All-Solution-Processed Quantum Dot Light Emitting Diodes Based on

Aug 6, 2018 - All-Solution-Processed Quantum Dot Light Emitting Diodes Based on Double Hole Transport Layers by Hot Spin-Coating with Highly Efficient...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

All-Solution-Processed Quantum Dot Light Emitting Diodes Based on Double Hole Transport Layers by Hot Spin-Coating with Highly Efficient and Low Turn-On Voltage Hongting Chen, Ke Ding, Lianwei Fan, Wei Liu, Rui Zhang, Songpo Xiang, Qing Zhang, and Lei Wang* Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China

ACS Appl. Mater. Interfaces 2018.10:29076-29082. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/30/18. For personal use only.

S Supporting Information *

ABSTRACT: Poly(N,N′-bis(4-butylphenyl)N,N′-bis(phenyl)benzidine) (poly-TPD) and poly(9-vinylcarbazole) (PVK) are both traditional hole transport layers (HTLs) in quantum dot light emitting diodes (QLEDs). Nevertheless, the low highest occupied molecular orbital of poly-TPD and poor hole mobility of PVK always result in poor performance of QLEDs when individually used as HTLs. Unfortunately, fabricating stepwise HTLs with poly-TPD and PVK faces technical problems until now. We demonstrate an effective method to construct the stepwise poly-TPD and PVK HTLs by utilizing the technique of hot spin-coating PVK in m-xylene on poly-TPD film. During this hot spin-coating process, the underlying poly-TPD remains unwashed and undamaged, benefiting the all-solution-processed QLED fabrication. The optimized all-solution-processed QLED with stepwise polyTPD and PVK HTLs shows a maximum external quantum efficiency (EQE) of 15.3% and a maximum luminance of 17 110 cd/m2 with a low turn-on voltage (Von) of 1.75 V. The maximum EQE is about 6.6 times higher than that of the reference QLED using a cold spin-coating process. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the hot spin-coating stepwise coHTLs based QLEDs. This work manifests the positive effect on performance boost by a hot spin-coating strategy toward stepwise co-HTLs formation and paves the way to fabricate highly efficient all-solution-processed light emitting diodes. KEYWORDS: quantum dot light emitting diodes, hot spin-coating, stepwise co-HTLs, all-solution-processed, charge injection balance

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INTRODUCTION Quantum dot light emitting diodes (QLEDs) have gone through wide research due to the great potential as the next generation of high-color-quality display and solid-state lighting.1,2 Besides, owing to the high photoluminescence quantum yield (PLQY),3−5 high color purity,6−9 size-tunable emission,10 and cost-effective processing11−18 of quantum dots (QDs) and the good compatibility with solution-processed techniques, the efficiency of QLEDs has been greatly improved from less than 0.01%19 to 20.5%20 in the past two decades. Especially, allsolution-processed QLEDs appeal to researchers due to their advantages of low cost and available for large-area and flexible displays fabrication.21 Nonetheless, it is still challenged for efficient performance. On one hand, the orthogonal solvents must be chosen to avoid damaging the underlying layers when depositing the overlayers and the overlayer should have good wettability onto the underlying layer to fabricate uniform films.21−23 On the other hand, hole injection capacity of traditional hole transport layers (HTLs) cannot match with electron injection capacity of traditional electron transport layers (ETLs), leading to charge injection imbalance. A red QLED with the best performance is obtained with all-solutionprocessed techniques containing a stepwise hole HTLs.20 © 2018 American Chemical Society

