Colloidal quantum dots (QDs) exhibit exceptional properties of narrow-band emission, tunable luminescent wavelength, high luminous efficiency, and remarkable material stability across the visible and infrared spectra, making them highly valuable in diverse applications like imaging, solar collection, displays, communications, and medical¹⁻⁶. The 2023 Nobel Prize in chemistry underscores the immense potential of QDs in basic scientific research, materials science, optics, optoelectronics, and even biomedical applications. Quantum dot light-emitting diodes (QLEDs) harness the synergies of QDs and organic light-emitting diodes, paving the way for them to emerge as the leading technology in next-generation solid-state lighting and flat panel display⁷⁻⁹.

Despite cadmium-based QLEDs having witnessed remarkable progress in recent decades, particularly in red and green QLEDs, challenges persist in improving the stability of blue QLEDs. Additionally, the stringent regulations on heavy metal elements in the European Union have posed significant hurdles to the commercialization prospects of cadmium-based QDs. To address this, research has intensified on heavy metal-free QDs, such as Cu-In-Zn-S QDs^10,11, Ag-In-Ga-S QDs¹²⁻¹⁴, Zn-Cu-Ga-S QDs¹⁵, and Zn-Ag-Ga-S-Se QDs¹⁶, which have been synthesized through the incorporation of homologous or heterogeneous ions. However, these QDs still face significant challenges, including a wide full width at half maximum (FWHM) (>45 nm), low external quantum efficiency (EQE) (<10%), and poor device stability (<10 h), significantly hindering their competitiveness in full-color display applications.

Indium phosphide (InP) QDs, as a substitute for cadmium selenide (CdSe) QDs, have been extensively studied to develop cadmium-free QLEDs¹⁷⁻²¹. The types of eco-friendly QDs are mainly divided into binary (heavy-metal-free group II-VI and III-V) system, multinary (heavy-metal-free group I-(Ⅱ)-III-VI) system. Currently, the red InP-QLEDs, being eco-friendly, have exhibited outstanding performance, rivaling that of traditional CdSe-QLEDs, in terms of efficiency (EQE=21.4%) and lifetime (T₉₅=615 h@1,000 cd·m⁻², i.e., the time taken for the brightness to decay to 95% at an initial brightness of 1,000 cd·m⁻²)²². Similarly, the green InP-QLEDs exhibit an EQE of 16.3% and a half-lifetime of 5944 hours (T₅₀=5944 h@100 cd·m⁻², i.e., the time taken for the brightness to decay to 50% at an initial brightness of 100 cd·m⁻²)²³. However, the utilization of InP for blue QLEDs remains challenging due to its narrow intrinsic bandgap of 1.35 eV, and although blue light emission can be achieved by controlling the particle size within 1–2 nm, this poses immense difficulties in the thermal injection synthesis process. Additionally, the instability and susceptibility to oxidation of InP severely hinder its application potential in high-performance blue QLEDs²⁴⁻²⁷.

Especially, by alloying zinc telluride (ZnTe) with a narrower bandgap of 2.25 eV and zinc selenide (ZnSe) with a wider bandgap of 2.70 eV, the resulting ZnSeTe QDs are considered the most promising candidates for cadmium-free blue QDs²⁸, offering flexible adjustment of the bandgap and spectral properties by manipulating the Te/Se ratio. Moreover, the adoption of a quasi-type II band structure can effectively weaken the quantum confinement effect within the ZnSeTe QDs core, suppressing low-energy tail emissions, which contributes to achieving a narrow FWHM, high color purity, and a high photoluminescence quantum yield (PLQY) for blue light emission²⁸. Unfortunately, by increasing the Te content in ZnSeTe QDs, their emission wavelength can also be tuned to emit green light (510–530 nm) and even red light (608–612 nm), accompanied by issues such as strain at the QD core-shell interface and broadening of the FWHM^29,30. Hence, harnessing the merits of InP QDs and ZnSeTe QDs while overcoming their limitations could pave a viable route for the advancement of cadmium-free, full-color QLED devices, unlocking novel prospects for future display technologies.

As seen in Fig. 1, Nozik's team first synthesized InP QDs as early as 1994 by heating a trioctylphosphine oxide precursor solution at 270 °C for several days³¹. In 2000, Banin's research group reported core/shell semiconductor nanocrystals, providing an effective solution to address the non-radiative decay of QDs excited states³². It was until 2011 that Lee announced the first green InP-QLED with an EQE of just 0.01% and an FWHM of 70 nm³³. However, the development of ZnSe-QDs lagged. In 2008, A. Pron's team achieved ZnSe-QDs with tunable photoluminescence within the spectral range of 390–440 nm and an emission FWHM of around 15 nm through the direct reaction of zinc stearate with selenium dissolved in trioctylphosphine³⁴. In 2014, Wedel presented the first blue ZnSe-QLED, exhibiting a maximum brightness of about 25 cd·m⁻² and an efficiency of approximately 0.02 cd/A³⁵. In 2019, Jang's research group debuted a blue QLED based on alloyed ternary ZnSeTe QDs, demonstrating a peak brightness of 1195 cd·m⁻² and an EQE of 4.2%³⁶. Nevertheless, when the initial brightness reached 200 cd·m⁻², the brightness decayed to 50% within only 5 minutes. Fortunately, cadmium-free InP-QLEDs and ZnSe-QLEDs have undergone significant development in the past five years, with cadmium-free red, green, and blue QLEDs achieving EQEs exceeding 15% and half-lifetime up to 1000 hours. Notably, cadmium-free blue ZnSeTe QDs, boasting their superior performance, durability, and color purity, offer promising solutions to the current obstacles hindering the development of blue QLEDs in current enterprises.

