Enhanced performances of quantum dot light-emitting diodes with PFN-adding emitting layer

Accepted Manuscript Enhanced performances of quantum dot light-emitting diodes with PFN-adding emitting layer Ling Chen, Shujie Wang, Kaixuan Zhang, Aqiang Wang, Yan Fang, Zuliang Du PII:

S1566-1199(18)30631-1

DOI:

https://doi.org/10.1016/j.orgel.2018.11.045

Reference:

ORGELE 5008

To appear in:

Organic Electronics

Received Date: 18 October 2018 Revised Date:

12 November 2018

Accepted Date: 29 November 2018

Please cite this article as: L. Chen, S. Wang, K. Zhang, A. Wang, Y. Fang, Z. Du, Enhanced performances of quantum dot light-emitting diodes with PFN-adding emitting layer, Organic Electronics (2018), doi: https://doi.org/10.1016/j.orgel.2018.11.045. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Enhanced performances of quantum dot lightemitting diodes with PFN-adding emitting layer

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Ling Chen, Shujie Wang, Kaixuan Zhang, Aqiang Wang, Yan Fang and Zuliang Du*

*E-mail: [email protected]

ABSTRACT

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Key Laboratory for Special Functional Materials, National & Local Joint Engineering Research Center for High-efficiency Display and Lighting Technology, School of Materials Science and Engineering, and Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, PR China.

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For solution-processed quantum dot light-emitting diodes (QLEDs), improving hole injection efficiency to balance carriers in the emitter is a priority for achieving high device performance. Aiming at this, we introduce a new strategy by utilizing advantage of special properties of poly in

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[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctyl-fluorene)(PFN),

which a small amount of PFN is simply mixed with CdSe/CdS/ZnS QDs (QDs@PFN) as

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emitting layer (EML). For QDs@PFN EML, the PFN additive not only can appreciably upshift the valence band maximum (VBM) of the QDs layer by 0.41 eV through interaction between its lone-pair electrons of the nitrogen atom and the QDs surface, but also maintains efficient electron injection/transport of QDs due to n-type semiconductor property. As a consequence, the hole injection efficiency has been remarkably improved in QDs@PFN EML-based device and then the optimized QLED has been successfully realized with a maximum current efficiency of 16.21 cd A-1 and corresponding to the EQE of 14.77%, meaning a 1.8-fold improvement. The

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results demonstrated that adding a small amount of polymer materials containing aliphatic amine groups into QDs EML could be a simple and effective way to improve hole injection while maintaining effective electron injection/transport in high-performance all-solution-processed

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QLEDs.

KEYWORDS: PFN, mixed EML of QDs and PFN, upshifting of QDs VBM, improving hole

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injection efficiency, QLED INTRODUCTION

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Quantum Dot light-emitting diodes (QLEDs) have attracted tremendous interest due to their outstanding properties such as tunable colors, wide color gamut, inherent photophysical stability, and low-cost solution processability, which highlight their potential applications in the next generation flat-panel displays and solid-state lighting.1-6 Since the first QLEDs reported by A.P.

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Alivisatos,1 to take full advantage of the above excellent characteristics of Quantum Dots (QDs) and thus improve performance of the QLEDs, multilateral efforts have been continued, such as material synthesis optimization,7-9 device structure design3,10-12 and the basic understanding of

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device physics.13,14 Up to now, unbalanced charge injection in the light emitting layer (EML) of the QLEDs is still one of the major obstacle to its development path. The main reason for this

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imbalance is that injection efficiency of holes is always much lower than that of electrons, duo to generally existing large energy barrier at the interface of the QDs and hole transport layer (HTL) in QLEDs.6,15

Hence, hole injection efficiency of the HTL/QDs interface have to be improved greatly in order to balance the carrier injection in QLEDs. For this purpose, three typical strategies have been developed, as follows. (i) Increasing the energy transfer between HTL and QDs EML, e.g.,

