Short-chain Ligands Research Articles

Facile Synthesis of Novel Short-Chain Ligand-Capped Colloidal Metal Oxide Nanoparticles for Printed Flexible Devices.

Colloidal metal oxide nanoparticles are key materials for achieving cost-effective and large-scale production of flexible devices, as they enable the formation of functional oxide thin films at low temperatures (<400 °C) through printing techniques such as inkjet printing, gravure coating, and microcontact printing. The conventional solvothermal synthesis of colloidal metal oxide nanoparticles through the thermal decomposition of precursors results in particles with bulky, long-chain ligands on their surfaces, which hinder the formation of dense oxide films when depositing the colloidal metal oxide nanoparticles. Herein, we have developed a simple and versatile method for synthesizing colloidal metal oxide nanoparticles using base-induced hydrolysis and the condensation of metal acetates as precursors. Various binary and ternary colloidal metal oxide nanoparticles (CuO, Mn3O4, Co3O4, CeO2, In2O3, Co1.8Mn1.2O4) were synthesized using short-chain acetate ligands on their surfaces. The thin acetate ligand-containing colloidal Co1.8Mn1.2O4 nanoparticle film exhibited lower resistivity than the same with long-chain oleate ligands. The films coated onto a polyimide substrate formed a flexible negative temperature coefficient thermistor that exhibited the temperature dependence of resistance comparable to bulk materials with a bending durability of up to 5 mm radius. These findings highlight the effectiveness of utilizing colloidal metal oxide nanoparticles with short-chain ligands in flexible devices.

Dion-Jacobson-Phase 2D Sn-Based Perovskite Comprising a High Dipole Moment of π-Conjugated Short-Chain Organic Spacers for High-Performance Solar Cell Applications.

The stability issue of Sn-based perovskite solar cells (PSCs) is expected to be resolved by involving a two-dimensional (2D) layered structure. However, Sn-based 2D PSCs, especially Dion-Jacobson (DJ)-phase ones with potentially good stability, have rarely been reported. Herein, superior DJ-phase Sn 2D perovskites with 3-aminobenzylamine (3ABA2+) or 4-aminobenzylamine (4ABA2+) π-conjugated short-chain ligands are reported to fabricate efficient 2D lead-free PSCs. Notably, the high dipole moment of the 3ABAI2 organic spacer is approved to possess faster charge transfer for forming (3ABA)FA4Sn5I16 2D perovskite with an extremely low exciton binding energy (only 84 meV). In combination with a diacetate partial substitution and methylamine iodide/bromide (MAI/MABr) post-treatment strategy to delay crystallization and improve compactness and coverage of the perovskite film, a record power conversion efficiency (PCE) of 6.81% and stability of 840 h (less than 5% degradation in a N2 atmosphere for unencapsulated devices) are acquired in eventual (3ABA)FA4Sn5I16 2D PSCs, which are among the highest PCE and the longest stability of Sn-based 2D PSCs reported to date. Our work provides a prospective molecule design and film preparation strategy of 2D Sn perovskites toward nontoxic high-performance tin-based PSCs, which pushes the almost stagnant research forward.

Orientation-Dependent Photoconductivity of Quasi-2D Nanocrystal Self-Assemblies: Face-Down, Edge-Up Versus Randomly Oriented Quantum Wells.

