Polyethylenimine Ethoxylated-Mediated All ... - ACS Publications

Feb 10, 2017 - of thick small molecules on AgNWs31 have been performed, which are ... were spin-deposited on top of a 27 nm-thick QD layer, ..... tum ...
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Polyethylenimine Ethoxylated-Mediated AllSolution-Processed High-Performance Flexible Inverted Quantum Dot-Light-Emitting Device Daekyoung Kim,† Yan Fu,‡ Sunho Kim,∥ Woosuk Lee,‡ Ki-Heon Lee,§ Ho Kyoon Chung,† Hoo-Jeong Lee,†,∥ Heesun Yang,*,§ and Heeyeop Chae*,†,‡ †

Sungkyunkwan Advanced Institute of Nanotechnology, ‡School of Chemical Engineering, and ∥School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Korea § Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea S Supporting Information *

ABSTRACT: We report on an all-solution-processed fabrication of highly efficient green quantum dot-lightemitting diodes (QLEDs) with an inverted architecture, where an interfacial polymeric surface modifier of polyethylenimine ethoxylated (PEIE) is inserted between a quantum dot (QD) emitting layer (EML) and a hole transport layer (HTL), and a MoOx hole injection layer is solution deposited on top of the HTL. Among the inverted QLEDs with varied PEIE thicknesses, the device with an optimal PEIE thickness of 15.5 nm shows record maximum efficiency values of 65.3 cd/A in current efficiency and 15.6% in external quantum efficiency (EQE). All-solutionprocessed fabrication of inverted QLED is further implemented on a flexible platform by developing a high-performing transparent conducting composite film of ZnO nanoparticles-overcoated on Ag nanowires. The resulting flexible inverted device possesses 35.1 cd/A in current efficiency and 8.4% in EQE, which are also the highest efficiency values ever reported in flexible QLEDs. KEYWORDS: quantum dot-light-emitting diode, all-solution-processed, inverted architecture, flexible device

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also made toward a higher-efficiency device specifically by adopting a blended or bilayered CTL of two types of either HTL10,11 or ETL materials12 or introducing an electron injection-boosting additive13 or electron injection-limiting layer.11 For instance, an ultrathin insulating poly(methyl methacrylate) (PMMA) layer was introduced between a red CdSe/CdS QD emitting layer (EML) and a ZnO NP ETL to optimally limit the electron flow into the EML region for the mitigation of AR-inducing QD charging, leading to a recordhigh external quantum efficiency (EQE) of a nearly theoretical maximum of 20.5%.11 Multilayered QLEDs are typically constructed with either standard or inverted device structure. An inverted structure is considered to be more advantageous over a standard one in a display manufacturing aspect, since the transparent bottom cathode of inverted QLED can be directly linked to the drain line of a n-type thin-film transistor.14 Meanwhile, with regard to

olloidal quantum dot-light-emitting diodes (QLEDs) has emerged as an alternative to organic light-emitting diodes (OLEDs) for next-generation displays. This is mainly because compared to fluorescent/phosphorescent organic emitters, the quantum dots (QDs) possess highly color-pure emissions, enabling the fabrication of QLEDs with superb color reproducibility. A notable improvement in QLED performance has been achieved by applying an inorganic ZnO nanoparticle (NP)-based electron transport layer (ETL) along with an organic hole transport layer (HTL).1 Since then, based on such a hybrid combination of charge transport layer (CTL), device performance in luminance and efficiency could be further upgraded by elaborately tailoring the compositiongradient core/shell interface, the composition of the intermediate shell,2−6 and the thickness of the outer shell of the QD structure,4,5,7−9 which are considered to play a key role in effectively suppressing nonradiative events of Auger recombination (AR) and Förster resonant energy transfer (FRET). Besides the rational redesigning of QD materials above, a parallel effort on the improved carrier balance via control of electron and/or hole flows across the device has been © 2017 American Chemical Society

Received: December 5, 2016 Accepted: February 10, 2017 Published: February 10, 2017 1982

DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990

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ACS Nano

which are involved with complicated and expensive processes. The surface roughness of AgNWs film might be somewhat improved by overcoating them with sputtered metal oxides, however, the sputter deposition could provide only a conformal coating without a complete filling of empty space between AgNWs, resulting in a still rugged morphology of the overcoated films.28,29 In this work, we exploit the same ZnO NPs as used for the formation of ETL of QLED to fill interAgNW voids via solution processing. This composite film of ZnO NP-overcoated AgNWs exhibits superiority to pristine AgNWs film in not only sheet resistance and surface roughness but also thermal and adhesive stability and is ultimately utilized as an FTE for the fabrication of a highly efficient all-solutionprocessed flexible inverted QLED.

device processing, a standard structured device can be fabricated by all-solution processing typically via a sequential spin-coating of individual layers, while in an inverted-structured one, usually ETL and QD EML are successively spin-casted, and then the following HTL and hole injection layer (HIL) are thermally evaporated.3,4,14,15 Such a partial solution processability of an inverted QLED mainly stems from the absence of orthogonal solvents between QDs and HTL materials. That is, upon solution processing of HTL onto the previously deposited QD EML, the HTL solvent severely, inhomogeneously dissolves the EML underlayer, leading to the destruction of EML and/or formation of mixed EML/HTL. Without solving the above issue, the fabrication of inverted QLEDs through sequential solution processing of EML and HTL was attempted, however, as expected, device performance of the resulting devices was quite limited, only reaching maximum current efficiencies of 0.69−1.53, 1.26−2.81, and 0.006−0.06 cd/A for red, green, and blue devices, respectively.16−18 Herein, we demonstrate an all-solution-processed fabrication of highly efficient inverted green QLEDs. In this multilayered device, a polymeric surface modifier of low-cost, environmentally friendly polyethylenimine ethoxylated (PEIE) is placed between the QD EML and HTL, and a HIL of MoOx is solution deposited on top of the HTL. Recently, a PEIE containing an aliphatic amine group has been universally employed to reduce the work functions of various electrodes, thereby enabling the efficient electron-selective electrodes in inorganic optoelectronic devices. The neutral amine groups of the PEIE physisorbed on the electrode are considered to induce the formation of the interface dipoles, leading to the substantial change in work function.19 In our inverted QLED, the presence of a thin PEIE layer indeed proved to be highly effective not only as a hydrophilic buffer layer to prevent predeposited QD EMLs from being damaged upon HTL processing but also as an interfacial dipole layer to upshift the valence band maximum (VBM) of QDs and thus facilitate a hole injection from HTL to QD EML. The present PEIE-mediated inverted green QLED possesses record high peak quantities of 65.3 cd/A in current efficiency and 15.6% in EQE, which are overwhelmingly higher than those of the control device without PEIE by over 20 times. Subsequently, we further demonstrate the fabrication of an all-solution-processed inverted QLED onto a flexible substrate through the development of an alternative transparent electrode to a brittle ITO cathode. As a flexible transparent electrode (FTE), silver nanowire (AgNW) may be a promising candidate compared to conductive polymer,20 carbon nanotube,21 graphene,22 and metal grid,23 considering the aspects of transmittance, sheet resistance, and fabrication cost. However, a simple assembly of AgNWs appears unsuited as an FTE film because of its still high sheet resistance from the contact resistance of the inter-NW junction, high surface roughness from the stacking of a few NW layers, and poor adhesion with flexible substrates. The sheet resistance of AgNWs film has been reduced by applying plasmonic welding24 or pulsed light sintering25 or forming their composites with graphene,26 graphene oxide,27 or sputtered/sol−gel-derived metal oxides.28,29 However, some of these approaches require a highenergy consumption or long-duration treatment, while others give some scalability and chemical damage issues. The film surface of multistacked AgNW networks is inherently rough and thus needs to be flattened for device fabrication. For this, the transfer of AgNWs from glass to polymer30 and evaporation of thick small molecules on AgNWs31 have been performed,

