Colloidal all‐inorganic perovskites nanocrystals (NCs) have emerged as a promising material for display and lighting due to their excellent optical properties. However, blue emissive NCs usually ...suffer from low photoluminescence quantum yields (PLQYs) and poor stability, rendering them the bottleneck for full‐color all‐perovskite optoelectronic applications. Herein, a facile approach is reported to enhance the emission efficiency and stability of blue emissive perovskite nano‐structures via surface passivation with potassium bromide. By adding potassium oleate and excess PbBr2 to the perovskite precursor solutions, potassium bromide‐passivated (KBr‐passivated) blue‐emitting (≈450 nm) CsPbBr3 nanoplatelets (NPLs) is successfully synthesized with a respectably high PLQY of 87%. In sharp contrast to most reported perovskite NPLs, no shifting in emission wavelength is observed in these passivated NPLs even after prolonged exposures to intense irradiations and elevated temperature, clearly revealing their excellent photo‐ and thermal‐stabilities. The enhancements are attributed to the formation of K‐Br bonding on the surface which suppresses ion migration and formation of Br‐vacancies, thus improving both the PL emission and stability of CsPbBr3 NPLs. Furthermore, all‐perovskite white light‐emitting diodes (WLEDs) are successfully constructed, suggesting that the proposed KBr‐passivated strategy can promote the development of the perovskite family for a wider range of optoelectronic applications.
High‐quality blue‐emitting CsPbBr3 nanoplatelets (NPLs) are synthesized via a facile potassium bromide‐enriched surface passivation. The resultant blue‐emitting (≈450 nm) CsPbBr3 NPLs show a high PLQY of 87% with excellent thermal stability and photostability. Furthermore, white light LEDs based on the mixture perovskite materials including the blue‐emitting NPLs are constructed, demonstrating a wide color gamut.
The Goodenough-Kanamori-Anderson (GKA) rules have been widely applied for explaining the magnetic properties induced by super-exchange interaction. As conclusions of the super-exchange theory, they ...reveal the antiferromagnetic (ferromagnetic) ordering along with bond angle of 180° (90°) in the cation-anion-cation interaction path, in which the theory sets a pre-condition that the electronic states of cations in all paths are identical. We observed that the GKA rules are in fact not universal and even invalid to materials containing anions with different valence states, for example, the layered CrOCl crystal (with two valence states of anions: O2− and Cl−). In this study, we propose an extended super-exchange theory (ESET) related to superposed electronic states of cation in a specific path. ESET is capable of predicting not only the sign and relative magnitude of magnetic exchange constants in different cation-anion-cation paths, but also the magnetic ground state. Through our proposed theory, we conclude that the magnetic ordering along with bond angle of 90° in Cr-Cl-Cr path is moderately antiferromagnetic and of 180° in Cr-O-Cr path is strongly ferromagnetic, which are opposite to the contents of GKA rules. Moreover, we clarify that monolayer CrOCl has antiferromagnetic ordering rather than ferromagnetic as reported recently. The reliability of ESET is verified via first-principles calculation and previous experimental report as well, and its universality is also demonstrated. Thus, our theory is powerful to predict the magnetic properties, which makes it possible to design new high Curie temperature two-dimensional semiconducting ferromagnets with polyvalent anion materials.
Lasing action is realized in a ZnO/GaN heterojunction by employing a MgO interlayer. The MgO layer can confine electrons in the ZnO layer, while holes can pass through the MgO layer and enter into ...the n‐ZnO layer from the p‐GaN layer. The threshold of the lasing action is as low as 0.8 mA..
Electrically pumped random lasers are realized in ZnO nanocrystallite films in a simple metal–oxide–semiconductor structure. By introducing an i‐ZnO layer, a threshold current of 6.5 mA is obtained. ...The reported results provide a simple route to electrically pumped random lasing (see figure) with relatively low threshold, a significant step towards the future applications of this kind of laser.
Interfacial quality of functional layers plays an important role in the carrier transport of sandwich-structured devices. Although the suppression of interface states is crucial to the overall device ...performance, our understanding on their formation and annihilation mechanism via direct characterization is still quite limited. Here, we present a thorough study on the interface states present in the electron transport layer (ETL) of blue quantum dot (QD) light-emitting diodes (QLEDs). A ZnO/ZnMgO bilayer ETL is adopted to enhance the electron injection into blue QDs. By probing the ETL band structure with photoelectron spectroscopy, we discover that substantial band bending exists at the ZnO/ZnMgO interface, elucidating the presence of a high density of interface states which hinder electron transport. By inserting a ZnO@ZMO interlayer composed of mixed ZnO and ZnMgO nanoparticles, the band bending and thus the interface states are observed to reduce significantly. We attribute this to the hybrid surface properties of ZnO@ZMO, which can annihilate the surface states of both the ZnO and ZnMgO layers. The introduction of a bridging layer has led to ∼40% enhancement in the power efficiency of blue QLEDs and noticeable performance boosts in green and red QLEDs. The findings here demonstrate a direct observation of interface states via detailed band structure studies and outline a potential pathway for eliminating these states for better performances in sandwich-structured devices.