Here, strongly orientation-dependent lateral photoconductivity of a CdSe monolayer colloidal quantum wells (CQWs) possessing short-chain ligands is reported. A controlled liquid-air self-assembly technique is utilized to deliberately engineer the alignments of CQWs into either face-down (FO) or edge-up (EO) orientation on the substrate as opposed to randomly oriented (RO) CQWs prepared by spin-coating. Adapting planar configuration metal-semiconductor-metal (MSM) photodetectors, it is found that lateral conductivity spans ≈2 orders of magnitude depending on the orientation of CQWs in the film in the case of utilizing short ligands. The long native ligands of oleic acid (OA) are exchanged with short-chain ligands of 2-ethylhexane-1-thiol (EHT) to reduce the inter-platelet distance, which significantly improved the photoresponsivity from 4.16, 0.58, and 4.79mAW-1 to 528.7, 6.17, and 94.2mAW-1, for the MSM devices prepared with RO, FO, and EO, before and after ligands exchange, respectively. Such CQW orientation control profoundly impacts the photodetector performance also in terms of the detection speed (0.061 s/0.074 s for the FO, 0.048 s/0.060 s for the EO compared to 0.10 s/0.16 s for the RO, for the rise and decay time constants, respectively) and the detectivity (1.7×1010, 2.3×1011, and 7.5×1011 Jones for the FO, EO, and RO devices, respectively) which can be further tailored for the desired optoelectronic device applications. Attributed to charge transportation in colloidal films being proportional to the number of hopping steps, these findings indicate that the solution-processed orientation of CQWs provides the ability to tune the photoconductivity of CQWs with short ligands as another degree of freedom to exploit and engineer their absorptive devices.

Strongly-Confined CsPbBr3 Perovskite Quantum Dots with Ultralow Trap Density and Narrow Size Distribution for Efficient Pure-Blue Light-Emitting Diodes.

The development of pure-blue perovskite light-emitting diodes (PeLEDs) faces challenges of spectral stability and low external quantum efficiency (EQE) due to phase separation in mixed halide compositions. Perovskite quantum dots (QDs) with strong confinement effects are promising alternatives to achieve high-quality pure-blue PeLEDs, yet their performance is often hindered by the poor size distribution and high trap density. A strategy combining thermodynamic control with a polishing-driven ligand exchange process to produce high-quality QDs is developed. The strongly-confined pure-blue (≈470nm) CsPbBr3 QDs exhibit narrow size distribution (12% dispersion) and are achieved in Br-rich ion environment based on growth thermodynamic control. Subsequent polishing-driven ligand exchange process removes imperfect surface sites and replaces initial long-chain organic ligands with short-chain benzene ligands. The resulting QDs exhibit high photoluminescence quantum yield (PLQY) to near-unity. The resulting PeLEDs exhibit a pure-blue electroluminescence (EL) emission at 472nm with narrow full-width at half-maximum (FWHM) of 25nm, achieving a maximum EQE of 10.7% and a bright maximum luminance of 7697cdm-2. The pure-blue PeLEDs show ultrahigh spectral stability under high voltage, a low roll-off of EQE, and an operational half-lifetime (T50) of 127min at an initial luminance of 103cdm-2 under continuous operation.

Modified surface ligand management of CsPbI3 perovskite quantum dots enables efficient and stable electroluminescent solar cells

Solution-processed CsPbI3 perovskite quantum dots (PQDs) have attracted enormous attention in both photovoltaic and electroluminescent applications in recent years. However, the surface defects of CsPbI3 PQDs in the ligand exchange procedure still suffer the challenge of imperfect passivation, leading to poor device performance and ambient stability. In this work, we propose a modified surface ligand management to deposit CsPbI3 PQD films using a layer-by-layer (LBL) solid-state exchange strategy, and its applications in photovoltaic and electroluminescent devices are studied. The conjugated phenethylammonium iodide (PEAI) ligand, acting as a short-chain ligand with a phenyl group (PEA+ ion) for encapsulating the CsPbI3 PQDs during the LBL procedure, which can both realize enhanced inter-dot coupling and defects passivation compared with conventional post-treatments. We further demonstrate the balanced transport and injection of electrons and holes within the PEAI-LBL based CsPbI3 PQD films. With this approach, the CsPbI3 PQD solar cells based on PEAI-LBL yielded a champion power conversion efficiency (PCE) of up to 14.18% with a high open voltage of 1.23 V, along with obviously electroluminescent red emission properties in one device. Meanwhile, the unencapsulated devices exhibited excellent stability under a high-humidity environment. This strategy should accelerate advancement in utilizing PQDs for bifunctional optoelectronic applications.

