Blue ZnSeTe quantum dot light-emitting diodes with low efficiency roll-off enabled by an in situ hybridization of ZnMgO nanoparticles and amino alcohol molecules

Abstract

ZnSeTe quantum dots (QDs) have been employed as promising emitters for blue QD-based light-emitting diodes (QLEDs) due to their unique optoelectronic properties and environmental friendliness. However, such QLEDs usually suffer from serious efficiency roll-off primarily stemming from exciton loss at the interface of the QD layer and the ZnMgO (ZMO) electron transport layer (ETL), which remarkably hinders their application in flat-panel displays. Herein, we propose an in situ hybridization strategy that involves the pre-introduction of amino alcohols into the reaction solution. This strategy effectively suppresses the nucleophilic condensation process by facilitating the coordination of ammonium and hydroxyl groups with metal cations (M²⁺, i.e. Zn²⁺ and Mg²⁺). It slows down the growth rate of ZMO nanoparticles (NPs) while simultaneously facilitating M–O coordination, resulting in the synthesis of small-sized and low-defect ZMO NPs. Notably, this in situ hybridization approach not only alleviates emission quenching at the QDs/ETL interface but also elevates the energy level of the ETL for enhancing carrier injection. We further investigated the impact of amino alcohols with varying carbon-chain lengths on the performance of ZMO NPs and the corresponding LED devices. The optimal blue ZnSeTe QLED demonstrates an impressive EQE of 8.6% with only an ∼11% drop when the current density is increased to 200 mA cm⁻², and the device operating lifetime extends to over 1300 h. Conversely, the device utilizing traditionally post-treated ZMO NPs as the ETL exhibits 45% efficiency roll-off and device lifetime of merely 190 h.

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Introduction

Quantum dot (QD) light-emitting diodes (QLEDs) are expected to achieve wide-color-gamut, low-cost, and flexible displays due to their advantages including high color-purity emission, size-dependent emission color, and low-cost solution processability.^1–5 However, the high toxicity of cadmium in well-established cadmium chalcogenide-based QDs poses sustainability concerns under the EU's Restriction of Hazardous Substances Directive and global environmental regulations. InP QDs emerge as non-toxic alternatives due to their similar optical properties to traditional CdSe QDs.⁶ Red and green-emitting InP-based QLEDs have achieved impressive external quantum efficiencies (EQEs) of 22.56% and 16.3%, respectively.^7,8 To achieve blue emission, small-sized InP QDs are requisite. Nonetheless, the presence of numerous core/shell interface defects renders InP QDs unsuitable for high-performance blue QLEDs.⁹ In another case, ZnSeTe alloyed QDs offer broad spectral adaptability, allowing them to cover the pure blue region (450–470 nm) by adjusting the Se/Te molar ratio, thereby making them the most promising environmentally friendly blue emitters.^10–12 Jang et al. demonstrated a highly efficient blue ZnSeTe-based QLED with an impressive EQE surpassing 20%.¹³ Notably, the reported ZnSeTe-based QLEDs commonly achieved peak EQEs at low driving current densities (∼1–10 mA cm⁻²) and low luminance, which significantly diminish at relatively high current densities.^14–17 This phenomenon is commonly known as the EQE roll-off or efficiency droop effect.^18,19 For display applications, it is crucial to address the efficiency roll-off issue, especially when high luminance and efficiency levels are required.²⁰

ZnMgO nanoparticles (ZMO NPs) are usually used as the electron transport layer (ETL) for blue QLEDs, commonly synthesized via a sol–gel strategy.^14,15,21 However, the room-temperature reaction conditions often yield ZMO NPs with poor crystallinity and numerous defects, particularly oxygen vacancies (V_O).^22,23 These defects acting as charge capture centers lead to exciton quenching at the interface between QDs and ETL,^24–27 which contributes to the efficiency roll-off observed in ZnSeTe-based blue QLEDs.²⁸ To address this issue, surface-modification strategies have been developed to mitigate V_O formation in ZMO NPs. Han et al. reported ZMO NPs surface-modified with Mg(OH)₂ to reduce electron mobility and alleviate emission quenching of the QD emitting layer (EML).¹⁶ Similarly, Yoon et al. achieved a comparable effect by the surface modification of ZMO NPs using acrylic functionalization. Although their EQEs have been remarkably improved, the efficiencies still drop by over 35% at a high current density of 100 mA cm⁻².²⁹ It is important to note that these post-treated processes only passivate the surface defects of ZMO NPs and cannot eliminate lattice defects generated during the growth process.

