1. Introduction

Semiconductor quantum dots (QDs) are considered as promising materials for next generation optoelectronics [1–8], owing to their unique optical and electronic properties, including tunable emission wavelengths by controlling the size [9], narrow emission widths, high photoluminescence quantum yields [10], high charge carrier mobility and ease of solution processing [11]. Over the past decades, colloidal QDs have been widely investigated for display applications. QD based light emitting diodes (LEDs) have advantages over traditional LEDs because of their superior color purity, full spectrum coverage and low cost [4, 12].

Highly efficient quantum dot light-emitting diodes (QLEDs) require high quality QD emitters. Recently, QLEDs based on the core-shell type-I QDs such as CdSe/CdS, CdSe/ZnS have attracted much attention, due to their extremely high PL QYs and non-blinking behaviors [13–15]. In the type-I architecture the outer shell with a large band gap materials such as ZnS can act as a passivating layer to deal with redundant dangling bonds, and therefore benefits the suppressing of the non-radiational recombination pathways [16]. It also functions well as a physical barrier that prevents the electron-hole pairs from polarizing and keeping the emission wavelengths of the QLEDs unaffected from the applying electric fields by its screening effects to the external fields. However, the turn-on voltage of the QLEDs will inevitably increase due to the elevation of the barrier of the charge carriers passing through the large band gap shell and reaching to the emitting core. Besides, the band gap of the type-I QDs can hardly be tuned effectively to achieve a full color, because both the electrons and holes are confined in the core and the red-shift phenomenon is weakening with the increase of the particle size. A reverse type-I system where a small band gap shell is coated onto a large band gap core may provide a new material of choice to overcome these obstacles. In a reverse type-I QDs, both electrons and holes are confined in the shell and emission comes from the radiative recombinations of these electron-hole pairs in the shell. Thereby, one can expect a smaller injection barrier of type-I QD based LEDs as compared with that of type-I QD, because the charge carriers have to pass through the physical barrier from the large band gap shell before they get ready to emit photons in the core. A recent study have shown that the CdS/CdSe core/shell favors the extraction of photogenerated electrons and enhances the electron injection rate in QD-sensitized solar cells [17]. Moreover, Maity et al. have demonstrated that the electron (hole) transfer rate between reverse type-I CdS/CdSe and electron (hole) acceptor are much higher than that of type-I CdSe/CdS [18].

To this end, we intend to employ reverse type-I QDs as an emitting layer to construct a QLED to reduce the charge carrier injection barrier from anode/cathode, thus maximizing the electrically driven QD emission. Currently, the reverse type-I QD investigations are mainly limited to binary core-shell systems, such as CdS/HgS, CdS/CdSe, ZnSe/CdSe, ZnS/CdS and etc [19–23]. The QYs of these QDs are quite low, the highest of which is below 50%, which restricts their further applications to electroluminescent devices [24]. In fact, in a core-shell system, the defect states may be formed when a shell is coated onto a core, possibly owing to the lattice strain, dangling bonds or others. These defect states provide numerous channels for non-radiative relaxations that quench the fluorescence. Recent studies on type-I systems suggest that an alloy hetero-structured interface between the core and the shell may prevent defect formation, because the lattice strain is relaxed in the continuous composition gradient. The ternary alloy Cd_xZn_1-xS with a band gap of about 3.0 eV, is a promising candidate for the wide band gap core material, owing to its sharp, distinctive feature in the absorption onset and high emission efficiency. The CdSe material proved a versatile shell to construct an efficient reverse type-I QDs. Besides, theoretical and experimental results reveal that combing these two materials will produce a reverse type-I QDs, because the conduction band (CB) of Cd_xZn_1-xS core and CdSe shell are about −3.60 eV and −3.88 eV, while their valence band (VB) are −6.60 eV and −5.70 eV, respectively [25, 26].

