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Abstract
Electron transport and electron-hole balance are the essential processes that determine the efficiency and luminance of quantum dot light emitting diodes (QLEDs). Those structures with a good capability of fast electron transport and charge balance are needed. We developed a novel composite electron transport layer (ETL) consisting of zinc oxide (ZnO) and zinc magnesium oxide (ZnMgO) nanoparticles for the QLED devices, which can enhance EQE to 13.5%. It is 1.29 times and 1.33 times compared with that of pure ZnO and ZnMgO nanoparticles, respectively. The luminance intensity was increased up to 22100 cd/m2 at a voltage of 8.8 V. The current-voltage of electron-only devices measurement results indicate that the composite ETL generates higher current than the nano-particulate ZnMgO layer. Meanwhile, the QLEDs with ZnMgO:ZnO ETLs exhibit lower leakage current densities at the turn on voltage than that with pure ZnO ETL. Transient measurement results indicate that the composite ETL can keep a charge balance more effectively than a conventional ZnO nano-particulate layer.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Colloidal quantum dot-based light emitting diodes (QLEDs) have attracted enormous attentions for their potential applications in low-cost large-scale displays and solid state lightings owing to their outstanding properties, including tunable emission wavelength, narrow emission bandwidth and simple solution processing [1–7]. The conventional QLED consists of hole transport layer (HTL), quantum dot light emitting layer (EML) and electron transport layer (ETL). The ETL is multilayer nanoparticles of metal oxide or organic complexes that move electrons from cathode to EML. By optimizing the material synthesis and structure design of the ETL, QLED devices have achieved impressive development in the last two decades [8–12], but there is still room for improvement in electron transport and electron-hole balance.
In QLEDs, the electron transport is the essential process that determines the brightness and efficiency. ZnO nanoparticles are usually used as charge transport layer due to their advantage of relatively high carrier mobility (2 × 10−3 cm2V−1s−1) compared with amorphous TiO2 or organic ETLs (typically ~1 × 10−4 cm2V−1s−1 or lower) [13–15]. The higher electron mobility of ZnO facilitates efficient electron transport, increased charge recombination and low driving voltage for QLEDs [13,15]. However, the high work function of ZnO nanoparticles limits the overall efficiency due to the charge unbalance in QD/ZnO interface by means of spontaneous charge transfer [15,16]. More electrons are accumulated than holes in recombination region result in fluorescence quenching of quantum dots. To compensate, structures with good capability of electron-hole balance are needed.
For charge balance, the strategy of tailoring the energy structure of the ZnO has been developed to include size control, surface modification and doping [15,17,18]. In particular, doping Mg2+ is an effective and low-cost method for modifying the energy structure because Mg2+ can effectively widen the band gap of ZnO by lifting the conduction band (CB) and lowering the valence band (VB) [19,20]. By using solution processed ZnMgO as ETL material, the energy barrier between the cathode and ZnMgO ETL were tuned for electron/hole balancing towards a better device performance [19]. Although it has the above advantage, the electron mobility of the ZnMgO is lower than ZnO nanoparticles due to the reduction of oxygen vacancies [19,20].
The ZnO nanoparticles have relatively high carrier mobility, whereas the ZnMgO nanoparticles have effectively wide band gap. Thus the composite materials including ZnO and ZnMgO nanoparticles are likely to exhibit relatively high carrier mobility and wide band gap. The QLED structure with ZnMgO:ZnO composite film as ETL can have the capability of both high charge balance and fast electron transport. Therefore, ZnMgO:ZnO composite film is an attractive and simple structure for ETL which has not been investigated previously.
In this work, we proposed a simple method to improve the electron transport and electron-hole balance by ZnO and ZnMgO composite as the electron transport layer. The composite ETLs were formed by mixing ZnO and ZnMgO nanoparticles together at different levels. The performance of the device has been significantly improved by composite ETL with a mass ratio of 2 part ZnMgO to 1 part ZnO. As a consequence, the QLED devices with ZnMgO:ZnO composite ETL exhibit the maximal luminance increases to 22100 cd/m2 at voltage of 8.8 V, meanwhile the external quantum efficiency (EQE) increase to 13.5% which is 1.29 times and 1.33 times compared with that of pure ZnO and ZnMgO nanoparticles, respectively. Furthermore, the current−voltage curves of electron-only devices and the time-resolved PL decay dynamics of QDs with ZnMgO, ZnMgO:ZnO (2:1) and ZnO ETL are measured and analysed to demonstrate the mechanism of the ZnMgO:ZnO ETL for improving the performance of the QLED device.