Here, we developed in situ hybrid amino alcohol ZMO NPs to resolve the aforementioned challenges. An amino alcohol was introduced into the reaction solution beforehand to mitigate the growth rate of ZMO NPs with the aid of the strong coordination of the ammonium and hydroxyl groups in the amino alcohol with the metal cations (M²⁺, i.e. Zn²⁺ and Mg²⁺).^30,31 The slow crystal growth allows for sufficient coordination of M–O composites, thus suppressing lattice defects. In addition, similar to traditional ligand post-treatment processes, the in situ hybrid enables amino alcohols to act as ligands to eliminate surface defects. At the same time, the in situ hybrid ZMO NPs not only alleviates the emission quenching at the QDs/ETL interface, but also shifts the energy level of the ZMO NPs upwards, lowering the energy barrier at the QDs/ZMO interface, and thus improves electron injection. We further investigated the impact of amino alcohols with various carbon-chain lengths (ethanolamine (EA), 3-aminopropanol (PA), and 4-aminobutanol (BA)) on the performance ZnSeTe QLEDs. The device based on a PA-treated ZMO ETL exhibited the optimal effective current ratio, which affects the electron–hole recombination zone and rate in the QD EML as well as at the hole transport layer (HTL)/QDs interface. Utilizing the optimal in situ PA-hybridized ZMO NPs as the ETL, the device achieved an EQE of 8.6%, maintaining a flat profile (<11%) across a wide current density range from 85 to 200 mA cm⁻². In contrast, the counterparts based on post-treated ZMO NPs experienced an EQE loss of more than 45%. Additionally, the device exhibits a long half-life (T₅₀) of over 1300 h when the initial luminance (L₀) is set at 100 cd m⁻², which is 6.8 times longer than that of the control device.

Results and discussion

Fig. 1a illustrates the synthesis protocol of in situ hybrid ZMO NPs, in which the amino alcohol (structural formula shown in Fig. S1†) as additive was introduced before the injection of tetramethylammonium hydroxide (TMAH) to regulate the reaction kinetics. To analyze the impact of amino alcohol on the growth dynamics of ZMO NPs, we took PA as a representative amino alcohol and monitored the evolution of UV-vis absorption spectra over time for both pristine and in situ hybrid ZMO NPs (Fig. 1b). As the reaction time increases, the absorption edge of the pristine ZMO NPs exhibits a rapid redshift. When the NPs undergo post-treatment with PA, their absorption spectrum remains almost identical.³² In contrast, the in situ hybrid ZMO NPs exhibit a much smaller redshift, indicating a slower crystallization process. Because the ammonium and hydroxyl groups of PA coordinate strongly with M²⁺ cations, the nucleophilic polycondensation process is delayed during the crystal growth.³³ The coordination effect of PA was confirmed by the distinct shift of N–H and O–H groups in Fourier transform infrared (FTIR) spectra of the reaction solution with PA molecules (Fig. S2†).^34,35 As a result, these in situ hybrid ZMO NPs exhibit the weakest emission associated with defects among the three samples (Fig. 1c and S3†). The corresponding FTIR spectra are shown in Fig. 1d. The peaks at 705 cm⁻¹ (rocking vibration of –CH₂), 951 cm⁻¹ (vibration peaks of O–H, N–H) and 1017 cm⁻¹ (stretching vibration of C–N) are absent in pristine ZMO NPs but become evident in post-treated and in situ hybrid counterparts, verifying that PA also acts as a surface ligand in the in situ hybrid ZMO NPs.^36,37 Consequently, in comparison with the post-treated ZMO NPs, the additional photoluminescence (PL) quenching observed in the in situ hybrid sample (Fig. 1c) can be attributed to sufficient coordination of M–O composites and consequent suppression of lattice defects. The transmission electron microscopy (TEM) images in Fig. S4a and b† reveal that the in situ hybrid ZMO NPs exhibit similar morphology and size distribution as their post-treated counterparts, except for a smaller average diameter (Fig. S4c and d†), which is consistent with the trends observed in Fig. 1b and the broadened diffraction peaks in Fig. S5.†

