π-π Stacking at the Perovskite/C60 Interface Enables High-Efficiency Wide-Bandgap Perovskite Solar Cells

Interface passivation is a key method for improving the efficiency of perovskite solar cells, and 2D/3D perovskite heterojunction is the mainstream passivation strategy. However, the passivation layer also produces a new interface between 2D perovskite and fullerene (C60), and the properties of this interface have received little attention before. Here, the underlying properties of the 2D perovskite/C60 interface by taking the 2D TEA2PbX4 (TEA = C6H10NS; X = I, Br, Cl) passivator as an example are systematically expounded. It is found that the 2D perovskite preferentially exhibits (002) orientation with the outermost surface featuring an oriented arrangement of TEACl, where the thiophene groups face outward. The outward thiophene groups further form a strong π-π stacking system with C60 molecule, strengthening the interaction force with C60 and facilitating the creation of a superior interface. Based on the vacuum-assisted blade coating, wide-bandgap (WBG, 1.77 eV) perovskite solar cells achieved impressive records of 19.28% (0.09 cm2) and 18.08% (1.0 cm2) inefficiency, respectively. This research not only provides a new understanding of interface processing for future perovskite solar cells but also lays a solid foundation for realizing efficient large-area devices.

Interlayer Carrier Dynamics in Two-Dimensional Perovskites Determined by the Length of Conjugated Organic Cations

Unlocking the restricted interlayer carrier transfer in a two-dimensional perovskite is a crucial means to achieve the harmonization of efficiency and stability in perovskite solar cells. In this work, the effects of conjugated organic molecules on the interlayer carrier dynamics of 2D perovskites were investigated through nonadiabatic molecular dynamics simulations. We found that elongated conjugated organic cations contributed significantly to the accelerated interlayer carrier dynamics, originating from lowered transport barrier and boosted π-p coupling between organic and inorganic layers. Utilizing conjugated molecules of moderate length as spacer cations can yield both superior efficiency and exceptional stability simultaneously. However, conjugated chains that are too long lead to structural instability and stronger carrier recombination. The potential of conjugated chain-like molecules as spacer cations in 2D perovskites has been demonstrated in our work, offering valuable insights for the development of high-performance perovskite solar cells.

Arising 2D Perovskites for Ionizing Radiation Detection

2D perovskites have greatly improved moisture stability owing to the large organic cations embedded in the inorganic octahedral structure, which also suppresses the ions migration and reduces the dark current. The suppression of ions migration by 2D perovskites effectively suppresses excessive device noise and baseline drift and shows excellent potential in the direct X-ray detection field. In addition, 2D perovskites have gradually emerged with many unique properties, such as anisotropy, tunable bandgap, high photoluminescence quantum yield, and wide range exciton binding energy, which continuously promote the development of 2D perovskites in ionizing radiation detection. This review aims to systematically summarize the advances and progress of 2D halide perovskite semiconductor and scintillator ionizing radiation detectors, including reported alpha (α) particle, beta (β) particle, neutron, X-ray, and gamma (γ) ray detection. The unique structural features of 2D perovskites and their advantages in X-ray detection are discussed. Development directions are also proposed to overcome the limitations of 2D halide perovskite radiation detectors.

Stereo-Hindrance Engineering of A Cation toward <110>-Oriented 2D Perovskite with Minimized Tilting and High-Performance X-Ray Detection

2D <100>-oriented Dion-Jacobson or Ruddlesden-Popper perovskites are widely recognized as promising candidates for optoelectronic applications. However, the large interlayer spacing significantly hinders the carrier transport. <110>-oriented 2D perovskites naturally exhibit reduced interlayer spacings, but the tilting of metal halide octahedra is typically serious and leads to poor charge transport. Herein, a <110>-oriented 2D perovskite EPZPbBr4 (EPZ = 1-ethylpiperazine) with minimized tilting is designed through A-site stereo-hindrance engineering. The piperazine functional group enters the space enclosed by the three [PbBr6 ]4- octahedra, pushing Pb─Br─Pb closer to a straight line (maximum Pb─Br─Pb angle ≈180°), suppressing the tilting as well as electron-phonon coupling. Meanwhile, the ethyl group is located between layers and contributes an extremely reduced effective interlayer distance (2.22 Å), further facilitating the carrier transport. As a result, EPZPbBr4 simultaneously demonstrates high µτ product (1.8 × 10-3 cm2 V-1 ) and large resistivity (2.17 × 1010 Ω cm). The assembled X-ray detector achieves low dark current of 1.02 × 10-10 A cm-2 and high sensitivity of 1240 µC Gy-1 cm-2 under the same bias voltage. The realized specific detectivity (ratio of sensitivity to noise current density, 1.23 × 108 µC Gy-1 cm-1 A-1/2 ) is the highest among all reported perovskite X-ray detectors.

Surface Crystallization Modulation toward Highly-Oriented and Phase-Pure 2D Perovskite Solar Cells

2D perovskites have shown great potential toward stable and efficient photovoltaic devices. However, the crystal orientation and phase impurity issues of 2D perovskite films originating from the anisotropic crystal structure and specific growth mechanism have demoted their optoelectronic performances. Here, the surface crystallization modulation technique is introduced to fabricate the high-quality 2D perovskite films with both vertical crystal orientation and high phase purity by regulating the crystallization dynamics. The solvent atmosphere condition is instituted during film processing, which promotes the formation of an oriented 2D perovskite layer in stoichiometric composition at the vapor-liquid interface and templates the subsequent film growth. The solar cells based on the optimized 2D perovskite films exhibit a power conversion efficiency of 15.04%, the record for 2D perovskites (with the perovskite slab thickness n ≤ 3 and high phase purity). The solar cells based on the highly-oriented and phase-pure 2D perovskite films also demonstrate excellent thermal and humidity stabilities.

Orbital Interactions in 2D Dion-Jacobson Perovskites Using Oligothiophene-Based Semiconductor Spacers Enable Efficient Solar Cells

2D Dion-Jacobson (DJ) perovskites have emerged as promising photovoltaic materials, but the insulating organic spacer has hindered the efficient charge transport. Herein, we successfully synthesized a terthiophene-based semiconductor spacer, namely, 3ThDMA, for 2D DJ perovskite. An interesting finding is that the energy levels of 3ThDMA extensively overlap with the inorganic components and directly contribute to the band formation of (3ThDMA)PbI4, leading to enhanced charge transport across the organic spacer layers, whereas no such orbital interactions were found in (UDA)PbI4, a DJ perovskite based on 1,11-undecanediaminum (UDA). The devices based on (3ThDMA)MAn-1PbnI3n+1 (nominal n = 5) obtained a champion efficiency of 15.25%, which is a record efficiency for 2D DJ perovskite solar cells using long-conjugated spacers (conjugated rings ≥ 3) and a 22.60% efficiency for 3ThDMA-treated 3D PSCs. Our findings provide an important insight into understanding the orbital interactions in 2D DJ perovskite using an organic semiconductor spacer for efficient solar cells.

