Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
www.acsami.org
Se/S Ratio-Dependent Properties and Application of GradientAlloyed CdSe1−xSx Quantum Dots: Shell-free Structure, Non-blinking Photoluminescence with Single-Exponential Decay, and Efficient QLEDs Huimin Zhang,† Fangfang Wang,† Yanmin Kuang,‡ Zhaohan Li,† Qingli Lin,*,† Huaibin Shen,† Hongzhe Wang,*,† Lijun Guo,‡ and Lin Song Li† Key Laboratory for Special Functional Materials of Ministry of Education and ‡Institute of Photo-biophysics, School of Physics and Electronics, Henan University, Kaifeng 475004, China
ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by TULANE UNIV on 01/31/19. For personal use only.
†
S Supporting Information *
ABSTRACT: Colloidal quantum dots (QDs) are promising optical and optoelectronic materials for various applications. The excited state properties are important indexes to assess the quality of QDs and may directly affect their applications. Different from controlling surface engineering (surface ligands, shell thickness, etc.) to adjust excited state properties, high-quality shell-free alloyed CdSe1−xSx (simplified as CdSeS) QDs with controlled excited state properties were synthesized by tuning the composition and using diphenylphosphine as a beneficial additive at a low temperature (∼180 °C). The optimized CdSeS shell-free alloyed QDs (Se/S = 1:8) exhibited excellent optical properties with tuning of the excited state, including single-exponential photoluminescence (PL) decay dynamics, a narrow full width at half maximum of 28 nm, and non-blinking emission behavior (>98% “on” time). Furthermore, all-solutionprocessed, multilayered quantum dot light-emitting diodes were fabricated using the conventional device structure to assess the performance of QDs with compositioncontrolled excited states. The best device displayed a maximum luminance of 92,330 cd m−2, a current efficiency of 50.3 cd A−1, and an external quantum efficiency of 14.5%. KEYWORDS: CdSeS, quantum dots, non-blinking, single-exponential decay, electroluminescence
■
INTRODUCTION Quantum dots (QDs) have been enthusiastically investigated with regard to their potential applications in lasers, biosensors, thin-film transistors, solar cells, and light-emitting diodes1−7 owing to their unique optical properties, including high photoluminescence quantum yield (PL QY), saturated pure color, easily tunable size-dependent emission wavelength, high optical and chemical stability, low-cost solution processability, and scalable production of high-class QDs.8−12 In particular, quantum dot light-emitting diodes (QLEDs) have attracted much attention since their first demonstration about two decades ago.8 Moreover, the maximum external quantum efficiency (EQE) of QLEDs has been elevated up to >20% from 0.1%,9,13−15 accompanied by a long operational lifetime of more than 300,000 and 480,000 h (at a luminance of 100 cd m−2) for red and green QLEDs by using hybrid device structures, respectively, which are comparable to the state-ofthe-art organic light-emitting diodes (OLEDs).16,17 The great improvement on the overall performance of QLEDs mainly results from both the improvement of the device structure and the targeted development of QD emitters. On the basis of the hybrid QLED structure with solutionprocessed n-type oxides as electron transport layers and © XXXX American Chemical Society
organic materials as hole transport layers, high-quality QDs with high PL QY, high stability, non-blinking behavior, and tailored excited state properties (single-exponential PL decay dynamics) are of great importance for the further achievement of highly efficient QLEDs from the perspective of QD active layers. PL blinking (PL intensity of single QDs turns “on” and “off” under continuous excitation) and multiple recombination channels (due to different defects) of QDs limit the performance of QLEDs. On the one hand, under the operational condition, QDs would be constantly excited by the injected electrons and holes and then the blinking phenomenon arising under the Auger effect, bringing further a negative effect on the efficiency. On the other hand, the multiexponential PL decay dynamics would reduce the color purity and efficiency because of the competition among the multiple recombination channels.18 Recently, Peng’s group achieved “ideal” CdSe nanoparticles, which possessed unity PL QY and unique single-channel PL decay dynamics by controlling the excited state properties through surface Received: November 1, 2018 Accepted: January 16, 2019
