Synthesis of Eco-Friendly Narrow-Band CuAlSe₂/Ga₂S₃/ZnS Quantum Dots for Blue Quantum Dot Light-Emitting Diodes

Abstract

Quantum dot light-emitting diodes (QLEDs) based on high-color-purity blue quantum dots (QDs) are crucial for the development of next-generation displays. I-III-VI type QDs have been recognized as eco-friendly luminescent materials for QLED applications due to their tunable band gap and high-stable properties. However, efficient blue-emitting I-III-VI QDs remain rare owing to the high densities of the intrinsic defects and the surface defects. Herein, narrow-band blue-emissive CuAlSe₂/Ga₂S₃/ZnS QDs is synthesized via a facile strategy. The resulting QDs exhibit a sharp blue emission peak at 450 nm with a full width at half maximum (FWHM) of 35 nm, achieved by coating a double-shell structure of Ga₂S₃ and ZnS, which is associated with the near-complete passivation of Cu-related defects (e.g., Cu vacancies) that enhances the band-edge emission, accompanied by an improvment in photoluminescence quantum yield up to 69%. QLEDs based on CuAlSe₂/Ga₂S₃/ZnS QDs are fabricated, exhibiting an electroluminescence peak at 453 nm with a FWHM of 39 nm, a current efficiency of 3.16 cd A⁻¹, and an external quantum efficiency of 0.35%. This research paves the way for the development of high-efficiency eco-friendly blue QLEDs.

1. Introduction

Colloidal semiconductor quantum dots (QDs) have garnered significant interest as promising optoelectronic materials for various applications, including solar cells [1,2], photocatalysis [3,4], QD light-emitting diodes (QLEDs) [5,6], and biological labels [7,8] owing to their outstanding photophysical properties, comprising tunable luminescence color, high photoluminescence (PL) quantum yield (QY), and solution processability. In recent years, the prevalent group II-VI cadmium-based and III-V InP-based QLEDs have demonstrated impressive performance in green and red emissions [9,10,11]. However, the low electroluminescence (EL) efficiency of blue-emitting QLEDs impairs their application in next-generation display technologies [12,13,14]. To address this, researchers have developed alternative lead-free and cadmium-free QDs, specifically group I-III-VI (I = Cu, Ag; III = Al, In, Ga; VI = S, Se, Te) QDs [15,16,17], which have emerged as luminescent materials for fabricating eco-friendly QLEDs due to their tunable composition, considerable PL QY, and high stability. Nonetheless, the PL emission of I-III-VI QDs possesses a wide full width at half maximum (FWHM) exceeding 100 nm [18,19], which is attributed to the intrinsic intragap defects such as vacancy defect states and substitution defect states [20,21]. These defects serve as primary radiative recombination centers leading to a broad spectral emission and reduced PL QY.