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 µC cm-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.

Enhancing Efficiency and Stability of Tin Halide Perovskite Light-Emitting Diodes via Engineered Alkali/Multivalent Metal Salts

Sn-based perovskite light-emitting diodes (PeLEDs) have emerged as promising alternatives to Pb-based PeLEDs with their rapid increase in performance owing to the various research studies on inhibiting Sn oxidation. However, the absence of defect passivation strategies for Sn-based perovskite LEDs necessitates further research in this field. We performed systematic studies to investigate the design rules for defect passivation agents for Sn-based perovskites by incorporating alkali/multivalent metal salts with various cations and anions. From the computational and experimental analyses, sodium trifluoromethanesulfonate (NaTFMS) was found to be the most effective passivation agent for PEA2SnI4 films among the explored candidate agents owing to favorable reaction energetics to passivate iodide Frenkel defects. Consequently, the incorporation of NaTFMS facilitates the formation of uniform films with relatively large crystals and reduced Sn4+. The NaTFMS-containing PEA2SnI4 PeLEDs demonstrate an improved luminance of 138.9 cd/m2 and external quantum efficiency (EQE) of 0.39% with an improved half-lifetime of more than threefold. This work provides important insight into the design of defect passivation agents for Sn-based perovskites.

Template-Assisted Synthesis of 2D Perovskite Grating Single Crystal Films at Low Temperatures for UV Polarization-Sensitive Photodetectors

2D perovskites have attracted tremendous attention due to their superior optoelectronic properties and potential applications in optoelectronic devices. Especially, the larger bandgap of 2D perovskite means that they are suitable for UV photodetection. However, the layered structure of 2D perovskites hinders the interlayer carrier transport, which limits the improvement of device performance. Therefore, nanoscale structures are normally used to enhance the light absorption ability, which is an effective strategy to improve the photocurrent in 2D perovskite-based photodetectors. Herein, a template-assisted low-temperature method is proposed to fabricate 2D perovskite ((C6 H5 C2 H4 NH3 )2 PbBr4 , (PEA)2 PbBr4 ) grating single crystal films (GSCFs). The crystallinity of the (PEA)2 PbBr4 GSCFs is significantly improved due to the slow evaporation of the precursor solution under low temperatures. Based on this high crystalline quality and extremely ordered microstructures, the metal-semiconductor-metal photodetectors are assembled. Finite-different time-domain (FDTD) simulation and experiment indicate that the GSCF-based photodetectors exhibit significantly improved performance in comparison with the plane devices. The optimized 2D perovskite photodetectors are sensitive to UV light and demonstrate a responsivity and detectivity of 28.6 mA W-1 and 2.4 × 1011 Jones, respectively. Interestingly, the photocurrent of this photodetector varies as the angle of the incident polarized light, resulting in a high polarization ratio of 1.12.

Fabrications of twisted moiré photonic crystal and random moiré photonic crystal and their potential applications in light extraction

Twisted moiré photonic crystal is an optical analog of twisted graphene or twisted transition metal dichalcogenide bilayers. In this paper, we report the fabrication of twisted moiré photonic crystals and randomized moiré photonic crystals and their use in enhanced extraction of light in light-emitting diodes (LEDs). Fractional diffraction orders from randomized moiré photonic crystals are more uniform than those from moiré photonic crystals. Extraction efficiencies of 76.5%, 77.8% and 79.5% into glass substrate are predicted in simulations of LED patterned with twisted moiré photonic crystals, defect-containing photonic crystals and random moiré photonic crystals, respectively, at 584 nm. Extraction efficiencies of optically pumped LEDs with 2D perovskite (BA)2(MA)n-1PbnI3n+1ofn= 3 and (5-(2'-pyridyl)-tetrazolato)(3-CF3-5-(2'-pyridyl)pyrazolato) platinum(II) (PtD) have been measured.

Trap or Triplet? Excited-State Interactions in 2D Perovskite Colloids with Chromophoric Cations

Movement of energy within light-harvesting assemblies is typically carried out with separately synthesized donor and acceptor species, which are then brought together to induce an interaction. Recently, two-dimensional (2D) lead halide perovskites have gained interest for their ability to accommodate and assemble chromophoric molecules within their lattice, creating hybrid organic-inorganic compositions. Using a combination of steady-state and time-resolved absorption and emission spectroscopy, we have now succeeded in establishing the competition between energy transfer and charge trapping in 2D halide perovskite colloids containing naphthalene-derived cations (i.e., NEA2PbX4, where NEA = naphthylethylamine). The presence of room-temperature triplet emission from the naphthalene moiety depends on the ratio of bromide to iodide in the lead halide sublattice (i.e., x in NEA2Pb(Br1-xIx)4), with only bromide-rich compositions showing sensitized emission. Photoluminescence lifetime measurements of the sensitized naphthalene reveal the formation of the naphthalene triplet excimer at room temperature. From transient absorption measurements, we find the rate constant of triplet energy transfer (kEnT) to be on the order of ∼109 s-1. At low temperatures (77 K) a new broad emission feature arising from trap states is observed in all samples ranging from pure bromide to pure iodide composition. These results reveal the interplay between sensitized triplet energy transfer and charge trapping in 2D lead halide perovskites, highlighting the need to carefully parse contributions from competing de-excitation pathways for optoelectronic applications.

2D Ruddlesden-Popper Polycrystalline PerovskitePyro-Phototronic Photodetectors

Two-dimensional (2D) Ruddlesden-Popper (RP) layered halide perovskite has attracted wide attentions due to its unique structure and excellent optoelectronic properties. With inserting organic cations, inorganic octahedrons are forced to extend in a certain direction, resulting in an asymmetric 2D perovskite crystal structure and causing spontaneous polarization. The pyroelectric effect resulted from spontaneous polarization exhibits a broad prospect in the application of optoelectronic devices. Herein, 2D RP polycrystalline perovskite (BA)2 (MA)3 Pb4 I13 film with excellent crystal orientation is fabricated by hot-casting deposition, and a class of 2D hybrid perovskite photodetectors (PDs) with pyro-phototronic effect is proposed, achieving temperature and light detection with greatly improved performance by coupling multiple energies. Because of the pyro-phototronic effect, the current is ≈35 times to that of the photovoltaic effect current under 0 V bias. The responsivity and detectivity are 12.7 mA W-1 and 1.73 × 1011 Jones, and the on/off ratio can reach 3.97 × 103 . Furthermore, the influences of bias voltage, light power density, and frequency on the pyro-phototronic effect of 2D RP polycrystalline perovskite PDs are explored. The coupling of spontaneous polarization and light facilitates photo-induced carrier dissociation and tunes the carrier transport process, making 2D RP perovskites a competitive candidate for next-generation photonic devices.