A
DOI: 10.1021/acsami.8b17127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces anion/cation stoichiometry and surface ligand engineering.18,19 Such “ideal” CdSe core nanocrystals could be a set of standard samples for surface science and spectroscopy related to semiconductor nanocrystals, both experimentally and theoretically. However, stability and durability are known to be problematic for such CdSe nanocrystals. For instance, purification by extraction and precipitation would greatly affect the excited state properties of those “perfect CdSe QDs”. This means that this kind of CdSe will have a blinking effect and become unstable under typical manipulation and application conditions, and the PL decay dynamics will turn into multiple recombination channels instead of single-channel decay. For better control of the excited state properties of the CdSe core, approaches based on surface engineering have been developed. These include controlling the interface between QDs and their surface ligands and coating thick wide-bandgap semiconductor shells onto the cores to prevent the excitons from delocalization to the surface.20 However, it is not conducive to precisely controlling the synthesis of QDs and reducing the preparation cost because of the complicated growth process of core/shell QDs. Therefore, it is more desirable to screen a simple approach to obtain the “perfect QD”, which is expected to have the features including high PL QY, high stability, single-channel PL decay dynamics, and nonblinking behavior. Recently, several groups have demonstrated that alloyed QDs have many advantages over binary QDs, especially for their natural formation of a gradient-alloyed outer part, which is beneficial to improving the stability and maintaining the good excited state properties.21 For example, Cao et al. described “non-blinking” CdSeS/CdZnSe QDs that have an alloyed composition.22 Our group reported green core/thickshell alloyed QDs that exhibit a narrow full width at half maximum (FWHM) of 20 nm, a high PL QY of >90%, and ensemble single-exponential PL decay.17 What is more, stateof-the-art high-efficiency blue and green QLEDs were fabricated using ternary and quaternary alloyed QDs, respectively.23,24 Therefore, it is enlightening to achieve a new kind of superior QDs by controlling the compositiondependent excited state properties within alloyed QDs, which not only possess high PL QY and high stability but also monoexponential decay dynamic properties. To the best of our knowledge, II−VI alloyed QDs without isolate shell growth but with ensemble single-exponential PL decay, controlled excited state properties, non-blinking or nearly non-blinking behavior (significantly suppressed blinking with average “on” time fractions of ∼95%.),25 and a comparatively high PL QY of >90% have not been reported to date. Herein, we report the synthesis of high-quality alloyed CdSe1−xSx QDs (simplified as CdSeS QDs) with controlled excited state properties by tuning the composition and using diphenylphosphine as a beneficial additive at a low temperature (∼180 °C). The optical properties (such as decay dynamics, FWHM, PL QY, and blinking behavior) of the gradient-alloyed CdSeS QDs can be effectively engineered by tuning the composition. With the optimal composition, such CdSeS QDs demonstrated surprising optical properties with ensemble single-exponential PL decay, a significant decrease of FWHM, and comparatively high PL QY. It should be pointed out that unlike the plain-core CdSe or CdSe QDs with thin CdS shells, the CdSeS QDs also have good stability even when their diameter is smaller than 5 nm because of the gradientalloyed structure formed under precision control. The alloyed
CdSeS QDs are more favorable for carrier injection than traditional core/shell QDs. By applying such CdSeS QDs into QLEDs with a conventional device structure, the best device displays maximum values of 92,330 cd m−2, 50.3 cd A−1, and 14.5% for luminance, current efficiency, and EQE, respectively. Such excellent performance should be attributed to the combined effects of several factors, including the high PL QY, monoexponential PL decay, and non-blinking characteristics of QDs.
