Multifunctional Silver Nanoparticle Interlayer-Modified ZnO as the

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Multi-Functional Silver Nanoparticles Interlayer Modified ZnO as Electron Injection Layer for Efficient Inverted Organic Light-Emitting Diodes Lei Zhou, Heng-Yang Xiang, Yufu Zhu, Qingdong Ou, Qiankun Wang, Juan Du, Rui Hu, Xian-Bo Huang, and Jian-Xin Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Materials & Interfaces

Multi-Functional Silver Nanoparticles Interlayer Modified ZnO as Electron Injection Layer for Efficient Inverted Organic Light-Emitting Diodes

Lei Zhou,1, * Heng-Yang Xiang,2 Yu-Fu Zhu,3 Qing-Dong Ou,2 Qian-Kun Wang,2 Juan Du, 1 Rui Hu,1 Xian-Bo Huang,1 Jian-Xin Tang 2, *

1Faculty

of Mathematics and Physics, Huaiyin Institute of Technology, Huai‫׳‬an

223003, PR China 2Institute

of Functional Nano & Soft Materials (FUNSOM), Soochow University,

Suzhou 215123, PR China 3Faculty

of Mechanical & Material Engineering, Huaiyin Institute of Technology,

Huai‫׳‬an 223003, China;

* Corresponding authors. E-mail addresses: [email protected] [email protected]

KEYWORDS: Inverted organic light-emitting diodes, ZnO, surface plasmon polariton, silver nanoparticles, electron injection

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ABSTRACT The insufficient electron injection constitutes the major obstacle to achieving high performance inverted organic light-emitting diodes (OLEDs). Here, a facile electron-injection architecture featuring silver nanoparticles (AgNPs) interlayer modified sol-gel-derived transparent zinc oxide (ZnO) ultrathin film is proposed and demonstrated. The optimized external quantum efficiencies of the developed inverted fluorescent and phosphorescent OLEDs capitalized on our proposed electron-injection structure reached 4.0% and 21.2% at current density of 20 mA cm-2, and increased by a factor of 1.90 and 2.86 relative to a reference device without AgNPs interlayer, while simultaneously reducing the operational voltage and substantially ameliorating the device efficiency. Detailed analyses reveal that the local surface plasmon resonance

(LSPR)

emanated

from

AgNPs

plays

three

meaningful

roles

simultaneously: suppressing the surface plasmon polariton (SPP) mode loss, aiding in energy level alignments, and inducing and reinforcing the local exciton-plasmon coupling electric field. Among these interesting and multi-functional roles, the enhanced local exciton-plasmon coupling electric field dominates the electron injection enhancement and substantial increase in device efficiency. Additionally, the light scattering effect also helps for recovering the trapped light energy flux and thus benefits for improving device efficiency. The proposed approach and findings provide an alternative path to fabricating high performance inverted OLEDs and other related organic electronic or optoelectronic devices.


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1. INTRODUCTION Since the pioneering work on organic light-emitting diode (OLED),1 remarkable achievements have been made during the past decades and these devices have evolved from scientific curiosity to commercial applications in full-color panel displays and interior lighting.2-4 In terms of displays, the active-matrix (AM) OLEDs are particularly suitable for commercial state-of-the-art display products (e.g., smart phones, televisions, and portable multimedia players) for high-resolution and high information-content due to their superior color balance and high aperture ratio. For AM-OLEDs flat panel display, the bottom-emission inverted OLEDs (IOLEDs) have been gained more interest since this device configuration can be easily integrated with n-type oxide thin film transistor (TFT) backplanes or low-temperature polycrystalline silicon (LTPS).5-7 Nevertheless, IOLEDs are still stymied by some challenges such as high operation voltage and low efficiency. The major issue in the IOLEDs is the insufficient electron injection between the indium tin oxide (ITO) cathode and the adjacent electron-injection layer (EIL) due to a relatively limited proper cathode materials,8,

9

which constitutes the particular problematic subject for constructing

desirable high performance IOLEDs. Consequently, numerous scientific and company efforts have been directed to modifying the ITO cathode for improving the electron injection such as aluminum (Al),10 cesium fluoride (CsF)11 or cesium carbonate (Cs2CO3)12 modified ITO cathodes, n-type dopant including alkali metal or alkali metal carbonates,13,