2. Experimental
2.1 Chemicals
Tetramethylammoniun hydroxide (TMAH, 98%), dimethyl sulfoxide (DMSO, 99.9%), zinc acetate dihydrate ((CH3COO)2Zn·2H2O, 98.5%) and magnesium acetate tetrahydrate ((CH3COO)2Mg · 4H2O, 99.9%) were purchased from Macklin reagent. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, 2.5-3.0% in H2O), Poly[bis(4-phenyl)(4-butylphenyl)amine] (Poly-TPD, Mn = 10,000-100,000 by GPC) were purchased from Xi’an polymer Light Technology Corp. All of dissolvent (ethanol, toluene, diethanolamine etc.) were purchased from Sinopharm chemical reagent. All chemicals were used directly as received without any further purification unless otherwise noted.
2.2 The synthesis of ZnO nanoparticles
Colloidal ZnO nanoparticles were synthesized by using a sol-gel method. First, TMAH (1.5 g) was dissolved in ethanol (30 ml). And zinc acetate dihydrate (1.5 g) was dissolved in DMSO (80 ml). Then the TMAH solution was dropped slowly in zinc acetate dihydrate solution. For growth of the ZnO nanoparticles, the reaction mixture was vigorously stirred at 60 °C for 120 min in ambient air. Finally, the ZnO nanoparticles were precipitated with toluene (120 ml) and then dispersed in ethanol.
2.3 The synthesis of Zn0.95Mg0.05O nanoparticles
Although the band gap widens as the content of Mg increases for Zn1-xMgxO (x = 0, 0.02, 0.05, 0.1) nanoparticles, more Mg content introduced in ZnMgO synthesis would form phase-separated MgO nanoparticles [19]. Thus we chose Zn0.95Mg0.05O as ZnMgO nanoparticles used in the QLEDs.
The TMAH solution was prepared by dissolving TMAH (1 g) in ethanol (10 ml). A mixture of magnesium acetate tetrahydrate (0.0322 g) and zinc acetate dihydrate (0.6256 g) was dissolved in DMSO (30 ml), and the mixture was added to a three-neck flask. The TMAH solution was added dropwise into the reaction flask, and the reaction solution was heated to 30 °C in 5 min and kept for another 60 min. The appearance of bluish yellow indicated the formation of ZnMgO nanoparticles, and then diethanol amine (160 ul) was injected into the reaction flask to make it stable. Subsequently, the ZnMgO nanoparticles were precipitated with toluene (120 ml) and then redispersed in ethanol.
2.4 QLED Fabrication
QLED devices were fabricated on indium tin oxide (ITO) coated glass (15 Ω/sq) substrates. The substrates were consecutively washed with deionized water, acetone, and isopropanol for 15 min each and then removed residual organics by treating with UV-ozone for 20 min. Subsequently, the cleaned substrates were transferred into an Ar-filled glove box for spin coating of PEDOT:PSS layer. PEDOT:PSS aqueous solution were spin coated onto the ITO at 3500 rpm for 50 s and then baked at 150 °C for 15 min. Then, the substrates were transferred into another Ar-filled glove box for the rest of processes. Poly-TPD dispersed in chlorobenzene was spin coated at 2500 rpm for 20 s, followed by baking at 120 °C for 30 min. Subsequently, CdSe/CdS/ZnS QDs were spin coated at 2000 rpm for 60 s and dried at room temperature. ZnMgO:ZnO nanoparticles dissolved in ethanol were spin coated at 4000 rpm for 40s, and baked at 80°C for 30 min. Finally, the Al films were evaporated on the samples at a rate of ≈0.1 nm/s with a vacuum pressure of 4 × 10−4 Pa.
2.5 Fabrication of electron-only device
The substrate of electron-only device is also ITO coated glass. These substrates were cleaned as mentioned in 2.4. The Al films were thermal evaporation on ITO layers via a thermal evaporation at a rate of ≈0.1 nm/s under a high vacuum of 4 × 10−4 Pa. Then, ZnMgO:ZnO nanoparticles were spin-coated onto the Al film at 2000 rpm for 40 s and annealed at 150 °C for 30 min. Finally, second Al film was evaporated on the same conditions as that first film.