A Binary Solution Strategy Enables High-Efficiency Quasi-2D Perovskite Solar Cells with Excellent Thermal Stability

Quasi-two-dimensional (2D) perovskites are highly promising light-harvesting materials for commercialization of perovskite solar cells (PSCs) owing to the excellent materials stability. However, the coexistence of multiple n-value species in 2D perovskites often causes increased complexities in crystallization that can negatively affect the eventual photovoltaic performance. Herein, we present a binary solution based strategy via introducing nontoxic and widely accessible CH₃COOH (HAc) as a co-solvent for preparing high-quality 2D perovskite films. Based on a 2D perovskite model system, (AA)₂MA₄Pb₅I₁₆ (n = 5), we show that the prenucleation and grain growth kinetics are appreciably modified with HAc, which benefits from the strong electron-donating ability of HAc with the key component of PbI₂, leading to formation of favorable cluster aggregates and resultant modulation of crystal growth. With the HAc-based method, the devices yield a boosted photovoltaic efficiency of 18.55% with an impressive photovoltage of 1.26 V. The champion cells exhibit a supreme thermal stability, showing <3% efficiency degradation under continuous thermal aging for 800 h.

Structural Isomer of Fluorinated Ruddlesden-Popper Perovskites Toward Efficient and Stable 2D/3D Perovskite Solar Cells

Defect passivation using two-dimensional (2D)-layered perovskites with organic spacers on 3D bulk perovskites has been proposed as an effective strategy to improve perovskite solar cell stability and efficiency. Specifically, fluorination of the organic spacers has been employed due to the resulting hydrophobic nature and the defect passivation characteristics. In addition to the type of functional groups attached to the spacer molecules, conformational changes of fluorine isomers on layered perovskites can provide an extended strategy to control a variety of opto-electrical properties related to the interlayer spacing. As a model system for the structural isomer of fluorinated spacers, meta-CF₃ and para-CF₃ groups anchored to phenethylammonium iodide (PEAI) spacer molecules are employed to synthesize 2D perovskites and to investigate their full potential as an interfacial modifier for perovskite solar cells. The fluorination position change leads to altered opto-electrical characteristics in layered perovskites. Although they possess identical functional groups, the different orientations of the functional groups used in the perovskite layer deposited on the 3D perovskite absorber result in distinct electrical properties of 2D/3D heterostructures due to dissimilar intermolecular interactions. The 2D perovskite with meta-CF₃-PEAI spacers exhibits an enhancement of the charge transport in the out-of-plane orientation and an improved suppression of the trap states of 3D perovskites while also providing a more favorable energy alignment for efficient charge transfers. Theoretical simulations are consistent with the experimental results. The structural isomers of fluorination anchoring to spacer cations alter the structural configuration of the spacer as well as the interlayer spacing that can improve the performance and the stability of 2D/3D perovskite solar cells.

Sub-Nanosecond 2D Perovskite Scintillators by Dielectric Engineering

Perovskite materials have demonstrated great potential for ultrafast scintillators with high light yield. However, the decay time of perovskite still cannot be further minimized into sub-nanosecond region, while sub-nanosecond scintillators are highly demanded in various radiation detection, including high speed X-ray imaging, time-of-flight based tomography or particle discrimination, and timing resolution measurement in synchrotron radiation facilities, etc. Here, a rational design strategy is showed to shorten the scintillation decay time, by maximizing the dielectric difference between organic amines and Pb-Br octahedral emitters in 2D organic-inorganic hybrid perovskites (OIHP). Benzimidazole (BM) with low dielectric constant inserted between [PbBr₆ ]^2- layers, resulting in a surprisingly large exciton binding energy (360.3 ± 4.8 meV) of 2D OIHP BM₂ PbBr₄ . The emitting decay time is shortened as 0.97 ns, which is smallest among all the perovskite materials. Moreover, the light yield is 3190 photons MeV^-1 , which is greatly higher than conventional ultrafast scintillator BaF₂ (1500 photons MeV^-1 ). The rare combination of ultrafast decay time and considerable light yield renders BM₂ PbBr₄ excellent performance in γ-ray, neutron, α-particle detection, and the best theoretical coincidence time resolution of 65.1 ps, which is only half of the reference sample LYSO (141.3 ps).

Mosaic CuI-CuII-InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method

The 2D perovskite (BA)4[CuII(CuIInIII)0.5]Cl8 (1BA; BA+ = butylammonium) allows us to study the high-pressure structural, optical, and transport properties of a mixed-valence 2D perovskite. Compressing 1BA reduces the onset energy of CuI/II intervalence charge transfer from 1.2 eV at ambient pressure to 0.2 eV at 21 GPa. The electronic conductivity of 1BA increases by 4 orders of magnitude upon compression to 20 GPa, when the activation energy for conduction decreases to 0.16 eV. In contrast, CuII perovskites achieve similar conductivity at ~50 GPa. The solution-state synthesis of these perovskites is complicated, with more undesirable side products likely from the precursor mixtures containing three different metals. To circumvent this problem, we demonstrate an efficient mechanochemical synthesis to expand this family of halide perovskites with complex composition by simply pulverizing together powders of 2D CuII single perovskites and CuIInIII double perovskites.

Fast, Multi-Bit, and Vis-Infrared Broadband Nonvolatile Optoelectronic Memory with MoS2 /2D-Perovskite Van der Waals Heterojunction

Nonvolatile optoelectronic memory (NVOM) integrating the functions of optical sensing and long-term memory can efficiently process and store a large amount of visual scene information, which has become the core requirement of multiple intelligence scenarios. However, realizing NVOM with vis-infrared broadband response is still challenging. Herein, the room temperature vis-infrared broadband NVOM based on few-layer MoS₂ /2D Ruddlesden-Popper perovskite (2D-RPP) van der Waals heterojunction is realized. It is found that the 2D-RPP converts the initial n-type MoS₂ into p-type and facilitates hole transfer between them. Furthermore, the 2D-RPP rich in interband states serves as an effective electron trapping layer as well as broadband photoresponsive layer. As a result, the dielectric-free MoS₂ /2D-RPP heterojunction enables the charge to transfer quickly under external field, which enables a large memory window (104 V), fast write speed of 20 µs, and optical programmable characteristics from visible light (405 nm) to telecommunication wavelengths (i.e., 1550 nm) at room temperature. Trapezoidal optical programming can produce up to 100 recognizable states (>6 bits), with operating energy as low as 5.1 pJ per optical program. These results provide a route to realize fast, low power, multi-bit optoelectronic memory from visible to the infrared wavelength.

