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A Layer-by-Layer Growth Strategy for Large-Size InP/ZnSe/ZnS CoreShell Quantum Dots Enabling High-Efficiency Light-Emitting Diodes Fan Cao, Sheng Wang, Feijiu Wang, Qianqian Wu, Dewei Zhao, and Xuyong Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03671 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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Chemistry of Materials

A Layer-by-Layer Growth Strategy for Large-Size InP/ZnSe/ZnS CoreShell Quantum Dots Enabling High-Efficiency Light-Emitting Diodes Fan Cao,1 Sheng Wang,1 Feijiu Wang,2 Qianqian Wu,1 Dewei Zhao,3 and Xuyong Yang1,* 1 Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Road, 200072, P. R. China 2 Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan 3 Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA ABSTRACT: Shell is of great significance to the enhancement in photoluminescence quantum yield (PLQY) and stability of core-shell-type quantum dots (QDs). InP/ZnS core-shell QDs without intrinsic toxicity have shown huge potential for them to replace the widely used cadmium-containing QDs, however it is still challenging to control the growth of InP-based coreshell QDs due to the lattice mismatch between InP core and ZnS shell. Here, we report on the synthesis of ~15 nm-size InP/ZnSe/ZnS QDs with a thick ZnS outer shell by a layer-by-layer shell growth strategy. The ZnS shell was prepared by a circularly gradient temperature-rise and long reaction procedure in each step, which not only ensures relatively low precursor concentration preventing the anisotropic growth of QDs, but also allows low-reactivity source to be decomposed sufficiently to achieve layer-by-layer growth of a thick ZnS shell. The resulting QDs show the highest PLQY of 73%, narrow emission linewidth of up to 40 nm, wide spectrum tunability and excellent stability. Furthermore, the thick ZnS shell also effectively suppresses non-radiative Förster resonant energy transfer and Auger recombination within QDs. As a result, these enable our quantum dot light-emitting diodes (QLEDs) to achieve a record external quantum efficiency of 6.6% in heavy-metal-free red QLEDs.

Semiconducting quantum dots (QDs) have attracted tremendous interest owing to their outstanding properties such as tunable emission, saturated color, high photoluminescence quantum yield (PLQY), and low-cost solution processing.1-7 Thus far, only cadmium (Cd)-based QDs can well satisfy the requirement of display technology.8-12 However, the toxicity of heavy metalcontaining QDs has severely limited its application in further development and commercialization.13-15 The rising demand on environmentally friendly QDs has greatly stimulated the study on Cd-free QDs, among which indium phosphide (InP) is of great interest as a potential alternative due to its nontoxic and broad emission range from visible to near-infrared region.16-21 Recently, great progress on core-shell-type InP QDs has been made including both photoluminescence (PL) and electroluminescence (EL).22-27 However, the size of InPbased core-shell QDs is still smaller than that of core-shelltype CdSe QDs due to a relatively large lattice mismatch between InP core and ZnS shell,28 resulting in more interface defects between core and shell and thus low PLQY and poor stability. Moreover, the thin-shell of QDs have negative effects on the performance of quantum dot light-emitting diodes (QLEDs) because of non-radiative Förster resonant energy transfer (FRET) between closelypacked QDs and Auger recombination (AR) in QDs.29,30 To date, the highest external quantum efficiency (EQE) of InP-

based QLEDs is only 3.46%,25 much lower than that of Cdcontaining QLEDs. Herein, we report a layer-by-layer strategy for synthesizing large-size InP/ZnSe/ZnS QDs (~15 nm) with a thick ZnS outer shell via a circularly gradient temperature-rise process. The ZnS shell was grown in periodic steps. At each step, relatively low precursor concentration could suppress the anisotropic growth of QDs. Meanwhile, gradient temperature-rise procedure accelerates the molecular thermal motion of materials, leading to extended reaction time that enables lowreactivity source (1-dodecanethiol, 1-DDT) to be decomposed sufficiently to achieve thick ZnS shell growth that suppresses non-radiative FRET and AR within QDs. The resulting QDs exhibit the high QY of 73%, narrow emission width of up to 40 nm, wide spectrum tunability (from 549 to 617 nm), and excellent stability. As a result, we obtain a record current efficiency of 13.6 cd/A and a record EQE of 6.6% for InP-based QLEDs. The schematic of synthesis route for InP-based QDs with thick ZnS shell is shown in Figure 1a. For a typical synthesis, indium acetate, palmitic acid (PA), and 1octadecene (ODE) were mixed in a three-neck flask before evacuation. The nucleation process of InP was completed with transient reaction at 290 °C under N2 atmosphere. The zinc stearate and TOP-Se were sequentially injected into InP core-containing solutions to grow ZnSe shell to reduce large lattice mismatch (7.7%) between InP core

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and ZnS shell (Figure 1b).28 Then, the mixture of Zn(OA)2 and 1-DDT with low-reactivity as S source of ZnS shell was injected at 230 °C and kept for 30 min. Additional ZnS layers were coated by repeating the procedure for seven times at the temperature increase by 10 °C for each time. The resulting QDs were washed with acetone for six times and dispersed in octane. The inset of Figure 1a shows the QD samples with different emission wavelengths under UV light illumination.

