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

Photolithographic Patterning of Organic Semiconductors for Organic Field-Effect Transistors

Over the past 25 years, three major photolithography strategies have been employed for patterning organic semiconductors in organic field-effect transistors (OFETs): modified photolithography (MPL), orthogonal photolithography (OPL), and direct photolithography (DPL). This Review examines key studies to highlight the strengths and weaknesses of each strategy. Two DPL methods, in particular, show great promise: organic azide/diazirine cross-linkers (Z-cross-linkers) and UV-cross-linkable organic semiconducting blends (X-blends). These methods not only simplify the photolithography process significantly but also enhance the chemical and physical resistance of patterned organic semiconductors against photolithographic chemicals and tandem solution-depositions. Consequently, these advancements enable the fabrication of organic integrated circuits entirely through solution processes. In conclusion, photolithography of organic semiconductors serves a dual purpose: it facilitates the patterning of active layers for OFETs and acts as an enabling technology for fabricating sophisticated and cost-effective organic integrated circuits.

Improved performance of all-solution-processed quantum dot light-emitting diodes with TFB/PVK double-hole transport layers using 1,2-dichloroethane

High-performance quantum dot light-emitting diodes (QD-LEDs) require balanced electron and hole injection into the QD emissive layer-an especially difficult task when using all-solution processes. One effective strategy for achieving this balance is to create a stepwise hole injection pathway via double-hole transport layers (D-HTLs). Poly(9-vinylcarbazole) (PVK) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine))] (TFB) are commonly employed in D-HTLs because of their favorable energy level alignment and suitable hole mobilities. However, constructing TFB/PVK D-HTLs demands careful attention to solvent orthogonality, as both polymers often dissolve in the same solvent. Thus, identifying a solvent system that enables the formation of TFB/PVK D-HTLs without damaging the TFB layer is crucial. In this study, we systematically investigate the solvent orthogonality of TFB/PVK and demonstrate high-performance QD-LEDs fabricated entirely by solution processes. We examine various PVK solvents-evaluating their polarity, solubility, and potential to damage the TFB layer-and identify 1,2-dichloroethane (1,2-DCE) as optimal for forming TFB/PVK D-HTLs with minimal damage. The resulting all-solution-processed QD-LEDs exhibit a 1.5-fold increase in external quantum efficiency compared to devices employing a single HTL. Furthermore, 1,2-DCE also proves effective in inverted QD-LED architectures, protecting the QD emissive layer during PVK deposition and demonstrating its versatility across multiple device architectures.

Optimization of the hole-injection layer for quantum dot light-emitting diodes

Quantum dot light-emitting diodes (QLEDs) are regarded as cornerstones of next-generation display technologies owing to their broad spectral tunability, high color purity, and exceptional efficiency. However, the deep valence band energy levels of quantum dots (QDs) result in a high hole-injection barrier, leading to a charge-injection imbalance and limiting the device performance. This review systematically summarizes the optimization strategies for the hole-injection layer (HIL) in QLEDs, focusing on the design and application of organic single-layer HILs (e.g., PEDOT : PSS), inorganic single-layer HILs (e.g., MoO3, NiOx, and V2O5), dual HIL structures (e.g., PEDOT : PSS/metal oxide), and doped HILs (e.g., metal-ion doping and organic-inorganic hybridization). Studies have demonstrated that dual HILs reduce the hole-injection barrier through stepped energy levels, doping strategies enhance the carrier mobility and interfacial stability, and metal oxide HILs exhibit superior thermal stability and environmental adaptability. Additionally, post-treatment processes such as rapid thermal annealing (RTA) can further optimize the interfacial properties. Although QLEDs possess immense potential in display and lighting applications, challenges remain in addressing the insufficient efficiency of cadmium-free blue QLEDs and the interfacial strain mismatch in flexible devices. This review provides a comprehensive reference for the rational design of HILs and outlines future directions for developing high-efficiency, stable, and scalable QLEDs.

Ligand-Engineered Direct Optical Lithography of Nanocrystals with Industrially Compatible Solvents

Nondestructive and precise patterning of colloidal semiconductor nanocrystals (NCs) is critical in the fabrication of solution-processable optoelectronic devices. Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is widely used for the high-resolution patterning of NCs. However, conventional DOLFIN chemistry relies on solvents incompatible with mainstream industrial lithography processes, which impedes DOLFN's widespread adoption as a universal technology for real-world additive manufacturing. In this work, we proposed specific criteria for ligand design and designed a series of multifunctional ligands combining methacrylate and carboxyl groups. Such ligands allowed us to colloidally stabilize and optically pattern NCs with i-line and h-line light sources by using industrially friendly solvents. We showed that single-color and multicolor patterns with a spatial resolution of 1 μm can be achieved without compromising the optical and optoelectronic properties. The patterned NC films showed photoluminescence (PL) and electroluminescence (EL) on par with those of unpatterned films. The red-emitting QLEDs showed a peak external quantum efficiency (EQE) of 22.0%. The ability to reliably pattern bright NCs from PGMEA will facilitate the adoption of DOLFIN as an industrialized system-level integration platform and will significantly impact the production of high-resolution, full-color QLED devices.

