Enhanced Efficiency of InP-Based Red Quantum Dot Light-Emitting

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Functional Inorganic Materials and Devices

Enhanced Efficiency of InP-Based Red Quantum Dot Light-Emitting Diodes Dong Li, Boris Kristal, Yunjun Wang, Jingwen Feng, Zhigao Lu, Gang Yu, Zhuo Chen, Yanzhao Li, Xinguo Li, and Xiaoguang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07437 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Applied Materials & Interfaces

Enhanced Efficiency of InP-Based Red Quantum Dot Light-Emitting Diodes 1‡Dong

Li, 1‡Boris Kristal, 2Yunjun Wang, 1Jingwen Feng, 1Zhigao Lu, 1Gang Yu, 1Zhuo Chen*, 1Yanzhao

Li, 1, 3Xinguo Li, 1Xiaoguang Xu*

1 BOE Technology Group Co., Ltd., Beijing, 100176, P. R. China 2 Suzhou Xingshuo Nanotech Co., Ltd. (Mesolight), Suzhou, 215123, P. R. China 3 School of Software & Microelectronics, Peking University, Beijing, 102600, P. R. China ‡These authors contributed equally Email: [email protected]; [email protected]; [email protected]

Keywords: Cd-free quantum dots, QLED, Mg-doped ZnO, PLQY, current efficiency Abstract: Due to the inherent toxicity of cadmium selenide (CdSe)-based quantum dots (QDs), Cd-free alternatives are being widely investigated. Indium phosphide (InP) QDs have shown a great potential as a replacement for CdSe QDs in display applications. However the performances of InP based QLEDs is still far behind that of the CdSe-based devices. In this study, we wanted to show the effects of different approaches to improving the performance of InP based QLED devices. We investigated the effect of magnesium (Mg) doping in ZnO nanoparticles, which is used as n-type electron transport layer (ETL), in balancing the charge transfer in InP-based QLED devices. We found that increasing Mg doping level can broaden ZnO band gap, shift its energy

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levels, but most importantly, increase its resistivity; as a result, the electron current density is significantly reduced and the device efficiency is improved. We also investigated the effect of high PLQY emitters and different QLED architecture on the device performance. Through optimizing QD structures and devices, red InP QLEDs with the current efficiencies as high as 11.6 cd/A were fabricated. INTRODUCTION Colloidal quantum dot based light-emitting didoes (QLEDs) have earned much attention as the next generation emitter to be used in displays and solid-state lighting due to their highly desirable properties, such as tunable color spectra, very narrow full-width half-maximum (FWHM, < 30 nm) and cost-effective solution based processing1-3. These unique optical and electrical properties of colloidal quantum dots (QDs) are determined by their particle size (2~10nm), resulting into electronic structure fundamentally different from bulk semiconductor materials due to the quantum confinement effect. Since the first emergence of QDs in 1994, the great progress has been made by many research groups to enhance performance of light-emitting devices.4-9 However, most of the QLED devices in recent years have been fabricated with cadmium (Cd) based II-VI semiconductor QDs. But in recent years the inherent toxicity of cadmium compounds became a big concern, making it critical to search for alternative Cd-free materials. Among possible core materials of Cd-free QDs, indium phosphide (InP) is an excellent candidate due to its wide spectrum range and well-matched energy levels.10-12 Achieving high and stable quantum yield (QY) in InP-based QDs can be accomplished by controlling the confinement of electron and hole wave functions under the electric field and the charged conditions in the device. Unfortunately, both efficiency and lifetime of InP-based QLED devices proved to be much lower compared to Cd-based QLEDs with similar device architecture.


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Over the past several years, many researches have focused on improvement of electroluminescent devices utilizing InP-based QDs. Wedel and co-workers have reported efficiency enhancement of indium phosphide (InP) QLEDs using polyethylenimine (PEI) surface modifier. They demonstrated a dramatic increase in current efficiency from 0.07 cd/A to 3.17 cd/A.13 Kim and co-workers investigated the optoelectronic characteristics of QLEDs by tuning ZnS shell thickness of InP/ZnSe/ZnS multishell QDs. Current efficiency of the QLEDs fabricated using QDs with thicker ZnS shell showed enhancement up to 4.65 cd/A.14 The most recent developments in efficiency improvement of InP QDs were mostly focused not only on modifying their core/shell structure, but also on the synthetic concepts utilized in growing those QDs. Yang et al. reported a synthesis of ~ 15 nm-size InP/ZnSe/ZnS RQDs with thick ZnS outer shell implementing a layerby-layer shell growth strategy, achieving 73% PLQY. They fabricating QLED devices utilizing these QDs with maximum current efficiency of 13.5 cd/A.15 Peng et al. also showed that different synthetic approach can be very effective, introducing a concept of stoichiometry-controlled synthesis of both core (III-V) and shell (II-VI) in InP/ZnSe/ZnS QDs. This led to fabrication of RQDs with >90% PLQY, resulting into QLED devices with maximum current efficiency of 14.7 cd/A. 16 The last two are some of the more promising results for Cd-free QLEDs reported thus far. In this study, we wanted to systematically study different effects, such as charge balance, QD PLQY, device structure and fabrication conditions, on the QLED device performance. We fabricated red QLED devices with emitting layer formed by Cd-free InP QDs and different electron transport layers utilizing all-solution process. The photoluminescence quantum yield (PLQY) of QD films deposited from the solutions of different concentration had been investigated. Further the effect of different electron transporting layers on electron injection was studied to optimize the