An effective strategy to tailor the blue emission of I-III-VI QDs (e.g., Cu-Ga-S [22], Ag-Ga-S [23], CuAlSe₂ [24]) involves controlling the nucleation temperature, adjusting the chemical composition, and performing surface engineering [22,25]. This method allows for tunable emission spectra ranging from 446 to 475 nm, with FWHM values between 22 and 100 nm. The nucleation temperature and composition play crucial roles in determining the photophysical characteristics of QDs. For instance, Niu et al. [22] prepared blue-emitting Cu-Ga-S-based QDs with a PL peak at 475 nm via a hot-injection approach. They observed a gradual narrowing of the FWHM in the PL spectrum to 29 nm and an increase in PL QY to 32% by adjusting the nucleation temperature to 290 °C, which was associated with the elevated mobility of Cu⁺ ions under high temperature, improving the ion exchange to create more luminous channels. Additionally, the femtosecond transient fluorescence studies confirmed that the blue emission originated from radiative recombination between the conduction band (CB) and Cu vacancies (V_Cu). A facile one-pot method was reported for synthesizing narrow-bandwidth Ag-Ga-Zn-S QDs that emit at 482 nm by varying the feeding ratios of Ag/Zn and Ag/Ga [26]. The blue QLEDs based on the narrow-bandwidth QDs achieved a luminance of 123 cd m⁻² and a peak external quantum efficiency (EQE) of 0.4%. Wide band gap semiconductors of ZnS and GaS_x are typically used as shell materials to reduce broad defect-related emissions from QDs by eliminating surface states. For example, after coating a ZnS shell, Zn-Ag-Ga-S/ZnS QDs were synthesized, presenting a blue emission that peaked at 450 nm with a FWHM of 85 nm. This emission is attributed to electron transitions from the conduction band to Ag vacancies (V_Ag) [27]. Thereafter, Tozawa et al. [23] developed core/shell-structured AgGaS₂/GaS_x QDs using Cl^- ions treated AgGaS₂ as the QD core and coating it with a GaS_x shell, which produced band-edge PL emission at 446 nm with a narrow FWHM of 22 nm, effectively eliminating intrinsic defects in the QDs core and/or on the QD surface, thus, enhancing the PL QY up to 12%. QLEDs based on AgGaS₂/GaS_x QDs were fabricated and exhibited a blue emission at 450 nm, accompanied by a weak broad emission peak fixed at 560 nm, obtaining a maximum luminance of 15 cd m⁻² and an EQE of 0.66%. A comprehensive understanding of the luminescence mechanism in blue-emitting QDs is essential for achieving high monochromatic PL properties, which is crucial for developing high-color-purity and high-efficiency blue QLEDs.

Herein, we synthesized blue-emitting CuAlSe₂ core QDs with a PL peak at 458 nm using a hot-injection method. By coating a double-shell structure of Ga₂S₃ and ZnS on the core QD surface, the band-edge emission was improved, resulting in a blue-shifted PL emission of 450 nm and a narrowing of the FWHM to 35 nm. The PL QY was increased from 56% for CuAlSe₂ core QDs to 69% for CuAlSe₂/Ga₂S₃/ZnS core/shell QDs. Consequently, color-pure blue QLEDs based on CuAlSe₂/Ga₂S₃/ZnS QDs were achieved, which exhibited a current efficiency (CE) of 3.16 cd A⁻¹ and an EQE of 0.35%. We believe that this study offers a feasible strategy for developing lead-free and cadmium-free blue QDs for the fabrication of highly efficient QLEDs.

2. Experimental

2.1. Materials

Zinc acetate dihydrate (Zn(OAc)₂·2H₂O, 98%), copper acetate (Cu(OAc)₂, 99.99%), gallium (III) acetylacetonate (Ga(acac)₃, 99.99%), aluminum isopropoxide (Al(OCH(CH₃)₂)₃, 99.998%), Se powder (99.99%), octadecene (ODE, 90%), oleic acid (OA, 90%), oleylamine (OLA, 90%), dodecanethiol (DDT, 98%), tributylphosphine (TBP, 95%), n-hexane (97%), and methanol (99.5%) were purchased from Aldrich Inc (Shanghai, China). S powder (99.5%) was purchased from Beijing Yinuokai Technology Co., Ltd. (Beijing, China). All the materials were used as received without further purification.

2.2. Preparation of Stock Solution

To prepare the Se stock solution, Se powder (0.16 mmol), ODE (6.0 mL), OA (0.5 mL), and DDT (0.5 mL) were loaded in a three-neck flask and sonicated for 10 min at 25 °C, then allowed to keep at room temperature for later use. A Ga₂S₃ stock solution was made by heating a mixture of Ga(acac)₃ (0.1 mmol), S powder (0.6 mmol), ODE (6 mL), and TBP (0.6 mL) in a three-neck flask at 100 °C for 15 min under an Ar atmosphere. A Zn stock solution was created by dissolving Zn(OAc)₂·2H₂O (0.6 mmol) into a mixture of ODE (3.0 mL), DDT (1.0 mL), and OLA (1.5 mL) in a three-neck flask at 130 °C for 10 min under an Ar atmosphere. The Ga and Zn stock solutions were then stored at 50 °C for further use.

