Beyond a Linker: The Role of Photochemistry of Crosslinkers in the Direct Optical Patterning of Colloidal Nanocrystals

Nondestructive Direct Patterning of Both Hole Transport and Emissive Layers for Pixelated Quantum-Dot Light-Emitting Diodes

Considering the increasing demand for high-resolution light-emitting diodes (LEDs), it is important that direct fine patterning technologies for LEDs be developed, especially for quantum-dot LEDs (QLEDs). Traditionally, the patterning of QLEDs relies on resin-based photolithography techniques, requiring multiple steps and causing performance deterioration. Nondestructive direct patterning may provide an easy and stepwise method to achieve fine-pixelated units in QLEDs. In this study, two isomeric tridentate cross-linkers (X8/X9) are presented and can be blended into the hole transport layer (HTL) and the emissive layer (EML) of QLEDs. Because of their photosensitivity, the in situ cross-linking process can be efficiently triggered by ultraviolet irradiation, affording high solvent resistance and nondestructive direct patterning of the layers. Red QLEDs using the cross-linked HTL demonstrate an impressive external quantum efficiency of up to 22.45%. Through lithographic patterning enabled by X9, line patterns of HTL and EML films exhibit widths as narrow as 2 and 4 μm, respectively. Leveraging the patterned HTL and EML, we show the successful fabrication of pixelated QLED devices with an area size of 3 × 3 mm2, alongside the successful production of dual-color pixelated QLED devices. These findings showcase the promising potential of direct patterning facilitated by engineered cross-linkers for the cost-effective fabrication of pixelated QLED displays.

Recent Progress of Quantum Dots Light-Emitting Diodes: Materials, Device Structures, and Display Applications

Colloidal quantum dots (QDs), as a class of 0D semiconductor materials, have generated widespread interest due to their adjustable band gap, exceptional color purity, near-unity quantum yield, and solution-processability. With decades of dedicated research, the potential applications of quantum dots have garnered significant recognition in both the academic and industrial communities. Furthermore, the related quantum dot light-emitting diodes (QLEDs) stand out as one of the most promising contenders for the next-generation display technologies. Although QD-based color conversion films are applied to improve the color gamut of existing display technologies, the broader application of QLED devices remains in its nascent stages, facing many challenges on the path to commercialization. This review encapsulates the historical discovery and subsequent research advancements in QD materials and their synthesis methods. Additionally, the working mechanisms and architectural design of QLED prototype devices are discussed. Furthermore, the review surveys the latest advancements of QLED devices within the display industry. The narrative concludes with an examination of the challenges and perspectives of QLED technology in the foreseeable future.

Photothermal Synergic Cross-Linking Hole Transport Layer for Highly Efficient RGB QLEDs

Developing an insoluble cross-linkable hole transport layer (HTL) plays an important role for solution-processed quantum dots light-emitting diodes (QLEDs) to fabricate a multilayer device with separated quantum dots layers and HTLs. In this work, a facile photothermal synergic cross-linking strategy is simultaneous annealing and UV irradiation to form the high-quality cross-linked film as the HTL without any photoinitiator, which efficiently reduces the cross-linking temperature to the low temperature of 130 °C and enhances the hole mobility of the 3-vinyl-9-{4-[4-(3-vinylcarbazol-9-yl)phenyl]phenyl}carbazole (CBP-V) thin films. The obtained high-quality cross-linked CBP-V films exhibited smooth morphology, excellent solvent resistance, and high mobility. Moreover, the high-performance red, green, and blue (RGB) QLEDs are successfully fabricated by using the photothermal synergic cross-linked HTLs, which achieved the maximum external quantum efficiency of 25.69, 24.42, and 16.51%, respectively. This work presents a strategy of using the photothermal synergic cross-linked HTLs for fabrication of high-performance QLEDs and advancing their related device applications.

