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Compositional engineering of multinary Cu–In–Zn-based semiconductor nanocrystals for efficient and solution-processed red-emitting quantum-dot light-emitting diodes
Organic Electronics 74 (2019) 46–51
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Compositional engineering of multinary Cu–In–Zn-based semiconductor nanocrystals for efficient and solution-processed red-emitting quantum-dot light-emitting diodes
T
Zhongyuan Guana, Fei Chena, Zhenyang Liua, Peiwen Lva, Mingjun Chenb, Mingxuan Guoc, Xu Lib, Feng Tenga, Song Chenc,**, Aiwei Tanga,* a b c
Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing, 100044, China Hebei Key Laboratory of Optic-electronic Information and Materials, College of Physics Science and Technology, Hebei University Baoding, 071002, China College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu-In-Zn-Se-S NCs Compositional engineering Red QD-LEDs Electroluminescence
For quantum-dot light-emitting diodes (QD-LED), replacing Cd-based II-VI nanocrystals (NCs) with the Cu-In-ZnVI counterparts combines the consideration of environmental benignity and device performance. To prove this concept, the chemical composition and nanostructures of Cu-In-Zn-VI nanocrystals need to be thoroughly explored aiming to competitive optoelectronic properties. Herein, we reported a detailed study of Cu–In–Zn–Se–S synthesis and demonstrated how the optical bandgap, emission full width at half-maximum (FWHM) and the performance of QD-LED were tuned by simply changing the dose of precursors in a non-injection synthesis. Evident by optical absorption, the optimization of Se and Cu doses enabled good dispersity and desired emission wavelength. Further analysis of photo-electron spectroscopy revealed the chemical composition from core to surface favored soft confinement of exciton by gradually increasing the loading of Zn element. Finally, we successfully demonstrated solution-processed QD-LEDs with the best external quantum efficiencies as high as 4.2% and emission wavelength centering at 663 nm. To our best knowledge, this is the most efficient solutionprocessed red QD-LED based on Cu-In-Zn-VI nanocrystals.
1. Introduction Semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have been extensively studied in the past several decades due to their narrow emission band, size-dependent optical spectra and high PL efficiencies [1–8]. Currently, the external quantum efficiency (EQE) of red- and green-emitting QD-LEDs has exceeded 20%, which is close to the performance of state-of-the-art phosphorescent organic light-emitting diodes (OLEDs) [9,10]. However, these highly efficient QD-LEDs depend on the excellent fluorescence from Cd-VI NCs, which give rise to environmental issues. To achieve sustainable development of QD-LED technology, semiconductor NCs without heavy-metal elements should be developed. Among different types of heavy-metal-free semiconductor NCs, Cu–In-based chalcogenide NCs have attracted much attention due to their excellent optoelectronic properties in the visible and near-infrared (NIR) range [11–14]. The strategy of forming core-shell (CuInS2–ZnS)
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structure achieved notable performance for yellow QD-LED, whose best EQE reached up to 7.3% through the engineering of surface ligands, shell thickness and devices architectures [15–17]. Later, it was realized that the core-shell structure actually could not form an abrupt surface between CuInS2 and ZnS, instead, the diffusion of Zn element resulted in a quasi-alloy structure [18–21]. As demonstrated for Cd-containing QD-LEDs [22–25], graded-alloying was a facile way to simultaneously achieve soft core-shell confinement and bandgap engineering, eventually enabling tunable emission colors with high efficiency. Inspired by these, our group recently applied this strategy to the synthesis of Cu-InVI NCs by introducing Zn elements during the growth of core, producing a so-called Cu-In-Zn-VI system [26,27]. Our Cu–In–Zn–S NCs showed tunable emission wavelength from 520 to 640 nm, enabling solution-processed yellow-emitting QD-LEDs with EQE of 2.4%. Modification of electron transport layer (ETL) improved the EQE beyond 4.1% [28,29]. To explore the effectiveness of graded-alloy structures in Cu-In-Zn-
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Chen), [email protected] (A. Tang).
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https://doi.org/10.1016/j.orgel.2019.06.024 Received 28 May 2019; Received in revised form 17 June 2019; Accepted 17 June 2019 Available online 20 June 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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VI system, as well as potential application for display technology, one needs to demonstrate efficient QD-LEDs emitting primary colors. Considering the optical band gap of bulk CuInSe2 (1.05 eV), CuInS2 (1.5 eV), ZnSe (2.7 eV) and ZnS (3.6 eV), the engineering of cations and anions is an effective approach to realize red emission from Cu-In-Zn-VI based NCs. Our previous study showed that the PL wavelength from multinary Cu–In–Zn–Se–S NCs red-shifted as the amount of Se increased [29]. Therefore, for the Cu-In-Zn-VI system, one could tune the optical band gap by adjusting the ratio of Se/S and Cu/Zn. In this paper, a series of multinary Cu–In–Zn–Se–S NCs were synthesized firstly using a non-injection method, and the emission wavelength was tunable from 624 to 665 nm by separately optimizing the dose of Se and Cu precursors. X-ray diffraction patterns showed that these NCs had a cubic zinc blende structure, XRD and TEM results revealed that when the initial feeding ratio of Se/(In + Zn) was 0.8/1.5, the produced NCs had an optimum FWHM, crystallinity and optical performance. The depth profile of X-ray photoelectron spectroscopy confirmed that the distribution of Zn element facilitated a soft core-shell confinement. Based on these results, solution-processed QD-LEDs were fabricated using these red Cu–In–Zn–Se–S NCs as the emitting layer. The EQE of redemitting QD-LEDs was 3.6 ± 0.6%, with a champion efficiency of 4.2%. To our best knowledge, these are the most efficient devices among solution-processed red-emitting QD-LEDs using Cu-In-Zn-VI QDs.
substrates, which were sequentially cleaned with deionized water, acetone and isopropanol. Afterwards, the substrates were subjected to ultraviolet-ozone treatment for 30 min. A poly(3,4-ethylenedioxythiophene)/polys-tyrenesulfonate (PEDOT:PSS) (AI 4083) layer was spin-coated onto the substrates at 5000 rpm for 60 s with a film thickness of 30–40 nm, and the film was baked at 150 °C for 30 min. Subsequently, poly(9,9-dioctylfluorene-co-N-(4-butylpheny-l)diphenylamine (TFB) solution (8 mg mL−1 in chlorobenzene), the Cu–In–Zn–Se–S NCs (10 mg mL−1, toluene), and the optimized ZnO NPs (Mg-doped ZnO nanoparticles modified with Cl ions, Cl@ZnO:Mg NPs, the detailed synthesis is given in supporting information) (30 mg mL−1, ethanol) were spin-coated onto PEDOT:PSS as functional layers. The thickness of different layers is TFB (30–40 nm), NCs (20–30 nm) and modified ZnO (40–50 nm). Different orthogonal solvents were used for deposition adjacent layers, which could prevent physical damage to the films caused by sequential casting processes in multilayered structures. After spin-coating each layer, the TFB layer was annealed at 160 °C for 30 min and the ZnO NPs layer was baked at 60 °C for 30 min. Spincoating and the baking/drying processes for different layers were carried out in a nitrogen-filled glove box. Finally, a top Al cathode (100 nm) was deposited with a custom high-vacuum deposition chamber through a shadow mask with an active device area of 4 mm2.