2.3. Synthesis of CuAlSe₂/Ga₂S₃/ZnS QDs

CuAlSe₂ QDs were synthesized during our previous study [24]. Briefly, Cu(OAc)₂ and Al(OCH(CH₃)₂)₃ with a Cu/Al molar ratio of 0.6 were mixed with OLA (0.5 mL), OA (1.0 mL), and ODE (6.0 mL) in a three-neck flask and heated to 120 °C under an Ar atmosphere, after which DDT (0.6 mL) was quickly injected into the mixture and kept at 120 °C for 15 min until a clear solution formed. The temperature was then rapidly raised to 200 °C, which was then injected into Se stock solution (1.5 mL), maintaining the temperature for 20 min under an Ar atmosphere. The temperature of the reaction mixture was subsequently reduced to 180 °C, followed by injection into Se stock solution (5.0 mL), which was kept at 180 °C for 15 min to form CuAlSe₂ core QD solution. After the completion of the reaction, the mixture was instantly cooled using liquid nitrogen and then gradually brought to room temperature.

To grow the Ga₂S₃ shell, the above system was heated to 210 °C at a rate of 10 °C/min and the Ga₂S₃ stock solution (1.2 mL) was added at a rate of 0.2 mL/min, maintaining the temperature for 30 min. The reaction solution was then immediately cooled with liquid nitrogen and allowed to gradually reach room temperature. For the subsequent growth of the ZnS shell, the as-prepared CuAlSe₂/Ga₂S₃ QD solution was heated to 230 °C at a rate of 10 °C/min and then the Zn stock solution (800 μL) was added at a rate of 0.2 mL/min. The reaction mixture was kept at 230 °C for another 30 min. After rapidly cooling the mixture with liquid nitrogen and allowing it to return to room temperature, the CuAlSe₂/Ga₂S₃/ZnS QD solution was obtained. The resulting product was precipitated with excess acetone. The purification process was performed twice by adding methanol and n-hexane, and the samples were then dispersed in n-hexane for micro-structure and optical characterizations.

2.4. Fabrication of QLEDs

QLEDs were fabricated on an indium tin oxide (ITO) glass substrate which was cleaned sequentially with detergent, deionized water, acetone, and ethanol with ultrasonic agitation, followed by ultraviolet ozone treatment for 10 min. A hole injection layer was formed by spin-coating poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Xi’an Polymer Light Technology Corporation, Xi’an, China) onto the ITO substrate at 5000 rpm for 60 s, then annealed at 150 °C for 20 min. Afterwards, a chlorobenzene solution of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4′-(N-(4-butylphenyl))] (TFB) (Xi’an Polymer Light Technology Corporation, Xi’an, China) at a concentration of 6.0 mg/mL was deposited on the PEDOT:PSS layer at 3000 rpm for 30 s and then baked at 150 °C for 30 min to form the hole transport layer. Subsequently, the CuAlSe₂/Ga₂S₃/ZnS QDs (dissolved in n-hexane at a concentration of 10 mg/mL) and ZnO nanoparticles (dissolved in ethanol at a concentration of 20 mg/mL) were spin-coated onto the TFB layer at 2000 rpm for 20 s and 3000 rpm for 30 s, respectively, to create the QD emitting layer (EML) and electron transport layer (ETL). The QD layer was then baked at 80 °C for 60 min, while the ETL layer was annealed at 60 °C for 30 min. Finally, an Al cathode (100 nm) was thermally evaporated at a pressure of 6 × 10⁻⁴ Pa through a shadow mask with an active device area of 4 mm².