In this work, we present the synthesis of Cd_xZn_1-xS/CdSe to construct a reverse type-I system for the first time. The defect states formed by lattice mismatch are expected to be suppressed in continuous composition gradient growth at the interface between Cd_xZn_1-xS and CdSe, thus leading to notable QYs ranging from 25% to 61% depending on the emission wavelengths. The QLEDs are fabricated by using the Cd_xZn_1-xS/CdSe and CdSe/Cd_xZn_1-xS QDs. Although a lower QY of reverse type-I Cd_xZn_1-xS/CdSe, the luminance and the external quantum efficiency (EQE) much are higher than its type-I CdSe/Cd_xZn_1-xS counterpart. Evidence show that the reverse type-I structure facilitates the injection of both the electrons and holes.

2. Experimental

2.1 Preparation of Cd_xZn_1-xS/CdSe QDs

Cd_xZn_1-xS core nanocrystals were synthesized according to previous reported method [27] but with some modifications. Briefly, 0.917 g of Zn acetate and 0.064 g CdO (the molar ratio of Zn and Cd is 10:1) were dissolved in a mixture solvent containing 4.0 ml of oleic acid (OA) and 10.0 ml of octadecene (ODE) in 50 ml of 3-neck reactor in 80 °C under the vacuum pump for 40 minutes. Then, the temperature of the reaction was persistent heated to 300 °C under N₂ flow. At this temperature, 1.0 mmol sulphur was dissolved in 2.0 ml of ODE, and then the solution was injected into the reaction system rapidly. Subsequently, the temperature of the reaction system was raised to 310 °C and kept for 12 minutes. Finally, Cd_xZn_1-xS cores were formed.

It is necessary to purity the Cd_xZn_1-xS cores and dry as powder before use. The purpose is to remove the excess Zn element drastically and other impurity. Here, proper n-hexane was added into the Cd_xZn_1-xS solution, followed by centrifugation and abandoning the white precipitate. Then, the supernatant containing QDs was added with excessive ethanol and centrifuged at 8000 r.p.m for 5 min. The above procedures should be repeated more than three times in order to obtain pure QDs. Finally, a slight yellow Cd_xZn_1-xS solid was obtained and dried to a powder.

The purified Cd_xZn_1-xS QD powder was mixed with 7.0 ml ODE and 3.0 ml of oleylamine (OAm) in a 50 ml 3-neck reactor (denoted as the main reaction). The reaction temperature was raised to 80 °C under the vacuum pump for 50 minutes, and then heated to 220 °C under N₂ flow. When the main reaction reached the condition, a Se stock solution (0.15 mmol Se, prepared by dissolving selenium in 0.5ml Trioctylphosphine (TOP)) and 1.5 ml of ODE was added into the reaction. After 20 minutes, an equimolar amount of Cd precursor stock solution (prepared by dissolving CdO in OA and ODE (volume ratio, 1:1) at 200 °C under N₂ flow) was added dropwisely (about 1 drop in every 2-3 s) to the main reaction. When the color of the solution showed no further changes, another cycle of Se/Cd precursor solution was added repeatedly until the Cd_xZn_1-xS/CdSe had grown to a wanted size. Then, the adding of the Se/Cd precursors was stopped and the reaction was terminated by lowering the reaction to room temperature.

2.2 QLED fabrication

QLEDs were fabricated with pre-patterned indium tin oxide (ITO) coated glass substrates (sheet resistance, 15Ω/sq).The substrates were consecutively cleaned with deionized water, acetone, and isopropanol for 15 min and then treated with ultraviolet-ozone for 20 min. Subsequently, the pure substrates were transferred into an Ar-filled glove box for spin coating of layers. Firstly, poly-(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) aqueous solution were spin coated onto the substrate at 3500 r.p.m for 50 s and annealed at 150 °C for 15 min. Then, poly(N,N'-bis(4-butylphenyl)-N, N'-bis(phenyl)benzidine) (poly-TPD) dispersed in chlorobenzene were spin coated at 2000 r.p.m for 30 s, followed by baking at 120 °C for 20 min. The Cd_0.1Zn_0.9S/CdSe QDs with emission wavelength at 625 nm and with PL QY>60% were dissolved in n-octane with a concentration of 20 mg/mL. QD layer was spin coated at 2000 r.p.m for 60 s and dried at room temperature. Zn_0.95Mg_0.05O nanoparticles dissolved in ethanol were spin-coated at 4000 r.p.m for 40 s, and annealed at 80 °C for 30 min. Finally, the multilayer samples were loaded into a vacuum evaporator with a base pressure of 4 × 10⁻⁴ Pa for deposition of an Ag cathode (100 nm).