Electron-Phonon Coupling Suppression by Enhanced Lattice Rigidity in 2D Perovskite Single Crystals for High-Performance X-Ray Detection

2D Dion-Jacobson (DJ) perovskite single crystals (PSCs) usually demonstrate better X-ray detection performance than Ruddlesden-Popper (RP) PSCs. However, the mechanism of the improved performance is still elusive. Here, by the aid of strong interactions between dimethylbiguanide (DGA) and PbI₂ , a novel DJ-perovskitoid (DGA)PbI₄ is designed. From the comparison of (DGA)PbI₄ to other 2D PSCs, it is discovered that the tiniest lattice distortion and increased hydrogen bonds in the atom-scaled analysis strengthen lattice rigidity and weaken electron-phonon coupling to suppress disordered scattering of carriers, resulting in significantly improved carrier transport and stability. Therefore, high carrier mobility (78.1 cm² V^-1 s^-1 ) and a pronounced sensitivity of 4869.0 µC Gy_air ^-1 cm^-2 are achieved using (DGA)PbI₄ , which are the best in 2D Pb-based PSC devices to date. Finally, the (DGA)PbI₄ devices exhibit good spatial resolution in X-ray imaging and excellent long-term stability to work as a promising candidate for medical diagnostics and nondestructive determination.

Interlayer Transition Induced Infrared Response in ReS2/2D Perovskite van der Waals Heterostructure Photodetector

The emerging Ruddlesden-Popper two-dimensional perovskite (2D PVK) has recently joined the family of 2D semiconductors as a potential competitor for building van der Waals (vdW) heterostructures in future optoelectronics. However, to date, most of the reported heterostructures based on 2D PVKs suffer from poor spectral response that is caused by intrinsic wide bandgap of constituting materials. Herein, a direct heterointerface bandgap (∼0.4 eV) between 2D PVK and ReS₂ is demonstrated. The strong interlayer coupling reduces the energy interval at the heterojunction region so that the heterostructure shows high sensitivity with the spectral response expanding to 2000 nm. The large type-II band offsets exceeding 1.1 eV ensure fast photogenerated carriers separation at the heterointerface. When this heterostructure is used as a self-driven photodetector, it exhibits a record high detectivity up to 1.8 × 10¹⁴ Jones, surpassing any reported 2D self-driven devices, and an impressive external quantum efficiency of 68%.

High Performance 0D ZnO Quantum Dot/2D (PEA)2PbI4 Nanosheet Hybrid Photodetectors Fabricated via a Facile Antisolvent Method

Two-dimensional (2D) organic-inorganic perovskites have great potential for the fabrication of next-generation photodetectors owing to their outstanding optoelectronic features, but their utilization has encountered a bottleneck in anisotropic carrier transportation induced by the unfavorable continuity of the thin films. We propose a facile approach for the fabrication of 0D ZnO quantum dot (QD)/2D (PEA)₂PbI₄ nanosheet hybrid photodetectors under the atmospheric conditions associated with the ZnO QD chloroform antisolvent. Profiting from the antisolvent, the uniform morphology of the perovskite thin films is obtained owing to the significantly accelerated nucleation site formation and grain growth rates, and ZnO QDs homogeneously decorate the surface of (PEA)₂PbI₄ nanosheets, which spontaneously passivate the defects on perovskites and enhance the carrier separation by the well-matched band structure. By varying the ZnO QD concentration, the Ion/Ioff ratio of the photodetectors radically elevates from 78.3 to 1040, and a 12-fold increase in the normalized detectivity is simultaneously observed. In addition, the agglomeration of perovskite grains is governed by the annealing temperature, and the photodetector fabricated at a relatively low temperature of 120 °C exhibits excellent stability after a 50-cycle test in the air condition without any encapsulation.

Surface-Tension-Dominant Crystallization of 2D Perovskite Single Crystals for Vertically Oriented Hetero-/Homo-Structure Photodetectors

2D halide perovskites feature solution processability and tunable optoelectronic properties for optoelectronic applications. However, the controllable fabrication of halide perovskite heterojunction still remains a challenge. Herein, through controlling surface tension and nucleation driving force, a fast and facile aqueous floating growth is demonstrated to obtain a series of large-area single-crystalline 2D perovskite microplates at room temperature. The optoelectronic performance of 2D perovskites can be tuned by composition engineering, and the best performance is realized for quantum well index n = 4, including a suppressed dark current with boosted photocurrent and an on/off ratio up to 3.5 orders of magnitude. Benefiting from a convenient transfer method onto arbitrary substrates, vertically oriented 2D perovskite hetero-/homo-junctions are gently stacked, which exhibit improved self-powered characteristics. This straightforward growth strategy is an universal solution-processed method for growing 2D perovskites, laying the foundation of the 2D perovskite hetero-/homo-junction for future miniaturization and functionalization of next-generation optoelectronics.

Light-Controlled Reconfigurable Optical Synapse Based on Carbon Nanotubes/2D Perovskite Heterostructure for Image Recognition

Two-dimensional (2D) halide perovskite material is characterized by a mixed conducting behavior that possesses both electronic and ionic conductivity. The study on the influence of the light on ion migration in the 2D perovskite is helpful to improve the performance of perovskite-based optoelectronic devices. Here, we constructed an exfoliated 2D perovskite/carbon nanotubes (CNTs) heterostructure optical synapse, in which CNTs can be used as nanoprobes to qualitatively observe the ion aggregation or dissipation process in 2D perovskite, and found that light significantly changes the memory curve of the reconfigurable optical synapses. Through the molecular dynamic simulation, the dynamic process of ion migration in the heterostructure was simulated and the electrostatic interaction effect of nonequilibrium charge distribution of CNTs on iodide ion was demonstrated. Finally, an effective light-controlled process was realized through the synapses, which in situ regulated the performance of the weight-value discretized BP (WD-BP) neural network. This work lays a foundation for the future development of intelligent nano-optoelectronic devices.

