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Abstract
Near-infrared (NIR) quantum dot-based light-emitting diodes (QLEDs) developed rapidly in the fields of biomedical applications, telecommunications, sensing and diagnostics. However, it remains an enormous challenge for the synthesis of high-quality NIR QD materials with low toxicity or non-toxicity, high photoluminescence (PL) quantum yields (QYs) and high stability. Herein, we used a facile method to synthesize large-sized (8 nm) and thick-shell NIR Zn:CuInSe2/ZnS//ZnS QDs by engineering a double ZnS shell. The resulting NIR QDs exhibited high PL QYs of 80%, and excellent photochemical stability, which could be ascribed to the decreased lattice mismatch of the core/shell interface by the introduced Zn element into CuInSe2 cores and the energetic defect passivation of the double ZnS shell engineering. Furthermore, the high-quality Zn:CuInSe2/ZnS//ZnS QDs based LEDs exhibited the maximum external quantum efficiency (EQE) of 3.0%, 4.0% and 2.5% for PL peaks located at 705, 719 and 728 nm, respectively. This efficiency is comparable to that of the outstanding PbS- and InAs-based NIR QLEDs, as well as the avoidance of toxic heavymetal and/or hazardous reagents in this work. The synthesized high-quality Zn:CuInSe2/ZnS//ZnS QDs could be expected to promote the potential applications of heavy-metal-free QDs in the NIR fields.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Quantum dots (QDs) have been extensively investigated for various applications, such as light-emitting diodes (LEDs), solar cells, lasers and biomedical labels, due to their easily tunable emission by controlling their size and composition, high photoluminescence (PL) quantum yields (QYs), high photochemical and thermal stability, and low-cost solution processing methods [1–8]. During the last two decades, II-VI (CdSe, CdTe, CdxHg1-xTe) and IV-VI (PbS, PbSe) QDs have been the representative compositions as high-quality visible and near-infrared (NIR) emitters [9–15]. However, the intrinsic toxicity of heavy-metal-containing QDs has severely restricted their applications in further development and commercialization, particularly in view of recent environmental regulations. Toward this end, other alternative materials including III-V semiconductor nanocrystals (e.g., InP), transition metal (Cu, Mn, Eu, etc.) doped ZnSe nanocrystals and I-III-VI2 nanocrystals (e.g., CuInS2, CuInSe2) have been developed [16–21]. For low-toxicity III-V semiconductor nanocrystals, amounts of extremely expensive and hazardous reagents are still adopted during the synthesizing process. While, for transition metal (Cu, Mn, Eu, etc.) doped ZnSe nanocrystals, both the raw materials and as-obtained products are low-toxic and/or non-toxic, but the emission wavelength is generally limited within the visible range [17–19]. Therefore, it is still a challenge to prepare the NIR QDs possessing low/non-toxicity, high PL QYs and high stability.
I-III-VI2 semiconductor nanocrystals have low toxicity, tunable emission in the wavelengths of the visible to NIR region, high absorption coefficient, large Stokes shift, and high emission intensity [20,22–26]. They are considered to be alternative low-toxicity materials for bio-imaging and solid-state lighting, as well as suitable candidates to act as solar harvesters for solution-processed photovoltaic devices [27–29]. In addition, CuInS2 QDs possess a direct bandgap of 1.45 eV, which guarantees a size-tunable NIR PL emission from 700 to 855 nm [30]. To improve the PL QYs and stability, CuInS2/ZnS core/shell QDs were successfully synthesized. Unfortunately, the PL emission peak emerged gradual blue-shift to the orange and even green ranges due to the permeated Zn2+ into CuInS2 cores during the growth of ZnS shell, which observably limited their applications in the deep-red and NIR fields [23,24,31]. While CuInSe2 QDs possess a smaller direct bandgap of 1.04 eV, thus, their emission in the deep-red and NIR region is independent from the shell growth, which makes them more potential deep-red and NIR emitting materials [32–34].