Figure 2. (a) Absorption (dash line) and PL spectra (solid line) of InP cores, InP/ZnSe, and InP/ZnSe/ZnS QDs synthesized following the reference protocols described in the experimental section. (b) TEM images of InP/ZnSe/ZnS, InP/ZnSe and InP QDs. The scale bar is 20 nm. (c) XRD pattern of InP, InP/ZnSe, and InP/ZnSe/ZnS QDs. Figure 1. (a) Schematic of our synthetic procedure of InP/ZnSe/ZnS core-shell QDs. Inset presents a fluorescent image of QDs under UV light illumination. (b) Band structure of InP/ZnSe/ZnS QDs and lattice mismatch of InP, ZnSe and ZnS layers. The absorption and PL spectra of the InP cores, InP/ZnSe core/shell and InP/ZnSe/ZnS core/shell/shell QDs are shown in Figure 2a. The resulting InP cores exhibit a narrow first excitonic transition peak, indicating a narrow size distribution. The average size of InP cores is ~2.8 nm (Figure 2b). After coating ZnSe shell, the first excitonic transition peak of QDs has a large red shift of 0.42 eV with the emission peak at 605 nm, ascribed to the translation of electrons (holes) from InP core to ZnSe shell due to relatively small band offset between them.31 The corresponding average diameter of InP/ZnSe QDs is increased to ~6.8 nm, leading to an estimated thickness of ~2 nm ZnSe shell. The InP/ZnSe QDs exhibit a PLQY of 46% with an emission full width at half maximum (FWHM) of 53 nm. With further coating of ZnS shell, electrons (holes) have been better confined by a larger band offset, leading to a slight red-shift of 2 nm for PL emission. The relatively narrow FWHM of 48 nm and higher PLQY of 73% are comparable to the best values of InP-based QDs reported,31 primarily attributed to the thick ZnS shell which results in better surface passivation of QDs.32 The average size of InP/ZnSe/ZnS QDs obtained through the efficient layer-by-layer ZnS shelling possess a bigger size of ~15.0 nm by overgrowing additional 4.1 nm-thick ZnS. To further confirm the crystal structure of the resulting QDs, X-ray diffraction (XRD) patterns of the InP cores, InP/ZnSe core/shell, and InP/ZnSe/ZnS core/shell/shell QDs are shown in Figure 2c, three distinct peaks at (111), (220) and (311) are in well agreement with InP phase (JCPDS No. 73-1983). With further growth of ZnSe and ZnS shells, these peaks have an obvious shift to the position of ZnSe phase (JCPDS No. 80-0021) and ZnS phase (JCPDS No. 990097), respectively.

The fluorescence property and morphology of QDs are highly dependent on the processes of ZnS shell growth. Three samples were prepared with different methods of forming ZnS shell (the same amount of ZnS precursor is fixed). The ZnS shell of sample 1 was prepared by injecting all mixtures of Zn(OA)2 and DDT at 230 °C and then the temperature was increased gradually to 300 °C. The ZnS shell of sample 2 was prepared by injecting the mixture at 300 °C and then the same procedure was repeated for seven times with an interval of 30 mins for each injection. The ZnS shell of sample 3 was prepared by combining two methods above that the mixture was injected at 230 °C and the procedure was repeated for seven times and the temperature was increased by 10 °C in each time. The PL spectra, FWHM, and PLQY of three InP/ZnSe/ZnS coreshell QDs samples are shown in Figure 3a,b. As compared to sample 1, sample 3 exhibits higher PL intensity and PLQY, as well as a narrower emission linewidth, which can be attributed to better layer-by-layer growth leading to sufficient decomposition of low-reactivity DDT so as to complete the shell growth at each stage and the low precursor concentration used for each injection that suppresses the anisotropic growth of QDs.33 The reasons for the enhancement in optical properties for sample 3 compared with sample 2 could be that the process for the gradient temperature-rise prevents the Ostwald ripening of InP cores occurring under high temperature resulting in a broad size distribution34 and thus facilities the coating of thicker ZnS shell. Moreover, the PL decays of three InP/ZnSe/ZnS QDs samples are displayed in Figure 3c. It can be found that sample 3 has a longer decay time of 51.9 ns than those of the other two samples (32.4 ns for sample 1 and 42.2 ns for sample 2), demonstrating effectively direct recombination of excitons, which is consistent with the PLQY trends of these QDs. Figure 3d-f show the TEM images of three InP/ZnSe/ZnS QDs samples. The diameter of sample 3 (14.8 nm) is larger than those of the other two samples (8.6 nm for sample 1 and 12.6 nm for sample 2), indicating effective growth of thick ZnS shell by the circularly gradient temperature-rise process. The detailed


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Chemistry of Materials parameters of these three QDs samples are summarized in Table S1.

