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

High-Throughput Computational Study and Machine Learning Prediction of Electronic Properties in Transition Metal Dichalcogenide/Two-Dimensional Layered Halide Perovskite Heterostructures

Heterostructures formed by transition metal dichalcogenides (TMDs) and two-dimensional layered halide perovskites (2D-LHPs) have attracted significant attention due to their unique optoelectronic properties. However, theoretical studies face challenges due to the large number of atoms and the need for lattice matching. With the discovery of more 2D-LHPs, there is an urgent need for methods to rapidly predict and screen TMDs/2D-LHPs heterostructures. This study employs first-principles calculations to perform high-throughput computations on 602 TMDs/2D-LHPs heterostructures. Results show that different combinations exhibit diverse band alignments, with MoS2 and WS2 more likely to form type-II heterostructures with 2D-LHPs. The highest photoelectric conversion efficiency of type-II structures reaches 23.26%, demonstrating potential applications in solar cells. Notably, some MoS2/2D-LHPs form type-S structures, showing promise in photocatalysis. Furthermore, we found that TMDs can significantly affect the conformation of organic molecules in 2D-LHPs, thus modulating the electronic properties of the heterostructures. To overcome computational cost limitations, we constructed a crystal graph convolutional neural network model based on the calculated data to predict the electronic properties of TMDs/2D-LHPs heterostructures. Using this model, we predicted the bandgaps and band alignment types of 9,360 TMDs/2D-LHPs heterostructures, providing a comprehensive theoretical reference for research in this field.

Ruddlesden-Popper 2D Perovskite-MoS2 Hybrid Heterojunction Photocathodes for Efficient and Scalable Photo-Rechargeable Li-Ion Batteries

Photo-rechargeable batteries (PRBs) can provide a compact solution to power autonomous smart devices located at remote sites that cannot be connected with the grid. The study reports the Ruddlesden-Popper (RP) metal halide perovskite (MHP) and molybdenum disulfide (MoS2) hybrid heterojunction-based photocathodes for Li-ion photo-rechargeable battery (Li-PRB) applications. Hybrid Lithium-ion batteries (LIBs) have demonstrated an average discharge specific capacity of 144.46 and 129.17 mAhg-1 for 50 cycles when operating at 176 and 294 mAg-1, respectively compared to the pristine LIBs which have shown specific capacity of 37.48 and 25.60 mAhg-1 under similar conditions. Hybrid Li-PRB has achieved an average dark discharge specific capacities of 128.66 mAhg-1 (capacity retention: 96.56%) which enhanced to 180.67 mAhg-1 under illumination (capacity retention: 97.39%; photo-enhancement: 40.42%) at 64 mAg-1. Excellent performance of hybrid Li-PRB is attributed to the formation of type-II heterojunction that leads to improved crystallinity and film morphology. The PRB has demonstrated a high photo conversion and storage efficiency (PC-SE) of 0.52% under standard 1 Sun illumination, which outperforms other previously reported MHP based LIBs and PRBs. This work provides a novel approach of harnessing the potential of MHPs for PRBs and offers new avenues for MHP photocathodes for various applications beyond PRBs.

High-Performance Broadband Self-Driven Photodetector Based on MoS2/Cs2CuBr4 Heterojunction

Few-layer transition metal dichalcogenides and perovskites are both promising materials in high-performance optoelectronic devices. Here, we developed a self-driven photodetector by creating a heterojunction between few-layer MoS2 and lead-free perovskite Cs2CuBr4. The detector shows a unique property of very high sensitivity in a broad spectral range of 400 to 800 nm with response speed in a millisecond order. Current-voltage characteristics of the heterojunction device show rectifying behavior, in contrast to the ohmic behavior of the MoS2-based device. The rectifying behavior is attributed to the type II band alignment of the MoS2/Cs2CuBr4 heterojunction. The device shows a broadband (400 to 800 nm) photodetection with very high responsivity reaching up to 2.8 × 104 A/W and detectivity of 1.6 × 1011 Jones at a bias voltage of 3 V. The detector can also operate in self-bias mode with sufficient response. The photocurrent, photoresponsivity, detectivity, and external quantum efficiency of the device are found to be dependent on the illumination power density. The response time of the device is found to be ∼32 and ∼79 ms during the rise and fall of the photocurrent. The work proposes a MoS2/Cs2CuBr4 heterostructure to be a promising candidate for cost-effective, high-performance photodetector.

The origins of dual-peak emission and anomalous exciton decay in 2D Sn-based perovskites

Two-dimensional (2D) Sn-based perovskites exhibit significant potential in diverse optoelectronic applications, such as on-chip lasers and photodetectors. Yet, the underlying mechanism behind the frequently observed dual-peak emission in 2D Sn-based perovskites remains a subject of intense debate, and there is a lack of research on the carrier dynamics in these materials. In this study, we investigate these issues in a representative 2D Sn-based perovskite, namely, PEA2SnI4, through temperature-, excitation intensity-, angle-, and time-dependent photoluminescence studies. The results indicate that the high- and low-energy peaks originate from in-face and out-of-face dipole transitions, respectively. In addition, we observe an anomalous increase in the non-radiative recombination rate as temperature decreases. After ruling out enhanced electron-phonon coupling and Auger recombination as potential causes of the anomalous carrier dynamics, we propose that the significantly increased exciton binding energy (Eb) plays a decisive role. The increased Eb arises from enhanced electronic localization, a consequence of weakened lattice distortion at low temperatures, as confirmed by first-principles calculations and temperature-dependent x-ray diffraction measurements. These findings offer valuable insights into the electronic processes in the unique 2D Sn-based perovskites.