14

and n-type metal oxides including ZnO,9 titanium dioxide

(TiO2),15 hafnium oxide (HfO2),16 zirconium oxide (ZrO2),17 and magnesium oxide



(MgO).18 Among these eminent metal oxides candidates, ZnO can be acquired via precursor routers at moderate annealing temperatures and exhibits some intriguing features such as high transparency, high stability, non-toxicity, high electron mobility, and low cost. However, the innate high work function of ZnO (~ 4.0 eV) always leads to a relatively high injection barrier for electrons between ZnO film and the adjacent organic functional layer.8,

9

To overcome this obstacle, various interlayers, such as

self-assembled monolayers (SAMs),19 Cs2CO3,20 conjugated polyelectrolytes,21 polyethylenimine ethoxylated (PEIE),22 and polyethyleneimine (PEI),9 have been extensively investigated for the purpose of higher electron injection between ZnO film and the neighboring organic electron-injection layers. Meanwhile, a number of studies have revealed that metal (e.g., Gallium (Ga), Zirconium (Zr) or Al) doped ZnO displays some promising electrical, optical and magnetic properties and can be adopted as an alternative EIL for air-stable IOLEDs.23-25 In particular, it has also been pointed out that silver (Ag) doped pristine ZnO manifests an attractive capability as a promising EIL for efficient hybrid organic-inorganic light-emitting diodes primarily owing to the improved energy level alignment.26 Unfortunately, to date, there are no detailed experimental and/or theoretical studies on Ag modified ZnO surface on the performance of IOLEDs. Additionally, it is well known that much of the emitted light (~ 80%) in the emissive layer is wasted inside the conventional flat OLEDs architecture due to the waveguide modes, substrate modes, and surface plasmon polariton (SPP) modes.27-29 Accordingly, various external substrate surface modification and/or internal structure design strategies have been explored to liberate


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the trapped light energy flow.27,

30

Particularly, metallic nanoparticles such as gold

nanoparticles,31 Ag-island nanostructures,32 and Al nanodisc33 have been conducted to release the trapped light photons. However, aforementioned past work on light steering in OLEDs mainly focus on the optical properties for suppressing the lossy surface waves, which always propagated along the metal surface and dissipated eventually, and the detailed interaction mechanism of SPP in composite electron-injection layer is still elusive and not very clear yet. Herein, it would be meaningful to explore the possibility of Ag modified ZnO film acting as an EIL for desired efficient IOLED and unveil the corresponding optoelectronic working mechanism. In this paper, we demonstrate a facile electron-injection structure via employing small-sized Ag nanoparticles (AgNPs) modified sol-gel-derived ZnO film for both inverted OLEDs with fluorescent and phosphorescent materials (hereafter termed IFOLEDs and IPOLEDs, respectively). The IFOLEDs and IPOLEDs with built modification EIL yield optimized highest current efficiencies (CE) of 8.4 cd A-1 and 95.3 cd A-1, and external quantum efficiencies (EQE) of 4.0% and 21.2% at current density of 20 mA cm-2, respectively, which are 2.50, 1.62, 1.90 and 2.86 times than that of a control device without evaporated AgNPs interlayer, while simultaneously reducing the operational voltage and substantially ameliorating the device efficiency. The combination of microscopic and optical characterization systematically reveals that the significant performance improvement stems from the multi-functional synergistic effects induced by the incorporated AgNPs interlayer. Here the AgNPs



interlayer presumably plays three meaningful roles simultaneously: suppressing the SPP mode loss, aiding in energy level alignments, and inducing and reinforcing the local exciton-plasmon coupling electric field. Among these multi-functional synergistic roles, the enhanced local exciton-plasmon coupling electric field dominates the electron injection enhancement and the substantial increase in device efficiency. In addition, the light scattering effect also helps for recovering the trapped light energy flux and thereby benefits for improving device efficiency. Given the simplicity of preparing AgNPs interlayer and corresponding benefits, the developed method can provide a practically viable route for building inverted OLEDs with desirable high performance.