RESULTS AND DISCUSSION A major concern in fabricating an inverted QLED via allsolution processing is physical damage of the QD EML underlayer occurring upon the subsequent HTL deposition, since the organic solvents (e.g., chlorobenzene and chloroform) commonly used to dissolve HTL conjugated polymers, such as poly(9-vinlycarbazole) (PVK), poly(N,N′-bis(4-butylphenyl)N,N′-bis(phenyl)benzidine (poly-TPD), and poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine) (TFB), also dissolve hydrophobic QDs, leading to the destruction of QD EML and/or interlayer mixing. To solve this matter, we spin-casted an interlayer of PEIE on a QD layer, followed by poly-TPD deposition. In the course of such successive spincasting steps, 2-methoxyethanol, the PEIE, did not dissolve QDs at all, and the PEIE formed functioned as a robust physical barrier to well preserve the QD layer underneath it upon contact with chlorobenzene for poly-TPD deposition. The PEIE solutions with different concentrations from 0.5 to 2 wt % were spin-deposited on top of a 27 nm-thick QD layer, resulting in the increased PEIE thickness from 7.5 to 27.6 nm (Figure S1). Among the above samples of glass/QD layer/ PEIE, 7.5 and 15.5 nm-thick PEIE ones were subjected to a solvent resistivity test by spin-casting a pure chlorobenzene on them. The degree of damage of QD layer from chlorobenzene rinsing was indirectly evaluated by measuring and comparing the photoluminescence maximum intensity (PMI) before versus after the rinsing (Figure S2). After chlorobenzene rinsing, the QD layer without PEIE displayed a dramatic PL reduction by 69%. In the case of the 7.5 nm-thick PEIE, PMI decreased by 33%, and such a noticeable PL reduction is attributable to the incomplete coverage of PEIE overlayer on QD layer. On the other hand, when applying a thicker PEIE of 15.5 nm, a negligible PMI reduction was observed after rinsing, suggesting that the reasonable PEIE thickness is in the range of 7.5−15.5 nm. In the case of II−VI QLEDs with hybrid CTLs of the organic HTL and the ZnO NP ETL, the band offset at the HTL/QD EML is much larger than that at the QD EML/ETL, leading to the unbalanced carrier injections to QD EML. Then, QDs turn charged with an excess of electrons, by which nonradiative AR becomes highly efficient, limiting the device performance.2,3,5 Therefore, to reduce the hole injection barrier at the HTL/QD EML, either lowering of the HTL VBM or upshifting of the QD VBM would be highly desirable for the improvement of device performance. The choice of HTL conjugated polymers (i.e., PVK, poly-TPD, TFB) used for QLED fabrication is quite limited, and their VBM levels similarly range from 5.2 to 5.4 eV. On the other hand, the VBM of QD could be rather 1983

DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990

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Figure 1. (a) UPS spectra showing high-binding energy secondary-electron cutoff (left) and valence band edge regions (right) of the QD layer as a function of PEIE overlayer thickness of 0−27.6 nm. (b) PEIE overlayer thickness-dependent variation of valence band maximum (VBM) level of QD layer. (c) Proposed energy band diagram of inverted multilayered QLED showing a VBM upshift of QDs by 0.6 eV when a 15.5 nm-thick PEIE was applied. (d) Cross-sectional TEM image of inverted device without PEIE interlayer. Inset: stacking sequence of the device. (e) Cross-sectional TEM image of inverted device with 15.5 nm-thick PEIE interlayer. Inset: stacking sequence of the device.

appreciably shifted by ∼0.6 eV toward the vacuum level by a cross-linking strategy using an organic surface modifier of 1,7diaminoheptane (DAH), favorably reducing the energy band offset between the QD EML and the neighboring HTL.32 Using ITO glass/QD layer/PEIE samples with different PEIE thicknesses, we examined the effects of PEIE on the VBM level of the QD layer by ultraviolet photoelectron spectroscopy (UPS) measurements. As shown in UPS spectra near the highbinding energy cutoff (Ecutoff) and the onset energy in valenceband edge (Eonset) (Figure 1a) regions, Ecutoff notably varied with the PEIE thickness from 18.55 eV for the sample without PEIE and to 19.3 eV for one with 27.6 nm-thick PEIE, while Eonset of 4.05 eV was nearly independent of PEIE thickness. The QD VBM levels were calculated according to the equation of VBM = 21.2 − (Ecutoff − Eonset) and found to be 6.7, 6.25, 6.1, and 5.95 eV for the samples without PEIE, with 7.5, 15.5, and 27.6 nm-thick PEIE, respectively (Figure 1b). Then, the CBM levels of QDs were determined by using the above VBM information and UV−visible absorption spectrum (Figure S3). The energetic alignments of green QDs without PEIE and with 15.5 nm-thick PEIE relative to poly-TPD are compared in Figure 1c, showing significant upshifts of VBM and CBM by 0.6 eV after PEIE introduction that enable a more efficient HTLto-QD EML hole injection by means of the reduced hole injection barrier. Note here that in spite of a physically