2.6 Characterizations
The absorption spectra of ZnMgO:ZnO nanoparticles were recorded by UV-vis spectrophotometer (Cary 300, Varian). The emission spectra were characterized by the steady-state photoluminescence system, Cary Eclipse Varian. Time resolved photoluminescence (TR-PL) spectra were acquired with a Bruker Optics 250IS/SM spectrometer equipped with an Andor IStar 740 intensified charge-coupled device (CCD) detector. The surface topographies of ZnMgO:ZnO films were recorded using an atomic force microscope (Bruker, Edge dimension) with tapping mode. X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (MiniFlex II X-ray diffractometer, Rigaku) at 50 kV and 250 mA using Cu Kα radiation (λ = 0.154 nm) at room temperature. Transmi-ssion electron microscope (TEM) images were acquired by the Tecnai F20 transmission electron microscope with acceleration voltage of 80 kV. Scanning electron microscope (SEM) images were obtained by the Hitachi HR-SEM SU8000 electron microscope with acceleration voltage of 5 kV.
The current density–voltage characterizations for QLEDs and electron-only devices were tested by using Keithley 2400 electrometer. Light output performances (EL, number of photons, etc.) of QLEDs were measured by using Labsphere 3P-GPS-033-SL integrating sphere coupled with an Ocean Optics Maya 2000spectrometer with driving voltage from 0 V to 10 V. The EQE was calculated, with the number of charge carriers () injected into the device divided by the number of photons () in the integration time (). The number of charge carriers was determined with current () multiplied by , and divided by the charge of an electron ().
3. Results and discussion
Figure 1 shows the X-ray diffraction spectra of as-prepared ZnMgO (ZMO) and ZnO nanoparticles. It can be seen that the diffraction peaks distribution of ZnO is in agreement with JCPDF card, 36-1451, the characteristic diffraction peaks have relatively high intensity, and these peaks are significantly broad. This indicates that ZnO nanoparticles have hexagonal wurtzite structure, the degree of crystallinity is relatively high, and the particle size is small. The XRD pattern of ZnMgO nanoparticles is also in agreement with JCPDF card, 36-1451, suggesting that ZnMgO nanoparticles have archetypal wurtzite-type ZnO structure. It may be caused by the fact that Mg2+ was incorporated into ZnO lattice without appreciable strain due to the ionic radii of Mg2+ (0.57 Å) being smaller than the ionic radii of Zn2+ (0.6 Å) [21,22]. ZnMgO nanoparticles have more broad diffraction peaks than ZnO, suggesting that the size of ZnMgO nanoparticles is smaller than that of ZnO, just as showed in high-resolution transmission electron microscope (HRTEM) images of them (Fig. 2). The size of ZnMgO is uniform and small, the average diameters of which is 3.0 nm, while that of ZnO 6.1 nm.
Fig. 2 TEM images of (a) ZnO and (b) ZnMgO nanoparticles. The insets are the particle-size distributions of them.
Figures 3(a) and 3(b) show the optical absorption and emission spectra for as-prepared ZnO and ZnMgO nanoparticles, and the mixtures of them. ZnMgO nanoparticles have a strong absorption peak at the range of 303 nm to 349 nm and an emission spectrum peak at 514 nm. By contrast, ZnO nanoparticles have significantly red shifted absorption peak (325 ~360 nm) and emission peak (540 nm), suggesting that ZnMgO has wider band gap than ZnO. The absorption and emission peaks of ZnMgO:ZnO mixtures lies between those of pure ZnMgO and ZnO nanoparticles, and the peak positions of mixtures have continuous blue shifts as ZnMgO proportion increases. These results indicate that the property of ZnMgO:ZnO composites can be tuned efficiently.
Fig. 3 Optical absorption and emission spectra of pure ZnMgO nanoparticles, ZnO nanoparticles and ZnMgO:ZnO mixtures.
Figure 4(a) shows the structure of the QLED devices, which consists of ITO/PEDOT:PSS/poly-TPD/QDs/ZnMgO:ZnO/Al. Here, PEDOT:PSS is used as the hole injection layer, poly-TPD as the hole transport layer, CdSe/CdS/ZnS QDs (PL quantum yield = 80%) as emitting layer, and Zn0.95Mg0.05O:ZnO as the electron transport layer. The energy level values (Fig. 4(b)) of Zn0.95Mg0.05O nanoparticles are referred to the paper [19]. The QLEDs were fabricated by the same processes as previous work [23]. In order to investigate the performance of QLEDs based on various mass mixing ratio for ZnMgO:ZnO, Zn0.95Mg0.05O nanoparticles blended with different ratio ZnO (ZnMgO:ZnO = 1:0, 3:1, 2:1, 1:1 and 0:1) were prepared as the electron transport layer.
Fig. 4 The structure and performance of QLED devices. (a) Architecture schematics, (b) energy level alignment, (c) electroluminescence (EL) spectra, inset: the picture of QLED working at 6 V, (d) current density-driving voltage, (e) luminance-driving voltage, and (f) EQE-driving voltage curves of the QLED devices.