At present, CuInSe2/ZnS core/shell QDs with high PL QYs were usually synthesized by a step-by-step method, namely, after synthesizing CuInSe2 core QDs, ZnS shells were coated onto the core surface [34–37]. However, there still existed two issues in the synthesis of the deep-red and NIR CuInSe2/ZnS nanocrystals. On one hand, the structural defects formed inevitably during the ZnS shell growth because of the large lattice mismatch between the CuInSe2 core and the ZnS shell [37,38]. Simultaneously, the continuous infiltration of Zn2+ into the CuInSe2 core caused a huge blue shift of the PL spectra, which could even been expanded to the visible region [21,34,39]. On the other hand, the penetration of the shell elements into the core makes the shell growth very difficult, which resulted in the smaller average size (< 6 nm), poor PL stability (especially after the post-treatment), and limited practical applications [34,36]. Therefore, it has become a critical issue to synthesize the high-quality CuInSe2/ZnS core/shell QDs with the tunable PL emission covering the NIR windows.
Herein, we first synthesized high-quality Zn:CuInSe2 alloyed core QDs via a “one-pot” approach, which effectively restrained the large blue shift of PL spectrum during the subsequent coating of ZnS shell. Due to the reduced lattice mismatch between Zn:CuInSe2 and ZnS, high PL QYs could be well maintained in the NIR region. Furthermore, thick ZnS shell was successfully grown on Zn:CuInSe2 core based on the double shell engineering, which has been used in the synthesis of visible QDs to increase the particle size and improve the PL QYs and photo/chemical stability [40–42]. For the first ZnS shell growth, we used the high reactive precursor (zinc oleate and octanethiol) with a slow addition rate to facilitate the formation of the gradient alloying core/shell interface. This stage contributed to passivating defects of core and improving PL QYs. For the growth of the second ZnS shell, the relatively low reactivity precursors (zinc stearate and 1-dodecanethiol) were employed with a fast addition rate to promote the increase of shell thickness, which was helpful to further enhance the PL QYs and stability. The shell thickness was modulated simply through the substantial extension of the shelling period time along with the timely replenishment of the shelling precursor. The average size of Zn:CuInSe2/ZnS//ZnS QDs increased to 8.8 nm from 3.7 nm of the thin-shell Zn:CuInSe2/ZnS QDs. Consequently, thick-shell Zn:CuInSe2/ZnS//ZnS QDs exhibited excellent PL QYs as high as 80% and robust stability. Furthermore, three types of Zn:CuInSe2/ZnS//ZnS QDs with PL peaks at 705, 719 and 728 nm were employed as the emissive layers to fabricate QLEDs according to the conventional device structure, which exhibited the maximum external quantum efficiency (EQE) of 3.0%, 4.0% and 2.5%, respectively. This efficiency is comparable to that of the outstanding PbS- and InAs-based NIR QLEDs. Especially, toxic heavy metal and/or hazardous reagents can be avoided during the entire process of synthesis.
2. Experimental section
2.1 Chemicals
Copper (I) iodide (CuI, 99.5%), indium acetate (In(Ac)3, 99.99%), zinc acetate (Zn(Ac)2, 99.99%), zinc stearate (Zn(St)2, 90%), selenium pellets (Se, 99.99%), 1-dodecanethiol (DDT, 98%), 1-Octanethiol (OT, 98%), Diphenylphosphine (DPP, 98%), oleylamine (OAm, 97%), octadecene (ODE, 90%), oleic acid (OA, 90%), zinc acetate dihydrate (99.999%), and chlorobenzene (HPLC grade) were all obtained from Aldrich. Tetramethylammonium hydroxide pentahydrate (TMAH, 98%), octane (99%, extra pure grade) and ethanol (99.8%) were purchased from Acros Organics. Dimethyl sulfoxide (DMSO, 99.9%) was purchased from Alfa Aesar. Hexane, toluene and methanol are all analytical grade and obtained from Beijing Chemical Reagent Ltd., China. Poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))-diphenylamine) (TFB) was purchased from American Dye Source. Poly (ethylenedioxythiophene):polystyrene sulphonate poly (ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) was purchased from Heraeus Deutschland GmbH & Co.KG. All chemicals were used as received without further purification.
