Perovskite Quantum Dots with Near Unity Solution and Neat‐Film Photoluminescent Quantum Yield by Novel Spray Synthesis

In this study, a novel perovskite quantum dot (QD) spray‐synthesis method is developed by combining traditional perovskite QD synthesis with the technique of spray pyrolysis. By utilizing this new technique, the synthesis of cubic‐shaped perovskite QDs with a homogeneous size of 14 nm is demonstrated, which shows an unprecedented stable absolute photoluminescence quantum yield ≈100% in the solution and even in the solid‐state neat film. The highly emissive thin films are integrated with light emission devices (LEDs) and organic light emission displays (OLEDs). The color conversion type QD‐LED (ccQD‐LED) hybrid devices exhibit an extremely saturated green emission, excellent external quantum efficiency of 28.1%, power efficiency of 121 lm W^?1, and extraordinary forward‐direction luminescence of 8 500 000 cd m^?2. The conceptual ccQD‐OLED hybrid display also successfully demonstrates high‐definition still images and moving pictures with a 119% National Television System Committee 1931 color gamut and 123% Digital Cinema Initiatives‐P3 color gamut. These very‐stable, ultra‐bright perovskite QDs have the properties necessary for a variety of useful applications in optoelectronics.     

Optimization of Low‐Dimensional Components of Quasi‐2D Perovskite Films for Deep‐Blue Light‐Emitting Diodes

Compared to efficient green and near‐infrared light‐emitting diodes (LEDs), less progress has been made on deep‐blue perovskite LEDs. They suffer from inefficient domain [various number of PbX₆^? layers (n)] control, resulting in a series of unfavorable issues such as unstable color, multipeak profile, and poor fluorescence yield. Here, a strategy involving a delicate spacer modulation for quasi‐2D perovskite films via an introduction of aromatic polyamine molecules into the perovskite precursor is reported. With low‐dimensional component engineering, the n₁ domain, which shows nonradiative recombination and retarded exciton transfer, is significantly suppressed. Also, the n₃ domain, which represents the population of emission species, is remarkably increased. The optimized quasi‐2D perovskite film presents blue emission from the n₃ domain (peak at 465 nm) with a photoluminescence quantum yield (PLQY) as high as 77%. It enables the corresponding perovskite LEDs to deliver stable deep‐blue emission (CIE (0.145, 0.05)) with an external quantum efficiency (EQE) of 2.6%. The findings in this work provide further understanding on the structural and emission properties of quasi‐2D perovskites, which pave a new route to design deep‐blue‐emissive perovskite materials.     

High‐Efficiency Perovskite Light‐Emitting Diodes with Synergetic Outcoupling Enhancement

Perovskite light‐emitting diodes (PeLEDs) show great application potential in high‐quality flat‐panel displays and solid‐state lighting due to their steadily improved efficiency, tunable colors, narrow emission peak, and easy solution‐processing capability. However, because of high optical confinement and nonradiative charge recombination during electron–photon conversion, the highest reported efficiency of PeLEDs remains far behind that of their conventional counterparts, such as inorganic LEDs, organic LEDs, and quantum‐dot LEDs. Here a facile route is demonstrated by adopting bioinspired moth‐eye nanostructures at the front electrode/perovskite interface to enhance the outcoupling efficiency of waveguided light in PeLEDs. As a result, the maximum external quantum efficiency and current efficiency of the modified cesium lead bromide (CsPbBr₃) green‐emitting PeLEDs are improved to 20.3% and 61.9 cd A^?1, while retaining spectral and angular independence. Further reducing light loss in the substrate mode using a half‐ball lens, efficiencies of 28.2% and 88.7 cd A^?1 are achieved, which represent the highest values reported to date for PeLEDs. These results represent a substantial step toward achieving practical applications of PeLEDs.     

Direct Optical Patterning of Quantum Dot Light-Emitting Diodes via In Situ Ligand Exchange

Precise patterning of quantum dot (QD) layers is an important prerequisite for fabricating QD light-emitting diode (QLED) displays and other optoelectronic devices. However, conventional patterning methods cannot simultaneously meet the stringent requirements of resolution, throughput, and uniformity of the pattern profile while maintaining a high photoluminescence quantum yield (PLQY) of the patterned QD layers. Here, a specially designed nanocrystal ink is introduced, “photopatternable emissive nanocrystals” (PENs), which satisfies these requirements. Photoacid generators in the PEN inks allow photoresist-free, high-resolution optical patterning of QDs through photochemical reactions and in situ ligand exchange in QD films. Various fluorescence and electroluminescence patterns with a feature size down to ≈1.5 µm are demonstrated using red, green, and blue PEN inks. The patterned QD films maintain ≈75% of original PLQY and the electroluminescence characteristics of the patterned QLEDs are comparable to thopse of non-patterned control devices. The patterning mechanism is elucidated by in-depth investigation of the photochemical transformations of the photoacid generators and changes in the optical properties of the QDs at each patterning step. This advanced patterning method provides a new way for additive manufacturing of integrated optoelectronic devices using colloidal QDs.     

