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Abstract
Quantum dots are a promising new candidate for use as emissive materials in the next generation of light-emitting diodes for lighting and display applications. One of the key issues in the solution preparation of inverted quantum dot light-emitting diodes (QDLEDs) is making a suitable sandwich structure of hydrophilic and hydrophobic layers. We solved this problem by inserting an ultrathin film of thermally evaporated MoO3 between a hydrophilic PEDOT:PSS layer and a hydrophobic PVK layer by controlling the delicate process. Inverted QD LEDs with an optimal MoO3 thickness of 5 nm exhibited a maximum current efficiency of nearly 4 cd A−1, a maximum EQE of 2.7682%, and a maximum luminance of 9317 cd m−2. Furthermore, the MoO3 interlayer extends the lifetime of the QDLED devices to approximately 300%.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Colloidal nanocrystal quantum dots (QDs) exhibit several unique properties as emitters, such as high photoluminescence (PL), high color purity, and high quantum yields (QYs). Furthermore, their emission wavelength can be easily tuned by controlling the core size of the QDs during synthesis. Much research has been done on QDs-based light-emitting diodes (QDLEDs) to increase the performance for commercialization [1,2].
QDLED designs can be classified as conventional and inverted structures. Recently, inverted-structured QDLED devices have been becoming more popular due to some advantages for future display applications. They feature an alignment of functional multi-layers sandwiched between a high-work-function top anode and a bottom cathode and exhibit higher luminous efficiency with better stability. In contrast to conventional-structure QDLED devices with a bottom anode, the transparent bottom cathode of inverted QDLEDs can be directly connected to n-channel thin film transistors (TFTs) backplane to reduce the driving voltage [3]. Direct connection to the drain line allows for the programming of the TFT gate-source voltage independently of the electrical characteristics of the QDLEDs.
Solution processing methods, such as spin-coating and ink-jet printing, are preferred for the fabrication of inverted QDLEDs because they are suitable for large-scale fabrication, have low cost, and are compatible with flexible devices. All-solution-processed inverted QDLEDs with a structure of ITO/ZnO/QDs/PVK/PEDOT:PSS/Al were demonstrated, but the device performance was unsatisfactory [4,5]. The most typical issue is that it is always difficult to deposit a hydrophilic PEDOT:PSS solution on a PVK layer, which is a hydrophobic material. Therefore, many have tried to improve the wetting properties of aqueous PEDOT:PSS solution by adding organic additives, such as isopropanol (IPA), fluorocarbon surfactant (FC-4430), dimethyl sulfoxide (DMSO), Triton X-100, and acetonitrile [5,6]. On the other hand, the organic additives generally decrease the work function of the PEDOT:PSS layer, leading to an undesirable increase in the energy barrier between the anode and the hole transport layer [7].
Therefore, inserting a very thin interlayer between the hydrophilic layer and the hydrophobic polymer hole transport layer has been considered. Among several transition metal oxides (TMOs), MoO3 was chosen as a p-type dopant because it is well known as a photochromic and electrochromic material with a thermodynamically stable orthorhombic phase. Moreover, the large work function of MoO3 (∼5.6 eV) is suitable for inverted QDLEDs for the alignment of energy levels. MoO3 has drawn attention in device engineering for its high transparency in the visible region of the spectrum, which is an essential requirement of light-emitting diodes. In addition, MoO3 is nontoxic and has moderate evaporation temperatures compared to other metal oxides [8].