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adopting hole transport material-added QDs EML.16-18 Li et al.16 firstly adopted poly vinyl(Ncarbazole)(PVK)-doped QDs as EML to achieve a well-balanced charge injection, in which the PVK dopant can reduce the hole injection barrier and act as a buffer medium between HTL and

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EML for exciton energy transfer. (ii) Lowering of the HTL valence band maximum (VBM) (e.g., adopting a novel HTL with high work functions19 or introducing the hole dopant in the HTL20,21). For instance, Shim et al.20 introduced hole dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-

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dimethane (F4TCNQ) into the HTL to enhance hole injection. Or (iii) Upshifting of the QDs VBM (i.e., introducing an QDs EML through surface modification22or ligand exchange,23-26 or

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inserting interface modification layer27,28). Chot et al.22 showed that cross-linking QDs EML using 1,7-diaminoheptane (DAH) resulted in an appreciably upshift in the QDs valence band level of 0.6 eV, advantageously reducing the hole injection barrier between QDs and HTL. However, the QDs need to be thermal annealed at 180℃, which is not compatible with the

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preparation of today's popular flexible devices. Chae et al.24 demonstrated the valance band shift of 0.3 eV by the benzenethiol ligand exchange, thus decreasing the hole injection barrier from the HTL to the QDs layer. While, the photoluminescence (PL) quantum yields (QYs) of QDs

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was usually affected, to a certain extent, by the means of ligand exchange.29 In 2017, Chae et al.27,28 used a thin insulating layer, polyethylenimine ethoxylated (PEIE) containing an aliphatic

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amine group, as a polymeric surface modifier for inverted QLED to energetically upshift QDs band levels by~0.6 eV, thereby facilitating the hole-injection from HTL to QDs EML. However, the insertion of modified layers generally increases the complexity of device fabrication. In addition, most of the modified layers applied to QLEDs are very thin, making it difficult to precisely control their thicknesses. Moreover, some of the modified materials used in QLEDs are organically insulated, which may increase the driving voltage of the device and result in low

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power efficiency. With these aspects in mind, in order to upshift the QDs VBM and improve hole-injection, finding more efficient materials and new strategies is necessary. As early as 2004, Cao et al.30 first reported the amino-functionalized conjugated (poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctyl-

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polyelectrolyte

fluorene) (PFN) and its quaternizated derivatives as new electron transfer and cathode interfacial modification materials to improve electron injection. They found that the strong interface dipole

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formed at the PFN/Al electrode interface due to metal-polymer interactions can reduce the work function of high work-function metals and promote electron injection from the cathode. Then it

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had been found that PFN could reduce the work function of ITO30,31 and ZnO32-34. In this work, we introduce a new strategy to improve charge injection balance by using advantage of PFN special properties, in which PFN additive is simply mixed with CdSe/CdS/ZnS QDs (QDs@PFN) as EML by solution processing. For QDs@PFN EML, the

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PFN additive not only can upshift the QDs VBM of 0.41 eV through interaction between its lone-pair electrons of the nitrogen atom and the QDs surface, but also maintains efficient electron injection/transport of QDs due to n-type semiconductor property. Additionally, our

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strategy, PFN and QDs co-solubilization as EML, does not complicate the device fabrication process and can also improve the film morphology of the device to some extent. In our

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QDs@PFN EML-based device, the hole injection efficiency of the HTL/QDs interface has been remarkably improved, resulting in enhancement of the device performance. As a consequence, optimized QLED has been successfully realized with a maximum current efficiency of 16.21 cd A-1 and corresponding to the EQE of 14.77%. The EQE is 1.8 times as high as that (8.32%) of the reference QLED. The results have demonstrated that adding a small amount of polymer materials containing aliphatic amine groups into QDs EML could be a simple and effective way

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to improve hole injection while maintaining effective electron injection in high-performance allsolution-processed QLEDs. EXPERIMENTAL SECTION

