Fully solution-processed red tandem quantum dot light-emitting diodes with an EQE exceeding 35%

Abstract

Compared to conventional quantum dot light-emitting diodes (QLEDs), tandem QLEDs (TQLEDs) have the advantages of long lifetime, high brightness and high efficiency, thereby making them potential candidates for display and lighting applications. In this work, ZnMgO/PEDOT:PSS was employed as the interconnecting layer (ICL) to fabricate TQLEDs, achieving brightness doubling at the equivalent current. Further, the devices exhibited a maximum external quantum efficiency (EQE) of 35.51%, which is currently the largest EQE reported for the full-solution process of manufacturing red TQLEDs. The improvement in TQLED efficiency can be attributed to the optimization of the electronic transport layer (ETL), which is achieved by adjusting the Mg doping level in ZnO. This process facilitated a balanced electron and hole injection and limited the photoluminescence (PL) quenching of QDs by ZnO. Furthermore, it can also be attributed to the effective charge generation capacity of ZnMgO/PEDOT:PSS along with the resistance of ZnMgO to acids.

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Introduction

In recent years, colloidal quantum dot light-emitting diodes have garnered significant attention as the anticipated next-generation display technology. Their burgeoning prominence can be attributed to their high color purity, tunable light-emitting wavelengths, and potential for low-cost solution-based preparation methods.^1–4 Since the advent of QLEDs, extensive research efforts have been devoted to enhancing device performance. Nevertheless, QLEDs still demonstrate low efficiency and inadequate stability in certain application scenarios.^5–8 A promising strategy to attain superior current efficiency (CE) and extended operational lifetime for QLEDs involves the use of a tandem structure. This structure utilizes an ICL to connect two or more electroluminescent units (ELUs) in series. Consequently, the brightness of tandem QLEDs significantly surpasses the single unit due to the consistent current passing through multiple ELUs. This means that tandem devices maintain equivalent brightness using less current, which contributes positively to their extended lifetime.^9–11 To date, numerous research groups have executed a succession of investigations on TQLEDs, yielding significant accomplishments.^12–19 For instance, a noteworthy achievement was reported by Yang et al., wherein they documented the highest EQEs of 40%, 50%, and 24% for red, green, and blue TQLEDs, respectively.¹⁵

Since 2011, Qian et al. first presented research employing inorganic zinc oxide (ZnO) nanoparticles (NPs) as an ETL for fully solution-processed QLEDs, which achieved a significant increase in efficiency and brightness.²⁰ The following ten years or so, most high-performance QLED work has been based on this structure.^7,21–23 The solution method is a simpler fabrication process for acquiring extensive areas and low cost, making it one of the main advantages of QLEDs.^24,25 A number of research works on the fabrication of TQLEDs by a full-solution process have also been carried out, and most of these works have used ICLs composed of PEDOT:PSS and ZnO. However, the performance remains notably inferior to TQLEDs prepared through the combined utilization of vacuum deposition and solution method deposition.^14,15,17,26

In this study, zinc magnesium oxide (ZnMgO) was synthesized with varying concentrations of magnesium (Mg) doping, and it was found that this process can effectively suppress the PL quenching effect of ZnO on quantum dots (QDs), which is consistent with the increase in magnesium doping. This process also elevates the conduction band minimum (CBM) while reducing the electron mobility of ZnO. Particularly, a 15% doping concentration creates equilibrium in electron and hole injection, resulting in the highest achievable device performance. The ICL based on ZnMgO/PEDOT:PSS demonstrates superior charge generation efficiency, recording an EQE of up to 35.51% for the fabricated red TQLED. This work has promoted the development of a low-cost and high-efficiency QLED, paving the way for the commercialization of QLED display and lighting technology.

Results and discussion

Fig. 1a shows the device structure of a single-junction QLED and its cross-sectional transmission electron microscopy (TEM) image with the conventional structure, which consists of an indium tin oxide (ITO) transparent anode, a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole injection layer (HIL), a poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(p-butylphenyl))diphenylamine)] (TFB) hole transport layer (HTL), a CdSe/ZnS quantum dot emission layer (EML), a ZnMgO NP ETL and a top Ag cathode. The conventional structure can be manufactured using mature solution processes, resulting in better economic benefits and providing good luminescence efficiency and stability.^7,27 TFB is a commonly used polymer hole-transporting material (HTM) with higher mobility (∼10⁻³ cm² V⁻¹ s⁻¹) than other HTMs. Red QLEDs employing TFB as the HTL have demonstrated good performance in the literature with low turn-on voltage and high EQE.²⁸ ZnO has become the most widely used material for QLED ETLs due to its advantageous energy level compatibility with QDs, superior electron mobility and its simple synthesis process.²⁹ However, it has been demonstrated in many studies that the structure exhibits excessive electron injection, which leads to electron accumulation at the QDs/ETL interface, subsequently causing exciton quenching and resulting in a decline in device performance.^25,30,31 To address the electron–hole injection imbalance issue, we attempted to replace ZnO with Mg-doped ZnMgO. Existing literature indicates that Mg doping can expand the bandgap and diminish the electron mobility of ZnO.^31–34