As shown in Fig. 4(c), when driving voltage increasing from 3 V to 8 V, the QLED device with ZnMgO:ZnO (2:1) as ETL exhibits saturated red emission with EL peak from 630 nm to 635 nm and the full width at half maximum of the mission (FWHM) from 31 nm to 35 nm. Other QLEDs with different ratio of ZnMgO:ZnO have similar EL spectra. Figure 4(d) reveals the current density-voltage (J-V) characteristics of the QLED devices. By blending ZnO into ZnMgO, the current density of the device is significantly improved than that of the QLED with pure ZnMgO ETL, which is caused by adding the high carrier mobility of ZnO. Significantly, the current density of QLED (ZnMgO:ZnO = 3:1) is much higher than that of the QLED with pure ZnO ETL. It indicates that a large proportion of ZnMgO in composite ETL can significantly increase the current density. The turn-on voltages of all the QLEDs are about 2 V. The inset picture in Fig. 4(d) shows that the QLEDs with ZnMgO:ZnO ETLs have lower current densities at the turn on voltage than that with pure ZnO ETL. That reduction means that, due to the wide band gap of ZnMgO, it could hinder the leakage. Owing to small CB difference between QD and ZnO, the electrons can spontaneously be transferred from ZnO layers to QD layers at zero bias. ZnMgO with higher CB can hinder the electron injecting to QD layers.
The luminance intensity and EQE curves are shown in Figs. 4(e) and 4(f), respectively. The ZnMgO:ZnO ETL can significantly enhance the luminance of QLEDs. The QLED with ZnMgO:ZnO = 2:1 ETL harvests the best luminance with a maximum of 22100 cd/m2 at voltage of 8.8 V. The voltage at the maximum of luminance increases from 8 V to 8.8 V and then to 9 V as ZnMgO does, which suggests that the more ZnMgO can create better charge balance of devices. The luminance is no more improved, and even falls when the ratio of ZnMgO:ZnO exceeds 2:1. The high current density of QLED with ZnMgO:ZnO (3:1) ETL (Fig. 4(d)) should be the primary reason causing the falling of luminance, because the over flow of carriers might lead up to the saturation of luminance intensity. Consequently, the EQE of the QLED with ZnMgO:ZnO of 3:1 ETL is lowest. In contrast, the ZnMgO:ZnO of 2:1 and 1:1 ETLs can both improve the EQE obviously. The EQE can be enhanced to 13.5% for the QLED with ZnMgO:ZnO of 2:1 ETL, which is 1.32 and 1.27 times the value of the QLEDs with pure ZnMgO and ZnO, respectively. The efficiency and brightness of these QLEDs have not exceed those of QLEDs with pure ZnO or ZnMgO ETL in Refs [13]. and [19], which might be caused by the fact that the structure and fabrication process of the device with ZnMgO:ZnO ETL have not yet been optimized completely. To further improve the efficiency and brightness, the thickness of each layer and the fabrication processes should be optimized.
Atomic force microscope (AFM) and scanning electron microscopy (SEM) images show the surface topographic of ZnMgO:ZnO films (Figs. 5-6). These films were prepared by spin coating ZnMgO:ZnO solutions of different mass mixing ratios on glass/ITO substrates while all other experimental conditions kept unchanged. Owing to smaller size, as shown in Figs. 5(a) and 5(e), the ZnMgO film exhibits smoother surface than ZnO film. Blending ZnO nanoparticles into ZnMgO leads to significantly increased surface roughness compared with that of pure ZnMgO film (Figs. 5(b)-5(d)). The root-mean-square (RMS) surface roughness are 1.71, 2.91, 5.12, 2.05 and 3.76 nm for 1:0, 1:1, 2:1, 3:1 and 0:1 ZnMgO:ZnO films, respectively. The ZnMgO:ZnO films (3:1 and 1:1) exhibit much smaller roughness than pure ZnO layers, suggesting that small ZnMgO nanoparticles are likely to fill gaps between ZnO nanoparticles, then resulting in a reduction of surface roughness. The surface coverage in the ZnMgO:ZnO film is also related to the mixing ratio of them (Fig. 6). The ZnMgO film shows good uniformity in surface coverage, while some defects (i.e., vacancies and partial aggregates) are observed in the other films. The ZnO and ZnMgO:ZnO (1:1) films both exhibit relatively less obvious defects compared with ZnMgO:ZnO (2:1) film, and the surface coverage of ZnMgO:ZnO (3:1) film is the highest among the three films. We note that the ZnMgO:ZnO (3:1) film shows better uniformity in both surface smoothness and coverage than that of ZnO film. The uniformity of ZnMgO:ZnO (3:1) provides good contact at the interface between ETL and QDs which contributes to the high current density of QLED device (Fig. 4(d)).
