Quantum dot light-emitting diodes (QD-LEDs) have made great development in the performance. However, the efficiency droop at high brightness limits their applications in daylight displays and outdoor lightings. Herein, we systematically regulate the shell structure and composition, and the results indicate that CdSe-based QDs with ZnSe interlayer and thinner ZnSeS outermost layer as emitting layers (EML) enable high-performance QD-LEDs. Accordingly, the devices exhibit peak external quantum efficiency (EQE) of 22.9% with corresponding brightness of 67,840 cd/m2, and this efficiency can be still maintained > 90% of the maximum value even at 100,000 cd/m2, which satisfies the requirements for high-brightness display and lighting applications. This strong performance is mainly attributed to the ZnSe/ZnSeS graded shell that smooths the injection barrier between QD EML and the adjacent hole transport layers (HTL), and then improves the hole injection and charge injection balance, in particular at the high luminance and/or at high current density.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Quantum dots (QDs) have many unique optical properties such as high photoluminescence (PL) quantum yield (QY), high color purity, size-tunable emission wavelength and solution-processed synthesis, which make them promising materials for next-generation light-emitting diodes (LEDs) in display and lighting applications [1–6]. Ever since the first demonstration of quantum dot light-emitting diodes (QD-LEDs), their performance has already been comparable with that of state-of-the-art organic LEDs (OLEDs), and satisfied the commercial demands for displays, due to the development of synthetic technology of QDs and the optimizing of the device structure [7–15]. While for lighting, a much higher threshold value of brightness (∼103-104 cd/m2) for peak external quantum efficiency (EQE) (> 6%) is required to provide a constant photon output . However, most QD-LEDs with high EQE so far still operate at low luminance (< 2,000 cd/m2) [7,8,11,12,17–21], which is much likely due to the band energy alignment of the popular organic/inorganic hybrid device structure. In such a structure, ZnO as the electron transport layer (ETL) can provide effective electron injection and transport, while, the large barrier between the QD emitting layer (EML) and the hole transport layer (HTL) impedes the hole injection. And then, hole-electron injection imbalance was brought, especially at high luminance and/or at high current, where more excitons may experience nonradiative recombination due to the charge accumulation. The accumulation of excess electron at the interface of HTL/QDs is considered as the main reason for Auger heating induced degradation of the organic HTL, which will drive an increase in voltage and a consequently efficiency droop at high current density 1and device damage . Thus, inefficient hole injection becomes a critical issue which hinders high efficiency obtained at high brightness for QD-LEDs and limits their application for outdoor display and lighting.
Since the demonstration of high-efficiency devices based on QDs with tailored nanostructure in 2015 , QD-level controls of hole injection have been an effective strategy to enhance the charge injection balance and improve the performance of QD-LEDs. Namely, high-quality QDs possessing matched energy level with HTL are synthesized by finely tuning the shell structure and composition, aiming to enhance the hole injection efficiency [11–15,23]. Recently, our group have reported a series of work about high-performance QD-LEDs based on CdSe/ZnSe QD systems [13–15] which suggests that shell engineering of QDs plays an important role in improving the hole injection efficiency, transport and charge-injection balance in QD-LEDs. However, the dependence of device performance such as luminance and EQE on the shell structure and composition of QDs still lacks systematic research, which is vital for designing QD-LEDs with high efficiency at high brightness.
For this purpose, based on the same-size CdSe cores, we synthesized a series of green core/shell QDs by regulating the shell structure and composition, including CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS. Adopting these QDs with distinct shells as EML of QD-LEDs, we systematically studied the effect of shell structure and composition on the performance of devices. The results suggest that the traditional CdSe/ZnS based QD-LEDs show a peak EQE of 16.3% at a low luminance of 1,250 cd/m2. While, the maximum EQE of 22.9% can be achieved at a high luminance of 67,840 cd/m2 for CdSe/ZnSe/ZnSeS based devices. Interestingly, the efficiency droop is less than 10% even at 100,000 cd/m2 (EQE=20.8% @ 100,000 cd/m2). Such an exceptional performance is due to QDs having the matched energy level with the HTL via shell engineering, which smooth the hole-injection barrier for a better hole injection efficiency. Consequently, charge injection becomes more balanced at higher brightness region resulting in a low efficiency droop.
