Role of Polymeric Metal Nucleation Inducers in Fabricating Large‐Area, Flexible…download.xuebalib.com/xuebalib.com.29882.pdf · ible optoelectronic devices by utilizing high-throughput

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Role of Polymeric Metal Nucleation Inducers in Fabricating Large-Area, Flexible, and Transparent Electrodes for Printable Electronics

Soyeong Jeong, Suhyun Jung, Hongkyu Kang,* Dasol Lee, Sang-Bae Choi, Seok Kim, Byoungwook Park, Kilho Yu, Jinho Lee, and Kwanghee Lee*

The advent of special types of transparent electrodes, known as “ultrathin metal electrodes,” opens a new avenue for flexible and printable electronics based on their excellent optical transparency in the visible range while main-taining their intrinsic high electrical conductivity and mechanical flexibility. In this new electrode architecture, introducing metal nucleation inducers (MNIs) on flexible plastic substrates is a key concept to form high-quality ultrathin metal films (thickness ≈ 10 nm) with smooth and continuous morphology. Herein, this paper explores the role of “polymeric” MNIs in fabricating ultrathin metal films by employing various polymers with dif-ferent surface energies and functional groups. Moreover, a scalable approach is demonstrated using the ionic self-assembly on typical plastic substrates, yielding large-area electrodes (21 × 29.7 cm2) with high optical transmittance (>95%), low sheet resistance (<10 Ω sq−1), and extreme mechanical flexibility. The results demonstrate that this new class of flexible and transparent elec-trodes enables the fabrication of efficient polymer light-emitting diodes.

DOI: 10.1002/adfm.201606842

S. Jeong, S. Jung, Dr. H. Kang, D. Lee, S. Kim, B. Park, K. Yu, J. Lee, Prof. K. LeeSchool of Materials Science and EngineeringDepartment of Nanobio Materials and ElectronicsHeeger Center for Advanced MaterialsResearch Institute for Solar and Sustainable EnergiesGIST-ICL International Collaboration R&D CentreGwangju Institute of Science and TechnologyGwangju 61005, Republic of KoreaE-mail: [email protected], [email protected]; [email protected]. H. KangDepartment of Chemistry and Centre for Plastic ElectronicsImperial College LondonLondon SW7 2AZ, UKDr. S.-B. ChoiDepartment of Nano-MechanicsKorea Institute of Machinery and MaterialsDaejeon 34103, Republic of Korea

achieve a considerable breakthrough in the manufacturing process of flex-ible optoelectronic devices by utilizing high-throughput roll-to-roll printing sys-tems, so-called “printable electronics.”[8,9] However, current transparent electrodes, which are mainly based on transparent conducting oxides (e.g., indium tin oxide, ITO), have a serious weakness in real-izing flexible and printable electronics due to their inherent mechanical brittleness under repeated bending stress and small bending radius.[10,11] Although nearly two decades of research efforts to address this issue have resulted in new FTE systems, such as metal meshes,[12,13] metal nanow-ires,[6,7,14–16] graphene,[3,17,18] carbon nano-tubes,[19,20] conducting polymers,[4,5,21–24] and organic–inorganic hybrid mate-rials,[25,26] most FTEs face tremendous challenges to increasing their area, which

must be overcome for practical application.The development of ultrathin metal electrodes (UMEs) has

been regarded as an effective approach to achieving high-per-formance, large-area FTEs.[12,27–29] These electrodes bear the outstanding characteristics of optical transmittance (Top), elec-trical conductivity (σdc) and mechanical flexibility; in particular, the high Top of the UMEs stems from the intrinsic features of ultrathin metal films with a percolation threshold thickness (τp: a minimum thickness required to form a continuous metal film) of less than 10 nm, where incident light can pass through the metal film without strong light absorption and reflection.[30] Furthermore, one of the remarkable advantages is that they can be manufactured on large-area, flexible plastic substrates using well-established easily controllable thermal evaporation/sputtering systems equipped with roll-to-roll machines. How-ever, the majority of the UMEs have serious difficulty forming continuous ultrathin metal films on plastic substrates due to 2D metal-island growth. The metal droplets evaporated onto the substrates disorderly migrate and aggregate to form dis-crete granular morphology containing numerous irregular islands at film thicknesses below 10 nm.[31] As the film thick-ness increases beyond ≈15 nm, the metal islands become inter-connected and create continuous metal films with low sheet resistance (Rsh), while the Top of the resulting metal film simul-taneously deteriorates through an inevitable trade-off with σdc.