2.2 Synthesis of Zn:CuInSe2 alloyed core QDs
Zn:CuInSe2 core QDs were synthesized through a “one-pot” approach, according to the previous report with a certain modification [43]. Typically, In(Ac)3 (175 mg, 0.6 mmol), CuI (114 mg, 0.6 mmol), OAm (6 mL) and ODE (6 mL) were mixed into a three-neck flask. The above system was heated to 100 °C and degassed for 60 min. Then, a Se stock solution (1.2 mmol Se pellets, 2 mL OAm and 0.6 mL DPP were mixed and stirred for 30 min in the N2 atmosphere) was fleetly injected into the above mixture at 210 °C and kept for 10 min. Further, a Zn stock solution (2 mmol Zn(Ac)2, 8 mL of ODE and 2 mL of OAm) was added into the system drop by drop (8 mL/h) at 180 °C. Then, the reaction was kept at 180 °C for 1 hour and set at 210 °C for 1 hour, respectively. The obtained Zn:CuInSe2 QDs were purified by repeated precipitation with methanol and redispersion in hexane three times, and lastly dispersed in toluene for shell growth.
2.3 Double ZnS shell growth
ZnS shell coating is realized via a two-step synthesis route. In the first step, the above purified Zn:CuInSe2 core QD solution was dispersed in a mixture of ODE (6 mL) and OAm (6 mL), and then degassed at 100 °C. Subsequently, a mixture of 2 mmol Zn(Ac)2, 8 mL ODE, 2 mL OA and 0.8 mL OT was added into the system drop by drop (6.4 mL/h) at 210 °C. In the second step, an additional ZnS stock solution (8 mmol Zn(St)2 was dissolved in 40 mL ODE, 20 mL OA and 20 mL DDT) was added into the above mixture drop by drop (8 mL/h) at 230 °C. The as-synthesized core/shell QDs were purified and dispersed in octane for further use. Herein, this two-step shell growth method has the following advantages. In the first ZnS shell growth, the condition of the high reactive precursor (zinc oleate and octanethiol) with a slow addition rate can effectively facilitate the formation of the gradient alloying core/shell interface. And, the relatively lower shell growth temperature (210 °C) can further alleviate the penetration of shell materials into cores to some extent, aiming to guarantee the successful caption of shells, and then improve the PL QYs and increase the particle size. In the second shell growth, the relatively low reactivity precursor (zinc stearate and 1-dodecanethiol) was employed with a fast addition rate to promote the increase of shell thickness, which was helpful to further enhance the PL QYs and stability. Shell growth temperature of 230 °C can not only promote the quick growth of shells, but also reduce the formation of lattice defects during the shell growth process and improve the crystallinity.
2.4 Synthesis of ZnO nanoparticles
ZnO was synthesized according to the previous report [44]. For a typical synthesis, 0.5 M TMAH in ethanol were stoichiometrically and dropwise added into 0.1 M zinc acetate in DMSO, and stirred for 1 h in air, then washed and dispersed in ethanol for device fabrication.
