Modifying the structure of quantum dots (QDs) is regarded as one of the promising way to improve the charge transfer balance of quantum dot light-emitting diodes (QLEDs). In this paper, we report highly bright Cd0.1Zn0.9S/CdSe/CdS quantum dots by optimizing the CdSe shell and CdS outer shell and explore their application in QLEDs. We find that with appropriate thicknesses of CdSe and CdS shell the charge transfer balance of the device can be improved. Comparable studies on two red QLEDs with Cd0.1Zn0.9S/CdSe/CdS and CdSe/CdS show that the external quantum efficiency (EQE) of the Cd0.1Zn0.9S/CdSe/CdS device is over 3 folds higher than its counterpart, implying that structure of the QDs plays an important role in controlling the charge transfer balance of the QLEDs.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Quantum dot light-emitting diodes (QLEDs) use the electroluminescence (EL) behaviors of QDs, which can harness the unique properties of QDs, such as size-dependent emission wavelengths, high quantum yields (QYs), pure colors, and narrow linewidths [1–4]. Highly efficient QLEDs require appropriate device structure and high quality QD emitter. Previous research of organic/inorganic hybrid QLEDs have made great efforts on adjusting the energy level of the electron and hole transport layer, aiming to balance electrons and holes and hence high device efficiency. However, the relatively large energy level offset between QDs and hole transport layer usually impedes hole injection. It is also a great challenge to design organic HTMs with deep enough energy levels to match the low lying valance band of QDs [5–10]. Interestingly, Wan Ki Bae and Victor I. Klimov et.al proposed that an extra outer shell is used to control the charge balance, in which case an extra CdZnS shell has been grown over a CdSe/CdS core/shell heterostructure, improving the LED efficiency by one order of magnitude. Recent researches reveal that engineering on quantum dot structure is capable of adjusting the charge carrier delocalization in heterostructures and therefore matching the device energy level requirements by exchanging the surface ligand or introducing thick-shell [12,13]. These heterostructure nanocrystals also show unique properties due to their special the charge carrier delocalization properties. For instance, a thick shell of the QDs such as ‘Giant’ CdSe/CdS can suppress the inter-dot interaction in the QD emitter and preserve its superior emission properties [15,16].
Our previous work show inverted type-I Cd0.1Zn0.9S/CdSe core/shell nanocrystals and their application to QLEDs , but the photoluminescence quantum yield (QY) is relatively low and the charge transfer is still unbalanced. We herein intend to improve the PL QY further and balance the electron and hole transfer by coating shell material onto Cd0.1Zn0.9S/CdSe core/shell QDs. After introducing a CdS onto Cd0.1Zn0.9S/CdSe to form a core/multishell system, the surface defects of CdSe emission layer are reduced and the QD achieves a quantum yield of 76%, which is much higher than the Cd0.1Zn0.9S/CdSe of 61%. Meanwhile, we take a comparison between Cd0.1Zn0.9S/CdSe and Cd0.1Zn0.9S/CdSe/CdS QLEDs. The corresponding performance of device is improved obviously. Furthermore, we attempt to compared the performance of QLEDs based on Cd0.1Zn0.9S/CdSe580/CdS(3 ML) QDs with commonly used CdSe/CdS QDs. We find that the charge transfer rate of the Cd0.1Zn0.9S/CdSe/CdS core/multishell device is higher than its CdSe/CdS counterpart, leading to a better charge transfer balance and a higher EQE.