2.3 Characterizations

The transmission electron microscopy (TEM) (JEM-2010 JEOL Ltd.), X-ray diffractometer (XRD) (Rigaku MiniFlex II X-ray diffractometer), photoluminescence (PL) spectrophotometer (Cary Eclipse, Varian) and UV-vis spectrophotometer (Cary 300, Varian) were employed to characterize the materials. Time resolved photoluminescence experiments were performed on a spectrometer (Bruker Optics 250IS/SM) with an intensified charge coupled device detector (CCD, Andor, IStar 740). The final concentration of each element was monitored by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Leeman Co., USA, PROFILE SPEC). Absolute PL QYs of the prepared QDs were tested by an absolute QY test system (Quantaurus-QY C11347-11, Hamamatsu Photonics Co., Ltd.). The luminescent and the visual performance of the fabricated QLEDs were measured on a spectrometer (Maya 2000, Ocean Optics) with an integrating sphere (3P-GPS-033-SL, Labsphere).

3. Results and discussion

3.1 Structural characterizations

X-ray powder diffraction (XRD) patterns of Cd_xZn_1-xS core and Cd_xZn_1-xS/CdSe core/shell nanocrystals with increasing shell thickness are shown in Fig. 1. Through the analytic comparison, the Cd_xZn_1-xS XRD pattern consists of the characteristic peaks 27.28°, 38.16°, 45.44°, 49.67° and 53.83°, which agrees well with the reported alloy QDs Zn_0.9Cd_0.1S peaks located at 26.83°, 39.40°, 47.27°, 51.50° and 56.06° (JCPDS 24-1136), corresponding to the planes (100) (102) (110) (103) and (112), respectively. Since the synthesis of the Cd_xZn_1-xS/CdSe QDs is based on the same core and the thickness of the CdSe shell is different, we denote the QDs as Cd_0.1Zn_0.9S/CdSe (x mononlayer, x ML) for simplicity. The XRD patterns of Cd_0.1Zn_0.9S/CdSe (2 ML, curve b) show characteristic peaks located at 25.61°, 42.89° and 50.52°, which roughly corresponds to the standard zinc blende CdSe peaks positioned at 25.36°, 42.01° and 49.71° (JCPDS 88-2346), respectively. A superposition of peaks at about 25.36° is observed when the CdSe shell presents. The Cd_0.1Zn_0.9S XRD pattern consists of the characteristics peaks of wurtzite structure. However, after coated with the CdSe shell (2 ML), the XRD pattern of the Cd_0.1Zn_0.9S/CdSe shifts to the wurtzite CdSe-like pattern and further to a zinc blend CdSe-like pattern with the increase of the CdSe shell thickness (4 ML). Some related works have discussed the phase changes of the QDs with the increase of CdSe shell in detail [24, 28]. With a thicker CdSe shell (4 ML), the diffraction peaks of the QDs get closer to that of standard CdSe, with peaks located at 25.35°, 42.32° and 49.94°, corresponding to the crystal planes (111) (220) and (311), respectively. The lattice constant of CdSe is smaller than that of Cd_0.1Zn_0.9S/CdSe, and therefore a thicker CdSe shell leads to a peak shift to smaller angles and becomes a much CdSe-like phase. Similar phenomena are also observed in other related core-shell systems [28–30]. With the increase of the thickness of the CdSe (from 2 to 4 ML), the full width at half maximum (FWHM) decreases from 3.54° to 3.39° at the 25.61° peak. It is worth noting that the diffraction peaks get sharper with the increase of CdSe shell, indicating a well epitaxial growth of the CdSe shell onto the Cd_0.1Zn_0.9S core.