2.5 Device fabrication
QLEDs were fabricated on the pre-patterned indium tin oxide (ITO) glass substrates with the sheet resistance of ∼20 Ω sq-1. These substrates were thoroughly cleaned with deionized water, acetone and isopropanol and treated with UV-Ozone for 15 min. PEDOT:PSS (AI 4083) was spin-coated on the substrates at 5000 rpm for 60 s and annealed at 140 °C for 15 min in air. After that, these coated ITO substrates were transformed into N2-filled glove-box for deposition of TFB hole transport layer, QD emitting layer and ZnO electron transport layer. TFB (8 mg/mL, in chlorobenzene) was spin-coated at 3000 rpm for 60 s, followed by annealing at 150 °C for 30 min. QDs (10 mg/mL, in octane) and ZnO (60 mg/mL, in ethanol) were spin-coated onto TFB layers at 2500 and 2000 rpm for 60 s, respectively, followed by annealing at 60 °C for 30 min. Finally, all these samples were transferred into a custom high-vacuum deposition chamber to deposit the top Al cathode (100 nm thick) patterned by an in situ shadow mask to form an active device area of 4 mm2.
2.6 Characterization
UV-vis absorption and PL spectra were recorded on Ocean Optics spectrophotometer (model PC2000-ISA) at room temperature. All PL QYs and time-resolved fluorescence spectra of QDs were collected using JY HORIBA FluoroLog-3 fluorescence spectrometer, and the PL QYs were recorded by an integrating sphere. The optical density (OD) values of the QD samples at the excitation wavelength were set the same in the range of 0.02∼0.05. The phase and the crystallographic structure of the QDs were investigated by X-ray diffraction (XRD, Bruker D8-ADVANCE, Germany equipped with a rotating anode and a Cu-Ka radiation source and wavelength 1.5406 Å). Transmission electron microscopy (TEM) studies were performed using a JEOL JEM-2100 electron microscope operating at 200 kV. UV photoelectron spectroscopy (UPS) was carried out on a Kratos AXIS Ultra DLD spectrometer by utilizing a HeI photon source (21.22 eV). Current density-voltage characteristics were recorded using a computer-controlled Keithley 2400 current/voltage source meter. Simultaneously, the front face EL power output through ITO is measured using a calibrated Newport silicon diode. The radiant emittance-voltage characteristics are calculated by assuming Lambertian emission and by accounting for the wavelength dependence of the photodiode’s responsivity. The electroluminescence (EL) spectra were obtained using an Ocean Optics spectrometer (USB2000) and a Keithley 2400 source meter. The long-time stability of NIR LEDs was measured using an OLED aging lifetime test system (model ZJZCL-2, Shanghai).
3. Results and discussion
CuInSe2 QDs were synthesized via a DDT-free synthetic approach [43]. Specifically, a mixture of the cation precursors (CuI and In(OAc)3) reacted with Se-DPP via a “one-pot” approach according to the reported method with a certain modification. Then, a Zn stock solution was gradually introduced into the system to form Zn:CuInSe2 QDs. The as-synthesized Zn:CuInSe2 as core QDs will significantly reduce the lattice mismatch between ZnS shell and CuInZnSe3 (lattice mismatch of ca.3%), compared with that between ZnS and CuInSe2 (lattice mismatch of ca.7%). Moreover, the introduction of Zn2+ into CuInSe2 core QDs is helpful to hinder the cation exchange between Zn2+ and Cu+ or In3+ during the shell growth, which can effectively avoid the pronounced blue-shift of PL spectrum and ensure the luminescence in the deep-red and NIR region. To improve the PL QYs and stability of Zn:CuInSe2 QDs, the alloying degree of the core/shell interface and the ZnS shell thickness were further regulated through the developed double ZnS shell engineering strategy, as shown in Fig. 1. This method is employed to significantly promote the growth of ZnS shells; therefore, the thick-shell NIR Zn:CuInSe2/ZnS//ZnS QDs were successfully synthesized.
