Investigation of Electrode Electrochemical Reactions in CH3NH3PbBr3 Perovskite Single‐Crystal Field‐Effect Transistors

Optoelectronic devices based on metal halide perovskites, including solar cells and light‐emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic–inorganic hybrid perovskite materials can enable high‐performance, solution‐processed field‐effect transistors (FETs) for next‐generation, low‐cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single‐crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source–drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such “ideal” interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom‐contact, bottom‐gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single‐crystal FETs with high mobility of up to ≈15 cm² V^?1 s^?1 at 80 K. This work addresses one of the key challenges toward the realization of high‐performance solution‐processed perovskite FETs.     

Enhancing the Performance of Perovskite Solar Cells by Hybridizing SnS Quantum Dots with CH3NH3PbI3

The combination of perovskite solar cells and quantum dot solar cells has significant potential due to the complementary nature of the two constituent materials. In this study, solar cells (SCs) with a hybrid CH₃NH₃PbI₃/SnS quantum dots (QDs) absorber layer are fabricated by a facile and universal in situ crystallization method, enabling easy embedding of the QDs in perovskite layer. Compared with SCs based on CH₃NH₃PbI₃, SCs using CH₃NH₃PbI₃/SnS QDs hybrid films as absorber achieves a 25% enhancement in efficiency, giving rise to an efficiency of 16.8%. The performance improvement can be attributed to the improved crystallinity of the absorber, enhanced photo‐induced carriers' separation and transport within the absorber layer, and improved incident light utilization. The generality of the methods used in this work paves a universal pathway for preparing other perovskite/QDs hybrid materials and the synthesis of entire nontoxic perovskite/QDs hybrid structure.     

Highly Reproducible Large‐Area Perovskite Solar Cell Fabrication via Continuous Megasonic Spray Coating of CH3NH3PbI3

A simple, low‐cost, large area, and continuous scalable coating method is proposed for the fabrication of hybrid organic–inorganic perovskite solar cells. A megasonic spray‐coating method utilizing a 1.7 MHz megasonic nebulizer that could fabricate reproducible large‐area planar efficient perovskite films is developed. The coating method fabricates uniform large‐area perovskite film with large‐sized grain since smaller and narrower sized mist droplets than those generated by existing ultrasonic spray methods could be generated by megasonic spraying. The volume flow rate of the CH₃NH₃PbI₃ precursor solution and the reaction temperature are controlled, to obtain a high quality perovskite active layer. The devices reach a maximum efficiency of 16.9%, with an average efficiency of 16.4% from 21 samples. The applicability of megasonic spray coating to the fabrication of large‐area solar cells (1 cm²), with a power conversion efficiency of 14.2%, is also demonstrated. This is a record high efficiency for large‐area perovskite solar cells fabricated by continuous spray coating.     

In Situ Monitoring of Laser‐Assisted Hydrothermal Growth of ZnO Nanowires: Thermally Deactivating Growth Kinetics

The laser‐assisted hydrothermal growth kinetics of a cluster of ZnO nanowires are studied based on optical in situ growth monitoring. The growth yields are orders of magnitude higher than those of conventional hydrothermal methods that use bulk heating. This remarkable improvement is attributed to suppression of precursor depletion occurring by homogeneous growth reactions, as well as to enhanced mass transport. The obtained in situ data show gradually decaying growth kinetics even with negligible precursor consumption. It is revealed that the growth deceleration is caused by thermal deactivation resulting from heat dissipation through the growing nanowires. Finally, it is demonstrated that the tailored temporal modulation of the input power enables sustained growth to extended dimensions. These results provide a key to highly efficient use of growth precursors that has been pursued for industrial use of this functional metal oxide semiconductor.     