 

Fig. 1 XRD patterns of the pure Cd_0.1Zn_0.9S with a diameter of 4.5 nm, Cd_0.1Zn_0.9S/CdSe core/shell nanocrystals with CdSe shell thickness of 2 ML and 4 ML. The line XRD spectra correspond to bulk wurtzite Cd_0.1Zn_0.9S (bottom) and bulk zinc blende CdSe (top), respectively.

Download Full Size | PDF

The TEM images of the pure Cd_0.1Zn_0.9S cores and Cd_0.1Zn_0.9S/CdSe with 2 ML CdSe shell are shown in Fig. 2. All the TEM images show high crystallinity and nearly monodisperse of the QDs. According to the statistical results over 10 TEM images, the average diameter of Cd_0.1Zn_0.9S core is estimated about 4.5 nm. After introducing a CdSe shell, the diameter of the Cd_0.1Zn_0.9S/CdSe QDs is increased to about 6.5 nm, indicating that thickness of 1 ML CdSe is about 1.0 nm. This agrees well with the shell thickness of CdSe from the reported ZnSe/CdSe system [24]. From the lattice fringes of the high resolution TEM images, the interplaner distances are estimated to be 0.303 nm and 0.272 nm for Cd_0.1Zn_0.9S and Cd_0.1Zn_0.9S/CdSe (2 ML), respectively. The distance spacing d = 0.303 nm of the core is consistent with the (112) interplanar spacing 0.305 nm of the alloy Cd_0.1Zn_0.9S (JCPDS 24-1136). The distance spacing d = 0.272 nm of the Cd_0.1Zn_0.9S/CdSe is consistent with the interplaner distance 0.272 nm of the (311) plane of the zinc blende CdSe (JCPDS 88-2346). It again confirms that the CdSe is well overcoated onto the core. In order to determine the final concentration of each element, we tested transparent solution of the QDs core using inductively coupled plasma atomic emission spectroscopy. By linear fit of the ICP peak intensity of standard liquid, the relationship between the peak intensity and the concentration is obtained. After equivalent mathematical calculation, the composition of the pure Cd_xZn_1-xS core is determined to be Cd_0.1Zn_0.9S. This agrees well with the XRD results. It is worth mentioning that the real Zn/Cd ratio (9:1) in Cd_xZn_1-xS core is higher than the initial adding ratio (10:1). This phenomenon is originated from stronger bond ability of Cd-S than Zn-S. Note that the core/shell QDs are synthesized on the same core. Therefore, it is reasonable to express the composition of the as-prepared core/shell QDs as Cd_0.1Zn_0.9S/CdSe.

 

Fig. 2 (a) TEM image of Cd_0.1Zn_0.9S; (b) High resolution TEM image of Cd_0.1Zn_0.9S; (c) TEM image of Cd_0.1Zn_0.9S/CdSe with two monlayer (2 ML) CdSe shell; (d) High resolution TEM image of Cd_0.1Zn_0.9S/CdSe. Insets highlight the size distributions.