Fig. 1 (a) Schematic illustration for the synthesis process for in situ hybrid ZMO NPs. (b) Evolution of UV-vis absorption spectra over time for the pristine and in situ hybrid ZMO NPs, along with the spectra of the post-treated ZMO NPs. (c) Comparison of PL intensity of the pristine, post-treated and in situ hybrid ZMO NPs. Inset: the corresponding photographs under UV irradiation. (d) FTIR spectra of the three NPs.

To investigate the mechanism of in situ amino alcohol inhibiting exciton quenching at the QDs/ZMO interface, we conducted steady-state PL and time-resolved PL decay (TRPL) analysis of QD films deposited on glass, and on substrates of both post-treated and in situ hybrid ZMO NPs, respectively. Compared to QDs directly deposited on glass substrates, those deposited on both in situ hybrid and post-treated ZMO films exhibit fluorescence quenching, indicating exciton quenching at the ZMO/QD interface. Remarkably, the glass/in situ hybrid ZMO/QD sample presents stronger PL emission than the glass/post-treated ZMO/QD sample, implying effective suppression of defects within the in situ hybrid ZMO NPs (Fig. 2a). The average exciton lifetime (τ_avg) was determined using a bi-exponential fitting function (Table S1†). We observed a reduction in τ_avg in the order of 24.30 ns for the QD film, 13.64 ns for in situ hybrid ZMO/QDs, and 8.49 ns for the post-treated ZMO/QDs (Fig. 2b), consistent with the trend of PL loss shown in Fig. 2a. It is foreseeable that this phenomenon can increase the effective current ratio.

Fig. 2 Variations in the (a) PL intensity and (b) time-resolved PL decay profile of ZnSeTe QD films on different substrates. XPS spectra of O 1s for the (c) post-treated ZMO and (d) in situ hybrid ZMO films.

The surface composition and chemical state of the post-treated and in situ hybrid ZMO NPs were analyzed using X-ray photoelectron spectroscopy (XPS). As shown in Fig. S6,† the higher binding energy observed for Zn 2p and Mg 1s in the in situ hybrid ZMO NPs compared to the post-treated ZMO NPs further confirms the reduction of defects in the former. To verify the change of V_O, the O 1s signals of both samples are depicted in Fig. 2c and d.³⁸ The core level spectra were deconvoluted with three sub-spectra: lattice oxygen, V_O, and hydroxide oxygen. This analysis clearly demonstrates a significant suppression of lattice defects through the in situ hybridization process.

We further explored the impact of amino alcohols with varying carbon-chain lengths (EA, PA, and BA) on device performance. Fig. 3a illustrates a typical QLED structure comprising indium tin oxide (ITO)/poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/poly(9,9-dioctylfluorence-co-N-(4-butyl phenyl) diphenylamine) (TFB)/ZnSeTe QDs/ETL/Al. The ZnSeTe QDs with an average size of 9 nm and a PL peak at 452 nm are used to assemble the EML (Fig. S7†). The band arrangements of blue QLEDs with a series of different ETLs are depicted in Fig. 3b, and the energy levels of QDs as well as ETLs were determined by ultraviolet photoelectron spectroscopy (UPS) and optical absorption spectroscopy measurements (Fig. S8–S10†). Compared to the post-treated ZMO NPs, the energy levels of the in situ hybrid ZMO NPs all shifted upward (Fig. 3b), which may be ascribed to the size reduction and defect suppression.^22,39,40 A slight redshift of EL (452 nm) relative to PL (456 nm) was observed in Fig. 3c, which is attributed to the quantum-confined Stark effect.⁴¹ The inset photograph shows the working device emitting blue light.