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electron-hole balance and realize high efficiency in QLED devices. We also investigated the effects of QD emitter PLQY and device architecture on the QLED device performance. RESULTS AND DISCUSSIONS Device structure and QD layer optimization A schematic device structure and an energy band diagram of the Cd-free QLEDs are shown in Figure 1. Energy levels of the materials were either provided by manufacturers (PEDOT:PSS and TFB) or measured by us directly, utilizing Riken-Keiki AC-2 photoelectron spectrometer (ITO, Al) or UPS spectrometer (InP RQD, ZnO, ZnMgO (a), ZnMgO (b)) to measure HOMO levels and UV-Vis absorption spectrometer to measure LUMO levels. We adopted a bottom-emission and an organic-inorganic hybrid device architecture for Cd-free QLEDs, where a transparent ITO acts as the anode and InP-based red QDs as the emitting layer, as shown in Figure 1a. In the device, the PEDOT:PSS and TFB are used as hole injection and transport layers respectively, and Zn1-xMgxO nanoparticles with different dopant ratio (x = 0, 0.05 and 0.15) are used as ETL. The holes and electrons are injected from the ITO anode and metallic cathode, respectively, and then recombined at the quantum dots layer to achieve luminescence.17,18 The energy band structure of the cadmiumfree QLED devices is shown in Figure 1b. We cannot draw any definite conclusions on the relative potential barrier for the electron and hole injection into QD layer without directly studying those, but based on the energy band diagram it is reasonable to assume that electron injection should be somewhat easier than the hole injection. In addition electron mobility in ZnO is much higher than hole mobility in TFB.19 We believe therefore, that the imbalance between hole and electron currents within the device happens due to superior electron mobility of ZnO ETL, resulting into negative charging of the QD emitter and significantly reduced current efficiency of the QLED


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devices. Doping ZnO with Mg results into band gap broadening and energy level shift with the increase of Mg concentration,20 producing a larger energy barrier between an Al cathode and ZnMgO ETL, effectively reducing electron injection. In addition Mg doping can significantly affect ZnO resistivity, resulting into lower electron current density leading to improved charge balance. The PLQY of InP QD films depends greatly on the concentration of the QD solution and annealing temperature.21 To optimize spin-coating parameters, we investigated the exact effect these two conditions have on the PLQY of QD film, as shown in Figure 2. These results indicate that the PLQY of QD film decreases as the QD concentration increases, which may be caused by self-quenching of QDs at higher concentrations through the Förster resonance energy transfer (FRET) between the particles due to small particle size and close packing in a thin film.22 With 10 mg/ml concentration being the only exception to this trend, we believe that a film formed from the solution with such low concentration is too thin to measure PLQY correctly, resulting into lower PLQY value than expected. Therefore, the highest PLQY for QD films was obtained with the concentration of the QD solution of 20 mg/mL, as shown in Figure 2a. The effect of annealing temperature can be seen in Figure 2b, indicating that PLQY decreases significantly with the increase of annealing temperature and that the highest PLQY can be achieved without annealing. Based on these experimental results, 20 mg/ml InP-RQD solution was chosen for spin-coating process, followed by drying in nitrogen-filled glove-box at room temperature without thermal annealing. Thin film analysis for ZnO and Zn1-xMgxO