bilayered structure of QD/PEIE, QD/PEIE was expressed as an one layer component in Figure 1c with an intention to highlight the effect of PEIE on the upshift of VBM/CBM levels of QD. Analogous to the function of the earlier DAH molecule on the modulation of the energetic positions,32 the reduction of QD VBM is presumably correlated to the formation of interfacial dipoles induced by the amine group of PEIE. To examine the effect of PEIE on the dynamics of hole transfer from QD to poly-TPD, the average PL lifetimes (τ) of the QD layer only, QD layer/poly-TPD, and QD layer/15.5 nm-thick PEIE/polyTPD were measured (Figure S4a), showing 8.22, 6.34, and 5.89 ns, respectively. It is noted that the τ value of QD layer/polyTPD, in which two phases of QDs and poly-TPD are intermixed to a certain degree (as sensed from Figure 1d), was comparable to that (5.4 ns) recorded from CdSe/ZnS core/shell QDs embedded in TPD small molecule.33According to the energy band diagram of Figure 1c, the insertion of PEIE led to the reduction of the valence band offset at QD/polyTPD. Bearing in mind that the energetic offset is a driving force for charge transfer at that interface, the PL lifetime of QD/ PEIE/poly-TPD with a smaller valence band offset is expected to be in principle longer than that of QD/poly-TPD, which is contrary to the experimental results. Such an inconsistency can be understood by taking into account that the insertion of PEIE layer would rather promote the trapping of the charges 1984

DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990

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ACS Nano photoexcited at QDs possibly at QD/PEIE interface and/or inside PEIE. This charge trapping may be a dominant process over the energetic offset in determining the QD exciton dynamics, consequently leading to a shortened τ from QD/ PEIE/poly-TPD versus QD/poly-TPD. The present green QDs consisted of composition gradientshelled CdSe@ZnS QDs plus an additional ZnS overcoat and exhibited an excellent PL quantum yield (QY) of >90% and an average diameter of 13.5 nm, which has been determined jointly by transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis (Figure S5). In particular, the resulting multishelled CdSe@ZnS/ZnS QDs comprised a thick ZnS outer shell along with a compositiongradient (CdSe-ZnS) alloyed intermediate shell, which may be a desirable QD structure toward the demonstration of highperformance QLEDs. The thick shell would function as a physical barrier to suppress inter-QD FRET among the densely packed QDs of the EML.5,7 The presence of an alloyed intermediate shell, which renders core/shell interfacial potential smooth, has been found to be effective in minimizing the probability of an AR event occurring efficiently at charged QDs of the EML.3,34,35 The surface of the as-coated QD EML is naturally expected to be somewhat bumpy mainly due to the large size of spherical QDs. The surface roughness of the QD EML was assessed by atomic force microscopy (AFM), showing a root-mean-square roughness (Rrms) of 2.72 nm (Figure S6a). Upon PEIE overcoating, the Rrms value tended to gradually decrease with increasing thickness of PEIE, i.e., from 2.39 to 1.09 nm for 7.5 nm- and 27.6 nm-thick PEIE overlayers, respectively (Figure S6b−d), indicating that the PEIE overcoating adjunctively contributes to the QD EML planarization, which can in turn afford the uniform hole injection. Three green inverted QLEDs with a layer sequence of ITO cathode/40 nm ZnO NP ETL/27 nm QD EML/7.5, 15.5, or 27.6 nm PEIE/40 nm poly-TPD HTL/10 nm MoOx HIL/Al anode along with the control device without PEIE interlayer were fabricated via all-solution processing except for Al. It should be stressed here that the MoOx HIL was also solutiondeposited using phosphomolybdic acid hydrate ((MoO3)12· H3PO4·(H2O)x, PMAH) to achieve a full-solution processing capability, which is in contrast to the earlier work on inverted QLEDs based on a vacuum-deposited MoOx.3,4,14 Some metal oxides such as MoO336,37 and WO338,39 have been solutiondeposited in the form of either precursor solution or NP dispersion as alternative charge generation layers to PEDOT:PSS in optoelectronic devices. Among them, very recently, PMAH-based MoOx has been successfully applied as a hole extraction layer in organic solar cells40 and as an electron acceptor layer in multistacked OLEDs.41 Figure 1d shows a cross-sectional TEM image of the control device above. The QD EML was markedly damaged and intermixed with polyTPD, as expected, and furthermore, a partial direct contact of poly-TPD with ZnO ETL cannot be also excluded. In a sharp contrast, both EML and HTL of an inverted device inserted with a thin (15.5 nm) PEIE layer at the QD EML/poly-TPD HTL interface remained undamaged (Figure 1e). Performance of all four QLEDs was compared by analyzing current density (J), luminance (L), and voltage (V) characteristics (Figure S7a−c). J−L−V characteristics of two representative devices without PEIE versus with 15.5 nm-thick PEIE were separately shown in Figure 2a,b to clearly highlight a dramatic effect of the presence of PEIE on device performance. The inverted QLED with a 15.5 nm-thick PEIE exhibited much

