Halide Perovskite Nanocrystals for Next‐Generation Optoelectronics

Colloidal perovskite nanocrystals (PNCs) combine the outstanding optoelectronic properties of bulk perovskites with strong quantum confinement effects at the nanoscale. Their facile and low‐cost synthesis, together with superior photoluminescence quantum yields and exceptional optical versatility, make PNCs promising candidates for next‐generation optoelectronics. However, this field is still in its early infancy and not yet ready for commercialization due to several open challenges to be addressed, such as toxicity and stability. Here, the key synthesis strategies and the tunable optical properties of PNCs are discussed. The photophysical underpinnings of PNCs, in correlation with recent developments of PNC‐based optoelectronic devices, are especially highlighted. The final goal is to outline a theoretical scaffold for the design of high‐performance devices that can at the same time address the commercialization challenges of PNC‐based technology.     

Localized Surface Plasmon Resonance Enhanced Light Absorption in AuCu/CsPbCl3 Core/Shell Nanocrystals

Localized surface plasmon resonance (LSPR) is shown to be effective in trapping light for enhanced light absorption and hence performance in photonic and optoelectronic devices. Implementation of LSPR in all-inorganic perovskite nanocrystals (PNCs) is particularly important considering their unique advantages in optoelectronics. Motivated by this, the first success in colloidal synthesis of AuCu/CsPbCl₃ core/shell PNCs and observation of enhanced light absorption by the perovskite CsPbCl₃ shell of thickness in the range of 2–4 nm, enabled by the LSPR AuCu core of an average diameter of 7.1 nm, is reported. This enhanced light absorption leads to a remarkably enhanced photoresponse in PNCs/graphene nanohybrid photodetectors using the AuCu/CsPbCl₃ core/shell PNCs, by more than 30 times as compared to the counterparts with CsPbCl₃ PNCs only (8–12 nm in dimension). This result illustrates the feasibility in implementation of LSPR light trapping directly in core/shell PNCs for high-performance optoelectronics.     

High‐Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light‐emitting devices, and photodiodes. In this work, high‐performance photovoltaic photodetectors based on MoTe₂/MoS₂ vertical heterojunctions are demonstrated by exfoliating‐restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>10⁵) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W^?1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications.     

Perovskite Materials for Light‐Emitting Diodes and Lasers

Organic–inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light‐emitting diodes, lasers, and light‐emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light‐emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A^?1 have been achieved. Although perovskite light‐emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light‐emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower‐dimensionality layered and three‐dimensional perovskites, nanostructures, charge‐transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light‐emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light‐emitting devices.     

Wrinkled 2D Materials: A Versatile Platform for Low‐Threshold Stretchable Random Lasers

A stretchable, flexible, and bendable random laser system capable of lasing in a wide range of spectrum will have many potential applications in next‐ generation technologies, such as visible‐spectrum communication, superbright solid‐state lighting, biomedical studies, fluorescence, etc. However, producing an appropriate cavity for such a wide spectral range remains a challenge owing to the rigidity of the resonator for the generation of coherent loops. 2D materials with wrinkled structures exhibit superior advantages of high stretchability and a suitable matrix for photon trapping in between the hill and valley geometries compared to their flat counterparts. Here, the intriguing functionalities of wrinkled reduced graphene oxide, single‐layer graphene, and few‐layer hexagonal boron nitride, respectively, are utilized to design highly stretchable and wearable random laser devices with ultralow threshold. Using methyl‐ammonium lead bromide perovskite nanocrystals (PNC) to illustrate the working principle, the lasing threshold is found to be ≈10 µJ cm^?2, about two times less than the lowest value ever reported. In addition to PNC, it is demonstrated that the output lasing wavelength can be tuned using different active materials such as semiconductor quantum dots. Thus, this study is very useful for the future development of high‐performance wearable optoelectronic devices.     