Defect Passivation through (α-Methylguanido)acetic Acid in Perovskite Solar Cell for High Operational Stability

Defect passivation has become essential in improving efficiency and stability in perovskite solar cells. Here, we report the use of (α-methylguanido)acetic acid, also known as creatine, as a passivation molecule. It is employed both as an additive and as a surface passivation layer of perovskite thin films, given its multiple functional groups, which could address different defect sites, and its size, which could inherently affect the material structure. We prove that the surface passivation is more efficiently working by removing vulnerable defects on the surface. Hole and electron defect densities were reduced, leading to the highest power conversion efficiency of 22.6%. In addition, it can effectively protect the perovskite thin film and improve the operational stabilities in high thermal (85 °C) and humid conditions (50% relative humidity), suggesting a strong stability of the surface passivation layer.

Polariton Dynamics in Two-Dimensional Ruddlesden-Popper Perovskites Strongly Coupled with Plasmonic Lattices

Strong coupling between light and matter can produce hybrid eigenstates known as exciton-polaritons. Although polariton dynamics are important photophysical properties, the relaxation pathways of polaritons in different coupling regimes have seen limited attention. This paper reports the dynamics of hybridized states from 2D Ruddlesden-Popper perovskites coupled to plasmonic nanoparticle lattices. The open cavity architecture of Al lattices enables the coupling strength to be modulated by varying either the lead halide perovskite film thickness or the superstrate refractive index. Both experiments and finite-difference time-domain simulations of the optical dispersion diagrams showed avoided crossings that are a signature of strong coupling. Our analytical model also elucidated the correlation between the exciton/plasmon mixing ratio and polariton coupling strength. Using fs-transient absorption spectroscopy, we found that both the upper and lower polaritons have shorter lifetimes than the excitons and that polaritons can show faster excited-state dynamics when they have access to additional energy transfer channels.

Tuning the optical properties of 2D monolayer silver-bismuth bromide double perovskite by halide substitution

Silver-bismuth double perovskites are promising replacement materials for lead-based ones in photovoltaic (PV) devices due to the lower toxicity and enhanced stability to environmental factors. In addition, they might even be more suitable for indoor PV, due to the size of their bandgap better matching white LEDs emission. Unfortunately, their optoelectronic performance does not reach that of the lead-based counterparts, because of the indirect nature of the band gap and the high exciton binding energy. One strategy to improve the electronic properties is the dimensional reduction from the 3D to the 2D perovskite structure, which features a direct band gap, as it has been reported for 2D monolayer derivates of Cs₂AgBiBr₆obtained by substituting Cs⁺cations with bulky alkylammonium cations. However, a similar dimensional reduction also brings to a band gap opening, limiting light absorption in the visible. In this work, we report on the achievement of a bathochromic shift in the absorption features of a butylammonium-based silver-bismuth bromide monolayer double perovskite through doping with iodide and study the optical properties and stability of the resulting thin films in environmental conditions. These species might constitute the starting point to design future sustainable materials to implement as active components in indoor photovoltaic devices used to power the IoT.

Zwitterion-Assisted Crystal Growth of 2D Perovskites with Unfavorable Phase Suppression for High-Performance Solar Cells

Despite two-dimensional (2D) Ruddlesden-Popper-phase layered perovskites (RPLPs) exhibiting excellent environmental stability, most solar cells based on 2D RPLP films are fabricated in a controlled inert atmosphere. Meanwhile, the poor charge transport of 2D RPLP films owing to the unfavorable phase arrangement and defects limits the efficiency of 2D RPLP solar cells. Here, we fabricate high-efficiency 2D RPLP solar cells in ambient air assisted by a zwitterion (ZW) additive. We show that the ZW additive suppresses the formation of the bottom 2D phases (n ≤ 2) and the top 3D-like phases in 2D RPLP films. These 2D phases usually grow parallel to the substrate and act as trap sites that inhibit charge transport in the vertical direction. The 3D-like phases, on the other hand, aggravate the long-term stability due to the intrinsic instability of MA⁺ cations. With improved phase distribution, crystal orientation, and reduced trap states in 2D RPLP films, efficient charge transport is obtained. Finally, a record-high open-circuit voltage (V_oc) of 1.19 V and a power conversion efficiency of 17.04% with an enhanced stability are achieved for (BA_0.9PEA_0.1)₂MA₃Pb₄I₁₃-based (n = 4) solar cells fabricated under high humidity (∼65% RH).

Interfacial Modification via a 1,4-Butanediamine-Based 2D Capping Layer for Perovskite Solar Cells with Enhanced Stability and Efficiency

Organic-inorganic perovskites face the issues of being vulnerable to oxygen and moisture and the trap sites located at the surface and grain boundaries. Integration of two-dimensional (2D) perovskites as a capping layer is an effective route to enhance both photovoltaic efficiency and environmental stability of the three-dimensional (3D) underlayer. Here, we employ 1,4-butanediammonium diiodide (BDADI), which has a short chain length and diammonium cations, to construct a 3D/2D stacking perovskite solar cells (PSCs). The introduction of BDA2+ could passivate surface defects in 3D perovskites by forming 2D Dion-Jacobson (DJ) phase perovskites and effectively suppressing nonradiative recombination, thus resulting in a longer carrier lifetime. The DJ 2D capping layer also regulate the energy level arrangement, enabling a better charge extraction and transport process. In addition, the water-resistance ability of 3D perovskite gets improved because of the hydrophobic characteristic of 1,4-butanediammonium cations. Consequently, the 3D/2D stacking PSCs yield an energy conversion efficiency of 20.32% in company with the enhanced long-term stability.

Highly Efficient and Stable Dion-Jacobson Perovskite Solar Cells Enabled by Extended π-Conjugation of Organic Spacer

2D Dion-Jacobson (DJ) perovskites have become an emerging photovoltaic material with excellent structure and environmental stability due to their lacking van der Waals gaps relative to 2D Ruddlesden-Popper perovskites. Here, a fused-thiophene-based spacer, namely TTDMAI, is successfully developed for 2D DJ perovskite solar cells. It is found that the DJ perovskite using TTDMA spacer with extended π-conjugation length exhibits high film quality, large crystal size and preferred crystal vertical orientation induced by the large crystal nuclei in precursor solution, resulting in lower trap density, reduced exciton binding energy and oriented charge transport. As a result, the optimized 2D DJ perovskite device based on TTDMA (nominal n = 4) delivers a champion PCE up to 18.82%. Importantly, the unencapsulated device based on TTDMA can sustain average 99% of their original efficiency after being stored in N₂ for 4400 h (over 6 months). Moreover, light, thermal, environmental and operational stabilities are also significantly improved in comparison with their 3D counterparts.