2. Experimental section
XRD patterns were recorded with a Bruker D8 Advance diffractometer (Cu Kα source at λ = 1.54056 A(o) ). X-ray photoelectron spectroscopy (XPS) measurements were performed using an Argon ion gun (EX06, 2 kV) with an emission current of 20 mA and an output power of 300 W. All binding energies for different elements were calibrated with respect to the C1s line from the carbon contaminant at 284.8 eV. TEM images were captured with a JEM-1400 transmission electron microscope at 100 kV acceleration voltage. Steady PL spectra were recorded using a FLUORAT-02-PANORAMA spectrophotometer. Absorption spectra were recorded with an Ocean Optics USB 2000 spectrophotometer. UV photoelectron spectra (UPS) were gathered with a Kratos AXIS Ultra DLD spectrometer using an He I photon source (21.22 eV). The current density-luminance-voltage (J-L-V) characteristics of QD-LEDs were recorded with a source-measure unit (SMU, Keithley 2400) and spectroradiometer (Photo Research-735) Electroluminescence (EL) spectra were gathered with an Ocean Optics USB 2000 spectrometer with the devices driven at constant current using the SMU. All measurements were conducted at room temperature. The relative PLQY was measured by using Rhodamine B as a standard reference (QY = 97%, in ethanol), which can be calculated as follows:
2.4. Characterization
2.1. Materials Copper(I) chloride (CuCl, 99.9%), indium(III) chloride (InCl3, 99.9%), zinc(II) acetate (Zn(OAc)2, 99.9%), Se powder (AR), 1-dodecanethiol (DDT), oleic acid (OA), and oleylamine (OM) were purchased from Shanghai Aladdin Reagent Company. 1-octadecene (ODE, 90%) was purchased from Alfa Aesar. Ethanol and toluene were purchased from Beijing Chemical Reagent, China. All chemicals were used without further purification. 2.2. Synthesis of multinary Cu–In–Zn–Se–S NCs Multinary Cu–In–Zn–Se–S NCs were synthesized by following a procedure that is very similar to that presented in our previous studies [26,27]. Typically, CuCl (0.15 mmol), InCl3 (0.5 mmol), Zn(OAc)2 (1 mmol), and Se powder (1 mmol) were mixed with OA (1 mL), OM (1 mL), DDT (3 mL), and ODE (10 mL) in a 25 mL four-necked flask. The mixture was then heated to 100 °C in a nitrogen flow. The powder dissolved completely after magnetic stirring for 20 min, and the reaction temperature was subsequently elevated to 220 °C for 10 min. The reaction was subsequently allowed to cool naturally to room temperature, followed by washing and purification with an ethanol/toluene mixture for three times. To study the effects of the amount of Se powder on the optical properties of Cu–In–Zn–Se–S NCs, different samples were synthesized by varying the amount of Se powder (0.8, 0.5 and 0.2 mmol) while the other reaction parameters remained unchanged. After obtaining the appropriate amount of Se, different Cu–In–Zn–Se–S NCs were synthesized by fine-tuning the amount of Cu precursors from 0.05 to 0.2 mmol while the amount of Se powder was kept at 0.8 mmol and other reaction conditions were kept unchanged. The synthesized Cu–In–Zn–Se–S NCs with different amount of Se powder are named Se-1, Se-0.8, Se-0.5 and Se-0.2 when the amount of Se powder was 1, 0.8, 0.5 and 0.2 mmol, respectively. Similarly, the synthesized Cu–In–Zn–Se–S NCs with different amount of Cu precursors are named Cu-0.05, Cu-0.075, Cu-0.1, Cu-0.125, Cu-0.15 and Cu-0.2.
QYS = QYR ×
n2 IS A × R × s2 IR AS nR
where the subscript R means Rhodamine B and S means the as-obtained products, all the data were measured under the excitation wavelength of 425 nm. QY is the quantum yield, I is the measured integrated area of PL intensity, n is refractive index (n = 1.497 for toluene; n = 1.362 for ethanol), and A is the absorbance at the excitation wavelength, which is typically lower than 0.10. All of the measurements were performed at room temperature. 3. Results and discussion We firstly explored the effect of anion engineering. Based on our previous work [27], multinary Cu–In–Zn–S NCs were synthesized by mixing precursors including CuCl, InCl3, Zn(OAc)2, S powder and DDT in solvents, in which DDT and S powder both offer S source in the quaternary NCs. For the synthesis of Cu–In–Zn–Se–S NCs, Se powder and DDT provide Se and S respectively, while S powder is not used [26,27]. Our proof of concept experiment produces a distinct red-shift
2.3. Fabrication of solution-processed QD-LEDs QD-LEDs were fabricated on indium tin oxide (ITO)-coated glass 47
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dispersed in toluene, and the PL maximum shifts from 610 to 643 nm as the amount of Cu increases from 0.05 to 0.125 mmol, and little shift is observed with a further increase of Cu dosage to 0.2 mmol. Accordingly, the emission color changes from orange to deep red under illumination with 365 nm light (Fig. 2d). When the amount of Cu exceeds 0.125 mmol, no obvious shift in the emission wavelength is observed, suggesting the incorporation of Cu is saturated. This is very different from the phenomenon observed in Cu–In–Zn–S NCs, whose formation mechanism is cationic exchange between Zn2+ and Cu+ [26]. Fig. 2b shows the corresponding XRD patterns with three main diffraction peaks assignable to Cu0.412In0.412Zn0.175S (JCPDS No. 47–1371) and Cu0.35In0.35Zn0.3Se (JCPDS No. 50–1240), suggesting the formation of an alloyed structure. The corresponding PLQYs of the samples with different Cu dose are plotted in Fig. 2b, and the PLQY is gradually increased to 48% with an increase of the Cu dose from 0.05 to 0.1 mmol, and further increasing Cu dose to 0.2 mmol leads a decrease of the PLQY to 20%. To visualize the result of anion and cation optimization, and understand the distribution of elements in the multinary Cu–In–Zn–Se–S NCs, XPS spectra were taken at different depths by etching with Ar ions in a given area. The Cu-0.15 sample was used as an example, and the corresponding high-resolution scan of Cu 2p, In 3d, Zn 2p, Se 3d, and S 2p are shown in Fig. 3. For the ease of comparison, the sample without any etching is denoted as etch 0, and the samples with increasing etching time are denoted as etch 1, etch 2, and etch 3. As shown in Fig. 3a, Cu 2p signals from the samples before and after etching fit the