2. EXPERIMENTAL DETAILS 2.1 Device fabrication. The ITO coated glass substrates with a sheet resistance of 20 Ω sq-1 were sequentially cleaned with detergent, acetone and deionized water in an ultrasonic bath (20 min). Afterwards the substrates were treated by UV-zone for 15 min. Prior to the inverted OLEDs fabrication, the high quality ultrathin ZnO film was constructed on pre-cleaned ITO glass substrates following our previously built method.6 Thereafter, the prepared ITO glass substrates with ZnO film were loaded in a high-vacuum chamber (base pressure = 10-6 Torr) thermal evaporation system for fabricating inverted OLEDs. Ultrathin monolayer Ag film were deposited on ZnO layer at a deposition rate of ~ 0.02 nm s-1, and then the AgNPs was successfully


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converged on the surface of ZnO film. To obtain reliable experimental results, each series of all four devices were simultaneously fabricated in the same batch. 2.2 Properties characterization. Optical properties including total transmittance, specular transmittance, and absorbance were measured by UV/vis/near-IR spectrometer (Shimadzu, UV3600). The haze values were calculated following the formula, haze = (total transmittance - specular transmittance)/total transmittance. The photoluminescence (PL) spectra were characterized by a Fluorescence spectrometer (Horiba Jobin Yvon FLW-oromax-4). The atomic force microscopy (AFM) images of various film surfaces were measured by Veeco Multimode V in tapping mode. The powder X-ray diffraction spectra of ZnO films with and without AgNPs were obtained by XRD (Bruker, D8 Advance). The ultraviolet photoemission spectroscopy (UPS) spectra were obtained in ultrahigh vacuum by ultraviolet photoelectron spectroscopy (Kratos AXIS Ultra-DLD). The current density-voltage-luminance (J-V-L) properties and electroluminescence (EL) characteristics were conducted by a programmable spectroradiometer of PhotoResearch PR-655 connected with a power source of Keithley model 2400. The fabricated devices were placed on a rotation stage for angular EL spectra characterization. 2.3 Theoretical Modeling and Simulation. The near field electric field radiation distribution and corresponding Poynting vector S distribution were calculated using the finite difference time domain (FDTD) method (RSoft FullWave, RSoft Design Group, Inc.) and corresponding post processing by ours Matlab codes. For near filed simulation, one single perpendicular-oriented dipole was set in the middle of the



emission layer for all the IOLEDs, where the AgNPs were randomly arranged in the range of ~ 100 nm, assuming that the AgNPs retain the same sizes with diameters of 0.1 nm, 0.2 nm 0.5 nm, and 1.0 nm, respectively. For convenience, the density of AgNPs was fixed at 40% with random distribution according to the AFM measurements. To obtain reliable far field quantitative light extraction efficiency, the simulation range was enlarged to 800 nm in three-dimensional. The unpolarized light source, which was composed of three individual incoherent orthogonal dipoles, was adopted to calculate far field quantitative optical out-coupling efficiency. In the FDTD simulation, the complex optical functions of metallic Ag and Al were fitted using the Drude-Lorentz model, and the values of frequency-dependent refractive index (n) and extinction coefficient (k) of the other dielectric materials involved the devices were attained by an ellipsometer in the wavelength range of 350-800 nm. The perfectly matched layer (PML) boundary conditions in all dimensions were employed because of the disorder distribution characters of the AgNPs, and the simulated mesh accuracy was set as λ/dx = 15 in the overall architecture.

3. RESULTS AND DISCUSSION 3.1. IFOLEDs Performance Characteristics


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Figure 1. Device architecture and performance characteristics of IFOLEDs. (a) Device structure of IFOLEDs, where AgNPs interlayer is deposited on ZnO layer by thermal evaporation. (b) Current density as a function of voltage. (c) CE versus current density. (d) EQE versus current density. (e) Relative electroluminescence (EL) spectra (@ 20 mA cm-2). (f) The measured angular-dependent normalized EL spectra intensities (@ 20 mA cm-2).



Figure 1a denotes a schematic IFOLED architecture comprising AgNPs interlayer prepared by thermal evaporation. For reference and comparison，five types of IFOLEDs without and with AgNPs modified ultrathin ZnO film were built simultaneously. The green fluorescent unit consists of electron-transporting layer of 4,7-diphenyl-1,10-phenanthroline (Bphen, 20 nm), a layer of tris(8-hydroxyquinoline) aluminum (Alq3, 45 nm) for green emission, and a hole-transporting layer (HTL) of N,N′-di(naphthalene-1-yl) -N,N′-diphenyl-benzidine (NPB, 40 nm), where the thickness of Bphen has been optimized in advance to yielding high device performance (Figure S1), and 6 nm-thick of MoO3 and 100 nm-thick of Al was employed as the bilayer top anode. The controllable thermally deposited thickness of Ag film enables a straightforward ‘tailoring’ of the only variable of Ag quantity on the ZnO layer. The current density-voltage (J-V) evolution curves of all fabricated devices are depicted in Figure 1b and the corresponding performance characteristics are presented in Table S1 in Supporting Information. It can be observed that the turn-on voltage of the ZnO-only device (reference device) is 5.2 V, which is rather high than all of the devices with the AgNPs interlayer. Obviously, the electrons are witnessed to be more efficiently injected and transported from the bottom cathode to the adjacent light photon emitting layer in the developed devices as compared to its counterparts of ZnO-only device. Surprisingly and importantly, the further deposition of Ag doesn’t lead to a monotonic voltage reduction, but gradually increasing again when the thickness of Ag exceeds an inflection point of 0.2 nm. As a consequence, the device constructed with 0.2 nm-thick Ag exhibits maximum CE of 8.4 cd A-1, and