Crosslinking-induced patterning of MOFs by direct photo- and electron-beam lithography

Metal-organic frameworks (MOFs) with diverse chemistry, structures, and properties have emerged as appealing materials for miniaturized solid-state devices. The incorporation of MOF films in these devices, such as the integrated microelectronics and nanophotonics, requires robust patterning methods. However, existing MOF patterning methods suffer from some combinations of limited material adaptability, compromised patterning resolution and scalability, and degraded properties. Here we report a universal, crosslinking-induced patterning approach for various MOFs, termed as CLIP-MOF. Via resist-free, direct photo- and electron-beam (e-beam) lithography, the ligand crosslinking chemistry leads to drastically reduced solubility of colloidal MOFs, permitting selective removal of unexposed MOF films with developer solvents. This enables scalable, micro-/nanoscale (≈70 nm resolution), and multimaterial patterning of MOFs on large-area, rigid or flexible substrates. Patterned MOF films preserve their crystallinity, porosity, and other properties tailored for targeted applications, such as diffractive gas sensors and electrochromic pixels. The combined features of CLIP-MOF create more possibilities in the system-level integration of MOFs in various electronic, photonic, and biomedical devices.

Nondestructive Direct Optical Patterning of Perovskite Nanocrystals with Carbene-Based Ligand Cross-Linkers

Microscale patterning of colloidal perovskite nanocrystals (NCs) is essential for their integration in advanced device platforms, such as high-definition displays. However, perovskite NCs usually show degraded optical and/or electrical properties after patterning with existing approaches, posing a critical challenge for their optoelectronic applications. Here we achieve nondestructive, direct optical patterning of perovskite NCs with rationally designed carbene-based cross-linkers and demonstrate their applications in high-performance light-emitting diodes. We reveal that both the photochemical properties and the electronic structures of cross-linkers need to be carefully tailored to the material properties of perovskite NCs. This method produces high-resolution (∼4000 ppi) NC patterns with preserved photoluminescent quantum efficiencies and charge transport properties. Prototype light-emitting diodes with patterned/cross-linked NC layers show a maximum luminance of over 60000 cd m-2 and a peak external quantum efficiency of 16%, among the highest for patterned perovskite electroluminescent devices. Such a material-adapted patterning method enabled by designs from a photochemistry perspective could foster the applications of perovskite NCs in system-level electronic and optoelectronic devices.

Safety Evaluation of a Prototypical Diazirine-Based Covalent Crosslinker and Molecular Adhesive

bis-Diazirine reagents are increasingly being used as polymer crosslinkers, adhesives, and photopatterning agents in the materials sciences literature, but little effort has been made thus far to document their chemical safety profile. Here, we describe the results of a detailed toxicity assessment of a representative bis-diazirine. Safety was evaluated by a series of in vitro assays, which found the product to be non-mutagenic in bacterial tester strains TA98 and TA100, non-corrosive and non-irritating to skin, and requiring no classification for eye irritation or serious damage. While in vitro tests do not capture the integrated whole animal system, and thus cannot completely rule out the possibility of adverse responses, the results of this study suggest a desirable safety profile for bis-diazirine reagents and provide a solid foundation upon which to add in vivo assessment of safety risk and dose-response studies.

Network of Inorganic Nanocrystals Can Swell: Study of Swelling-Induced Surface Instability

A unique organic-inorganic hybrid network composed of inorganic nanocores (ranging from semiconductors to metallic ones) interconnected through organic molecules can be produced by crosslinking the organic ligands of colloidal inorganic nanocrystals in assemblies. This work reports that this network, which is conventionally considered an inorganic film, can swell when exposed to a solvent because of the interaction between the solvent and the organic linkage within the network. Intriguingly, this work discovers that drying the solvent of the swollen organic-inorganic hybrid network can significantly affect the morphology owing to the swelling-induced compress stress, which is widely observed in various organic network systems. This work studies the surface instability of crosslinked organic-inorganic hybrid networks swollen by various organic solvents, which led to buckling delamination. Specifically, this work investigates the effects of the i) solvent-network interaction, ii) crosslinking density of the network, and iii) thickness of the film on the delamination behavior of the crosslinked network.