doublets located at 932.4 and 952.1 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. No satellite peaks were observed between the two peaks, indicating that the oxidation state of Cu is +1. The signals from In 3d and Zn 2p shown in Fig. 3b and c confirm the presence of In3+ and Zn2+, and the valence do not change from the surface to interior of the NCs. As shown in Fig. 3d, the Se 3d signals can be fitted to two doublets, which are can be assigned to Se 3d3/2 and Se 3d5/2, respectively. According to an analysis of Se 3d in previous works, the two doublets suggest the existence of two different Se species in the samples: Se2− ions at lower binding energy and an intermediate Se species (e.g., diselenide) at higher binding energy [30–33]. It should be noted that the position of the Se 3d peaks do not change before and after etching, suggesting that Se is uniformly distributed in the samples. Apart from Se 3d, intense Se 3p signals were also detected in the binding energy region where S 2p and Se 3p signals overlap, which can be fitted to Se 3p3/2 and Se 3p1/2 (Fig. 3e). Furthermore, the S 2p signal can be fitted to two doublets using a spin-orbit splitting of 1.2 eV, which indicates two different sulfur species in the sample. In the synthesis, DDT acts as a surface ligand and sulfur source, thus the two doublets can be assigned to thiols bound to the surface and sulfide ions in the lattice [27,33]. It should be also noted that the position of the doublets shifts to higher binding energy and the intensity of the doublets from sulfide intensifies as the etching time increases, which indicates that the concentration of sulfide increases from the surface to the interior, and sulfur on the surface is attributed to thiols. The concentration of different elements from the surface to the interior is summarized in Table S1 of the SI, and the plots show the concentration at different etching times are shown in Fig. S3 of SI. One can clearly see that Cu, In, and Se are evenly distributed throughout the sample, and Zn ions is enriched on the surface while lattice S increases from the surface to the interior of the alloyed NCs. The Zn-rich surface with wider bandgap the interior serves as a soft shell that passivates the alloyed core with a gradient distribution of S elements. Based on the different red Cu–In–Zn–Se–S NCs, corresponding QDLEDs were fabricated in solution-processed with the NCs as emitting layer and these QD-LEDs with a forward structure: ITO/PEDOT:PSS/ TFB/NCs/Cl@ZnO:Mg/Al. In these devices, TFB functions as the hole transport layer, and the Cl@ZnO:Mg NPs are modified by Mg and Cl during synthesis to serve as the electron transport layer. It has been demonstrated in our previous report that the Cl@ZnO:Mg NPs could
Fig. 1. (a) Absorption and PL spectra from Cu–In–Zn–Se–S NCs with different amount of Se; (b) a typical TEM image of sample Se-0.8; (c) XRD patterns of Cu–In–Zn–Se–S NCs with different Se doses, where the bottom lines show the standard diffraction lines from cubic Cu0.35In0.35Zn0.3Se (JCPDS No.50–1240) and tetragonal Cu0.412In0.412Zn0.175S (JCPDS No.47–1371).
in the absorption band edge and the PL emission peak, which are shown in Fig. S1 of supporting information (SI). Inspired by this result, fabricating highly efficient red-emitting Cu–In–Zn-based QD-LEDs through anion engineering may be a feasible approach. Herein, the synthesis Cu–In–Zn–Se–S NCs with red emission is firstly explored by fine-tuning the amount of Se powder from 0.2 to 1 mmol while the amount of Cu precursors was kept constant at 0.15 mmol and the DDT dosage was 3 mL. The corresponding absorption and PL spectra are shown in Fig. 1a. The absorption band edge and PL emission maximum exhibit an obvious red-shift as the amount of Se powder increases from 0.2 to 1 mmol. Moreover, these absorption spectra show no distinct excitonic peak, and the emission peaks are adjustable in the red region from 624 to 665 nm. It should be noted that a PL shoulder of 670 nm is also observed in the Se-0.2, which also appeared in Cu–In–Zn–S NCs of our previous work [26]. As a matter of fact, the decrease of Se content leads to an increase constituent of Cu–In–Zn–S in the alloyed NCs. Thus the shoulder of 670 nm may arise from the emission of Cu–In–S NCs. Moreover, the relative PLQYs of the samples increase from less than 5%–36% and then decreases to 30% as the Se content increases from 0.2 to 1 mmol. A typical TEM image of Se-0.8 is shown in Fig. 1b, and the sample exhibits an undefined morphology with a very small size. The TEM images of other samples with different Se doses are shown in Fig. S2 of SI. The XRD patterns shown in Fig. 1c exhibit three distinct diffraction peaks which locate between the diffraction peaks of cubic Cu0.35In0.35Zn0.3Se (JCPDS No.50–1240) and tetragonal Cu0.412In0.412Zn0.175S (JCPDS No.47–1371). The diffraction peaks shift slightly toward higher 2θ angles as the Se doses decreases, which confirms the formation of an alloyed structure after partial substitution of S for Se anions. Moreover, the FWHM of the diffraction peaks increases as the Se doses decreases, indicating the reduction of average size per Scherrer equation. Combining the above results, we learn that the introduction of Se not only changes the chemical composition but also increases average particle size, enabling effective turning of optical properties. In the following, further exploration on cation engineering was carried out by fixing the amount of Se at 0.8 mmol and varying the Cu dose from 0.05 to 0.2 mmol. Other reaction conditions were kept unchanged. Herein, we discuss the effects of cation engineering. Fig. 2a shows the corresponding absorption and PL spectra measured from NCs
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Fig. 2. (a) Absorption and PL spectra of Cu–In–Zn–Se–S NCs with Cu doses ranging from 0.05 to 0.2 mmol; (b) The corresponding XRD patterns, where the bottom lines show the standard diffraction lines from cubic Cu0.35In0.35Zn0.3Se (JCPDS No.50–1240) and tetragonal Cu0.412In0.412Zn0.175S (JCPDS No.47–1371); (c) The PLQYs versus the Cu dose from 0.05 to 0.2 mmol; (d) the digital photographs of the corresponding samples under UV illumination.