Recent advances in two-dimensional perovskite materials for light-emitting diodes

Light-emitting diodes (LEDs) are an indispensable part of our daily life. After being studied for a few decades, this field still has some room for improvement. In this regard, perovskite materials may take the leading role. In recent years, LEDs have become a most explored topic, owing to their various applications in photodetectors, solar cells, lasers, and so on. Noticeably, they exhibit significant characteristics in developing LEDs. The luminous efficiency of LEDs can be significantly enhanced by the combination of a poor illumination LED with low-dimensional perovskite. In 2014, the first perovskite-based LED was illuminated at room temperature. Furthermore, two-dimensional (2D) perovskites have enriched this field because of their optical and electronic properties and comparatively high stability in ambient conditions. Recent and relevant advancements in LEDs using low-dimensional perovskites including zero-dimensional to three-dimensional materials is reported. The major focus of this article is based on the 2D perovskites and their heterostructures (i.e., a combination of 2D perovskites with transition metal dichalcogenides, graphene, and hexagonal boron nitride). In comparison to 2D perovskites, heterostructures exhibit more potential for application in LEDs. State-of-the-art perovskite-based LEDs, current challenges, and prospects are also discussed.

Enhanced Photodetection Performance of an In Situ Core/Shell Perovskite-MoS2 Phototransistor

While two-dimensional transition metal dichalcogenides (TMDCs)-based photodetectors offer prospects for high integration density and flexibility, their thinness poses a challenge regarding low light absorption, impacting photodetection sensitivity. Although the integration of TMDCs with metal halide perovskite nanocrystals (PNCs) has been known to be promising for photodetection with a high absorption coefficient of PNCs, the low charge mobility of PNCs delays efficient photocarrier injection into TMDCs. In this study, we integrated MoS2 with in situ formed core/shell PNCs with short ligands that minimize surface defects and enhance photocarrier injection. The PNCs/MoS2 heterostructure efficiently separates electrons and holes by establishing type II band alignment and consequently inducing a photogating effect. The synergistic interplay between photoconductive and photogating effects yields a high responsivity of 2.2 × 106 A/W and a specific detectivity of 9.0 × 1011 Jones. Our findings offer a promising pathway for developing low-cost, high-performance phototransistors leveraging the advantages of two-dimensional (2D) materials.

Recent advances in 2D transition metal dichalcogenide-based photodetectors: a review

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a highly promising platform for the development of photodetectors (PDs) owing to their remarkable electronic and optoelectronic properties. Highly effective PDs can be obtained by making use of the exceptional properties of 2D materials, such as their high transparency, large charge carrier mobility, and tunable electronic structure. The photodetection mechanism in 2D TMD-based PDs is thoroughly discussed in this article, with special attention paid to the key characteristics that set them apart from PDs based on other integrated materials. This review examines how single TMDs, TMD-TMD heterostructures, TMD-graphene (Gr) hybrids, TMD-MXene composites, TMD-perovskite heterostructures, and TMD-quantum dot (QD) configurations show advanced photodetection. Additionally, a thorough analysis of the recent developments in 2D TMD-based PDs, highlighting their exceptional performance capabilities, including ultrafast photo response, ultrabroad detectivity, and ultrahigh photoresponsivity, attained through cutting-edge methods is provided. The article conclusion highlights the potential for ground-breaking discoveries in this fast developing field of research by outlining the challenges faced in the field of PDs today and providing an outlook on the prospects of 2D TMD-based PDs in the future.

Optical Enhancement of Indirect Bandgap 2D Transition Metal Dichalcogenides for Multi-Functional Optoelectronic Sensors

The unique electrical and optical properties of transition metal dichalcogenides (TMDs) make them attractive nanomaterials for optoelectronic applications, especially optical sensors. However, the optical characteristics of these materials are dependent on the number of layers. Monolayer TMDs have a direct bandgap that provides higher photoresponsivity compared to multilayer TMDs with an indirect bandgap. Nevertheless, multilayer TMDs are more appropriate for various photodetection applications due to their high carrier density, broad spectral response from UV to near-infrared, and ease of large-scale synthesis. Therefore, this review focuses on the modification of the optical properties of devices based on indirect bandgap TMDs and their emerging applications. Several successful developments in optical devices are examined, including band structure engineering, device structure optimization, and heterostructures. Furthermore, it introduces cutting-edge techniques and future directions for optoelectronic devices based on multilayer TMDs.

Spin Texture Sensitive Photodetection by Dion-Jacobson Tin Halide Perovskites

The organic spacer molecule is known to regulate the optoelectronic properties of two-dimensional (2D) perovskites. We show that the spacer layer thickness determines the nature of optical transitions, direct or indirect, by controlling the structural properties of the inorganic layer. The spin-orbit interactions lead to different electron spin orientations for the states associated with the conduction band minimum (CBM) and the valence band maximum (VBM). This leads to a direct as well as an indirect component of the transitions, despite them being direct in momentum space. The shorter chains have a larger direct component, leading to a better optoelectronic performance. The mixed halide Sn2+ Dion-Jacobson (DJ) perovskite with the shortest 4-C diammonium spacer outshines the photodetection parameters of those having longer (6-C and 8-C) spacers and the corresponding Ruddlesden-Popper (RP) phases. The DJ system with a 4-C spacer and equimolar Br/I embodies an unprecedentedly high responsivity of 78.1 A W-1 under 3 V potential bias at 485 nm wavelength, among the DJ perovskites. Without any potential bias, this phase manifests the self-powered photodetection parameters of 0.085 A W-1 and 9.9 × 1010 jones. The unusual role of electron spin texture in these high-performance photodetectors of the lead-free DJ perovskites provides an avenue to exploit the information coded in spins for semiconductor devices without any ferromagnetic supplement or magnetic field.