Improving the Quality and Luminescence Performance of All‐Inorganic Perovskite Nanomaterials for Light‐Emitting Devices by Surface Engineering

Lead halide perovskites and their applications in the optoelectronic field have garnered intensive interest over the years. Inorganic perovskites (IHP), though a novel class of material, are considered as one of the most promising optoelectronic materials. These materials are widely used in detectors, solar cells, and other devices, owing to their excellent charge‐transport properties, high defect tolerance, composition‐ and size‐dependent luminescence, narrow emission, and high photoluminescence quantum yield. In recent years, numerous encouraging achievements have been realized, especially in the research of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) and surface engineering. Therefore, it is necessary to summarize the principles and effects of these surface engineering optimization methods. It is also important to scientifically guide the applications and promote the development of perovskites more efficiently. Herein, the principles of surface ligands are reviewed, and various surface treatment methods used in CsPbX₃ NCs as well as quantum‐dot light‐emitting diodes are presented. Finally, a brief outlook on CsPbX₃ NC surface engineering is offered, illustrating the present challenges and the direction in which future investigations are intended to obtain high‐quality CsPbX₃ NCs that can be utilized in more applications.     

Size‐Controlled Soft‐Template Synthesis of Carbon Nanodots toward Versatile Photoactive Materials

Size‐controlled soft‐template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue‐shifted photoluminescence (PL) or so‐called an inverse PL shift. Using time‐correlated single photon counting, ultraviolet photoelectron spectroscopy, and low‐temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND‐based optoelectronic devices, including multicolored light‐emitting diodes and air‐stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.     

Gap‐Mode Surface‐Plasmon‐Enhanced Photoluminescence and Photoresponse of MoS2

2D materials hold great potential for designing novel electronic and optoelectronic devices. However, 2D material can only absorb limited incident light. As a representative 2D semiconductor, monolayer MoS₂ can only absorb up to 10% of the incident light in the visible, which is not sufficient to achieve a high optical‐to‐electrical conversion efficiency. To overcome this shortcoming, a “gap‐mode” plasmon‐enhanced monolayer MoS₂ fluorescent emitter and photodetector is designed by squeezing the light‐field into Ag shell‐isolated nanoparticles–Au film gap, where the confined electromagnetic field can interact with monolayer MoS₂. With this gap‐mode plasmon‐enhanced configuration, a 110‐fold enhancement of photoluminescence intensity is achieved, exceeding values reached by other plasmon‐enhanced MoS₂ fluorescent emitters. In addition, a gap‐mode plasmon‐enhanced monolayer MoS₂ photodetector with an 880% enhancement in photocurrent and a responsivity of 287.5 A W^?1 is demonstrated, exceeding previously reported plasmon‐enhanced monolayer MoS₂ photodetectors.     

Broadband MoS2 Field‐Effect Phototransistors: Ultrasensitive Visible‐Light Photoresponse and Negative Infrared Photoresponse

Inverse photoresponse is discovered from phototransistors based on molybdenum disulfide (MoS₂). The devices are capable of detecting photons with energy below the bandgap of MoS₂. Under the illumination of near‐infrared (NIR) light at 980 and 1550 nm, negative photoresponses with short response time (50 ms) are observed for the first time. Upon visible‐light illumination, the phototransistors exhibit positive photoresponse with ultrahigh responsivity on the order of 10⁴–10⁵ A W^?1 owing to the photogating effect and charge trapping mechanism. Besides, the phototransistors can detect a weak visible‐light signal with effective optical power as low as 17 picowatts (pW). A thermally induced photoresponse mechanism, the bolometric effect, is proposed as the cause of the negative photocurrent in the NIR regime. The thermal energy of the NIR radiation is transferred to the MoS₂ crystal lattice, inducing lattice heating and resistance increase. This model is experimentally confirmed by low‐temperature electrical measurements. The bolometric coefficient calculated from the measured transport current change with temperature is ?33 nA K^?1. These findings offer a new approach to develop sub‐bandgap photodetectors and other novel optoelectronic devices based on 2D layered materials.     