reports [3,29]. The energy level in modified ZnO NPs was estimated using the UPS results shown in Fig. S4 of the SI. Fig. 5a presents J-L-V curves of the QD-LEDs with different Cu–In–Zn–Se–S NCs as the emitting layers. The cation engineering of NCs significantly impacts the device performance. The current density of these devices is not significantly changed by varying the amount of Cu, however, samples comprising more Cu show less luminance under the same bias, resulting in luminance-turn-on voltages ranging from 2.6 to 3.3 V. We speculate that excess Cu could result in quenching centers for luminescence despite of insignificant effect on carrier transport. The EQE as a function of luminance in these QD-LEDs is shown in Fig. 5b, and the best EQE is observed in the device using Cu-0.1 NCs. Specifically, the EQE of this sample reaches the maxima of 3.6% at 30 cd m−2 and decreases to 2.2% at 100 cd m−2. Although the luminance intensity is not very high due to the deep red emission, to the best of our knowledge, this result is the highest EQE value for solution-processed red QD-LEDs based on Cu-In-Zn-VI materials. It is also encouraging that such a device can maintain a relatively high EQE (> 1%) at luminance ranging from 1 to 500 cd m−2, indicating that the efficiency roll-off is low. Fig. 5c shows histograms of the peak EQE from 20 QD-LEDs with Cu-0.1 produced from different batches. The measured EQE is
facilitate the electron injection balance and exciton recombination in the emitting layer [29]. Since QDs used for QD-LED are not intensively doped, charge-transporting layers with appropriate energy levels are required for efficient charge injection and efficient luminescence. Here we carry out UPS measurement to determine the energy levels of each functional layers. Taking the sample Cu-0.15 as an example, the secondary electron cut-off and electron occupation in the valence band are shown in Fig. 4a. The energy of valence-band maximum (VBM) can be calculated from the incident photon energy (21.22 eV), the onset energy of the valence-band signal (Eonset), and the secondary electron cut-off energy (Ecutoff) using the following equation: EVBM = 21.22 - (Ecutoff Eonset). As a result, the VBM energy of the Cu-0.15 sample is approximately 6.33 eV below the vacuum level. From the absorption spectrum (Fig. 4b), the optical band gap is estimated to be approximately 1.84 eV. Thus, the energy of conduction band minimum (CBM) can be further estimated to be 4.49 eV below the vacuum level. Fig. 4c illustrates the multilayer structure of our QD-LEDs, where all functional layers were spin-coated on patterned ITO substrates with orthogonal solvents, and the Al cathode was deposited via vacuum thermal evaporation. The energy level diagram of the device is shown in Fig. 4d, where the energy levels in ITO, PEDOT:PSS, and TFB were taken from previous
Fig. 3. Depth profiles of the XPS signals for (a) Cu 2p, (b) In 3d, (c) Zn 2p, (d) Se 3d, and (e) S 2p from sample Cu-0.15. 49
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different amount of Cu. The FWHM and emission profiles are very similar to the PL spectra. Moreover, emission from other functional layers is not observed, indicating good management of charge balance and exciton confinement in the emission layer. The insets in Fig. 5e show that the emission color observed from different QD-LEDs changes from orange to deep red by varying the amount of Cu, which is consistent with the photoluminescence results of these NCs (Fig. 2a). The EL spectra from different QD-LEDs at various driving voltages are shown in Fig. S5 of SI, and the EL intensity increases steadily as the voltage increases from 3 to 7 V with little shift in the EL maximum. In sum, the device efficiency of the QD-LEDs based on Cu–In–Zn–Se–S NCs can be effectively improved through the optimization of anion and cation dosage during the non-injection-based synthesis. The best red QD-LEDs with Cu–In–Zn–Se–S NCs exhibit a competitive mean peak EQE of 3.6 ± 0.6%, which is the highest reported EQE value for solutionprocessed red-emitting Cu–In–Zn-based QD-LEDs with a forward structure. In the course of preparing this article, we noticed the report of red-emitting QD-LEDs based on Cu–In–S NCs. Although the maximum EQE is over 7%, the device utilized an inverted structure wherein the hole-transporting layer was deposited by vacuum deposition [34]. In contrast, all the charge-transporting layers and emission layer in our devices are solution-processed. Moreover, we believe that the performance of these devices could be further improved through a combination of interfacial engineering, surface-ligand exchange strategies, and nanostructure optimization, which will be covered in our future work.
Fig. 4. (a) UPS spectrum from the high-binding energy secondary electron cutoff region of Cu-0.15, where the inset shows the valence-band edge region; (b) absorption spectrum from sample Cu-0.15, where the inset shows the corresponding optical band gap; (c) schematic device structure and d) energy level diagram for different layers in the QD-LEDs.
3.6 ± 0.6%, and the champion device reaches 4.2%, demonstrating good reproducibility of the device performance. The inset in Fig. 5b summarizes the effect of Cu on the EQE of QD-LED which increases from 1.9% to 3.6% as the amount of Cu increases from 0.05 to 0.1 mmol and decreases to 1.5% as the amount of Cu further increases to 0.15 mmol. Further increasing the Cu doses to 0.2 mmol reduces the EQE of the corresponding device to as low as 0.3%. The current efficiency-power efficiency-luminance characteristics of QD-LEDs with Cu0.1 are shown in Fig. 5d; the peak current efficiency is 0.76 cd A−1, corresponding to a peak power efficiency of 0.66 lm W−1. Fig. 5e shows the EL spectra of QD-LEDs comprising Cu–In–Zn–Se–S NCs with
4. Conclusions In summary, we explored the effects of Cu and Se concentration in a non-injection synthesis of Cu–In–Zn–S–Se NCs, which aims to obtain high red emission by rational design the chemical compositions. As confirmed by depth profiles of XPS, these NCs were featured of a soft core/shell confinement with a gradient distribution of Zn and S from the surface to the interior of NCs. The tuning of anion and cation concentration enabled the optimization of optical properties. Solutionprocessed QD-LEDs were fabricated using these Cu–In–Zn–Se–S NCs as an emitting layer. The best performing devices exhibited EQE of
Fig. 5. (a) J-L-V characteristics of the red QD-LEDs based on Cu–In–Zn–Se–S NCs with different Cu content; (b) corresponding EQE as a function of luminance; (c) statistical histograms of the peak EQE from 20 QD-LEDs based on Cu-0.1; (d) the current and power efficiencies as functions of luminance for the device based on Cu0.1; (e) EL spectra from the corresponding QD-LEDs, where the inset shows photographs of emission from the device operating at 6 V. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 50
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3.6 ± 0.6% with a champion EQE of 4.2%, which was the highest reported value from all-solution-processed red-emitting Cu–In–Znbased QD-LEDs. Our results demonstrate the feasibility of constructing efficient QD-LEDs using Cu–In–Zn–Se–S NCs.