Mixing cage cations in 2D metal-halide ferroelectrics enhances the ferro-pyro-phototronic effect for self-driven photopyroelectric detection

The ferro-pyro-phototronic (FPP) effect, coupling photoexcited pyroelectricity and photovoltaics, paves an effective way to modulate charge-carrier behavior of optoelectronic devices. However, reports of promising FPP-active systems remain quite scarce due to a lack of knowledge on the coupling mechanism. Here, we have successfully enhanced the FPP effect in a series of ferroelectrics, BA2Cs1-xMAxPb2Br7 (BA = butylammonium, MA = methylammonium, 0 ≤ x ≤ 0.34), rationally assembled by mixing cage cations into 2D metal-halide perovskites. Strikingly, chemical alloying of Cs+/MA+ cations leads to the reduction of exciton binding energy, as verified by the x = 0.34 component; this facilitates exciton dissociation into free charge-carriers and boosts photo-activities. The crystal detector thus displays enhanced FPP current at zero bias, almost more than 10 times higher than that of the x = 0 prototype. As an innovative study on the FPP effect, this work affords new insight into the fundamental principle of ferroelectrics and creates a new strategy for self-driven photodetection.

2D/3D Perovskite Photodetectors with High Response Frequency and Improved Stability Based on Thiophene-2-ethylamine and Dual Additives

Organic-inorganic lead halide perovskite materials have received great attention in recent years. However, the poor stability of these materials severely limits the commercial application of perovskite devices. Here, we used thiophene-2-ethylammonium iodide (TEAI) material as the organic spacer NH₄SCN and NH₄Cl as the dual additives to realize high-stability two-dimensional (2D)/three-dimensional (3D) perovskite thin films for perovskite photodetectors. Then, we investigated different effects of the dual additives on the orientation and crystallinity of the perovskite films. At room temperature, the optimized 2D/3D perovskite photodetectors exhibit good performance with high external quantum efficiency (EQE) (72%), large responsivity (0.36 A/W), high detectivity (2.46 × 10¹² Jones at the bias of 0 V), high response frequency (1.7 MHz), and improved stability (retains 90% photocurrent after 2000 h storage in RT and 10% RH conditions). Based on these devices, a dual-channel optical transport system and a light-intensity adder are achieved. The results of this study indicate that, with a simple process, the TEAI and dual-additives based 2D/3D perovskite photodetectors have promising applications in light-intensity adder and optical communication systems.

Pinhole-free 2D Ruddlesden-Popper perovskite layer with close packed large crystalline grains, suitable for optoelectronic applications

Here, we achieved pinhole-free 2D Ruddlesden-Popper Perovskite (RPP) BA₂PbI₄ layers with close packed crystalline grains with dimension of about 30 × 30 µm², which have been demonstrated to be favorable for optoelectronic applications, such as fast response RPP-based metal/semiconductor/metal photodetectors. We explored affecting parameters in hot casting of BA₂PbI₄ layers, and proved that oxygen plasma treatment prior to hot casting plays a significant role to achieve high quality close packed polycrystalline RPP layers at lower hot cast temperatures. Moreover, we demonstrate that crystal growth of 2D BA₂PbI₄ can be dominantly controlled by the rate of solvent evaporation through substrate temperature or rotational speed, while molarity of the prepared RPP/DMF precursor is the dominant factor that determines the RPP layer thickness, and can affect the spectral response of the realized photodetector. Benefiting from the high light absorption and inherent chemical stability of 2D RPP layers, we achieved high responsivity and stability, and fast response photodetection from perovskite active layer. We achieved a fast photoresponse with rise and fall times of 189 µs and 300 µs, and the maximum responsivity of 119 mA/W and detectivity of 2.15 × 10⁸ Jones in response to illumination wavelength of 450 nm. The presented polycrystalline RPP-based photodetector benefits from a simple and low-cost fabrication process, suitable for large area production on glass substrate, a good stability and responsivity, and a promising fast photoresponse, even around that of exfoliated single crystal RPP-based counterparts. However, it is well known that exfoliation methods suffer from poor repeatability and scalability, which make them incompatible with mass production and large area applications.

Photosensitive Dielectric 2D Perovskite Based Photodetector for Dual Wavelength Demultiplexing

Stacked 2D perovskites provide more possibilities for next generation photodetector with more new features. Compared with its excellent optoelectronic properties, the good dielectric performance of metal halide perovskite rarely comes into notice. Here, a bifunctional perovskite based photovoltaic detector capable of two wavelength demultiplexing is demonstrated. In the Black Phosphorus/Perovskite/MoS₂ structured photodetector, the comprehensive utilization of the photosensitive and dielectric properties of 2D perovskite allows the device to work in different modes. The device shows normal continuous photoresponse under 405 nm, while it shows a transient spike response to visible light with longer wavelengths. The linear dynamic range, rise/decay time, and self-powered responsivity under 405 nm can reach 100, 38 µs/50 µs, and 17.7 mA W^-1 , respectively. It is demonstrated that the transient spike photocurrent with long wavelength exposure is related to the illumination intensity and can coexist with normal photoresponse. Two waveband-dependent signals can be identified and used to reflect more information simultaneously. This work provides a new strategy for multispectral detection and demultiplexing, which can be used to improve data transfer rates and encrypted communications. This work mode can inspire more multispectral photodetectors with different stacked 2D materials, especially to the optoelectronic application of the wide bandgap, high dielectric photosensitive materials.

Defect recombination suppression and carrier extraction improvement for efficient CsPbBr3/SnO2heterojunction photodetectors

Perovskite materials with excellent optical and electronic properties have huge potential in the research field of photodetectors. Constructing heterojunctions and promoting carrier transportation are significant for the development of perovskite-based optoelectronics devices with high performances. Herein, we demonstrated a CsPbBr₃/SnO₂heterojunction photodetector and improved the device performances through post-annealing treatment of SnO₂film. The results indicated that the electrical properties of SnO₂films will make an important impact on carrier extraction, especially for type-II heterojunction. As the electrons transfer layer in CsPbBr₃/SnO₂type-II heterojunction, defects related to oxygen vacancy should be the key factor to affect carrier concentration, induce carriers' limitation and recombination rate. Under proper annealing temperature for SnO₂layer, the recombination rate can decrease to 1.37 × 10²¹cm³s and the spectral responsivity will be highly increased. This work can enhance the understanding on the photoresponse of perovskite photodetectors, and will be helpful for the further optimization and design of optoelectronic devices based on the perovskite heterojunction.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

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.

