Revealing Grain Boundaries and Defect Formation in Nanocrystal Superlattices by Nanodiffraction

X‐ray nanodiffraction is applied to study the formation and correlation of domain boundaries in mesocrystalline superlattices of PbS nanocrystals with face‐centered cubic structure. Each domain of the superlattice can be described with one of two mesocrystalline polymorphs with different orientational orders. Close to a grain boundary, the lattice constant decreases and the superlattice undergoes an out‐of‐plane rotation, while the orientation of the nanocrystals with respect to the superlattice remains unchanged. These findings are explained with the release of stress on the expense of specific nanocrystal–substrate interactions. The fact that correlations between adjacent nanocrystals are found to survive the structural changes at most grain boundaries implies that the key to nanocrystal superlattices with macroscopic domain sizes are strengthened interactions with the substrate.     

Trap‐State Suppression and Improved Charge Transport in PbS Quantum Dot Solar Cells with Synergistic Mixed‐Ligand Treatments

The power conversion efficiency of colloidal PbS‐quantum‐dot (QD)‐based solar cells is significantly hampered by lower‐than‐expected open circuit voltage (V_OC). The V_OC deficit is considerably higher in QD‐based solar cells compared to other types of existing solar cells due to in‐gap trap‐induced bulk recombination of photogenerated carriers. Here, this study reports a ligand exchange procedure based on a mixture of zinc iodide and 3‐mercaptopropyonic acid to reduce the V_OC deficit without compromising the high current density. This layer‐by‐layer solid state ligand exchange treatment enhances the photovoltaic performance from 6.62 to 9.92% with a significant improvement in V_OC from 0.58 to 0.66 V. This study further employs optoelectronic characterization, X‐ray photoelectron spectroscopy, and photoluminescence spectroscopy to understand the origin of V_OC improvement. The mixed‐ligand treatment reduces the sub‐bandgap traps and significantly reduces bulk recombination in the devices.     

In Situ Passivation for Efficient PbS Quantum Dot Solar Cells by Precursor Engineering

Current efforts on lead sulfide quantum dot (PbS QD) solar cells are mostly paid to the device architecture engineering and postsynthetic surface modification, while very rare work regarding the optimization of PbS synthesis is reported. Here, PbS QDs are successfully synthesized using PbO and PbAc₂ · 3H₂O as the lead sources. QD solar cells based on PbAc‐PbS have demonstrated a high power conversion efficiency (PCE) of 10.82% (and independently certificated values of 10.62%), which is significantly higher than the PCE of 9.39% for PbO‐PbS QD based ones. For the first time, systematic investigations are carried out on the effect of lead precursor engineering on the device performance. It is revealed that acetate can act as an efficient capping ligands together with oleic acid, providing better surface coverage and replace some of the harmful hydroxyl (OH) ligands during the synthesis. Then the acetate on the surface can be exchanged by iodide and lead to desired passivation. This work demonstrates that the precursor engineering has great potential in performance improvement. It is also pointed out that the initial synthesis is an often neglected but critical stage and has abundant room for optimization to further improve the quality of the resultant QDs, leading to breakthrough efficiency.     

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.     

Ligand effects on electronic and optoelectronic properties of two-dimensional PbS necking percolative superlattices

The inter-nanocrystal (NC) distance,necking degree,ordering level,and NC surface ligands all affect the electronic and optoelectronic properties of NC solids.Herein,we introduce a unique PbS structure of necking percolative superlattices to exclude the morphological factors and study the effect of ligands on the NC properties.X-ray photoelectron spectroscopy data indicate that 1,2-ethanedithiol (EDT),oxalic acid,mercaptopropionic acid,and NH4SCN (SCN)ligands were attached to the surface of NCs by substrate-supported ligand exchange.Field-effect transistors were tested and photodetector measurements were performed to compare these NC solids.An SCN-treated film had the highest mobility and responsivity under high-power intensity irradiation owing to its high carrier density,whereas an EDT-treated film had the lowest mobility,photocurrent,and dark current.These findings introduce new avenues for choosing suitable ligands for NC applications.     