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Conflicts of interest The authors declare no competing financial interests. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 61674011, 61735004, 51802069) and Natural Science Foundation of Beijing Municipality, China (No. 4172050). The author (A. W.) appreciates the Fundamental Research Funds for the Central Universities, China (No. 2019RC020). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.06.024. References [1] W. Nan, Y. Niu, H. Qin, F. Cui, Y. Yang, R. Lai, W. Lin, X. Peng, J. Am. Chem. Soc. 134 (2012) 19685–19693. [2] C. Coughlan, M. Ibáñez, O. Dobrozhan, A. Singh, A. Cabot, K.M. Ryan, Chem. Rev. 117 (2017) 5865–6109. [3] Y. Yang, Y. Zheng, W. Cao, A. Titov, J. Hyvonen, J.R. Manders, J. Xue, P.H. Holloway, L. Qian, Nat. Photon. 9 (2015) 259–266. [4] Q. Zhang, Y.D. Yin, ACS Cent. Sci. 4 (2018) 668–679. [5] D. Aldakov, A. Lefrançois, P. Reiss, J. Mater. Chem. C 1 (2013) 3756–3776. [6] F. Chen, Z. Guan, A. Tang, J. Mater. Chem. C 6 (2018) 10958–10981. [7] J. Lim, M. Park, W.K. Bae, D. Lee, S. Lee, C. Lee, K. Char, ACS Nano 7 (2013) 9019–9026. [8] P. Ramasamy, K.-J. Ko, J.-W. Kang, J.-S. Lee, Chem. Mater. 30 (2018) 3643–3647. [9] X. Dai, Z. Zhang, Y. Jin, Y. Niu, H. Cao, X. Liang, L. Chen, J. Wang, X. Peng, Nature
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Compositional engineering of multinary Cu–In–Zn-based semiconductor nanocrystals for efficient and solution-processed red-emitting quantum-dot light-emitting diodes
T
Zhongyuan Guana, Fei Chena, Zhenyang Liua, Peiwen Lva, Mingjun Chenb, Mingxuan Guoc, Xu Lib, Feng Tenga, Song Chenc,**, Aiwei Tanga,* a b c
Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing, 100044, China Hebei Key Laboratory of Optic-electronic Information and Materials, College of Physics Science and Technology, Hebei University Baoding, 071002, China College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu-In-Zn-Se-S NCs Compositional engineering Red QD-LEDs Electroluminescence
For quantum-dot light-emitting diodes (QD-LED), replacing Cd-based II-VI nanocrystals (NCs) with the Cu-In-ZnVI counterparts combines the consideration of environmental benignity and device performance. To prove this concept, the chemical composition and nanostructures of Cu-In-Zn-VI nanocrystals need to be thoroughly explored aiming to competitive optoelectronic properties. Herein, we reported a detailed study of Cu–In–Zn–Se–S synthesis and demonstrated how the optical bandgap, emission full width at half-maximum (FWHM) and the performance of QD-LED were tuned by simply changing the dose of precursors in a non-injection synthesis. Evident by optical absorption, the optimization of Se and Cu doses enabled good dispersity and desired emission wavelength. Further analysis of photo-electron spectroscopy revealed the chemical composition from core to surface favored soft confinement of exciton by gradually increasing the loading of Zn element. Finally, we successfully demonstrated solution-processed QD-LEDs with the best external quantum efficiencies as high as 4.2% and emission wavelength centering at 663 nm. To our best knowledge, this is the most efficient solutionprocessed red QD-LED based on Cu-In-Zn-VI nanocrystals.
1. Introduction Semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have been extensively studied in the past several decades due to their narrow emission band, size-dependent optical spectra and high PL efficiencies [1–8]. Currently, the external quantum efficiency (EQE) of red- and green-emitting QD-LEDs has exceeded 20%, which is close to the performance of state-of-the-art phosphorescent organic light-emitting diodes (OLEDs) [9,10]. However, these highly efficient QD-LEDs depend on the excellent fluorescence from Cd-VI NCs, which give rise to environmental issues. To achieve sustainable development of QD-LED technology, semiconductor NCs without heavy-metal elements should be developed. Among different types of heavy-metal-free semiconductor NCs, Cu–In-based chalcogenide NCs have attracted much attention due to their excellent optoelectronic properties in the visible and near-infrared (NIR) range [11–14]. The strategy of forming core-shell (CuInS2–ZnS)
*
structure achieved notable performance for yellow QD-LED, whose best EQE reached up to 7.3% through the engineering of surface ligands, shell thickness and devices architectures [15–17]. Later, it was realized that the core-shell structure actually could not form an abrupt surface between CuInS2 and ZnS, instead, the diffusion of Zn element resulted in a quasi-alloy structure [18–21]. As demonstrated for Cd-containing QD-LEDs [22–25], graded-alloying was a facile way to simultaneously achieve soft core-shell confinement and bandgap engineering, eventually enabling tunable emission colors with high efficiency. Inspired by these, our group recently applied this strategy to the synthesis of Cu-InVI NCs by introducing Zn elements during the growth of core, producing a so-called Cu-In-Zn-VI system [26,27]. Our Cu–In–Zn–S NCs showed tunable emission wavelength from 520 to 640 nm, enabling solution-processed yellow-emitting QD-LEDs with EQE of 2.4%. Modification of electron transport layer (ETL) improved the EQE beyond 4.1% [28,29]. To explore the effectiveness of graded-alloy structures in Cu-In-Zn-
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Chen), [email protected] (A. Tang).
**
https://doi.org/10.1016/j.orgel.2019.06.024 Received 28 May 2019; Received in revised form 17 June 2019; Accepted 17 June 2019 Available online 20 June 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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VI system, as well as potential application for display technology, one needs to demonstrate efficient QD-LEDs emitting primary colors. Considering the optical band gap of bulk CuInSe2 (1.05 eV), CuInS2 (1.5 eV), ZnSe (2.7 eV) and ZnS (3.6 eV), the engineering of cations and anions is an effective approach to realize red emission from Cu-In-Zn-VI based NCs. Our previous study showed that the PL wavelength from multinary Cu–In–Zn–Se–S NCs red-shifted as the amount of Se increased [29]. Therefore, for the Cu-In-Zn-VI system, one could tune the optical band gap by adjusting the ratio of Se/S and Cu/Zn. In this paper, a series of multinary Cu–In–Zn–Se–S NCs were synthesized firstly using a non-injection method, and the emission wavelength was tunable from 624 to 665 nm by separately optimizing the dose of Se and Cu precursors. X-ray diffraction patterns showed that these NCs had a cubic zinc blende structure, XRD and TEM results revealed that when the initial feeding ratio of Se/(In + Zn) was 0.8/1.5, the produced NCs had an optimum FWHM, crystallinity and optical performance. The depth profile of X-ray photoelectron spectroscopy confirmed that the distribution of Zn element facilitated a soft core-shell confinement. Based on these results, solution-processed QD-LEDs were fabricated using these red Cu–In–Zn–Se–S NCs as the emitting layer. The EQE of redemitting QD-LEDs was 3.6 ± 0.6%, with a champion efficiency of 4.2%. To our best knowledge, these are the most efficient devices among solution-processed red-emitting QD-LEDs using Cu-In-Zn-VI QDs.