High‐Performance,Solution‐Processed,and Insulating‐Layer‐Free Light‐Emitting Diodes Based on Colloidal Quantum Dots

Quantum‐dot light‐emitting diodes (QLEDs) may combine superior properties of colloidal quantum dots (QDs) and advantages of solution‐based fabrication techniques to realize high‐performance, large‐area, and low‐cost electroluminescence devices. In the state‐of‐the‐art red QLED, an ultrathin insulating layer inserted between the QD layer and the oxide electron‐transporting layer (ETL) is crucial for both optimizing charge balance and preserving the QDs' emissive properties. However, this key insulating layer demands very accurate and precise control over thicknesses at sub‐10 nm level, causing substantial difficulties for industrial production. Here, it is reported that interfacial exciton quenching and charge balance can be independently controlled and optimized, leading to devices with efficiency and lifetime comparable to those of state‐of‐the‐art devices. Suppressing exciton quenching at the ETL–QD interface, which is identified as being obligatory for high‐performance devices, is achieved by adopting Zn_0.9Mg_0.1O nanocrystals, instead of ZnO nanocrystals, as ETLs. Optimizing charge balance is readily addressed by other device engineering approaches, such as controlling the oxide ETL/cathode interface and adjusting the thickness of the oxide ETL. These findings are extended to fabrication of high‐efficiency green QLEDs without ultrathin insulating layers. The work may rationalize the design and fabrication of high‐performance QLEDs without ultrathin insulating layers, representing a step forward to large‐scale production and commercialization.     

Inkjet‐Printed High‐Efficiency Multilayer QLEDs Based on a Novel Crosslinkable Small‐Molecule Hole Transport Material

Quantum dots light‐emitting diodes (QLEDs) have attracted much interest owing to their compatibility with low‐cost inkjet printing technology and potential for use in large‐area full‐color pixelated display. However, it is challenging to fabricate high efficiency inkjet‐printed QLEDs because of the coffee ring effects and inferior resistance to solvents from the underlying polymer film during the inkjet printing process. In this study, a novel crosslinkable hole transport material, 4,4′‐bis(3‐vinyl‐9H‐carbazol‐9‐yl)‐1,1′‐biphenyl (CBP‐V) which is small‐molecule based, is synthesized and investigated for inkjet printing of QLEDs. The resulting CBP‐V film after thermal curing exhibits excellent solvent resistance properties without any initiators. An added advantage is that the crosslinked CBP‐V film has a sufficiently low highest occupied molecular orbital energy level (≈?6.2 eV), high film compactness, and high hole mobility, which can thus promote the hole injection into quantum dots (QDs) and improve the charge carrier balance within the QD emitting layers. A red QLED is successfully fabricated by inkjet printing a CBP‐V and QDs bilayer. Maximum external quantum efficiency of 11.6% is achieved, which is 92% of a reference spin‐coated QLED (12.6%). This is the first report of such high‐efficiency inkjet‐printed multilayer QLEDs and demonstrates a unique and effective approach to inkjet printing fabrication of high‐performance QLEDs.     

Slow‐Photon‐Effect‐Induced Photoelectrical‐Conversion Efficiency Enhancement for Carbon‐Quantum‐Dot‐Sensitized Inorganic CsPbBr3 Inverse Opal Perovskite Solar Cells