A spin-coated MoO3 interlayer was inserted in regular-structure devices between the hole injection layer (HIL) and hole transport layer (HTL) to improve the hole transport and device performance for all-solution-processed QDLEDs [9]. In an inverted device, a 5-nm thickness of s-MoO3 was used in solution-processed inverted QDLEDs with a device configuration of ITO/ZnO/QDs/PVK/s-MoO3/PEDOT:PSS:s-MoO3 (3:1)/AgNWs and a composite anode [7]. However, most of the research has focused on solution processes to obtain MoO3 nanoparticles, with which it is a challenge to obtain uniform nanoparticles and to precisely control the extremely thin MoO3 interlayer by spin coating. Furthermore, the roughness of spin-coated film is a little high and leads to unsatisfactory device performance [7]. However, inserting a very thin MoO3 buffer layer by thermal evaporation in an inverted QDLED device with a sandwiched structure of ITO/ZnO/QDs/PVK/Evaporated-MoO3/PEDOT:PSS/Al are rare to find and could be an alternative approach.
To solve the issues of the hydrophilic and hydrophobic interface, we introduced thermally evaporated MoO3 thin film between PVK and PEDOT in an inverted QDLED for the first time. We precisely control the thickness of thermally evaporated MoO3 as a key way to improve the uniformity and performance of inverted QDLEDs. The operational lifetime was also dramatically improved by avoiding moisture and oxygen.
2. Experimental details
Pre-patterned indium tin oxide (ITO) glass substrates were prepared by a photolithography process. The patterned ITO glass was cleaned in an ultrasonic bath with IPA, acetone, and de-ionized water, and then treated in a UV-O3 chamber for 30 min. ZnO nanoparticles were synthesized by following the previous reports [10,11]. High-conductive PEDOT:PSS polymer (Heraus-CLEVIOS P VP.AI 4083), Poly (9-vinylcarbazole) (PVK, Sigma-Aldrich), Red CdSe/CdS/ZnS core/shell/shell colloidal QDs (oleic acid ligand in hexan, QDs from Global Zeus Co. Ltd., Korea), and molybdenum trioxide (99.99% MoO3, Sigma-Aldrich) were used as received without further purification.
The hybrid inverted QDLEDs have a device configuration of glass/ITO/ZnO NPs/red QDs/PVK/evaporated-MoO3/PEDOT:PSS/Al. A cross-sectional view is shown in Fig. 1(a). A dispersion solution of ZnO nanoparticles with a concentration of 20 mg mL−1 was spin-coated at 2500 rpm for 45 s as an electron transport layer (ETL) and thermally annealed on a hot plate at 110°C for 30 min. Red QDs dispersed in n-hexan with a concentration of 10 mg mL−1 were spin-coated at 4000 rpm for 30 s on top of the ZnO layer, and then dried at room temperature for 10 min.
Fig. 1. (a) Cross-sectional view of the inverted QDLED structure and (b) energy band diagram of the inverted QDLED.
Next, PVK (10 mg/ml in chlorobenzene) was spin-coated onto the QD layer at 3000 rpm for 60 s and baked with the same conditions as the ZnO layer. After drying, the MoO3 buffer layer was thermally evaporated at a rate of 0.3 $\dot{\rm A}$ sec−1 to achieve various thicknesses. The thermal MoO3 films were exposed to oxygen and air at 20 °C with a relativity humility of 40% for 90 min [12]. PEDOT:PSS was then spin-coated at 3000 rpm for 60 sec onto the MoO3 buffer layer and baked at 150°C for 30 min. Finally, an Al anode was thermally evaporated under high vacuum pressure of 2 × 10−6 Torr. To protect against humidity and oxygen, encapsulation was applied using aluminum tape (square of 15 × 15 mm2).
We measured the current density-voltage-luminance (J-V-L) properties, electroluminescence (EL) spectra, International Commission on Illumination (CIE) color chromaticity, and half-life using a Keithley 2400 source measurement unit and a SpectraScan PR 670 Spectroradiometer. The surface morphology was observed using an optical microscope (OM), NanoSystem in white light scattering interferometry (WSI) mode, a scanning electron microscopy (SEM, Hitachi S-4700), and an atomic force microscope (AFM, PAFM Nano Xpert II) in tapping mode.