substrates, which were sequentially cleaned with deionized water, acetone and isopropanol. Afterwards, the substrates were subjected to ultraviolet-ozone treatment for 30 min. A poly(3,4-ethylenedioxythiophene)/polys-tyrenesulfonate (PEDOT:PSS) (AI 4083) layer was spin-coated onto the substrates at 5000 rpm for 60 s with a film thickness of 30–40 nm, and the film was baked at 150 °C for 30 min. Subsequently, poly(9,9-dioctylfluorene-co-N-(4-butylpheny-l)diphenylamine (TFB) solution (8 mg mL−1 in chlorobenzene), the Cu–In–Zn–Se–S NCs (10 mg mL−1, toluene), and the optimized ZnO NPs (Mg-doped ZnO nanoparticles modified with Cl ions, Cl@ZnO:Mg NPs, the detailed synthesis is given in supporting information) (30 mg mL−1, ethanol) were spin-coated onto PEDOT:PSS as functional layers. The thickness of different layers is TFB (30–40 nm), NCs (20–30 nm) and modified ZnO (40–50 nm). Different orthogonal solvents were used for deposition adjacent layers, which could prevent physical damage to the films caused by sequential casting processes in multilayered structures. After spin-coating each layer, the TFB layer was annealed at 160 °C for 30 min and the ZnO NPs layer was baked at 60 °C for 30 min. Spincoating and the baking/drying processes for different layers were carried out in a nitrogen-filled glove box. Finally, a top Al cathode (100 nm) was deposited with a custom high-vacuum deposition chamber through a shadow mask with an active device area of 4 mm2.
2. Experimental section
XRD patterns were recorded with a Bruker D8 Advance diffractometer (Cu Kα source at λ = 1.54056 A(o) ). X-ray photoelectron spectroscopy (XPS) measurements were performed using an Argon ion gun (EX06, 2 kV) with an emission current of 20 mA and an output power of 300 W. All binding energies for different elements were calibrated with respect to the C1s line from the carbon contaminant at 284.8 eV. TEM images were captured with a JEM-1400 transmission electron microscope at 100 kV acceleration voltage. Steady PL spectra were recorded using a FLUORAT-02-PANORAMA spectrophotometer. Absorption spectra were recorded with an Ocean Optics USB 2000 spectrophotometer. UV photoelectron spectra (UPS) were gathered with a Kratos AXIS Ultra DLD spectrometer using an He I photon source (21.22 eV). The current density-luminance-voltage (J-L-V) characteristics of QD-LEDs were recorded with a source-measure unit (SMU, Keithley 2400) and spectroradiometer (Photo Research-735) Electroluminescence (EL) spectra were gathered with an Ocean Optics USB 2000 spectrometer with the devices driven at constant current using the SMU. All measurements were conducted at room temperature. The relative PLQY was measured by using Rhodamine B as a standard reference (QY = 97%, in ethanol), which can be calculated as follows:
2.4. Characterization
2.1. Materials Copper(I) chloride (CuCl, 99.9%), indium(III) chloride (InCl3, 99.9%), zinc(II) acetate (Zn(OAc)2, 99.9%), Se powder (AR), 1-dodecanethiol (DDT), oleic acid (OA), and oleylamine (OM) were purchased from Shanghai Aladdin Reagent Company. 1-octadecene (ODE, 90%) was purchased from Alfa Aesar. Ethanol and toluene were purchased from Beijing Chemical Reagent, China. All chemicals were used without further purification. 2.2. Synthesis of multinary Cu–In–Zn–Se–S NCs Multinary Cu–In–Zn–Se–S NCs were synthesized by following a procedure that is very similar to that presented in our previous studies [26,27]. Typically, CuCl (0.15 mmol), InCl3 (0.5 mmol), Zn(OAc)2 (1 mmol), and Se powder (1 mmol) were mixed with OA (1 mL), OM (1 mL), DDT (3 mL), and ODE (10 mL) in a 25 mL four-necked flask. The mixture was then heated to 100 °C in a nitrogen flow. The powder dissolved completely after magnetic stirring for 20 min, and the reaction temperature was subsequently elevated to 220 °C for 10 min. The reaction was subsequently allowed to cool naturally to room temperature, followed by washing and purification with an ethanol/toluene mixture for three times. To study the effects of the amount of Se powder on the optical properties of Cu–In–Zn–Se–S NCs, different samples were synthesized by varying the amount of Se powder (0.8, 0.5 and 0.2 mmol) while the other reaction parameters remained unchanged. After obtaining the appropriate amount of Se, different Cu–In–Zn–Se–S NCs were synthesized by fine-tuning the amount of Cu precursors from 0.05 to 0.2 mmol while the amount of Se powder was kept at 0.8 mmol and other reaction conditions were kept unchanged. The synthesized Cu–In–Zn–Se–S NCs with different amount of Se powder are named Se-1, Se-0.8, Se-0.5 and Se-0.2 when the amount of Se powder was 1, 0.8, 0.5 and 0.2 mmol, respectively. Similarly, the synthesized Cu–In–Zn–Se–S NCs with different amount of Cu precursors are named Cu-0.05, Cu-0.075, Cu-0.1, Cu-0.125, Cu-0.15 and Cu-0.2.