■
RESULTS AND DISCUSSION Ternary colloidal CdSeS QDs are excellent emitter candidates with wide coverage of the whole visible spectrum from blue to red.26,27 For zinc-blende CdSe and CdS, their lattice constants at room temperature are 6.050 and 5.835 Å, respectively. Such a small difference in lattice cell promises that the formation of CdSeS-alloyed QDs by the introduction of S during the synthesis process is a practical way to engineer blue-green emitters with high band-edge emission efficiency. According to the growth kinetics, it is beneficial to form gradient-alloyed CdSeS QDs because of the lower activity of S compared with Se.28 The alloyed composition of CdSeS QDs are also understood to have a low nonradiative Auger recombination rate, which is helpful to suppress the blinking effect. This has been explained theoretically by smoothing the shape of the confinement potential that is believed to occur in these alloyed QDs.22,29−31 A recent study has also shown that gradientalloyed CdSeS QDs are a compositionally inhomogeneous nanoheterostructure designed to decouple the exciton from the nanocrystal surface.32 Therefore, the synthesis of alloyed CdSeS QDs is expected to obtain non-blinking, high-QY, and high-stability QDs. Herein, we synthesized green emission gradient-alloyed CdSeS QDs by adjusting the molar ratio of Se/S at lower reaction temperatures with diphenylphosphine as a beneficial additive,33 and the results confirmed the excellent optical properties of the as-prepared QDs. As shown in Figure S1, CdSeS QDs with Se/S = 1:8 present an asymmetrical PL spectrum along with a very low PL QY (typically no more than 10%) at the early stage of the reaction, which is ascribed to the small-size CdSeS QDs with more surface defects. As the reaction time was prolonged, the PL spectra became symmetric gradually. Simultaneously, there was a continuous increase in PL QY with the increase of reaction time to 80 min, and nearly no further variation could be observed with even longer reaction time from 80 to 120 min, as shown in Figure S2. This trend is similar to that observed for CdSe/CdS core/shell nanocrystals reported in the literature.34 The CdSe-rich core QDs were presumably formed at the early stage of nucleation, and the CdSeS gradient-alloyed QDs gradually formed on the base of the CdSe-rich cores with the consumption of Se. This can be evidenced by the XRD patterns of samples obtained at different reaction times in Figure S3, where the reference XRD patterns of bulk zincblende CdSe and CdS are also shown at the bottom and top, respectively. As can be seen from Figure S3, at the early stage of nucleation, the diffraction peaks of QDs are more close to those of zinc-blende CdSe. As the reaction time increased, the diffraction peak positions shifted gradually to those of zincblende CdS. This result suggests that the alloyed QDs are not a homogeneous system but more like a “core/shell” structure with a composition gradient, which can effectively minimize the stress caused by lattice mismatch in traditional core/shell systems. Also, it explains why both the PL peak and first B
DOI: 10.1021/acsami.8b17127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces absorption feature shifted toward low energy with extended reaction time (Figure S1), which is mostly as a consequence of exciton delocalization into the CdS-rich outer region. To identify the structure and chemical composition of the as-prepared QDs, the energy-dispersive spectroscopy (EDS) elemental mapping and ICP-AES spectra for the samples were recorded. Taking the ratio of Se/S of 1:8 as a model system, EDS elemental mapping (as shown in Figure S4) suggests the typical compositions of Cd, Se, and S. The Se element was mainly distributed in the inner part of the QDs, and Cd and S were distributed all throughout the particles. Because of the competition relationship between Se and S in CdSeS, the Serich cores means the gradual increase of S along the radial direction, and the S-rich outer part results in the total coverage of S all throughout the QDs. Furthermore, the elemental composition data as a function of reaction time are given in Figure S5, which is obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES). At an early reaction stage (within the first 30 s), the amount of reacted Se was higher than that of reacted S even if the initial concentration of the S precursor was 8 times higher than that of the Se precursor, which suggested that the reactivity of Se was much higher than that of S when the reaction started. This result verified our previous hypothesis that a large amount of Se precursor was consumed during the early stage of the reaction, leading to the formation of CdSe-rich