All‐inorganic cesium lead halide perovskite is suggested as a promising candidate for perovskite solar cells due to its prominent thermal stability and comparable light absorption ability. Designing textured perovskite films rather than using planar‐architectural perovskites can indeed optimize the optical and photoelectrical conversion performance of perovskite photovoltaics. Herein, for the first time, this study demonstrates a rational strategy for fabricating carbon quantum dot (CQD‐) sensitized all‐inorganic CsPbBr₃ perovskite inverse opal (IO) films via a template‐assisted, spin‐coating method. CsPbBr₃ IO introduces slow‐photon effect from tunable photonic band gaps, displaying novel optical response property visible to naked eyes, while CQD inlaid among the IO frameworks not only broadens the light absorption range but also improves the charge transfer process. Applied in the perovskite solar cells, compared with planar CsPbBr₃, slow‐photon effect of CsPbBr₃ IO greatly enhances the light utilization, while CQD effectively facilitates the electron–hole extraction and injection process, prolongs the carrier lifetime, jointly contributing to a double‐boosted power conversion efficiency (PCE) of 8.29% and an increased incident photon‐to‐electron conversion efficiency of up to 76.9%. The present strategy on CsPbBr₃ IO to enhance perovskite PCE can be extended to rationally design other novel optoelectronic devices.     

Stability of Quantum Dots,Quantum Dot Films,and Quantum Dot Light‐Emitting Diodes for Display Applications

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color‐conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light‐emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.     

Ultrathin,Core–Shell Structured SiO2 Coated Mn2+‐Doped Perovskite Quantum Dots for Bright White Light‐Emitting Diodes

All‐inorganic semiconductor perovskite quantum dots (QDs) with outstanding optoelectronic properties have already been extensively investigated and implemented in various applications. However, great challenges exist for the fabrication of nanodevices including toxicity, fast anion‐exchange reactions, and unsatisfactory stability. Here, the ultrathin, core–shell structured SiO₂ coated Mn²⁺ doped CsPbX₃ (X = Br, Cl) QDs are prepared via one facile reverse microemulsion method at room temperature. By incorporation of a multibranched capping ligand of trioctylphosphine oxide, it is found that the breakage of the CsPbMnX₃ core QDs contributed from the hydrolysis of silane could be effectively blocked. The thickness of silica shell can be well‐controlled within 2 nm, which gives the CsPbMnX₃@SiO₂ QDs a high quantum yield of 50.5% and improves thermostability and water resistance. Moreover, the mixture of CsPbBr₃ QDs with green emission and CsPbMnX₃@SiO₂ QDs with yellow emission presents no ion exchange effect and provides white light emission. As a result, a white light‐emitting diode (LED) is successfully prepared by the combination of a blue on‐chip LED device and the above perovskite mixture. The as‐prepared white LED displays a high luminous efficiency of 68.4 lm W^?1 and a high color‐rendering index of Ra = 91, demonstrating their broad future applications in solid‐state lighting fields.     

Highly Emissive Green Perovskite Nanocrystals in a Solid State Crystalline Matrix

Perovskite nanocrystals (NCs) have attracted attention due to their high photoluminescence quantum yield (PLQY) in solution; however, maintaining high emission efficiency in the solid state remains a challenge. This study presents a solution‐phase synthesis of efficient green‐emitting perovskite NCs (CsPbBr₃) embedded in robust and air‐stable rhombic prism hexabromide (Cs₄PbBr₆) microcrystals, reaching a PLQY of 90%. Theoretical modeling and experimental characterization suggest that lattice matching between the NCs and the matrix contribute to improved passivation, while spatial confinement enhances the radiative rate of the NCs. In addition, dispersing the NCs in a matrix prevents agglomeration, which explains their high PLQY.     

Synergies of Electrochemical Metallization and Valance Change in All‐Inorganic Perovskite Quantum Dots for Resistive Switching

The in‐depth understanding of ions' generation and movement inside all‐inorganic perovskite quantum dots (CsPbBr₃ QDs), which may lead to a paradigm to break through the conventional von Neumann bottleneck, is strictly limited. Here, it is shown that formation and annihilation of metal conductive filaments and Br^? ion vacancy filaments driven by an external electric field and light irradiation can lead to pronounced resistive‐switching effects. Verified by field‐emission scanning electron microscopy as well as energy‐dispersive X‐ray spectroscopy analysis, the resistive switching behavior of CsPbBr₃ QD‐based photonic resistive random‐access memory (RRAM) is initiated by the electrochemical metallization and valance change. By coupling CsPbBr₃ QD‐based RRAM with a p‐channel transistor, the novel application of an RRAM–gate field‐effect transistor presenting analogous functions of flash memory is further demonstrated. These results may accelerate the technological deployment of all‐inorganic perovskite QD‐based photonic resistive memory for successful logic application.     