Ligand-Nondestructive Direct Photolithography Assisted by Semiconductor Polymer Cross-Linking for High-Resolution Quantum Dot Light-Emitting Diodes

The photolithographic patterning of fine quantum dot (QD) films is of great significance for the construction of QD optoelectronic device arrays. However, the photolithography methods reported so far either introduce insulating photoresist or manipulate the surface ligands of QDs, each of which has negative effects on device performance. Here, we report a direct photolithography strategy without photoresist and without engineering the QD surface ligands. Through cross-linking of the surrounding semiconductor polymer, QDs are spatially confined to the network frame of the polymer to form high-quality patterns. More importantly, the wrapped polymer incidentally regulates the energy levels of the emitting layer, which is conducive to improving the hole injection capacity while weakening the electron injection level, to achieve balanced injection of carriers. The patterned QD light-emitting diodes (with a pixel size of 1.5 μm) achieve a high external quantum efficiency of 16.25% and a brightness of >1.4 × 105 cd/m2. This work paves the way for efficient high-resolution QD light-emitting devices.

Patterning Quantum Dots via Photolithography: A Review

Pixelating patterns of red, green, and blue quantum dots (QDs) is a critical challenge for realizing high-end displays with bright and vivid images for virtual, augmented, and mixed reality. Since QDs must be processed from a solution, their patterning process is completely different from the conventional techniques used in the organic light-emitting diode and liquid crystal display industries. Although innovative QD patterning technologies are being developed, photopatterning based on the light-induced chemical conversion of QD films is considered one of the most promising methods for forming micrometer-scale QD patterns that satisfy the precision and fidelity required for commercialization. Moreover, the practical impact will be significant as it directly exploits mature photolithography technologies and facilities that are widely available in the semiconductor industry. This article reviews recent progress in the effort to form QD patterns via photolithography. The review begins with a general description of the photolithography process. Subsequently, different types of photolithographical methods applicable to QD patterning are introduced, followed by recent achievements using these methods in forming high-resolution QD patterns. The paper also discusses prospects for future research directions.

3D printing of inorganic nanomaterials by photochemically bonding colloidal nanocrystals

3D printing of inorganic materials with nanoscale resolution offers a different materials processing pathway to explore devices with emergent functionalities. However, existing technologies typically involve photocurable resins that reduce material purity and degrade properties. We develop a general strategy for laser direct printing of inorganic nanomaterials, as exemplified by more than 10 semiconductors, metal oxides, metals, and their mixtures. Colloidal nanocrystals are used as building blocks and photochemically bonded through their native ligands. Without resins, this bonding process produces arbitrary three-dimensional (3D) structures with a large inorganic mass fraction (~90%) and high mechanical strength. The printed materials preserve the intrinsic properties of constituent nanocrystals and create structure-dictated functionalities, such as the broadband chiroptical responses with an anisotropic factor of ~0.24 for semiconducting cadmium chalcogenide nanohelical arrays.

Stimuli-Responsive Surface Ligands for Direct Lithography of Functional Inorganic Nanomaterials

ConspectusColloidal nanocrystals (NCs) have emerged as a diverse class of materials with tunable composition, size, shape, and surface chemistry. From their facile syntheses to unique optoelectronic properties, these solution-processed nanomaterials are a promising alternative to materials grown as bulk crystals or by vapor-phase methods. However, the integration of colloidal nanomaterials in real-world devices is held back by challenges in making patterned NC films with the resolution, throughput, and cost demanded by device components and applications. Therefore, suitable approaches to pattern NCs need to be established to aid the transition from individual proof-of-concept NC devices to integrated and multiplexed technological systems.In this Account, we discuss the development of stimuli-sensitive surface ligands that enable NCs to be patterned directly with good pattern fidelity while retaining desirable properties. We focus on rationally selected ligands that enable changes in the NC dispersibility by responding to light, electron beam, and/or heat. First, we summarize the fundamental forces between colloidal NCs and discuss the principles behind NC stabilization/destabilization. These principles are applied to understanding the mechanisms of the NC dispersibility change upon stimuli-induced ligand modifications. Six ligand-based patterning mechanisms are introduced: ligand cross-linking, ligand decomposition, ligand desorption, in situ ligand exchange, ion/ligand binding, and ligand-aided increase of ionic strength. We discuss examples of stimuli-sensitive ligands that fall under each mechanism, including their chemical transformations, and address how these ligands are used to pattern either sterically or electrostatically stabilized colloidal NCs. Following that, we explain the rationale behind the exploration of different types of stimuli, as well as the advantages and disadvantages of each stimulus.We then discuss relevant figures-of-merit that should be considered when choosing a particular ligand chemistry or stimulus for patterning NCs. These figures-of-merit pertain to either the pattern quality (e.g., resolution, edge and surface roughness, layer thickness), or to the NC material quality (e.g., photo/electro-luminescence, electrical conductivity, inorganic fraction). We outline the importance of these properties and provide insights on optimizing them. Both the pattern quality and NC quality impact the performance of patterned NC devices such as field-effect transistors, light-emitting diodes, color-conversion pixels, photodetectors, and diffractive optical elements. We also give examples of proof-of-concept patterned NC devices and evaluate their performance. Finally, we provide an outlook on further expanding the chemistry of stimuli-sensitive ligands, improving the NC pattern quality, progress toward 3D printing, and other potential research directions. Ultimately, we hope that the development of a patterning toolbox for NCs will expedite their implementation in a broad range of applications.