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Fully solution processed QLED devices utilizing InP based QDs can be fabricated using organic hole injection (HIL) and hole transport (HTL) layers, such as PEDOT:PSS and TFB respectively, with ZnO or doped ZnO nanoparticles forming an electron transport layer (ETL) on top of QDs. More details for QLED fabrication are given in the “Device fabrication” section, and it is noted that the preparation conditions such as annealing temperature and annealing time for the thin-films are the same as those for the device fabrication. Here, core-shell InP/ZnSe QDs are used as an emitting material in R-QLED devices. These RQDs show the PL peak at 621 nm, FWHM of 54 nm and PLQY of 35%. A series of ZnMgO materials (supplied by PolyOE, China) with different Mg doping concentration has been studied using XPS spectroscopy to obtain the exact Mg doping concentration. Figure 3 shows Mg 1s ZnMgO surfaces from which Mg doping concentration has been estimated for all samples. The results are summarized in Table 1. Based on the findings we were able to conclude that three ETL materials used in this study are ZnO with 0% Mg doping concentration, ZnMgO with 5% Mg doping concentration, further referred as ZMO (a) or Zn0.95Mg0.05O, and ZnMgO with 15% Mg doping concentration, further referred as ZMO (b) or Zn0.85Mg0.15O. It’s been reported previously that PLQY of InP-based QDs although can be very high in solution environment, decreases significantly after depositing into a film.21 It has also been shown that when more than one films are deposited consequentially, a donor-acceptor relationship can be formed between them.23 To find the effect of the Mg doping concentration in ZnMgO ETL material on photoluminescence of InP-RQDs, photoluminescence quantum yield (PLQY) measurements had been performed on the samples consisting of a ZnMgO ETL film and an InP-RQD film formed


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consequentially on a glass substrate. The results can be found in Figure 4. It is evident that regardless of the Mg doping concentration there is a pronounced effect on PLQY of QD emitters whenever a ZnMgO ETL is present, however the strongest negative effect can be observed when undoped ZnO is used - PLQY drops from 35% (single RQD layer emission) to 18% - nearly 50% efficiency reduction. With the increase of Mg concentration the efficiency reduction is less severe, resulting into 22%, and 25% PLQY when Zn0.95Mg0.05O, and Zn0.85Mg0.15O ETLs are used respectively. It has been reported that the process of QD quenching by ZnO nanoparticles has two components – dynamic quenching that has a contribution of electron transfer from InP/ZnSe core/shell QDs to ZnO nanoparticles, and static quenching that is related to the decrease of emitting centers in InP/ZnSe core/shell QDs, possibly through tethering of QDs to the surface of ZnO nanoparticles.24 Yet another possibility for the quenching process is FRET between the InP/ZnSe QDs and ZnO nanoparticles. To further investigate the quenching mechanism time resolved photoluminescence measurements were conducted (Figure 5). The average exciton decay times (avg) of InP RQD films on different underlayers differ insignificantly, resulting into 42.9 ns, 38.8 ns, 40.5 ns, and 41.0 ns for the QD films on glass, ZnO, Zn0.95Mg0.05O, and Zn0.85Mg0.15O underlayers respectively (Table 2). Such small changes in exciton decay times suggest that Forster resonance energy transfer (FRET) between InP RQDs and ZnMgO films, though present, is not the major contributor to the changes in PLQY seen in Figure 4. We believe that a much slower process of electron transfer from InP/ZnSe core/shell QDs to ZnO nanoparticles is mainly responsible for the PLQY quenching. When QDs are excited, the electrons in their valence band can be transferred to LUMO levels of ZnO or ZnMgO nanoparticles, resulting into a charge separation on the QD/ZnO


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interface. In such a case the discrepancies between LUMO energy levels of QDs and ZnO or ZnMgO nanoparticles would directly affect the electron transfer probability. Increasing Mg doping concentration in ZnO results into raised LUMO levels, creating higher energy barrier for electron transfer and therefore reducing quenching efficiency. In short, the experimental data seems to suggest that there are multiple quenching pathways present in this system, but the main contributor to the QD PLQY quenching by ZnO or ZnMgO nanoparticles is the dynamic quenching through the electron transfer between them. The electronic structures of ZnO, Zn0.95Mg0.05O, and Zn0.85Mg0.15O films were investigated by ultraviolet photoelectron spectroscopy (UPS) measurements, and the resulting secondary electron cut-off and valence-band regions are shown in Figure 6 (a) and (b) respectively. The valence band maximum (VBM) level is estimated by using the incident photon energy (21.2 eV), the highbinding energy cut-off (Ecut-off) (Figure 6 (a)), and the onset energy in valence-band region (Eonset) (Figure 6 (b)) according to the equation of VBM = 21.2 − (Ecutoff − Eonset) (see Table 3 for detailed UPS values). The VBM positions of ZnO, Zn0.95Mg0.05O (ZMO (a)), and Zn0.85Mg0.15O (ZMO (b)) films are calculated to be 7.8, 7.8, and 7.5 eV below the vacuum level, respectively, exhibiting a relatively low variation of the VBM level with changing Mg content. Using the band gap values obtained from UV−visible absorption spectra of individual nanoparticle solid films (Figure 7) the conductive band minimum (CBM) levels were estimated to be 4.2, 4.1, and 3.8 eV below the vacuum level for ZnO, Zn0.95Mg0.05O (ZMO (a)), and Zn0.85Mg0.15O (ZMO (b)) films, respectively. Energy levels of InP RQDs were also measured using UPS and UV-Vis absorption spectroscopies, providing a value of 6.2 eV below the vacuum level for the VBM and 3.6 eV below the vacuum level for CBM (Table 3). When comparing energy levels alignment between InP RQD emitting layer, ZnO, Zn0.95Mg0.05O, and Zn0.85Mg0.15O electron transporting layers (Figure 1 b), it is