Figure 2(a) presented the evolution of UV-vis and PL spectra of Zn:CuInSe2-based core/shell QDs with the ZnS shell growth. As shown in Fig. 2(a) and 2(b), both the absorption and PL peaks gradually blue-shifted during the shell growth. Simultaneously, taking into account the structural resemblance between tetragonal chalcopyrite Zn:CuInSe2 and cubic zinc blende ZnS, these two phases of Zn:CuInSe2 and ZnS have approximate lattice constants, which could facilely form Zn:CuInSe2-ZnS intermediate alloyed shell through an interdiffusion manner at an elevated temperature of 230 °C. This process relieved the lattice mismatch of core/shell interface and reduced the formation of interfacial defects, and then enhanced the PL QYs. With the continuous growth of ZnS shells, the PL QYs increased from 20% of core QDs to 80% of ZnS-shelled QDs, as shown in Fig. 2(c). Meanwhile, the continually narrowed PL full width at half maximum (FWHM) could be observed in Fig. 2(d), which was attributable to the narrower size distribution of 4% for the thick-shell than that of 6% for thin-shell QDs (shown in the inset of Supplement 1, Fig. S1). TEM images of Zn:CuInSe2-based QDs (Supplement 1, Fig. S1, showed that the average size could be tuned continuously from 3.7 to 8.8 nm by increasing the shell thickness.
Fig. 2. (a) Evolution of UV-Vis and PL spectra upon the consecutive growth of Zn:CuInSe2-based core/shell QDs. (b) PL peak, (c) PL QYs and (d) FWHM of Zn:CuInSe2-based core/shell QDs as functions of shell growth time. F-1 h and F-2h: growth 1 h and 2 h after first-time growth of ZnS, respectively; S-4 h, S-8 h and S-12h: growth 4 h, 8 h and 12 h after second-time growth of ZnS, respectively.
As shown in Fig. 3(a) and 3(b), the high-resolution transmission electron microscopy (HRTEM) images of thin-shell Zn:CuInSe2/ZnS and thick-shell Zn:CuInSe2/ZnS//ZnS QDs revealed continuous lattice fringes throughout the whole particles, indicating good crystallinity. Simultaneously, the lattice spacing of the two adjacent planes was 0.31 nm for both of these QD samples, which corresponded to the (1 1 1) plane of ZnS. Thereafter, ZnS shell was well epitaxial grown on the Zn:CuInSe2 cores. To further characterize the evolution of structures of Zn:CuInSe2-based core/shell QDs, their crystallographic properties were determined by XRD (Fig. 3(c)). With the successive growth of ZnS shell, the diffraction peak positions of the QDs shifted gradually from chalcopyrite CuInZnSe to zinc blend ZnS, and the shape of peaks gradually became sharper. These results further support that thick ZnS shell was successfully epitaxial grown on Zn:CuInSe2 core QDs, accompanied by the increased average size and improved PL QYs of core/shell QDs. By systematically adjusting the reaction temperature and the precursor concentration, the thick-shell QDs showed continuously tunable PL spectra from 640 to 800 nm covering the deep-red and NIR region as shown in Fig. 3(d). In brief, the strategy of Zn:CuInSe2 core QDs and double ZnS shell engineering assisted successfully in synthesizing Zn:CuInSe2/ZnS//ZnS QDs with a larger average size and high PL QYs in the deep-red and NIR region.
Fig. 3. The representative HRTEM images of (a) Zn:CuInSe2/ZnS and (b) Zn:CuInSe2/ZnS//ZnS core/shell QDs. (c) XRD of Zn:CuInSe2 core, Zn:CuInSe2/ZnS and Zn:CuInSe2/ZnS//ZnS core/shell QDs. (d) PL spectra of thick-shell Zn:CuInSe2/ZnS//ZnS QDs synthesized by adjusting the reaction temperature and the precursor concentration.