2. Experimental section
Cadmium oxide (CdO, 99.99%), zinc oxide (ZnO, 99.9%, powder), zinc acetate (Zn(AC)2, 99.99%), selenium (Se, 99.99%, powder), sulfur (S, 99.998%, powder), oleic acid (OA, 90%), oleylamine (OAm, 98%), Octadecylamine (ODA, 90%), 1-octadecene (ODE, 90%), zinc acetate dihydrate (99.999%), dimethyl sulfoxide (DMSO, 99.7%) and chlorobenzene (HPLC grade) were purchased from Aldrich. Tetramethylammonium hydroxide pentahydrate (TMAH, 99%), octane (99%) and ethanol (99.8%) were purchased from Acros Organics. Hexane and methanol were all analytical grade and obtained from Beijing Chemical Reagent Ltd., China. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(p-butylphenyl))diphenylamine)] (TFB) was purchased from American Dye Source. Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) (CLEVIOS P VP AI4083) was purchased from Heraeus Deutschland GmbH & Co.KG. The above reagents were used as received.
2.2 Synthesis of CdSe core
The CdSe cores were prepared by the following procedures according to the literature . Preparation of Se-ODA-ODE precursor: 2.4 mmol Se, 7.2 mmol ODA and 18 mL ODE were loaded in a 50 mL three-neck flask and degassed at 100 °C for 10 min, then, the solution was heated to 240 °C and kept for 3 hours at this temperature. Preparation of Cd precursor: 0.12 mmol CdO, 0.36 mmol OA and 4.0 g ODE were added in a 25 mL three-neck flask and degassed at 100 °C for 10 min. Then, the mixture was heated to 240 °C under nitrogen until it turned transparent. 2 mL Se-ODA-ODE was quickly injected into the above solution and CdSe cores began to grow. After reaction, the CdSe QDs was purified by methanol and hexane.
2.3 Synthesis of CdSe based core/shell QDs
Preparation of Zn precursor: 20 mmol ZnO, 30 mL OA and 20 mL ODE were degassed in a 100 mL three-neck flask at 100 °C for 15 min and then heated to 300 °C until the solution became colorless. Finally, the solution temperature was lowered to 150 °C for stocking. Preparation of Se (0.4 M) precursor: 20 mmol Se and 50 mL ODE were placed in a 100 mL three-neck flask and degassed at 100 °C for 10 min. Then, the mixture was heated to 260 °C and kept for 4 hours until the Se powder dissolved. Preparation of S (0.4 M) precursor: 20 mmol S and 50 mL ODE were loaded in a 100 mL three-neck flask and degassed at 100 °C for 10 min. Then, the mixture was heated to 150 °C until the solution turned transparent. For the coating of ZnS or ZnSe shell on the CdSe core, 5 mL CdSe in hexane, 5 mL OA, 5 mL OAm and 10 mL ODE were mixed together and the hexane was eliminated by degassing at 60 °C for 15 min, 4 mL of the precursor mixture (the volume ration for the Zn precursor to the S or Se precursor is 1:1) was injected into the former solution at a rate of 5 mL/h at 240 °C. After reaction, the solution was kept at 300 °C for 30 minutes. The CdSe/ZnSe/ZnS QDs were synthesized by adding the mixture of Zn and Se precursor to the CdSe solution followed by the Zn and S precursor injection. Synthesis of CdSe/ZnSe/ZnSeS QDs: the CdSe solution was degassed at 100 °C for 15 min and then heated to 300 °C. 10 mL Zn precursor was added swiftly in 1 min followed by the injection of 6 mL Se precursor at a rate of 4 mL/h. Finally, 4 mL of the mixture of Se and S precursor (1:1) was injected at a rate of 4 mL/h to form the out shell.
2.4 Fabrication of QD-LEDs devices
ZnO nanoparticles were synthesized according to the previously reported method . Typically, a solution of zinc acetate dihydrate in DMSO (0.5 M) and 30 mL of a solution of TMAH in ethanol (0.55 M) were mixed and stirred for 1 h in ambient conditions, then washed and dispersed in ethanol at a concentration of 30 mg/mL to be used as the ETL.
QD-LEDs were fabricated on the pre-patterned indium tin oxide (ITO) glass substrates with the sheet resistance of ∼15 Ω sq-1. Before use, these substrates were thoroughly cleaned with deionized (DI) water, acetone, and isopropanol for 15 min each, followed by treatment with UV-ozone for 15 min. Next, PEDOT:PSS as the hole injection layer (HIL) was spin-deposited onto the cleaned substrates and baked at 140 °C for 15 min in air. Then, these coated substrates were transferred into a N2-filled glove box for spin-coating of the TFB, QDs, and ZnO layers. TFB serving as the HTL was spin-casted at 3000 rpm using an 8 mg/mL solution in chlorobenzene and baked at 150 °C for 30 min. The as-prepared CdSe-based QDs with the concentration of 18 mg/mL in octane were spin coated onto TFB layers at 2500 rpm. ZnO was spin coated at 2000 rpm for 30 s and followed by baking at 60 °C for 30 min. Finally, the samples were loaded into a high-vacuum chamber to evaporate a 100-nm-thick Al cathode. All the devices are 4 mm2 in active area, and encapsulated in commercially available ultraviolet-curing epoxy and cover glass.