Figure 2. (a) Current density−luminance−voltage characteristics of inverted QLEDs without and with 15.5 nm-thick PEIE interlayer. (b) Current efficiency−EQE−luminance characteristics of those two devices. (c) A representative EL spectrum of 15.5 nm PEIEbased inverted device collected at 7 V (left) and corresponding CIE (1931) color coordinates (0.157, 0.786) marked with a star (right).

lower current densities than the control device without PEIE (Figure 2a), even though the hole injection from HTL to QD EML became facilitated from the former device by an interfacial PEIE layer-mediated QD VBM upshift (Figure 1c). This can be unambiguously understood with a crucial role of PEIE as a robust physical barrier to well preserve QD EML against solvent attack during the following HTL deposition, leading to significantly reduced leakage currents across the device. The device without PEIE exhibited a maximum luminance of 14,643 cd/m2 at an excessively high current density of 3474 mA/cm2. Meanwhile, one with 15.5 nm PEIE showed a maximum 1985

DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990

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Figure 3. (a) SEM images of pristine AgNWs films deposited with 0.25 (left), 0.5 (middle), and 1 wt % (right) dispersion on PU-coated PET substrate. (b) Variations of transmittance and sheet resistance of pristine AgNWs and ZnO NPs-AgNWs films as a function of AgNW concentration. Note that the transmittance values of (b) correspond to the films only with PU-coated PET substrate excluded. (c) Low- (left) and high-magnification (right) TEM images of ZnO NPs-AgNWs composite, showing that AgNWs were well in contact with ZnO NPs, and inter-NW voids were nearly continuously filled with ZnO NPs.

luminance of 110,205 cd/m2 at a relatively low current density of 574 mA/cm2 (Figure 2a). As sensed from such greatly different J−L−V results, the QLED with 15.5 nm PEIE outperformed the control device without PEIE with respect to device efficiency. The maximum current efficiency, EQE, and power efficiency of the latter device only reached 2.7 cd/A, 0.6%, and 0.9 lm/W, respectively, whereas the former device displayed huge improvements in maximum values, i.e., current efficiency of 65.3 cd/A, EQE of 15.6% at a current density of ∼3 mA/cm2, a luminance of ∼2000 cd/m2 (Figures 2b and S7d), and power efficiency of 29.3 lm/W (Figure S8). These outstanding peak efficiencies of our PEIE-mediated green QLED are higher by nearly three times as compared to the earlier green inverted device (maximum current efficiency and EQE of 19.2 cd/A and 5.8%, respectively) fabricated by the sequential thermal evaporation of 4,4′-bis(carbazole-9-yl)biphenyl (CBP) HTL and MoO3 HIL14 and comparable to the recent record values (maximum current efficiency and EQE of 63 cd/A and 14.5%, respectively) obtained from a standard green device based on hybrid CTLs of TFB HTL/ZnO NP ETL and PEDOT:PSS HIL.6 This is attributable to the key multiple roles of PEIE of (i) a robust physical barrier to prevent the interlayer mixing and EML damage upon successive HTL deposition, (ii) an interfacial dipole layer to upshift the VBM of QD and thus to facilitate the hole injection from HTL to QD EML and improve the charge balance at QD EML, and (iii) a surface smoother to level the QD EML underneath it, favorable for a uniform charge injection. Compared to the turn-on voltage (∼5.2 V) (here, defined as a driving voltage at a luminance of ∼1 cd/m2) of the QLED without PEIE, those of the devices with 7.5 and 15.5 nm PEIE were appreciably reduced similarly to 3.1−3.2 V (Figure S9), being primarily