Figure 2 clearly illustrates the unique advantages and inherent limitations of InP-based QDs and ZnSe-based QDs. Notably, a shared cause of surface defects in both is the steric hindrance of long-chain oleic acid ligands and the incomplete passivation of some surface sites during shell formation due to redox reactions, as well as the relatively weak interaction between QDs and ligands. Furthermore, a high PLQY does not inherently guarantee long-lifetime QLEDs. Hence, a thorough analysis of multiple factors, including the photoelectric properties of QDs, the stability of carrier transport layer, and device structure, provides a solid foundation for exploring the similarities and differences among QDs in order to comprehensively enhance QLED performance.

Herein, we review the progress in the field of eco-friendly QD materials and device technologies. Special attention is given to the promising InP-based QDs and ZnSe-based QDs, including their current status, advantages, and inherent limitations. With regard to the challenges in achieving long-lifetime, cadmium-free, full-color QLEDs, we have proposed practical and effective solutions to tackle technical barriers and proposed future research avenues. In particular, this comprehensive review offers an insightful perspective on the intricate relationship between the photoelectric properties of QDs, the carrier transport layer stability, and the device lifetime.

QLED is a typical injection-type optoelectronic device that generates electroluminescence under the driving current. The operational process of QLEDs can be simply divided into three stages: (i) the injection and transport of carriers; (ii) the formation and recombination of excitons; and (iii) the emission and extraction of photons. Meanwhile, various factors impact the performance of QLEDs, including carrier balance, interface defects, and deceive structure.

Especially, the EQE of QLEDs is a crucial parameter to evaluate their performance, and the calculation formula for EQE is as follows:

1EQE=γ×ηrad×ηout.

The PLQY of the QDs, denoted as η_rad, quantifies the fraction of excitons that undergo radiative decay, which is not only related to the radiative recombination of excitons in QDs but also to the quenching mechanism during the operation of QLEDs. On one hand, the Auger recombination and Förster resonance energy transfer (FRET) within QDs can be effectively suppressed by adopting strategies such as gradient alloyed QDs, thick-shell QDs, and ligand exchange, resulting in a PLQY of up to 100%. On the other hand, the EQE of the devices can be reduced by various processes during operation, including defect-related non-radiative recombination, Auger recombination, and thermally activated defect-assisted recombination.

γ represents the charge balance factor, defining the proportion of injected charges that contribute to the formation of excitons. In traditional QLED architectures, the main influential factors of γ are the hole injection barrier, low hole mobility, and electron leakage.

η_out denotes the out-coupling efficiency, indicating the percentage of the produced photons that are capable of escaping the device. Since QLEDs feature a sandwich structure composed of stacked multilayers with varying refractive indices, some of the light emitted from the QDs encounters total internal reflection due to the significant refractive index difference between the device layers and the air interface. Typically, only 20%–30% of the total photons can be emitted to the exterior of the device.

Focusing on the key parameters mentioned, various methods have been proposed to significantly enhance the performance of the devices. A comprehensive comparative analysis of these optimized parameters is presented in Table 1, providing a clear insight into the effectiveness of each approach. With improvements in QD core-shell engineering and ligand exchange methods, the PLQY of QDs has reached near-perfect levels of 100%. However, the EQE of QLEDs still remains at around 20%. Meanwhile, it is essential to recognize that a high PLQY does not inherently guarantee a prolonged device life, due to the intricate relationship between the electric properties of QDs, the stability of carrier transport layers, and QLED packaging technology. This article explores the approaches to boost QLED performance in terms of QD architecture, the charge balance of the device, and the intricate aging mechanisms, paving the way for more stable and efficient devices.

The advancements in QLED research can be categorized into two stages. Before 2015, researchers focused on establishing the QD core-shell structure and creating QDs with superior PLQY. Since 2015, the focus has shifted to optimizing device architectures to enhance efficiency and achieve long lifetimes for industrial applications.

Initially, indium trichloride (InCl₃) and tris-(trimethylsilyl) phosphine ((TMS)₃P) served as the primary phosphorus sources for InP QDs³³, as depicted in Fig. 3(a). However, the high reactivity of the phosphorus source led to a significant number of defects on the QD surfaces. Additionally, the high reactivity of (TMS)₃P not only contributed to an uneven distribution of particle sizes but also resulted in the generation of harmful gases. To address this issue, the H. Yang research team has pioneered the use of (DMA)₃P as a substitute for (TMS)₃P. The adoption of the more economical and safer (DMA)₃P significantly boosted the progress in InP QD research^79,80. Sun's research discovered that green InP/ZnSeS/ZnS QDs, synthesized with (DMA)₃P, exhibit a remarkable PLQY of 95% and an FWHM of 45 nm (Fig. 3(b))⁵⁶.

In 2001, Weller et al.⁸¹ first synthesized core-shell InP/ZnS QDs, which greatly reduced surface defects and consequently boosted band edge emission efficiency. Later, InP/ZnSe and InP/ZnSeS structures were also developed. However, due to the thin shell thickness, a satisfactory solution with a high PLQY and photochemical stability has not yet been achieved. In 2012, Kim et al.⁸² introduced a GaP shell that effectively passivated the surface and eliminated traps, while also minimizing the lattice mismatch between the InP core and ZnS shell, as illustrated in Fig. 3(c). Moreover, Bae et al.⁴⁰ revealed that the potential distribution and energy level shifts in core-shell heterostructured QDs play a crucial role in their photophysical properties and charge transport characteristics. By controlling the atomic ratio of QDs to alter the interfacial dipole density, the potential distribution of QDs can be further reshaped. As the interfacial dipole density increases from 0.09 nm to 1.50 nm, the oxidative stability is enhanced, as depicted in Fig. 3(d). In 2019, the Peng group⁴² demonstrated that zinc chalcogenides (ZnSe and ZnS) with wide bandgaps can be epitaxially deposited onto InP QDs, effectively confining photo- and electro-generated electron-hole pairs (excitons) primarily within the InP cores. This results in a remarkable PLQY of up to 95%, as shown in Fig. 3(e).