Co-regulation strategy dominated by double short molecules permitting the regrowth of quantum dots for efficient deep-blue perovskite light-emitting diodes

Recently, blue perovskite quantum dots (PeQDs) have garnered considerable attention for use in full-color optoelectronic displays. However, the large specific surface area caused by the small particle size of blue PeQDs results in high defect state density, which severely limits their optical pursuit. To address this issue, a co-regulation strategy using double short-chain molecules of tetraoctylammonium fluoride and tetraethylammonium perfluorooctanesulfonate is proposed to enhance the regrowth of blue CsxRb1−xPb(ClyBr1−y)3 PeQDs. Long-chain insulated oleylamine and oleic acid ligands, which restrict the growth of PeQDs, are substantially replaced using short-chain ligands with higher adsorption energy. This replacement promotes PeQDs to regrow considerably from ∼8 to ∼18 nm, and the defect-state density decreases by 3–5 times. Consequently, highly efficient blue PeQDs with a photoluminescent quantum yield of unity are obtained. Moreover, the PeQDs exhibit lower insulating ligand density and improved charge injection property. As a result, the synthesized deep-blue PeQD light-emitting diodes (PeQLEDs) exhibit a maximum external quantum efficiency of 5.7% at the CIE coordinate of (0.148, 0.032). According to the International Telecommunication Union Radiocommunication Sector television standard (Rec. 2100), the obtained PeQLEDs are state-of-the-art among colloidal deep-blue PeQDs. This study provides a novel strategy to improve the optical performance of deep-blue PeQLEDs and paves the way for developing high-end wide color-gamut displays.

Surface defect mitigation via alkyl-ligand-controlled purification for stable and high-luminescence perovskite quantum dots.

Perovskite quantum dots (PQDs) have received considerable attention as fluorescent materials due to their excellent optical properties. However, because PQDs contain ionic bonds, they have the disadvantage of being vulnerable to environmental conditions, so improving their stability is essential. Indeed, recent research has focused on improving both the stability and luminescence of PQDs by mixing them with methyl acetate (MeOAc) to suppress surface defects via purification. MeOAc reacts with the surface ligands of PQDs, resulting in ligand-controlled purification. However, while the ligands are limited for the PQD synthesis, the effect of ligand alkyl-chain length has not been reported. Therefore, we report herein a strategy for obtaining stable PQDs with tunable performances by using amine ligands of various chain lengths. The amine ligand is selected because it is very effective in interacting with the halide vacancies present on the surface of the perovskite crystal structure. The results indicate that MeOAc becomes less effective as the chain length of the ligand is increased, and more effective as the chain length is decreased. Consequently, PQDs treated with MeOAc and a short-chain ligand afford a quantum yield (QY) of 79.2% and are highly stable when exposed to thermal and ambient conditions. Therefore, we suggest a facile approach to suppressing the degradation of PQDs during the fabrication process.

Obviating Ligand Exchange Preserves the Intact Surface of HgTe Colloidal Quantum Dots and Enhances Performance of Short Wavelength Infrared Photodetectors.

A large volume, scalable synthesis procedure of HgTe quantum dots (QDs) capped initially with short-chain conductive ligands ensures ligand exchange-free and simple device fabrication. An effective n- or p-type self-doping of HgTe QDs is achieved by varying cation-anion ratio, as well as shifting the Fermi level position by introducing single- or double-cyclic thiol ligands, that is, 2-furanmethanethiol (FMT) or 2,5-dimercapto-3,4-thiadiasole (DMTD) in the synthesis. This allows for preserving the intact surface of the HgTe QDs, thus ensuring a one order of magnitude reduced surface trap density compared with HgTe subjected to solid-state ligand exchange. The charge carrier diffusion length can be extended from 50 to 90nm when the device active area consists of a bi-layer of cation-rich HgTe QDs capped with DMTD and FMT, respectively. As a result, the responsivity under 1340nm illumination is boosted to 1 AW-1 at zero bias and up to 40 AW-1 under -1V bias at room temperature. Due to high noise current density, the specific detectivity of these photodetectors reaches up to 1010 Jones at room temperature and under an inert atmosphere. Meanwhile, high photoconductive gain ensures a rise in the external quantum efficiency of up to 1000% under reverse bias.