In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High‐Speed Visible–Near‐Infrared Photodetectors

c2D transition metal dichalcogenides (TMDCs)‐based heterostructures have been demonstrated to achieve superior light absorption and photovoltaic effects theoretically and experimentally, making them extremely attractive for realizing optoelectronic devices. In this work, a vertical multilayered n‐MoS₂/n‐silicon homotype heterojunction is fabricated, which takes advantage of multilayered MoS₂ grown in situ directly on plane silicon. Electrical characterization reveals that the resultant device exhibits high sensitivity to visible–near‐infrared light with responsivity up to 11.9 A W^–1. Notably, the photodetector shows high‐speed response time of ≈30.5 µs/71.6 µs and capability to work under higher pulsed light irradiation approaching 100 kHz. The high response speed could be attributed to a good quality of the multilayer MoS₂, as well as in situ device fabrication process. These findings suggest that the multilayered MoS₂/Si homotype heterojunction have great potential application in the field of visible–near‐infrared detection and might be used as elements for construction of high‐speed integrated optoelectronic sensor circuitry.     

Elucidation of Photoluminescence Blinking Mechanism and Multiexciton Dynamics in Hybrid Organic–Inorganic Perovskite Quantum Dots

Halide perovskites (ABX₃) have emerged as promising materials in the past decade owing to their superior photophysical properties, rendering them potential candidates as solar cells, light‐emitting diode displays, and lasing materials. To optimize their utilization into optoelectronic devices, fundamental understanding of the optical behaviors is necessary. To reveal the comprehensive structure–property relationship, CH₃NH₃PbBr₃ (MAPbBr₃) perovskite quantum dots (PQDs) of three different sizes are prepared by controlling the precipitation temperature. Photoluminescence (PL) blinking, a key process that governs the emission efficiency of the PQD materials, is investigated in detail by the time‐resolved spectroscopic measurements of individual dots. The nature of the generated species in the course of blinking events is identified, and the mechanism governing the PL blinking is studied as a function of PQD sizes. Further, the practical applicability of MAPbBr₃ PQDs is assessed by studying the multiexciton dynamics under high photoexcitation intensity under which most of the display devices work. Ultrafast transient absorption spectroscopy helped in uncovering the volume‐dependent Auger recombination rates, which are further explored by comparing the early‐time transitions related to surface trap states and higher band states.     

A Highly Sensitive Single Crystal Perovskite–Graphene Hybrid Vertical Photodetector

Organolead trihalide perovskites have attracted significant attention for optoelectronic applications due to their excellent physical properties in the past decade. Generally, both grain boundaries in perovskite films and the device structure play key roles in determining the device performance, especially for horizontal‐structured device. Here, the first optimized vertical‐structured photodetector with the perovskite single crystal MAPbBr₃ as the light absorber and graphene as the transport layer is shown. The hybrid device combines strong photoabsorption characteristics of perovskite and high carrier mobility of flexible graphene, exhibits excellent photoresponse performance with high photoresponsivity (≈1017.1 A W^?1) and high photodetectivity (≈2.02 × 10¹³ Jones) in a low light intensity (0.66 mW cm^?2) under the actuations of 3 V bias and laser irradiation at 532 nm. In particular, an ultrahigh photoconductive gain of ≈2.37 × 10³ is attained because of fast charge transfer in the graphene and large recombination lifetime in the perovskite single crystal. The vertical architecture combining perovskite crystal with highly conductive graphene offers opportunities to fulfill the synergistic effect of perovskite and 2D materials, is thus promising for developing high‐performance electronic and optoelectronic devices.     

Dual‐Phase CsPbBr3–CsPb2Br5 Perovskite Thin Films via Vapor Deposition for High‐Performance Rigid and Flexible Photodetectors

Inorganic perovskites with special semiconducting properties and structures have attracted great attention and are regarded as next generation candidates for optoelectronic devices. Herein, using a physical vapor deposition process with a controlled excess of PbBr₂, dual‐phase all‐inorganic perovskite composite CsPbBr₃–CsPb₂Br₅ thin films are prepared as light‐harvesting layers and incorporated in a photodetector (PD). The PD has a high responsivity and detectivity of 0.375 A W^?1 and 10¹¹ Jones, respectively, and a fast response time (from 10% to 90% of the maximum photocurrent) of ≈280 µs/640 µs. The device also shows an excellent stability in air for more than 65 d without encapsulation. Tetragonal CsPb₂Br₅ provides satisfactory passivation to reduce the recombination of the charge carriers, and with its lower free energy, it enhances the stability of the inorganic perovskite devices. Remarkably, the same inorganic perovskite photodetector is also highly flexible and exhibits an exceptional bending performance (>1000 cycles). These results highlight the great potential of dual‐phase inorganic perovskite films in the development of optoelectronic devices, especially for flexible device applications.     