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possible to observe that energy barrier between InP RQD and ETL is reduced with the increase in Mg doping concentration, while the barrier between Al cathode and ETL follows the opposite trend. As a result, we cannot tell with any certainty whether electron injection becomes easier or more difficult when the Mg doping concentration is increased. On the other hand, it has been shown that doping ZnO with Mg increases its resistivity and as a result reduces its conductivity and carrier density,25 thus improving charge balance within the device. To check if this effect indeed takes place, we measured sheet resistance of ZnO, Zn0.95Mg0.05O (ZMO (a)), and Zn0.85Mg0.15O (ZMO (b)) films using Jandel 4 point probe. Films with the approximate thickness of 1 m were fabricated on a glass substrate using drop-casting. The results were normalized to the film thickness and are reported in the Table 4. It can be clearly seen that sheet resistance of these films is significntly increased with the rise of Mg doping concentration and goes from 1.55*107 Ω/□ for pure ZnO to 5.9*107 Ω/□ with 5% Mg doping, and to 9.16*108 Ω/□ with 15% Mg doping. Single carrier Devices Since charge balance between electrons and holes in QDs is one of the more important factors to improve device performance26-30, hole current and electron current within QLED devices had to be investigated. In order to verify the electron injection and transport capabilities of ZnMgO films and compare them to hole injection and transport capabilities of HIL/HTL, the electron-only (EOD) and hole only (HOD) devices were fabricated. The fabrication conditions, such as thickness and annealing conditions, for EODs and HOD are identical to the full device fabrication conditions and are explained in “Device fabrication” section of this paper. Device structures were as following:

ITO/Zn0.85Mg0.15O/InP-RQD/ZnO/Al

(ZnO),

ITO/Zn0.85Mg0.15O/InP-


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RQD/Zn0.95Mg0.05O/Al (ZMO (a)), ITO/Zn0.85Mg0.15O/InP-RQD/Zn0.85Mg0.15O/Al (ZMO (b)), and ITO/PEDOT:PSS/TFB/InP-RQD/Au (HOD). It is well understood that electron current and hole current in full devices is different from those in EODs and HODs, but the device structures we chose for EODs and HODs in our opinion provide a reasonable enough approximation of electrons and holes dynamics in real devices. Thus, the injection and transport capabilities can be adequately enough characterized by measuring J–V characteristics of these devices as shown in Figure 8. It is evident that with the increase of Mg doping concentration in ZnMgO ETL, electron density in the devices decreases. These findings are in good agreement with the increase of ZnMgO resistance corresponding to higher Mg doping concentration. This increase in resistance has a positive effect on QLED device performance, since it results into more balanced charge characteristics of the device. It can be noted from the Figure 8, that EOD devices with Zn0.85Mg0.15O show current densities, most similar to HOD devices among tested EODs at lower voltage, leading to the effective electron and hole current matching in QLEDs. However, at higher voltage hole current density in HODs becomes higher than electron current density in EODs with Zn0.85Mg0.15O. We were unable to prove the mechanism of this phenomenon to our satisfaction, but we have a notion that due to very high resistivity of Zn0.85Mg0.15O in comparison to both ZnO and Zn0.95Mg0.05O electron density in these devices becomes significantly lower at high operating voltage than the hole density. Device Performance of QLEDs with ZnO and ZnMgO ETLs To test the effectiveness of electron current density reduction in QLED devices with raised Mg doping ratio in ZnMgO ETL on QLED performance we fabricated a series of QLED devices with the

following

structures:

ITO/PEDOT:PSS/TFB/InP-RQD/ZnO/Al

(ZnO+Al),


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ITO/PEDOT:PSS/TFB/InP-RQD/Zn0.95Mg0.05O /Al (ZMO (a) +Al), and ITO/PEDOT:PSS/TFB/ InP-RQD/Zn0.85Mg0.15O /Al (ZMO (b) +Al). Electroluminescence performance of these devices is summarized in Figure 9. Looking at current density and brightness of the devices (Figure 9 (a)), two trends can be observed – with the increase of Mg doping concentration in ZnMgO ETL current density is decreased, while brightness is increased. Therefore, it logically follows that with the increase of Mg doping concentration, current efficiency of the R-QLED devices increases (Figure 9 (b)). These findings are in very good agreement with the data obtained from single carrier devices (Figure 8). With the increase in Mg doping concentration electron density in QLED devices is reduced, resulting into improved hole-electron balance and therefore more efficient emissive charge recombination. And more efficient charge recombination is responsible for increased current efficiency of the QLED devices. There is another mechanism at play that is more noticeable at higher voltage. Due to higher electron density in conventional QLED devices, excess electrons cause QD charging at higher voltage resulting into non-emissive Auger recombination becoming more pronounced and playing a major role in severe efficiency roll-off. By increasing Mg doping concentration in ZnMgO ETL and lowering electron current density we can reduce QD charging, and therefore contribution of Auger recombination and efficiency roll-off. By controlling Mg doping concentration in ZnMgO ETL we were able to achieve an almost 3fold efficiency improvement – from 1.2 cd/A with ZnO ETL to 3.0 cd/A with Zn0.85Mg0.15O ETL. Device Performance of QLEDs utilizing R-QDs with higher PLQY


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Notwithstanding the pronounced effect balanced charge injection has on the electro-luminescent display device efficiency, it is still impossible to achieve very high efficiency when QLED devices utilize QD emitters with low PLQY. Unfortunately, despite some recent successes in developing high PLQY InP-based QDs15,16

nearly all commercially available InP-based materials are

characterized by photoluminescence quantum yield, which is still far behind that of CdSe-based QDs. While the latter can reach PLQY in excess of 80%, most of the InP-based QD films have PLQY close to 40%. Recently we were able to synthesize a sample of InP RQDs, in which the outer part of the ZnSe shell had been replaced by ZnS, giving them a core/shell/shell structure of InP/ZnSe/ZnS. These RQDs exhibited a much higher PLQY compared to those with InP/ZnSe core/shell structure (60% vs. 35%). In order to compare device performances, QLEDs with the following compositions had been

fabricated:

ITO/PEDOT:PSS/TFB/InP-RQD(35%)/Zn0.85Mg0.15O/Ag,

and

ITO/PEDOT:PSS/TFB/ InP-RQD(60%)/Zn0.85Mg0.15O /Ag. The silver cathode had been chosen for these devices, because due to higher work function of Ag compared to Al (4.5 eV vs. 4.2 eV) an injection barrier from cathode to ZnMgO ETL is higher, resulting into further reduction of electron current density. Low current density is very important to the InP RQD with 60% PLQY we used, since it exhibits much lower stability at high current densities compared to InP RQD with 35% PLQY. Utilizing Ag as a cathode material causes a small current efficiency improvement for the devices with 35% PLQY InP RQD emitter, but at the same time a severe brightness decrese. The results of IVL measurements are summarized in Figure 10. Despite small differences in current densities between devices with 35% and 60% PLQY emitters, there is a significant difference in brightness (Figure 10 (a)), resulting into almost one order of magnitude increase in brightness when 60% PLQY emitter is utilized. The improved luminescence of the RQDs also


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results into much higher efficiency – it increased from 3.0 cd/A for 35% PLQY to 11.6 cd/A for 60% PLQY. This is a promising result, suggesting that more efforts being spent on researching and developing new InP-based QDs now, when CdSe-based QDs toxicity has become an important issue urging people toward Cd-free QDs, a significant improvement in InP-based QLED performance is possible with increase in high PLQY InP QDs availability. Device Performance of QLEDs utilizing top-emitting architecture One of the bigger inherent advantages of QLED devices over OLED devices is narrow emission spectrum with full width at half-maximum (FWHM) 35 nm for CdSe-RQDs, compared to 56 nm for red emitting organic materials. Unfortunately, this is one of the areas where current state of InP-based QDs is still behind CdSe-based QDs - as reported above, InP-RQDs we synthesized have FWHM of 54 nm, which is almost as broad as that of organic red emitters. In order to improve FWHM of the QLED devices we fabricated devices with top emitting inverted structures. We performed optical simulations using SETFOS semiconducting thin film optics simulation software to determine optimal layer thicknesses to achieve both high efficiency and narrow spectrum of the top-emitting R-QLED devices. Based on the simulations the following devices were

fabricated

-

1)

bottom

emitting

reference

devices

with

the

structure

of

Glass/ITO/PEDOT:PSS/InP-RQD/Zn0.85Mg0.15O/Ag, in which PEDOT:PSS and TFB deposition conditions are the same as listed in “Device fabrication” section, InP-RQD was spincoated from a 20mg/ml solution in octane at 3000 rpm for 40 s, followed by 5 min annealing at 80C in N2filled glovebox, ZnMgO was spincoated in glovebox from 20 mg/ml solution in ethanol at 3000 rpm for 40 s without annealing, 100 nm of Ag cathode was deposited in a vacuum chamber through