However, the traditional HTLs named poly(N,N′-bis(4butylphenyl)N,N′-bis(phenyl)benzidine) (poly-TPD) and poly(9-vinylcarbazole) (PVK),24−29 are found difficult to form stepwise poly-TPD/PVK co-HTLs, caused by solvent selection difficulties. Poly-TPD and PVK can dissolve in almost the same solvent, causing film damage during the spin-coating process. Although the reported orthogonal solvent for the HTLs like m-xylene and chlorobenzene (CB) can solve the problem,20 the poor solubility of PVK in m-xylene and rarely feasible film formation process still limit the stepwise co-HTLs application in all-solution-processed QLEDs. It is well-known that charge injection balance is the critical factor in the performance of QLEDs.20,30 As for all-solutionprocessed QLEDs, ZnO with good electron mobility and suitable energy level with QDs has become the most popular ETL for QLEDs.20,27−29,31,32 But HTLs like poly-TPD or PVK possess lower hole mobility and unsuitable energy level with QDs (the highest occupied molecular orbital (HOMO) of poly-TPD, PVK and the valence band (VB) of QD are 5.4, 5.6, Received: April 27, 2018 Accepted: August 6, 2018 Published: August 6, 2018 29076

DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

Research Article

ACS Applied Materials & Interfaces

CB) were spin-coated at 3000 rpm and 2000 rpm on PEDOT:PSS, respectively. Next, CdSe/CdS/ZnS QDs (from Poly OptoElectronics Co., Ltd.) in octane (15 mg/mL) was spin-coated and baked at 100 °C for 5 min. Last, ZnO NPs in ethanol (50 mg/mL) were spincoated and baked at 100 °C for 15 min. Finally, the substrates were transferred to a vacuum chamber for the overlying organic film and Al electrode deposition with the base pressure of 4 × 10−4 Pa. Characterizations. For QLED characteristics, the current density−voltage and luminance−voltage were measured by a Keithley 2400 source measurement unit and in the meanwhile a PhotoResearch spectroradiometer (PR655) with the efficiency measured, and all the tests were conducted in the air without encapsulation. The morphology of the films was gained by a Veeco Dimension 3100 NanoScope in a tapping mode measured by Bruker RTESPA-300 probes. The thickness of the films was measured by a Bruker DektakXT profilometer. To overcome the precision limit, for ultrathin film with the thickness less than 20 nm, we measured their thicknesses by depositing them on a thickness-known film (>30 nm) and obtained thicknesses through a simple subtraction.

and 6.8 eV respectively), leading to charge injection imbalance in this kind of all-solution-processed QLEDs. Utilizing the stepwise energy level of poly-TPD and PVK can reduce the energy barrier and promote hole injection. Thus, the stepwise poly-TPD/PVK HTLs can also benefit the charge injection balance in QLEDs.27,28,30 Herein, we report a serviceable technique to fabricate allsolution-processed QLEDs based on poly-TPD and PVK as stepwise co-HTLs. The solvent for PVK was chosen as mxylene,20,33−35 and hot spin-coating which required both hot PVK m-xylene solution and hot substrate was introduced. Furthermore, when the substrate temperature was ∼60 °C and the solution temperature was ∼80 °C, the best film was obtained and the RMS surface roughness was as small as 1.01 nm. Utilizing the hot spin-coating technique, the stepwise coHTLs based deep-red QLED with CdSeCdSZnS quantum dot as the emitter and ZnO nanoparticles (NPs) as ETL was fabricated. For comparison, QLEDs with stepwise co-HTLs by cold spin-coating (with ambient-temperature substrate and hot solution when spin-coating PVK) and QLEDs with poly-TPD or PVK as single HTL were also fabricated. The stepwise coHTLs based QLEDs by hot spin-coating show a lower turn-on voltage (Von) of 1.75 V than that of cold spin-coating (1.9 V), and exhibits better maximum external quantum efficiency (EQE) and luminance (15.3% and 17 110 cd/m2, respectively) than QLEDs with cold spin-coating (2.3% and 10 990 cd/m2, respectively). Furthermore, the maximum EQE and luminance of the QLED with stepwise co-HTLs exhibit about 2.9-fold and 3.5-fold enhancement compared to that of poly-TPD as HTL (5.2%, 4813 cd/m2). Meanwhile, the turn-on voltage of the device based on stepwise co-HTLs is much lower than that based on PVK (2.6 V). The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge balance resulting from the hot spincoating stepwise co-HTLs, which manifests the hot spincoating strategy toward stepwise co-HTLs formation an effective method to achieve high performance and low turnon voltage QLEDs.