3. Results and discussion
3.1 Good wettability on the interface
Figure 1(b) shows the energy band diagram alignment of the fabricated inverted QDLEDs. Unlike a conventional structure of QDLEDs, the electrons are injected from the ITO electrode as the cathode, and the holes are injected at the aluminum electrode as an anode. The electrons are transported through the ZnO ETL to the QD emissive materials layer (EML). ZnO nanoparticles were used as the ETL due to their high electron mobility and matching energy level with that of QDs. QDs dissolved in n-hexan can be spin-coated onto ZnO without destruction of the interlayer.
The holes are injected at the aluminum electrode through the PEDOT:PSS hole injection layer (HIL). They then pass through the MoO3 buffer layer and PVK hole transport layer (HTL). The energy band diagram showed that the highest occupied molecular orbital (HOMO) level is a reasonable value of 5.6 eV between the PEDOT:PSS and the PVK. This makes MoO3 a very good material for the transport of holes from the anode to the EML.
The characteristics of thermal evaporated MoO3 thin film were investigated (Fig. 2). Figure 2(a) shows a picture of pristine hydrophilic PEDOT:PSS dropped onto an evaporated-MoO3/PVK hydrophobic/ITO glass pattern. The aqueous PEDOT:PSS apparently only gathers on to the evaporated MoO3 area, but not on the PVK layer. Figure 2(b) shows an illustration of the wettability examination of the structure. Interestingly, almost pristine PEDOT:PSS will be flying out of the hydrophobic PVK surface after spin-coating. So we investigated mixed solvents to have better wettability of the PEDOT:PSS solution. The optical microscope (OM) images in Fig. 2(c) show poor uniformity across the surface with circular masks and trails of the spin-coated solution (PEDOT:PSS.IPA:acetonitrile) on the PVK pattern, which could be a consequence of disconnected granular phases and poor device performance [4]. In contrast, and surprisingly, when inserting the evaporated MoO3 thin film, the spin-coated pristine PEDOT:PSS showed excellent wetting and a very uniform and smooth surface, as shown on the right side of the borderline in Fig. 2(d).
Fig. 2. (a) A picture of pristine PEDOT:PSS solution dropped onto a MoO3 pattern, (b) structure of wettability examination, (c) OM image of spin-coated solution of (PEDOT:PSS.IPA:acetonitrile = 9:9:1 vol%) on PVK. (d) OM image; left side area: spin-coated PVK/ ITO glass, right side area: spin-coated PEDOT:PSS/8-nm MoO3/PVK/ITO. (e) Contact angle of a pristine PEDOT:PSS droplet on PVK. (f) Contact angle of a pristine PEDOT:PSS droplet on MoO3.
We also analyzed the contact angle for the effect of thermally evaporated MoO3 thin film on the wettability of the PEDOT:PSS interface. A pristine PEDOT:PSS droplet was dropped out from the tip of a syringe pump. The shape was photographed by a high-resolution camera in front of a light source. After that, the values of the various contact angles were analyzed by the ImageJ software package. Figures 2(e) and 2(f) show images of a drop of PEDOT:PSS on the PVK surface and MoO3 surface, respectively. The result shows a difference in the contact angle of PEDOT:PSS on the PVK layer of 86.445° (Fig. 2(e)) and the contact angle of PEDOT:PSS on the MoO3 layer of 37.16° (Fig. 2(f)). Dramatic improvement of the wettability was achieved by the MoO3 layer between the PEDOT and PVK, which completely solved the issue of hydrophilic and hydrophobic contact. Thus, pristine PEDOT:PSS can be easily coated on the surface of a MoO3/hole injection layer without any further solvent modification, which improves the interfacial contact quality and facilitates charge injection through the interfacial layer.