2.5. Characterization

High-resolution transmission electron microscopy (HRTEM) images of QDs were evaluated using an FEI Talos F200X operating at 300 kV (Thermo Fisher Scientific, Waltham, MA, USA). The elemental contents of the QDs were quantified with a PerkinElmer ELAN DRC-e inductively coupled plasma mass spectrometer (ICP-MS) (PerkinElmer, Woodbridge, ON, Canada). X-ray diffraction (XRD) measurements of QDs were performed by a Rigaku D/max 2500v/pc diffractometer with Cu Kα radiation (Rigaku Corporation, Tokyo, Japan). UV–Vis absorption spectra were measured using a Hitachi UV−4100 spectrophotometer (Hitachi High-Tech Science Corporation, Tokyo, Japan). PL spectra and decay profiles were recorded on a Jobin-Yvon FluoroLog-3 fluorescence spectrometer (Horiba, Longjumeau, France) with a 450 W Xenon lamp (Hitachi High-Tech Science Corporation, Tokyo, Japan) and a laser with a wavelength of 376 nm (Hitachi High-Tech Science Corporation, Tokyo, Japan) employed as excitation sources, respectively. For UV–Vis absorption spectra and PL characteristic measurements, each QD solution was diluted to achieve an optical density of approximately 0.1 at the excitation wavelength. The relative PL QY of QDs was determined by using Rhodamine 6G as a standard reference (with a 95% QY in ethanol) according to the following equation [28]:

$${\phi }_S={\phi }_R\times {\displaystyle \frac{I_S}{I_R}}\times {\displaystyle \frac{A_R}{A_S}}\times {\displaystyle \frac{{n_S}^2}{{n_R}^2}}$$

(1)

where φ_S and φ_R represent the PL QY of QDs and Rhodamine 6G, respectively, I is the integrated PL intensity at 370 nm, A is the identical optical density of 0.05 at 370 nm, n is the refractive index of the solvent (n = 1.375 for n-hexane; n = 1.36 for ethanol). Thermogravimetric and differential scanning calorimetry (TG-DSC) analyses were performed with a PerkinElmer STA 6000 instrument (PerkinElmer, Massachusetts, United States of America) with a heating rate of 20 °C/min under a N₂ atmosphere. EL spectra of QLEDs were measured using a spectroradiometer (Konica Minolta CS-2000, Tokyo, Japan). The current density-voltage-luminance (J-L-V) characteristics of QLEDs were examined with a Keithley 2400 source meter (Keithley, Cleveland, OH, USA) and a Konica Minolta CS-2000 luminance meter (Konica Minolta, Tokyo, Japan). All measurements were conducted at room temperature under relatively constant ambient conditions.

3. Results and Discussion

3.1. Micro-Structure Analysis

HRTEM measurements were conducted on CuAlSe₂ core, CuAlSe₂/Ga₂S₃ core/shell, and CuAlSe₂/Ga₂S₃/ZnS core/shell/shell QDs to confirm their morphologies, as displayed in Figure 1. The image analysis (Figure 1a–c) of different areas indicates that the CuAlSe₂ core QDs possess a quasi-spherical shape with an average diameter of 4.8 ± 0.31 nm (Figure 1d). This size increases to 5.4 ± 0.32 nm (Figure 1e) and 5.9 ± 0.31 nm (Figure 1f) following the growth of Ga₂S₃ and ZnS shells, respectively. The average size increase of 1.1 nm corresponds to the formation of approximately one monolayer of Ga₂S₃ and ZnS, respectively [29]. This size increase indirectly proves that the double-shell of Ga₂S₃ and ZnS can grow on the surface of CuAlSe₂ core QDs. There is an invariable morphology, and relatively uniform particle size distribution, revealing homogeneous shell growth. For CuAlSe₂ QDs, the interlattice spacing of 0.31 nm determined via image analysis is attributed to the (112) plane of the chalcopyrite CuAlSe₂ structure [30], which remains unchanged after the growth of Ga₂S₃ and ZnS shells. Additionally, ICP-MS measurements of CuAlSe₂/Ga₂S₃ and CuAlSe₂/Ga₂S₃/ZnS QDs were performed to quantify the elemental contents in QDs. As shown in

Table 1. Elemental contents of CuAlSe₂/Ga₂S₃ and CuAlSe₂/Ga₂S₃/ZnS QDs determined by ICP-MS.