2D/3D Halide Perovskites for Optoelectronic Devices

The traditional three-dimensional (3D) halide perovskites (HPs) have experienced rapid development due to their highly power conversion efficiency (PCE). However, the instability of 3D perovskite on humidity and UV irradiation blocks their commercialization. In the past few years, two-dimensional (2D) halide perovskites attract much attention because they behave better stability due to the water resistance of the aliphatic carbon chains in the 2D perovskite lattice. In this review, we categorize the 2D/3D perovskites based on the applications [i.e., solar cells (SCs), light-emitting diodes (LEDs) and photodetectors (PDs)]. We further discuss the recent efforts in the performance enhancement of the 2D/3D perovskite-based devices. However, there are still some difficulties before 2D/3D HPs is fully commercialized. We will provide some scientific and technical challenges and prospects in the article to point out the future direction.

Creating a Dual-Functional 2D Perovskite Layer at the Interface to Enhance the Performance of Flexible Perovskite Solar Cells

Flexible perovskite solar cells (f-PSCs) have been attracting tremendous attention due to their potentially commercial prospects in flexible energy system and mobile energy system. Reducing the energy barriers and charge extraction losses at the interfaces between perovskite and charge transport layers is essential to improve both efficiency and stability of f-PSCs. Herein, 4-trifluoromethylphenylethylamine iodide (CF₃ PEAI) is introduced to form a 2D perovskite at the interface between perovskite and hole transport layer (HTL). It is found that the 2D perovskite plays a dual-functional role in aligning energy band between perovskite and HTL and passivating the traps in the 3D perovskite, thus reducing energy loss and charge carrier recombination at the interface, facilitating the hole transfer from perovskite to the Spiro-OMeTAD. Consequently, the photovoltaic performance of f-PSCs is significantly improved, leading to a power conversion efficiency (PCE) of 21.1% and a certified PCE of 20.5%. Furthermore, the long-term stability of f-PSCs is greatly improved through the protection of 2D perovskite layer to the underlying 3D perovskite. This work provides an excellent strategy to produce efficient and stable f-PSCs, which will accelerate their potential applications.

Robust and Swiftly Reversible Thermochromic Behavior of a 2D Perovskite of (C6H4(CH2NH3)2)(CH3NH3)[Pb2I7] for Smart Window and Photovoltaic Smart Window Applications

Highly robust, swiftly reversible thermochromic nature of a two-dimensional (2D) perovskite of (PDMA)(CH₃NH₃)_n-1Pb_nI_3n+1, nominally prepared as n = 2 is found, where PDMA = C₆H₄(CH₂NH₃)₂. A wide band gap variation from 700 to 430 nm is observed between room temperature and >60 °C under ambient conditions, resulting from moisture absorption and desorption. X-ray diffraction and Fourier-transform infrared spectroscopy are performed to analyze the hydrated and dehydrated states. Furthermore, the (PDMA)(CH₃NH₃)_n-1Pb_nI_3n+1 film is demonstrated as an active material for smart windows and thermochromic solar cells, which could lower the inside air temperature in an enclosed space and supply a power conversion efficiency of more than 0.5% at a high ambient temperature, respectively. Overall, we may pave a pathway for exploring the novel phenomena and applications of Dion-Jacobson 2D perovskites.

Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure

Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS₂) monolayer/2D perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of -0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS₂/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS₂ region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 μA/W and 5.7 × 10^-5, respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.

Excitonic Energy Transfer in Heterostructures of Quasi-2D Perovskite and Monolayer WS2

Quasi-two-dimensional (2D) organic-inorganic hybrid perovskite is a re-emerging material with strongly excitonic absorption and emission properties that are attractive for photonics and optoelectronics. Here we report the experimental observation of excitonic energy transfer (ET) in van der Waals heterostructures consisting of quasi-2D hybrid perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) and monolayer WS₂. Photoluminescence excitation spectroscopy reveals a distinct ground exciton resonance feature of perovskite, evidencing ET from perovskite to WS₂. We find unexpectedly high photoluminescence enhancement factors of up to ∼8, which cannot be explained by single-interface ET. Our analysis reveals that interlayer ET across the bulk of the layered perovskite also contributes to the large enhancement factor. Further, from the weak temperature dependence of the lower-limit ET rate, which we found to be ∼3 ns^-1, we conclude that the Förster-type mechanism is responsible.

Two-Dimensional Cs2Pb(SCN)2Br2-Based Photomemory Devices Showing a Photoinduced Recovery Behavior and an Unusual Fully Optically Driven Memory Behavior

The rapid development of Internet of Things and big data has made the conventional storage devices face the need of reforming. Rather than using electrical pulses to store data in one of two states, photomemory exploiting optical stimulation to store light information emerges as a revolutionary candidate for the optoelectronic community. However, fully optically driven photomemory with fast data transmission speed and outstanding energy saving capability suffers from less exploration. Herein, a transistor-type photomemory using a 2D Cs₂Pb(SCN)₂Br₂/polymer hybrid floating gate is explored and three host polymers, polystyrene, poly(4-vinylphenol), and poly(vinylpyrrolidone) (PVP), are investigated to understand the relationship between polymer matrix selection and memory performance. All devices show a photoinduced recovery memory behavior but with two distinctly different photomemory behaviors. In addition to the demonstration of a regular nonvolatile photomemory showing a high on/off ratio of >10⁶ over 10⁴ s, an unusual fully optically driven memory behavior is intriguingly accomplished in the Cs₂Pb(SCN)₂Br₂/PVP photomemory. Using white light as the driver of programming and a blue laser as the main performer of erasing, this device can be switched between two distinguishable states and possesses acceptable data discriminability, as evidenced by its fully optically driven writing (programing)-reading-erasing-reading switching function that shows an on/off current ratio of 10³. This study not only presents the first 2D perovskite-based photomemory but also shows a novel fully optically driven memory that has been rarely reported in the literature.

Spatial Charge Separation as the Origin of Anomalous Stark Effect in Fluorous 2D Hybrid Perovskites

2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long-lived correlated electron-hole pairs. Such states have a marked charge-transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities.