Revealing Giant Exciton Fine-Structure Splitting in Two-Dimensional Perovskites Using van der Waals Passivation

Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials. Low crystal quality usually broadens the exciton line width, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach to reducing the effect of spectral diffusion by means of hBN capping on layered perovskites, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral line width with the population of charge fluctuation centers present in the organic spacer. van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence the amplitude of the overall spectral diffusion effect is reduced. Our work provides a low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission.

Insertion of metal cations into hybrid organometallic halide perovskite nanocrystals for enhanced stability: eco-friendly synthesis, lattice strain engineering, and defect chemistry studies

In this work, we developed a facile and environmentally friendly synthesis strategy for large-scale preparation of Cr-doped hybrid organometallic halide perovskite nanocrystals. In the experiment, methylammonium lead bromide, CH₃NH₃PbBr₃, was efficiently doped with Cr³⁺ cations by eco-friendly method at low temperatures to grow crystals via antisolvent-crystallization. The as-synthesized Cr³⁺ cation-doped perovskite nanocrystals displayed ∼45.45% decrease in the (100) phase intensity with an enhanced Bragg angle (2θ) of ∼15.01° compared to ∼14.92° of pristine perovskites while retaining their cubic (221/Pm-cm, ICSD no. 00-069-1350) crystalline phase of pristine perovskites. During synthesis, an eco-friendly solvent, ethanol, was utilized as an antisolvent to grow nanometer-sized rod-like crystals. However, Cr³⁺ cation-doped perovskite nanocrystals display a reduced crystallinity of ∼67% compared to pristine counterpart with ∼75% crystallinity with an improved contact angle of ∼72° against water in thin films. Besides, as-grown perovskite nanocrystals produced crystallite size of ∼48 nm and a full-width-at-half-maximum (FWHM) of ∼0.19° with an enhanced lattice-strain of ∼4.52 × 10^-4 with a dislocation-density of ∼4.24 × 10¹⁴ lines per m² compared to pristine perovskite nanocrystals, as extracted from the Williamson-Hall plots. The as-obtained stable perovskite materials might be promising light-harvesting candidates for optoelectronic applications in the future.

A high-performance polarization-sensitive and stable self-powered UV photodetector based on a dendritic crystal lead-free metal-halide CsCu2I3/GaN heterostructure

Polarization-sensitive photodetectors are the core of optics applications and have been successfully demonstrated in photodetectors based on the newly-emerging metal-halide perovskites. However, achieving high polarization sensitivity is still extremely challenging. In addition, most of the previously reported photodetectors were concentrated on 1D lead-halide perovskites and 2D asymmetric intrinsic structure materials, but suffered from being external bias driven, lead-toxicity, poor stability and complex processes, severely limiting their practical applications. Here, we demonstrate a high-performance polarization-sensitive and stable polarization-sensitive UV photodetector based on a dendritic crystal lead-free metal-halide CsCu₂I₃/GaN heterostructure. By combining the anisotropic morphology and asymmetric intrinsic structure of CsCu₂I₃ dendrites with the isotropic material GaN film, a high specific surface area and built-in electric field are achieved, exhibiting an ultra-high polarization selectivity up to 28.7 and 102.8 under self-driving mode and -3 V bias, respectively. To our knowledge, such a high polarization selectivity has exceeded those of all of the reported perovskite-based devices, and is comparable to, or even superior to, those of the conventional 2D heterostructure materials. Interestingly, the unsealed device shows outstanding stability, and can be stored for over 2 months, and effectively maintained the performance even after repeated heating (373K)-cooling (300K) for different periods of time in ambient air, indicating a remarkable temperature tolerance and desired compatibility for applications under harsh conditions. Such excellent performance and simple method strongly show that the CsCu₂I₃/GaN heterojunction photodetector has great potential in practical applications with high polarization-sensitivity. This work provides a new insight into designing novel high-performance polarization-sensitive optoelectronic devices.

Two-Dimensional Cs2AgBiBr6/WS2 Heterostructure-Based Photodetector with Boosted Detectivity via Interfacial Engineering

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers have been widely used for optoelectronic devices because of their ultrasensitivity to light detection acquired from their direct gap properties. However, the small cross-section of photon absorption in the atomically thin layer thickness significantly limits the generation of photocarriers, restricting their performance. Here, we integrate monolayer WS₂ with 2D perovskites Cs₂AgBiBr₆, which serve as the light absorption layer, to greatly enhance the photosensitivity of WS₂. The efficient charge transfer at the Cs₂AgBiBr₆/WS₂ heterojunction is evidenced by the shortened photoluminescence (PL) decay time of Cs₂AgBiBr₆. Scanning photocurrent microscopy of Cs₂AgBiBr₆/WS₂/graphene reveals that improved charge extraction from graphene leads to an enhanced photoresponse. The 2D Cs₂AgBiBr₆/WS₂/graphene vertical heterostructure photodetector exhibits a high detectivity (D*) of 1.5 × 10¹³ Jones with a fast response time of 52.3 μs/53.6 μs and an on/off ratio of 1.02 × 10⁴. It is worth noting that this 2D heterostructure photodetector can realize self-powered light detection behavior with an open-circuit voltage of ∼0.75 V. The results suggest that the 2D perovskites can effectively improve the TMDC layer-based photodetectors for low-power consumption photoelectrical applications.