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RESULTS AND DISCUSSION CB, ODCB, and toluene were the traditional solvents for PVK; thus they were first selected when we constructed stepwise coHTLs based on poly-TPD and PVK. PEDOT:PSS which was insoluble in the three solvents was deposited in the traditional method, and the thickness of its film was about 40 nm. The poly-TPD film spin-coated on PEDOT:PSS from CB was about 30 nm. When PVK in these solvents was deposited, the thickness of multilayers of PEDOT:PSS/poly-TPD/PVK was only about 40 nm, which was the thickness of PEDOT:PSS (Table S1). It means almost all the poly-TPD film was washed up apparently. Hence, solvent selection was the first key step. It is found that m-xylene has been used as the solvent of PVK to fabricate stepwise co-HTLs of poly-TPD and PVK.33−35 However, as we followed that, it was surprising that PVK was insoluble in it at ambient temperature, while it can be dissolved above ∼70 °C but precipitated when cooled below ∼70 °C, which is shown in Figure S1a. In that case, the 80 °C PVK m-xylene solution was spin-coated on the ambienttemperature substrate of the multilayers of ITO/PEDOT:PSS/ poly-TPD. Nevertheless, the PVK film was extremely inhomogeneous. It may result from the rapid precipitation of PVK due to the sudden cooling of hot PVK m-xylene solution, which can be verified from the atomic force microscopy (AFM) morphology image presented in Figure S1b. The aggregation of PVK is obvious, and root-mean-square (RMS) roughness is as high as 4.63 nm. It is considered that a lower precipitate rate may decrease the aggregation, and the precipitate rate is related to the temperature of both substrate and solution. Hence, temperature is also the critical step in the stepwise co-HTLs fabrication. To prove the conjecture, the technique of hot spin-coating was introduced to the stepwise co-HTLs fabrication, and the schematic representation of the fabrication procedure is shown in Figure 1 with cold spin-coating as comparison. For cold spin-coating, the substrate was at ambient temperature, while, for hot spin-coating, the substrate was warmed up to a certain temperature. To further figure out the effect of the temperature, the substrate and PVK m-xylene solution were heated to different temperatures. The temperature of the substrate ranged from 50, 60 to 70 °C, and the temperature of PVK m-xylene solution ranged from 70, 80 to 90 °C. For the sake of consistency with that in QLEDs, the substrate was the multilayers of ITO/PEDOT:PSS/poly-TPD.

EXPERIMENTAL SECTION

Synthesis of ZnO Nanoparticles. ZnO nanoparticles were synthesized through a solution-precipitation process with some modifications.30 QLEDs Fabrication. Patterned indium tin oxide (ITO) glass substrates with the sheet resistance less than 15 Ω/sq were cleaned with alkaline cleaner diluted with deionized water, acetone, isopropanol successively and then oven-dried for more than 3 h after dried with N2. After UV ozone treatment in air for 5 min, poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) solutions (Baytron PVP Al 4083, filtered through a 0.45 μm N66 filter) was spin-coated onto the ITO-coated glass substrates at 2000 rpm and baked at 120 °C for 30 min in the air. The PEDOT:PSScoated substrates were transferred into a nitrogen-filled glovebox (O2 < 1 ppm, H2O < 1 ppm). For stepwise co-HTLs devices, poly-TPD (10 mg/mL in CB) was spin-coated at 2500 rpm and baked at 110 °C for 20 min. After that, for the solution of CB o-dichlorobenzene (ODCB) and toluene, PVK (1.5 mg/mL) was spin-coated at 2000 rpm, and baked at 170 °C for 30 min. For the solution of m-xylene with hot spin-coating, the substrates were heated to different temperatures, and PVK (1.5 mg/mL) was spin-coated at 2000 rpm onto the hot substrates after heated to a certain temperature, and then baked at 170 °C for 30 min. For the solution of m-xylene with cold spin-coating, PVK (1.5 mg/mL) was spin-coated at 2000 rpm onto the cold substrates, and then baked at 170 °C for 30 min. For single HTLs devices, poly-TPD (10 mg/mL in CB) and PVK (10 mg/mL in 29077

DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

Research Article

ACS Applied Materials & Interfaces

scan profiles in Figure S2). As for the substrate, when the temperatures was defined to 50 °C, the precipitate rate of PVK was still fast; hence there was obvious aggregation as presented in the red circle in Figure 2a,d,g. Actually, poly-TPD was soluble in 70 °C m-xylene and the film of poly-TPD was more sensitive to substrate temperature than solution temperature; hence, even when the temperatures for the substrate was defined to 70 °C, poly-TPD was easier to be damaged, and the thickness of the multilayers of PEDOT:PSS/poly-TPD/PVK was as thin as 60 nm (Table S2). Therefore, the temperature of the substrate was set to 60 °C. As for the solution, 70 °C was a bit low for PVK, in that the precipitate rate was still fast, and 90 °C was too high for poly-TPD, in that the poly-TPD film was washed and damaged. Considering the morphology and thickness of HTL synthetically, the optimized temperatures for substrate and PVK m-xylene solution were defined to be 60 and 80 °C, respectively. In this condition, the RMS surface roughness of PVK film was only 1.01 nm. In general, the poor morphology of PVK resulted from the fast precipitation of PVK when hot PVK m-xylene solution was spin-coated onto a cold substrate for cold spin-coating. However, as for hot spincoating, PVK can precipitate uniformly without damaging the substrate material, as long as an appropriate temperature was chosen. To further confirm the significance of PVK film morphology for the QLEDs devices, stepwise co-HTLs based QLEDs by cold spin-coating and hot spin-coating were fabricated. The device structure was as follows: ITO/PEDOT:PSS (40 nm)/ poly-TPD and PVK (25 nm)/CdSeCdSZnS QDs (30 nm)/ ZnO NPs (75 nm)/aluminum (Al, 100 nm). Here, cold spincoating was an ambient-temperature substrate and PVK mxylene solution heated to 80 °C when depositing PVK. Hot spin-coating was the substrate heated to 60 °C and PVK mxylene solution heated to 80 °C when depositing PVK. The schematic diagram and the energy level diagram of the multilayered QLED device is shown in Figure 3. The energy

Figure 1. Schematic representation of stepwise co-HTLs fabrication procedure: (a) cold spin-coating and (b) hot spin-coating.

It is obvious that both two temperatures have a significant impact on the uniformity of PVK films, which is proved by the AFM morphology images illustrated in Figure 2. Compared

Figure 2. AFM morphology images of PVK films with different temperatures of substrate-solution. (a) 50-70 °C, (b) 60-70 °C, (c) 70-70 °C, (d) 50-80 °C, (e) 60-80 °C, (f) 70-80 °C, (g) 50-90 °C, (h) 60-90 °C, (i) 70-90 °C. Note that the substrate is the multilayers of ITO/PEDOT:PSS/poly-TPD and the solution is PVK m-xylene solution. Figure 3. (a) The device structure of the multilayered QLED device. (b) The energy level diagram for the various layers.

