Simultaneous Improvement of Efficiency and Lifetime of Quantum Dot

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Surfaces, Interfaces, and Applications

Simultaneous Improvement of Efficiency and Lifetime of Quantum Dot Light-Emitting Diodes with a Bilayer Hole InjectionLayer Consisting of PEDOT:PSS and Solution-Processed WO3 Ling Chen, Shujie Wang, Dongdong Li, Yan Fang, Huaibin Shen, Lin Song Li, and Zu-liang Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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ACS Applied Materials & Interfaces

Simultaneous Improvement of Efficiency and Lifetime of Quantum Dot Light-Emitting Diodes with a Bilayer Hole Injection-Layer Consisting of PEDOT:PSS and Solution-Processed WO3 Ling Chen, Shujie Wang, Dongdong Li, Yan Fang, Huaibin Shen*, Linsong Li and Zuliang Du* Key Laboratory for Special Functional Materials, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, PR China KEYWORDS: inorganic/organic bilayer hole injection layers, tungsten oxide, quantum dot lightemitting diodes, solution-processed, balance charge

ABSTRACT

Even though chemical stability metal oxides (MOs), as the substitutes for PEDOT:PSS, have been successfully adopted for improving device stability in solution-processed quantum dot light-emitting diodes (QLEDs), the efficiencies of QLEDs are at a relatively low level. In this work, a novel architecture of QLEDs has been introduced in which inorganic/organic bilayer hole injection layers (HILs) were delicately designed by inserting an amorphous WO3 interlayer between PEDOT:PSS and the ITO anode. As a result, the efficiency and operational lifetime of

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QLEDs were improved simultaneously. The results show that the novel architecture QLEDs relative to conventional PEDOT:PSS-based QLEDs have an enhanced external quantum efficiency (EQE) by a factor of 50%, increasing from 8.31% to 12.47%, meanwhile exhibit the relative long operational lifetime ( 12551 hours ) and high maximum brightness (>40 000 cd m-2 ), resulting from a better pathway for hole injection with staircase energy level alignment of the HILs and reduction of surface roughness. Our results demonstrate that the novel architecture QLEDs by using bilayer MO/PEDOT:PSS HILs can achieve long operational lifetime without sacrificing efficiency.

INTRODUCTION

Colloidal quantum dots light-emitting diodes (QLEDs) have received significant attention due to its excellent properties, such as, tunable emission wavelength covering the entire visible region, narrow linewidth, wide gamut and low-cost solution processability, since was first reported by A. P. Alivisatos in 1994.1-4 In the last few years, the performances of QLEDs have been achieved great improvement through improving quantum yields (QYs) of QDs, adjusting the devices architectures, optimizing carrier transport layer (CTL) materials, promoting and balancing the carriers injection efficiency, and so on. Latest results show that the efficiency and brightness of QLEDs can compared with those of the best phosphorescent organic LEDs (OLEDs). This makes QLEDs have great potential applications in next-generation solid-state lighting and displays.5-11 So far high-efficiency QLEDs generally adopt organic − inorganic composite multi-layer structures, in which the CTLs consist of the organic polymers as hole injection layer (HIL) and hole transport layer (HTL) and inorganic metal oxide (MO) ZnO nanoparticles (NPs) as the electron transport layer (ETL), exhibiting outstanding device


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performance.5,7,12-16

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ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was also frequently used as a HIL (or hole extraction layer (HEL)) in QLED to improve device efficiency.17-23 However, in this case the carrier mobility of ZnO ETLs can reach the order of 10-3 cm2V-1s-1 far exceeding the hole mobility of most organic HTLs.5, 7, 23 As a result, the limited hole transport efficiency is one of the large obstacle to further improve QLED performance. On the other hand, the inherent acidity of PEDOT:PSS can damage indium tin oxide (ITO) anodes by dissolving indium species and the hygroscopic nature also accelerates the diffusion of indium which strongly affect the stability of the device. 24-26

In order to address these problems, inorganic MOs (NiO27-30, MoO331-33, WOx34-36, and V2O53739

) as alternatives of PEDOT:PSS have been successfully adopted in QLEDs due to their high

work function, chemical stability and good carrier transfer capability. For instance, Caruge et al. used NiO as the HTL through RF sputtering technique, but the EQE (~0.18%) is quite low.27,28 Meanwhile, these deposited MOs using RF sputtering techniques or vacuum thermal evaporation cannot be matched to the inexpensive solution-processing techniques. Considering low-cost industrial production, therefore, there is a significant demand for development of MOs prepared by solution-processed methods. From this point of view, Tang et al. used solution-processed MoO3 thin film as HIL and their operation lifetime showed 9691 h.33 Sun et al. have reported the green QLED showing the ηEQE of 3.32% and the high brightness of 30 006 cd m-2, with significantly improved operating lifetime, in which solution-processed WO3 NPs at low annealing temperatures were used as the HILs to replace PEDOT:PSS.34 They also demonstrated all-solution-processed QLED with the use of inorganic WO3 and ZnO NPs as the HTLs and ETLs, respectively. The resulting green QLED showed a ηA (current efficiency) of 4.4 cd/A and