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a mask; 2) top emitting devices with the structure Glass/Ag/ITO/Zn0.85Mg0.15O/In-P RQD/TCTA/NPB/HAT-CN/Ag, in which ZnMgO was spincoated in glovebox from 20 mg/ml solution in ethanol at 4000 rpm for 40 s without annealing, InP-RQD was spincoated from a 20 mg/ml solution in octane at 3000 rpm for 40 s, followed by 5 min annealing at 80C in N2-filled glovebox, a 10 nm layer of 4,4’,4”-tris(carbazol-9-yl)-triphenylamine (TCTA), a 25 nm layer of N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4”-diamine (NPB), and a 15 nm layer of silver (Ag) were deposited consequentially in a vacuum chamber. All the devices were then encapsulated according to the description in section “Device fabrication”. Emission spectra of the bottom and top emitting devices along with the simulation of top emission are presented in Figure 11. Measured emission spectrum of top-emitting device is in a very good agreement with simulation results, and both show significant improvement in FWHM compared to bottom emitting devices – it was reduced from 54 nm to 32 nm, which is comparable to the FWHM of CdSe-based R-QLEDs. This result can be explained by a microcavity formation between a reflective anode (Ag/ITO) and a semi-reflective cathode (Ag, 15nm) of devices with top-emitting architecture. To support our claim about microcavity formation we performed optical simulations of device luminance dependence on thickness of ETL and HIL layers for both top- and bottom-emitting devices using SETFOS software (Figure 12). It can be clearly seen that bottom-emitting devices (Figure 12a) are much less affected by changes in both HIL and ETL thicknesses than top-emitting devices (Figure 12b), suggesting that top-emitting devices have a much stronger optical cavity formed. We can consider this as a Fabry-Perot cavity, in which emitter is situated between a bottom reflective mirror formed by Ag/ITO cathode and a top semi-transparent mirror formed by a thin (15 nm) Ag anode. In such a cavity there are two types of interferences can be found – (1) interference between directly emitted light and the light reflected from the bottom mirror with the


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same wave vector (wide-angle interference); and (2) interference between the light multiply reflected from bottom and top mirrors (multiple-beam interference). If the resonant conditions are reached, the constructive interference is achieved resulting into enhanced emission. Resonant conditions are dependent on the emitting wavelength of the emitter, optical length of the cavity, and position of the emitter within the cavity relative to the reflective and semi-transparent mirrors. If resonant conditions are not reached, the interference stops being constructive, and the emission from micro-cavity is suppressed. Formation of the microcavity thus can explain the narrowing of emission spectra in top-emitting devices since only relatively narrow distribution of the emission wavelengths can satisfy resonant conditions. It is evident, that top-emitting devices in Figure 12b have a microcavity with resonant conditions reached only for relatively narrow thickness distribution of HIL and ETL layers. In order to experimentally support results of our optical simulations LIV characteristics of the top and bottom devices reported above were measured (Figure 13). It can be seen that top-emitting devices exhibit higher luminance at the same current density than the bottom-emitting devices, resulting into improved current efficiency, with maximum current efficiency increasing from 2.9 cd/A to 7.1 Cd/A. Unfortunately the necessity of maintaining the resonant optical conditions resulted into less than ideal electrical characteristics of top-emitting devices, resulting into higher driving voltage compared to bottom-emitting devices, making direct comparison of these two structures difficult. To further support the evidence of micro-cavity formation in top-emitting devices we fabricated two types of top-emitting devices: Top Emitting 1, the same recipe as reported above; and Top Emitting 2, which has the same recipe as Top Emitting 1, accept ZnMgO was spincoated at 3000 rpm instead of 4000 rpm, resulting into ZnMgO ETL thickness of 30 nm in Top Emitting 1 and 40


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nm in Top Emitting 2. The LIV characteristics of these devices are reported in Figure 14. It is evident that in device Top Emitting 1 optical conditions are much closer to resonant conditions than in devices Top Emitting 2, resulting into much higher luminance at the same current density and as a result higher current efficiency

Conclusions In conclusion, we reported a study of the factors responsible for InP-based R-QLED device performance. We showed that electron current density in these devices is generally much higher than hole current density, resulting into unbalanced charge injection. By controlling band gap, mobility and electron injection barriers of ZnMgO ETLs through changing Mg doping concentration we were able to achieve much more balanced charge injection resulting into nearly 3-fold efficiency improvement. It has been also shown that current state of InP QD development for QLED applications is still far behind that of CdSe QDs. We demonstrated that improving PLQY of InP QD emitters in conjunction with balanced device structure can result into significant efficiency improvement. By increasing InP RQD PLQY from 35% to 60% we were able to achieve a very high efficiency of 11.6 cd/A for InP based R-QLED devices. We also showed that topemitting architecture of InP based R-QLED devices can lead to improved efficiency and to significant reduction of FWHM compared to bottom-emitting architecture (from 54 nm to 32 nm), resulting into much higher color purity, which is very important for use of this materials in display applications.