core QDs. As the reaction proceeded, the composition percentage of S increased and eventually a S-based outer layer was formed. As the reaction time was extended to 15 and 80 min, the ratio of Se/S was close to 1:6 and 1:8, respectively, indicating the gradual decrease of Se from the center core to the outermost range within the Se1−xSx, and finally the outer part of the QDs was mainly composed of Cd and S. The elemental mapping data along with the ICP-AES results prove the radial composition gradient of CdSeS QDs produced by our single-step synthesis. XRD patterns of the CdSeS QDs with different compositions (Figure S6a) show that all the CdSeS QDs can be indexed to a zinc-blende crystal structure as compared to the referenced peak positions of bulk zinc-blende CdS and CdSe. Taking CdSeS QDs with Se/S = 1:8 as an example, their zinc-blende phase structure can also be evidenced by the well-resolved lattice fringes with an interplanar spacing of 0.341 nm, which can be indexed to the (111) planes of the cubic phase because this value lies between the (111) lattice spacing of cubic CdS (0.336 nm) and that of cubic CdSe (0.351 nm), as shown in Figure S6b. The corresponding fast Fourier transform (FFT) pattern (Figure S6c) shows the (111) lattice planes of the zincblende structure perpendicular to the incident electron beam. The diffraction peaks shifted gradually toward higher angles along with the percentage increase of S, indicating the domination of CdS in the outer part. Also, the average sizes of the QDs estimated from the XRD peaks based on the Scherrer equation are consistent with the values obtained from the TEM observation (Figure S7). The optical properties of CdSeS QDs are largely dependent on the molar ratio of Se/S. As shown in Figure 1a,b, the first exciton absorption peak and the band-edge PL peak of the as-prepared CdSeS QDs redshifted continuously with the increase of S amount. As mentioned above, this red shift could be attributed to the bigger radius of exciton delocalization as a result of the increased shell thickness with the increase of the amount of S.35 On the premise of a narrow size distribution of 90% was obtained, which was also in accord with the elimination of the surfacestate-related PL decay channel when the ratio of Se/S was 1:8 because the surface channel should possess a low PL QY. With the further increase of S, as shown in Figure 2e,f and Table S1, the long-lived component reappeared, which means that the large amount of S interfered with the smooth transition of the gradient composition in alloyed QDs, then leading to the reappearance of the long-lived component of surface-traprelated recombination. Furthermore, to demonstrate the degree of Auger decay suppression, measurements of transient absorption spectra of CdSeS QDs were conducted. As shown in Figure S9, the Auger times are 54, 150, and 30 ps for CdSeS with Se/S = 1:6, 1:8 and 1:10, respectively, which suggests that the Auger time of CdSeS with Se/S = 1:8 is 3−5 times longer than that of CdSeS with Se/S = 1:6 and 1:10, indicating a very considerable suppression of Auger decay for QDs with the Se/ S ratio of 1:8. The PL decay dynamics of the QD film is similar to that of the QD solution. For CdSeS QDs obtained with a reaction molar ratio of Se/S of ≤1:10 and >1:8, the PL decay dynamics of these composition-dependent QD films showed the characteristics of biexponential decay, consisting of the intrinsic PL decay channel (band-edge exciton states) and the long-lived decay channel related to the shallow electron traps. Meanwhile, for CdSeS QDs synthesized with Se/S = 1:8, the PL decay dynamics of the QD film showed a mere singleexponential decay channel. It is notable that the PL lifetime of the QD film was still maintained at 86.9% that of the QD solution when the ratio of Se/S was 1:8. This result coincides with the PL QY of QD in forms of solution (91%) and a solid film (77%), as shown in Figure S10. The aforementioned exciton decay behavior changing with the different ratios of Se/S means that the excited state properties can be adjusted by composition tuning. When the element ratio of Se/S reached 1:8, the new synthetic scheme produced characteristics of monoexponential PL decay dynamics, intrinsic PL peak width, and near-unity PL QY. It is necessary to state that the efficiency of QLEDs is strongly dependent on the PL QY of QD films but not on that of QD solution.37 However, QDs frequently exhibit high PL QY in solution but severely reduced QY in the form of a closely packed thin film as a result of the efficient nonradiative Fö rster resonance energy transfer (FRET), typically retaining 95%.25 Herein, CdSeS QDs with Se/S = 1:8 show an average “on” time