Iodine Vacancy Redistribution in Organic–Inorganic Halide Perovskite Films and Resistive Switching Effects

Organic–inorganic halide perovskite (OHP) materials, for example, CH₃NH₃PbI₃ (MAPbI₃), have attracted significant interest for applications such as solar cells, photodectors, light‐emitting diodes, and lasers. Previous studies have shown that charged defects can migrate in perovskites under an electric field and/or light illumination, potentially preventing these devices from practical applications. Understanding and control of the defect generation and movement will not only lead to more stable devices but also new device concepts. Here, it is shown that the formation/annihilation of iodine vacancies (V_I's) in MAPbI₃ films, driven by electric fields and light illumination, can induce pronounced resistive switching effects. Due to a low diffusion energy barrier (≈0.17 eV), the V_I's can readily drift under an electric field, and spontaneously diffuse with a concentration gradient. It is shown that the V_I diffusion process can be suppressed by controlling the affinity of the contact electrode material to I^? ions, or by light illumination. An electrical‐write and optical‐erase memory element is further demonstrated by coupling ion migration with electric fields and light illumination. These results provide guidance toward improved stability and performance of perovskite‐based optoelectronic systems, and can lead to the development of solid‐state devices that couple ionics, electronics, and optics.     

Flexible Visible‐Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films

Two new flexible visible‐light photodetectors based on ZrS₃ nanobelts films are fabricated on a polypropylene (PP) film and printing paper, respectively, by an adhesive‐tape transfer method, and their light‐induced electric properties are investigated in detail. The devices demonstrate a remarkable response to 405 to 780 nm light, a photocurrent that depends on the optical power and light wavelength, and an excellent photoswitching effect and stability. This implies that ZrS₃ nanobelts are prospective candidates for high‐performance nanoscale optoelectronic devices that may be practically applied in photodetection of visible to near infrared light. The facile fabrication method is extendable to flexible nanodevices with different nanostructures.     

Vacuum‐Drying Processed Micrometer‐Thick Stable CsPbBr3 Perovskite Films with Efficient Blue‐To‐Green Photoconversion

Metal halide perovskite materials have attracted great attention owing to their fascinating optoelectronic characteristics and low cost fabrication via facile solution processing. One of the potential applications of these materials is to employ them as color‐conversion layers (CCLs) for visible blue light to achieve full‐color displays. However, obtaining thick perovskite films to realize complete color conversion is a key challenge. Here, the fabrication of micrometer‐level thick CsPbBr₃ perovskite films is presented through a facile vacuum drying approach. An efficient green photoconversion is realized in a 3.8 µm thick film from blue light @ 463 nm. For a back luminance of 1000 cd m^?2, the brightness of the resulting green emission can reach as high as 200 cd m^?2. Furthermore, only ≈2% of decay in brightness is observed when the films are tested after 18 days of exposure to ambient environment. In addition, a potential design is also proposed for full‐color displays with perovskite materials incorporated as CCLs.     

A 2D Titanium Carbide MXene Flexible Electrode for High-Efficiency Light-Emitting Diodes

Although several transparent conducting materials such as carbon nanotubes, graphene, and conducting polymers have been intensively explored as flexible electrodes in optoelectronic devices, their insufficient electrical conductivity, low work function, and complicated electrode fabrication processes have limited their practical use. Herein, a 2D titanium carbide (Ti₃C₂) MXene film with transparent conducting electrode (TCE) properties, including high electrical conductivity (≈11 670 S cm⁻¹) and high work function (≈5.1 eV), which are achieved by combining a simple solution processing with modulation of surface composition, is described. A chemical neutralization strategy of a conducting-polymer hole-injection layer is used to prevent detrimental surface oxidation and resulting degradation of the electrode film. Use of the MXene electrode in an organic light-emitting diode leads to a current efficiency of ≈102.0 cd A⁻¹ and an external quantum efficiency of ≈28.5% ph/el, which agree well with the theoretical maximum values from optical simulations. The results demonstrate the strong potential of MXene as a solution-processable electrode in optoelectronic devices and provide a guideline for use of MXenes as TCEs in low-cost flexible optoelectronic devices.     