Alkali Cation Doping for Improving the Structural Stability of 2D Perovskite in 3D/2D PSCs

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs), whereas undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which causes serious hysteresis of PSCs after aging. This issue is, however, rarely studied. The stability study for 3D/2D hybrid systems to date is all under the one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long-chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, a large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in an ambient environment, even though the reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations, K⁺ and Rb⁺, are introduced into the 2D perovskite to exquisitely modulate the crystal formation, which gives rise to a higher crystallinity of 2D perovskite and a better film morphology with fewer defects. We achieved PCE beyond 21% due to the preferable charge transfer process and reduced nonradiative recombination losses. The structural features also bring about impressive moisture stability, which results in the corresponding PSCs retaining 93% of its initial PCE and negligible hysteresis after aging in an ambient atmosphere for 1200 h.

Exciton-Polariton Properties in Planar Microcavity of Millimeter-Sized Two-Dimensional Perovskite Sheet

In this work, the self-organized two-dimensional (2D) perovskite crystal sheet was synthesized for exciton-polariton studies. The layered 2D perovskite sheets naturally form Fabry-Pérot (F-P) optical cavities and exhibit evident cavity polariton modes. In the region of F-P cavity polariton modes, an exciton-polariton is formed by the superposition of strongly coupled exciton and photon states and the decreased spacing between two adjacent oscillation modes can be well explained by the formation of exciton polarities. Rabi splitting energy (Ω) evaluated from the energy-wavevector (E-k) dispersion relation of exciton-polariton coupling was about 259 ± 10 meV. For edge emissions of the laminated structure, the thickness-dependent quantum well exerts great influence on the band-gap absorption and emission. This work studies exciton-polariton properties in layered 2D perovskite sheets, making it possible application for polariton devices in the strong light-matter interaction region.

Probing Phase Distribution in 2D Perovskites for Efficient Device Design

Two-dimensional (2D) lead halide perovskite has recently been recognized as a promising candidate to stabilize perovskite solar cells due to its extraordinary moisture resistance. These 2D perovskite films often consist of multiple phases with layered (n) lead halide (from n = 1, 2, 3 to ≈∞). However, a convincing evidence is still lacking to clarify the phase distribution with respect to different n, thus causes the misleading for device design. Herein, confocal photoluminescence (PL) spectroscopy was applied to probe the inhomogeneity of 2D perovskite films along the vertical direction to construct a clear-phase distribution mapping consequently. It reveals that the 2D perovskite phases (n = 2, 3, 4) locate preferentially near the substrate, while large n phases are predominantly near the top surface. Moreover, we successfully developed a simple method to manipulate the phase distribution in 2D perovskite thin films, which results in a dramatic increase of device efficiency from 4.95 to 11.6%. Our findings thus provide insights to the understanding of 2D perovskite film growth. The utilization of visualized phase distribution data could also guide the further development of 2D perovskite materials for optoelectronic devices.

A New Organic Interlayer Spacer for Stable and Efficient 2D Ruddlesden-Popper Perovskite Solar Cells

Two-dimensional (2D) perovskite materials have exhibited great possibilities toward the fabrication of highly efficient and stable solar cell devices. The large degree of structural versatility due to the viable choices of organic interlayer spacers promises new and valuable 2D perovskite species. Herein, phenyltrimethylammonium (PTA⁺) is successfully employed as the organic interlayer spacer to prepare the 2D Ruddlesden-Popper perovskite films that exhibit exceptional optoelectronic properties. By adding Cl^- ions during film growth, the (PTA)₂(MA)₃Pb₄I₁₃ (MA = methylammonium) perovskite films are effectively prepared with a tunable crystal orientation and film morphology. The optimized devices fabricated with the assistance of Cl^- ions deliver the power conversion efficiency up to 11.53%, which is ascribed to the simultaneous reductions of charge transfer resistance and defect-induced charge recombination. Moreover, the PTA-based 2D perovskite solar cells demonstrate remarkable environmental and thermal stabilities.

Microstructural Study of Two-Dimensional Organic-Inorganic Hybrid Perovskite Nanosheet Degradation under Illumination

Two-dimensional (2D) organic-inorganic hybrid perovskite materials have received substantial attention because of their exceptional optoelectronic properties. Although the applications of 2D perovskite nanosheets are promising in various optoelectronic devices, which all face harsh working conditions of light exposure, little is known about the photo-stability and degradation mechanisms of these 2D perovskite nanosheets. In this work, degradation of (C₄H₉NH₃)₂PbBr₄ (BA₂PbBr₄) nanosheets when exposed to ultraviolet (UV) light and white light is explored. The morphology, optical properties, and microstructure of the nanosheets, under different conditions of light exposure, was studied in detail. UV light is more destructive compared to white light, which both led to a nanosheet breakdown. A combination of transmission electron microscopy (TEM) imaging and electron diffraction revealed that the organic moieties are most sensitive to light exposure and partial disorder toward complete disorder takes place during light exposure. Moreover, excessive light exposure further causes a [PbBr₆]⁴⁻ octahedron tilt and re-ordering within the perovskite structure. This study could enrich the understanding of 2D perovskite nanosheets and their photostability, offer a new perspective in interpreting the light–perovskite interaction, and further help the design of robust and light-tunable 2D perovskite-based optoelectronic devices.

Oriented Growth of Ultrathin Single Crystals of 2D Ruddlesden-Popper Hybrid Lead Iodide Perovskites for High-Performance Photodetectors

Compared with their 3D counterparts, layered 2D Ruddlesden-Popper perovskites (2D RPPs) with the formula of (A')₂A _n-1Pb _nX_{3 n+1} exhibit improved photo and moisture stability. To develop flexible single-crystalline electronics and wearable devices, cost-effective growth of large-area single crystals of 2D RPPs with large lateral size, ultrathin thickness, parallel orientation, and well-defined and controlled n values are highly desirable, but it remains still a challenge. Here, we modified the space-confined aqueous solution growth method to fabricate single-crystalline films of (BA)₂(MA) _n-1Pb _nX_{3 n+1} and n = 1, 2, 3, respectively, with the lateral size reaching millimeter and thickness about 400-1000 nm. Moreover, the quantum well layers are found to be parallel to the substrate, well suitable for lateral transport requirement. We successfully integrated ultrathin single crystals of (BA)₂(MA) _n-1Pb _nX_{3 n+1} ( n = 1-3) into a metal-semiconductor-metal two-terminal photodetector. The photoresponsive wavelength range was extended from 525 nm for n = 1 and 590 nm for n = 2 to 630 nm for n = 3 as a result of the reduced exciton band gap. The device of n = 2 single crystals shows the highest responsivity of 2.96 A/W and a detectivity of 10¹³ jones, while the device of n = 3 shows the lowest dark current 10^-14 A, and therefore, its on/off ratio reaches 2862 at the bias voltage of 1 V.