Additionally, the cation exchange during the ZnSe shell formation of InP QDs leads to Zn traps in the lattice, which not only act as lattice defects but also increase charge scattering and hinder charge transport due to local band bending. Fortunately, Yang et al.⁸³ achieved InP/ZnSe/ZnS QDs with a PLQY of up to 87% by constructing a Se-rich shielding layer on the surface of the InP core (Fig. 3(f)). More recently, Li et al.⁸⁴ proposed a one-pot synthesis method that employs a temperature gradient solution growth strategy ranging from 240 °C to 280 °C to grow the inner shell layer, enabling the production of high-brightness, narrow-emission InP/ZnSeS/ZnS multi-shell QDs. This approach enables a high PLQY of 91% and a FWHM of 36 nm, as illustrated in Fig. 3(g). Moreover, the Yang group⁵² proposed the quasi-shell-growth strategy, which utilizes a quasi-ZnSe shell to minimize surface defects in the InP core through the passivation of in-terminated vacancies. It further hinders the Ostwald ripening of the InP core at high temperatures, enabling the synthesis of InP/ZnSe/ZnS QDs with a remarkable PLQY of 91% and a narrow emission FWHM of 36 nm. As a result, the green QLED achieved a peak EQE of 10.6% and an impressive half-lifetime exceeding 5,000 hours, as shown in Fig. 3(h).

Particularly, the high reactivity of InP towards water and oxygen poses challenges in precisely controlling the QD core-shell interface. Traditional synthesis methods for InP QDs involve the use of carboxylic acid-based precursor solutions or ligands at temperatures above 188 °C. Unfortunately, these carboxylic acids tend to decarboxylate and generate water at high temperatures, leading to the formation of an oxide layer of InPO_x that hinders the epitaxial growth of the ZnS shell. Jang et al.²² employed hydrofluoric acid (HF) to eliminate uncoordinated surface dangling bonds and etch away the InPO_x oxide layer, thereby passivating defects in the InP core and promoting a more uniform QD morphology with high PLQY. To reduce the harm caused by HF, Ji's group³⁷ proposed a method where HF can be generated in situ through the reaction of ZnF₂ with carboxylic acid at high temperatures. This HF then effectively removes the surface oxide layer of InP QDs, enabling subsequent shell growth of ZnSe and ZnS on oxide-free InP QDs. The resulting InP/ZnSe/ZnS QDs exhibit a PLQY of 90% and a FWHM red emission of 36 nm. Furthermore, InP/ZnSe/ZnS QLEDs achieve a remarkable EQE of 22.2% and a T₉₅ lifetime exceeding 32,000 hours at 100 cd·m⁻², as depicted in Fig. 3(i). In brief, HF treatment in QD synthesis offers superiorities such as selective etching for purification, controlled size and shape tuning, and enhanced stability. However, it also poses limitations including corrosiveness and safety concerns, environmental impact, complexity in the synthesis process, and limitation in material selection.

The current reports on eco-friendly QD materials like CuInS₂, InP, and Si face significant challenges in achieving blue light emission due to the necessity of ultra-small particle diameters, which pose stability issues. In 2014, Wedel's group³⁵ explored ZnSe QDs for their potential as emissive materials in cadmium-free blue QLEDs. By manipulating the chemical composition during synthesis, they were able to adjust the emission wavelength to fall within the range of 390–435 nm. However, ZnSe QDs primarily exhibit photoluminescence in the violet range, where the wavelengths are insufficient for practical use. Nevertheless, increasing the size of ZnSe QDs can alleviate the effects of quantum confinement, enabling blue light emission, as shown in Fig. 4(a).

Du's group first provide spectroscopic evidence of oxygen contamination on the synthesized ZnSe QDs when they are subjected to oxygen exposure⁸⁵. Meanwhile, they group presented high-quality ZnSe-based core (~5.0 nm)/shell (~1 nm) QDs with a radius greater than the bulk Bohr radius⁷⁰. Through the epitaxial growth of a thin ZnS layer, the ZnSe/ZnS core-shell QDs achieved a PLQY of 95%, and an extremely narrow PL width of around 9.6 nm, as shown in Fig. 4(b). The QLEDs using these bulk-like ZnSe/ZnS core/shell QDs boast a vibrant blue emission centered at 445 nm, a high EQE of up to 12.2%, and a half-lifetime of 237 hours for an initial brightness of 100 cd·m⁻². Recently, Zhong et al.⁸⁶ proposed a novel reaction-controlled epitaxial growth strategy for ultra-large ZnSe nanocrystals, as shown in Fig. 4(c). By injecting precursor monomer solutions with different reactivity at high temperatures and adjusting the injection rate, they successfully suppressed the secondary nucleation during the epitaxial growth process. This was followed by a ZnS shell coating, yielding nearly spherical ZnSe/ZnS nanocrystals with a diameter of 35 nm, which emit blue light with a peak wavelength of 470 nm and a PLQY of 60%. This strategy provides new insights for synthesizing other types of ultra-large semiconductor nanocrystals. Recently, Wu's research group presented a novel core-shell interface, unintentionally alloyed, which aids in mitigating non-radiative Auger recombination in compact QDs (~7.8 nm)⁸⁷. These QDs show biexciton lifetimes that are comparable to larger CdSe-based systems. Their small size and prolonged gain lifetime make them incredibly versatile, similar to laser dyes, enabling powerful blue amplified spontaneous emission (ASE) and adjustable laser outputs with a narrow linewidth. This groundbreaking research holds promise for promoting the practical deployment of eco-friendly colloidal QDs in the laser field.