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EQE of 4.0% (@ J = 20 mA cm-2), which increased by a factor of 1.62 and 1.90 relative to a reference device without AgNPs interlayer (Figure 1c, d). Moreover, as presented in Figure 1e and Figure S2, all the devices clearly show very similar electroluminescence (EL) spectra pattern, revealing that the evaporated small-size AgNPs offer a very favorable broadband wavelength-independent enhancement in efficiency without introducing any spectral distortion. In particular, it seems somewhat interesting that the measured angular-dependent normalized EL spectra intensity of control device displays ideal Lambertian intensity profile, whereas all the devices with AgNPs interlayer exhibit noticeable sider-stronger distributions beyond the glass substrate critical angle (~ 41). It is thus reasonable to deduce that local surface plasmon polaritons resonance and/or light scattering must be inevitably induced by the AgNPs formed by the evaporated AgNPs, which always lead to the key feature of stronger side patterns in high viewing angles.34, 35

3.2. Morphological and Optical Characteristics To explore the morphology of AgNPs interlayer with different deposited thickness, the atomic force microscopy (AFM) images of various film surface profiles were characterized and are displayed in Figure 2. It is clearly found that small and uniform AgNPs were successfully converged on the bare ZnO surfaces, and the corresponding depth profiles gradually increased as the increment of the thickness of the evaporated Ag film. Simultaneously, the surface root-mean-square (RMS) roughness of the attained ZnO film was slightly increased from ~ 1.1 nm to ~ 1.8 nm upon the



deposited thickness of Ag film. In contrast, the turn-on voltage and modified devices performance didn’t exhibit monotonic variation as the Ag quantity increases as discussed in Figure 1. It is herein revealed that the substantially improved device performance can’t be only ascribed to the morphological change.

Figure 2. AFM images of different fabricated film surfaces with the corresponding roughness profile: (a) bare ZnO film, (b) 0.2 nm-thick Ag, (c) 0.5 nm-thick Ag film and (d) 1 nm-thick Ag film.

To further confirm the Ag modification situation, the XRD patterns of ZnO film


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without and with AgNPs were measured and are presented in Figure 3a. Note that the XRD patterns of AgNPs modified ZnO exhibit distinct feeble reflection of 111- and 200- planes of metallic face-centered cubic (FCC) Ag, signifying successful Ag-modification. The intense peaks of planes 110, 100 and 201 of ZnO without and with AgNPs also indicate that crystallinity of ZnO keeps unchanged after the thermal deposition of the AgNPs interlayer.

Figure 3. (a) The measured X-ray diffraction (XRD) patterns of ZnO film without and with evaporated Ag film. (b) Total transmittance of ZnO-coated ITO/glass substrates w/o AgNPs and the corresponding inset haze values.



For an intuitive comparison of optical transmittance characteristics, the optical total transmission and the corresponding haze values of different substrates without and with AgNPs were denoted in Figure 3b. We can observe that the total transmittance decreases as the thickness of evaporated Ag layer increases monotonically, and thus the corresponding haze values slightly increases as the increment of the thick of the Ag film. The average haze for ZnO coated ITO glass substrate is only ~ 0.5% over the entire visible range, and it progressively increases to ~ 3.4% for the modified substrate with 1.0 nm-thick Ag film. It demonstrates that the light photon scattering is excited and thus certainly assists in the luminous flux enhancement because the AgNPs acted as scattering center of photons, which is consistent with our previous speculation and has also been extensively investigated and confirmed by several other scientific literatures.36-39 However, it should be emphasized that although this light scattering effect is objectively excited, it serves as a non-crucial role for aiding in improving device efficiency, because the increased haze values are not prominent and the increasing trend also conflicts with the experimental results of the IFOLED discussed above.