Element	CuAlSe2/Ga2S3	CuAlSe2/Ga2S3/ZnS
	Atomic Percentage (%)
Cu	13.3	9.62
Al	21.53	16.5
Se	27.7	10.19
Ga	17.35	12.02
S	20.12	35.87
Zn	—	15.8

, there is a high S content after the growth of the ZnS shell, accompanied by a decrease in Se content. In the presence of all shelling precursors, the weak Se-S bond results in the rapid nucleation rate of ZnS, facilitating the formation of a ZnS shell on the surface of CuAlSe₂/Ga₂S₃ QDs, which effectively confines excitons.

The XRD characterizations were carried out to analyze the structure of CuAlSe₂ QDs before and after the shell growth, as illustrated in Figure 2. For CuAlSe₂ QDs, the representative diffraction peaks at 27.73°, 46.23°, and 54.34° are matched with the (112), (204), and (312) planes of the chalcopyrite structure of CuAlSe₂ [30]. The diffraction peak at 64.60° corresponds to the (411) plane of cubic Cu₂Se [31]. After the growth of the Ga₂S₃ shell, the XRD pattern retains the primary diffraction peaks diffraction peaks but shows a slight shift to the higher angles, approaching the peak position of the hexagonal Ga₂S₃ structure. This shift indicates that the Ga₂S₃ shell formation is induced by compressive strain effect or the diffusion of Ga³⁺ ions to the surface of core QDs. Moreover, the disappearance of the peak at 64.60° suggests that the Ga₂S₃ shell can suppress the formation of the Cu₂Se phase, leading to an increased intensity of the diffraction peak at 28.15°. The subsequent growth of the ZnS shell leads to sharper XRD peaks compared to the CuAlSe₂/Ga₂S₃ QDs, indicating that the CuAlSe₂/Ga₂S₃/ZnS QDs have a high crystallinity and relatively larger sizes due to the shell treatment. Additionally, both curves show no appearance of new diffraction peaks, but a slight shift to higher angles towards the peak of zinc blende ZnS indicates that the Cu⁺ ion sites in the core QDs replaced by Zn²⁺ ions, resulting in the formation of ZnS shell. The interplanar spacing of the lattice fringes is 0.32 nm, which aligns well with the 0.321 nm spacing of the (112) crystal face in chalcopyrite CuAlSe₂.

3.2. Absorption Analysis and Luminescence Properties

Figure 3 presents the evolution of the absorption spectra of CuAlSe₂ QDs before and after shell growth. After the coating with a Ga₂S₃/ZnS double-shell, a blue shift in the absorption onset is observed compared to the CuAlSe₂ core QDs. Meanwhile, an obvious excitonic absorption peak at 450 nm is noticeable. By extrapolating the Tauc’s relation, the band gap is widened from 2.79 to 2.97 eV. Considering the direct band gap of Ga₂S₃ (E_g = 2.8 eV) [23,32] and ZnS (E_g = 3.6 eV) [33], as well as the XRD results (Figure 2), the widened band gap is associated with the improved component of Ga₂S₃ and ZnS in the phase structures, the alloying of CuAlSe₂ with Ga₂S₃ and ZnS, and the lattice strain between the core and the shells of QDs, which slightly shift the band alignments.