A Two‐Stage Annealing Strategy for Crystallization Control of CH3NH3PbI3 Films toward Highly Reproducible Perovskite Solar Cells

The solvent‐engineering method is widely used to fabricate top‐performing perovskite solar cells, which, however, usually exhibit inferior reproducibility. Herein, a two‐stage annealing (TSA) strategy is demonstrated for processing of perovskite films, namely, annealing the intermediate phase at 60 °C for the first stage then at 100 °C for the second stage. Compared to conventional direct annealing temperature (DHA) at 100 °C, using this strategy, MAPbI₃ films become more controllable, leading to superior film uniformity and device reproducibility with the champion device efficiency reaching 19.8%. More specifically, the coefficient of variation of efficiency for 49 cells is reduced to 5.9%, compared to 9.8% for that using DHA. The TSA process is carefully studied using Fourier transform infrared spectroscopy, X‐ray diffraction, and UV–vis absorption spectroscopy. It is found that in comparison with DHA the formation of hydrogen bonding and crystallization of perovskite are much slower and can be better controlled when using TSA. The improvements in film uniformity and device reproducibility are attributed to: 1) controllable MAPbI₃ crystal growth stemming from the progressive formation of hydrogen bonding between methylammonium and halide; 2) suppression of intermediate phase film dewetting, which is believed to be due to its decreased mobility at the initial low‐temperature annealing stage.     

Space‐Confined Growth of Individual Wide Bandgap Single Crystal CsPbCl3 Microplatelet for Near‐Ultraviolet Photodetection

Perovskite photodetectors (PDs) with tunable detection wavelength have attracted extensive attention due to the potential application in the field of imaging, machine vision, and artificial intelligence. Most of the perovskite PDs focus on I‐ or Br‐based materials due to their easy preparation techniques. However, their main photodetection capacity is situated in the visible region because of their narrower bandgap. Cl‐based wide bandgap perovskites, such as CsPbCl₃, are scarcely reported because of the bad film quality of the spin‐coated Cl‐based perovskite, due to the poor solubility of the precursor. Therefore, ultraviolet detection using high‐quality full inorganic perovskite films, especially with high thermal stability of materials and devices, is still a big challenge. In this work, high‐quality single crystal CsPbCl₃ microplatelets (MPs) synthesized by a simple space‐confined growth method at low temperature for near‐ultraviolet (NUV) PDs are reported. The single CsPbCl₃ MP PDs demonstrate a decent response to NUV light with a high on/off ratio of 5.6 × 10³ and a responsivity of 0.45 A W^?1 at 5 V. In addition, the dark current is as low as pA level, leading to detectivity up to 10¹¹ Jones. Moreover, PDs possess good stability and repeatability.     

Controlled Substitution of Chlorine for Iodine in Single‐Crystal Nanofibers of Mixed Perovskite MAPbI3–xClx

Longer carrier diffusion length and improved power conversion efficiency have been reported for thin‐film solar cell of organolead mixed‐halide perovskite MAPbI_{3– x} Cl _x in comparison with MAPbI₃. Instead of substituting I in the MAPbI₃ lattice, Cl‐incorporation has been shown to mainly improve the film morphology of perovskite absorber. Well‐defined crystal structure, adjustable composition (x), and regular morphology, remains a formidable task. Herein, a facile solution‐assembly method is reported for synthesizing single‐crystalline nanofibers (NFs) of tetragonal‐lattice MAPbI_{3– x} Cl _x with the Cl‐content adjustable between 0 ≤ x ≤ 0.75, leading to a gradual blueshift of the absorption and photoluminescence maxima from x = 0 to 0.75. The photoresponsivity (R) of MAPbI₃ NFs keeps almost unchanging at a value independent of the white‐light illumination intensity (P). In contrast, R of MAPbI_{3– x} Cl _x NFs decreases rapidly with increasing both the x and P values, indicating Cl‐substitution increases the recombination traps of photogenerated free electrons and holes. This study provides a model system to examine the role of extrinsic Cl ions in both perovskite crystallography and optoelectronic properties.     

Flexible Photodetector Arrays Based on Patterned CH3NH3PbI3−xClx Perovskite Film for Real‐Time Photosensing and Imaging

The quest for novel deformable image sensors with outstanding optoelectronic properties and large‐scale integration becomes a great impetus to exploit more advanced flexible photodetector (PD) arrays. Here, 10 × 10 flexible PD arrays with a resolution of 63.5 dpi are demonstrated based on as‐prepared perovskite arrays for photosensing and imaging. Large‐scale growth controllable CH₃NH₃PbI_3?xCl_x arrays are synthesized on a poly(ethylene terephthalate) substrate by using a two‐step sequential deposition method with the developed Al₂O₃‐assisted hydrophilic–hydrophobic surface treatment process. The flexible PD arrays with high detectivity (9.4 × 10¹¹ Jones), large on/off current ratio (up to 1.2 × 10³), and broad spectral response exhibit excellent electrical stability under large bending angle (θ = 150°) and superior folding endurance after hundreds of bending cycles. In addition, the device can execute the functions of capturing a real‐time light trajectory and detecting a multipoint light distribution, indicating that it has widespread potential in photosensing and imaging for optical communication, digital display, and artificial electronic skin applications.     