Absorption and PL spectra were collected with an Ocean Optics spectrophotometer (model PC2000-ISA). All QY data of QD solutions was measured using a JY HORIBA FluoroLog-3 fluorescence spectrometer, coupled with an integrating sphere. Transmission electron microscope (TEM) images of QDs were obtained using JEOL JEM-2100 operating at an accelerating voltage of 200 kV. The cross-sectional image of QD-LEDs was collected using a FEI Talos F200X TEM. The phase and the crystallographic structure of the QDs were investigated by X-ray diffraction (XRD, Bruker D8 Advance). Ultraviolet photoemission spectroscopy (UPS) (Thermo Fisher Scientific ESCALAB 250 XI) was performed using with a He I discharge lamp (hv = 21.22 eV) under high vacuum (2.5 × 10−8 mbar). Impedance spectroscopy (IS) data was recorded with the electrochemical workstation (AUTOLAB, Metrohm Ltd.), in the frequency range of 1 Hz to 105 Hz with 5 mV amplitude. The current density-voltage-luminance (J-L-V) characteristics were tested using a Keithley 2400 source meter and a Keithley 6485 picoammeter coupled with a Si photodetector (Newport 818-UV), and the luminance was calibrated using a Minolta CS-100 Chroma Meter under ambient conditions. EL spectra were recorded with an Ocean Optics spectrometer (USB 2000) and a Keithley 2400 power source.
3. Results and discussion
To study the effect of shell structure and composition on the performance of QD-LEDs, the same-size CdSe cores were used to synthesize CdSe-based core/shell QDs with distinct shells (CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS), according to the reported method [13–15,26]. Figure 1(a) presents the UV-vis and PL spectra of CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs. These QD samples have the PL QYs of nearly 100%, and their emission wavelengths locate at 525∼535 nm along with full width at half maximum (FWHM) ≤ 27 nm. TEM images in Fig. 1(b)-(e) show that all these QDs have narrow size distribution, and the average size is ∼9, ∼12, ∼11 and ∼11 nm for CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs, respectively. Correspondingly, the high resolution transmission electron microscopy (HRTEM) images (inset of Fig. 1(b)-(e)) and XRD patterns (Fig. S1 in Supplement 1) indicate that all the QD samples exhibit the zinc-blend structure with relatively high crystallinity.
As reported, the charge-injection efficiency and balance in QD-LEDs can be affected greatly by the energy levels of EML, which is mainly determined by the shell structure and composition of QDs. Therefore, the UPS spectra were carried out to study the band structure of CdSe-based QDs capped by different shells (ZnS, ZnSe, ZnSe/ZnS and ZnSe/ZnSeS), as shown in Fig. 2. The valance-band maximum (VBM) level can be assumed by the incident photon energy (21.22 eV), the high-binding energy cut-off (Ecut-off) (Fig. 2(a)), and the onset energy in valence-band region (Eonset) (Fig. 2(b)), according to the equation of VBM=21.22-(Ecut-off-Eonset) . Thus, the VBM position of traditional CdSe/ZnS QDs is calculated to be 6.42 eV. When ZnSe was used as the outer shell or intermediate layer, the VBM levels are 6.01, 6.12 and 6.03 eV for CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs, respectively, which are shallower about 0.3∼0.4 eV than that of CdSe/ZnS QDs. The elevated VBM is mainly due to the ZnSe possessing a shallower valance band of ∼0.3 eV than that of ZnS [28,29]. Moreover, compared to CdSe/ZnSe/ZnS QDs with thicker ZnS outermost shell, CdSe/ZnSe/ZnSeS QDs with thinner ZnSeS outmost shell have a shallower VBM, similar to that of CdSe/ZnSe QDs. These results suggest that the valance band of QDs was elevated by finely tuning the shell structure and composition, which will be favorable for hole injection of QD-LEDs, especially for CdSe/ZnSe and CdSe/ZnSe/ZnSeS QDs. Combination with the optical bandgaps (Eg) determined by the UV-vis spectra shown in Fig. 1(a), the conduction-band minimum (CBM) levels are estimated to be 4.19, 3.80, 3.89 and 3.81 eV for CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs, respectively. The band alignments of these QD samples are shown in Fig. 2(c). The detailed UPS parameters are summarized in Table S1.