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an operation lifetime of 95 h.35 Very recently, to further improve the performance of MO HILbased QLED, by using two-dimensional WOn-WX2 heterostructures as the HIL, high efficient green QLEDs with the ηEQE of 8.53% has been also fabricated by Sun et al.36 This ηEQE reported here is the maximum value of the green MO HIL-based QLED, which can be close to that of PEDOT:PSS-based QLED. In 2017, Zhang et al. demonstrated that using a solution method, QLED with V2O5 HIL significantly improved the lifetime and efficiency of the device, exhibiting a peak EQE of 7.25%.38 Our recent work also showed that the lifetime of QLEDs applied NiO thin film as HIL could be increased 6 times than that of conventional PEDOT:PSS.29 Although the lifetime of the device has been improved obviously with solutionprocessed inorganic MOs, the efficiency of the device is still at a rather low level (EQE 40 000 cd m-2). The proposed device architecture show that using MO/PEDOT:PSS inorganic/organic bilayer HIL in QLED is an uncomplicated-experimental


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procedure, reliable and low-cost avenue to achieve long operational lifetime without sacrificing efficiency. 

EXPERIMENTAL SECTION Synthesis of WO3, ZnO and CdSe/CdS/ZnS core–shell nanoparticles (NPs). Tungsten

oxide precursor were prepared by dissolving tungsten powder (Aladdin, 99.99%) in hydrogen peroxide (Aladdin, 30%-35.5%) according the sol-gel method reported previously.40,41 The tungsten powder (0.5g-0.9g) was slowly dissolved in 10 ml hydrogen peroxide with stirring under a constant rate at room temperature. Then, the solution stirred for 24 h with 150 ul of IPA added as stabilizer. Before use, the prepared solution was diluted with IPA to control the WO3 concentration. The ZnO NPs were prepared using previously reported solution precipitation method.5 The as-synthesized ZnO NPs was washed twice with heptane and the particles were finally dispersed in ethanol for our experiment (approx. 30 mg mL-1). Details of the synthesis of CdSe/CdS/ZnS core/shell QDs could be found in our previous works42 and the details of QDs were provided in the Figure S1. Fabrication of QLED Devices. The ITO-coated glass substrates (sheet resistance ~ 15 Ω/square) were washed in ultrasonic bath of detergent, de-ionized water, acetone, and IPA for 15 minutes in sequence, then treated with ultraviolet ozone for 15 minutes. Immediately after the plasma treatment, PEDOT:PSS (Heraeus, Clevios™ P VP.AI 4083) solution was spin-coated on the ITO substrates at 4000 rpm for 60 s and thermally annealed at 130 ℃ for 15 min (for Type A and D QLEDs), or various concentrations of the prepared precursor were spin-coated at 3000 rpm for 30 s on ITO glass in ambient air and heated at 120 ℃ for 10 min, and the substrate with WO3 HIL was treated with UV-ozone for 10 min (for Type B, Type C and Type E). In the case of the Type B and Type E device, the PEDOT:PSS solution was spin-coated on the WO3 layer


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under the same conditions, and the sample was heated at 120 ℃ for 15 min. While, for Type D and Type E, the another WO3 layer was also spin-coating under the same conditions. Subsequently, for all samples, TFB (ADS 254BE) dissolved in chlorobenzene with a concentration of 8 mg mL-1 was deposited, followed by a bake-out at 150 ℃ for 30 min in the nitrogen glovebox. CdSe/CdS/ZnS QDs were dispersed in toluene at 15mg/mL and then spincoated at 2000 rpm for 60s. After that, the ZnO NPs as the ETL was spin-coated at 2000 rpm for 45s and baked at 60 ℃ for 30 min. In the end, the substrates were loaded into a vacuum chamber (~1 × 10−6 mbar) to thermally deposit the 100 nm-thick Al cathodes. The active areas of QLEDs were 0.04cm2. Characterization. The UV-Vis absorption and photoluminescence (PL) spectra of the QDs solutions were measured by using an UV-vis spectrometer (Lambda 950, PerkinElmer, USA) and a spectrofluorometer (JY HORIBA FluoroLog-3), respectively. Transmission Electron Microscope (TEM) image of the QDs was obtained via a JEOL JEM-2100 electron microscope. Field emission-SEM (Nova Nano SEM 450) and AFM (Dimension Icon) were used to obtain the surface topography images. X-ray diffraction (XRD) patterns were taken with a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ=0.15418 nm). X-ray photoelectron spectroscopy (XPS) measurement was performed with monochromatized Al K X-ray photons (hv = 1486.6 eV) using an AXIS ULTRA System (Kratos Inc.). Ultraviolet photoemission spectroscopy (UPS) spectra were collected on a Thermo Scientific ESCALAB 250 XI equipment with a He I discharge lamp (hv=21.22eV) at a base pressure of about 2.5×10-8 mbar. The current density-voltage-luminance (J-V-L) characteristics of the QLEDs were measured using a characterization system comprising a Keithley 2400 voltmeter together with a PhotoResearch SpectraScan PR-735 spectrometer under ambient conditions.