Download Full Size | PDF

3.2 Optical properties

Notable red shifts of the absorption and PL spectra are observed from Fig. 3, which gives direct evidence for the shell growth of CdSe. Figure 3(a) shows the digital photo of the as-prepared QDs in toluene. The resulting QDs exhibit bright full color emissions from blue to red, depending on the thickness of the CdSe shell. Detailed analysis of the PL and absorption spectra would give direct evidence for the shell growth of CdSe. The single PL emission peaks in the PL spectra together with nonobvious absorption features of CdSe in the absorption spectra demonstrates that there is no separate nucleation of CdSe nanocrystals. The Cd_0.1Zn_0.9S cores show a very sharp first excitonic absorption onset at 435 nm as shown in Fig. 3(b). After overcoated with CdSe shells, the excitonic aborption features are also distinctive, suggesting that a very narrow size distribution of the Cd_0.1Zn_0.9S/CdSe nanocrystals. Accordingly, the absorption onsets of the QDs are extended to wavelengths of λ = 556, 577, 587, 601, 620, and 635 nm for 1, 1.5, 2, 2.5, 3 and 4 ML CdSe, respectively. There are not any aborption tails in the lower energies, suggesting good optical properties of these QD suspensions without light scattering. Figure 3(c) shows the evolution of the PL spectra of the samples extracted from the reaction during the overcoating CdSe shell onto the same batch of Cd_0.1Zn_0.9S cores. With the CdSe shell growing on the Cd_0.1Zn_0.9S cores, the PL emission peaks significantly shift to the red range, which is consistent with the absorption band edges. In detail, the PL peaks of the as-prepared QDs are λ = 570, 591, 604, 616, 638, 650 nm, for 1, 1.5, 2, 2.5, 3 and 4 ML CdSe, respectively. Small Stokes shifts of about 15 nm between the emission peaks and the corresponding first excitonic absorption onsets are observed. This implies that the reverse type-I Cd_0.1Zn_0.9S/CdSe system prefers a band-edge luminescence rather than deep trap emission at the long-wavelength side, which is essential for high emission efficiency. The PL curve of the green QDs (1 ML, peak at 535 nm) shows a little unsymmetrical profile, indicating some trap states are formed due to the imperfectness outer shell. With a thicker shell, the PL curves become symmetric and the full width at half maximum (FWHM) decreased from 45 to 30 nm, indicating that the size focusing occurs with the shelling process. However, further growth of the CdSe leads to a FWHM increase and a QY decline (>3ML). The lower QYs and wider FWHM of the thicker shelled Cd_0.1Zn_0.9S/CdSe QDs may originate from the weaker quantum confinement effect, because the thickness of the shell may exceed the exciton Bohr radius of CdSe (5.4 nm). The optical parameters of the as-prepared reverse type-I Cd_0.1Zn_0.9S/CdSe are listed in Table 1. The red Cd_0.1Zn_0.9S/CdSe QDs (3 ML), shows a QY of 61%, a FWHM of 30 nm, which are comparable to those best reverse type-I reported elsewhere [24]. Since the red QDs exhibit best optical properties, we chose the red QDs for further LED investigations unless particularly claimed.

 

Fig. 3 (a) Digital photographs of Cd_0.1Zn_0.9S/CdSe QD solutions with different CdSe shell thickness under the irradiation of a UV lamp. The absorption (b) and PL (c) spectra of the QDs.

Download Full Size | PDF

Table 1. Optical parameters of the QDs.

View Table | View all tables in this article

In order to study the effects of the shell thickness on the emission properties, time-resolved PL measurements were performed. After careful analysis of the decays, we found that a bi-exponential function was satisfactory in determining the kinetic parameters [31, 32]:

As shown in Fig. 4, the decays of QDs can be clearly decomposed into two parts: a fast decay within the first 10 ns followed by a long tail. Besides, Table 2 shows that the goodness-of-fit χ² is below 1.3, indicating that our bi-exponential fitting is reliable. The long decay dominates the contribution to the whole decay curves, suggesting that the radiation mainly originates from the slow recombinations. Therefore, QDs with high emission efficiency requires enhancing the slow decay component and suppress the fast decay component. The amplitude and the long decay component τ₂ are sensitive to the thickness of the CdSe shell, as listed in Table 2. The QD with 3 ML CdSe exhibits the highest amplitude 0.88, implying that the long decay component becomes more competitive and the non-radiative relaxation pathways are greatly suppressed. Besides the QD with 3ML CdSe exhibits a τ₂ of 35 ns, which is the closest one to its average PL lifetime 34.4 ns. In other words, the PL decay curve is a more single-exponential like profile than its counterparts, which is essential for high PL QY.

 

Fig. 4 Time-resolved PL decays of the Cd_0.1Zn_0.9S/CdSe core/shell QDs with 1 ML, 2 ML, 3 ML and 4 ML CdSe shell in n-hexane, excited at λ = 573, 602, 632 and 653 nm, respectively.

Download Full Size | PDF

Table 2. Kinetic parameters of the QDs obtained from time-resolved PL decays.