2.1 Material synthesis
The quantum dots were synthesized according to the methods previously reported with appropriate modifications [15,17]. For a typical synthesis of Cd0.1Zn0.9S (r = 2.3 nm) pure core QDs, 0.5 mmol of CdO, 5 mmol of Zn(Ac)2 and 4 mL of OA were placed in 50 mL flask and heated to 150 °C under the vacuum pump for 40 min, then 10 mL of 1-ODE was added to the flask and the temperature was heated to 300 °C under N2 flow. A stock solution containing 1.05 mmol of sufur (S) powder dissolved in 3 mL of 1-ODE was quickly injected into the flask at 300 °C and the reaction mixture was cooled to room temperature after 8 min. Subsequently, the core QDs were purified by appropriate hexane and excess ethanol, but it is mentioning that multiple steps of the solution are required to remove excess unreacted Zn. Finally, the obtained QDs resolved in 8 mL toluene preparation for next step. Preparation for Cd0.1Zn0.9S/CdSe/CdS QDs need to prepared stock precursor solutions 0.5 M cadmium oleate Cd(OA)2 (CdO resolved in OA and 1-ODE, the volume of ratio of OA and 1-ODE was 1:1); 1 M TBP-S (S powder resolved in Tributylphosphine/TBP). For Cd0.1Zn0.9S/CdSe (2.3/1.3 nm), 2 mL of Cd0.1Zn0.9S-toluene mixed with 10 mL of 1-ODE in a 3-neck flask and heated to 100 °C under vacuum pump for 30 min to exclude excess toluene. Then the temperature was heated up to 300 °C and a stock solution of 0.15 mmol of Se dissolved in 0.5 mL of TBP mixed with 0.5 mL of Cd(OA)2 were dropwise injected into the reaction mixture within 5 min. For the growth of CdS shell, after 10 min for epitaxial growth of CdSe layer, the mixture of 0.5 M Cd(OA)2 and 1 M TBP-S were injected at a rate of 8 mL/h into the reaction at 300 °C under N2 flow. After the injection completed, the reaction flask was kept 300 °C for 10 min and then cooled to room temperature. The synthesized QDs were purified and dissolved in n-octane or toluene for further characterization.
2.2 Device Fabrication
Electroluminescent quantum dot LEDs (QLEDs) were fabricated with pre-patterned indium tin oxide (ITO) glass substrates that were ultrasonic cleaned and then treated in an ultraviolet-ozone for 20 min. Subsequently, all the following deposition procedures were performed in nitrogen-filled glove box with controlled oxygen and water level below 1 ppm. PEDOT:PSS layer was spin-coated on the ITO at 3500 r.p.m for 50 followed by annealing at 150 °C first 15 min. Then, poly[(9,9-dioctylfluorenyl)-2,7diyl)-alt-(4,4’-(N-(4-butylphenyl)))] (TFB) dispersed in chlorobenzene were spin coated at 3000 r.p.m for 30 s, followed by baking at 150 °C for 30 min. The Cd0.1Zn0.9S/CdSe/CdS or CdSe/CdS QDs (QDs in n-octane, 20 mg/mL) and ZnO layer (ZnO in ethanol, 30 mg/mL) were sequentially spin-coated onto the TFB at 2000 r.p.m for 60 s and 4000 r.p.m for 40 s, respectively, followed by baking at 80 °C for 30 min. After that, Al anode electrode was deposited onto the top of ZnO under a high vacuum (5 × 10-6 Torr) with an evaporation rate of 0.1 nm/s.
A higher solution transmission electron microscope (HR-TEM, JEM-2010, JEOL Ltd.) X-ray diffractometer (XRD) (Rigaku MiniFlex II X-ray diffractometer) Absolute QYs of the QDs were measured by an absolute QY system (Quantaurus-QY C11347-11, Hamamatsu Photonics Co., Ltd.) photoluminescence (PL) spectrophotometer (Cary Eclipse, Varian) and UV-vis spectrophotometer (Cary 300, Varian) were employed to characterize the materials. The final concentration of each element was monitored by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Leeman Co., USA, PROFILE SPEC). An Ocean Optic UV–vis-NIR spectrophotometer was used to study the electroluminescence (EL) spectrum. The luminance and current efficiency were then calculated from the known portion of the forward emission and the LEDs output spectra. All the measurements were performed under ambient condition.