In Situ Growth of Pd Nanosheets on g‐C3N4 Nanosheets with Well‐Contacted Interface and Enhanced Catalytic Performance for 4‐Nitrophenol Reduction

Loading novel metal nanosheets onto nanosheet support can improve their catalytic performance, but the morphological incompatibility makes it difficult to construct a well‐contacted interface, which is of particular interest in supported catalysts. Herein, Pd nanosheets (Pd NSs) are supported onto graphitic carbon nitride nanosheets (CNNSs) with intimate face‐to‐face contact through an in situ growth method. This method overcomes the limitations of the morphological incompatibility and ensures the intimate interfacial contact between Pd NSs and CNNSs. The nitrogen‐rich nature of CNNSs endows Pd NSs with abundant anchoring sites, which optimizes the electronic structure and improves the chemical and morphological stability of Pd NSs. The supported Pd NSs demonstrate high dispersion and exhibit largely enhanced activity toward the reduction of 4‐nitrophenol. The concentration‐normalized rate constant is up to 3052 min^?1 g^?1 L, which is 5.4 times higher than that obtained by unsupported Pd NSs. No obvious deactivation is observed after six runs of the recycling experiments. It is believed that the supported novel metal nanosheets with the intimately contacted interface may show promising applications in catalysis.     

Epitaxial Growth of 6 in. Single‐Crystalline Graphene on a Cu/Ni (111) Film at 750 °C via Chemical Vapor Deposition

The future electronic application of graphene highly relies on the production of large‐area high‐quality single‐crystal graphene. However, the growth of single‐crystal graphene on different substrates via either single nucleation or seamless stitching is carried out at a temperature of 1000 °C or higher. The usage of this high temperature generates a variety of problems, including complexity of operation, higher contamination, metal evaporation, and wrinkles owing to the mismatch of thermal expansion coefficients between the substrate and graphene. Here, a new approach for the fabrication of ultraflat single‐crystal graphene using Cu/Ni (111)/sapphire wafers at lower temperature is reported. It is found that the temperature of epitaxial growth of graphene using Cu/Ni (111) can be reduced to 750 °C, much lower than that of earlier reports on catalytic surfaces. Devices made of graphene grown at 750 °C have a carrier mobility up to ≈9700 cm² V^?1 s^?1 at room temperature. This work shines light on a way toward a much lower temperature growth of high‐quality graphene in single crystallinity, which could benefit future electronic applications.     

One‐Step In Situ Growth of Iron–Nickel Sulfide Nanosheets on FeNi Alloy Foils: High‐Performance and Self‐Supported Electrodes for Water Oxidation

Efficient and durable oxygen evolution reaction (OER) catalysts are highly required for the cost‐effective generation of clean energy from water splitting. For the first time, an integrated OER electrode based on one‐step direct growth of metallic iron–nickel sulfide nanosheets on FeNi alloy foils (denoted as Fe? Ni₃S₂/FeNi) is reported, and the origin of the enhanced OER activity is uncovered in combination with theoretical and experimental studies. The obtained Fe? Ni₃S₂/FeNi electrode exhibits highly catalytic activity and long‐term stability toward OER in strong alkaline solution, with a low overpotential of 282 mV at 10 mA cm^?2 and a small Tafel slope of 54 mV dec^?1. The excellent activity and satisfactory stability suggest that the as‐made electrode provides an attractive alternative to noble metal‐based catalysts. Combined with density functional theory calculations, exceptional OER performance of Fe? Ni₃S₂/FeNi results from a combination of efficient electron transfer properties, more active sites, the suitable O₂ evolution kinetics and energetics benefited from Fe doping. This work not only simply constructs an excellent electrode for water oxidation, but also provides a deep understanding of the underlying nature of the enhanced OER performance, which may serve as a guide to develop highly effective and integrated OER electrodes for water splitting.