attributable to the facilitated HTL-to-QD EML hole injection enabled by PEIE-meditated band offset reduction. Individual QLEDs in Figure S7a,b were repeatedly fabricated (eight devices for each) as a part of a reproducibility test, and the resulting distributions in luminance, current efficiency, and EQE are shown in Figure S7c. The peak electroluminescence (EL) wavelength of 528 nm with a bandwidth of 21 nm (Figure 2c), corresponding to (0.157, 0.786) in the Commission Internationale de l’Eclairage (CIE) (1931) color coordinates, means a highly color-saturated emission, which is well satisfactory for an ultrahigh-definition (UHD) display application based on Rec. 2020 standard. AgNW films were first spin-deposited onto polyurethane (PU)-coated polyethylene terephthalate (PET) substrate by using AgNW dispersions with different concentrations (Figure 3a). With increasing concentration from 0.25 to 1 wt %, the sheet resistances of these pristine films naturally decreased from 3811 to 131 Ω sq−1. The subsequent ZnO NPs overcoating led to substantially reduced sheet resistances of 120, 47, and 18 Ω sq−1 for the composite electrodes prepared with 0.25, 0.5, and 1 wt % NW dispersions, respectively (Figure 3b). This evidently indicates that highly conducting ZnO NPs are capable of electrically interconnecting neighboring AgNWs by effectively filling inter-NW voids. To offer insight into such a composite structure, a TEM sample was prepared by sequentially dropping the dispersions of AgNWs and ZnO NPs. As shown in Figure 3c, AgNWs were well in contact with ZnO NPs, and inter-NW voids were nearly continuously filled with ZnO NPs. As seen in the surface and cross-sectional morphologies of the composite electrode film (Figure S10), AgNWs were well covered with and embedded in ZnO NPs, leading to a smoothened film surface. The visible transmittance of pristine AgNWs films 1986

DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990

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Figure 4. (a) Overall preparation procedures of ZnO NPs-AgNWs composite cathode for the following fabrication of an all-solution-processed flexible inverted QLED and (b) its electroluminescent images at 8 V driving in unbent (left), mildly bent (middle), and highly bent states (right), which are compiled in Supporting Movie S1.

While pristine AgNWs film was severely peeled off, ZnO NPsAgNWs composite well-retained film integrity against ultrasonic agitation (Figure S12). These improvements in thermal stability and adhesion characteristic of AgNWs film enabled by ZnO NPs overcoating can be also found in the composites of AgNWs with sputter-29 and sol−gel-deposited metal oxide films.44 Among a series of composite films above, one with 1 wt % AgNWs having the lowest sheet resistance was utilized as a flexible transparent cathode for the fabrication of flexible inverted QLEDs. As shown in overall fabrication procedures (Figure 4a), ∼500 nm-thick PMMA overlayer was precoated on PU-coated PET substrate, and then the composite electrode was formed. Subsequently, a heat-assisted flat-plate pressing was applied to reduce the surface roughness by embedding AgNWs into the softened PMMA overcoat,45 resulting in an Rrms of 2.89 nm (Figure S13), followed by a laser patterning. Reflecting the same all-solution processing along with an optimum PEIE thickness of 15.5 nm as in ITO-based QLED (Figure 2), the fabrication of a flexible inverted device was implemented. Eight V-driven electroluminescent images of the resulting flexible in different bending conditions are shown in Figure 4b. As shown in J−L−V results (Figure S14a,b), the flexible device exhibited maximum values of 50,560 cd/m2 in luminance, 35.1 cd/A in current efficiency, and 8.4% in EQE, which were somewhat inferior to those from the earlier glass/ ITO-based device. This may be primarily attributable to the lowering of annealing temperatures of the constituent layers for flexible QLED (see Supporting Information) compared to a glass-based one, which was unavoidable to prevent the thermal deformation of PET substrate. Nevertheless, the present flexible inverted QLED far surpasses the previous best flexible devices with a standard structure based on a brittle ITO46 and ultrathin (18 nm) Ag anode,47 whose maximum EQEs only ranged in 2.35−4.03%.