3. Results and Discussion
3.1 Structural characterizations and optical properties
In previous study, Cd0.1Zn0.9S/CdSe QLEDs exhibits excellent characteristics compared to CdSe/Cd0.1Zn0.9S QLEDs . There is no need to overcome the physical barrier from the outer shell material as compare to the type-I CdSe/Cd0.1Zn0.9S QDs or CdSe/CdS system, because the electrons and holes reside in the CdSe shell for the reverse type-I Cd0.1Zn0.9S/CdSe QDs, which benefits the direct injection of the charge carriers. However, the unbalanced charge injection issue still needs to resolve. In Cd0.1Zn0.9S/CdSe system, we found that the electrons and holes are all confined to the CdSe shell layer. Despite the charge carriers reside in the outer shell CdSe, which facilitates the charge injection, but the injection rate of electrons is about several orders higher than holes. In order to modifying the charge injection rates of the electrons and holes, we introduced a CdS outer shell with different thicknesses.
CdS shell has proved a robust material to adjust the energy band between the QD and electron transport layer to modifying the charge injection rate . In the present study, we designed Cd0.1Zn0.9S/CdSe/CdS core/multishell nanocrystals, where the intermediate CdSe emission layer and the outer CdS shell were grown via successive layer epitaxy technique (Fig. 1(a)). After the CdS shell coated onto the Cd0.1Zn0.9S/CdSe QDs, a red shift (50 nm) of the band-edge absorption and PL peak position are observed in Fig. 1(b) due to the delocalization or confinement of electron and hole . The corresponding TEM and XRD of Cd0.1Zn0.9S core (radius of r = 2.3 nm), Cd0.1Zn0.9S/CdSe with 1.3 nm CdSe layer and Cd0.1Zn0.9S/CdSe/CdS with 5.0 nm CdS outer shell are showed in Fig. 1(c)-(d). With the growth of CdS shell and the radius of QDs increasing, the Cd0.1Zn0.9S/CdSe/CdS QDs retain a nearly regular shape and the FWHM of 33 nm PL spectra shows the CdS shell coated onto the Cd0.1Zn0.9S/CdSe QDs primely. The characteristic peaks of Cd0.1Zn0.9S core locate between the CdS (JCPD NO. 65-3414) and zinc blend ZnS (JCPDS NO. 65-0309) phases, which confirms that Cd0.1Zn0.9S forms a ternary alloy compound rather than a mixture of CdS and ZnS . Equivalent calculation results reveal that the composition of the resulting core is expressed as Zn0.9Cd0.1S, which is consistent to XRD result. The XRD patterns of Cd0.1Zn0.9S/CdSe get close to the standard zinc-blende CdSe, implying the well coated of CdSe. After coating with CdS shell, the signal of the wurtzite structure of CdSe becomes weak and the diffraction peaks coincide with the standard diffraction patterns of CdS in wurtzite (bottom, blue lines), indicating the outer shell of the CdS [12,13].
3.2 Carrier transport properties
In order to explore electron and hole injection properties of these NCs, we fabricated and tested the electron- and hole-only devices. We set variant parameters in the form of Cd0.1Zn0.9S/CdSe(x nm)/CdS(X ML) for simplicity, that is, x represents the CdSe shell emission wavelength and X represents the number of CdS shell monolayer. The current densities of the electron-only devices based on Cd0.1Zn0.9S/CdSe(600-620)/CdS(X ML) QDs keep almost unchanged as compare to pure Cd0.1Zn0.9S/CdSe(600-620) QDs, but the current densities of the hole-only devices of the Cd0.1Zn0.9S/CdSe(600–620)/CdS are much higher than that of without CdS shell. Overall, the current density of electron-only device is also nearly 30 fords greater than and the hole-only device. Figure 2a and b show that the electron-only and hole-only devices tests based on the optimized Cd0.1Zn0.9S/CdSe580/CdS (3 ML) QDs and the Cd0.1Zn0.9S/CdSe QDs. The current densities of electron-only devices based on Cd0.1Zn0.9S/CdSe580/CdS(3 ML) QDs have a substantial reduction in current density, resulting in notable decrease of the electron and hole current density ratio. However, when the outer CdS shell exceeds three mononlayers, the PL QY of the QDs will decline greatly.