Fig. 5 AFM images of (a) ZnMgO film, (b) ZnMgO:ZnO = 1:1 film, (c) ZnMgO:ZnO = 2:1 film, (d) ZnMgO:ZnO = 3:1 film and (e) ZnO film.
Fig. 6 SEM images of (a) ZnMgO film, (b) ZnMgO:ZnO = 1:1 film, (c) ZnMgO:ZnO = 2:1 film, (d) ZnMgO:ZnO = 3:1 film and (e) ZnO film.
In order to demonstrate the mechanism of the ZnMgO:ZnO ETL introduced to improve the performance of the QLED device, we investigated the current density-voltage (J-V) curves of the electron-only devices and the time-resolved PL decay dynamics of QDs with ZnMgO, ZnMgO:ZnO (2:1) and ZnO ETL, respectively. The J-V curves clearly indicate that the current density of QLED with the ZnMgO:ZnO ETL is higher than that of the pure ZnMgO nanoparticles device (Fig. 7(a)). It can be concluded that blending ZnO into ZnMgO can improve the electron transport of the device. Figure 7(b) shows time-resolved PL decay spectra of QD films with glass/QDs, ITO/QDs/ZnMgO, ITO/QDs/ZnMgO:ZnO and ITO/QDs/ZnO structures, respectively. The PL decay lifetimes of QDs on glass is 20.2 ns (Table 1) which is longer than that of the QDs with sandwiched structure between ITO and ETL, which means that many electrons transport to ETL and exciton numbers are reduced, thus depressing the PL lifetime of QDs. The PL lifetimes of QDs being in contact with ZnMgO, ZnMgO:ZnO and ZnO are 11.9 ns, 13.9 ns and 10.2 ns, respectively. The defects in the ZnMgO:ZnO film and effective wide band gap of ZnMgO could both contribute to the longest PL lifetime of ZnMgO:ZnO ETL. The surface coverage of the ZnMgO:ZnO film is estimated to be 0.8 by image analysis of the SEM picture. The PL lifetime of uniform ZnMgO:ZnO (2:1) film without defect is about 11.1 ns obtained by multiplying original value by 0.8, which is longer than that of pure ZnO ETL. This indicates that ZnMgO:ZnO (2:1) ETL could depress the charge transfer from QDs to ETL more efficiently than pure ZnO ETL could do. The spontaneous charge transfer rate () and transfer efficiency () can be calculated by the method in the literature [24]:
The spontaneous charge transfer rate () and transfer efficiency () are summarized in Table 1. The charge transfer rate and efficiency of ITO/QDs/ZnMgO:ZnO = 2:1 are both lower than that of ITO/QDs/ZnO, which suggests that the charge transfer between the QDs and ETL is restrained. Those results verify that blending ZnMgO into ZnO can help decrease electron transport and maintain the charge neutrality of the QD film.Fig. 7 (a) The current density-voltage of the electron-only device. (b) Time-resolved PL decay of QDs with Glass/QDs, ITO/QDs/ZnMgO, ITO/QDs/ZnMgO:ZnO = 2:1 and ITO/QDs/ZnO structures.
Table 1. The PL decay lifetime of QDs and the rate () and efficiency () of spontaneous charge transfer from QDs to ETL.
4. Conclusions
In summary, we demonstrated that the composite ETL consisting of ZnO and ZnMgO nanoparticles can increase the electron transport and balance the electron-hole. By mixing ZnO and ZnMgO nanoparticles by the ratio 2:1, electron transport of QLED device can be enhanced dramatically, compared with pure ZnMgO ETL QLED, and charge balance can also be significantly improved, compared with pure ZnO ETL QLED. As a result, the QLED with composite ETL exhibits luminance intensity of 22100 cd/m2 at 8.8 V, and enhanced EQE of 13.5% which is 1.29 and 1.33 times the value of QLEDs with pure ZnO and ZnMgO ETLs, respectively. We will continue to optimize the structure and fabrication process of the QLED device with ZnMgO:ZnO ETL to further improve the efficiency and brightness. The results presented here have widely applications in the optoelectronic devices such as light-emitting devices or flat-panel displays.
Funding
National Natural Science Foundation of China (NSFC) (11564026, 61366003, 61765011, 11774141, 21563013); Natural Science Foundation of Jiangxi Province (20151BBE50114, 20151BAB212001, 20171BAB202036, 20161BAB212035); Outstanding Youth Funds of Jiangxi Province (20171BCB23051, 20171BCB23052); Science and Technology Project of the education department of Jiangxi Province (GJJ150727, GJJ160681).
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