View Table | View all tables in this article

3.3 Electron and hole injection properties

The reverse type-I QDs are considered to have a small physical barrier to overcome, and therefore electrons and holes are easy to inject into the QDs from the hole transport layer (HTL). Because the electrons and holes contact directly to the outer shell CdSe, then they are not required to get into the core. They just reside in CdSe shell before undergoing radiative recombinations. On the contrary, electrons and holes intended to accumulate at the QDs/HTL interface with difficulty to inject into the electron transport layer (ETL) for CdSe/Cd_0.1Zn_0.9S QDs, due to the large physical barrier of Cd_0.1Zn_0.9S. In this architecture, both the electron and the hole wave functions are confined to the CdSe core, due to its type-I behaviors. As a result, electrons and holes should overcome the large barrier of the outer Cd_0.1Zn_0.9S to inject in the CdSe core where radiative recombination occurs. In order to reveal the charge carrier (electron and hole) transfer properties of the red Cd_0.1Zn_0.9S/CdSe reverse type-I QDs, we prepared CdSe/Cd_0.1Zn_0.9S type-I QDs with same emission wavelength for comparison. The optical properties and the synthesis method of this type-I QDs have been detailedly discussed in previous report [33]. Based on these two types of QDs, the electron-only devices with the structure of ITO/ZnMgO (80 nm)/QDs (20 nm)/ZnMgO/Al and hole-only device with the structure of ITO/PEDOT: PSS (~20 nm)/QDs (~20 nm)/Poly-TPD (~30nm)/Al were fabricated. The current densities of the electron-only devices based on both types of the QDs are much higher than that of hole-only devices, suggesting a higher mobility of electrons than that of holes (Fig. 5). This will cause the unbalanced transfer of electrons and holes and eventually causes an emission decline due to the unbalanced recombination and other undesired effects such as polaron-exciton quenching. However, the current density of the Cd_0.1Zn_0.9S/CdSe based hole-only device is almost 3 orders higher than that of the CdSe/Cd_0.1Zn_0.9S based one. Therefore, we conclude that reverse type-I Cd_0.1Zn_0.9S/CdSe QDs significantly reduce the hole injection barrier and facilitates the hole transport, thus leading to a better electron-hole balance as compared with its counterpart.

 

Fig. 5 The current density versus driving voltage of the electron-only (a) and the hole-only (b) devices.

Download Full Size | PDF

3.4 Device performances

The structure of the Cd_0.1Zn_0.9S/CdSe based LEDs is schematically shown in Fig. 6(a), and the corresponding to the cross-sectional TEM image of a typical device is shown in Fig. 6(b). In brief, the Cd_0.1Zn_0.9S/CdSe core/shell structure QDs-LEDs were fabricated by successive coating or evaporation a hole injection layer PEDOT: PSS, as a hole transport layer poly-TPD and a QD emitting layer, an electron transport layer ZnMgO and an Al electrode. The Al layer was deposited by thermal vacuum evaporation, and the other layers were sequentially spin-coated on a pre-patterned ITO transparent.

 

Fig. 6 (a) Schematic of the device structure; (b) Cross-sectional TEM image of a typical device.