To gain a deeper insight into the role of CdS thick-shell coated on Cd0.1Zn0.9S/CdSe, we fabricated Cd0.1Zn0.9S/CdSe for comparison. Meanwhile, we also synthesized traditional CdSe/CdS to reveal the advantages of our Cd0.1Zn0.9S/CdSe/CdS QDs with thin CdSe and thick CdS shell. We have found that the electrons need overcome the large physical barrier of the CdS shell to reach the spatially indirect, but the holes just need to reach the CdS outer shell because there is no extra barriers as electrons need to overcome. Hence, we study the time resolved PL dynamics based on Cd0.1Zn0.9S/CdSe/CdS and CdSe/CdS QDs. As is shown in Fig. 3(a), (b) and (c), there are the PL decay curves of the QD film and QD/ZnO on glass substrates. It is worth noting that after interfacing with ZnO, the τave value of the QD film further decreased from 14.84 to 13.67 ns, 19.04 to 13.18 ns and 27.3 to 12.1 ns, which is corresponding to Cd0.1Zn0.9S/CdSe/CdS, Cd0.1Zn0.9S/CdSe and CdSe/CdS QDs respectively. In CdSe/CdS and Cd0.1Zn0.9S/CdSe system, such a notable decrease in PL lifetime suggests a fast and sever transport of the electrons from QDs to ETL. However, the insertion of thick CdS shell modifies the QD/ZnO interacial interaction, and slows down the decrease in PL lifetime greatly. Besides, electron-only and hole-only devices based on CdSe/CdS core/shell QDs, Cd0.1Zn0.9S/CdSe QDs and optimized Cd0.1Zn0.9S/CdSe580/CdS 3 core/multishell QDs were provided in Fig. 3(d). The structure of electron- and hole-only device are ITO/ZnO/QDs/Al and ITO/PEDOT:PSS/TFB/QDs/MoO3/Al, respectively. For the CdSe/CdS QDs, the current density of the electron-only device is over 80 times greater than the hole-only device. Similarly, the ratio of the current density (electro/hole) is 103:1 in Cd0.1Zn0.9S/CdSe system. In contrast, the ratio of the current density between the electron- and hole-only devices is only 4:1. Besides, the current densities of electron-only based on Cd0.1Zn0.9S/CdSe580/CdS 3 QDs is about 1/5 to that of CdSe/CdS, owing to the electron blocking behavior of the thick CdS outer shell. Such a great decline in electron/hole current ration implies that the CdS outer shell functions well in blocking excess electron injection, leading to a better charge transfer balance.
3.3 Device performances
To elucidate the advantages of Cd0.1Zn0.9S/CdSe580/CdS QDs, we fabricated the QLEDs base on CdSe/CdS QDs, Cd0.1Zn0.9S/CdSe and Cd0.1Zn0.9S/CdSe/CdS QDs. The structure and the corresponding cross-sectional TEM image of a typical QLED device are shown in Fig. 4. It is worth mentioning that the preparations of these quantum dot light-emitting diodes (QLEDs) are kept on consistent conditions without any modification on organic layers.
The electroluminescence (EL) and PL spectrum of QLED device based on Cd0.1Zn0.9S/CdSe/CdS, Cd0.1Zn0.9S/CdSe and CdSe/CdS QDs are shown in Fig. 5(a), (b) and (c), where the inset highlights the operated light-emitting device driven at the voltage of 5.0 V. The slightly EL spectra and the red-shifts of the peak position about 6, 5 and 14 nm to the initial solution PL spectrum (629, 627 and 626 nm), respectively, can be easily observed. Such obvious red-shifts are attributed to two factors. One is the combination of interdot interactions, especially closely packed quantum dot solids, another is the strong electric field, which acts to reduce the energy of exciton recombination through the Stark effect [8,20,21]. In general, EL spectra of conventional QD-LEDs are usually red-shifted with increasing voltage act as Fig. 5(f), which is ascribed to the so-called electric-field-induced Stack effect . But as shown in Fig. 5(d), EL spectra of Cd0.1Zn0.9S/CdSe/CdS QD-LEDs peaked at 635 nm show no red-shift and their bandwidth nearly remains unchanged, indicating the independence of emission wavelength from the applied voltage. This is maybe similar to the