3.3. Modeling and Structural Properties In a bid to further investigate the physical mechanism of the devices, the electric field distributions and corresponding poynting vector S (photon flux) patterns were judiciously simulated by virtue of the finite-difference-time-domain (FDTD) method. For the control device, a noticeable strong electric field is confined around the area


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close to the Al anode, which corresponds to well-known SPP mode (Figure 4a). This unwanted optical mode directly leads to the majority of emitted light photons that are unavoidably trapped and dissipated within the flat architecture eventually.40,

41

In

contrast, the field localized around the metallic anode surface is notably suppressed and thus the intensity of electric field beyond the substrate/air interface is enhanced as the increase of the thickness of the evaporated Ag film owing to LSPR excited by the AgNPs (Figure 4b,c and Figure S3a,b), implying that the larger AgNPs result in more escaping photon flux. To better reveal the optical working mechanism, Poynting vector S (photon flux) distributions, which distinctly manifest how the generated light propagates before it is escaped from the device or totally dissipated, were also simulated for all the devices. It is noteworthy that the emitted energy coupled into SPP as a trapped surface wave was strongly bounded to the interface between the Al anode and neighboring dielectric layer for the flat ZnO-only device, clearly displaying a non-radiative outlet profile because of the mismatched momentum between the freely propagating light photons and the excited SPPs.29, 42 In contrast, the converged AgNPs nanostructures at this interface induce the LSPR and thus change the wavevector of the unwanted SPP, directly allowing them to escape into the air (Figure 4d-f and Figure S3c, d). Furthermore, it also can be notably observed that the lossy surface waves on the metallic anode are undergoing much more suppression and more emitted photons are scattered out of the device simultaneously as the increment of the size AgNPs, directly resulting in the calculated EQE enhanced factors increased monotonically from 1.08 to 1.19 (Figure S4). These calculated facts provide direct



theoretical support that LSPR and light scattering effects originated from incorporated Ag particles occurred and contributed to the improvement of device performance. This feature rendered a good correlation between optical transmittance and side-stronger angular emission intensity distribution as discussed above and also implied that the suppressed SPP mode loss acted as a non-crucial role for aiding in improving device efficiency because of the apparent contradictory trend between the measured non-monotonically increasing performances of the IFOLED discussed in Figure 1.

Figure 4. Optical simulations for IFOLEDs without and with AgNPs interlayer at 520 nm. Calculated electric field distributions (|E|2) for control device (a), device with 0.2 nm-thick Ag film (b), and device with 1.0 nm-thick Ag film (c), respectively. (d-f) The corresponding simulated poynting vector S (photon flux) distributions of the devices in a-c, respectively. A longer and thicker arrow represents a stronger intensity of the photon flux energy and the solid lines depict the layer interfaces.


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Indeed, LSPR excited by metallic AgNPs could lead to increasing light recovering in terms of suppressing SPP mode loss and light scattering. Meanwhile, high density electrons of the Ag accumulated at the AgNPs/ZnO interface form an electron cloud and oscillate when LSPR occurs, and thus straightly results in downward band bending at the ZnO side. Moreover, the work function of Ag (4.26 eV) is smaller than that of ZnO (4.0 eV) due to the relatively higher Fermi level of Ag (Figure S5a, Supporting Information). Consequently, transfer of the electrons from the Fermi level of metallic Ag to the conduction band of ZnO accordingly becomes much easier since the formation of Ag-modified ZnO structure, leading to the bending of the conduction band of ZnO to a higher value and that of metallic Ag to a relative lower value, which thereby leads to an equivalent Fermi level. Obviously, this desirable obtained feature, which was paid little attention before, allows for much easy electron injection from the metallic Ag side to the ZnO (Figure 5a). Nevertheless, the ideal work function reduction could maximally approach 0.26 eV between the AgNPs and ZnO interface, distinctly implying that this favorably tuned energy level alignment induced by the LSPR effect is impossible to cause a significant drop in the turn-on voltage. To further probe the physical mechanism of the enhanced electron injection property induced by the modification AgNPs interlayer, electronic structures of Bphen/Ag interface were examined by UPS characterization. The deposition of metallic nanoscale Ag particles onto Bphen films exhibits a profile of spectral shift of only ~ 0.39 eV (Figure S5b, Supporting Information), which directly points to that



relatively lower interface electron-injection barrier was achieved between the Ag and Bphen layer.43,

44

Thereby, the UPS feature indeed supports our argument on the

limited energy level tuning merely giving rise to finite contribution to the low operational driving voltage and improved device performance.