To enhance the standard blue light emission, Yang et al.³⁶ reported in 2019 the first device based on ZnSeTe QDs (441 nm), achieving a high PL QY of 70% and a FWHM of 32 nm by optimizing the Te/Se ratio and ZnSe inner shell thickness, as displayed in Fig. 4(d). However, it was found that the introduction of Te leads to lattice mismatch issues between ZnSe and ZnTe, and the uneven distribution of Te results in low-energy tail emissions in the spectrum. To address these issues, researchers have focused on controlling interfacial defects and achieving a more uniform distribution of Te. They also precisely tailored the Te/Se ratio of the ZnSeTe core, achieving moderate PL tunability in the deep-blue region⁶⁶. Moreover, a detailed analysis of the ZnS shell thickness variations revealed their influence on QLED operational efficiency, as shown in Fig. 4(e). Devices with thicker ZnS-based QDs exhibited a peak EQE of 18.6%. Nevertheless, their half-life (T₅₀@1000 cd·m⁻²) was limited to just 25 minutes. Similarly, Zhao's team⁶⁸ refined the ZnSe inner shell thickness to reduce the exciton-LO phonon coupling and trap states in the QDs, as seen in Fig. 4(f). Furthermore, increasing the QD shell thickness suppressed the FRET process in the QD film. Consequently, the ZnSeTe QLED exhibited an EQE of 18% at the 452 nm electroluminescence peak, but the lifetime stands at a comparatively low 49.1 h (T₅₀@100 cd·m⁻²). Recently, Tian et al.⁷¹ designed a step-by-step shell growth technique that controls the concentration of monomers and ensures sufficient growth intervals, thereby facilitating the controlled and uniform epitaxial growth of ZnSe and ZnS shells on ZnSeTe cores, as indicated in Fig. 4(g). This approach significantly reduces lattice distortions and defects, thus greatly inhibiting non-radiative charge recombination. The resulting deep blue ZnSeTe/ZnSe/ZnS QDs (448 nm) have a PLQY close to 100%, while the EQE is only 10.9%. Thus, the achievement of both high efficiency and long lifetime in cadmium-free blue QLEDs has proven to be a challenge.

By optimizing the Te doping ratio and the thickness of each shell layer (Fig. 4(h)), Lee's team⁶⁵ ultimately achieved tunable emission wavelengths of ZnTeSe/ZnSe/ZnS QDs ranging from 452 to 457 nm, with PLQY exceeding 90% and FWHM ranging from 23 to 27 nm. It is worth mentioning that the subsequent Cl⁻ treatment substitutes the native aliphatic ligands, thereby improving thermal stability and enhancing charge injection/transport. Additionally, the emitting layer features a unique double-stack design with a scaled Cl⁻ concentration, which promotes optimal charge recombination. As a result, the device achieves an EQE of up to 20.2% and a T₅₀ of 15850 hours at 100 cd·m⁻², representing the highest values ever reported for blue QLEDs. Furthermore, to alleviate the hazards of using HF, a novel synthesis approach has been introduced that employs benzoyl fluoride as a chemical additive for the in-situ production of anhydrous HF, as shown in Fig. 4(i). This approach ensures a controlled release of HF during the nucleation and growth stages, leading to a uniform distribution of Te in the ZnSeTe lattices⁸⁸. The resulting core/shell/shell QDs exhibit emission in the natural blue region (457–463 nm), with an FWHM ranging from 22 to 25 nm and a PLQY of 85% ± 5%.

The conventional synthesis of QDs (including InP and ZnSe types) is typically based on the use of carboxylic acid precursor solutions or ligands at high temperatures (>188 °C)^13,89−91. Unfortunately, carboxylic acids undergo decarboxylation at high temperatures, which can lead to the regeneration of defects at the QD core-shell interface, thereby increasing non-radiative recombination channels in QDs⁹²⁻⁹⁴. Additionally, the feeble interaction between long-chain carboxylic acid ligands and metal ions undermines the chemical stability of QDs^57,95,96. Moreover, the intrinsic low conductivity of the alkyl chains in carboxylic acids hinders the charge transport properties of QDs^97,98. To address these issues, researchers have proposed a ligand exchange strategy for QDs, which not only enhances the PLQY of QDs but also achieves long-lifetime QLEDs.

Many ligand exchange preparation strategies exist for InP-based QDs, facilitating the control of their surface properties, stability, and optoelectronic performance, as shown in Fig. 5. For instance, Jang et al.²² achieved a 63% substitution of long-chain oleic acid with short-chain hexanoic acid (HA), forming QD-3R-HA, which improved the charge injection in QLED. The hole current of short-chain ligands increased by four times, which facilitated exciton recombination and reduced Auger recombination, thus inhibiting the voltage increase at the interface between TFB and QDs. As depicted in Fig. 5(a), the fabricated HA-QLED exhibited a more stable operating voltage during device operation. In addition, by dispersing the pristine QDs in chlorobenzene with the aid of a semiconducting diblock copolymer, Kwak's research⁹⁹ observed that the copolymer units surrounding the QDs widened the inter-dot distances, minimizing FRET and consequently reducing PLQY. Meanwhile, the carbazole groups within the copolymer contributed to efficient hole transport into the QDs while blocking excessive electron leakage. As a result, the hybrid QLED device exhibited a half-lifetime of 120 hours at an initial brightness of 1000 cd·m⁻², which is four times longer than that of the unmodified QLED, as seen in Fig. 5(b).

Similarly, the Chou group²³ integrated the InP QDs into a prepared stock solution, which consisted of a mixture of EDA, BDA, or HAD toluene solutions and an anhydrous ethanol solution containing ZnCl₂, ZnBr₂, and ZnI₂. As illustrated in Fig. 5(c), the emitting layer was subsequently treated with passivating agents, which comprised a variety of alkyl diamines and zinc halides. This strategy led to a reduction in electron mobility and an enhancement in hole transport. Additionally, Wedel et al.⁹⁵ explored the interplay between the diverse ligands, including zinc octoate, 1-octanoate, zinc stearate, 1-trioctylphosphine, acetone, ethanol, and 1-octanethiol and the metal ions in InP/ZnSe/ZnS QDs, as illustrated in Fig. 5(d). The findings revealed the presence of solvent ligands, such as ethanol and acetone, on the imperfect surfaces generated during QDs synthesis, and the carboxylate ligands were mostly present as carboxylates on the QD surface. Furthermore, surface defects in these QDs give rise to electronic states within the bandgap, ultimately hindering the charge transport properties of QLEDs.