QYS = QYR ×
n2 IS A × R × s2 IR AS nR
where the subscript R means Rhodamine B and S means the as-obtained products, all the data were measured under the excitation wavelength of 425 nm. QY is the quantum yield, I is the measured integrated area of PL intensity, n is refractive index (n = 1.497 for toluene; n = 1.362 for ethanol), and A is the absorbance at the excitation wavelength, which is typically lower than 0.10. All of the measurements were performed at room temperature. 3. Results and discussion We firstly explored the effect of anion engineering. Based on our previous work [27], multinary Cu–In–Zn–S NCs were synthesized by mixing precursors including CuCl, InCl3, Zn(OAc)2, S powder and DDT in solvents, in which DDT and S powder both offer S source in the quaternary NCs. For the synthesis of Cu–In–Zn–Se–S NCs, Se powder and DDT provide Se and S respectively, while S powder is not used [26,27]. Our proof of concept experiment produces a distinct red-shift
2.3. Fabrication of solution-processed QD-LEDs QD-LEDs were fabricated on indium tin oxide (ITO)-coated glass 47
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dispersed in toluene, and the PL maximum shifts from 610 to 643 nm as the amount of Cu increases from 0.05 to 0.125 mmol, and little shift is observed with a further increase of Cu dosage to 0.2 mmol. Accordingly, the emission color changes from orange to deep red under illumination with 365 nm light (Fig. 2d). When the amount of Cu exceeds 0.125 mmol, no obvious shift in the emission wavelength is observed, suggesting the incorporation of Cu is saturated. This is very different from the phenomenon observed in Cu–In–Zn–S NCs, whose formation mechanism is cationic exchange between Zn2+ and Cu+ [26]. Fig. 2b shows the corresponding XRD patterns with three main diffraction peaks assignable to Cu0.412In0.412Zn0.175S (JCPDS No. 47–1371) and Cu0.35In0.35Zn0.3Se (JCPDS No. 50–1240), suggesting the formation of an alloyed structure. The corresponding PLQYs of the samples with different Cu dose are plotted in Fig. 2b, and the PLQY is gradually increased to 48% with an increase of the Cu dose from 0.05 to 0.1 mmol, and further increasing Cu dose to 0.2 mmol leads a decrease of the PLQY to 20%. To visualize the result of anion and cation optimization, and understand the distribution of elements in the multinary Cu–In–Zn–Se–S NCs, XPS spectra were taken at different depths by etching with Ar ions in a given area. The Cu-0.15 sample was used as an example, and the corresponding high-resolution scan of Cu 2p, In 3d, Zn 2p, Se 3d, and S 2p are shown in Fig. 3. For the ease of comparison, the sample without any etching is denoted as etch 0, and the samples with increasing etching time are denoted as etch 1, etch 2, and etch 3. As shown in Fig. 3a, Cu 2p signals from the samples before and after etching fit the doublets located at 932.4 and 952.1 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. No satellite peaks were observed between the two peaks, indicating that the oxidation state of Cu is +1. The signals from In 3d and Zn 2p shown in Fig. 3b and c confirm the presence of In3+ and Zn2+, and the valence do not change from the surface to interior of the NCs. As shown in Fig. 3d, the Se 3d signals can be fitted to two doublets, which are can be assigned to Se 3d3/2 and Se 3d5/2, respectively. According to an analysis of Se 3d in previous works, the two doublets suggest the existence of two different Se species in the samples: Se2− ions at lower binding energy and an intermediate Se species (e.g., diselenide) at higher binding energy [30–33]. It should be noted that the position of the Se 3d peaks do not change before and after etching, suggesting that Se is uniformly distributed in the samples. Apart from Se 3d, intense Se 3p signals were also detected in the binding energy region where S 2p and Se 3p signals overlap, which can be fitted to Se 3p3/2 and Se 3p1/2 (Fig. 3e). Furthermore, the S 2p signal can be fitted to two doublets using a spin-orbit splitting of 1.2 eV, which indicates two different sulfur species in the sample. In the synthesis, DDT acts as a surface ligand and sulfur source, thus the two doublets can be assigned to thiols bound to the surface and sulfide ions in the lattice [27,33]. It should be also noted that the position of the doublets shifts to higher binding energy and the intensity of the doublets from sulfide intensifies as the etching time increases, which indicates that the concentration of sulfide increases from the surface to the interior, and sulfur on the surface is attributed to thiols. The concentration of different elements from the surface to the interior is summarized in Table S1 of the SI, and the plots show the concentration at different etching times are shown in Fig. S3 of SI. One can clearly see that Cu, In, and Se are evenly distributed throughout the sample, and Zn ions is enriched on the surface while lattice S increases from the surface to the interior of the alloyed NCs. The Zn-rich surface with wider bandgap the interior serves as a soft shell that passivates the alloyed core with a gradient distribution of S elements. Based on the different red Cu–In–Zn–Se–S NCs, corresponding QDLEDs were fabricated in solution-processed with the NCs as emitting layer and these QD-LEDs with a forward structure: ITO/PEDOT:PSS/ TFB/NCs/Cl@ZnO:Mg/Al. In these devices, TFB functions as the hole transport layer, and the Cl@ZnO:Mg NPs are modified by Mg and Cl during synthesis to serve as the electron transport layer. It has been demonstrated in our previous report that the Cl@ZnO:Mg NPs could
Fig. 1. (a) Absorption and PL spectra from Cu–In–Zn–Se–S NCs with different amount of Se; (b) a typical TEM image of sample Se-0.8; (c) XRD patterns of Cu–In–Zn–Se–S NCs with different Se doses, where the bottom lines show the standard diffraction lines from cubic Cu0.35In0.35Zn0.3Se (JCPDS No.50–1240) and tetragonal Cu0.412In0.412Zn0.175S (JCPDS No.47–1371).
in the absorption band edge and the PL emission peak, which are shown in Fig. S1 of supporting information (SI). Inspired by this result, fabricating highly efficient red-emitting Cu–In–Zn-based QD-LEDs through anion engineering may be a feasible approach. Herein, the synthesis Cu–In–Zn–Se–S NCs with red emission is firstly explored by fine-tuning the amount of Se powder from 0.2 to 1 mmol while the amount of Cu precursors was kept constant at 0.15 mmol and the DDT dosage was 3 mL. The corresponding absorption and PL spectra are shown in Fig. 1a. The absorption band edge and PL emission maximum exhibit an obvious red-shift as the amount of Se powder increases from 0.2 to 1 mmol. Moreover, these absorption spectra show no distinct excitonic peak, and the emission peaks are adjustable in the red region from 624 to 665 nm. It should be noted that a PL shoulder of 670 nm is also observed in the Se-0.2, which also appeared in Cu–In–Zn–S NCs of our previous work [26]. As a matter of fact, the decrease of Se content leads to an increase constituent of Cu–In–Zn–S in the alloyed NCs. Thus the shoulder of 670 nm may arise from the emission of Cu–In–S NCs. Moreover, the relative PLQYs of the samples increase from less than 5%–36% and then decreases to 30% as the Se content increases from 0.2 to 1 mmol. A typical TEM image of Se-0.8 is shown in Fig. 1b, and the sample exhibits an undefined morphology with a very small size. The TEM images of other samples with different Se doses are shown in Fig. S2 of SI. The XRD patterns shown in Fig. 1c exhibit three distinct diffraction peaks which locate between the diffraction peaks of cubic Cu0.35In0.35Zn0.3Se (JCPDS No.50–1240) and tetragonal Cu0.412In0.412Zn0.175S (JCPDS No.47–1371). The diffraction peaks shift slightly toward higher 2θ angles as the Se doses decreases, which confirms the formation of an alloyed structure after partial substitution of S for Se anions. Moreover, the FWHM of the diffraction peaks increases as the Se doses decreases, indicating the reduction of average size per Scherrer equation. Combining the above results, we learn that the introduction of Se not only changes the chemical composition but also increases average particle size, enabling effective turning of optical properties. In the following, further exploration on cation engineering was carried out by fixing the amount of Se at 0.8 mmol and varying the Cu dose from 0.05 to 0.2 mmol. Other reaction conditions were kept unchanged. Herein, we discuss the effects of cation engineering. Fig. 2a shows the corresponding absorption and PL spectra measured from NCs
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Fig. 2. (a) Absorption and PL spectra of Cu–In–Zn–Se–S NCs with Cu doses ranging from 0.05 to 0.2 mmol; (b) The corresponding XRD patterns, where the bottom lines show the standard diffraction lines from cubic Cu0.35In0.35Zn0.3Se (JCPDS No.50–1240) and tetragonal Cu0.412In0.412Zn0.175S (JCPDS No.47–1371); (c) The PLQYs versus the Cu dose from 0.05 to 0.2 mmol; (d) the digital photographs of the corresponding samples under UV illumination.