High‐Performance,Room Temperature,Ultra‐Broadband Photodetectors Based on Air‐Stable PdSe2

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high‐performance ultra‐broadband photodetector based on PdSe₂ with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid‐infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid‐infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W^?1 and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe₂, anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.     

Ultrabroadband Photodetectors up to 10.6 µm Based on 2D Fe3O4 Nanosheets

The ultrabroadband spectrum detection from ultraviolet (UV) to long-wavelength infrared (LWIR) is promising for diversified optoelectronic applications of imaging, sensing, and communication. However, the current LWIR-detecting devices suffer from low photoresponsivity, high cost, and cryogenic environment. Herein, a high-performance ultrabroadband photodetector is demonstrated with detecting range from UV to LWIR based on air-stable nonlayered ultrathin Fe₃O₄ nanosheets synthesized via a space-confined chemical vapor deposition (CVD) method. Ultrahigh photoresponsivity (R) of 561.2 A W⁻¹, external quantum efficiency (EQE) of 6.6 × 10³%, and detectivity (D*) of 7.42 × 10⁸ Jones are achieved at the wavelength of 10.6 µm. The multimechanism synergistic effect of photoconductive effect and bolometric effect demonstrates the high sensitivity for light with any light intensities. The outstanding device performance and complementary mixing photoresponse mechanisms open up new potential applications of nonlayered 2D materials for future infrared optoelectronic devices.     

Edge‐Epitaxial Growth of InSe Nanowires toward High‐Performance Photodetectors

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO₂/Si substrate due to lattice mismatch. Here, a catalysis‐free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO₂/Si substrates through edge‐homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO₂/Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self‐driven vapor–liquid–solid process, which is distinct from the conventional metal‐catalytic vapor–liquid–solid method widely used for growing Si and III–V nanowires. Furthermore, it is demonstrated that the as‐grown InSe nanowire‐based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W^?1, ultrahigh detectivity of 1.57 × 10¹⁴ Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as‐grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge‐homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO₂/Si substrates.     

Phase-Modulated Multidimensional Perovskites for High-Sensitivity Self-Powered UV Photodetectors

2D Ruddlesden-Popper perovskites (PVKs) have recently shown overwhelming potential in various optoelectronic devices on account of enhanced stability to their 3D counterparts. So far, regulating the phase distribution and orientation of 2D perovskite thin films remains challenging to achieve efficient charge transport. This work elucidates the balance struck between sufficient gradient sedimentation of perovskite colloids and less formation of small-n phases, which results in the layered alignment of phase compositions and thus in enhanced photoresponse. The solvent engineering strategy, together with the introduction of poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS) and PC₇₁BM layer jointly contribute to outstanding self-powered performance of indium tin oxide/PEDOT:PSS/PVK/PC₇₁BM/Ag device, with a photocurrent of 18.4 µA and an on/off ratio up to 2800. The as-fabricated photodetector exhibits high sensitivity characteristics with the peak responsivity of 0.22 A W⁻¹ and the detectivity up to 1.3 × 10¹² Jones detected at UV-A region, outperforming most reported perovskite-based UV photodetectors and maintaining high stability over a wide spectrum ranging from UV to visible region. This discovery supplies deep insights into the control of ordered phases and crystallinity in quasi-2D perovskite films for high-performance optoelectronic devices.     