Interestingly, the evaporation of MoO3 was very delicate, and the physical characteristics of ultra-thin MoO3 films depended heavily on the conditions of the evaporation. During the evaporation process, MoO3 powder was slowly evaporated with a rate of 0.1 to 0.3 $\dot{\rm A}$.sec−1 to reach uniform thickness of 2 to 12 nm [13]. With this very slow rate, the MoO3 thin film is completely transparent. However, the color of the MoO3 film becomes pale yellow if the rate is higher than 1 $\dot{\rm A}$.sec−1. MoO3 powder was evaporated at temperatures between 500 to 550°C on a Mo boat. To obtain a very transparent film, the evaporation was done in a high-vacuum (pressure less than 8 × 10−7 Torr).
When depositing MoO3 by thermal evaporation on organic films, MoO3 can diffuse into the underlying layer, exhibiting a trend of increasing diffusion with decreasing molecular molar mass [14]. The diagrams shown in Fig. 3(a) illustrate the effect of MoO3 deposition on the physical interface formation. Right after the deposition MoO3 onto PVK organic materials started, a small part of the evaporated MoO3 film was absorbed. Heavy MoO3 clusters have enough kinetic energy to penetrate into the underlying PVK layer and diffuse through the low density and amorphous structure of the PVK film. In contrast, with PEDOT:PSS film deposited on the MoO3 layer, substantial diffusion does not take place at the interface due to a higher density and stable metal or metal oxide of the layer underneath.
Fig. 3. (a) Illustration of absorbed MoO3 layer on a PVK organic material and (b) OM image of PEDOT:PSS/1.5-nm nm MoO3/PVK and possible image (inset).
The evaporated ultrathin MoO3 film was transparent. Thus, it was a challenge to observe the diffusion of MoO3 on the PVK organic polymer. However, when the thickness of the evaporated MoO3 thin film was less than 2 nm, some holes were detected on the surface of the PEDOT:PSS/1.5-nm evaporated-MoO3/PVK/ITO-glass pattern (Fig. 3(b)). The OM image clearly shows two areas: holes and a base with different colors. The base is the PEDOT:PSS layer with brown color, and the yellow holes are the exposed PVK layer. The inset in Fig. 3(b) shows an image of the formation of the holes. Firstly, the absorption of MoO3 on PVK revealed the hydrophobic area. Then after dropping and spin-coating, PEDOT:PSS flew out at hydrophobic/ hydrophilic contacts and left the holes.
The energy level alignment was also considered for fabricating inverted QDLEDs. The high work function (WF) of MoO3 is a crucial factor in the performance enhancement of QDLED devices. However, numerous values of surface WFs have been reported, such as 5.3 eV, 5.68 eV, 6.86 eV, and so on [15–17]. One interesting thing is that the energy level of thermally evaporated MoO3 film will be gradually decreased from high WFs (9.7 and 6.7 eV) to lower WFs (5.3 and 2.3 eV), which depends on the air and oxygen exposure effects [12,18]. Therefore, to have a suitable WF of MoO3 between the PEDOT:PSS and PVK energy levels, the evaporated MoO3 film needs 90 min of oxygen and air exposure in ambient air after thermal evaporation.
3.2 Surface morphology
To observe morphological surface of evaporated MoO3 thin film, 3D images were obtained and analyzed. To have a 2 × 2 mm2 active area in inverted QDLED devices, the evaporated MoO3 thin film was required to smoothly cover the whole area at a large scale. Therefore, we scanned MoO3 film with more than 500 × 500 µm2 scales of the X- and Y-axis, and at the nanoscale on the Z-axis. Figure 4(a) shows a 3D image of evaporated MoO3 on ITO glass with a thickness of 9 nm (magnification of ×100). At a scale of 124 × 93 µm2, the average thickness difference was around 1 nm to 2 nm and was observed by the changing color (color bar scale) of the surface. The average roughness Ra was 0.5 nm. Figure 4(b) shows a 3D image of 3-nm MoO3 deposited on a Si wafer with a magnification of ×10 at a scale of 619 × 463 µm2. The image displays an edge with two smooth areas of evaporated MoO3 and the wafer base. We can precisely measure the MoO3 thickness to confirm the deposition thickness from the thermal evaporator. The roughness of MoO3 on wafer were 0.42, 0.51, and 0.54 nm for 5, 8 and 12 nm of MoO3 thin films, respectively.