Thick film high-performance carbon-based CsPbIBr2 perovskite solar cells vis nano-perovskite modified hot casting method

All-inorganic carbon-based CsPbIBr2 perovskite solar cells (PSCs) have attracted attention due to their more balanced performance and stability. The performance of the PSCs is highly dependent on the quality of the absorber films. Unfortunately, most high-performance films are prepared in a glove box and conventional preparation methods often produce perovskite films within 400 nm of thickness, which is detrimental to large area device fabrication and limits their commercial application. Here we had demonstrated a short chain ligands nano-CsPbIBr2 particles modified hot-casting method to deposit a perovskite film with thickness up to 800 nm. It was carried out by spin-coating a hot CsPbIBr2 precursor solution onto nano-CsPbIBr2 particles film on an electron transport layer (ETL), and then depositing nano-CsPbIBr2 particles on the precursor film again before annealing. The nano-CsPbIBr2 particles on the ETL promotes precursor solution crystallization and increased film thickness, and the nano-CsPbIBr2 particles on the precursor film promoted grain growth and reduced defects due to compositional deviations from stoichiometric ratios. The power conversion efficiency (PCE) of the PSCs fabricated by this method is up to 10.66 %. The unencapsulated PSCs still retain more than 85 % of the initial PCE when stored at ambient atmosphere (RH 50–60 %) for 10 days.

Ligand Engineering of CsPbI3 Quantum Dots for Efficient Solar Cells

Perovskite quantum dots (PQDs) have gained significant attention due to their exceptional optical and electronic properties, rendering them promising materials for various optoelectronic applications, such as single-junction solar cells because of the multiple exciton generation effects. To improve the electrical properties of CsPbI3 PQDs, ligand engineering was explored by introducing short-chain ligands to exchange the original long-chain ligands on the PQD surface during the purification process. However, the short-chain ligands suffer from low solubility in the anti-solvent, resulting in insufficient ligand exchange. To address this issue, we developed an in-situ ligand exchange (ILE) strategy to promote the replacement of long-chain ligands with short-chain ones, leading to improved surface chemistry. In this work, 4-methoxyphenethylammonium iodide (CH3O-PEAI) was demonstrated as the short-chain ligand in the ILE strategy, thereby modifying halide vacancy trap states, and promoting carrier transportation and extraction. As a result, the ILE-CsPbI3-based solar cells achieved a higher power conversion efficiency of 14.4% with a high open circuit voltage (VOC) of 1.23 V and exceptional long-term durability due to higher hydrophobicity.

Stabilized Perovskite Quantum Dot Solids via Nonpolar Solvent Dispersible Covalent Ligands.

The ligand exchange procedure of CsPbI3 perovskite quantum dots (PQDs) enables the fabrication of thick and conductive PQD solids that act as a photovoltaic absorber for solution-processed thin-film solar cells. However, the ligand-exchanged CsPbI3 PQD solids suffer from deterioration in photovoltaic performance and ambient stability due to the surface traps, such as uncoordinated Pb2+ sites on the PQD surface, which are generated after the conventional ligand exchange process using ionic short-chain ligands dissolved in polar solvents. Herein, a facile surface stabilization is demonstrated that can simultaneously improve the photovoltaic performance and ambient stability of CsPbI3 PQD photovoltaic absorber using covalent short-chain triphenylphosphine oxide (TPPO) ligands dissolved in a nonpolar solvent. It is found that the TPPO ligand can be covalently bound to uncoordinated Pb2+ sites and the nonpolar solvent octane can completely preserve the PQD surface components. Owing to their synergetic effects, the CsPbI3 PQD photovoltaic absorber stabilized using the TPPO ligand solution dissolved in octane exhibit higher optoelectrical properties and ambient stability than the control absorber. Consequently, CsPbI3 PQD solar cells composed of PQD photovoltaic absorbers fabricated via surface stabilization strategy provide an improved power conversion efficiency of 15.4% and an enhanced device stability.