As we know, the PL properties and stability of QDs can be affected by the shell thickness [45–47]. As shown in Supplement 1, Fig. S2, the PL lifetime increased from 336.9 ns of thin-shell Zn:CuInSe2/ZnS to 420.6 ns of thick-shell Zn:CuInSe2/ZnS//ZnS QDs, which suggests that the surface defect can be effectively passivated by the thick shell to some extent. Furthermore, the photostability of Zn:CuInSe2-based core/shell QDs was inspected by multi-time washing and long-time exposure under UV light. As shown in Supplement 1, Fig. S3, after 10 times of purification processes, the PL intensity of thin-shell Zn:CuInSe2/ZnS QDs decreased to ∼50% of its initial value. While, for thick-shell Zn:CuInSe2/ZnS//ZnS QDs, the PL QYs still retained ∼90% of the initial value. In addition, after continuous UV irradiation for 240 min, the PL intensity of thick-shell Zn:CuInSe2/ZnS//ZnS QDs sustained over 80% of their initial value. This indicated that the thick shell-capped QDs possessed excellent optical stability.
For the application of Zn:CuInSe2-based QDs in the NIR QLEDs, the devices were fabricated according to the device structure shown in Fig. 4(a). The corresponding energy level diagram is shown in Fig. 4(b). To confirm the energy level of Zn:CuInSe2-based QD layers, the valence-band maximum (VBM) was assessed using UPS spectra. As presented in Fig. 4(c) and 4(d), the VBM for thin-shell Zn:CuInSe2/ZnS is 4.75 eV and that for thick-shell Zn:CuInSe2/ZnS//ZnS QDs is 5.19 eV, respectively. Combined with their optical band-gap, the conduction band minima (CBM) was estimated as 2.88 eV for thin-shell Zn:CuInSe2/ZnS, and 3.50 eV for thick-shell Zn:CuInSe2/ZnS//ZnS QDs, respectively. As a result, there is a larger potential barrier for electron injection in I-III-VI2 QLEDs but a negligible hole injection barrier, which is contrary to the Cd-QLEDs based on the organic-inorganic hybrid structure. Based on the device structure shown in Fig. 4(a), we compared the performance of devices based on thin-shell and thick-shell Zn:CuInSe2-based QDs. As shown in Supplement 1, Fig. S4, thin-shell QD-based devices showed the maximum radiant emittance of 1290 μW/cm2 and EQE of 2.46%. While thick-shell QD-based devices showed much more remarkable performance in terms of radiant emittance and EQE. Accordingly, the maximum radiant emittance and EQE increased by >160% and 60%, respectively. This is attributable to the thick shell that not only passivated the surface defect of QDs, but also improved the charge injection balance in Zn:CuInSe2-based QLEDs.
Fig. 4. (a) The schematic of multilayered QLEDs. (b) The energy level diagram of materials used in this study. UPS spectra of (c) high-binding energy secondary electron cut-off and (d) the valence-band edge regions of thin- and thick-shell Zn:CuInSe2-based QDs, respectively.
Figure 5(a) presented the voltage-dependent variations of current density and radiant emittance of three NIR QLEDs based on Zn:CuInSe2/ZnS//ZnS QDs with PL peaks located at 705, 719 and 728 nm, respectively. The maximum radiant emittance is 1910, 3440 and 1680 μW/cm2 for these three devices based on Zn:CuInSe2/ZnS//ZnS QDs with emission wavelengths of 705, 719 and 728 nm, respectively. Correspondingly, the turn-on voltage of all devices was less than 2 V, which was close to their according photo voltages. Figure 5(b) showed the variation of EQE as a function of current density. The maximum EQE of 3.0%, 4.0% and 2.5% could be obtained at the current density of 0.1, 0.15 and 0.2 mA/cm2 for three QLEDs with PL peaks of 705, 719 and 728 nm, respectively. This efficiency is 8 times more higher than that of CuInSe/ZnS-based QLEDs [34]. Compared with recently reported heavy-metal-containing and heavy-metal-free QDs-based NIR LEDs, this efficiency is comparable to that of outstanding PbS/CdS- and In(Zn)As-In(Zn)P-GaP-ZnS-based NIR QLEDs [48,49] (as illustrated in Supplement 1, Table S1). Moreover, this QD material is more eco-friendly and the adoption of hazardous reagents can be avoided during the synthesis. The detailed performance parameters of QLEDs based on Zn:CuInSe2/ZnS//ZnS QDs have been summarized in Table 1. Taking the device with PL peak of 719 nm for example, the histograms in Supplement 1, Fig. S5, showed an average peak EQE of 3.6% with a low standard deviation of 1.4%, suggesting the relatively high reproducibility. Although the maximum efficiency was just obtained at the low current density due to the imbalanced charge injection, these results facilitated the application of heavy-metal-free Zn:CuInSe2-based QDs in NIR fields. After being simply sealed with a glass cover using UV resin, the stability of this NIR LED was further conducted in air at the constant current density of 45 mA/cm2, corresponding to an initial intensity of 618 μW/cm2. As shown in Supplement 1, Fig. S6, the intensity of devices reduced to 70% of the initial intensity after 12.6 h. In accordance, the driving voltage slightly increased from 2.9 V to 3.0 V. Although this stability still fell behind that of Cd-based visible QLEDs, the long-time stability of the as-fabricated NIR LEDs based on heavy-metal free QDs was still rarely reported so far.