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Synthesis of CdSe/CdS/ZnS core–shell and ZnO Nanoparticles. The red CdS/CdSe/ZnS QDs were synthesized according to our previous works.35 The colloidal ZnO nanoparticles (NPs) was synthesized following the reported method.36

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Fabrication of QLED Device. The ITO-coated glass substrates (sheet resistance ~ 15 Ω sq-1)

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were cleaned, sequentially, in ultrasonic baths of detergent, de-ionized water, acetone, and isopropanol for 15 min each, and then were immediately exposed to an ultraviolet ozone ambient for 15 min. After the above cleaning, PEDOT:PSS solutions (Heraeus, Clevios™ P VP.AI 4083) were spin-coated onto the substrates at 5000 rpm and baked at 140 oC for 15min. The substrates were then transferred to a N2-filled glovebox for spin-coating of TFB (ADS 254BE), QDs and

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ZnO NPs layers. The TFB dissolved in chlorobenzene with a concentration of 8 mg mL-1 was spin-coated at 3000 rpm and annealed at 150 oC for 30 min. Then, the red QDs or QDs@PFN (purchased from 1-materials Inc., with different weight ratios) were dispersed in chloroform with

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a concentration of 15 mg mL-1and then spin-coated at 3000 rpm for 60s. After that, the ZnO NPs

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as electron transport layers (ETLs) were deposited at a speed of 1500 rpm for 45 s from a 30 mg/mL ethanol solution. The multilayer samples were then loaded into a high-vacuum deposition chamber (at a pressure of ≤1 × 10−6 mbar) to thermally deposit the Al cathode (100 nm). Finally, the devices were simply encapsulated by the cover glasses using ultraviolet-curable resin.

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Characterization Transmission Electron Microscope (TEM) images of QDs were recorded by using a JEOL JEM-2100 electron microscope. The absorption spectra of the QDs, PFN, QDs@PFN and ZnO solutions was measured by using an UV-vis spectrometer (Lambda 950,

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PerkinElmer, USA). PL spectra and time-resolved PL of samples were carried out using a spectrofluorometer (JY HORIBA FluoroLog-3). A scanning electron microscope (SEM) (Nova Nano SEM 450) was used to measure the thickness of the individual layer of the multilayered

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QLED. Atomic force microscopy (AFM) (Dimension Icon) was used to obtain the surface topography images. Ultraviolet photoemission spectroscopy (UPS) was performed using the He I

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discharge lamp (hν = 21.2 eV) at under high vacuum∼2.5*10-8 mbar (Thermo Scientific ESCALAB 250 XI). The J–V–L characteristics and electroluminescence spectra of QLEDs were collected under ambient condition using a Keithley 2400 sourcemeter and a PhotoResearch spectrometer PR-735.

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RESULTS AND DISCUSSION

A schematic of the device structure of the multilayer red-emitting QLEDs studied herein and

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the corresponding cross-sectional scanning electron microscopy (SEM) image are shown in Figure 1(a) and Figures S1, respectively. In this work, QLEDs with typical structures of

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ITO/PEDOT:PSS/poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,40-(N-(4-butylphenyl)))](TFB) /QDs or QDs@PFN /ZnO/Al were fabricated by solution processing, where a transparent ITO electrode serves as an anode and Al as a cathode. We adopted ZnO NPs as electron transport layer (ETL) and the conjugated polymer TFB as the HTL. It is well known that the ZnO NPs is highly advantageous for our QLEDs with conventional structures due to its high electron mobility (2 × 10-3 cm2V-1 s-1) and low temperature treatment characteristics.2 TFB was chosen for HTL as a counterpart of ZnO ETL and its hole mobility (1 ×10-2cm2V-1 s-1) is comparable to

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or even better than the electron mobility of ZnO ETL.37,38 Aiming at improving the hole injection efficiency at TFB/QDs interface and thus achieving carriers balance of the emitter, we introduced CdSe /CdS /ZnS QDs mixed with different weight ratios of PFN as EMLs. The