Understanding the ligand-assisted reprecipitation of CsPbBr3 nanocrystals via high-throughput robotic synthesis approach

•High-throughput automated experimental workflow for LARP-PNC synthesis is employed •Effects of chemical and processing parameters on PNC functionalities are explored •SHAP, a machine learning method, assesses the impact of synthesis parameters •Ligand diffusion during reaction determines the functionalities of PNCs Inorganic cesium lead bromide (CsPbBr3) perovskite nanocrystals (PNCs) have shown promise in optoelectronic applications. A simpler method of synthesizing high-quality PNCs is the ligand-assisted reprecipitation (LARP) method, but it is susceptible to instability. This study used a high-throughput automated experimental platform to explore the growth behaviors and colloidal stability of LARP-synthesized PNCs. The influence of ligands on particle growth and functionalities was systematically explored using two distinctive acid-base pairs. The study found that short-chain ligands cannot make functional PNCs with the desired sizes and shapes, whereas long-chain ligands provide homogeneous and stable PNCs. The study also found that excessive amines or polar antisolvents can cause PNCs to transform into a Cs-rich non-perovskite structure with poorer emission functionalities and larger size distributions. The diffusion of ligands in a reaction system is crucial in determining the structures and functionalities of the PNCs. This study provides detailed guidance on synthesis routes for desired PNCs. Inorganic cesium lead bromide (CsPbBr3) perovskite nanocrystals (PNCs) have shown promise in optoelectronic applications. A simpler method of synthesizing high-quality PNCs is the ligand-assisted reprecipitation (LARP) method, but it is susceptible to instability. This study used a high-throughput automated experimental platform to explore the growth behaviors and colloidal stability of LARP-synthesized PNCs. The influence of ligands on particle growth and functionalities was systematically explored using two distinctive acid-base pairs. The study found that short-chain ligands cannot make functional PNCs with the desired sizes and shapes, whereas long-chain ligands provide homogeneous and stable PNCs. The study also found that excessive amines or polar antisolvents can cause PNCs to transform into a Cs-rich non-perovskite structure with poorer emission functionalities and larger size distributions. The diffusion of ligands in a reaction system is crucial in determining the structures and functionalities of the PNCs. This study provides detailed guidance on synthesis routes for desired PNCs.

Overcoming the Ambient Manufacturability-Performance Bottleneck in Perovskite Nanocrystal Emitters for Efficient Light-Emitting Diodes.

Colloidal perovskite nanocrystals (NCs) have risen rapidly in luminescence efficiency and color purity. However, their high performance requires careful and complex pre-treatment of precursors and precise regulation of the reaction atmosphere; otherwise, their emission will be weak and broad. To overcome these limitations, we develop a facile ligand exchange method using a new type of bidentate ligand, which is obtained by reacting cheap sulfur with tributylphosphine (S-TBP). During ligand exchange, the double bond between P and S atoms breaks and a single bond is formed between them, after which S-TBP switches into a bidentate ligand and binds to a perovskite NC at two points. With short-chain S-TBP ligands that have high spatial position resistance, both NC spacing and surface ligand density can be reduced, thereby improving carrier injection and transport. On the NC surface after ligand exchange, halogen vacancies were substantially filled, leading to a PbSP (Pb, S, and P elements) component-dominated shell that greatly decreases trap density and enhances material stability. The resulting perovskite NCs are stable and bright with a photoluminescence quantum yield of ~96%, and an external quantumefficiency of 22%. Note that our ligand-exchange strategy remains effective even when scaling up, which should accelerate commercialization.

Polymer-doped perovskite nanocrystals for efficient single active layer white light-emitting diodes through energy transfer

White-light emitting diodes (WLEDs) are mainly constructed by stacking different light-emitting layers with complementary emission spectra. However, this not only requires complex device fabrication processes but also needs comprehensive charge transport regulation, limiting the development and application of WLEDs. In this work, efficient single-layer WLEDs are achieved via doping tiny amount of a red polymer poly[[9-(1-octylnonyl)-9H- carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5 thiophenediy (PCDTBT8) into the perovskite nanocrystals (NCs) matrix. The 490 nm sky blue perovskite nanocrystals (FA0.08Cs0.92PbBr2Cl) are synthesized and passivated with a short-chain ligand 3-methoxyphenethylamine (MPEA), which enhances luminance and stability at high bias. Energy transfer from NCs to PCDTBT8 significantly enhances red-light emitting in the range from 600 to 750 nm, enabling white-light emission. By tuning of the NCs:PCDTBT8 weight ratio from 400:1 to 150:1, a continuous transition of emission from cold white-light to warm white-light is achieved. With the NCs:PCDTBT8 weight ratio of 250:1, a standard white-light emission with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.33, 0.33), a luminance of 752 cd/m2 and an external quantum efficiency (EQE) of 1.59% are obtained at a fairly low voltage of 4.8 V.