Fig. 1 (a) Schematic of the layers in the device structure (ITO/PEDOT:PSS/TFB/QDs/ZnMgO/Ag) and the corresponding cross-sectional TEM image. (b) Energy level diagram of the device with ZnMgO compared to ZnO. PL emission intensity (c) and PL lifetime (d) of the QDs, ZnO NPs deposited on QDs, and ZnMgO-20% NPs deposited on QDs.

The synthesis of ZnO NPs, as previously reported,²⁰ can be adapted to produce ZnMgO NPs through the partial substitution of magnesium acetate tetrahydrate for zinc acetate dihydrate. Further, we adjusted the concentration of the Mg dopant in ZnMgO by controlling the molar ratio of magnesium acetate tetrahydrate throughout the synthesis process, ranging from 0 to 20%. Subsequently, we analyzed the influence of this dopant concentration on the energy levels of ZnMgO using ultraviolet photoelectron spectroscopy (UPS). The valence band maximum (VBM) of ZnMgO can be estimated considering both the position of the secondary electron cut-off (E_cut-off) and the onset of the valence band edge (E_onset) (VBM = 21.22 eV − E_cut-off + E_onset). As shown in Fig. S1a and b (ESI‡), with the increase of Mg doping concentration, both the E_cut-off and E_onset of ZnMgO are shifted to the left, but the change of E_cut-off is more pronounced, resulting in a shallower VBM. Consequently, utilizing the VBM and the bandgap of materials obtained from their absorption spectra (Fig. S2, ESI‡), it can be calculated that, with an increase in Mg content, the CBM of ZnMgO also increases (Table S1 and Fig. S3, ESI‡). Based on the known energy bands of ITO, PEDOT:PSS, TFB and QD in the literature,^12,15,35 and the energy bands of ZnO and ZnMgO-15% from Table S1 (ESI‡), the energy level structure of QLEDs using two different ETLs is shown in Fig. 1b, where an increased energy barrier between ZnMgO and the electrode may restrict electron injection, thereby enhancing the charge balance within the EML.

In addition to considerably enhancing the electrical properties of the ZnO ETL, the doping of Mg into ZnO concurrently diminishes the PL quenching incited by ZnO NPs to a certain extent. Typically, the mechanism of PL quenching is attributed to the exciton dissociation and interfacial charge transfer from QDs to the trap states in ZnO which act as non-radiative recombination centers and promote the Auger recombination in QDs.^36–38 In order to analyse the PL quenching of QDs by the ETL, we deposited ZnO films and ZnMgO films with a doping concentration of 20% on QD films, respectively, and performed the PL spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy of the two sets of samples. As indicated in Fig. 1c, the PL intensity significantly diminished following the deposition of ZnO NPs on the QDs, which is the PL quenching effect of ZnO on the QDs. Replacing ZnO with ZnMgO resulted in approximately a 20% increase in PL intensity, which suggests that Mg doping suppresses the PL quenching of QDs by ZnO. This could be attributed to Mg doping enhancing the energy barrier between ZnO and QDs, thereby limiting charge transfer. And at the same time, the doping of Mg has a passivating effect on the ZnO defects, which suppresses the non-radiative recombination caused by the defects. TRPL tests show that the PL lifetime of QDs deposited on ZnMgO films is significantly shorter than that of pure QD films, but longer than that of QDs deposited on ZnO films, as shown in Fig. 1d. These results are consistent with the PL results.