Apart from the ligand exchange methods utilized for InP QDs, there exists an extensive suite of preparation protocols specifically tailored for ZnSe QDs, offering new avenues for property improvement. For example, Jang et al.⁶⁵ uncovered a mechanism where halogen anions stabilize Zn dangling bonds on the QDs surface upon exchanging ligands with ZnCl₂, enhancing their stability. In essence, the PLQY of QDs improved from an initial 50% to 76% through liquid-phase ligand exchange and further to 90% via solid-phase ligand exchange. Meanwhile, the lifetime of QLEDs incorporating ligand-exchanged QDs was extended by nearly ten times, as shown in Fig. 6(a). The bromide decoration technique, as demonstrated by Chen's group⁷⁶, simultaneously tackles QD surface imperfections by the passivation of unsaturated Zn sites, which can enhance carrier radiative recombination and eliminate excess oleic acid through ligand exchange, facilitating efficient carrier transport. These benefits lead to a substantial improvement in PLQY, increasing from 39.7% to 86.2%, as shown in Fig. 6(b). Moreover, through the substitution of oleic acid with various alkanethiol ligands, Woo et al.¹⁰⁰ successfully improved the water and oxygen stability, particularly for QDs modified with dodecanethiol (DDT), as illustrated in Fig. 6(c). Upon exposure to air for 2 months, the PLQY of these QDs maintained a high level, decreasing from 94% to 75%.

Despite the excellent surface defect passivation properties of zinc carboxylate and zinc chloride for QDs, they are still plagued by issues such as carboxylic acid ligand detachment and QDs aggregation. To overcome these issues, 4-methylbenzylzinc chloride ligands were utilized on the surface of ZnSeTe/ZnSe/ZnSeS/ZnS QDs, resulting in enhanced PLQY and PL stability¹⁰¹. The zinc-chlorine component of the 4MBZC ligand provides dual passivation and strong binding affinities to the QDs surfaces, while the aromatic group promotes stable dispersion in solution. Importantly, the 4MBZC-coated QD solutions demonstrated remarkable stability, maintaining 84.8% of their initial PLQY (99%) after 1,728 hours of storage, as shown in Fig. 6(d).

With a profound understanding of QLEDs' operational mechanism, the device structure has undergone significant evolution. Back in 1994, the Alivisatos' group¹⁰² at UC Berkeley pioneered electroluminescence devices incorporating nanocrystals and polymers, initiating a new era in QD lighting, which was categorized as Type-I QLEDs, as exemplified in Fig. 7. With the rapid advancements in QLED technology, three distinct structural variations emerged. Among them, placing the QD emission layer between two organic transport layers gave rise to Type-II QLEDs¹⁰³. Meanwhile, the incorporation of inorganic materials for charge transport led to the designation of Type-III QLEDs¹⁰⁴. Additionally, the innovative combination of organic and inorganic components was named Type-IV QLEDs, reflecting the synergy between these disparate materials¹⁰⁵.

In traditional sandwich structures, it is found that excessive electron injection in QLEDs triggers Auger recombination within the QDs. This process not only undermines the stability of the QDs but also causes efficiency roll-off in QLEDs under high current operation. Moreover, the excessive electrons can leak into the HTL, accelerating its aging and thereby impacting the device's lifetime. Meanwhile, the disparity in injection efficiencies between electrons and holes in conventional QLED designs underscores the importance of carrier dynamics optimization. To tackle this, key strategies involve enhancing the injection and transport capabilities of the HTL, optimizing the hole injection efficiency of QDs, suppressing electron injection, and achieving a balanced charge distribution within the device.

Since 2011, the introduction of ZnO NPs as an ETL in QLED by Qian et al.¹⁰⁵ has sparked widespread interest in type-IV QLEDs integrating organic and inorganic materials. ZnO NPs possess versatile surface functional groups and allow for straightforward solution synthesis, facilitating diverse chemical modifications. Additionally, various strategies, such as controlling the size of ZnO nanocrystals¹⁰⁶, metal ion doping^107−110, and organic functionalization (depicted in Fig. 8(a))⁵⁴, have proven effective in mitigating exciton quenching at QD/ETL interfaces, minimizing ZnO oxygen vacancies, and regulating electron injection, ultimately optimizing charge balance for superior QLED performance. Additionally, dipole moments have proven effective in mitigating defects in ZnO. Yang et al.¹¹¹ utilized a dielectric layer composed of phenylethylammonium bromide (PEABr) and methylammonium bromide (MABr), which not only shifts the conduction band minimum of ZnMgO upwards to prevent the injection of extra electrons but also passivates its defect states via Br filling, enhancing charge balance in QLEDs (Fig. 8(b)). Consequently, the QLEDs achieved an operational lifetime of more than 400 hours.

Although ZnMgO thin films are commonly utilized as ETLs in QLEDs for their electron transport properties, they often lead to challenges including interface exciton quenching and excessive electron injection, affecting device performance. In particular, in cadmium-free green QLEDs, our group has found that PO-T2T with its shallower conduction band and lower defect density, outperforms ZnMgO as an ETL, as illustrated in Fig. 8(c)⁴⁹. This not only diminishes interfacial exciton quenching but also improves charge balance. In contrast to traditional organic/QD/inorganic structures, organic/QD/organic architectures demonstrate potential for efficient and stable cadmium-free QLEDs.