of > 98%, and the volume of these non-blinking CdSeS QDs is much smaller than the nonblinking volume threshold (390 nm3) reported previously by Chen et al.35 Moreover, to test the performance of these high-quality alloyed QDs in electroluminescence devices, CdSeS QD-based QLEDs have been constructed according to the conventional device scheme via an all-solution processes except for the Al cathode. As shown in Figure 5a, the device structure of QLEDs consists of an indium−tin oxide (ITO) transparent anode, a PEDOT:PSS hole injection layer (HIL), a TFB hole-transport layer (HTL), a QD emissive layer (EML), an electrontransport layer (ETL) of ZnO nanoparticles, and an Al cathode. According to the schematic energy-level diagram shown in Figure 5b, TFB was chosen as the HTL due to the relatively lower HOMO of −5.3 eV and higher hole mobility of 1.0 × 10−2 cm2 V−1 s−1,45,46 which is profitable for the hole injection and transport. ZnO nanoparticles with an ionization potential of ∼7.6 eV and relatively high electron mobility of ∼2.0 × 10−3 cm2 V−1 s−1 have been proven to be an efficient electron-injection/transport material, which improves the radiative recombination probability within QD active layers.47
purification. Moreover, QDs with other ratios of Se/S (such as Se/S = 1:3 and 1:15) show similar robustness to ligand washing, as shown in the inset of Figure S11a. This remarkable stability is mainly ascribed to the good crystallinity even at the low reaction temperature because of the use of the secondary phosphines (HPPh2) to improve the reactivity of the precursors, and the gradient-alloyed structure minimizes the lattice mismatching along the radial direction of QD particles, which can strongly reduce the interfacial defects. At the same time, after exposure to UV light for 7 days, their PL intensities can all be kept at ∼83% of the initial PL intensity. Essentially, their excited state properties can still be maintained because of the elevated reaction temperature to form a highly crystalline gradient-alloyed structure. This is supported by the results of the variation of PL intensity and PL decay of the original and washed QDs, as shown in Figure 3. Experiments are actively in
Figure 3. PL spectra (left) and PL decay kinetics (right) of CdSeS QDs with Se/S = 1:8 before (top) and after (bottom) washing to remove some of the surface ligands.
place to identify the nature of the single-exponential PL decay kinetics for the perfect composition-dependent QDs with Se/S = 1:8. Preliminary results revealed that a typical compositiondependent CdSeS QD with Se/S = 1:8 sample possesses an ensemble single-exponential lifetime of about 12.9 ns. The PL decay could still maintain a single-exponential characteristic even if the nanocrystals had been washed several times to remove part of the surface ligands, which means that the excited state properties have good stability. At the same time, the PL QY was almost invariable from its typical value compared to that after the partial removal of the surface ligands. This reveals that the reduced surface ligands have no influence on the composition-dependent QDs, which differs from the pure core and/or thin-shell QDs that suffer from sharp declines in PL QY and bi-/multiexponential PL decay after multiple washing treatments.42 For these small nanoparticles, such high PL stability is mainly attributed to the perfect composition transition and high crystallinity of the asprepared QDs. PL blinking of single QDsPL intensity switching between different brightness states under constant excitationis a roadblock for most applications of QDs. Currently, the most widely accepted explanation is that blinking events arise because of illumination-induced charging (on to off), followed by reneutralization (off to on) of the QDs.43,44 The blinking behavior of QDs with different ratios of Se/S was studied at a E
DOI: 10.1021/acsami.8b17127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a−f) Representative PL intensity time traces (red curves) of a single CdSeS QD with different ratios of Se/S varied from 1:1 to 1:15 under continuous wave (CW) laser excitation with background noises (black curves). The dashed gray lines indicate the value chosen as the threshold between on and off states in calculating the “on” time fraction. Bin time is 20 ms. Correspondingly, the distribution of the PL intensity is plotted on the right side of each trace. Laser at 458 nm with an excitation power of 150 μW was selected to characterize this property. (g−l) Histograms of the blinking “on” time fraction for CdSeS QDs with Se/S = 1:1, 1:3, 1:6, 1:8, 1:10, and 1:15.
current density at a voltage of >3 V and lower leakage current density at a voltage of