One-pot synthesis of novel ligand-free tin(II)-based hybrid metal halide perovskite quantum dots with high anti-water stability for solution-processed UVC photodetectors

Recently, lead-based halide perovskites have gained extensive attention due to their outstanding optoelectronic properties. However, the toxicity of lead would seriously limit its future application. To address these issues, in this work novel ligand-free organic-inorganic hybrid metal halide TBASnCl₃ (C₁₆H₃₆NSnCl₃) quantum dots are synthesized by a one-pot method at room temperature, and they showed high anti-water stability and high potential applications for high-performance UVC photodetectors. Our experimental data showed that the responsivity of the lateral photodetectors Au/TBASnCl₃/Au, in which the active layer (i.e. TBASnCl₃) was synthesized by further introducing SnF₂ as a precursor besides SnCl₂, reached 7.3 mA W^-1 with a specific detectivity of 1.67 × 10¹¹ Jones under 0.36 mW cm^-2 254 nm illumination at -5 V, and it showed a long lifetime even in an environment with an air humidity of 60%. Therefore, it laid a solid foundation for further fabricating lead-free metal halide optoelectronic devices.

Perovskite-Type 2D Materials for High-Performance Photodetectors

Photodetectors are light sensors in widespread use in image sensing, optical communication, and consumer electronics. In current smart optoelectronic technology, conventional semiconductors have encountered a bottleneck caused by inflexibility and opacity. With the ever-increasing demands for versatile optoelectronic applications, perovskite-type 2D materials demonstrate great potential for advanced photodetectors inspired by molecularly thin 2D materials. Through the reduction of thickness to thin or molecularly thin levels, single-crystalline 2D perovskites can exhibit superior optoelectronic performance characteristics, such as tunable absorption property by chemical design, enhanced carrier separation by remarkable photosensing capability, and improved carrier extraction by versatile band engineering. More importantly, perovskite-type 2D materials exhibit great potential for large-scale monolithic integration to achieve all-in-one sensing-memory-computing optoelectronic devices. In this Perspective, recent progress in 2D perovskite-based photodetectors is presented in detail. The focus is on growth strategies for reducing thickness, thickness-dependent optical and electrical properties, device engineering, heterojunction fabrication, and device performance. Finally, the current challenges and future prospects in this field are presented.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Strong Edge Stress in Molecularly Thin Organic-Inorganic Hybrid Ruddlesden-Popper Perovskites and Modulations of Their Edge Electronic Properties

Organic-inorganic hybrid Ruddlesden-Popper perovskites (HRPPs) have gained much attention for optoelectronic applications due to their high moisture resistance, good processability under ambient conditions, and long functional lifetimes. Recent success in isolating molecularly thin hybrid perovskite nanosheets and their intriguing edge phenomena have raised the need for understanding the role of edges and the properties that dictate their fundamental behaviors. In this work, we perform a prototypical study on the edge effects in ultrathin hybrid perovskites by considering monolayer (BA)₂PbI₄ as a representative system. On the basis of first-principles simulations of nanoribbon models, we show that in addition to significant distortions of the octahedra network at the edges, strong edge stresses are also present in the material. Structural instabilities that arise from the edge stress could drive the relaxation process and dominate the morphological response of edges in practice. A clear downward shift of the bands at the narrower ribbons, as indicative of the edge effect, facilitates the separation of photoexcited carriers (electrons move toward the edge and holes move toward the interior part of the nanosheet). Moreover, the desorption energy of the organic molecule can also be much lower at the free edges, making it easier for functionalization and/or substitution events to take place. The findings reported in this work elucidate the underlying mechanisms responsible for edge states in HRPPs and will be important in guiding the rational design and development of high-performance layer-edge devices.

Two-Dimensional Hybrid Perovskite-Based van der Waals Heterostructures

Two-dimensional (2D) hybrid perovskites, as newly emerging materials, have become the center of attention in optoelectronic fields within a few years because of their unique optoelectronic properties, including tunable bandgap, long carrier diffusion length, and high absorption coefficient. 2D perovskite-based van der Waals heterostructures via integration of 2D perovskites with other layered materials provide new platforms for many optoelectronic devices with prominent performance, such as photodetectors, light-emitting diodes (LEDs), and phototransistors. In this Perspective, the unique properties of 2D perovskites will be first introduced to explore why this material is attractive and popular. Subsequently, various types of heterostructures based on 2D perovskites will be illustrated, including the heterostructure construction approaches as well as their optical and optoelectronic applications. Finally, potential research directions based on 2D perovskite heterostructures are also proposed.

2D Ruddlesden-Popper Perovskite with Ordered Phase Distribution for High-Performance Self-Powered Photodetectors

2D Ruddlesden-Popper perovskites exhibit great potential in optoelectronic devices for superior stability compared with their 3D counterparts. However, to achieve a high level of device performance, it is crucial but challenging to regulate the phase distribution of 2D perovskites to facilitate charge carrier transfer. Herein, using a solvent additive method (adding a small amount of dimethyl sulfoxide (DMSO) in N,N-dimethylformamide (DMF)) combined with a hot-casting process, the phase distribution of (PEA)₂ MA₃ Pb₄ I₁₃ (PEA⁺  = C₆ H₅ CH₂ CH₂ NH₃ ⁺ , MA⁺  = CH₃ NH₃ ⁺ ) perovskite can be well controlled and the Fermi level of perovskites along the film thickness direction can achieve gradient distribution. The increased built-in potential, oriented crystal, and improved crystal quality jointly contribute to the high photoresponse of devices in the entire response spectrum range. The optimum device exhibits a characteristic detection peak at 570 nm with large responsivity/detectivity (0.44 A W^-1 /3.38 × 10¹² Jones), ultrafast response speed with a rise/fall time of 20.8/20.6 µs, and improved stability. This work suggests the possibility of manipulating the ordered phase distribution of 2D perovskites toward high-performance and stable optoelectronic conversion devices.