The PL measurements were carried out at an excitation wavelength of 370 nm at room temperature for CuAlSe₂, CuAlSe₂/Ga₂S₃, and CuAlSe₂/Ga₂S₃/ZnS QDs. The normalized PL spectra and PL QY of the QD samples are shown in Figure 4a. The PL spectra of CuAlSe₂ QDs display an emission peak at 458 nm with an FWHM of 53 nm, attributed to a combination of transitions from the band-edge and defect-related transitions. After coating with a Ga₂S₃ shell, the PL emission peak shifts to 455 nm, accompanied by a weak peak at 492 nm, which is attributed to radiative transitions from the direct band gap hexagonal Ga₂S₃ phase [34]. A further blue-shift to 450 nm in the emission peak is observed after the QDs are coated with a double-shell of Ga₂S₃ and ZnS, with no significant emission at a longer wavelength, and the FWHM is narrowed to 35 nm. This indicates that some defect sites in the CuAlSe₂ core QDs and/or on their surface are eliminated, enhancing the band-edge emission. The reproducibility of the PL QY was evaluated by analyzing the statistics of 8 QDs from each batch, as presented in Figure 4b. A PL QY increase from 56 to 69% was achieved through double-shell passivation, confirming an improvement in excitonic recombination probability. This result is comparable to previous studies that reported PL QY of blue I-III-VI QDs to be below 50%, with PL emissions ranging from 450 to 475 nm [22,27].

A Gaussian fitting analysis was performed (Figure 5) to further investigate the origin of luminescence. Considering the long-wavelength tail of the absorption spectra and the emission band with an FWHM exceeding 40 nm of CuAlSe₂ and CuAlSe₂/Ga₂S₃ QDs, the PL spectra can be well decomposed into three subspectra, as depicted in Figure 5a,b. For CuAlSe₂/Ga₂S₃/ZnS QDs, the PL emission exhibits a symmetric spectral shape with an FWHM of less than 40 nm, which can be divided into two subspectra (Figure 5c) including higher-energy excitonic and lower-energy trap emission components [35]. As discussed in our previous research [24], the multiple electronic transitions in CuAlSe₂ QDs involve the transitions from the conduction band to the valence band (CB-VB), from the Al_Cu defect level to the valence band (Al_Cu-VB), and from the conduction band to Cu-related levels (e.g., V_Cu) (CB-V_Cu). After the growth of the Ga₂S₃ shell, the CB-VB emission dominates the PL spectra of QDs while a consistent decrease in CB-V_Cu emission is observed alongside an increase in Al_Cu-VB emission (Figure 5a,b). Moreover, the three fitted emission peaks are noticeably shifted towards the red region (Figure 5d) which is associated with the substitution of some Al³⁺ ions with Ga³⁺ ions at the surface of the core QD during the shelling process. The fitted emission peaks of CuAlSe₂/Ga₂S₃ QDs are located at 453, 473, and 506 nm, respectively, which may account for the weak peak at 492 nm in the PL emission (Figure 4a) ascribing to the complex transition behavior. When the ZnS shell is subsequently deposited, the increase in the CB-VB emission component primarily contributes to the narrow-band blue emission. This suggests that the Cu-related levels are passivated, leading to a suppression of the longer-wavelength spectral component. This finding supports the notion of improved PL QY.

To investigate the effect of shell growth on the electronic transition process in QDs, time-solved PL decay spectra of CuAlSe₂, CuAlSe₂/Ga₂S₃, and CuAlSe₂/Ga₂S₃/ZnS QDs were obtained by monitoring the emission wavelengths at 458, 455, and 450 nm, respectively. As shown in Figure 6a, the decay spectra are well-fitted by the following triexponential function [36]:

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) + A₃ exp(−t/τ₃)

(2)

where τ₁, τ₂, and τ₃ represent the decay time constants for the emission components, while A₁, A₂, and A₃ denote their relative amplitudes, respectively. The average lifetimes (τ_av) were calculated using τ_av = ∑A_iτ_i²/∑A_iτ_i, revealing that CuAlSe₂/Ga₂S₃/ZnS QDs possess a longer average lifetime of 316 ns compared to the 259 ns lifetime of CuAlSe₂ QDs. This indicates that the Ga₂S₃/ZnS double-shell structure enhances the radiative decay pathways, a finding that can be supported by the dependence of PL QY on the growth of the shells.