Figure 5. (a) The schematic energy band configuration of metallic Ag and ZnO. (b)


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Absorption spectra of AgNPs, and ZnO film without and with AgNPs. (c) The photography

of

an

electron-only

device

based

on

the

architecture

of

ITO/ZnO/Ag/Bphen/MoO3/Al under dark ambient environment.

To better unveil the role of AgNPs interlayer, the absorption spectra of AgNPs, and ZnO film without and with AgNPs were measured and are provided in Figure 5b. It can be observed that the absorption curve of the ZnO shows a characteristic interband absorptivity start at ~ 370 nm and the absorption curve of the AgNPs interlayer exhibits a typical SPP absorbtivity peak located at ~ 420 nm. In contrast, the absorptivity spectra of the AgNPs modified ZnO film displays a conspicuous splitting peak and thus the distinct exciton-plasmon coupling induced by the LSPR.45, 46

This hybrid coupling reinforces the local electric field (E) around AgNPs, which

can be approximately described as the equation:10 𝑄

E = 4𝜋𝜀𝑅2 Where the 𝑄 denotes the charge quantity in the AgNPs, and 𝜀 is the dielectric constant of the ETL material. Obviously, the local exciton-plasmon coupling electric field is proportional to

𝑄 𝑅2

, which demonstrate that the smaller AgNPs induce larger

enhancement in local electric filed and thereby more easier electron injection and lower operational driving voltage in the devices. This changing trend is in accordance with the experimental optoelectronic results as denoted in Figure 1, indicating that the local exciton-plasmon coupling electric field stemmed from the LSPR plays the key role in improving the device performance. However, the turn-on voltage of the device



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with 0.1 nm thick Ag film was apparently larger than that of the device with 0.2 nm thick Ag film as discussed in Figure 1b. We surmise this discrepancy probably originates from the trade-off between this enhanced exciton-plasmon coupling electric field and the suppressed SPP mode loss as well as the light scattering within the devices. Additionally, the visualized electron-only device under dark ambient environment exhibits a vivid purple halo profile around the edge of the device emanating from the hybrid visible and ultra-violate (UV) emission (Figure 5c) induced by the coupling-generated hybrid excitons (Figure S6, Supporting Information), verifying the accuracy of the aforementioned conclusion. Therefore, the experimental and theoretical analysis demonstrated that the excited LSPR plays three interesting roles: suppressing the SPP mode loss, aiding in energy level alignments, and inducing and reinforcing the local exciton-plasmon coupling electric field.

3.4. PIOLED Performance Characteristics To further validate our proposed approach, AgNPs interlayer modified ZnO film was also introduced into inverted phosphorescent green OLEDs (IPOLED) and are compared with that of the control ZnO-only device. As presented in Figure 6a, the phosphorescent

emitter

electron-transporting

unit

layer,

was a

15

consist nm-thick

of

15 8wt%

nm-thick

Bphen

as

bis(2-phenylpyridine)

(acetylacetonate) iridium(III)[Ir(ppy)2(acac)]-doped -(4,4'-N,N'-dicarbazole)biphenyl (CBP) for green emission, 30 nm-thick 4’,4”-tris-(N-carbazolyl)-triphenylamine (TCTA) as hole injection layer, and a MoO3 (6 nm)/Al (100 nm) bilayer was


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employed as the top anode. As plotted in Figure 6b and presented in Table 1, the J-V evolution curves of all the devices show a notably clean diode behavior without any abnormal discontinuities, the operational voltages of the devices with 0.2 nm-thick and 1.0 nm-thick Ag film are 3.5 V and 3.8 V, respectively, which are rather lower than a control device (5.5 V). This tread of operational voltage corresponds very well to that of the discussed IFOLEDs, indicating that the desirable electron injection effect of the AgNPs on sol-gel ZnO film can be driven to a general mechanism. Correspondingly, as provided in Figure 6c, Figure S7 and Table 1, the device incorporating modified EIL interlayer with thickness of 0.2 nm and 1.0 nm yields maximum CE of 95.3 cd A-1 and 56.4 cd A-1, PE of 87.2 lm W-1 and 48.5 lm W-1, and EQE of 21.2% and 12.6% (@ J = 20 mA cm-2), respectively, which are 2.5 and 1.48, 2.94 and 1.63, 2.86 and 1.70 times that of the ZnO-only device (CE = 38.1 cd A-1, PE = 29.7 lm W-1 and EQE = 7.4%). Noticeably, although this device performance improvement trend is in good agreement with the experimental and theoretical conclusions as presented in previously explored IFOLEDs, the enhanced factors of the IPOLEDs with AgNPs interlayer are generally higher than that of counterparts of IFOLEDs. We surmise that this stronger enhancement is attributed to the fact than an OLED using phosphorescent materials usually has a large SPP enhancement than a fluorescent one because of the inherent longer lifetime of their excited state,47,