Photo-Patternable and Healable Polymer Semiconductor Enabled by Dynamic Covalent Disulfide Bonding

Apart from charge transport property, polymer semiconductors with patternable and healable functions are highly demanding for the fabrication of organic circuits. Herein, by leveraging the dynamic covalent disulfide bonding of thioctic acid (TA) groups, we successfully integrate photo-patterning and thermal-healing capabilities into a single diketopyrrolopyrrole (DPP)-based polymer semiconductor for the first time. The results show that the thin film of DPP-based conjugated donor-acceptor polymer with TA groups in the side chains exhibits excellent photo-patterning capability under 365 nm UV light irradiation, with sensitivity (S) of 210 mJ⋅cm-2 and contrast (γ) of 1.2. Importantly, the patterning process has minimal impact on thin film morphology, interchain stacking, and charge transport mobility. Moreover, the patterned thin films, which were initially scratched, can be well healed after exposure to chloroform vapor and further thermal annealing, and simultaneously the charge mobility can be restored. In comparison, the scratched thin film of PDPP4T without TA groups in the side chains achieves only 82.6 % recovery of scratch depth and 54.5 % recovery of charge mobility under the same conditions. These results demonstrate the feasibility of constructing multifunctional polymer semiconductors by incorporating TA groups in the side chains, offering a new pathway to lithography-compatible, self-healing smart flexible devices.

Hole Delayed-Release Effect of Inorganic Interfacial Dipole Layer on Charge Balance for Boosting CsCu2I3 Light-Emitting Diodes

CsCu2I3 light-emitting diodes (LEDs) have attracted tremendous interest due to their environmental friendliness, low-cost processing, and broadband luminescence properties. However, their hole injection efficiency usually towers over electron injection efficiency owing to the intrinsic Cu vacancy-mediated excess hole concentration and wide bandgap of CsCu2I3, generating severe charge imbalance. Here, we designed an inorganic interfacial dipole layer (IDL) based on lithium chloride to exert a hole delayed-release effect for charge balance in CsCu2I3 LEDs by selectively modulating the energy levels of the contact layers. Accordingly, the IDL-based CsCu2I3 LEDs exhibit a record champion brightness of 2620 cd/m2 along with a three times enhanced peak external quantum efficiency of 3.73% at 565 nm. Meanwhile, the universality of the hole delayed-release effect of inorganic IDLs is verified by demonstrating enhanced CsCu2I3 LEDs incorporated with other alkali metal salt-based IDLs. This work provides a comprehensive guideline for optimizing the charge transport of lead-free LEDs by charge delayed-release effect of IDLs toward next-generation eco-friendly display applications.

Photothermally Cross-Linkable Polymeric Hole Transport Material Functionalized with Azide for High-Performance Quantum Dot Light-Emitting Diodes

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl)))] (TFB) is a widely used hole transport material (HTM) in quantum dot light-emitting diodes (QLEDs). However, TFB-based solution-processed QLEDs face several challenges, including interlayer erosion, low hole mobility, shallow energy level of the highest occupied molecular orbital, and current leakage, which compromise the device efficiency and stability. To overcome these challenges, bromine and azide-based photothermally cross-linkable TFB derivatives, i.e., TFB-Br and TFB-N3, were designed and synthesized. TFB-N3 photothermally cross-linked under 254 nm ultraviolet light at 140 °C exhibited excellent solvent resistance within 30 s. Furthermore, the photothermally cross-linked TFB-N3 formed a compact three-dimensional (3D) network in QLEDs, enhancing hole transport and reducing the leakage current. Moreover, the HOMO energy level in photothermally cross-linked TFB-N3 decreased to -5.39 eV from that in TFB (-5.30 eV), reducing the hole transport energy barrier. Thus, the charge balance in the quantum dot (QD) layer was enhanced, and the current leakage was reduced, improving the overall QLED performance. The photothermally cross-linked TFB-N3-based QLEDs achieved a maximum external quantum efficiency of 19.53%, i.e., 61% higher than that of devices using TFB. Moreover, the T90 lifetime of the photothermally cross-linked TFB-N3-based QLEDs was 4.49 times longer than that of TFB-based devices. The proposed strategy demonstrates that incorporating azide groups into polymeric HTMs can considerably enhance their hole transport and solvent resistance and reduce leakage current, improving QLED efficiency and stability.