Efficient photovoltaic effect in graphene/h-BN/silicon heterostructure self-powered photodetector

Graphene (Gr)/Si-based optoelectronic devices have attracted a lot of academic attention due to the simpler fabrication processes, low costs, and higher performance of their two-dimensional (2D)/three-dimensional (3D) hybrid interfaces in Schottky junction that promotes electron-hole separation. However, due to the built-in potential of Gr/Si as a photodetector, the I_ph /I_dark ratio is often hindered near zero-bias at relatively low illumination intensity. This is a major drawback in self-powered photodetectors. In this study, we have demonstrated a self-powered van der Waals heterostructure photodetector in the visible range using a Gr/hexagonal boron nitride (h-BN)/Si structure and clarified that the thin h-BN insertion can engineer asymmetric carrier transport and avoid interlayer coupling at the interface. The dark current was able to be suppressed by inserting an h-BN insulator layer, while maintaining the photocurrent with minimal decrease at near zero-bias. As a result, the normalized photocurrent-to-dark ratio (NPDR) is improved more than 10⁴ times. Also, both I_ph/I_dark ratio and detectivity, increase by more than 10⁴ times at −0.03 V drain voltage. The proposed Gr/h-BN/Si heterostructure is able to contribute to the introduction of next-generation photodetectors and photovoltaic devices based on graphene or silicon.

     

Hydrogen Terminated Germanene for a Robust Self‐Powered Flexible Photoelectrochemical Photodetector

As a rising star in the family of graphene analogues, germanene shows great potential for electronic and optical device applications due to its unique structure and electronic properties. It is revealed that the hydrogen terminated germanene not only maintains a high carrier mobility similar to that of germanene, but also exhibits strong light–matter interaction with a direct band gap, exhibiting great potential for photoelectronics. In this work, few‐layer germanane (GeH) nanosheets with controllable thickness are successfully synthesized by a solution‐based exfoliation–centrifugation route. Instead of complicated microfabrication techniques, a robust photoelectrochemical (PEC)‐type photodetector, which can be extended to flexible device, is developed by simply using the GeH nanosheet film as an active electrode. The device exhibits an outstanding photocurrent density of 2.9 µA cm^?2 with zero bias potential, excellent responsivity at around 22 µA W^?1 under illumination with intensity ranging from 60 to 140 mW cm^?2, as well as short response time (with rise and decay times, t_r = 0.24 s and t_d = 0.74 s). This efficient strategy for a constructing GeH‐based PEC‐type photodetector suggests a path to promising high‐performance, self‐powered, flexible photodetectors, and it also paves the way to a practical application of germanene.     

Investigations on the morphology,optical and photoresponse properties of PbS/CdS binary colloidal quantum dot thin film

Binary thin film exhibits not only the quantum features of the individual building blocks but also novel collective properties through coupling of colloidal quantum dot components. In this paper, lead sulfide (PbS) and cadmium sulfide (CdS) colloidal quantum dots (CQDs) were synthesized by using oleate and oleylamine as ligand. The as-synthesized PbS and CdS CQDs were monodispersity and well passivation. The average diameter of as-synthesized PbS and CdS CQDs were about 3 nm and 6 nm, respectively. By blending PbS with CdS CQDs and utilizing ethanedithiol for ligand passivation, the responsivity and detectivity of PbS CQDs thin film was enhanced with the weight ratio of CdS CQDs increased, the optimum responsivity and detectivity were 21.9 mA/W and 2.1 × 10¹⁰ Jones, respectively. The desirable properties of binary colloidal quantum dot thin films have important applications in future electronic and optoelectronic devices.     

Hybrid PbS Quantum‐Dot‐in‐Perovskite for High‐Efficiency Perovskite Solar Cell

In this study, a facile and effective approach to synthesize high‐quality perovskite‐quantum dots (QDs) hybrid film is demonstrated, which dramatically improves the photovoltaic performance of a perovskite solar cell (PSC). Adding PbS QDs into CH₃NH₃PbI₃ (MAPbI₃) precursor to form a QD‐in‐perovskite structure is found to be beneficial for the crystallization of perovskite, revealed by enlarged grain size, reduced fragmentized grains, enhanced characteristic peak intensity, and large percentage of (220) plane in X‐ray diffraction patterns. The hybrid film also shows higher carrier mobility, as evidenced by Hall Effect measurement. By taking all these advantages, the PSC based on MAPbI₃‐PbS hybrid film leads to an improvement in power conversion efficiency by 14% compared to that based on pure perovskite, primarily ascribed to higher current density and fill factor (FF). Ultimately, an efficiency reaching up to 18.6% and a FF of over ≈0.77 are achieved based on the PSC with hybrid film. Such a simple hybridizing technique opens up a promising method to improve the performance of PSCs, and has strong potential to be applied to prepare other hybrid composite materials.     