Near‐Infrared Photodetectors Based on MoTe2/Graphene Heterostructure with High Responsivity and Flexibility

2D transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their impressively high performance in optoelectronic devices. However, efficient infrared (IR) photodetection has been significantly hampered because the absorption wavelength range of most TMDCs lies in the visible spectrum. In this regard, semiconducting 2D MoTe₂ can be an alternative choice owing to its smaller band gap ≈1 eV from bulk to monolayer and high carrier mobility. Here, a MoTe₂/graphene heterostructure photodetector is demonstrated for efficient near‐infrared (NIR) light detection. The devices achieve a high responsivity of ≈970.82 A W^?1 (at 1064 nm) and broadband photodetection (visible‐1064 nm). Because of the effective photogating effect induced by electrons trapped in the localized states of MoTe₂, the devices demonstrate an extremely high photoconductive gain of 4.69 × 10⁸ and detectivity of 1.55 × 10¹¹ cm Hz^1/2 W^?1. Moreover, flexible devices based on the MoTe₂/graphene heterostructure on flexible substrate also retains a good photodetection ability after thousands of times bending test (1.2% tensile strain), with a high responsivity of ≈60 A W^?1 at 1064 nm at V _DS = 1 V, which provides a promising platform for highly efficient, flexible, and low cost broadband NIR photodetectors.     

Electron Mobility of 24 cm2 V−1 s−1 in PbSe Colloidal‐Quantum‐Dot Superlattices

Colloidal quantum dots (CQDs) are nanoscale building blocks for bottom‐up fabrication of semiconducting solids with tailorable properties beyond the possibilities of bulk materials. Achieving ordered, macroscopic crystal‐like assemblies has been in the focus of researchers for years, since it would allow exploitation of the quantum‐confinement‐based electronic properties with tunable dimensionality. Lead‐chalcogenide CQDs show especially strong tendencies to self‐organize into 2D superlattices with micrometer‐scale order, making the array fabrication fairly simple. However, most studies concentrate on the fundamentals of the assembly process, and none have investigated the electronic properties and their dependence on the nanoscale structure induced by different ligands. Here, it is discussed how different chemical treatments on the initial superlattices affect the nanostructure, the optical, and the electronic‐transport properties. Transistors with average two‐terminal electron mobilities of 13 cm² V^?1 s^?1 and contactless mobility of 24 cm² V^?1 s^?1 are obtained for small‐area superlattice field‐effect transistors. Such mobility values are the highest reported for CQD devices wherein the quantum confinement is substantially present and are comparable to those reported for heavy sintering. The considerable mobility with the simultaneous preservation of the optical bandgap displays the vast potential of colloidal QD superlattices for optoelectronic applications.     

Highly Oriented Thin Films of 2D Ruddlesden‐Popper Hybrid Perovskite toward Superfast Response Photodetectors

2D hybrid perovskites have shown great promise in the photodetection field, due to their intriguing attributes stemming from unique structural architectures. However, the great majority of detectors based on this 2D system possess a relatively low response speed (≈ms), making it extremely urgent to develop new candidates for superfast photodetection. Here, a new organic–inorganic hybrid perovskite, (PA)₂(FA)Pb₂I₇ (EFA, where PA is n‐pentylaminium and FA is formamidine), which features the 2D Ruddlesden–Popper type perovskite framework that is composed of the corner‐sharing PbI₆ octahedra is reported. Significantly, photodetectors fabricated on highly oriented thin films, which exhibit a perfect orientation parallel to 2D inorganic perovskite layers, exhibit a superfast response time up to ≈2.54 ns. To the best of the knowledge, this figure‐of‐merit catches up with that of the top‐ranking commercial materials, and sets a new record for 2D hybrid perovskite photodetectors. Moreover, extremely high photodetectivity (≈1.73 × 10¹⁴ Jones, under an incident power intensity of ≈46 µW cm^?2), considerable switching ratios (>10³), and low dark current (≈10 pA) are also achieved in the detector, indicating its great potential for high‐efficiency photodetection. These results shed light on the possibilities to explore new 2D candidates for assembling future high‐performance optoelectronic devices.