Fig. 4. (a) NanoSystem WSI 3D image of 9 nm thickness of evaporated MoO3/ ITO glass and (b) 3-nm thickness of evaporated MoO3/ SiO2 wafer. (c) 3D AFM images of MoO3/SiO2 wafer. (d) 3D AFM images of PEDOT:PSS/8-nm MoO3 /PVK/ITO-glass.
Figures 4(c) and 4(d) show the 3D AFM surface topographies of the MoO3/SiO2 wafer and PEDOT:PSS/5nm-MoO3/PVK/ITO glass pattern. In Fig. 4(c), the AFM height image reveals that the MoO3 /SiO2 wafer substrate is very smooth with an average roughness Ra of 0.42 nm at a scan scale of 5 × 5 µm2. This value is similar to the value from the NanoSytem WSI analysis. Figure 4(d) presents the AFM images of PEDOT:PSS/5nm-MoO3/PVK/ITO glass. The film surface became a little rough with an average roughness Ra of 0.71 nm. Some sharp peaks and small pin-holes were also observed on the PEDOT:PSS surface. The average surface roughness of PEDOT: PSS on 8 nm and 12 nm of MoO3 thin films are 0.82 and 0.85 nm, respectively. However, the surface of the PEDOT:PSS thin film deposited on the evaporated MoO3 interlayer was much smoother than that of previously spin-coated MoO3 [9].
Figure 5(a) presents a top-view SEM image of thermally evaporated MoO3 on a wafer with a magnification of × 50k. Figure 5(b) shows a cross-sectional SEM image of the PEDOT:PSS/8-nm-MoO3/PVK/ITO-glass configuration. The MoO3 film has good surface morphologies as an interfacial layer for inverted QDLEDs. The smooth surface morphology of the PEDOT:PSS/MoO3 film provides more effective hole injection paths and hence accelerates hole injection to the PVK HTL.
Fig. 5. (a) Top-view SEM image of evaporated-MoO3 on SiO2 wafer and (b) cross-sectional-view SEM image of a pristine PEDOT:PSS/8-nm MoO3/PVK/ITO-glass configuration.
3.3 Device performance
To investigate the effects of an evaporated MoO3 buffer layer on the device performance, we fabricated red-emitting inverted QDLEDs with two different types of structures: Type 1: ITO/ZnO/QDs/PVK/×-nm MoO3/pristine PEDOT:PSS/Al (× = 2, 3, 5, 8, 12 nm corresponding to devices 1 to 5) and Type 2: ITO/ZnO/QDs/PVK/PEDOT:PSS:IPA:acetonitrile (9:9:1)/Al (device 6), as shown in Fig. 6(a) and Table 1. In general, direct coating of pristine PEDOT:PSS on a PVK surface is not possible due to the different surface properties. Therefore, this wetting issue was solved by adding various solvents in previous work [6]. Various solvent compositions were chosen to fabricate all-solution inverted QDLEDs for device 6, and we obtained a luminescence of 3115 cd m−2.
Fig. 6. (a) Luminance-voltage characteristics of inverted QDLEDs with and without a MoO3 interlayer. The inset picture is an illuminated red inverted QDLED device. (b) Operational lifetime of devices 3 and 6 at initial luminance of 200 cd.m−2. The inset graph is CIE 1931 color chromaticity.
Table 1. Device characteristics of QDLED with and without MoO3 inter layer.