Doping Mg in ZnO can change its energy level structure, slow down electron injection, and suppress PL quenching of QDs. To determine the appropriate doping concentration for ZnMgO, we first fabricated single unit devices using ZnMgO with different Mg doping concentrations. The improvement of the electrical characteristics of ZnMgO is further confirmed by the current density–voltage (J–V) curves of the single unit devices shown in Fig. 2a, where the current density of the device with ZnMgO decreases significantly compared to that with ZnO, and the current density decreases with the increase in the amount of Mg doped, and the leakage current of the device is also better suppressed at low voltages. Fig. 2b presents the luminance–voltage (L–V) characteristics of devices with varying Mg doping concentrations. It can be observed that the luminance of devices at the same voltage decreases with an increase in Mg doping concentration. This suggests that Mg doping not only mitigates leakage current but also diminishes the radiative recombination current, potentially due to a reduced exciton formation rate. Jin et al. established that the QDs of electron-dominated devices have a tendency to accrue electrons, leading to a negative charge state,³⁹ thus enhancing hole injection via Coulombic attraction. In contrast, the decrease in electron accumulation in QDs makes hole injection more difficult, and in the presence of fewer electrons, it reduces the possibility of electron–hole pair formation, thereby reducing the radiative recombination rate and luminance. Despite a decline in device luminance with increased Mg doping at constant voltage, the current efficiency rises with heightened Mg levels when measured under identical current densities, peaking at a 15% doping level as depicted in Fig. 2c. Consequently, as the radiative recombination current decreases, its proportion in the total current increases. This is because ZnMgO with a 15% Mg concentration promotes the improvement of charge transfer balance and the reduction of PL quenching. Imbalances in charge transfer have been demonstrated to escalate the leakage current, whereas the quenching of fluorescence is correlated with an enhancement in the non-radiative recombination current. Therefore, the device doped with ZnMgO-15% achieved an optimal EQE of 17.31%, as shown in Fig. 2d. Therefore we preferred ZnMgO (15% doping concentration) with PEDOT:PSS as the ICL to fabricate tandem devices.

Fig. 2 (a) J–V, (b) L–V, (c) CE–L, and (d) EQE–L curves of the resulting QLEDs.

We first verified the charge generation capability of the ICL. As shown in Fig. 3a, we fabricated capacitor devices with the structure of glass/ITO/LiF (20 nm)/ICL/LiF (20 nm)/Ag, in which the double insulating layer of LiF blocks charge injection, and under forward bias, the ICL generates charges, which accumulate at the LiF interface on both sides, thus leading to the formation of a capacitance. Fig. 3b shows the capacitance–voltage (C–V) curve and current–voltage (I–V) curve of the ICL-ZnMgO device. The C–V characteristics of the device were measured at a frequency of 1 kHz. In the C–V curve, the capacitance at lower voltages, termed geometric capacitance, remains fixed. With increasing voltage, the electric field strength at the ZnMgO/PEDOT:PSS junction grows sufficiently to surmount the potential barrier, inducing electron transfer from PEDOT:PSS to the conduction band of ZnMgO. This results in charge separation, which is then driven to the LiF layers on both sides under the influence of the electric field, leading to charge accumulation and consequent sharp escalation in capacitance.^40–42 The I–V curve shows that the current increases with increasing voltage, reaching 3 mA at 5 V.

Fig. 3 (a) Schematic diagram of the ICL capacitor device structure. I–V–C characteristics of the (b) ZnMgO-15%/PEDOT:PSS ICL and (c) ZnO/PEDOT:PSS ICL.

This indicates that a large amount of charge is injected into the device, and as the voltage increases, the charge injection gradually accelerates. The injected charges combine with those produced by the ICL, causing a reduction in capacitance. Therefore, the relationship between capacitance and voltage is the result of competition between the charge generation rate and the charge injection and recombination rate. A comparison of Fig. 3b and c reveals a precipitous rise in capacitance between 2 and 4 V for the ZnMgO device, indicating superior charge generation relative to ICL-ZnO. It should be noted that the C–V curves show a capacitance increase phenomenon from 0 to 2 V, which may be related to ion migration. There have been early studies on the ion mobility of PEDOT:PSS at lower voltages,^43,44 and interactions between ZnO and PEDOT:PSS have been acknowledged in the literature.^45,46

In addition, we fabricated pure ICL devices using a glass/ITO/ICL/Ag structure and analyzed their J–V curves. When a negative electric field is applied, electrons and holes are injected from the corresponding electrodes and combined at the interface between ZnMgO and PEDOT:PSS. Conversely, when a positive electric field is applied, electrons and holes are generated at the interface between ZnMgO and PEDOT:PSS, and transported through ZnMgO and PEDOT:PSS to the electrodes. Fig. 4 presents the data across all Mg doping concentrations. Under negative bias, the current density decreases progressively with increasing Mg doping, a trend attributable to diminishing electron mobility in ZnO due to Mg incorporation. These results are in agreement with the data shown in Fig. 2a. Conversely, in the forward bias scenario, the current density increases incrementally up to a 15% Mg doping level, indicating enhanced charge generation efficiency in the ICL. Beyond this concentration, a Mg doping of 20% results in a substantial decrease in current density, signaling a considerable decline in electron mobility. Notably, few studies have reported ZnMgO NPs with a 20% Mg doping.⁴⁷

Fig. 4 J–V characteristics of ICL-only devices with different Mg doping concentrations.