Furthermore, the single-carrier devices were fabricated to investigate the effect of energy level of QDs on the charge injection. The corresponding characterization of electronic properties are illustrated in Fig. 2(d) and (e). With the introduction of ZnSe outershell or intermediate layer, the corresponding hole-only devices exhibit much higher current density than that of CdSe/ZnS-based hole-only devices (Fig. 2(d)). Among them, CdSe/ZnSe-based hole-only devices have the highest single-hole current, followed by the CdSe/ZnSe/ZnSeS QDs, and then CdSe/ZnSe/ZnS QDs. These results indicate that hole injection efficiency is greatly improved for CdSe/ZnSe and CdSe/ZnSe/ZnSeS because of the higher VBM levels, while, it has a just slight increase for devices based on CdSe/ZnSe/ZnS QDs with thicker ZnS outmost shell. Correspondingly, for the electron-only devices based on CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs, the single-electron current has a little decrease, compared with that of CdSe/ZnS-based electron-only devices, which can be reflected in Fig. 2(e). This result indicates that the elevated CBM makes the electron injection less to some extent. Consequently, the balance of charge injection is expected to be enhanced ascribed to the improved energy levels by controlling shell structure and composition, and eventually improve the performance of QD-LEDs, in particular for CdSe/ZnSe- and CdSe/ZnSe/ZnSeS-based devices.
To confirm our conjecture, multi-layered QD-LEDs with a structure of ITO (∼90 nm)/PEDOT:PSS (∼50 nm)/TFB (∼30 nm)/QDs (∼55 nm)/ZnO (∼25 nm)/Al (100 nm) (Fig. 3(a) and (b)) were fabricated to measure the electroluminescence (EL) properties of the four kinds of QD-LEDs based on CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs. Figure 3(c) illustrates the energy levels of materials. Such a structure designed will increase the probability of hole-electron recombination within the QD EML, which is ascribed to the following factors: (1) TFB as the HTL can be favorable for the hole injection and transport, due to the deep HOMO energy level of -5.4 eV and the relatively high hole mobility of 1.0 × 10−2 cm2 V-1 s-1 ; (2) ZnO possesses an electron affinity of ∼4.3 eV and an ionization potential of ∼7.6 eV . Thus, it can facilitate the electron injection and impedes the hole transport to the cathode when it was used as the ETL.
The current density-voltage-luminance (J-V-L) curves for QD-LEDs based on CdSe/ZnS, CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs are shown in Fig. 4(a). Compared with CdSe/ZnS-based devices, QD-LEDs based on CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS show much higher current density at given voltage throughout the measurement range, which is due to the introduction of ZnSe shell that elevates the VBM of QDs and then improve the hole injection efficiency. Correspondingly, CdSe/ZnS-based QD-LEDs show the maximum luminance of 89,150 cd/m2 at 9.4 V. While, the maximum luminance are much higher for QD-LEDs based on CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs, which reach up to 345,970, 272,540 and 496,980 cd/m2, respectively, and these values can be obtained at the voltage of 6.2∼6.3 V. Simultaneously, these three kinds of devices exhibit lower turn-on voltage of 1.9∼2.1 V than that of CdSe/ZnS-based devices. Although CdSe/ZnSe/ZnS-based QD-LEDs possess a slightly higher current density and luminance at the low voltage of 2.5∼4 V, the maximum luminance is lower than that of devices based on CdSe/ZnSe and CdSe/ZnSe/ZnSeS QDs. This is mainly due to the thicker ZnS outermost shell makes the VBM deeper of ∼0.1 eV than that of CdSe/ZnSe and CdSe/ZnSe/ZnSeS, and then undermines the charge injection balance, while CdSe/ZnSe and CdSe/ZnSe/ZnSeS QDs as the EML enable an enhanced hole injection efficiency and better injection balance at the high voltage, and consequently a much higher luminance. As well, this results indicate that the thickness of ZnS outer layer can reducem the surface trap states and nonradiative Auger recombination of QDs, ensuring the PL QYs. But the thicker ZnS shells impair the charge injection efficiency and charge-injection balance in QD-LEDs. What’s more, impedance spectroscopy (IS) is a sensitive and efficient characterization technique to understand the behavior of the device,  therefore, IS measurement of these LED devices are carried out, as shown in Fig. S2 in Supplement 1. As reported by reference 31, CdSe/ZnSe- and CdSe/ZnSe/ZnSeS-based devices exhibits the less hindrance to exciton recombination than that of CdSe/ZnS- and CdSe/ZnSe/ZnS- based devices, accountable for balanced charge injection, leading to better device performance, which is supported the above results.