Exploring the Role of Short Chain Acids as Surface Ligands in Photoinduced Charge Transfer Dynamics from CsPbBr3 Perovskite Nanocrystals

The most commonly used surface capping ligands, like oleic acid and oleylamine, passivate the surface of perovskite nanocrystals (PNCs) to enhance their stability and optical properties. However, due to their inherent insulating nature, charge transport across the surface of the PNCs is hindered, limiting their application in devices. In this study, we have post-treatment CsPbBr3 PNCs with short chain ligands benzoic acid (BA) and ascorbic acid (AA) and observed that both acid-treated PNCs show enhanced stability and optical properties. Still, BA-treated PNCs show the highest charge transport rate due to their conjugating nature. The photoelectrochemical measurements also show the most efficient electron flow across the surface of the PNC with BA-treated PNCs. A longer carrier lifetime and fast charge transfer make BA-treated PNCs ideal candidates for application in real-life devices.

Passivation of Surface Defects by X-type Short-Chain Ligands for High Quantum Yield and Stable CsPbX3 Quantum Dots Synthesis and Application in WLEDs

All-inorganic CsPbX3 perovskite quantum dots have excellent optoelectronic properties and are excellent candidates for optoelectronic applications. However, the surface ligands (oleyl ammonium, oleic acid) commonly used in traditional inorganic perovskite quantum dots easily fall off from the quantum dots and their protective power is relatively weak, resulting in poor quantum yield and stability of the quantum dots, limiting their application. In this paper, we introduce an X-type short-chain ligand (DDEAB) passivation strategy to enhance the optoelectronic properties of CsPbX3 quantum dots through a facile room-temperature synthesis method and the strong affinity of X-type ligands for surface atoms. The results of TRPL and X-ray photoelectron spectroscopy showed longer lifetime and stronger surface affinity. This result confirms that didecyldimethylammonium bromide (DDEAB) has better binding to CsPbBr3 QDs and can effectively reduce surface defects and improve quantum yield. As a result, the introduction of short-chain ligands can more effectively passivate the surface halogen defects of CsPbX3 QDs, achieving high quantum yield (>90%) and long-term storage environmental stability. In addition, DDEAB-passivated CsPbBr3 QDs have been successfully applied in white light emitting diodes (WLEDs) with wider color gamut (123%), and good EL stability compared to NTSC TV color standard. Our proposed short-chain ligand DDEAB passivation strategy may provide a reference guideline for the preparation of quantum dots and photovoltaic devices with high photoluminescence and environmental stability.

Planar Perovskite Solar Cells Using Perovskite CsPbI3 Quantum Dots as Efficient Hole Transporting Layers.

Perovskite CsPbI3 quantum dots (QDs) were synthesized as a hole-transporting layer (HTL) of a planar perovskite solar cell (PSC). By using the Octam solution during the ligand engineering, CsPbI3 QDs exhibits a denser grain and a larger grain size due to the short-chain ligands of Octam. In addition, CsPbI3 QDs with the Octam solution showed a smooth and uniform surface on MAPbI3 film, indicating the QDs improved the microstructure of the MAPbI3 perovskite film. As a result, the PSC with CsPbI3 QDs as an HTL has the optimal open-circuit voltage as 1.09 V, the short-circuit current as 20.5 mA/cm2, and the fill factor (FF) as 75.7%, and the power conversion efficiency (PCE) as 17.0%. Hence, it is inferred that introducing QDs as a HTL via the ligand engineering can effectively improve the device performance of the PSC.