48

directly resulting in more band bending of the ZnO and more

recovery of trapped photons. In addition, by thoroughly scrutinizing the normalized wavelength-dependent EL spectra profiles of all the devices, it is worth noting that



all the devices with developed interlayer show nearly identical pattern compared with that of the control ZnO-only device, clearly signifying no anomalous spectra distortion effects within the entire luminescence spectrum range.

Figure 6. Device structure and performance characteristics of IPOLEDs. (a) Device architecture of IPOLEDs. (b) J-V characteristics. (c) CE-J curves. (d) Normalized EL spectra from a normal viewing angle relative to the substrate (@ 20 mA cm-2). Inset: a photography of IPOLEDs constructed on ITO glass substrate with AgNPs.

Table 1. Performance of fabricated green IPOLEDs. The CE, and PE are presented at their maximum values (depicted in the parenthesis) and at a current density of 20 mA cm-2, respectively. The calculated REQE data are the enhanced factor relative to that of the control ZnO-only device (@ J = 20 mA cm-2).


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Device structures

Turn-on voltage [V]

CE

PE

[cd A-1]

EQE

[lm W-1]

REQE

[%]

Ref

5.5

30.8 (38.1)

12.8 (29.7)

7.4

1

0.2 nm

3.5

71.3 (95.3)

42.9 (87.2)

21.2

2.86

1.0 nm

3.8

39.9 (56.4)

21.6 (48.5)

12.6

1.70

4. CONCLUSIONS In summary, we have demonstrated a facile unique type of electron-injection architecture comprising nanoscale Ag particles modified sol-gel ZnO layer, enabling the effective injection of electron from the bottom ITO cathode in both IFOLEDs and IPOLEDs. The modified devices exhibit favorable superior performance compared with that of the counterparts of control devices. The IFOLEDs and IPOLEDs with built modification EIL yield optimized maximum current efficiencies of 8.4 cd A-1 and 95.3 cd A-1, and external quantum efficiencies (EQE) of 4.0% and 21.2% at current density of 20 mA cm-2, respectively, and without introducing spectra distortion.

Systematically

analyses

demonstrate

that

the

enhanced

local

exciton-plasmon coupling electric field stemming from the developed AgNPs interlayer dominates the electron injection enhancement and the substantial increase in device efficiency. Besides, the suppressed SPPs mode loss, the tuned energy level alignment as well as the light scattering effect, although non-crucial and always negligible, also helps for improving device efficiency. Moreover, the AgNPs are prepared simply by thermal evaporation and directly compatible with the



cost-effective industrial manufacture. Thereby, the present approach and findings may provide some valuable insight for achieving high performance of inverted OLEDs and other related optoelectronic devices.

ASSOCIATED CONTENT Supporting Information Table for performance properties of green IFOLEDs, device performance properties of the control IFOLEDs with various thickness of the Bphen, the normalized EL spectra of the IFOLEDs, the calculated electric field distributions of the devices with 0.1 nm-thick and 0.5 nm-thick Ag film, the calculated EQE of the inverted OLED devices as a function of the average diameter of the AgNPs, the steady-state photoluminescence (PL) spectrum of ZnO film without and with AgNPs, the EQE versus current density of the fabricated IPOLEDs are also presented. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS L. Zhou and H. Y. Xiang contributed equally to this work. We acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 61775076, 61774070), the project of the Six Talent Peaks of Jiangsu Province (Grant Nos. DZXX-011, XNY-008), the Natural Science Foundation of Jiangsu Province (Grant Nos. BK20161303, BK20161300), and “333 High-level Talents Training Program” of Jiangsu Province.

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159-165.

ToC figure


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Multifunctional Silver Nanoparticle Interlayer-Modified ZnO as the

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