with the substrate at ambient temperature, the aggregation of PVK reduced apparently and the film roughness was reduced substantially in all the heating conditions (the AFM morphology images and line-scan profiles in Figure S2). Moreover, the AFM morphology images present that the RMS surface roughnesses are less than 1.5 nm, which is 3 times improvement of that with cold spin-coating. Particularly, for hot spin-coating, when the substrate temperature rises to 60 °C, the aggregation of PVK practically disappeared; on the contrary, the RMS surface roughness showed a slight increase when the substrate temperature was up to 70 °C or the solution temperature reached 90 °C. The peak to valley roughness of PVK film was ∼40 nm for cold spin-coating, while it decreased to ∼4 nm for the substrate temperature up to 60 °C and the solution temperature up to 80 °C. Then it increased to ∼7 nm as the temperature continued to rise (line-

bands of all the materials were referred from literature reports.27,28,30,36 As it is presented in Figure 3b that the coHTLs form stepwise HTLs, which can promote the hole injection to the EML, so that the hole injection matches the electron injection better, furtherly improves the performance of QLEDs. For high-performance QLED devices, the quality of films is essential. Specially, it is more challenging for all-solutionprocessed QLEDs. The five layers, including PEDOT:PSS, poly-TPD, PVK, QDs, and ZnO, were all deposited by spincoating and only the top Al electrodes were fabricated through vacuum deposition. Figure S3 shows the AFM morphology images of the five solution-processed layers of QLEDs with hot 29078

DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

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ACS Applied Materials & Interfaces

Figure 4. Electroluminescence performance of QLEDs with different stepwise co-HTLs fabrication of cold spin-coating and hot spin-coating. (a) J−V characteristics. (b) L−V characteristics. (c) EQE−L and PE−L characteristics. (d) The CIE coordinate and its enlarged view of QLEDs with cold spin-coating at different voltage. (e) The CIE coordinate and its enlarged view of QLEDs with hot spin-coating at different voltage.

Table 1. Summarized Device Performance of the QLEDs with Different HTLs devices

EL peaka [nm]

Vonb [V]

Lumax [cd/m2]

CEmaxc [cd/A]

PEmax [lm/W]

EQEmax [%]

cold spin-coating co-HTL hot spin-coating co-HTL poly-TPD only PVK only

624 632 632 632

1.9 1.75 1.9 2.6

10990 17110 4813 560

3.1 16.8 5.3 12.9

4.3 27.8 8.6 11.9

2.3 15.3 5.2 10.9

a

Electroluminescence peak at maximum EQE. bMeasured voltage when luminance was 1 cd/m2. cMaximum current efficiency.

by cold spin-coating and hot spin-coating before lighted up is shown in Figure S4. There was no leakage current for hot spincoating QLEDs, which proves the uniformity in another light. In addition, the Commission International de l’Eclairage (CIE) (x,y) (Figure 4d,e) changes more obviously for QLEDs with cold spin-coating, which moves from (0.67,0.32) at 2 V then to (0.66,0.33) at 4 V ultimately to (0.68,0.31) at 6 V. It may be due to the change of carrier recombination center resulting from the poor morphology. However, for hot spin-coating, the CIE color coordinates are basically steady. The detail is shown in Table S3. It was further proved the device stability of QLEDs by hot spin-coating through the consistency of CIE color coordinates. Besides, to confirm that the substrate defined as 60 °C and PVK m-xylene solution defined as 80 °C was the best condition of hot spin-coating, QLEDs with co-HTLs on different temperature conditions of hot spin-coating were also fabricated except the conditions of substrates were defined as 50 °C, since the aggregation of PVK still existed obviously (circled in red in Figure 2a,d,g) on these conditions. The devices performance is shown in Figure S5. It is apparent that there is little difference on the performance between QLED with co-HTLs on different temperature conditions of hot spincoating. All the luminance is beyond 17 000 cd/m2, and all the maxima EQE are more than 10%, which is much higher than