Additionally, the thermal property of CuAlSe₂/Ga₂S₃/ZnS QDs was investigated by TG-DSC analysis, as illustrated in Figure 6b. A notable mass loss of approximately 17% occurs at 336 °C, corresponding to an exothermal peak identified by DSC, which is attributed to the degradation of ligand molecules on the QD surface. At 800 °C, the weight retention is 65.2%, indicating nearly complete volatilization of the decomposition products from the ZnS shell. This result suggests that the ZnS capping shell material contributes to the stability of QDs during the operation of QLED devices.

3.3. Fabrication of QLEDs

With the QDs before and after shell growth, three types of QLEDs based on CuAlSe₂, CuAlSe₂/Ga₂S₃, and CuAlSe₂/Ga₂S₃/ZnS QDs were fabricated and termed as device A, B, and C, respectively. These devices were constructed with an architecture of ITO/PEDOT:PSS/TFB/QDs/ZnO/Al, as shown in Figure 7a. All three devices demonstrate blue emission (Figure 7b and

Table 2. Performance of blue QLEDs based on CuAlSe₂, CuAlSe₂/Ga₂S₃, and CuAlSe₂/Ga₂S₃/ZnS QDs^a.

Device	QDs	EL Peak (nm)	EL FWHM (nm)	Lmax (cd m−2)	CE (cd A−1)	EQEmax (%)
A	CuAlSe2	463	60	1.6	0.03	0.02
B	CuAlSe2/Ga2S3	460	45	53.7	1.93	0.21
C	CuAlSe2/Ga2S3/ZnS	453	39	138.9	3.16	0.35

L_max: luminance; CE: current efficiency.

) at 463, 460, and 453 nm, respectively, at an applied voltage of 5 V, which is a slight red-shift compared to their PL spectra owing to the quantum confinement Stark effect [37]. For device C, the J-L-V characteristic curves, as well as the CE and EQE as functions of the current density, are presented in Figure 7c,d. Device C has a turn-on voltage of 2.8 V and obtained a maximum luminance of 138.9 cd m⁻² at an applied voltage of 5 V. The maximum CE and EQE achieved are 3.16 cd A⁻¹ and 0.35%, respectively, which are superior to those of devices A and B (Table 2). The CE and EQE values for device C are comparable to previous studies [23,26], attributed to a more balanced charge distribution in the QD EML. These results indicate that CuAlSe₂/Ga₂S₃/ZnS QDs possess significant potential for use in QLEDs as blue-emitting QDs, and further improvements in EL performance are needed through optimization of the structural design of the blue-emitting QDs and the interface energy level structure of the device.

4. Conclusions

In this study, blue-emitting CuAlSe₂/Ga₂S₃/ZnS QDs were synthesized using a hot-injection approach. Through overgrowth with a Ga₂S₃/ZnS double-shell, the PL emission exhibited a blue-shifted from 458 to 450 nm compared to the CuAlSe₂ core QDs. This shift was attributed to the suppression of intrinsic defect states (e.g., V_Cu) and an enhancement in radiative recombination from CB-VB transitions, yielding a PL QY increase of up to 69%. The resulting color-pure blue QLEDs based on CuAlSe₂/Ga₂S₃/ZnS QDs (450 nm) achieved a CE of 3.16 cd A⁻¹ and an EQE of 0.35%. Accordingly, our research provides an approach for passivating intrinsic defects to narrow the FWHM in the PL spectra and enhance band-edge emission, and further improve the EL performance of eco-friendly blue QLEDs for display applications.