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Experimental Section Synthesis of InP-QDs with 35% PLQY: Typically, 0.4 g of InCl3, 1.2 g of ZnCl2, and 20 mL oleylamine were added to a flask, and the mixture was heated to 120~180℃ under Ar flow until a clear solution formed. Then the reaction mixture was heated to 200~240℃. At this temperature, 2 mL (DEA)3P solution was injected into the reaction flask. After 10~60 min, a certain amount (4 mL) of the as-prepared stock solution for Se precursor(2M) and 100 mL stock solution for Zn precursor(2) (0.25 M) were added and the reaction were kept for 30~120 min. At this temperature, another 15 mL of the as-prepared stock solution for Se precursor (2M) was swiftly injected into the reaction flask. The reaction mixture was then heated to 270~320℃ and 30 mL stock solution for Zn precursor(1) (0.4M) and 3 mL DDT were added and kept for more than 30min. At the end, the heating was removed to stop the reaction and allow the flask to cool to room temperature. Synthesis of InP-QDs with 60% PLQY: Typically, 0.4 g of InCl3, 1.2 g of ZnCl2, and 15 mL oleylamine were added to a flask, and the mixture was heated to 120~180℃ under Ar flow until a clear solution formed. Then the reaction mixture was heated to 200~240℃. At this temperature, 2 mL (DEA)3P solution was injected into the reaction flask. After 10~60 min, a certain amount (3 mL) of the as-prepared stock solution for Se precursor (2M) and 100 mL stock solution for Zn precursor(2) (0.25 M) were added and the reaction were kept for 30~120 min. The reaction mixture was then heated to 270~320℃, and 15 mL of the as-prepared stock solution for S precursor (2M) was swiftly injected into the reaction flask. After another 30~120 min, 20 mL stock solution for


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Zn precursor (1) (0.4M) and 3 mL DDT were added and kept for more than 30min. At the end, the heating was removed to stop the reaction and allow the flask to cool to room temperature. All manipulations were performed using standard air-free techniques. Stock solution for Se precursor: The selenium precursor solurion (2M) was prepared by hearing a mixture of selenium (7.9g, 100 mmol) and TOP (50 mL) to 150℃ under nitrogen. Stock solution for S precursor: The sulfur precursor solurion (2M) was prepared by hearing a mixture of sulfur (1.28g, 40 mmol) and TOP (20 mL) to 150℃ under nitrogen. Stock solution for 0.4 M Zn precursor (1): A mixture of ZnO (4.0 g, 50 mmol), oleic acid (70 mL),and 55 mL ODE was loaded into a 250 mL three-necked flask and heated to 280℃ under nitrogen to obtain a colorless, clear solution. Stock solution for 0.25 M Zn precursor (2): A mixture of ZnO (5.1 g, 62.5 mmol), stearic acid (130 mL),and 120 mL ODE was loaded into a 500 mL three-necked flask and heated to 280℃ under nitrogen to obtain a colorless, clear solution.

Device fabrication: The QLED devices were fabricated through spin-coating and evaporation on glass substrates with pre-patterned indium-tin oxide (ITO) anode. The substrates were carefully cleaned in deionized water, acetone and ethyl alcohol for 15min each, and then treated with UVozone for another 10 min. Subsequently, a hole injection (HI) material of poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) AI 4083 was spin-coated on the substrate at 4000 rpm for 40 s, and then annealed at 135oC for 20 min in air. Then the HI coated