Figure 3. (a) PL spectra, (b) FWHM and PLQY, (c) PL lifetimes, and (d-f) TEM images of InP/ZnSe/ZnS QDs with different growth methods. The scale bar in TEM images is 50 nm. The shell thickness of QDs can significantly influence the fluorescent properties and stability of core-shell QDs.35,36 To verify the optimal thickness of ZnS shell, the QDs were synthesized by cyclic injection with variable number of repeated injections to control the thickness of QDs. As shown in the TEM images (Figure S1), when the number of injections is 1, 3, 5, and 7 times, the ZnS shell thickness is estimated to be 9.4, 11.6, 13.5, and 15.0 nm, respectively, corresponding to a ZnS shell of 4, 7, 10, and 12 layers. Although the PL and UV-Vis spectra of QDs have no obvious change with the increase of ZnS shell thickness (Figure 4a), the thick ZnS shell could passivate the surface defects at the core-shell interface of QDs, and thus resulting in improved PLQY (73%) and narrower FWHM (48 nm) of QDs with 10 layer-ZnS shell (13.5 nm). However, as the ZnS shell further increases, the PLQY has a slight decrease owing to formation of internal defects caused by dislocations and low-angle grain boundaries.37 The corresponding average PL lifetime of QDs with 4, 7, 10, 12 layer-ZnS shell of QDs are 43.5, 48.6, 67.3, and 56.0 ns, respectively (Figure 4b). It is obvious that thicker ZnS shell enables an efficient recombination of excitons resulting from the effective defect passivation and the PL lifetimes for the QDs with overgrown ZnS shell are decreased.38 The photostability and thermal stability of QDs were measured to visually get the relationship between the shell thickness and stability of QDs (Figure 4c,d). The QDs with thinner ZnS shell have a rapid decrease of their PL intensity to 85% of its initial value after 128 h of continuous UV irradiation. As for the QDs with thicker ZnS

shell, even after 256 h of continuous UV irradiation, the PL intensity is still higher than those of QDs with thin ZnS shell via 128 h UV irradiation. In addition, when the temperature is raised up to 150 °C, the PL intensity of the thick-shell InP/ZnSe/ZnS QDs can still remain at above 50% of its initial value, exhibiting better stability compared with thin-shell QDs. These results suggest that the thicker ZnS shell facilitates QDs to possess the enhancement in fluorescent stability.

Figure 4. (a) Absorption (dash line) and PL spectra (solid line), (b) PL lifetime, (c) photo stability and (d) thermal stability tests of InP/ZnSe/ZnS QDs with different ZnS layers.



The large-size InP/ZnSe/ZnS QDs in solution have better performance in both PL lifetime and PLQYs than those in the dry films because of the FRET in the solid state (Figure 5). However, the FRET for the QDs with thick shell is significantly reduced due to the increase in the distance between QDs.30,39 Figure 5a shows the PL decays of InP/ZnSe/ZnS QDs with thin (1.3 nm) and thick (4.1 nm) ZnS shell in solution and solid film. The PL decay time of the large-size QDs decreases by ~30% from solution (55.6 ns) to solid film (38.7 ns), compared with that of the thinshell QDs decreasing by ~39% (43.4 ns for solution and 26.6 ns for solid film), which originates from effective prevention on non-radiative FRET by the thicker ZnS shell. Furthermore, Figure 5b compares PLQYs of QDs with thick and thin shell in solution and solid film. The PLQY (37%) of QD film with thick ZnS shell is reduced by ~43% compared with that (~65%) in solution. However, the decrease in PLQY for the QDs with thin ZnS shell from solution to solid film is more severe compared with the case in the QDs with thick shell (from PLQY of 61% in solution to PLQY of 22% in solid film, decreased by ~64%). This is attributed to the high degree of non-radiative FRET in closely packed QD films. The results indicate that the QDs with thicker ZnS outer shell significantly reduce the negative influence of QDs in film because the thick ZnS shell serves as an effective spacer enabling better suppression of nonradiative FRET between QDs.28

The PL emission wavelength of the large-size QDs can be continuously tuned from green to red region by adjusting the amount of phosphorus and indium sources while keeping the other reaction conditions unchanged. Figure 6a shows the luminescence photograph of the QDs samples with different colors, displaying bright emissions under UV illumination. The absorption and PL spectra of QDs emitting from 549 nm to 617 nm are shown in Figure 6b,c. The red-shift in both the absorption and PL spectra is observed with increasing the amount of phosphorus and indium sources. The long-wavelength QDs exhibit higher PL QYs compared with those with short-wavelength because the excess of indium precursor in reaction system could reduce the surface defects (Figure 6d).40 Meanwhile, it is also found the FWHM is broadened from 40 to 52 nm with the increase in emission wavelength due to the inhomogeneous size distribution of larger size InP QDs.41 With the amount of InP sources further increasing, the PLQY of QDs gradually decreases due to the difficulty in controlling the growth of QDs with the larger size. The highest PLQY of 73% is achieved as the peak emission position of 607 nm with a narrow FWHM of 48 nm.