Furthermore, to compare the performance of different LEDs in more quantitative terms, we analyze the dependence of EQE on luminance (Fig. 4(b)) and current density (Fig. 4(c)). CdSe/ZnS-based QD-LEDs show the peak EQE of 16.2% at 1,250 cd/m2 (at 1.5 mA/cm2), and this efficiency decreases by ∼30% at the luminance of 10,000 cd/m2. Correspondingly, the maximum values of current efficiency and luminous power efficiency is 69.2 cd/A and 53.9 lm/W, respectively (Fig. S3 in Supplement 1). When ZnSe was used as the outmost shell or intermediate shell, the peak EQE has improved up to 21.7%, 19.8% and 22.9% for QD-LEDs based on CdSe/ZnSe, CdSe/ZnSe/ZnS and CdSe/ZnSe/ZnSeS QDs with corresponding luminance of 39,600 (at 43.6 mA/cm2), and 6,390 (at 6.8 mA/cm2) and 67,840 cd/m2 (at 69 mA/cm2), respectively. Furthermore, the efficiency droop has been suppressed in these devices compared with the CdSe/ZnS-based QD-LEDs. For example, the efficiency sustained > 95% of peak EQE at 10,000 cd/m2 for CdSe/ZnSe/ZnS-based QD-LEDs. Remarkably, EQE of CdSe/ZnSe- and CdSe/ZnSe/ZnSeS-based QD-LEDs still maintained about 90% of the peak values even at 100,000 cd/m2. Usually, the critical luminance (L0) or critical current density (J0) at which EQE reduces to half of its maximum value, is used to evaluate the efficiency droop for different devices . In our devices, as shown in Fig. 4(b) and (c), L0 and J0 increased from 16,140 cd/m2 and 46 mA/cm2 for CdSe/ZnS-based QD-LEDs to 485,480 cd/m2 and 1,020 mA/cm2 for CdSe/ZnSe/ZnSeS-based QD-LEDs, satisfying the requirements of high-brightness display and lighting. This strong performance is a direct consequence of ZnSe as the intermediate gradient shell and thinner ZnSeS as the outermost shell that enhance the hole-injection efficiency and facilitate the charge injection balance at the high luminance and/or high current density. Correspondingly, the maximum current efficiency and power efficiency of 100.8 cd/A and 85.9 lm/W, 94.7 cd/A and 107.4 lm/W and 98.4 cd/A and 81.5 lm/W can be achieved for QD-LEDs based on CdSe/ZnSe, CdSe/ZnSe/ZnS, CdSe/ZnSe/ZnSeS, respectively (Fig. S4 in Supplement 1). Table 1 summarizes the performance parameters of these devices in detail.
As shown in Fig. S4, in all fabricated devices, EL spectra shift 5∼7 nm toward the low energy region as the driving voltage spanning from 3 to 9 V resulting from quantum confined Stark effect (QCSE). But EL is exclusively due to QDs without measurable parasitic emission, which indicates excitons were well confined within QD layers.
Green QD-LEDs with high efficiency at high luminance have been demonstrated by precise controlling the shell structure and composition. QD-LEDs based on traditional CdSe/ZnS QDs show the peak EQE of 16.3% at 1250 cd/m2. With the introduction of ZnSe as the intermediate gradient shell and/or outermost shell, the corresponding devices exhibit remarkable performance. Especially, the maximum EQE of 22.9% can be achieved at the luminance of 67,840 cd/m2 for CdSe/ZnSe/ZnSeS-based QD-LEDs, and this efficiency can be still sustained about 90% of the peak values even at 100,000 cd/m2. Moreover, L0 and J0 were promoted to 485,480 cd/m2 and 1,020 mA/cm2, compared with that of CdSe/ZnS-based devices (L0 = 16,140 cd/m2 and J0 = 46 mA/cm2). This improved performance is mainly attributed to the ZnSe/ZnSeS graded shell that elevates valance band of QDs, and then facilitates the hole injection and enhances the charge injection balance, in particular at the high luminance and/or high current density. These results promote the application of QD-LEDs in the field of high-brightness displays and lighting.
National Natural Science Foundation of China (51802079, 61874039, 61922028); Science and Technology Innovation Talents in Universities of Henan Province (20IRTSTHN020).
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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