Confinement of Perovskite‐QDs within a Single MOF Crystal for Significantly Enhanced Multiphoton Excited Luminescence

The development of the photostable higher‐order multiphoton‐excited (MPE) upconversion single microcrystalline material is fundamentally and technologically important, but very challenging. Here, up to five‐photon excited luminescence in a host–guest metal–organic framework (MOF) and perovskite quantum dot (QD) hybrid single crystal ZJU‐28?MAPbBr₃ is shown via an in situ growth approach. Such a MOF strategy not only results in a high QD loading concentration, but also significantly diminishes the aggregation‐caused quenching (ACQ) effect, provides effective surface passivation, and greatly reduces the contact of the QDs with the external bad atmosphere due to the confinement effect and protection of the framework. These advantages make the resulting ZJU‐28?MAPbBr₃ single crystals possess high PLQY of ≈51.1%, a high multiphoton action cross‐sections that can rival the current highest record (measured in toluene solution), and excellent photostability. These findings liberate the excellent luminescence and nonlinear optical properties of perovskite QDs from the solution system to the solid single‐crystal system, which provide a new avenue for the exploitation of high‐performance multiphoton excited hybrid single microcrystal for future optoelectronic and micro–nano photonic integration applications.     

Efficient and Tunable Electroluminescence from In Situ Synthesized Perovskite Quantum Dots

Semiconductor quantum dots (QDs) are among the most promising next‐generation optoelectronic materials. QDs are generally obtained through either epitaxial or colloidal growth and carry the promise for solution‐processed high‐performance optoelectronic devices such as light‐emitting diodes (LEDs), solar cells, etc. Herein, a straightforward approach to synthesize perovskite QDs and demonstrate their applications in efficient LEDs is reported. The perovskite QDs with controllable crystal sizes and properties are in situ synthesized through one‐step spin‐coating from perovskite precursor solutions followed by thermal annealing. These perovskite QDs feature size‐dependent quantum confinement effect (with readily tunable emissions) and radiative monomolecular recombination. Despite the substantial structural inhomogeneity, the in situ generated perovskite QDs films emit narrow‐bandwidth emission and high color stability due to efficient energy transfer between nanostructures that sweeps away the unfavorable disorder effects. Based on these materials, efficient LEDs with external quantum efficiencies up to 11.0% are realized. This makes the technologically appealing in situ approach promising for further development of state‐of‐the‐art LED systems and other optoelectronic devices.     

Molecule‐Induced p‐Doping in Perovskite Nanocrystals Enables Efficient Color‐Saturated Red Light‐Emitting Diodes

Color‐saturated red light‐emitting diodes (LEDs) with emission wavelengths at around 620–640 nm are an essential part of high‐definition displays. Metal halide perovskites with very narrow emission linewidth are promising emitters, and rapid progress has been made in perovskite‐based LEDs (PeLEDs); however, the efficiency of the current color—pure red PeLEDs—still far lags behind those of other‐colored ones. Here, a simple but efficient strategy is reported to gradually down‐shift the Fermi level of perovskite nanocrystals (NCs) by controlling the interaction between NCs and their surface molecular electron acceptor—benzyl iodide with aromatic rings—and realize p‐doping in the color‐saturated 625 nm emitting NCs, which significantly reduces the hole injection barrier in devices. Besides, both the luminescence efficiency and electric conductivity of perovskite NCs are enhanced as additional advantages as the result of surface defects passivation. As a result, the external quantum efficiency for the resulting LED is increased from 4.5% to 12.9% after benzyl iodide treatment, making this device the best‐performing color‐saturated red PeLED so far. It is further found that the hole injection plays a more critical role than the photoluminescence efficiency of perovskite emitter in determining the LED performance, which implies design principles for efficient thin‐film planar LEDs.     

Highly Efficient and Bright Inverted Top‐Emitting InP Quantum Dot Light‐Emitting Diodes Introducing a Hole‐Suppressing Interlayer

InP quantum dots (QDs) based light‐emitting diodes (QLEDs) are considered as one of the most promising candidates as a substitute for the environmentally toxic Cd‐based QLEDs for future displays. However, the device architecture of InP QLEDs is almost the same as the Cd‐based QLEDs even though the properties of Cd‐based and InP‐based QDs are quite different in their energy levels and shapes. Thus, it is highly required to develop a proper device structure for InP‐based QLEDs to improve the efficiency and stability. In this work, efficient, bright, and stable InP/ZnSeS QLEDs based on an inverted top emission QLED (ITQLED) structure by newly introducing a “hole‐suppressing interlayer” are demonstrated. The green‐emitting ITQLEDs with the hole‐suppressing interlayer exhibit a maximum current efficiency of 15.1–21.6 cd A^?1 and the maximum luminance of 17 400–38 800 cd m^?2, which outperform the recently reported InP‐based QLEDs. The operational lifetime is also increased when the hole‐suppressing interlayer is adopted. These superb QLED performances originate not only from the enhanced light‐outcoupling by the top emission structure but also from the improved electron–hole balance by introducing a hole‐suppressing interlayer which can control the hole injection into QDs.     