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.     

Bifacial illuminated PbS quantum dot-sensitized solar cells with translucent CuS counter electrodes

This paper reports the optimization of the TiO₂ photoanode and the fabrication of bifacial illuminated PbS quantum dot-sensitized solar cells (QDSSCs) with translucent CuS counter electrodes. TiO₂ photoanode is prepared by introducing a compact TiO₂ layer between FTO substrate and TiO₂ film with titanium diisopropoxide bis(acetylacetonate) in 1-butanol and post-treatment with TiCl₄ aqueous solution; then, PbS quantum dots (QDs) are deposited on the surface of TiO₂ film by successive ionic layer adsorption and reaction method; also, by means of control of the TiO₂ surface charge, QD density of TiO₂ film is improved by adding triethanolamine into the cationic precursor solution. Using this optimized TiO₂ photoanode and a translucent CuS counter electrode, a bifacial illuminated PbS QDSSC is fabricated. The preparation conditions are optimized and the surface morphology and electrochemical properties of TiO₂ photoanodes are characterized. The bifacial illuminated PbS QDSSC achieves a power conversion efficiency of 2.16 %, which is increased by 48.97 % compared with the single illuminated QDSSC.     

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.     

Morphology and energy transfer in PbS quantum dot arrays formed with supercritical fluid deposition

Lead sulfide (PbS) quantum dots (QDs) were synthesized in our lab with controllable and tunable sizes. Supercritical fluid CO₂ (sc-CO₂) provides a useful tool to deposit PbS QDs on substrate surfaces with lateral uniformity. Either in the PbS/toluene solution or in the sc-CO₂ fabricated film, the absorbance maxima of the PbS QDs do not show an obvious dependence on the PbS QD concentrations. Fluorescence spectra of PbS QDs obtained from the films prepared by the sc-CO₂ method indicate energy transfer between PbS QDs with different sizes, the small QDs serving as energy donors and large ones as acceptors. Samples formed with sc-CO₂ method measured by photoluminescence (PL) show a red-shift and an enhanced emission intensity with respect to samples formed with solution deposition method (SDM), specifically at cryogenic temperatures.     

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.     

Direct Growth of Perovskite Crystals on Metallic Electrodes for High‐Performance Electronic and Optoelectronic Devices

Metal halide perovskite has attracted enhanced interest for its diverse electronic and optoelectronic applications. However, the fabrication of micro‐ or nanoscale crystalline perovskite functional devices remains a great challenge due to the fragility, solvent, and heat sensitivity of perovskite crystals. Here, a strategy is proposed to fabricate electronic and optoelectronic devices by directly growing perovskite crystals on microscale metallic structures in liquid phase. The well‐contacted perovskite/metal interfaces ensure these heterostructures serve as high‐performance field effect transistors (FETs) and excellent photodetector devices. When serving as an FET, the on/off ratio is as large as 10⁶ and the mobility reaches up to ≈2.3 cm² V^?1 s^?1. A photodetector is displayed with high photoconductive switching ratio of ≈10⁶ and short response time of ≈4 ms. Furthermore, the photoconductive response is proved to be band‐bending‐assisted separation of photoexcited carriers at the Schottky barrier of the silver and p‐type perovskites.     

Solution‐Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for High‐Performance Solar Cells

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution‐processed perovskite films. Here, solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is reported. Under transmission electron microscopy, in‐plane coupling between the perovskite and the MoS₂ crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS₂ surface. In perovskite solar cells, when perovskite active layers are grown on MoS₂ flakes coated on hole‐transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high‐performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.