Self-Assembled Monolayer-Functionalized NiO Hole Injection layer for Improved Charge Injection in Quantum Dot Light-Emitting Diodes

The development of quantum dot light-emitting diodes (QLEDs) represents a promising advancement in next-generation display technology. However, there are challenges, especially in achieving efficient hole injection, maintaining charge balance, and replacing low-stability organic materials such as PEDOT:PSS. To address these issues, in this study, self-assembled monolayers (SAMs) were employed to modify the surface properties of NiO, a hole injection material, within the structure of ITO/HIL/TFB/QDs/ZnMgO/Al QLEDs. Specifically, using Br-2PACz-based SAMs resulted in surface defect passivation, improved hole injection, reduced exciton quenching, and enhanced electrical characteristics. Notably, QLEDs based on (NiO+Br-2PACz) demonstrated a turn-on voltage of 2.4 V, a maximum external quantum efficiency (EQE) of 8.30%, a maximum luminance of 88,831 cd/m2, and a maximum current efficiency of 32.78 cd/A. Compared to NiO-based QLEDs, these results represent a reduction in turn-on voltage by approximately 1.5 V, a 1.99-fold increase in EQE, and a 3.63-fold increase in luminance, indicating significantly enhanced performance with notable improvements in turn-on voltage, EQE, and luminance. They also showed higher EQE and luminance than PEDOT:PSS-based QLEDs; this could be attributed to the downshifting of energy levels by Br-2PACz, which reduced the hole injection barrier, increased the conductivity, and improved charge balance. In particular, the reduction in exciton quenching and the increase in electrical conductivity contributed significantly to the overall performance enhancement of the (NiO+Br-2PACz)-based QLEDs. This paper proposes a simple method for inorganic hole injection layer functionalize and application.

Conjugated Polymer-Based Photo-Crosslinker for Efficient Photo-Patterning of Polymer Semiconductors

Photo-patterning of polymer semiconductors using photo-crosslinkers has shown potential for organic circuit fabrication via solution processing techniques. However, the performance of patterning, including resolution (R), UV light exposure dose, sensitivity (S), and contrast (γ), remains unsatisfactory. In this study, a novel conjugated polymer based photo-crosslinker (PN3, Figure 1a) is reported for the first time, which entails phenyl-substituted azide groups in its side chains. Due to the potential π-π interactions between the conjugated backbone of PN3 and those of polymer semiconductors, PN3 exhibits superior miscibility with polymer semiconductors compared to the commonly used small molecule photo-crosslinker 4Bx (Figure 1a). Consequently, photo-patterning of polymer semiconductors with PN3 demonstrates improved performance with much lower UV light exposure dose, higher S and higher γ compared to 4Bx. By utilizing electron beam lithography, patterned arrays of polymer semiconductors with resolutions down to 500 nm and clearer edges are successfully fabricated using PN3. Furthermore, patterned arrays of PDPP4T, the p-type semiconductor (Figure 1b), after being doped, can function as source-drain electrodes for fabricating field-effect transistors (FETs) with comparable charge mobility and significantly lower sub-threshold swing value compared to those with gold electrodes.

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.

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.

Ultralow Roll-Off Quantum Dot Light-Emitting Diodes Using Engineered Carrier Injection Layer

Quantum dot (QD) light-emitting diodes (QLEDs) have attracted extensive attention due to their high color purity, solution-processability, and high brightness. Due to extensive efforts, the external quantum efficiency (EQE) of QLEDs has approached the theoretical limit. However, because of the efficiency roll-off, the high EQE can only be achieved at relatively low luminance, hindering their application in high-brightness devices such as near-to-eye displays and lighting applications. Here, this article reports an ultralow roll-off QLED that is achieved by simultaneously blocking electron leakage and enhancing the hole injection, thereby shifting the recombination zone back to the emitting QDs layer. These devices maintain EQE over 20.6% up to 1000 mA cm-2 current density, dropping only by ≈5% from the peak EQE of 21.6%, which is the highest value ever reported for the bottom-emitting red QLEDs. Furthermore, the maximum luminance of the optimal device reaches 320 000 cd m-2 , 2.7 times higher than the control device (Lmax : 128 000 cd m-2 ). A passive matrix (PM) QLED display panel with high brightness based on the optimized device structure is also demonstrated. The proposed approach advances the potential of QLEDs to operate efficiently in high-brightness scenarios.

Highly Efficient ITO-Free Quantum-Dot Light Emitting Diodes via Solution-Processed PEDOT:PSS Semitransparent Electrode

We present a study on the potential use of sulfuric acid-treated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a viable alternative to indium tin oxide (ITO) electrodes in quantum dot light-emitting diodes (QLEDs). ITO, despite its high conductivity and transparency, is known for its disadvantages of being brittle, fragile, and expensive. Furthermore, due to the high hole injection barrier of quantum dots, the need for electrodes with a higher work function is becoming more significant. In this report, we present solution-processed, sulfuric acid-treated PEDOT:PSS electrodes for highly efficient QLEDs. The high work function of the PEDOT:PSS electrodes improved the performance of the QLEDs by facilitating hole injection. We demonstrated the recrystallization and conductivity enhancement of PEDOT:PSS upon sulfuric acid treatment using X-ray photoelectron spectroscopy and Hall measurement. Ultraviolet photoelectron spectroscopy (UPS) analysis of QLEDs showed that sulfuric acid-treated PEDOT:PSS exhibited a higher work function than ITO. The maximum current efficiency and external quantum efficiency based on the PEDOT:PSS electrode QLEDs were measured as 46.53 cd/A and 11.01%, which were three times greater than ITO electrode QLEDs. These findings suggest that PEDOT:PSS can serve as a promising replacement for ITO electrodes in the development of ITO-free QLED devices.