Fig. 3 (a) Schematic structure and (b) energy-level alignment of the blue ZnSeTe QLEDs with different ETLs. (c) Normalized PL and EL spectra; the inset shows a photograph of the device operating at 4.5 V. Performance comparison of QLEDs using the post-treated ZMO, ZMO-EA, ZMO-PA and ZMO-BA as ETLs: (d) current density–voltage, (e) luminance–voltage, and (f) EQE–luminance.

As depicted in Fig. 3d, the three devices, based on in situ hybrid ZMO with EA, PA and BA, present higher current density values than that of the post-treated ZMO-based device. The variation in current density being dependent on the ETL was further confirmed through electron-only devices (EODs), demonstrating a consistent trend as depicted in Fig. S11.† This phenomenon is attributed to the reduced potential barrier at the QDs/ZMO interface, thereby enhancing electron injection. This is also evidenced by the drop in the turn-on voltage (V_t) from 3.5 V to 2.5 V for the in situ hybrid ZMO-based QLED. The control device presents its highest luminance of 6668 cd m⁻² and peak EQE of 4.5% (Fig. 3e and f). In contrast, the devices based on in situ hybrid ZMO ETLs exhibit higher current density and luminance values at lower voltages. The peak luminance of the ZMO-EA-, ZMO-PA-, and ZMO-BA-based QLEDs reached 12 410 cd m⁻², 14 125 cd m⁻², and 13 109 cd m⁻², with their peak EQE of 7.0%, 8.6% and 6.7%, respectively (Table S2†).

Fig. S12a† shows the PL spectrum and EQE of a QLED based on an in situ hybrid ZMO ETL collected at 0.25 V intervals. When the current density is increased from 3 mA cm⁻² to 110 mA cm⁻², the relative PL intensity decreases by about 30%, while the EQE continues to increase and reaches its maximum. The reduced PL efficiency is primarily attributed to charged QDs. However, benefiting from the balanced charge injection, the EQE is improved. In the QD emitting layer, the injection of excess electrons initially generates long-lived intermediate QDs.⁴² The increase in the number of electrons leads to the potential moving towards higher energy (Coulomb charging effect), which is manifested macroscopically as the energy level of the QD layer moving upwards (Fig. S12b†).^42,43 Consequently, excessive electron injection is prevented and hole injection is significantly enhanced through Coulomb interaction.^42–44 To verify our hypothesis, we inserted a labelling layer of red QDs between the TFB and ZnSeTe QDs to investigate the carrier balance and the effective current ratio of the devices (Fig. S13a†). At a current density of 20 mA cm⁻², the EL intensity of the red QDs in the in situ hybrid ZMO-based QLED was higher than that of the post-treated ZMO-based QLED (Fig. S13b†). This can be directly attributed to the improved hole injection efficiency from the TFB to the red QDs in the in situ hybrid ZMO-based QLED, resulting in more electron–hole recombinations in the red QDs and thus an increase of the effective current ratio of the device.⁴⁵ The EL intensity of red QDs in the ZMO-PA-based device is the highest, which indicates that it obtains the best effective current ratio, thus obtaining the maximum EQE and luminance. ZMO-BA NPs, having the longest carbon-chain and weakest group coordination ability, possess more defects (Fig. S14†), resulting in a decrease in its effective current ratio and further leading to a decrease in EQE. ZMO-EA NPs have the fewest defects, which could reduce the carrier mobility (Fig. S11†), and they fail to reach the optimal effective current ratio, resulting in a certain impact on device performance.