Direct photocatalytic patterning of colloidal emissive nanomaterials

We present a universal direct photocatalytic patterning method that can completely preserve the optical properties of perovskite nanocrystals (PeNCs) and other emissive nanomaterials. Solubility change of PeNCs is achieved mainly by a photoinduced thiol-ene click reaction between specially tailored surface ligands and a dual-role photocatalytic reagent, pentaerythritol tetrakis(3-mercaptopropionate) (PTMP), where the thiol-ene reaction is enabled at a low light intensity dose (~ 30 millijoules per square centimeter) by the strong photocatalytic activity of PeNCs. The photochemical reaction mechanism was investigated using various analyses at each patterning step. The PTMP also acts as a defect passivation agent for the PeNCs and even enhances their photoluminescence quantum yield (by ~5%) and photostability. Multicolor patterns of cesium lead halide (CsPbX3)PeNCs were fabricated with high resolution (<1 micrometer). Our method is widely applicable to other classes of nanomaterials including colloidal cadmium selenide-based and indium phosphide-based quantum dots and light-emitting polymers; this generality provides a nondestructive and simple way to pattern various functional materials and devices.

Direct Optical Patterning of Quantum Dots: One Strategy, Different Chemical Processes

Patterning, stability, and dispersion of the semiconductor quantum dots (scQDs) are three issues strictly interconnected for successful device manufacturing. Recently, several authors adopted direct optical patterning (DOP) as a step forward in photolithography to position the scQDs in a selected area. However, the chemistry behind the stability, dispersion, and patterning has to be carefully integrated to obtain a functional commercial device. This review describes different chemical strategies suitable to stabilize the scQDs both at a single level and as an ensemble. Special attention is paid to those strategies compatible with direct optical patterning (DOP). With the same purpose, the scQDs' dispersion in a matrix was described in terms of the scQD surface ligands' interactions with the matrix itself. The chemical processes behind the DOP are illustrated and discussed for five different approaches, all together considering stability, dispersion, and the patterning itself of the scQDs.

Direct Photo-Patterning of Efficient and Stable Quantum Dot Light-Emitting Diodes via Light-Triggered, Carbocation-Enabled Ligand Stripping

Next generation displays based on quantum dot light-emitting diodes (QLEDs) require robust patterning methods for quantum dot layers. However, existing patterning methods mostly yield QLEDs with performance far inferior to the state-of-the-art individual devices. Here, we report a light-triggered, carbocation-enabled ligand stripping (CELS) approach to pattern QLEDs with high efficiency and stability. During CELS, photogenerated carbocations from triphenylmethyl chlorides remove native ligands of quantum dots, thereby producing patterns at microscale precision. Chloride anions passivate surface defects and endow patterned quantum dots with preserved photoluminescent quantum yields. It works for both cadmium-based and heavy-metal-free quantum dots. CELS-patterned QLEDs show remarkable external quantum efficiencies (19.1%, 17.5%, 12.0% for red, green, blue, respectively) and a long operation lifetime (T₉₅ at 1000 nits up to 8700 h). Both are among the highest for patterned QLEDs and approach the records for nonpatterned devices, which makes CELS promising for building high-performance QLED displays and related integrated devices.