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molecule structure of the PFN was also shown in the inset of Figure 1a. We can see that the lonepair electrons of the nitrogen atom in amine group of PFN could be prone interact with QDs surface,39 leading to the change in work function of QDs. Figure 1(b) shows the absorption and

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PL spectra of the core–shell QDs dispersed in chloroform, as well as the PL spectrum of PFN in methanol. The QDs shows PL peak at 630 nm with a narrow full width at half maximum

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(FWHM) of about 32 nm. It can be observed from the transmission electron microscope (TEM) image shown in Figure S2 that the QDs has a uniform size distribution with an average diameter of 8 nm and exhibits lattice fringes throughout the whole QDs.

The effect of the QDs@PFN EML on device performance is exemplified based on a series of

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QLEDs. We compared the device characteristics of the QLEDs based on the QDs@PFN with varied weight ratios (0.33wt%, 0.67wt% and 1.0wt%) or the QDs only. Except for the above differences, all devices were fabricated with the same parameters. As the current density-voltage-

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luminance (J-V-L) characteristics of QLEDs with different EMLs shown in Figure 2(a), when the drive voltage is lower than the turn-on voltage, i.e., Ohmic conduction region, the current density

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decreases as the mixing concentration of PFN increases, indicating that the leakage current of the QLEDs can be suppressed by mixing PFN into QDs. This reduction in leakage current is likely due to the improved morphology of the EMLs, which will be discussed in later sections. Meanwhile, from the J-V curve, we also found that the turn-on voltages for all devices is almost the same value of about 1.7 V. This may be thanks to the non-insulating characteristics of the PFN itself and its small amount of mixing. The turn-on voltage appears to be slightly less than

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the QDs bandgap voltage (or bandgap energy ~1.92 eV), which may be due to the Auger-assisted energy up-conversion process at the QDs/ZnO interface.40 Under the maximum voltage (approximately 5V), the maximum brightness of the devices based on QDs@PFN EMLs

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exhibited varying degrees of improvement compared to that of the control device. Especially for the improved QLEDs with 0.67 wt% PFN shows an Lmax of 54 360 cd m-2, that is, a 1.6-fold

PFN leads to a notable decrease in luminance.

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increase, compared to the reference device. However, a further increase in the weight ratios of

Figures 2(b) shows electroluminescence (EL) spectrum of the QLED based on QDs@PFN

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EML under an applied voltage of 4 V. The spectrum shows a main peak (λ =638 nm) with a narrow FWHM of 33 nm arising from only QDs without contribution from the PFN additive or TFB layer, which corresponds to the Commission International de l’Eclairage (CIE) color coordinates of (0.69, 0.31), as shown in Figure 2(c). Further, the EL spectra of QLEDs based on

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QDs only and the 0.67 wt % QDs@PFN EMLs are also shown at half-exponential coordinates in Figure S3, and no EL signals from 400 to 550 nm are observed. The photograph of the high brightness QLED is displayed in the inset of Figure 2(c). In short, by mixing PFN into the EML,

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it does not “disturb” the color purity of the red-emitting QLED. Figures 2(d-f) demonstrate the external quantum efficiency (ηEQE), current efficiency(ηC) and

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power efficiency (ηP) versus luminance of these devices with the different EMLs, respectively. A more detailed comparison of these device performances is listed in Table 1. It is not difficult to see that the trends of the ηEQE-L, ηC-L and ηP-L curves are similar, and the enhancement of efficiencies is very sensitive to the amount of mixed PFN. The best performance is achieved in 0.67 wt% QDs@PFN EML-based device. Correspondingly, the maximum ηEQE, ηC and ηP reaches 14.77 %, 16.21cd A-1 and 19.59 lm W-1, respectively, yielding an enhancement factor of

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about 2 compared with the reference device (a maximum ηEQE of 8.12 %, ηC of 9.13 cd A-1and corresponding ηP of 11.95 lm W-1). The excellent reproducibility of device performance has also been fully demonstrated, as shown in Figure S4. Notably, the efficiencies of improved QLEDs