A comparison was made between two representative QLEDs utilizing post-treated ZMO and ZMO-PA ETLs in terms of device operational stability. Fig. 4a depicts the normalized EQE as a function of current density. The peak EQE of the post-treated ZMO-based device reaches 4.5% at 6 mA cm⁻², accompanied by a luminance of 357 cd m⁻², whereas at 200 mA cm⁻², the EQE experiences a rapid decline to 2.93%, marking a 45% decrease from the peak EQE value, which is consistent with previously reported observations.^10,14,16 For ZMO-PA-based devices, the peak EQE (8.6%) was achieved at a current density of 84.4 mA cm⁻² and a luminance of 4583 cd m⁻². When the current density was increased to 200 mA cm⁻², EQE only decreased by 11%. Under the same voltage or current density, the in situ hybrid ZMO-based devices exhibit higher luminance and efficiency than the post-treated ZMO-based device. Fig. S15† shows the maximum EQE histogram for 33 MZO-PA-based QLEDs where the average EQE of these devices is 7.3%, which proves the feasibility of MZO-AA as the ETL in QLEDs. At a constant current of 10 mA, the changes in luminance and driving voltage over time are shown in Fig. 4b and c, respectively. L₀ values of the post-treated ZMO- and ZMO-PA-based QLEDs are 5048 cd cm⁻² and 13 031 cd cm⁻², respectively. The T₅₀ values of the post-treated ZMO-based QLED and ZMO-PA-based QLED are 9.85 min and 12.2 min, respectively. Using the formula (an acceleration factor of 1.8) to convert T₅₀ at L₀ of 100 cd cm⁻², the device lifetime with a 6.8-fold improvement was extended from 190 h for the post-processed ZMO-based devices to 1301 h for ZMO-PA-based devices. Notably, the driving voltage of the ZMO-PA-based QLED exhibits a more linear trend, which can improve the colour uniformity and energy efficiency of the QLEDs. Additionally, it makes control of the device performance easier, thus being more conducive to long-term device operation and practical applications.^46,47 The improved stability is attributed to the effective suppression of exciton quenching at the QDs/ETL interface and the enhancement of the effective current ratio.

Fig. 4 (a) Normalized EQE–current density of blue QLEDs integrated with the post-treated ZMO and ZMO-PA ETLs. (b) Luminance decay vs. time and (c) driving voltage vs. time of QLEDs with the post-treated ZMO and ZMO-PA ETLs operated at a constant current of 10 mA.

Conclusions

In summary, a blue ZnSeTe QLED with low efficiency roll-off was achieved by using in situ amino alcohol-hybridized ZMO ETLs. The results showed that the coordination of ammonium and hydroxyl groups with M²⁺ ions slowed down the growth rate of ZMO NPs, effectively reducing the V_O in ZMO NPs, and suppressing the fluorescence quenching phenomenon at the QDs/ETL interface. Due to size reduction and defect suppression, the energy levels of ZMO NPs shift upwards, lowering the energy barrier at the QDs/ZMO interface and thus improving electron injection. Meanwhile, this process also increases the effective current ratio. Finally, we investigated the effect of amino alcohols with different carbon-chain lengths hybrid ZMO ETLs on device performance. The optimal QLED presents a high EQE of 8.6% with low efficiency roll-off even at a current density of 200 mA cm⁻², and the device operating lifetime extends to over 1300 h, which is over 6 times longer than that of the control device.

Author contributions

X. Yang participated in supervision and coordination throughout the process. S. Ma conceived the work, conducted the experiments and wrote the initial manuscript. Q. Wu and F. Cao provided the ZnSeTe QDs and contributed to the measurement of devices. S. Wang and Q. Wu analyzed the experimental results and revised the final draft. All authors have contributed to the editing and revision of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Key Research and Development Program of China (2022YFB3602902). This work was also supported by the National Natural Science Foundation of China (62105195, 62174104, and 61735004), the Australian Research Council (ARC) Future Fellowship Scheme (FT210100509), the ARC Discovery Project Scheme (DP220101959), the Shuguang Program of the Shanghai Education Development Foundation and Shanghai Municipal Education Commission (no. 22SG40), the Shanghai Natural Science Foundation (23ZR1423300), the Program of the Shanghai Academic/Technology Research Leader (22XD1421200) and the China Postdoctoral Science Foundation (2023M742197, 2023M742198).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr01515k

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