spin-coating stepwise co-HTLs, all of which are pinhole-free. Besides, the RMS surface roughnesses are approximately 1 nm and most of them are smaller than 1 nm, which ensures low leakage current for highly efficient QLEDs. Figure 4 shows the device performance of stepwise co-HTLs based QLEDs by different techniques. Figure 4a,b presents the current density−voltage and luminance−voltage curves, respectively. In contrast, the turn-on voltage of hot spincoating is 1.75 V, which is the state-of-the-art in deep-red QLEDs, but for cold spin-coating, it rises up to 1.9 V. The current density of cold spin-coating is low; what’s more, the luminance of cold spin-coating is lower at all voltages compared to hot spin-coating technique. The poor performance of the cold spin-coating technique QLEDs was attributed to the poor morphology, which impedes charge injection. Figure 4c presents the EQE and power efficiency (PE) as a function of luminance. The maximum EQE and PE of hot spin-coating are 15.3% and 27.8 lm/W, respectively, which are improved more than 6.5-fold and 6-fold compared to that of cold spin-coating, respectively. The summarized device performance of the QLEDs with different techniques is represented in Table 1. The poor PVK film leads to crude QDs and ZnO films; therefore, the nonradiation recombination was aggravated which resulted in ineffectiveness inevitably. The current density versus voltage characteristics for QLEDs 29079

DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

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Figure 5. Electroluminescence performance of the fabricated QLEDs with single and stepwise co-HTLs. (a) Current and (b) luminance vs voltage characteristics. (c) The EL spectra at a luminance of about 100 cd/m2. (d) EQE−J characteristics. (e) PE−L characteristics. (f) CE−J characteristics.

addition, the conduction band (CB) of ZnO NPs and QDs is 4.4 and 4.8 eV, respectively,30 and the HOMO of poly-TPD, PVK and valence band (VB) of QDs are 5.4, 5.8, and 6.8 eV, respectively.27,28,30 The energy levels and charge mobility were referenced from the previous reports. The energetic barrier of ZnO NPs and QDs (∼0.4 eV) is moderate than that of polyTPD and QDs (∼1.4 eV). So the serious charge imbalance appeared in QLEDs with single HTL. Nevertheless, it was ingenious for stepwise co-HTLs to eliminate the drawbacks of the two materials. On one hand, PVK was thin (∼5 nm) in the co-HTLs structure; therefore, the hole transport capacity of poly-TPD was dominant in co-HTLs, and the hole capacity of co-HTLs was enhanced compared to PVK. On the other hand, the energetic barrier between HTL and QDs became more moderate due to the stepwise construction; hence, the hole injection to QDs was accelerated. Further, the charge injection balance supposed to be essential in efficient QLEDs was ameliorated and the efficiency was promoted eventually. As presented in Figure S6, hole mobility of PVK and poly-TPD was measured by space-charge limited current (SCLC) with the structure of ITO/PEDOT:PSS (40 nm)/HTL (100 nm/ MoO3 (10 nm)/Al (100 nm). The hole mobility was obtained by fitting the space-charge-limited-current region with Child’s law which is described as39

that of the QLED with cold spin-coating co-HTLs. Furthermore, when the substrate is defined as 60 °C and PVK m-xylene solution is defined as 80 °C, the EQE is the highest. The above results demonstrate that hot spin-coating is a practicable technique to fabricate co-HTLs, and the optimized performance can be obtained by temperature regulation to substrate and solution. To further verify the advantage of stepwise co-HTLs, QLEDs with a single HTL of poly-TPD and PVK were fabricated as comparison. The current density−voltage and luminance−voltage curves are shown in Figure 5a,b, respectively. The current density of stepwise co-HTLs based QLEDs was higher than that of single ones, and the turn-on voltage (Von) is much lower than that of PVK only, which indicated that stepwise co-HTLs ensured more charge injection. It was attributed that the step configuration energy level of HTLs provided a buffer for hole transport so that holes transferred to EML more efficiently. The maximum luminance of stepwise co-HTLs based QLEDs is 17 110 cd/m2. Compared with single HTL based QLEDs, the luminance is boosted a lot, in particular for PVK only QLEDs. Figure 5c presents the electroluminescence (EL) spectra at a luminance of about 100 cd/m2; it is obvious that the EL spectra of different HTLs were with the same peak wavelength of 632 nm and the full-width at half-maximum (FWHM) of 39 nm, which demonstrated the comparability between QLEDs based on stepwise co-HTLs and single HTL. The efficiency curves of QLEDs are exhibited in Figure 5d−f. The maximum EQE of the QLED with stepwise co-HTLs represents more than 2.9fold enhancement compared to that of the QLEDs with only poly-TPD as HTL. The summarized device performance of the QLEDs with different HTLs is presented in Table 1. In our QLEDs, the electron mobility20 of ZnO NPs was approximately 1.8 × 10−3 cm2 v−1 s−1, while the hole mobilities of poly-TPD36 and PVK37,38 were 1 × 10−4 cm2 v−1 s−1 and 1× 10−5 ∼ 10−8 cm2 v−1 s−1, respectively. The charge mobility of PVK is 3 orders of magnitude lower than that of ZnO NPs. In