Electronic and Optical Properties of van der Waals Heterostructures Based on Two-Dimensional Perovskite (PEA)2PbI4 and Black Phosphorus

Combining two-dimensional (2D) perovskites with other 2D materials to form a van der Waals (vdW) heterostructure has emerged as an intriguing way of designing electronic and optoelectronic devices. The structural, electronic, and optical properties of the 2D (PEA)2PbI4/black phosphorus (BP) [PEA:(C4H9NH3)+] vdW heterostructure have been investigated using first-principles calculations. We found that the (PEA)2PbI4/BP heterostructure shows a high stability at room temperature. It is demonstrated that the (PEA)2PbI4/BP heterostructure exhibits a type-I band arrangement with high carrier mobility. Moreover, the band gap and band offset of (PEA)2PbI4/BP can be effectively modulated by an external electric field, and a transition from semiconductor to metal is observed. The band edges of (PEA)2PbI4 and BP in the (PEA)2PbI4/BP heterostructure, which show significant changes with the external electric field, provide further support. Furthermore, the BP layers can enhance the light absorption of the (PEA)2PbI4/BP heterostructures. Our results indicate that the 2D perovskite and BP vdW heterostructures are competitive candidates for the application of low-dimensional photovoltaic and optoelectronic devices.

MoS2 and CdMoS4 nanostructure-based UV light photodetectors

We have developed MoS2 nanosheets and CdMoS4 hierarchical nanostructures based on a UV light photodetector. The surface morphologies of the as-prepared samples were investigated via field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The performance parameters for the present photodetectors are investigated under the illumination of UV light having a wavelength of ∼385 nm. Upon the illumination of UV light, the CdMoS4-based photodetector device showed a better response to UV light compared to the MoS2 device in terms of photoresponsivity, response time (∼72 s) and recovery time (∼94 s). Our results reveal that CdMoS4 hierarchical nanostructures are practical for enhancing the device performance.

Two-dimensional halide perovskites: synthesis, optoelectronic properties, stability, and applications

Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.

Preparation Engineering of Two-Dimensional Heterostructures via Bottom-Up Growth for Device Applications

Two-dimensional heterostructures with tremendous electronic and optoelectronic properties hold great promise for nanodevice integrations and applications owing to the wide tunable characteristics. Toward this end, developing construction strategies in allusion to large-scale production of high-quality heterostructures is critical. The mainstream preparation routes are representatively classified into two categories of top-down and bottom-up approaches. Nonetheless, the relatively low reproductivity and the limitation for lateral heterostructure formations of top-down methods at the present stage inherently impeded their further developments. To surmount these obstacles, assembling heterostructures via miscellaneous bottom-up preparation protocols has emerged as a potential solution, attributed to the controllability and clean interface. Three typical approaches of chemical/physical vapor deposition, solution synthesis, and growth under ultrahigh vacuum conditions have shown promise due to the possibilities for preparing heterostructures with predesigned structures, clean interfaces, and the like. Therefore, bottom-up preparation engineering of heterostructures in two dimensions for further device applications is of vital importance. Moreover, heterostructure integrations by these methods have experienced a period of flourishing development in the past few years. In this review, the classical bottom-up growth routes, characterization methods, and latest progress of diverse heterostructures and further device applications are overviewed. Finally, the challenges and opportunities are discussed.

Lead-Free Perovskite Photodetectors: Progress, Challenges, and Opportunities

State-of-the-art photodetectors which apply hybrid perovskite materials have emerged as powerful candidates for next-generation light sensing. Among them, lead-based ones are the most popular beyond doubt on account of their unique and superior optoelectronic properties. Nevertheless, trade-off toward commercialization exists between nontoxicity and high performance, with the poor stability of lead-based perovskites, indicating that it is indispensable to substitute lead with nontoxic element meanwhile bringing about a comparable figure of merit of photodetectors and relatively long-term stability. Herein, recent advances in lead-free perovskite photodetectors are reviewed, analyzing the principle while designing new materials and highlighting some remarkable progress, which are comparable, even superior, to lead-based photodetectors. Furthermore, their potential strategy in optical communication, image sensing, narrowband photodetection, etc., is examined and a perspective on developing new materials and photodetectors with superior properties for more practical applications is provided.

Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

The past decade has witnessed the great success achieved by metal halide perovskites (MHPs) in photovoltaic and related fields. However, challenges still remain in further improving their performance, as well as, settling the stability issue for future commercialization. Recently, MHP/2D material heterostructures that combining MHPs with the low-cost and solution-processable 2D materials have demonstrated unprecedented improvement in both performance and stability due to the distinctive features at hetero-interface. The diverse fabrication techniques of MHPs and 2D materials allow them to be assembled as heterostructures with different configurations in a variety of ways. Moreover, the large families of MHPs and 2D materials provide the opportunity for the rational design and modification on compositions and functionalities of MHP/2D materials heterostructures. Herein, a comprehensive review of MHP/2D material heterostructures from syntheses to applications is presented. First, various fabrication techniques for MHP/2D material heterostructures are introduced by classifying them into solid-state methods and solution-processed methods. Then the applications of MHP/2D heterostructures in various fields including photodetectors, solar cells, and photocatalysis are summarized in detail. Finally, current challenges for the development of MHP/2D material heterostructures are highlighted, and future opportunities for the advancements in this research field are also provided.

Low-Dimensional Metal Halide Perovskite Photodetectors

Metal halide perovskites (MHPs) have been a hot research topic due to their facile synthesis, excellent optical and optoelectronic properties, and record-breaking efficiency of corresponding optoelectronic devices. Nowadays, the development of miniaturized high-performance photodetectors (PDs) has been fueling the demand for novel photoactive materials, among which low-dimensional MHPs have attracted burgeoning research interest. In this report, the synthesis, properties, photodetection performance, and stability of low-dimensional MHPs, including 0D, 1D, 2D layered and nonlayered nanostructures, as well as their heterostructures are reviewed. Recent advances in the synthesis approaches of low-dimensional MHPs are summarized and the key concepts for understanding the optical and optoelectronic properties related to the PD applications of low-dimensional MHPs are introduced. More importantly, recent progress in novel PDs based on low-dimensional MHPs is presented, and strategies for improving the performance and stability of perovskite PDs are highlighted. By discussing recent advances, strategies, and existing challenges, this progress report provides perspectives on low-dimensional MHP-based PDs in the future.