Comparing ICL-ZnO across all tested Mg doping levels, the ICL with ZnMgO-15% showcases improved symmetry in its J–V characteristics at both positive and negative biases in Fig. 4, signifying a superior charge generation capacity.^15,48

Fig. 3 and 4 demonstrate that ICL-ZnMgO-15% exhibits an enhanced charge generation capacity in comparison to ICL-ZnO. However, interpreting this observation from the energy level configurations of ZnO, ZnMgO, and PEDOT:PSS presents challenges. We hypothesised that, compared to ZnO, ZnMgO has better acid resistance, which may lead to better interface contact between ZnMgO and PEDOT:PSS in the preparation of the ICL.^45,46 To illustrate this, Fig. S5 (ESI‡) shows the morphology changes of ZnO and ZnMgO-15% films before and after the deposition of PEDOT:PSS. The average roughness of ZnO increased from 1.18 nm to 1.59 nm, while the morphology change of ZnMgO-15% was relatively small. The average roughness before and after deposition was 1.69 nm and 1.66 nm, respectively. This morphological change indicates a reaction between ZnO and PEDOT:PSS. Consequently, it is deduced that the improved charge generation of ICL-ZnMgO-15% can be attributed to the acid resistance of ZnMgO, which preserves superior interfacial contact quality. Furthermore, the pronounced alterations in surface morphology indicate a chemical reaction has taken place. From this, we have strong grounds to suspect that the reaction products have accumulated at the interface between ZnO and PEDOT:PSS, potentially adversely affecting the charge generation characteristics. However, due to the complexity of interfacial analysis, we have not yet been able to obtain conclusive data to fully substantiate this hypothesis. In addition, ICL-ZnMgO has better transmission in the red light band than ICL-ZnO, which significantly benefits light outcoupling enhancement (Fig. S4, ESI‡).

ZnMgO with 15% Mg doping concentration exhibits a more balanced electron–hole injection in single unit QLEDs to achieve improved device performance. Furthermore, the ICL incorporating ZnMgO exhibits significantly superior charge generation and transport capabilities in comparison to the ICL incorporating ZnO. Drawing upon the aforementioned validation, we prepared tandem QLEDs with the structure of glass/ITO/PEDOT:PSS/TFB/QDs/ZnMgO15%/PEDOT:PSS/TFB/QDs/ZnMgO-15%/Ag by the full-solution process. One of the main challenges in the fabrication of TQLEDs using the solution process is solvent damage to the lower films during the deposition of the upper films.^9,12Fig. 5a shows a TEM cross-sectional image of the tandem device. All the functional films were deposited by the solution method, but the interfaces between all the functional layers can be clearly observed and free of pinholes, indicating that solvent damage was well avoided.

Fig. 5 (a) Device architecture of the resulting double-junction tandem device and its corresponding TEM image. (b) Tandem device energy level diagram. (c) Normalized EL spectra, (d) J–V, (e) L–J, (f) CE–J, and (g) EQE–J curves of the tandem device, bottom unit device, and top unit device and the theoretical superposition value of two single units. (h) Operating lifetime characteristics of the single and tandem devices at the same initial luminance of 1000 cd m⁻². The photograph in (c) shows a working tandem device (with an emitting area of 2 mm × 2 mm operated at a current density of 100 mA cm⁻²).

The energy level structure of the tandem device is shown in Fig. 5b, and the ICL can well connect two sub-QLED units in series for efficient carrier transport. Under the action of an electric field, the electrons thermally hop in PEDOT:PSS onto the CBM of ZnMgO, which leads to the dissociation of electron–hole pairs.⁴⁹ This dissociation leads to the generation of holes and electrons, which can be injected into the bottom and top EMLs.