J=

9 V2 ε0εr μh 3 8 D

where J is the measured current density, ε0 is vacuum permittivity, εr is relative permittivity (assumed to be 3.0), and D is the thickness of HTL. The hole mobility of poly-TPD was determined to be 1.7 × 10−4 cm2 v−1 s−1, and the hole mobility of PVK was determined to be 3 × 10−9 cm2 v−1 s−1, which were almost coincident with previous reports.36−38 It is further verified by the hole-only devices (ITO/PEDOT:PSS/HTLs/ QDs/MoO3/Al) in Figure S7 that the current density of the hole-only device with stepwise co-HTLs became higher than 29080

DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

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61674064). In addition, we wish to thank the support facilities at the Center for Nanoscale Characterization & Devices, Wuhan National Laboratory for Optoelectronics of Huazhong University of Science and Technology (HUST) and the Analytical and Testing Center of HUST.

that of the hole-only devices with single HTL, proving the hole injection improvement of stepwise co-HTLs.

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CONCLUSIONS In summary, this work demonstrated a technique to use polyTPD and PVK as stepwise co-HTLs for the highly efficient and low turn-on voltage all-solution-processed deep-red QLED. Stepwise co-HTLs made full use of the merits of poly-TPD and PVK to get more balance of the charge injection in QLEDs. Moreover, by means of the hot spin-coating technique, stepwise co-HTLs were fabricated in high quality. For hot spin-coating, m-xylene was chosen as the appropriate solvent and the best condition of hot spin-coating was explored to that the substrate was heated up to ∼60 °C; meanwhile, the PVK m-xylene solution was warmed up to ∼80 °C. In this condition, the RMS surface roughness of PVK film was as small as 1.01 nm, which was of extremely high quality. As a result, the improved QLEDs performance with a low turn-on voltage of 1.75 V and a peak EQE of 15.3% was achieved. In this work, we provided a technique which can be applied to solutionprocessed multilayer fabrication of the materials insoluble at low temperature and possessing similar dissolution properties. Furthermore, it was manifested that the hot spin-coating strategy was an effective technique to improve the surface morphology and charge balance for high performance and low turn-on voltage QLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06917. Summarized thickness of the multilayers of PEDOT:PSS/poly-TPD/PVK with different solvents for PVK, summarized thickness of the multilayers of PEDOT:PSS/poly-TPD/PVK with the substrate and PVK m-xylene solution at different temperatures, photograph of PVK in m-xylene above and below 70 °C, AFM images and the line-scan profiles of PVK films with different temperatures of substrate-solution when spin-coating PVK, the AFM height images of the multilayers of the films in the device configuration, the J−V characteristics before lighted up for QLEDs with cold spin-coating stepwise co-HTLs and hot spincoating stepwise co-HTLs, the CIE (x,y) for QLEDs with cold spin-coating and hot spin-coating at different operating voltage, the electroluminescence performance of the fabricated QLEDs with co-HTLs on different temperature conditions, the J−V characteristics of holeonly devices of different HTL (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lei Wang: 0000-0002-2141-3256 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (51573065, 51727809, 29081

DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082

Research Article

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DOI: 10.1021/acsami.8b06917 ACS Appl. Mater. Interfaces 2018, 10, 29076−29082