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substrates were transferred into a N2-filled glovebox and further baked at 135oC for 5 min to remove any residual moisture. Next, hole transporting layer (HTL) of poly[(9,9-dioctylfluorenyl2,7-diyl)-co-(4,4’-(N-(4-sec-butylphenyl)diphenylamine)] (TFB) had been spin-coated at 3000 rpm for 40 s in glovebox using 8 mg/ml solution in chlorobenzene, followed by a thermal annealing at 245 °C for 30 min. Subsequently, InP red QDs and ZnO (or ZnMgO) nanocrystals were sequentially deposited on the substrates under identical conditions (at 2500 rpm for 40 s if not specified otherwise). Furthermore, the thickness of each layer had been adjusted by changing the solution concentration from which the material was being spincoated. InP RQDs were deposited from an octane solution, ZnO and ZnMgO were deposited from ethanol solution. After the deposition of the solution-processed layers, all samples were transferred to a vacuum deposition chamber with chamber pressure below 10−6 torr (P < 10−6 torr) for Al or Ag cathode deposition ( ≧ 100 nm), followed by the encapsulation with a UV-curable epoxy and cover glasses in N2 atmosphere. All the devices had the emitting area of 4 mm2 that was defined by the overlapping of ITO anodes and metallic cathodes. PEDOT:PSS AI 4083 solution had been purchased from Heraeus Precious Metals GmbH & Co. KG filtered through the 0.2m PVDF filter before use. TFB had been purchased from SigmaAldrich. ZnO, ZnMgO (5%), and ZnMgO (15%) were supplied by Poly OptoElectronics TECH. Ltd. (PolyOE, China).


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Figure 1. a) Basic device structure of InP QLED with ITO anode; b) Energy band diagrams of InP QLEDs with ZnO, Zn0.95Mg0.05O (a), and Zn0.85Mg0.15O (b) ETLs

Figure 2. InP RQD films PLQY dependence on spincoating solution concentration (a) and on annealing temperature (b).


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Figure 3. Mg 1s XPS intensity spectra of ZnMgO with 5% (a), and 15% (b) Mg doping concentration.

Figure 4. Mg doping concentration in ZnO ETL effect on InP-RQD PLQY.


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Figure 5. Time resolved photoluminescence (TRPL) of InP RQD films on different underlayers.

Figure 6. UPS spectra of high binding energy secondary electron cut-off (a) and valance-band (b) regions for ZnO (I), Zn0.95Mg0.05O (II) and Zn0.85Mg0.15O (III)


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Figure 7. Tauc plot of ZnO (black), Zn0.95Mg0.05O (red), and Zn0.85Mg0.15O (blue) films on glass.

Figure 8. Current density vs. voltage characteristics of EOD and HOD devices.


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Figure 9. Current efficiency of the InP R-QLED devices utilizing ZnO, Zn0.95Mg0.05O, and Zn0.85Mg0.15O ETLs

Figure 10. Characteristics of QLED devices, utilizing InP-based RQDs with 35% and 60% PLQY: (a) Current density and brightness; (b) Current efficiency.


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Figure 11. Emission spectra of InP-based R-QLED devices: bottom-emitting devices have a broad emission spectrum (black line) with FWHM 54 nm, while both simulated (dashed line) and experimentally measured (red line) emissions of top-emitting devices have much narrower spectra with FWHM 32 nm.

Figure 12. SETFOS simulation results of InP R-QLED devices luminance dependence on HIL and ETL thickness: (a) Bottom emitting device with normal structure; (b) Top emitting device with inverted structure.


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Figure 13. Characteristics of InP R-QLED devices in bottom-emitting and top-emitting configurations: (a) Current density and brightness; (b) Current efficiency.

Figure 14. Characteristics of InP R-QLED top-emitting devices with different ZnMgO ETL thickness: (a) Current density and brightness; (b) Current efficiency. Top Emission 1 devices have 30 nm thick ZnMgO ETL; Top Emission 2 devices have 40 nm thick ZnMgO ETL.

Table 1. Summary of the Mg doping concentrations obtained by XPS spectroscopy. Film

Mg XPS Area, CPS

Mg doping, %

ZnO

0

0

ZMO (a)

5587.9

5.0  0.3

ZMO (b)

16891.3

15.0  0.3


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Table 2. InP RQD exciton decay times measured on different underlayers. Underlayer

Glass

ZnO

avr (ns)

42.9

38.8

ZnMgO (a) 40.5

ZnMgO (b) 41.0

Table 3. Energy levels of series of ZMO films obtained from UPS and absorption spectra. Eonset,

Ecut-off,

Band Gap,

VBM,

CBM,

eV

eV

eV

eV

eV

ZnO

4.4

17.8

3.6

7.8

4.2

ZMO (a)

4.2

17.6

3.6

7.8

4.1

ZMO (b)

3.9

17.6

3.7

7.5

3.8

InP RQD

2.7

17.7

2.6

6.2

3.6

Film

Table 4. Sheet resistance of series of ZMO films. Film

ZnO

ZnMgO (a)

ZnMgO (b)

Rs (Ω/□)

1.550.12*107

5.900.23*107

9.161.2*108


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Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China under Grant 2016YFB0401700.

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Enhanced Efficiency of InP-Based Red Quantum Dot Light-Emitting

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