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based on QDs@PFN EML are always higher than those of control device over the entire brightness range of 100-50000 cd m-2. Moreover, the introduction of PFN can also effectively suppress the efficiency roll-off phenomenon of the devices under high brightness (or high

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voltage). However, a further increase in the amount of the PFN added will lead to a decline in the ηEQE, ηC and ηP. This may be due to the presence of the wider barrier width caused by excess

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PFN itself.32

To obtain insights into the role of PFN-induced energy level shift in the efficiency improvement, the energy level values of the QDs only and QDs@PFN were characterized by ultraviolet photoemission spectroscopy (UPS) measurement, as displayed in Figure 3(a). From

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the UPS spectra, the high-binding energy secondary-electron cutoff (Ecutoff) of QDs obviously varied from 18.52 eV to 18.93 eV after mixing with a small amount of PFN. According to the equation of φ = hν - |Ecutoff – Eonset|, where Eonset is the valence band onset energy, the QDs VBM

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levels were calculated and shifted by ∼0.41 eV from 6.45 to 6.04 eV. It has been reported that conjugated polyelectrolyte PFN is commonly used as an interface dipole layer to achieve a

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vacuum level shift of more than 0.5 eV.30-34 Therefore, the reduction in QDs VBM may be related to the formation of interfacial dipoles induced by the amine groups of PFN. Then, the energetic alignments of the multilayer QLEDs with and without PFN are compared in Figure 3(b). The energy levels of the ZnO ETL was obtained using UPS and optical measurements (See Figures S5, Supporting Information), while other energy level values were taken from the literature.6,32 From the energy level diagram of the device, the upshift VBM of the QDs caused

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by PFN additive can effectively reduce the hole injection barrier of 0.41eV at the interface of HTL/QDs, thereby effectively improving the balanced injection of carriers. To reveal the carriers injection/transport characteristics of QDs EMLs with or without PFN,

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hole-only devices with structures of ITO/PEDOT:PSS/TFB/QDs or QDs@PFN /MoO3/Al and electron-only devices with structures of Al/ZnO/QDs or QDs@PFN /ZnO/Al were fabricated and characterized. Figure 3c shows the J−V characteristics of the electron-only and hole-only

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devices. The current density of the electron-only device based on QDs only is much larger (about 3 orders of magnitude) than that of the hole-only device based on QDs only, as reported by many

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groups.6,18 Clearly, with the mixing of PFN, the hole current is remarkably increased and the electron current remains almost unchanged, that is, PFN can simultaneously induce the increase of hole injection from TFB to QDs EML and maintain the effective injection/transport of electrons in emitter. Furthermore, in the high voltage region (4-5 V), the electron current is larger

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than the hole current by only less than one order of magnitude. This trend may explain the suppressed efficiency roll-off of QLEDs based on QDs@PFN EML at high voltage or high brightness. In short, the upshift of QDs energy level after the introduction of PFN into the EML

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does allow for a more balanced carrier injection for our device and, thus improving device performance, as we expected.

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Herein, we also performed atomic force microscopy(AFM) measurements to investigate the roughness/morphology of various EMLs and ETLs, as shown in Figures 4(a-b). It can be observed that both QDs only and QDs@PFN films exhibit similar relatively low roughness. The measured of surface root-mean-square (RMS) roughnesses for QDs only and QDs@PFN films were 1.60 and 1.42 nm, respectively. It should be noted here that the small amount of PFN mixed into the QDs is not sufficient to cause significant phase separation of the EML. Conversely, a

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slightly improvement in the morphology of QDs film indicates that mixing a small amount of polymer into QDs may reduce the probability of electrons being transferred directly from ZnO ETL to TFB HTL and, thereby reducing leakage current of device (See Figure 2(a)). After being

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covered with ZnO ETL with good film forming properties, the RMS values of the samples showed a further decrease (Figure 4(c-d)). Obviously, simply mixing PFN with QDs EMLs can effectively control the morphology of the films in the device and reduce the leakage current of

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device.