However, after inserting the MoO3 interlayer, the highest luminance was 9317 cd m−2 at 10.5 V for device 3 with a 5-nm thickness of MoO3. Furthermore, 2050 cd m−2, 5420 cd m−2, 7021 cd m−2, 5672 cd m−2 were obtained for 2 nm, 3 nm, 8 nm, and 12 nm of MoO3, respectively. Among the six devices, the device with a 2-nm thickness of MoO3 exhibited the lowest luminance. This happened because the absorbed MoO3 can form isolated holes on the surface of PVK, as discussed above and shown in Fig. 3(b). The deficient PEDOT:PSS on this layer leads to lower density of injected holes. This is also the reason for the reduction of device performance.
The turn-on voltages (Von) for devices 1 and 5 gradually increased from 2.5 V to 4.5 V with inserted MoO3 film thicknesses of 2 to 12 nm. This linear relationship between the thickness of MoO3 and Von suggests that the observed increase in Von is a consequence of the enhancement of the built-in potential generated between Al/PEDOT:PSS/MoO3 and ITO. The maximum luminance goes up with increasing thickness of the MoO3 buffer layer from 2 to 5 nm and saturates. It then slows down at thicknesses of around 5 to 12 nm. This is also due to the delayed hole transfer in the thicker interlayer, resulting in reduce recombination of holes and electrons at the emitting layer. The property of device 3 with the optimum MoO3 thickness of 5 nm shows a maximum EQE of 2.7682%, and maximum current efficiency of nearly 4 cd A−1, which is much higher than that of device 2 with a 3 nm-thick MoO3 layer.
The inset picture in Fig. 6(a) shows an illuminating inverted QDLED device at 4.5 V. The EL peak was 627 nm with a full width at half maximun around 45 nm, which was shifted from the PL peak to a longer wavelength due to the Stark effect (not shown). The inset graph in Fig. 6(b) shows the CIE 1931 color chromaticity, which revealed coordinate of (0.6977, 0.3021) with a saturated deep red color.
3.4 Lifetime
QDLEDs with an organic-inorganic multi-layer structure are sensitive to moisture and oxygen. In ambient air, exposure to moisture causes severe oxidation of the electrodes and organic semiconductor, resulting in fast degradation of the quantum efficiency and the life of the QDLEDs. As a metal oxide, the MoO3 thin film can prevent the penetration of moisture well. Thus, the deposition of MoO3 on the PVK layer can form a metal/organic interface and can be expected to improve the efficiency and lifetime of electronic devices [19]. More recently, multiayered hole transport struture including MoO3 was reported to improve life time [20].
The operational lifetime of the inverted QDLEDs was observed at the initial luminance of 200 cd m−2, as shown in Fig. 6(b). The half-life was predicted by data fitting to the stretched exponential function L(t) with the shape parameter β=1;
where Lo is the initial luminance, t is the operating time, and τ is the decay time [21,22]. The half-life of device 3 is expected to be about 500 min, which is almost 3 times longer than that of device 6. The lifetime enhancement of device 3 proves the critical role of the buffer layer. The dyad structure of PVK/ MoO3/PEDOT:PSS is the key to avoiding moisture and oxygen. Therefore, we conclude that inserting an ultrathin MoO3 interlayer in the inverted QDLEDs is a decent method to ameliorate the efficiency, performance, and lifetime of the devices.4. Conclusions
In summary, an inverted structure of QDLEDs was fabricated with MoO3 buffer layer between hydrophilic PEDOT:PSS and hydrophobic PVK. The devices with thermally evaporated 5-nm of MoO3 exhibited the best performance with a low turn-on voltage of 3 V, maximum current efficiency of nearly 4 cd A−1, maximum EQE of 2.7682%, and maximum luminance of 9317cd m−2. The half-life of devices with ultrathin MoO3 was 3 times longer than that of the device without an interlayer. These results indicate that the evaporated MoO3 interlayer is mainly responsible for the smoother surface morphology of PEDOT:PSS/MoO3/PVK and the enhanced performance and operational lifetime of QDLEDs.
Acknowledgment
This research was supported by the Academic Research Fund of Hoseo University in 2018 (20180363).
Disclosures
The authors declare no competing interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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