To date, considerable efforts have been devoted to tackling the challenge of charge imbalance within QLEDs. For instance, Kwakthe et al.¹¹² employed CzSi as a hole-blocking interlayer, effectively diminishing the exciton-hole quenching process. Additionally, an ultrathin LiF layer is employed to tailor the work function of ITO, effectively preventing electron leakage while promoting hole injection¹¹³. Moreover, Yang et al.⁴⁷ developed an energy-level regulated QLED using BFTP self-assembled monolayers on the surface of the TFB, which has a backbone benzene ring and functional head group, as illustrated in Fig. 8(d). The molecular dipole layer at the interface of QDs and HTL accomplishes two key functions: it lessens the energy barrier for hole injection into QDs via vacuum level offset, and it stops the HTL from quenching the fluorescence of QDs. To hinder electron transfer, they recently have also inserted an ultra-thin interlayer of polyvinylpyrrolidone (PVP) between TFB and QD. With the PVP interlayer⁵², parasitic emissions from TFB were eliminated, resulting in QLEDs with a lifespan exceeding five times that of the reference device.

In particular, novel hole transport materials, such as B-PTAA³⁸ and DBTA⁴¹ have been developed, which feature rigid dibenzothiophene and tertiary amine units, as illustrated in Fig. 8(e) and Fig. 8(f). These materials offer high hole mobility and a deep highest occupied molecular orbital level, enhancing hole injection efficiency into InP QDs. Furthermore, a homogeneous emitting layer has been created by combining an organic hole-transporting material (DOFL-TPD) with InP/ZnSe/ZnS QDs, as presented in Fig. 8(g). This combination offers exceptional hole mobility (1.1×10^–3 cm²/V·s) and optimal LUMO/HOMO energy levels (2.4 eV/5.4 eV), respectively, effectively blocking electron leakage and enabling efficient hole injection into the QD EML³⁹.

The disparity in energy band offsets, where the offset between the anode Fermi level and the valence band maximum of QDs significantly exceeds that between the cathode Fermi level and the conduction band of QDs, is particularly pronounced in blue QLEDs. As both InP-based and ZnSe-based QLEDs adopt a “p-i-n” sandwich design, the operation of the device involves electrons and holes being injected into the QD layer through charge transport layers, ultimately recombining within the QDs to emit light. Consequently, strategies to optimize charge balance can be similarly applied in both cases.

One aspect involves the HOMO level of PVK (–5.8 eV), which aligns favorably with QDs for hole injection, in contrast to TFB (–5.3 eV), as shown in Fig. 9(a). Nevertheless, TFB's superior hole transfer stability necessitates consideration when selecting the optimal material¹¹⁴. Simultaneously, reducing the electron mobility of the electron transport layer, as demonstrated in Fig. 9(b) and 9(c), has proven effective in balancing charge injection within QLEDs^69,72. Beyond that, Chen's group⁶⁷ has developed a top-emitting device structure, as shown in Fig. 9(d). By integrating transparent indium zinc oxide as both the top electrode and a phase-tuning layer, this design achieves a saturated blue light emission that closely aligns with the Rec.2020 color standard. Additionally, due to its remarkable external coupling efficiency and balanced charge transport, the ZnSeTe-based QLEDs for blue emission exhibit an impressive EQE of up to 18.16%.

Due to the distinctive benefits and inherent constraints of InP-based and ZnSe-based QDs, the diversification of QD core-shell engineering and QLED interface modification techniques enables the PLQY of QDs to nearly reach 100%, whereas the EQE of QLEDs remains around 20%. Moreover, high PLQY does not inherently guarantee long-lifetime QLEDs. It is imperative to tackle the issue of charge injection imbalance and field-enhanced electron delocalization in Cd-free devices during operation, as these factors cause a decrease in brightness. Nevertheless, the primary challenge in commercializing QLEDs revolves around developing cadmium-free QDs with prolonged lifetimes for full-color, cadmium-free displays, which requires QDs to serve as the self-luminous layer to possess not only high PLQY but also to maintain steady brightness during operation.

Meanwhile, unlike their red and green QLEDs, blue QLEDs exhibit unique degradation mechanisms that are yet to be fully elucidated, primarily hindered by their markedly inferior lifetime. Firstly, their intrinsic properties, such as a deep valence band and large bandgap, lead to an elevated hole injection barrier and increased surface defects, which can reduce performance and PLQY. Secondly, structural differences in device design and material compatibility for blue QDs necessitate specific considerations to optimize carrier injection and transport. Thirdly, degradation mechanisms such as charge accumulation and leakage, non-radiative recombination, and interfacial stability issues are more prevalent in blue QLEDs, leading to reduced efficiency and stability. Lastly, achieving high efficiency and accurate chromaticity for blue QLEDs is particularly challenging, further highlighting their unique degradation challenges compared to red and green QLEDs. Consequently, we embark on an overview of the degradation mechanisms of QLEDs, along with strategies to improve their lifetime performance.

Despite attempts to mitigate surface traps through ligand passivation and minimize lattice mismatch issues with core-shell structures, the inherent chemical reactivity of In and P is too strong to prevent oxygen-induced traps in InP QDs completely. In QLEDs, these traps hinder exciton generation and radiative recombination by capturing injected carriers. Additionally, they contribute to the formation of trapped excitons with excessively rapid nonradiative decay rates, thus posing a significant challenge to achieving long lifetimes. For instance, Du’s group¹¹⁵ has elucidated the distinct loss mechanisms of InP-based QLEDs under low and high bias voltages. With the increase in shell thickness, exciton quenching caused by surface traps and spontaneous energy transfer independent of the field are effectively suppressed. Conversely, the field-enhanced electron delocalization that leads to highly efficient energy transfer is one of the primary reasons for the sharp decline in EQE in InP-based QLEDs under high bias (Fig. 10(a)).