reports [3,29]. The energy level in modified ZnO NPs was estimated using the UPS results shown in Fig. S4 of the SI. Fig. 5a presents J-L-V curves of the QD-LEDs with different Cu–In–Zn–Se–S NCs as the emitting layers. The cation engineering of NCs significantly impacts the device performance. The current density of these devices is not significantly changed by varying the amount of Cu, however, samples comprising more Cu show less luminance under the same bias, resulting in luminance-turn-on voltages ranging from 2.6 to 3.3 V. We speculate that excess Cu could result in quenching centers for luminescence despite of insignificant effect on carrier transport. The EQE as a function of luminance in these QD-LEDs is shown in Fig. 5b, and the best EQE is observed in the device using Cu-0.1 NCs. Specifically, the EQE of this sample reaches the maxima of 3.6% at 30 cd m−2 and decreases to 2.2% at 100 cd m−2. Although the luminance intensity is not very high due to the deep red emission, to the best of our knowledge, this result is the highest EQE value for solution-processed red QD-LEDs based on Cu-In-Zn-VI materials. It is also encouraging that such a device can maintain a relatively high EQE (> 1%) at luminance ranging from 1 to 500 cd m−2, indicating that the efficiency roll-off is low. Fig. 5c shows histograms of the peak EQE from 20 QD-LEDs with Cu-0.1 produced from different batches. The measured EQE is
facilitate the electron injection balance and exciton recombination in the emitting layer [29]. Since QDs used for QD-LED are not intensively doped, charge-transporting layers with appropriate energy levels are required for efficient charge injection and efficient luminescence. Here we carry out UPS measurement to determine the energy levels of each functional layers. Taking the sample Cu-0.15 as an example, the secondary electron cut-off and electron occupation in the valence band are shown in Fig. 4a. The energy of valence-band maximum (VBM) can be calculated from the incident photon energy (21.22 eV), the onset energy of the valence-band signal (Eonset), and the secondary electron cut-off energy (Ecutoff) using the following equation: EVBM = 21.22 - (Ecutoff Eonset). As a result, the VBM energy of the Cu-0.15 sample is approximately 6.33 eV below the vacuum level. From the absorption spectrum (Fig. 4b), the optical band gap is estimated to be approximately 1.84 eV. Thus, the energy of conduction band minimum (CBM) can be further estimated to be 4.49 eV below the vacuum level. Fig. 4c illustrates the multilayer structure of our QD-LEDs, where all functional layers were spin-coated on patterned ITO substrates with orthogonal solvents, and the Al cathode was deposited via vacuum thermal evaporation. The energy level diagram of the device is shown in Fig. 4d, where the energy levels in ITO, PEDOT:PSS, and TFB were taken from previous
Fig. 3. Depth profiles of the XPS signals for (a) Cu 2p, (b) In 3d, (c) Zn 2p, (d) Se 3d, and (e) S 2p from sample Cu-0.15. 49
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different amount of Cu. The FWHM and emission profiles are very similar to the PL spectra. Moreover, emission from other functional layers is not observed, indicating good management of charge balance and exciton confinement in the emission layer. The insets in Fig. 5e show that the emission color observed from different QD-LEDs changes from orange to deep red by varying the amount of Cu, which is consistent with the photoluminescence results of these NCs (Fig. 2a). The EL spectra from different QD-LEDs at various driving voltages are shown in Fig. S5 of SI, and the EL intensity increases steadily as the voltage increases from 3 to 7 V with little shift in the EL maximum. In sum, the device efficiency of the QD-LEDs based on Cu–In–Zn–Se–S NCs can be effectively improved through the optimization of anion and cation dosage during the non-injection-based synthesis. The best red QD-LEDs with Cu–In–Zn–Se–S NCs exhibit a competitive mean peak EQE of 3.6 ± 0.6%, which is the highest reported EQE value for solutionprocessed red-emitting Cu–In–Zn-based QD-LEDs with a forward structure. In the course of preparing this article, we noticed the report of red-emitting QD-LEDs based on Cu–In–S NCs. Although the maximum EQE is over 7%, the device utilized an inverted structure wherein the hole-transporting layer was deposited by vacuum deposition [34]. In contrast, all the charge-transporting layers and emission layer in our devices are solution-processed. Moreover, we believe that the performance of these devices could be further improved through a combination of interfacial engineering, surface-ligand exchange strategies, and nanostructure optimization, which will be covered in our future work.
Fig. 4. (a) UPS spectrum from the high-binding energy secondary electron cutoff region of Cu-0.15, where the inset shows the valence-band edge region; (b) absorption spectrum from sample Cu-0.15, where the inset shows the corresponding optical band gap; (c) schematic device structure and d) energy level diagram for different layers in the QD-LEDs.
3.6 ± 0.6%, and the champion device reaches 4.2%, demonstrating good reproducibility of the device performance. The inset in Fig. 5b summarizes the effect of Cu on the EQE of QD-LED which increases from 1.9% to 3.6% as the amount of Cu increases from 0.05 to 0.1 mmol and decreases to 1.5% as the amount of Cu further increases to 0.15 mmol. Further increasing the Cu doses to 0.2 mmol reduces the EQE of the corresponding device to as low as 0.3%. The current efficiency-power efficiency-luminance characteristics of QD-LEDs with Cu0.1 are shown in Fig. 5d; the peak current efficiency is 0.76 cd A−1, corresponding to a peak power efficiency of 0.66 lm W−1. Fig. 5e shows the EL spectra of QD-LEDs comprising Cu–In–Zn–Se–S NCs with
4. Conclusions In summary, we explored the effects of Cu and Se concentration in a non-injection synthesis of Cu–In–Zn–S–Se NCs, which aims to obtain high red emission by rational design the chemical compositions. As confirmed by depth profiles of XPS, these NCs were featured of a soft core/shell confinement with a gradient distribution of Zn and S from the surface to the interior of NCs. The tuning of anion and cation concentration enabled the optimization of optical properties. Solutionprocessed QD-LEDs were fabricated using these Cu–In–Zn–Se–S NCs as an emitting layer. The best performing devices exhibited EQE of
Fig. 5. (a) J-L-V characteristics of the red QD-LEDs based on Cu–In–Zn–Se–S NCs with different Cu content; (b) corresponding EQE as a function of luminance; (c) statistical histograms of the peak EQE from 20 QD-LEDs based on Cu-0.1; (d) the current and power efficiencies as functions of luminance for the device based on Cu0.1; (e) EL spectra from the corresponding QD-LEDs, where the inset shows photographs of emission from the device operating at 6 V. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 50
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3.6 ± 0.6% with a champion EQE of 4.2%, which was the highest reported value from all-solution-processed red-emitting Cu–In–Znbased QD-LEDs. Our results demonstrate the feasibility of constructing efficient QD-LEDs using Cu–In–Zn–Se–S NCs.
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