With many advantages including superior color saturation and efficiency, quantum dot light-emitting diodes (QLEDs) are considered a promising candidate for the next-generation displays. Emission ...uniformity over the entire device area is a critical factor to the overall performance and reliability of QLEDs. In this work, we performed a thorough study on the origin of dark spots commonly observed in operating QLEDs and developed a strategy to eliminate these defects. Using advanced cross section fabrication and imaging techniques, we discovered the occurrence of voids in the organic hole transport layer and directly correlated them to the observed emission nonuniformity. Further investigations revealed that these voids are thermal damages induced during the subsequent thermal deposition of other functional layers and can act as leakage paths in the device. By inserting a thermo-tolerant 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) interlayer with an optimized thickness, the thermally induced dark spots can be completely suppressed, leading to a current efficiency increase by 18%. We further demonstrated that such a thermal passivation strategy can work universally for various types of organic layers with low thermal stability. Our findings here provide important guidance in enhancing the performances and reliability of QLEDs and also other sandwich-structured devices via the passivation of heat-sensitive layers.
The amine-induced very fast liquefaction and recrystallization (L&R) process on organic–inorganic hybrid perovskites has been demonstrated to be an effective way to improve the quality of the ...as-deposited MAPbI3 films and increase the feasibility of preparing large area perovskite solar cells (PSCs). Despite the effectiveness, the reaction mechanism of the L&R process is yet to be well understood. In this work, we investigate the fundamental influences of amines on the structural change of MAPbI3 perovskite by studying the reaction between ammonia and a bulk MAPbI3 crystal. By employing various techniques including in situ X-ray diffraction, Raman spectroscopy and Fourier transform infrared spectroscopy, we find that NH3 effectively substitutes iodine in the perovskite crystal lattice, transforming the 3-dimensional (3D) lattice into a MAPb(NH3)nI3 coordination complex-based system. The resulting coordination complex octahedral structures are only held together by weak forces of attraction, leading to the experimentally observed L&R process. First-principles calculations further confirm the feasibility of such L&R at room temperature. As a proof of concept, we demonstrated the use of liquefaction for substrate recycling of MAPbI3 based solar cells with ammonia. Our results here provide deep insights for understanding the interaction mechanism, which can surely provide important guidance on improving the quality of perovskite films and related devices, as well as the recycling of toxic lead for long-term sustainability.
Metal halide perovskites (MHPs) have shown unparalleled potential for optoelectronic applications. In particular, the quasi one-dimensional perovskite structures have attracted great attention due to ...their unique optoelectronic properties which can open up new possibilities for this exciting materials system. However, it is challenging to acquire one-dimensional cubic-phase MHPs in high yield due to their isotropic crystallization nature and instability towards conventional fabrication processes. Here, we demonstrate the feasibility of synthesizing single-crystalline perovskite microwires from an intrinsic 1D structured lattice framework which defines the size and shape of the resulting microwires. By incorporating the appropriate monovalent ions into the lattice of self-assembled PbBr2 microwires, single-crystal CH3NH3PbBr3 microwires are successfully synthesized. The excellent crystal quality of the microwires has led to a high carrier mobility of 36 cm2 V−1 s−1 and a long charge carrier lifetime of ∼100 ns. As a preliminary demonstration of their optoelectronics functionality, single CH3NH3PbBr3 microwire-based photodetectors are fabricated and the devices exhibit a fast response time (a rise time of 113 μs and a decay time of 295 μs), stable light switching behavior, high sensitivity, and good spatial resolution. The method described here proposes a novel and facile strategy for synthesizing single-crystal perovskite microwires with high yield and repeatability, showing the great promise of MHPs in micro-optoelectronic applications.
Here, we report the extraordinary electrochemical energy storage capability of NiMoO4@NiMoO4 homogeneous hierarchical nanosheet-on-nanowire arrays (SOWAs), synthesized on nickel substrate by a ...two-stage hydrothermal process. Comparatively speaking, the SOWAs electrode displays superior electrochemical performances over the pure NiMoO4 nanowire arrays. Such improvements can be ascribed to the characteristic homogeneous hierarchical structure, which not only effectively increases the active surface areas for fast charge transfer, but also reduces the electrode resistance significantly by eliminating the potential barrier at the nanowire/nanosheet junction, an issue usually seen in other reported heterogeneous architectures. We further evaluate the performances of the SOWAs by constructing an asymmetric hybrid supercapacitor (ASC) with the SOWAs and activated carbon (AC). The optimized ASC shows excellent electrochemical performances with 47.2 Wh/kg in energy density of 1.38 kW/kg at 0–1.2 V. Moreover, the specific capacity retention can be as high as 91.4% after 4000 cycles, illustrating the remarkable cycling stability of the NiMoO4@NiMoO4//AC ASC device. Our results show that this unique NiMoO4@NiMoO4 SOWA has great prospects for future energy storage applications.
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