In addition, the challenges posed by the broad FWHM, size irregularity, and core-shell interfacial non-uniformity of InP QDs have been at the forefront of scientific inquiry. Kim et al.²⁰ have highlighted the strong dependence of charge carrier behavior in InP-QLEDs on the morphology of QDs, as depicted in Fig. 10(b). Compared to anisotropic QDs, isotropic counterparts exhibit significantly reduced hole trapping in InP/ZnSe/ZnS QDs and enhanced PLQY due to the effective elimination of stacking faults within the QDs. In anisotropic QDs, photo-generated charges are more prone to entering non-radiative recombination centers, and charge capture by defect sites occurs more readily. Remarkably, the difference in PLQY between the two types of QDs is not attributed to surface defects but rather to the extent of hole trapping.

Contrary to epitaxial III-V or colloidal II-VI QDs, phonon-mediated scattering among bright exciton states prevails as the primary dephasing mechanism in colloidal core-shell InP/ZnSe QDs. For example, Bayer et al.¹¹⁶ provided evidence for the emission of positive trions through the analysis of Zeeman splitting into the spectral lines of individual InP/ZnSe/ZnS, as clearly illustrated in Fig. 10(c). By employing both one-photon and two-photon photoluminescence excitation on nanocrystal ensembles, the research conclusively designates the 1Sh state as the lowest energy hole level, in contrast to the 1Ph state. Moreover, individual InP/ZnSe QDs exhibit minimal spectral fluctuations, facilitating the observation of emission spectra with line widths limited by dephasing effects, as illustrated in Fig. 10(d)¹¹⁷.

Another issue is that the high susceptibility to oxidation of InP/ZnSe/ZnS QDs significantly impacts their emission efficiency, making emission quenching under the environment a persistent challenge. For instance, Park's team¹¹⁸ observed that when individual InP/ZnSe/ZnS QDs are exposed to UV light, oxidation of the ZnS shell leads to the formation of ZnO, causing lattice mismatch with the host core lattice. This in turn creates dislocations and strains within the QD, prompting the diffusion of In atoms from the core to the surface. In particular, oxidative defects within the QD further contribute to incomplete passivation of the ZnSe layer, ultimately leading to the emergence of dangling bonds, as demonstrated in Fig. 10(e).

Recent developments in cadmium-free QD-LEDs have been significant and promising. The research team, comprising Professor Shen, Professor Feng, and Professor Tang, used their custom-developed electrically excited transient absorption technology to analyze the electronic transport characteristics of QLEDs¹¹⁹. Their study revealed that the ZnSeS intermediate layer hindered the efficiency of green InP-QLEDs. By experimentally and theoretically proving that a thicker ZnSe layer could replace ZnSeS to improve electron injection and reduce leakage, they achieved a peak EQE of 26.68% and a T₉₅ lifetime of 1,241 hours for green InP-based QLEDs with an initial brightness of 1000 cd/m² at 543 nm.

By taking advantage of the ZnS bandgap, the influence of the quantum confinement effect can be effectively mitigated through a thick shell that confines the electrons within the particles and thereby improve the PL efficiency and stability against changes in the pH of the solution (Fig. 11(a))¹²⁰. Meanwhile, the integration of ZnTe with a narrower bandgap and ZnSe with a broader one allows for the tuning of the bandgap, which can be tailored to produce blue-emitting ZnTeSe QDs, as shown in Fig. 11(b)¹²¹. Moreover, the prevention of blue light leakage from light-emitting chips significantly relies on the absorption efficiency of QDs under blue excitation. Importantly, both the emission spectra and absorption capacity are exquisitely sensitive to changes in core size. Yang et al⁵⁸ revealed that by evaluating the size-dependent molar absorption coefficients at 450 nm for a range of ZnSeTe cores and producing green-emitting ZnSeTe cores of varying sizes while preserving a uniform Se/Te ratio, as shown in Fig. 11(d).

In particular, the challenges in achieving high color purity blue QLEDs lie in the low-energy tail emission and spectral broadening observed in PL spectra. Wang et al.¹²² found that with low Te doping (Te/Se < 20%), the asymmetric low-energy tail arises from localized state recombination by Te clusters, with hot carrier localization occurring within ~500 fs. Above 20% Te/Se, the decrease in electronegativity contrast results in a homogenous charge density distribution, leading to carrier delocalization and the elimination of tail emission. The broadening of emission FWHM primarily stems from inhomogeneous broadening effects, as seen in Fig. 11(c). Moreover, the typically poor PLQY in ZnSeTe QDs is due to ultrafast hot carrier/band-edge carrier trapping. In thick-shell samples, rapid decay is suppressed, while the slow component relates to radiative exciton recombination of long lifetimes. Effective passivation of band-edge electron traps in thicker shells minimizes photoinduced absorption from electron trapping states, as illustrated in Fig. 11(e)¹²³.

Recently, Bae's group¹²⁴ has illuminated the morphological inhomogeneity within ZnSe_1–xTe_x alloy layers, specifically the presence of nearest-neighbor Te pairs, rather than size or compositional inhomogeneity, is the primary cause of dispersion observed in emission spectra and decay kinetics (Fig. 11(f)). Due to the difference in electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe_1–xTe_x alloys create localized hole states, leading to the formation of spatially separated excitons (delocalized electrons and localized holes in traps), which explains the inhomogeneous and homogeneous FWHM broadening with delayed recombination dynamics. In addition, HF treatment is a well-established method to remove dangling bonds and stacking faults in QDs. As illustrated in Fig. 11(g), Kim et al.¹²⁵ demonstrated that the addition of HF promotes the formation of Zn-F bonds, which in turn reduces surface energy and favors the preferential growth of (100) facets. This not only enhances the lattice order but also improves the PL emission properties. Additionally, an increase in HF concentration results in a more stable PL blinking behavior and a decrease in spectral diffusion.