increased from 91.9 to 97.3% at 550 nm with decreasing AgNWs concentration. Intriguingly, the transmittance could be rather improved slightly to 93.6 and 98.7% for the composite films prepared with 1 and 0.25 wt % AgNW dispersions, respectively, after the deposition of ZnO NPs (Figure 3b), presumably attributable to the additional function of ZnO NP overcoat as an antireflection layer.42 On the basis of sheet resistance (Rs) and transmittance (T), transparent conducting performance of ZnO NPs-AgNWs composite film can be expressed as a figure of merit (FoM), defined as the ratio of the DC conductivity (σDC) to the optical conductivity at 550 nm (σOp (λ)), according to the equation of σDC/σOp (λ) = 188.5/ [Rs × (T−1/2 − 1)].43 ZnO NPs-1 wt % AgNWs composite film possessed an excellent FoM of 311, which was nearly 10 times higher than that (33) of bare 1 wt % AgNWs film and even better than that (246) of the commercial ITO having T of 89.9% and Rs of 14 Ω sq−1. Considering that solution-processed QLED fabrication usually accompanies baking processes of constituent layers at certain temperatures, it is worth examining the thermal stability of AgNW-based transparent electrode. Thermal stability of pristine AgNWs and ZnO NPs-AgNWs films was compared by measuring the variation of sheet resistance as a function of temperature applied (Figure S11a). The sheet resistance of pristine AgNWs film increased suddenly from around 145 °C and became unmeasurably high at 200 °C as a result of the fragmentation of AgNWs into Ag particles and consequent disconnection of their network (Figure S11b). On the contrary, the sheet resistance of ZnO NPs-AgNWs film remained almost unchanged up to 175 °C, implying that the embedding of AgNWs with ZnO NPs was highly effective in maintaining the AgNWs network without the fragmentation into particles at a relatively high temperature (Figure S11c). Additionally, pristine AgNWs and ZnO NPs-AgNWs films on a PET substrate were subjected to ultrasonication for 1 min in a water bath to qualitatively compare the adhesive strength. 1987

DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990

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CONCLUSIONS We for the first time explored the fabrication of highperformance inverted green QLEDs via all-solution processing, which was simply enabled by the insertion of a PEIE interlayer between a QD EML and a poly-TPD HTL. The PEIE interlayer was found to multiply function as a physical buffer layer against QD EML damage, interfacial dipole layer for energetically upshifting QD band levels, and surface smoothing of QD EML, thereby drastically improving the device performance. As a result, the present inverted green QLED with a PEIE interlayer thickness of 15.5 nm optimized with respect to device efficiency exhibited maximum efficiencies of 65.3 cd/A in current efficiency and 15.6% in EQE. Furthermore, the fabrication of a high-performance flexible inverted QLED was demonstrated by employing a newly developed ZnO NP-AgNW composite electrode with a sheet resistance of 18 Ω sq−1 and a visible transmittance of 93.6% and applying the same all-solution processing as in a glass/ITObased device. Although the resulting flexible QLED did not fully reach the rigid device in performance mainly due to the plastic substrate-derived limitation of baking temperature, we believe that this work offers a major step toward the realization of a high-performance all-solution-processsed flexible inverted QLED display device.

Fiber laser (3SP Technologies). Subsequently, a flat-plate pressing under 300 psi for 30 s with a heating at 130 °C was applied to embed AgNWs into the softened PMMA layer. The following solutionprocessing details of a flexible inverted QLED were the same as those of glass/ITO-based one above except for the baking conditions of the constituent layers, which were reduced equally to 110 °C for ETL, HTL, and HIL in order to prevent the thermal deformation of PET substrate.