type-II structure QDs . Figure 6(a) show the current density and luminance as a function of driving voltage of the Cd0.1Zn0.9S/CdSe/CdS, Cd0.1Zn0.9S/CdSe and CdSe/CdS QLEDs. A relative lower turn-on voltage (31.46 cd/m2 at VT=2.4 V at) is achieved for Cd0.1Zn0.9S/CdSe/CdS than its CdSe/CdS counterpart (21.10 cd/m2 at VT=3.6 V at) and Cd0.1Zn0.9S/CdSe system (16.88 cd/m2 at VT=2.4 V). It is because of the CdSe/CdS QD-LEDs with lower injection rate of hole. In other word, exceeding carriers accumulate at the barrier interface during the device operation, not only acting as the non-radiative centers but also rising the driving voltage and limiting device lifetime [24,25]. The current densities of CdSe/CdS QLEDs are much higher than that of Cd0.1Zn0.9S/CdSe/CdS based devices about three times. This will be benefit for the device lifetime, due to the less unnecessary thermal power. The luminance of Cd0.1Zn0.9S/CdSe/CdS (21664 cd/m2) is also higher than its counterpart (11511 cd/m2). At the driving of 5.6 V, EQE of the Cd0.1Zn0.9S/CdSe/CdS based device shows a maximum value of 12.44% (Fig. 6(b)), which is higher than its counterpart (4.1%), owing to its facile charge carrier injection and the better balance of charge carrier transmission.Although the CdSe/CdS QDs have a higher PL QY (90%)than that of Cd0.1Zn0.9S/CdSe/CdS QDs, the device performance are much lower than Cd0.1Zn0.9S/CdSe/CdS QDs. Besides, the current density of Cd0.1Zn0.9S/CdSe/CdS device is 5 times lower than Cd0.1Zn0.9S/CdSe device, but the luminance is 1.5 times higher than its counterpart. These results agree well with electron- and hole- only device tests. Due to the lower current density but the higher luminance of the Cd0.1Zn0.9S/CdSe/CdS device, its EQE achieves 12.44%, which is 1.5-hold higher than that of Cd0.1Zn0.9S/CdSe device (8.23%). The current efficiency of the Cd0.1Zn0.9S/CdSe/CdS device is 5.5 Cd/A, which is 5.5-fold higher its counterpart. Our results suggest that the current efficiency depends much on the charge transfer balance. Some relevant QLEDs also exhibit remarkable improvements in CE by using insulating blocking layers to gain better charge transfer balance[11,26].
The present work proposes an idea that by tuning the energy level of the QDs to improve the balance of electron and hole in QLEDs and obtains highly stable QLEDs. Hence, we designed the Cd0.1Zn0.9S/CdSe/CdS nanocrystal QDs which better reduced the injection rate of electron and increased that of hole. A series of electron- or hole- only experiments and PL decay dynamics measurement reveal that Cd0.1Zn0.9S/CdSe/CdS QDs with 1.3 nm CdSe emission shell and 5.0 nm CdS outer shell can block the injection rate of electron compared with its Cd0.1Zn0.9S/CdSe counterpart and the well-developed CdSe/CdS core/shell QDs. Subsequently, three red QLEDs containing Cd0.1Zn0.9S/CdSe/CdS, CdSe/CdS and Cd0.1Zn0.9S/CdSe QDs have been identically fabricated for comparison which used a simple architecture. Obviously, with the CdS shell coated onto Cd0.1Zn0.9S/CdSe, its PL QY is achieved 76% compared to 61%, meanwhile the corresponding performance of device is improved greatly. In addition, although the PL QY of Cd0.1Zn0.9S/CdSe/CdS (76%) is lower than CdSe/CdS QDs (90%), the characteristic of energy level cause the luminance and EQE of the Cd0.1Zn0.9S/CdSe/CdS QLED is much higher than that of CdSe/CdS one. We believe that such concepts to improve the balance of electron and hole with energy level design strategies of QDs will open minds for other QLEDs further optimized in the future.
National Natural Science Foundation of China (NSFC) (11564026, 11774141); Natural Science Foundation of Jiangxi Province (20171BAB202036, 20171BCB23051, 20171BCB23052, 20181BAB201016); Graduate Innovation Special Fund Project of Nanchang Hangkong University (YC 2017037).
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