Mixed halide perovskite nanocrystals with surface engineering based on pseudohalide passivation and Short-Chain ligand exchange for High-Performance blue Light-Emitting diodes

Perovskite nanocrystals (PeNCs) have attracted considerable interest in light-emitting diodes (LEDs) owing to their unique optoelectronic properties. Although significant progress has been made in green and red perovskite LEDs (PeLEDs) with PeNCs, the performance of blue PeLEDs is still limited. The main obstacles for blue PeLEDs based on PeNCs are their Cl vacancy-induced low quantum efficiency in mixed halide (Br/Cl) systems and high electrical resistance from long organic capping ligands. In this study, to overcome these issues and achieve high-performance blue PeLEDs, a surface engineering (SE) strategy based on pseudohalide defect passivation and short-chain ligand exchange was introduced. By passivating Cl vacancies with SCN– and exchanging long-chain oleic acid and oleylamine ligands with short octylphosphonic acid and 3,3-diphenylpropyl amine, respectively, the optoelectronic properties of the PeNCs were significantly enhanced. The blue PeLEDs based on PeNCs, using SE, exhibited a maximum luminance of 1874 cd/m2 and maximum external quantum efficiency of 4.9 % at an electroluminescence wavelength of 467 nm, which satisfies the blue primary color requirement (467 nm) in Rec. 2020 color space.

Effects of the surface ligands of quantum dots on the intaglio transfer printing process

High-resolution semiconductor nanocrystal quantum dot (QD) patterns are required for applications in display devices. For this, the dry transfer printing of QDs is promising because it does not degrade the inherent properties of QDs. However, the effect of the surface ligands on this process remains poorly understood, despite its importance. Herein, we investigate the effect of the surface ligands on the intaglio transfer printing process. Colloidal QDs with organic (C8–C18) or inorganic (I−) ligands are prepared. In various pattern-printing tests, including patterns with a sub-10-μm width, the patterning yield is ∼100% for QDs with long-chain ligands (C18). However, the patterning yield decreases with decrease in the chain length for the organic-ligand-passivated QDs and is the lowest for the QDs passivated with I−. Using surface energy characterization and finite element method simulations, we suggest two printing failure mechanisms: (i) Inorganic-ligand-passivated QDs are not effectively picked-up by the stamp because of poor adhesion, and, (ii) for the organic-ligand-passivated QDs, internal crack formation is easier for QDs having short-chain ligands because of the weak interparticle attraction between QDs. Our findings reveal previously unknown defects of the intaglio printing process and provide guidance for mitigating these problems for preparing high-resolution QD patterns.

The efficient green light-emitting diodes based on low-toxicity Zr-Pb alloy perovskite quantum dots passivated by inorganic ligand

Perovskite quantum dots (PeQDs) are emerging as one of the most promising candidates for next-generation display technology due to their outstanding optical and electrical properties. However, the inherent instability, lead-toxicity and poor electrical conductivity of PeQDs films have seriously hindered their commercial applications. To solve these problems, we adopted the strategy of Zr ions doping and in-situ passivation of CsPbBr3 PeQDs with the short-chain inorganic ligand potassium thiocyanate (KSCN), which not only enhanced the stability and boosted the photoluminescence quantum yield (PLQY) of PeQDs films, but also improved their electrical conductivity. To further solve the problem of charge injection balance and improve the operating lifetime of the devices, we simultaneously used all-inorganic metal oxide semiconductor materials Mg-doped ZnO (Mg-ZnO) and Mg-doped NiOx (Mg-NiOx) as the electron and hole transport layers of QLEDs (ETL and HTL), respectively. The operating stability of the devices was greatly improved while being able to withstand higher current densities. Therefore, using Zr-doped and KSCN passivated CsPbBr3 (Zr doped CsPbBr3-KSCN) PeQDs as the light-emitting layer, we fabricated highly efficient and stable green-light all-inorganic quantum dots light-emitting diodes (QLEDs) with the maximum external quantum efficiency (EQE) of 13.8% and the maximum brightness of 24800 cd/m2. These green perovskite QLEDs provide a working half-life of 360 min (initial brightness is 1000 cd/m2) and the operating half-life of the devices is more than two times higher than that of the reference devices. Our proposed strategy based on doping and in-situ inorganic ligand passivation will open a new avenue for fabricating efficient, stable and other colored perovskite QLEDs.