Figure 6. (a) Photograph of InP/ZnSe/ZnS QDs with different emission wavelengths under UV illumination. (b) Absorption and (c) PL spectra of the resulting InP/ZnSe/ZnS QDs with different amounts of InP precursors. (d) PLQYs and FWHMs of QDs with different emission wavelengths.

Figure 5. (a) PL lifetimes and (b) PL QYs of InP/ZnSe/ZnS with thin or thick ZnS shell in solution and solid film.

To evaluate the suitability for our large-size QDs in QLEDs, the resulting InP/ZnSe/ZnS QDs with emission wavelength at 607 nm were selected as the emissive layer (Figure S2). A common inverted device structure consisting of a multilayer thin film architecture of ITO/ZnO/QDs/4,4'-bis(carbazol-9-yl)biphenyl (CBP)/dipyrazino [2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11hexacarbonitrile (HAT-CN)/Al is adopted (Figure 7a). Figure 7b shows the corresponding schematic energy level diagram of device. UV photoelectron spectroscopy (UPS) was measured to understand the energy band structure of the synthesized large-size InP/ZnSe/ZnS QDs (Figure S3a) and the work function of the QDs is estimated to be 4.93 eV. The valence band edge region demonstrates that the


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Chemistry of Materials energy separated between the valence band level and the Fermi level is 1.40 eV, thus the valence band maximum (VBM) of the QDs is 6.33 eV. The band gap of QDs is 2.72 eV, determined from the absorption spectrum (Figure S3b). Therefore, the conduction band minimum (CBM) value of the QDs is 6.33 - 2.72 = 3.61 eV. Other functional layers are obtained from previous reports.42,43 The EL peak is located at 607 nm (Figure 7c) in a good agreement with the PL peak, indicating an effective suppression of quantum confined Stark effect (QCSE) by preventing the polarization of electron-hole pairs.44 The inset of Figure 7c shows the image of QLED operating at 7 V, yielding a bright emission at ~700 cd/m2. Figure 7d,e show the current

density-luminance-voltage (J-L-V) and current efficiencyexternal quantum efficiency-voltage (CE-EQE-L) characteristics of InP QDs-based QLED. Note that the luminance increases gradually once the voltage reaches a low turn-on voltage of 2.0 V, achieving the maximum brightness of over 1600 cd/m2 at an applied voltage of 8.5 V. The peak CE is 13.6 cd/A, corresponding to an external quantum efficiency (EQE) of 6.6%. Moreover, the devices can maintain a relatively high EQE (>1%) in the range of 1700 cd/m2, indicating the low efficiency roll-off of our devices. The histogram of peak EQEs shows an average peak EQE of 5.27%, which is still the record value in heavymetal-free red QLEDs (Figure 7f).

Figure 7. (a) Schematic device structure of multilayered InP QLED and (b) energy levels of individual layers of device. (c) Normalized EL and PL spectra of the device (d) J-L-V and (e) CE-EQE-L characteristics of device. (f) Histogram of peak EQEs measured from 36 devices. In summary, the large-size InP/ZnSe/ZnS QDs with 15 nm-thick shell were synthesized by a layer-by-layer shell growth process via an effective circularly gradient temperature-rise method. Both the PLQY and stability of InP QDs are dramatically improved after the efficient ZnS shell coating. Meanwhile, the thicker ZnS shell enables better suppression of non-radiative FRET and AR process within InP QD films, which are beneficial to achieve highperformance electroluminescent devices. The maximum EQE of 6.6% for our red QLEDs is the highest efficiency value ever reported in heavy-metal-free QLEDs to date. This work suggests an effective approach to further improve heavy-metal-free QLED performance and moves a significant step towards the commercialization of QLEDs.

ASSOCIATED CONTENT Supporting Information. The Support Information is available free of charge on the ACS publications website at DOI:

Experimental section, additional characterizations of QDs including particle size, optical properties, TEM and HRTEM images, as well as AFM images and UPS spectra of InP based QD films.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT The authors would like to thank the financial support from National Natural Science Foundation of China (Nos. 51675322, 61605109, and 61735004), National Key Research and Development Program of China (No. 2016YFB0401702), Shanghai Rising-Star Program (No. 17QA1401600), and The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2015037).

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