In order to investigate the effect of PFN on fluorescence emission, the time-resolved (TR) PL

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curves of the ITO/HIL/HTL/TFB/QDs and ITO/HIL/HTL/TFB/ QDs @PFN samples were measured and plotted in Figure 5. The lifetimes obtained from biexponential fitting are listed in Table S1. The excitation source wavelength used is 470 nm, which is lower than the absorption edge of TFB or PFN, avoiding the contribution of energy transfer from TFB or PFN to QDs. The

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PL decay lifetime of QDs films increases only slightly from 12.18 ns to 12.77 ns after mixing with a small amount of PFN. Generally, PL lifetime of QDs would increase in varying degree when mixed with polymer26,41, which would reduce the fluorescence emission efficiency of QDs.

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The results of TR PL show that the adding PFN has little effect on the emission property of device. That is to say, the adding PFN has more beneficial to improve the device performance, in

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which the hole injection efficiency can remarkably increase resulting from upshifting the VBM of QDs.

CONCLUSIONS

We have demonstrated that simply mixing a small amount of PFN into the QDs EML can simultaneously improve hole-injection efficiency and ensure efficient electron-injection, thereby remarkably increasing the carriers balance in the emitter and boosting the device performance.

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Impressively, with an optimal QDs@PFN EML (0.67wt%), the device shows record maximum efficiency values of 16.21 cd A-1 in ηC and 14.7% in ηEQE. The ηEQE is 1.8 times as high as that of

QLEDs for next-generation display and solid-state lighting. Notes

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The authors declare no competing financial interest.

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the reference QLED. Our results show a simple but efficient strategy toward high-performance

ACKNOWLEDGMENT

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This research was supported by National Natural Science Foundation of China (Grant No. U1604261, 61605041), Program for Changjiang Scholars and Innovative Research Team in University (No. PCSIRT15R18).

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(2009) 2636-2640.

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Figure Captions

Figure 1. (a) A schematic of the device structure of the multilayer red-emitting QLEDs studied herein. The inset shows the molecules structure of PFN and schematic illustration of the red CdSe/CdS/ZnS core–shell QDs. (b) PL and absorption spectra of the QDs in chloroform, and PL

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spectrum of PFN in methanol. The inset shows the image of the QDs dispersed in chloroform.

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Figure 2. (a) Current density−voltage− luminance characteristics of QLEDs. (b) EL spectra of

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QLEDs at 4 V. (c) The CIE color coordinates of QLEDs. The inset is the photograph of device. (d)The external quantum efficiency, (e) current efficiency and (f) power efficiency versus luminance of QLEDs.

Lmax (cd.m-2)

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Table 1. Summary of the device performances of QLEDs. ηP (lm W-1)

ηA (cd A-1)

ηEQE (%)

Control

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11.95

9.13

8.32

0.33 wt% PFN

39860

16.45

11.52

10.56

0.67 wt% PFN

54360

19.59

16.21

14.77

1.00 wt% PFN

34030

18.28

15.13

13.79

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Figure 3. (a) Ultraviolet photoemission spectroscopy (UPS) spectra of high-binding energy

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secondary-electron cutoff and valence band edge regions of the QDs layer without and with PFN. (b) Proposed energy band diagram of the multilayer red-emitting QLED. (c) Current

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density − voltage characteristics of the hole-only and electron-only devices.

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Figure 4. AFM images of (a)QDs and (b) [email protected] wt % PFN EMLs. AFM images of the

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formed ZnO ETLs on (c) the QDs only and (d) [email protected] wt % PFN EMLs.

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Figure 5. Time-resolved PL decay and the fitting curve of the solid film samples of ITO/HIL/

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HTL/QDs and ITO/HIL/HTL/QDs@PFN (λexc = 470 nm, λdet = 630 nm).

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4. Carrier injection balance improves quantum dot light-emitting diode’s performance.