A General Approach to Stabilize Nanocrystal Superlattices by Covalently Bonded Ligands

Self-assembled inorganic nanocrystal (NC) superlattices are powerful material platforms with diverse structures and emergent functionalities. However, their applications suffer from the low structural stability against solvents and other stimuli, due to the weak interparticle interactions. Existing strategies to stabilize NC superlattices typically require the design and incorporation of special ligands prior to self-assembly and are only applicable to superlattices of certain NCs, ligands, and structures. Here we report a general method to stabilize superlattices of various NC compositions and structures via strong, covalently bonded ligands. The core is the use of light-triggered, nitrene-based cross-linkers that do not interfere the self-assembly process while nonspecifically and effectively bonding the native ligands of NCs. The stabilized 2D and 3D superlattices of metal, semiconductor, and magnetic NCs retain their structures when being exposed to solvents of different polarities (from toluene to water) and show high thermal stability and mechanical rigidity. This can further stabilize binary NC superlattices, beyond those achievable in previous methods. Stabilized superlattices show robust and reproducible functionalities, for instance, when serving as reusable substrates for surface enhanced Raman spectroscopy. These results create more possibilities in exploiting the impressive library of NC superlattices in a broad scope of applications.

Direct Patterning of Colloidal Nanocrystals via Thermally Activated Ligand Chemistry

Precise patterning with microscale lateral resolution and widely tunable heights is critical for integrating colloidal nanocrystals into advanced optoelectronic and photonic platforms. However, patterning nanocrystal layers with thickness above 100 nm remains challenging for both conventional and emerging direct photopatterning methods, due to limited light penetration depths, complex mechanical and chemical incompatibilities, and others. Here, we introduce a direct patterning method based on a thermal mechanism, namely, the thermally activated ligand chemistry (or TALC) of nanocrystals. The ligand cross-linking or decomposition reactions readily occur under local thermal stimuli triggered by near-infrared lasers, affording high-resolution and nondestructive patterning of various nanocrystals under mild conditions. Patterned quantum dots fully preserve their structural and photoluminescent quantum yields. The thermal nature allows for TALC to pattern over 10 μm thick nanocrystal layers in a single step, far beyond those achievable in other direct patterning techniques, and also supports the concept of 2.5D patterning. The thermal chemistry-mediated TALC creates more possibilities in integrating nanocrystal layers in uniform arrays or complex hierarchical formats for advanced capabilities in light emission, conversion, and modulation.

Molecular Design of Diazo Compound for Carbene-Mediated Cross-Linking of Hole-Transport Polymer in QLED with Reduced Energy Barrier and Improved Charge Balance

Polymeric hole-transport materials (HTMs) have been widely used in quantum-dot light-emitting diodes (QLEDs). However, their solution processability normally causes interlayer erosion and unstable film state, leading to undesired device performance. Besides, the imbalance of hole and electron transport in QLEDs also damages the device interfaces. In this study, we designed a bis-diazo compound, X1, as carbene cross-linker for polymeric HTM. Irradiated by ultraviolet and heating, a poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl))] (TFB)/X1 blend can achieve fast "electronically clean" cross-linking with ∼100% solvent resistance. The cross-linking reduced the stacking behaviors of TFB and thus led to a lower hole-transport mobility, whereas it was a good match of electron mobility. The carbene-mediated TFB cross-linking also downshifted the HOMO level from -5.3 to -5.5 eV, delivering a smaller hole-transport energy barrier. Benefiting from these, the cross-linked QLED showed enhanced device performances over the pristine device, with EQE, power efficiency, and current efficiency being elevated by nearly 20, 15, and 83%, respectively. To the best of our knowledge, this is the first report about a bis-diazo compound based carbene cross-linker built into a polymeric HTM for a QLED with enhanced device performance.