METHODS

Notes

Fabrication of Glass/ITO-Based Inverted QLED. A windmillpatterned indium tin oxide (ITO) glass substrate was cleaned sequentially with DI water, acetone, and isopropanol and then UVozone-treated for 15 min. 40 nm-thick ZnO NP ETL was generated by spin-casting the above ZnO NP ethanol dispersion (80 mg/mL) at 1500 rpm for 30 s and baking at 150 °C for 30 min. Onto the underlying ETL, using a CdSe@ZnS/ZnS QD octane/hexane dispersion (OD = 1.5 at 512 nm, corresponding to ∼25 mg/mL), a 27 nm-thick QD EML was spin-deposited (3000 rpm, 30 s), followed by drying at room temperature. By varying the PEIE concentration from 0 to 2 wt % in 2-metoxyethanol, 0−27.6 nm-thick PEIE layers were spin-deposited (3000 rpm, 30 s) on EML, followed by baking at 120 °C for 10 min. Using poly-TPD (Solaris Chem, Inc.) in chlorobenzene (10 mg/mL), a 40 nm-thick HTL was spin-deposited (3000 rpm, 30 s) and baked at 120 °C for 20 min. For 10 nm-thick HIL formation, (MoO3)12·H3PO4·(H2O)x (PMAH) acetonitrile solution (10 mg/mL) filtered through polyvinylidene fluoride (PVDF) membrane was spin-coated (3000 rpm, 30 s) onto a PEIE layer, followed by baking at 120 °C for 10 min. All spin-coating and baking processes were performed in a nitrogen-filled glovebox. Device fabrication was finalized by depositing 100 nm-thick Al anode by a high-vacuum thermal evaporation with a line-patterned metal mask. Preparation of ZnO NPs-AgNWs Composite Film and Fabrication of Flexible Inverted QLED. AgNWs (Nanopyxis), having the mean length of 30 μm and diameter of 25 nm, were dispersed in a solvent mixture of isopropanol and butanol (volume ratio of 3:1) with different concentrations of 0.25−1 wt % to control the film density of AgNWs network. These AgNW dispersions were spin-deposited with 600 rpm for 5 s on PU-coated PET substrate, followed by vacuum drying at 76 mTorr for 30 s. Then, a ZnO NP ethanol dispersion, which was prepared identically to that used for the above ETL formation, with a concentration of 30 mg/mL was overcoated onto a predeposited AgNW film by spin-casting at 3000 rpm for 30 s. Among three ZnO NPs-AgNWs composite films prepared with a 1 wt % AgNW dispersion, one was chosen as a transparent flexible electrode for the fabrication of inverted QLED considering its lowest sheet resistance. For the fabrication of flexible inverted QLED, an ∼500 nm-thick PMMA overlayer was deposited by bar-coating a 5 wt % PMMA-anisole solution onto a PU-coated PET substrate, and then ZnO NPs-AgNWs composite film was generated. This blanket film was patterned by an infrared (1080 nm) ytterbium

The authors declare no competing financial interest.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08142. Experimental procedures and Figure S1−S13 (PDF) Movie S1: an all-solution-processed flexible inverted QLED and its electroluminescent images at 8 V driving in unbent, mildly bent, and highly bent states (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Heesun Yang: 0000-0002-2827-2070

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2015R1A2A2A01003520) and by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2016-H8501-16-1009) supervised by the IITP (Institute for Information & communications Technology Promotion). This work was also supported by Basic Science Research Program through the NRF funded by the Ministry of Education (no. 2015R1A6A1A03031833) and by Creative Materials Discovery Program through the NRF funded by the MSIP (2015M3D1A1069755). REFERENCES (1) Qian, L.; Zheng, Y.; Xue, J.; Holloway, P. H. Stable and Efficient Quantum-Dot Light-Emitting Diodes Based on Solution-Processed Multilayer Structures. Nat. Photonics 2011, 5, 543−548. (2) Lee, K. H.; Lee, J. H.; Song, W. S.; Ko, H.; Lee, C.; Lee, J. H.; Yang, H. Highly Efficient, Color-Pure, Color-Stable Blue Quantum Dot Light-Emitting Devices. ACS Nano 2013, 7, 7295−7302. (3) Bae, W. K.; Park, Y. S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I. Controlling the Influence of Auger Recombination on the Performance of QuantumDot Light-Emitting Diodes. Nat. Commun. 2013, 4, 2661. (4) Lim, J.; Park, M.; Bae, W. K.; Lee, D.; Lee, S.; Lee, C.; Char, K. Highly Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes Enabled by the Direct Formation of Excitons within InP@ZnSeS Quantum Dots. ACS Nano 2013, 7, 9019−9026. (5) Lee, K. H.; Lee, J. H.; Kang, H. D.; Park, B.; Kwon, Y.; Ko, H.; Lee, C.; Lee, J.; Yang, H. Over 40 cd/A Efficient Green Quantum Dot Electroluminescent Device Comprising Uniquely Large-Sized Quantum Dots. ACS Nano 2014, 8, 4893−4901. (6) Yang, Y. X.; Zheng, Y.; Cao, W. R.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J. G.; Holloway, P. H.; Qian, L. High-Efficiency 1988

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DOI: 10.1021/acsnano.6b08142 ACS Nano 2017, 11, 1982−1990