Download Full Size | PDF

The electroluminescence (EL) and PL spectrum of a QLED device based on Cd_0.1Zn_0.9S/CdSe and CdSe/Cd_0.1Zn_0.9S QDs are shown in Fig. 7(a) and 7(b), where bright EL are observed with an emission peak located at 631 nm with high color purity (full width at half-maximum, FWHM 30nm), demonstrating that such QLED has inherited the PL color purity. The inset highlights the operated light-emitting device driven at the applied voltage of 5.0 V. Both of the devices show an EL peak of 631 nm, a 6-nm red shift to the solution PL spectrum (625 nm), owing to the inter-dot interaction enhancement from the close-packed film or dielectric dispersion of solvent [34]. It is worth mentioning that the FWHM line widths of the EL spectra are almost unchanged despite the driving voltages changing from 2.5 to 6.0 V. As shown in Fig. 7(c), a slight red shift of the peak is observed with the increase of driving voltages for the Cd_0.1Zn_0.9S/CdSe based device, which is ascribed to the so-called electric-field-induced Stark effect [35]. Whereas, the EL peak positions almost keep unchanged with the diriving voltages (Fig. 7(d)), probably due to the screening effect to the electric field from the wide band gap shell Cd_0.1Zn_0.9S. Figure 8(a) and 8(b) show the current density and luminance as a function of driving voltage of the Cd_0.1Zn_0.9S/CdSe and CdSe/Cd_0.1Zn_0.9S QLEDs. A lower turn-on voltage (V_T = 1.8 V at 3.29 cd/m²) is achieved for Cd_0.1Zn_0.9S/CdSe than its CdSe/Cd_0.1Zn_0.9S counterpart (V_T = 2.0 V at 1.96 cd/m²). Because the electrons and holes reside in the outer shell CdSe for Cd_0.1Zn_0.9S /CdSe QDs, due to its intrinsic reverse type-I behaviors, which facilitates the charge injection as compared to the type-I QDs. The luminance of Cd_0.1Zn_0.9S/CdSe is also higher than its counterpart, which is consistent with the variation of current density. At the driving voltage of 4.6 V, EQE of the Cd_0.1Zn_0.9S/CdSe based device shows a maximum value of 8.23% (Fig. 8(c)), which is 14.2% higher than its counterpart (5.8%), owing to its facile charge carrier injection. Although the type-I CdSe/Cd_0.1Zn_0.9S QDs have a higher PL QY than that of reverse type-I Cd_0.1Zn_0.9S/CdSe QDs, the device performances are much lower than reverse type-I Cd_0.1Zn_0.9S/CdSe QDs. This implies that charge injection barrier plays an important role in determining the final emission efficiency of the QLEDs. The reverse type-I Cd_0.1Zn_0.9S/CdSe based QLED reported here shows a luminance of 11412 cd/m² at the EQE maximum of 8.23%, which are moderate in alloy QD based LED devices so far [36–39].

 

Fig. 7 PL and EL spectra of the Cd_0.1Zn_0.9S/CdSe (a) and CdSe/Cd_0.1Zn_0.9S (b) based LED; Evolution of EL spectra with driving voltages of the Cd_0.1Zn_0.9S/CdSe (c) and CdSe/Cd_0.1Zn_0.9S (d) based devices.

Download Full Size | PDF

 

Fig. 8 Current density, luminance of the Cd_0.1Zn_0.9S/CdSe (a), CdSe/Cd_0.1Zn_0.9S (b) based devices; (c) EQEs of the two devices.

Download Full Size | PDF

4. Conclusion

In summary, we have synthesized reverse type-I Cd_0.1Zn_0.9S/CdSe QDs with a high PL QY (625 nm, 61%) and a widely tunable emission, ranging from 450 to 670 nm. Two red QLEDs containing Cd_0.1Zn_0.9S/CdSe versus CdSe/Cd_0.1Zn_0.9S have been identically fabricated for comparison. Although the PL QY of Cd_0.1Zn_0.9S/CdSe is lower than CdSe/Cd_0.1Zn_0.9S QDs, the luminance and EQE of the Cd_0.1Zn_0.9S/CdSe QLED is much higher than its counterpart. In detail, Cd_0.1Zn_0.9S/CdSe QLED exhibits an EQE maximum of 8.23%, which is nearly 1.4 times higher as compared to its CdSe/Cd_0.1Zn_0.9S counterpart (5.8%). Electron- or hole- only experiments reveal that the Cd_0.1Zn_0.9S/CdSe with a reverse type-I structure facilitates the charge carrier injection. There is no need to overcome the physical barrier from the outer shell material as compare to the type-I CdSe/Cd_0.1Zn_0.9S QDs, because the electrons and holes reside in the CdSe shell for the reverse type-I Cd_0.1Zn_0.9S/CdSe QDs, which benefits the direct injection of the charge carriers. We hope that with further construction of the heterostructure system, the PL QYs and the charge injection ability of reverse type-I QDs will be enhanced further, making them comparable to the well-established type-I QLEDs.