (LES) in 2D hybrid organic–inorganic perovskites demonstrate a prolonged carrier lifetime for better performance of optoelectronic devices. However, the fundamental understanding of LES in 2D perovskites is still inconclusive. Herein, a photoluminescence (PL) study of LES in 2D Ruddlesden–Popper perovskites is presented with n = 2 and n = 3 from their cleaved cross sections that are more stable than the natural edge. The PL measurements clearly observe reversible, and irreversible surface relaxations (case I and case II) in three laser intensity ranges, further supported by a PL excitation cycle from low to high laser intensity, and vice versa. The PL wavelength of LES is tunable with laser intensity and blueshifts with increasing laser intensity during irreversible surface relaxation process (case I). Fluorescence lifetime imaging (FLIM) shows that the LES has a longer lifetime than the band‐edge emission in the sample without a photodegradation, while the BE lifetime becomes relatively longer in the area with a photodegradation. The presented laser tunable LES and the related irreversible relaxation process provide a new insight that can help improve the photostability in 2D perovskites and understand roles of LESs in optoelectronic device performance.

Unveiling the Fatigue Behavior of 2D Hybrid Organic-Inorganic Perovskites: Insights for Long-Term Durability.

2D hybrid organic-inorganic perovskites (HOIPs) are commonly found under subcritical cyclic stresses and suffer from fatigue issues during device operation. However, their fatigue properties remain unknown. Here, the fatigue behavior of (C4 H9 -NH3 )2 (CH3 NH3 )2 Pb3 I10 , the archetype 2D HOIP, is systematically investigated by atomic force microscopy (AFM). It is found that 2D HOIPs are much more fatigue resilient than polymers and can survive over 1 billion cycles. 2D HOIPs tend to exhibit brittle failure at high mean stress levels, but behave as ductile materials at low mean stress levels. These results suggest the presence of a plastic deformation mechanism in these ionic 2D HOIPs at low mean stress levels, which may contribute to the long fatigue lifetime, but is inhibited at higher mean stresses. The stiffness and strength of 2D HOIPs are gradually weakened under subcritical loading, potentially as a result of stress-induced defect nucleation and accumulation. The cyclic loading component can further accelerate this process. The fatigue lifetime of 2D HOIPs can be extended by reducing the mean stress, stress amplitude, or increasing the thickness. These results can provide indispensable insights into designing and engineering 2D HOIPs and other hybrid organic-inorganic materials for long-term mechanical durability.

Unraveling the Effect of Stacking Configurations on Charge Transfer in WS2 and Organic Semiconductor Heterojunctions.

Photoinduced interfacial charge transfer plays a critical role in energy conversion involving van der Waals (vdW) heterostructures constructed of inorganic nanostructures and organic materials. However, the effect of molecular stacking configurations on charge transfer dynamics is less understood. In this study, we demonstrated the tunability of interfacial charge separation in a type-II heterojunction between monolayer (ML) WS2 and an organic semiconducting molecule [2-(3″',4'-dimethyl-[2,2':5',2':5″,2″'-quaterthiophen]-5-yl)ethan-1-ammonium halide (4Tm)] by rational design of relative stacking configurations. The assembly between ML-WS2 and the 4Tm molecule forms a face-to-face stacking when 4Tm molecules are in a self-aggregation state. In contrast, a face-to-edge stacking is observed when 4Tm molecule is incorporated into a 2D organic-inorganic hybrid perovskite lattice. The face-to-face stacking was proved to be more favorable for hole transfer from WS2 to 4Tm and led to interlayer excitons (IEs) emission. Transient absorption measurements show that the hole transfer occurs on a time scale of 150 fs. On the other hand, the face-to-edge stacking resulted in much slower hole transfer without formation of IEs. This inefficient hole transfer occurs on a similar time scale as A exciton recombination in WS2, leading to the formation of negative trions. These investigations offer important fundamental insights into the charge transfer processes at organic-inorganic interfaces.

Chirality-Induced Spin Selectivity of Chiral 2D Perovskite Enabling Efficient Spin-Dependent Oxygen Evolution Reaction.

The sluggish and complex multi-step oxygen evolution reaction remains an obstacle to bias-free photoelectrochemical water-splitting systems. Several theoretical studies have suggested that spin-aligned intermediate radicals can significantly enhance the kinetic rates for oxygen generation. Herein, it is reported that the chirality-induced spin selectivity phenomena can become an impressive approach by adopting chiral 2D organic-inorganic hybrid perovskites as a spin-filtering layer on the photoanode. This chiral 2D perovskite-based water-splitting device achieves enhanced oxygen evolution performance with a reduced overpotential of 0.14V, high fill factor, and 230% increased photocurrent compared to a device without a spin-filtering layer. Moreover, combined with a superhydrophobic patterning strategy, this device realizes excellent operational stability by sustaining ≈90% of the initial photocurrent, even after 10h.

Radiative pumping of exciton-polaritons in 2D hybrid perovskites

In addition to their attractive technological applications in photovoltaics and light emitters, the perovskite family of semiconductors has recently emerged as an excellent excitonic material for fundamental studies. Specifically, the 2D hybrid organic-inorganic perovskite (HOIP) offers the added advantage of room temperature investigations owing to their large exciton binding energy. In this work, we strongly couple excitons in 2D HOIP crystals to planar microcavity photons sustaining exciton-polaritons under ambient conditions resulting in a Rabi splitting of 290 meV. Dark excitons directly pump the polariton branch along its dispersion in resonance with the Stokes shifted emission state (radiative pumping), creating a high density of polaritons at higher in-plane momentum (k||). We further probe the nonlinear polariton dispersion dynamics at varying input laser fluence, which indicates efficient polariton-polariton scattering and decay to k|| = 0 from higher k||. The observation of Stokes shift-assisted energy exchange of dark states with lower polaritons coupled with evidence of efficient polariton-polariton scattering makes 2D HOIPs an attractive platform to study exciton-polariton many-body physics and Bose-Einstein like condensation (BEC) at room temperature.

Multiexciton Generation from a 2D Organic-Inorganic Hybrid Perovskite with Nearly 200% Quantum Yield of Red Phosphorescence.

2D organic-inorganic hybrid perovskites (OIHPs) show obvious advantages in the field of optoelectronics due to their high luminescent stability and good solution processability. However, the thermal quenching and self-absorption of excitons caused by the strong interaction between the inorganic metal ions lead to a low luminescence efficiency of 2D perovskites. Herein, a 2D Cd-based OIHP phenylammonium cadmium chloride(PACC) with a weak red phosphorescence (ΦP < 6%) at 620nm and a blue afterglow is reported. Interestingly, the Mn-doped PACC exhibits very strong red emission with nearly 200% quantum yield and 15ms lifetime, thus resulting in a red afterglow. The experimental data prove that the doping of Mn2+ not only induces the multiexciton generation (MEG) process of the perovskite, avoiding the energy loss of inorganic excitons, but also promotes the Dexter energy transfer from organic triplet excitons to inorganic excitons, thus realizing the superefficient red-light emission of Cd2+ . This work suggests that guest metal ions can induce host metal ions to realize MEG in 2D bulk OIHPs, which provides a new idea for the development of optoelectronic materials and devices with ultrahigh energy utilization.