Fig. 5. Characteristics of three QLEDs based on Zn:CuInSe2/ZnS//ZnS QDs with PL peaks at 705, 719 and 728 nm. (a) Variation of current density and radiant emittance as a function of voltage. (b) EQE as a function of current density. (c) EL spectra and (d) normalized EL intensity with an increasing voltage of QLEDs based on Zn:CuInSe2/ZnS//ZnS QDs with PL peak at 719 nm. The inset of (d) is the magnified spectra corresponding to a shaded area of (c).
Table 1. Summary of performance parameters of three types of NIR QLEDs based on Zn:CuInSe2/ZnS//ZnS QDs with PL peaks at 705, 719 and 728 nm, including λPL/FWHM, λEL, turn-on voltage (Von), maximum values of radiant emittance and EQE.
Figure 5(c) showed the typical EL spectra as a function of driving voltage of QLEDs based on Zn:CuInSe2/ZnS//ZnS with PL peak of 719 nm. Some parasitic emission from TFB could be observed at the high energy region, which was due to a relatively deep-lying unoccupied molecular orbital (LUMO) level of -2.3 eV (Fig. 4(b)) that could not completely block an electron flow from EML to HTL. According to the normalized EL spectra shown in Fig. 5(d), TFB emission tended to increase with driving voltage, as well, the areal contribution in the overall EL was < 5%. As shown in Supplement 1, Fig. S7, EL spectra of the other two devices showed a similar trend to that of device #2 (PL@ 719 nm).
4. Summary
In summary, we developed a facile approach to synthesize the high-quality NIR Zn:CuInSe2/ZnS//ZnS core/shell QDs by double ZnS shell engineering, which possessed high PL QYs of 80%, wide spectrum tunability and high photochemical stability. Furthermore, the NIR EL devices were successfully fabricated, which showed the maximum EQE of 3.0%, 4.0% and 2.5% for QLEDs based on Zn:CuInSe2/ZnS//ZnS QDs with PL peaks of 705, 719 and 728 nm, respectively. These results suggested that thick-shell Zn:CuInSe2/ZnS//ZnS QDs act as excellent candidates with promising potential for the exploration of novel and efficient NIR QLEDs
Funding
National Natural Science Foundation of China (51802079, 61874039, 61922028); the Innovation Research Team of Science and Technology in Henan province (20IRTSTHN020); Undergraduate Teaching Reform Research and Practice Project of Henan University (YB-JFZX-2022-11).
Acknowledgments
This work was supported by the research project of the National Natural Science Foundation of China (No. 51802079, 61874039, 61922028), the Innovation Research Team of Science and Technology in Henan province (20IRTSTHN020), and Undergraduate Teaching Reform Research and Practice Project of Henan University (YB-JFZX-2022-11).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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