Multistate Memory Enabled by Interface Engineering Based on Multilayer Tungsten Diselenide

The diversification of data types and the explosive increase of data size in the information era continuously required to miniaturize the memory devices with high data storage capability. Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for flexible and transparent electronic and optoelectronic devices with high integration density. Multistate memory devices based on TMDs could possess high data storage capability with a large integration density and thus exhibit great potential applications in the field of data storage. Here, we report the multistate data storage based on multilayer tungsten diselenide (WSe₂) transistors by interface engineering. The multiple resistance states of the WSe₂ transistors are achieved by applying different gate voltage pulses, and the switching ratio of the memory can be as large as 10⁵ with high cycling endurance. The water and oxygen molecules (H₂O/O₂) trapped at the interface between the SiO₂ substrate and WSe₂ introduce the trap states and thus the large hysteresis of the transfer curves, which leads to the multistate data storage. In addition, the laminated Au thin film electrodes make the contact interface between the electrodes and WSe₂ free of dangling bond and Fermi level pinning, thus giving rise to the excellent performance of memory devices. Importantly, the interface trap states can be easily controlled by a simple oxygen plasma treatment of the SiO₂ substrate, and subsequently, the performance of the multistate memory devices can be manipulated. Our findings provide a simple and efficient strategy to engineer the interface states for the multistate data storage applications and would motivate more investigations on the trap state-associated applications.

Design and Integration of a Layered MoS2/GaN van der Waals Heterostructure for Wide Spectral Detection and Enhanced Photoresponse

Molybdenum disulfide (MoS₂) as a typical two-dimensional (2D) transition-metal dichalcogenide exhibits great potential applications for the next-generation nanoelectronics such as photodetectors. However, most MoS₂-based photodetectors hold obvious disadvantages including a narrow spectral response in the visible region, poor photoresponsivity, and slow response speed. Here, for the first time, we report the design of a two-dimensional MoS₂/GaN van der Waals (vdWs) heterostructure photodetector consisting of few-layer p-type MoS₂ and very thin n-type GaN flakes. Thanks to the good crystal quality of the 2D-GaN flake and the built-in electric field in the interface depletion region of the MoS₂/GaN p-n junction, photogenerated carriers can be rapidly separated and more excitons are collected by electrodes toward the high photoresponsivity of 328 A/W and a fast response time of 400 ms under the illumination of 532 nm light, which is seven times faster than pristine MoS₂ flake. Additionally, the response spectrum of the photodetector is also broadened to the UV region with a high photoresponsivity of 27.1 A/W and a fast response time of 300 ms after integrating with the 2D-GaN flake, exhibiting an advantageous synergetic effect. These excellent performances render MoS₂/GaN vdWs heterostructure photodetectors as promising and competitive candidates for next-generation optoelectronic devices.

Self-trapped excitons in two-dimensional perovskites

With strong electron-phonon coupling, the self-trapped excitons are usually formed in materials, which leads to the local lattice distortion and localized excitons. The self-trapping strongly depends on the dimensionality of the materials. In the three-dimensional case, there is a potential barrier for self-trapping, whereas no such barrier is present for quasi-one-dimensional systems. Two-dimensional (2D) systems are marginal cases with a much lower potential barrier or nonexistent potential barrier for the self-trapping, leading to the easier formation of self-trapped states. Self-trapped excitons emission exhibits a broadband emission with a large Stokes shift below the bandgap. 2D perovskites are a class of layered structure material with unique optical properties and would find potential promising optoelectronic. In particular, self-trapped excitons are present in 2D perovskites and can significantly influence the optical and electrical properties of 2D perovskites due to the soft characteristic and strong electron-phonon interaction. Here, we summarized the luminescence characteristics, origins, and characterizations of self-trapped excitons in 2D perovskites and finally gave an introduction to their applications in optoelectronics.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Improved photoemission and stability of 2D organic-inorganic lead iodide perovskite films by polymer passivation

2D organic-inorganic lead iodide perovskites hold great promise for functional optoelectronic devices. However, their performances have been seriously limited by poor long-term stability in ambient environment. Here, we perform a systematic study for the stability improvement of a typical 2D organic-inorganic lead iodide perovskite (PEA)2PbI4. The degradation of the (PEA)2PbI4 films can be attributed to the interaction with the humidity in environment, which leads to decomposition of the perovskite components. Then, we demonstrate that polymer passivation provides an effective approach for improving the crystal quality and stability of the (PEA)2PbI4 films. Correspondingly, the photoemission of the polymer-passivated (PEA)2PbI4 films has been enhanced due to the decreased trap states. More importantly, a hydrophobic polymer (Poly(4-Vinylpyridine), PVP) will protect the (PEA)2PbI4 films from humidity in ambient environment, which can greatly improve the physical and chemical stability of the 2D perovskite films. As a result, the PVP-passivated (PEA)2PbI4 films can produce a bright emission even after long-term (>15 d) exposure to ambient environment (25 °C, 80% RH) and continuous UV illumination. This work provides a convenient and effective approach for improving the long-term stability of 2D organic-inorganic lead iodide perovskites, which shows great promise for fabricating large-area and versatile optoelectronic devices.