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RESULTS AND DISCUSSION In this work, the film thickness of WO3 layer was controlled by varying the concentration of the precursor solution during spin-coating process. The composition of WO3 films were recorded by X-ray photoelectron spectroscopy (XPS) spectra. The XPS spectrum of W4f core level shown in Figure 1(a) indicates that the decomposition of the W4f photoemission peak is carried out with a sharp doublet peak at around 35.5 eV. And a broad peak of 5p3/2 is approximately at 42.0 eV. These peak positions and shapes have been assigned to the WO3 compound in W6+.43,44 The corresponding O 1s peak is found at 530.1 eV. According to the integrated XPS signal intensity, the atomic ratio of W vs. O is 1:3, which matches the stoichiometric composition of WO3 well. The crystal structures of sol-gel WO3 thin layers annealed at 120 ℃ and 300 ℃ were characterised via XRD, as illustrated in Figure 1(b). XRD patterns show that the crystallinity of WO3 film is increased with the increased annealing temperature.The broad peak centered at 26° indicates the typical amorphous structure of the WO3 film (annealed at 120℃). While the most intensive diffraction peaks of the WO3 layer annealed at 300℃ match with the typical hexagonal WO3 (JCPDS no. 33-1387). The absorption spectrum in Figure 1(c) shows that the WO3 thin film maintains a very high transmission over the entire visible spectrum. And the high transmittance of the WO3 film as HIL is a key requirement for our thin film QLED designs, as we will discuss later in this article. In addition, the band gap energy Eg of WO3 layer can be determined by the following relation45: αhν = (hν - Eg)m, where m = ½ for direct bandgap semiconductors. The linear section of the curve (in Figure 1(c), inset) is extrapolated to the zero absorption coefficient to yield an estimated bandgap of Eg = 3.52 eV. In order to investigate the performance of a QLED device with solution-processed WO3 film serving as the HIL, two different types of QLED devices were prepared: ITO/PEDOT:PSS/


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TFB/QDs/ZnO/Al (Type A) and ITO/WO3/TFB/QDs/ZnO/Al (Type B). Here WO3 film is used as a direct replacement for PEDOT:PSS, a hole-injection material commonly used in optoelectronic devices. Meanwhile, TFB and ZnO NPs are chosen as the HTL and ETL, respectively. The red CdSe/CdS/ZnS core/shell QDs is used as emitter for the fabrication of QLED and the details of QDs were provided in the Figure S1. The device performance was then verified by measuring the luminance, current density and current efficiency alongside the voltage in the Type A and Type B QLEDs, as plotted in Figure S2. From the double logarithmic current density-voltage (J-V) characteristic curves shown in the inset of Figure S2(a), in the low voltage range, i.e. Ohmic region, the leakage current of the Type B QLED is about ten times higher than that of Type A. This means that Type B device has more phenomena of micro electrical shorts.46 And this relatively large leakage current (in Type B device) is probably because of the insufficient coverage of the WO3 layer on ITO as a HIL, or owing to of the nonideal adhesion of the TFB layer formed on top of the WO3 HIL.47,48 Furthermore, based on the luminance-voltage (L-V) and current efficiency–luminance (ηC-L) measurements shown in Figure S2, Type B device with the WO3 HIL shows much worse performance (Lmax = 15160 cd m-2, ηC

-1

max

= 1.40 cd A ) compared with PEDOT:PSS-based

device. Considering the previous investigation,47 there was a relatively larger (hole) barrier, i.e. EF-HOMO edge, at the interfaces of metal oxide/TFB, compared to PEDOT:PSS/TFB. Therefore, to understand the reasons of poor performances of the WO3-based QLED, steady state PL spectra of TFB, WO3/TFB, WO3/PEDOT:PSS/TFB, and PEDOT:PSS/TFB on glass substrates were investigated. As illustrated in the Figure S3(a), under the same measurement conditions, the magnitude of PL response for TFB layer is reduced after introducing the PEDOT:PSS, WO3 or WO3/PEDOT:PSS bilayer. These PL quenching is believed to be mainly attributed to the charge


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transfer from TFB to PEDOT:PSS or WO3. Note that WO3 shows a higher quenching efficiency, which can be ascribed to a relatively larger band offset, i.e., a larger (hole) barrier at WO3/TFB interface. Here, the PL quenching efficiency of the PEDOT:PSS and/or WO3 followed the same orders when TCTA was taken the place of TFB, as shown in Figure S3(b). So, perhaps, that explains why the Type B device shows the less-than-ideal performance by virtue of the “big” leakage current and/or the larger band offset at WO3/TFB interface. According to the above results, and to combine the advantages of a WO3 HIL layer and a PEDOT:PSS layer, the solution-processed QLEDs were fabricated with another type of HILs, as depicted in Figure 2: Type A: ITO(135 nm)/PEDOT:PSS (35 nm)/TFB(40 nm)/QDs(30 nm)/ZnO(40 nm)/Al(100 nm); Type C: ITO(135 nm)/WO3(

Simultaneous Improvement of Efficiency and Lifetime of Quantum Dot

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