High-Performance Photodetectors Based on Lead-Free 2D Ruddlesden-Popper Perovskite/MoS2 Heterostructures

Two-dimensional (2D) Ruddlesden-Popper perovskites have attracted great interest for their promising applications in high-performance optoelectronic devices owing to their greatly tunable band gaps, layered characteristics, and better environmental stability over three-dimensional (3D) perovskites. Here, we for the first time report on photodetectors based on few-layer MoS₂ (n-type) and lead-free 2D perovskite (PEA)₂SnI₄ (p-type) heterostructures. The heterojunction device is capable of sensing light over the entire visible and near-infrared wavelength range with a tunable photoresponse peak. By using few-layer graphene flakes as the electrical contact, the performance of the heterostructures can be improved with a responsivity of 1100 A/W at 3 V bias, a fast response speed of ∼40 ms under zero bias, and an excellent rectification ratio of 500. Importantly, the quantum efficiency can achieve 38.2% at zero bias, which is comparable or even higher than that of 3D perovskite/2D material photodetectors. Importantly, the spectral response peak of heterojunctions gradually shifts in a wide spectral range from the band edge of MoS₂ toward that of (PEA)₂SnI₄ with the external bias. We believe these 2D perovskite/2D material heterostructures with a great diversity represent an interesting system for investigating the fundamental optoelectronic properties and open up a new pathway toward 2D perovskite-based optoelectronic devices.

All-Perovskite Emission Architecture for White Light-Emitting Diodes

We demonstrate all-perovskite light-emitting diodes (PeLEDs) with white emission on the basis of simultaneously solving a couple of issues including the ion exchanges between different perovskites, solvent incompatibility in the solution process of stacking different perovskites and carrier transport layers, as well as the energy level matching between each layer in the whole device. The PeLEDs are built with a two-dimensional (CH₃CH₂CH₂NH₃)₂CsPb₂I₇ perovskite that emits red light, CsPb(Br,Cl)₃ quantum dots that emit a cyan color, and an interlayer composed of bis(1-phenyl-1H-benzo[ d]imidazole)phenylphosphine oxide (BIPO) and poly(4-butylphenyl-diphenyl-amine) (Poly-TPD). The interlayer is designed to realize desirable white electroluminescence by tuning the electron and hole transportation and distribution in-between multilayers. With this PeLED configuration, we achieve the typical white light with chromaticity coordinates of (0.32, 0.32) in Commission Internationale de L'Eclairage (CIE) 1931 color space diagram and steady CIE coordinates in a wide range of driving current densities (from 2.94 to 59.29 mA/cm²). Consequently, our work, as the starting point for future research of all-perovskite white PeLEDs, will contribute to the future applications of PeLEDs in lighting and display. In addition, we believe that the proposed material and all-perovskite concept will leverage the design and development of more perovskite-based devices.

Controllable Synthesis of 2D Perovskite on Different Substrates and Its Application as Photodetector

Perovskites have recently attracted intense interests for optoelectronic devices application due to their excellent photovoltaic and photoelectric properties. The performance of perovskite-based devices highly depends on the perovskite material properties. However, the widely used spin-coating method can only prepare polycrystalline perovskite and physical vapor deposition (PVD) method requires a higher melting point (>350 °C) substrate due to the high growth temperature, which is not suitable for low melting point substrates, especially for flexible substrates. Here, we present the controlled synthesis of high quality two-dimensional (2D) perovskite platelets on random substrates, including SiO₂/Si, Si, mica, glass and flexible polydimethylsiloxane (PDMS) substrates, and our method is applicable to any substrate as long as its melting point is higher than 100 °C. We found that the photoluminescence (PL) characteristics of perovskite depend strongly on the platelets thickness, namely, thicker perovskite platelet has higher PL wavelength and stronger intensity, and thinner perovskite exhibits opposite results. Moreover, photodetectors based on the as-produced perovskite platelets show excellent photoelectric performance with a high photoresponsivity of 8.3 A·W⁻¹, a high on/off ratio of ~10³, and a small rise and decay time of 30 and 50 ms, respectively. Our approach in this work provides a feasible way for making 2D perovskite platelets for wide optoelectronic applications.

High Color Rendering Index White-Light Emission from UV-Driven LEDs Based on Single Luminescent Materials: Two-Dimensional Perovskites (C6H5C2H4NH3)2PbBr xCl4- x

Two-dimensional (2D) white-light-emitting hybrid perovskites (WHPs) are promising active materials for single-component white-light-emitting diodes (WLEDs) driven by UV. However, the reported WHPs exhibit low quantum yields (≤9%) and low color rendering index (CRI) values less than 85, which does not satisfy the demand of solid-state lighting applications. In this work, we report a series of mixed-halide 2D layered WHPs (C₆H₅C₂H₄NH₃)₂PbBr _xCl_{4- x} (0 < x < 4) obtained from the phenethylammonium cation. Unlike the reported WHPs including (C₆H₅C₂H₄NH₃)₂PbCl₄, the mixed-halide perovskites display morphology-dependent white emission for the different extents of self-absorption. Additionally, the amount of Br has a huge influence on the photophysical properties of mixed-halide WHPs. With the increasing content of Br, the quantum yields of WHPs increase gradually from 0.2 to 16.9%, accompanied by tunable color temperatures ranging from 4000 K ("warm" white light) to 7000 K ("cold" white light). When applied to the WLEDs, the mixed-halide perovskite powders exhibit tunable white electroluminescent emission with very high CRI of 87-91.

Two-Dimensional CH3NH3PbI3 Perovskite Nanosheets for Ultrafast Pulsed Fiber Lasers

Even though the nonlinear optical effects of solution processed organic-inorganic perovskite films have been studied, the nonlinear optical properties in two-dimensional (2D) perovskites, especially their applications for ultrafast photonics, are largely unexplored. In comparison to bulk perovskite films, 2D perovskite nanosheets with small thicknesses of a few unit cells are more suitable for investigating the intrinsic nonlinear optical properties because bulk recombination of photocarriers and the nonlinear scattering are relatively small. In this research, we systematically investigated the nonlinear optical properties of 2D perovskite nanosheets derived from a combined solution process and vapor phase conversion method. It was found that 2D perovskite nanosheets have stronger saturable absorption properties with large modulation depth and very low saturation intensity compared with those of bulk perovskite films. Using an all dry transfer method, we constructed a new type of saturable absorber device based on single piece 2D perovskite nanosheet. Stable soliton state mode-locking was achieved, and ultrafast picosecond pulses were generated at 1064 nm. This work is likely to pave the way for ultrafast photonic and optoelectronic applications based on 2D perovskites.