Extremely Vivid,Highly Transparent,and Ultrathin Quantum Dot Light‐Emitting Diodes

Displaying information on transparent screens offers new opportunities in next‐generation electronics, such as augmented reality devices, smart surgical glasses, and smart windows. Outstanding luminance and transparency are essential for such “see‐through” displays to show vivid images over clear background view. Here transparent quantum dot light‐emitting diodes (Tr‐QLEDs) are reported with high brightness (bottom: ≈43 000 cd m^?2, top: ≈30 000 cd m^?2, total: ≈73 000 cd m^?2 at 9 V), excellent transmittance (90% at 550 nm, 84% over visible range), and an ultrathin form factor (≈2.7 µm thickness). These superb characteristics are accomplished by novel electron transport layers (ETLs) and engineered quantum dots (QDs). The ETLs, ZnO nanoparticle assemblies with ultrathin alumina overlayers, dramatically enhance durability of active layers, and balance electron/hole injection into QDs, which prevents nonradiative recombination processes. In addition, the QD structure is further optimized to fully exploit the device architecture. The ultrathin nature of Tr‐QLEDs allows their conformal integration on various shaped objects. Finally, the high resolution patterning of red, green, and blue Tr‐QLEDs (513 pixels in.^?1) shows the potential of the full‐color transparent display.     

Aqueous Phase Exfoliating Quasi‐2D CsPbBr3 Nanosheets with Ultrahigh Intrinsic Water Stability

All‐inorganic cesium lead halide perovskite nanocrystals (NCs) have emerged as attractive optoelectronic materials due to the excellent optical and electronic properties. However, their environmental stability, especially in the presence of water, is still a significant challenge for their further commercialization. Here, ultrahigh intrinsically water‐stable all‐inorganic quasi‐2D CsPbBr₃ nanosheets (NSs) via aqueous phase exfoliation method are reported. Compared to conventional perovskite NCs, these unique quasi‐2D CsPbBr₃ nanosheets present an outstanding long‐term water stability with 87% photoluminescence (PL) intensity remaining after 168 h under water conditions. Moreover, the photoluminescence quantum yields (PLQY) of quasi‐2D CsPbBr₃ NSs is up to 82.3%, and these quasi‐2D CsPbBr₃ NSs also present good photostability of keeping 85% PL intensity after 2 h under 365 nm UV light. Evidently, such quasi‐2D perovskite NSs will open up a new way to investigate the intrinsic stability of all‐inorganic perovskites and further promote the commercial development of perovskite‐based optoelectronic and photovoltaic 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.     

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.     

Ultrastable CsPbBr3 Perovskite Quantum Dot and Their Enhanced Amplified Spontaneous Emission by Surface Ligand Modification

The poor stability and aggregation problem of CsPbBr₃ quantum dots (QDs) in air are great challenges for their future practical application. Herein, a simple and effective ligand‐modification strategy is proposed by introducing 2‐hexyldecanoic acid (DA) with two short branched chains to replace oleic acid (OA) with long chains during the synthesis process. These two short branched chains not only maintain their colloidal stability but also contribute to efficient radiative recombination. The calculations show that CsPbBr₃ QDs with DA modification (CsPbBr₃‐DA QDs) have larger binding energy than CsPbBr₃ QDs with OA (CsPbBr₃‐OA QDs), resulting in significantly enhanced stability. Due to the strong binding energy between DA ligands and QDs, CsPbBr₃‐DA QDs exhibit no aggregation phenomenon even after stored in air for more than 70 d, and CsPbBr₃‐DA QDs films can maintain 94.3% of initial PL intensity after 28 d, while in CsPbBr₃‐OA QDs films occurs a rapid degradation of PL intensity. Besides, the enhanced amplified spontaneous emission (ASE) performance of CsPbBr₃‐DA QDs films has been demonstrated under both one‐ and two‐photon laser excitation. The ASE threshold of CsPbBr₃‐DA QDs films is reduced by more than 50% and their ASE photostability is also improved, in comparison to CsPbBr₃‐OA QDs films.