Fig. 5c shows the normalized EL spectra of the devices. The EL spectra of the single and tandem devices overlap almost exactly, with the luminescence peaks both at 621 nm and a full width at half-maximum (FWHM) of about 22 nm. The inset in Fig. 5c shows a photograph of the red TQLEDs operating at a current density of 100 mA cm⁻².

Fig. 5d–g shows the J–V, L–J, CE–J, and EQE–J characteristics of the tandem devices as well as the bottom and top sub-devices. In addition, a theoretical prediction curve was constructed by summing data from two sub-devices at the same current density. The “Bottom Unit” and “Top Unit” leveraged the parameters of the bottom and top ELUs within the tandem device, respectively, leading to performance disparities due to film thickness variations. The actual tandem device (labeled 'Tandem') exhibited a larger operating voltage in comparison to the predicted tandem (labeled 'Predicted Tandem'). This discrepancy is attributed to the voltage drop caused by the barrier difference between the ZnMgO and PEDOT:PSS layers. Fig. 5e illustrates that the tandem device exhibits higher luminance at a specific current density, which can be attributed to the reduction in leakage current resulting from the tandem configuration. This enhanced luminance signifies the effective series connection of the ELUs by ICL-ZnMgO. Analogous trends were observed in both CE and EQE.

We have also compared the device characteristics of TQLEDs with ZnO and ZnMgO as ICLs, as shown in Fig. S6 (ESI‡). The TQLED with ZnMgO as the ICL exhibits a markedly reduced current density at equivalent voltage levels, accompanied by a slight increase in turn-on voltage of 0.2 V. These observations align with those presented in Fig. 2a and can be attributed to the elevated electron injection barrier associated with ZnMgO. Nevertheless, the TQLED with ICL-ZnMgO shows significant improvements in luminance, EQE and CE at equivalent current densities compared to the TQLED with ICL-ZnO. It is posited that these improvements are the result of a synergistic effect of electron and hole injection balance, decreased PL quenching, augmented charge generation, and enhanced acid resistance.

Benefiting from the high luminance at the same current density, the TQLEDs show excellent operating stability. Fig. 5h illustrates that, under ambient conditions, the operational lifetime of TQLEDs significantly surpasses that of single devices. The single devices show a lifetime (T₅₀) of merely 4 khr, whereas the tandem devices boast a T₅₀ of 12 khr, nearly tripling in endurance at an initial luminance of 1000 cd m⁻². With an acceleration factor of n = 1.7,⁵⁰ a 1-fold increase in brightness increases the lifetime by a factor of about 3. The results are consistent with this, so the increase in lifetime is mainly due to the increase in luminance. The tandem device can achieve a maximum brightness of approximately 400 000 cd m⁻² (Fig. S7, ESI‡), a maximum EQE of 35.51%, and a maximum CE of 51.84 cd A⁻¹. To the best of our knowledge, this result sets a new EQE record for red TQLEDs fabricated by the full-solution process, as shown in Fig. 6.^14,15,51,52

Fig. 6 The best performance of red tandem QLEDs based on different manufacturing methods.

Conclusions

In summary, by modifying the doping concentration of Mg in ZnMgO and combining it with PEDOT:PSS to construct an ICL, we prepared and obtained highly efficient fully solution-processed red TQLEDs with an EQE of up to 35.51%. The excellent EL performance of the TQLEDs is attributed to the efficient charge generation capability of the ICL, more balanced carrier injection/transport, suppression of PL quenching of QDs, and reduced leakage current. It is our conclusion that these results will be an important step towards the realization of high-efficiency QLEDs based on the solution process and facilitate the commercialization of TQLEDs in display and illumination applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the programs supported by the National Key Research and Development Program of China (2022YFB3606501 and 2022YFB3602902), the Key projects of National Natural Science Foundation of China (62234004), the National Natural Science Foundation of China (Grant No. U23A2092), the “Pioneer” and “Leading Goose” R&D Program of Zhejiang (2024C01092, 2024C01191), the Innovation and Entrepreneurship Team of Zhejiang Province (2021R01003), the Ningbo Key Technologies R & D Program (2022Z085), the Ningbo 3315 Program (2020A-01-B), the YONGJIANG Talent Introduction Program (2021A-038-B), the Ningbo Jiangbei District public welfare science and technology project (2022C07), and the Natural science foundation of Ningbo (2021J204).

Notes and references

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Footnotes

† This paper is dedicated in memory of Prof. Lei Qian, who was a great mentor, colleague, and friend. He made many significant scientific contributions during his highly productive career and will be remembered.

‡ Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01175a

§ These authors contributed equally to this work.

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