The advancement in QLED technology, featuring heightened efficiency and stability, is fostering a highly promising future for large-area display applications. Nevertheless, the endeavor to achieve high-resolution pixel arrays through the patterning of QD layers remains a formidable obstacle. An ideal QD patterning approach should embody high resolution, impeccable uniformity, minimal degradation of QDs, cost-effectiveness, and other favorable characteristics. At present, the development of full-color display solutions based on cadmium-free QDs is in its nascent stages, a topic we will explore further in subsequent sections.

Typically, QLEDs produced via spin-coating methods require an inert atmosphere for fabrication, whereas the inkjet printing technique operates optimally under ambient conditions. However, eco-friendly InP QLEDs, unlike their stable Cd-based counterparts, encounter challenges during production under ambient conditions, primarily due to the degradation of QD. In 2022, Li et al.¹²⁶ reported the first efficient and high-color-purity inkjet-printed QLED using blue InP/ZnS/ZnS QDs as the emission layer. A two-step heating and thick shell strategy was utilized to prepare large-sized (10.6 nm) blue InP/ZnS/ZnS QDs. The substantial ZnS shell layer effectively suppresses non-radiative FRET among densely packed QDs, leading to high PLQY in InP/ZnS/ZnS QD films. However, the pixelated blue InP QLED achieved a maximum brightness of 91 cd·m⁻² and an EQE of 0.15%, as shown in Fig. 12(a).

Recent innovations in multi-component QD ink formulation have led to the development of inks with superior stability and printability, suitable for inkjet printing of InP QLED arrays. By carefully managing the dynamic interplay between capillary and Marangoni flows during the printing process, it is possible to achieve high-quality QD films on diverse substrates, including those featuring raised banks. Furthermore, the integration of ZnO microlens arrays, fabricated by nanoimprinting, has significantly improved the light extraction efficiency, leading to a maximum luminance of 17759 cd·m⁻², an EQE of 8.1%, and a current efficiency of 11.1 cd/A (Fig. 12(b))¹²⁷. These advancements are poised to accelerate the adoption of inkjet-printed, environmentally friendly QLEDs.

In addition, mitigating solubility issues of the underlying hole transport layer during inkjet printing is essential for achieving high pixel resolution and uniformity. For example, Kim et al.¹²⁸ employed a dual HTL layer consisting of TFB and PVK, effectively mitigating QD ink-induced HTL corrosion while preserving hole transport efficiency. Utilizing InP/ZnSeS red and green QDs along with ZnTeSe/ZnSe/ZnS blue QDs, a cadmium-free RGB inkjet-printed QLED was realized. Based on photolithography, high-resolution RGB color QLED pixels with dimensions of 60 μm×160 μm were presented, as illustrated in Fig. 12(c). Furthermore, they employed hexane, which has a low boiling point, as the primary solvent, adding a small amount of octane (less than 10% by volume) to moderate the evaporation rate of the droplets. Upon depositing the QD ink containing the photoinitiator 2-hydroxy-2-methylpropiophenone onto the desired surface, the film underwent in-situ polymerization induced by the formation of radicals through ultraviolet irradiation (Fig. 12(d)). The crosslinking reaction among the ligands in the QDs reduced the inter-particle distances, leading to a flattened surface and enhanced environmental stability in the air. The maximum luminance of the printed InP-based green QLED reached 3600 cd·m⁻²¹²⁹.

For two decades, QDs have sparked a revolution in displays owing to their distinctive luminescent properties and vivid color range. Researchers have diligently refined and innovated the materials and designs of QLEDs, propelling a remarkable surge in their capabilities. Currently, cadmium-free QLEDs in red, green, and blue QLED are nearly on par with Cd-QLEDs in terms of EQE. However, ensuring their stability poses the primary obstacle to achieving fully environmentally friendly, full-color displays. Furthermore, to expand the versatility of QLEDs into areas such as photomedicine, biosensing, lasers, and visual light communication, we encourage researchers to focus their efforts on the following directions.

i) Formulating precise and technical specifications. The methodologies employed by various laboratories for characterizing the same material exhibit similarities, yet there persists an absence of standardized definitions for QLED performance indicators, such as "device lifetime" and "device stability."

ii) Device performance of cadmium-free green QLEDs. So far, there is still a lot of room for improvement in the efficiency and lifespan of cadmium-free QLEDs, especially for green QLEDs. The exchange of surface ligands for cadmium-free QDs (including InP and ZnSn types) represents an effective pathway toward achieving high-performance QLEDs. Possible effective strategies include crystal engineering and surface ligands with excellent optoelectronic stability for cadmium-free green QDs.

iii) Exploring the degradation mechanism of cadmium-free devices. Enhancing QLED stability requires a deep comprehension of their degradation mechanisms. It is essential to recognize that a high PLQY does not inherently guarantee a prolonged device life, due to the intricate relationship between QD electric capabilities, carrier transport layer stability, and QLED packaging technology. Possible effective strategies include reducing the sensitivity of QDs to electric fields, and material chemistry for high-performance HTLs.

iv) Exploring the patterning technologies. To meet the demands of ultra-high resolution, the patterning of the QD layer must adhere to high pixel density, uniformity, and low roughness. However, the stability of cadmium-free QDs remains a challenge, and full-color displays utilizing inkjet printing technology are still in their developmental stages. Therefore, it is crucial to explore and develop alternative patterning technologies that offer high resolution, low cost, and high yield for realizing full-color cadmium-free QLED displays.

This work was supported by the Research Projects of Department of Education of Guangdong Province-024CJPT002, Special Project of Guangdong Provincial Department of Education in Key Areas (No. 6021210075K), Shenzhen Polytechnic University Research Fund. (No. 6024310006K).

Z. Y. supervised the work and made substantial revisions to the manuscript. P. G. wrote the draft of the manuscript. All authors discussed the results and reviewed the manuscript.

The authors declare no competing financial interests.