Large Optical Anisotropy in Two-Dimensional Perovskite [CH(NH2)2][C(NH2)3]PbI4 with Corrugated Inorganic Layers

Optical anisotropy plays an indispensable role in a variety of optical components. Organic halide perovskites often rely on artificially oriented nanostructures to enhance optical anisotropy due to their in-plane isotropic crystal structure, which results in unnecessary optical losses and fabrication difficulties. Here, we report the large optical anisotropy in two-dimensional perovskite [CH(NH₂)₂][C(NH₂)₃]PbI₄ crystals. Without specially designing their morphology, we achieved a large photoresponse linear dichroic ratio of 2 and a photoluminescence linear dichroic ratio of 4.7. Furthermore, we identified that the polarization orientation is parallel to the corrugated inorganic layers on every crystal plane by density functional theory calculations. The anisotropy of the ab-plane and ac-plane changes in opposite trend with temperature, suggesting that the perovskite can selectively generate polarized light or unpolarized light from different crystal planes by tuning the temperature. Our studies provide a new platform toward two-dimensional perovskite-based optical polarization devices.

The strain effects in 2D hybrid organic-inorganic perovskite microplates: bandgap, anisotropy and stability

Strain engineering provides an efficient strategy to modulate the fundamental properties of semiconducting structures for use in functional electronic and optoelectronic devices. Here, we report on how the strain affects the bandgap, optical anisotropy and stability of two-dimensional (2D) perovskite (BA)₂(MA)_n-1Pb_nI_3n+1 (n = 1-3) microplates, using photoluminescence spectroscopy. Upon applying external strain, the bandgap decreases at a rate of -5.60/-2.74/-1.38 meV per % for n = 1, 2, and 3 2D perovskites, respectively. This change of the bandgap can be ascribed to the distortion of the octahedra (Pb-I bond contraction) in 2D perovskites, supported by a study on emission anisotropy, which increases with the increase of strain. In addition, the external strain can significantly deteriorate the stability of 2D perovskites due to the strain induced distortion which would make the penetration of moisture and oxygen into the perovskite microplates easier, resulting in much faster degradation rates. Our findings not only provide insights into the design and optimization of functional devices, but also provide a new approach to improve the stability of 2D perovskite based devices.

Lateral Photodetectors Based on Double-Cable Polymer/Two-Dimensional Perovskite Heterojunction

Double-cable conjugated polymers and two-dimensional (2D) perovskites are both promising materials for next-generation photodetectors (PDs) due to their solution processibility and tunable optoelectronic properties. In this work, a lateral PD is designed by layering a double-cable conjugated polymer film atop a 2D Ruddlesden-Popper perovskite film. Compared to the corresponding single-layer polymer and perovskite PDs, the heterojunction device exhibits greatly improved performance with a high responsivity of 27.06 A W^-1, an on/off ratio of 1379, and a short rise/decay time of 3.53/3.78 ms. In addition, a flexible device using polyimide as the substrate is successfully fabricated and exhibits comparable performance with the device on glass. This work demonstrates the great potential of double-cable polymer/2D perovskite heterojunctions in future flexible optoelectronics.

Hole Localization Inhibits Charge Recombination in Tin-Lead Mixed Perovskites: Time-Domain ab Initio Analysis

Using time domain density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that the Sn dopants favor forming localized hole states with different extent at low and high doping concentrations, mimicking the small and large polarons, while retain the electron wave functions comparable with the pristine system, leading to nonadiabatic coupling decreasing by a factor of 45% and 38% and bandgap reduction by 0.04 and 0.27 eV, respectively. Furthermore, replacing heavier Pb with lighter Sn increases atomic fluctuations and accelerates loss of quantum coherence, in particular even faster at higher Sn doping concentration. As a result, the interplay among the bandgap, NA coupling, and decoherence time delays the electron-hole recombination by a factor of 3.5 and 1.3 at low and high doping concentration. Our study reveals the atomistic mechanisms of suppressed recombination dependence on Sn doping concentration, providing a new way to design high performance mixed perovskites.

Ultrathin Ruddlesden-Popper Perovskite Heterojunction for Sensitive Photodetection

Thanks to their unique optical and electric properties, 2D materials have attracted a lot of interest for optoelectronic applications. Here, the emerging 2D materials, organic-inorganic hybrid perovskites with van der Waals interlayer interaction (Ruddlesden-Popper perovskites), are synthesized and characterized. Photodetectors based on the few-layer Ruddlesden-Popper perovskite show good photoresponsivity as well as good detectivity. In order to further improve the photoresponse performance, 2D MoS₂ is chosen to construct the perovskite-MoS₂ heterojunction. The performance of the hybrid photodetector is largely improved with 6 and 2 orders of magnitude enhancement for photoresponsivity (10⁴ A W^-1 ) and detectivity (4 × 10¹⁰ Jones), respectively, which demonstrates the facile charge separation at the interface between perovskite and MoS₂ . Furthermore, the contribution of back gate tuning is proved with a greatly reduced dark current. The results demonstrated here will open up a new field for the investigation of 2D perovskites for optoelectronic applications.

Completely Solvent-free Protocols to Access Phase-Pure, Metastable Metal Halide Perovskites and Functional Photodetectors from the Precursor Salts

Mechanochemistry is a green, solid-state, re-emerging synthetic technique that can rapidly form complex molecules and materials without exogenous heat or solvent(s). Herein, we report the application of solvent-free mechanochemical ball milling for the synthesis of metal halide perovskites, to overcome problems with solution-based syntheses. We prepared phase-pure, air-sensitive CsSnX3 (X = I, Br, Cl) and its mixed halide perovskites by mechanochemistry for the first time by reactions between cesium and tin(II) halides. Notably, we report the sole examples where metastable, high-temperature phases like cubic CsSnCl3, cubic CsPbI3, and trigonal FAPbI3 were accessible at ambient temperatures and pressures without post-synthetic processing. The perovskites can be prepared up to "kilogram scales." Lead-free, all-inorganic photodetector devices were fabricated using the mechanosynthesized CsSnBr1.5Cl1.5 under solvent-free conditions and showed 10-fold differences between on-off currents. We highlight an essentially solvent-free, general approach to synthesize metastable compounds and fabricate photodetectors from commercially available precursors.