Electronics

1. Introduction

Flexible and transparent electrodes (FTEs) are a key com-ponent for the realization of flexible optoelectronic devices, such as solar cells, touch screens, smart windows, and light-emitting diodes (LEDs).[1–7] Moreover, FTEs are expected to

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To overcome this problem, pioneering researchers have explored metal nucleation inducers (MNIs), including metal seeds,[28,32–34] metal oxides,[35–37] organic small molecules,[38] or hybrids thereof.[27] To control the surface energies of the sub-strates or induce chemical interactions with the evaporated metals, MNIs are precoated on the plastic substrate before metal evaporation, thereby producing highly smooth and con-tinuous ultrathin metal films with τp of less than 10 nm. Most recently, a new type of MNI based on polymers, such as poly-ethyleneimine (PEI)[25] and poly(N-vinylcarbazole) (PVK),[39] has been developed. UMEs fabricated with the polymeric MNIs have shown high electrode performance and excellent device applicability to high-end devices, such as organic solar cells and organic LEDs. In addition, polymeric inducers have an impor-tant advantage with respect to large-area film fabrication because their solution processability allows for scaling up UMEs through high-throughput roll-to-roll processing without using time-consuming vapor deposition or a high-temperature annealing process, which are applied to form conventional MNIs on sub-strates. Despite these beneficial features of polymeric MNIs, systematic studies have not explored the detailed mechanism of polymeric MNIs for creating ultrathin metal films and have not successfully demonstrated large-area UMEs on flexible plastic substrates.

In this work, we investigate the role of polymeric MNIs in producing high-quality large-area UMEs. By modifying the sur-faces of plastic substrates with various polymers that possess different surface energies and functional groups, we confirm that the chemical interaction between the evaporated metals and the functional amines dominantly governs the formation of continuous metal films and prove that polymers containing more functional amines lead to higher electrode performance. Furthermore, using the ionic self-assembly property of the poly-mers, we demonstrate a scalable method to fabricate large-area UMEs (21 × 29.7 cm2) with high uniformity while maintaining their high performance (Top > 95% and Rsh < 10 Ω sq−1) in small-area electrodes. We successfully fabricate flexible white LEDs on the UMEs, which exhibit performance comparable to ITO-based devices.

2. Results and Discussion

2.1. Polymeric Metal Nucleation Inducers

A schematic illustration of ultrathin silver (Ag) films (thick-ness ≈ 9 nm) on plastic substrates (polyethylene naphthalate, PEN) without and with MNIs and their surface images taken by scanning electron microscopy (SEM) are shown in Figure 1a. The Ag film on the bare PEN substrate represents a discrete morphology with numerous granular defects containing mobile metal clusters, islands, and voids, which were formed as a result of the disordered migration and aggregation of the evaporated metal. Because the isolated defects confine the free electrons of the metal within their boundaries and scatter incident light, the ultrathin metal film becomes electrically insulated and optically opaque.[31] By contrast, introducing polymeric MNIs on the PEN substrate enables the creation of well-connected conductive metal films with thicknesses less than ≈10 nm. By employing a variety of polymers, including PEI, polyallylamine (PAA), polyvinylpyrrolidone (PVP), PVK, polymethylmeth-acrylate (PMMA), and polyethyleneoxide (PEO), which have different hydrophilicity and functional groups, we intended to clarify the effect of polymeric MNIs on the formation of a high-quality ultrathin Ag film on a plastic substrate (Figure 1b).

Considering previous studies based on other MNIs, we assumed that the surface energies of the substrates coated with polymeric MNIs and/or the chemical interaction between the functional groups of the MNIs and evaporated metal atoms would be major factors in the formation of ultrathin metal films.[40] For example, Schubert et al. demonstrated that increasing the surface energy of metal seeds can improve the wettability of evaporated metal droplets on the substrate and have a positive effect on metal growth for continuous metal films.[28] As another example of MNIs, Hatton’s group demonstrated that thiol-functionalized silanes can fix evaporated metal atoms through strong chemical interactions and suppress surface diffusion of the metal droplets, thereby promoting the growth of continuous metal films at low thickness.[22] In addition to the MNI-related researches, Elam and co-workers similarly reported that chemical interactions between

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Figure 1. a) Schematic illustration of the UMEs and SEM images without and with an MNI layer on PEN substrates. b) Chemical structures of the MNIs.



metal species and organic materials have a significant effect on forming film morphology in a research field using sequential infiltration synthesis techniques.[41,42]

We investigated the surface energies of MNIs by measuring the contact angles of water droplets on the surfaces of PEN/MNI samples (Figure 2a; Figure S1, Supporting Information). Because of the polar functional groups (i.e., amine or ether) in the repeating unit, PEI, PAA, PVP, and PEO have high surface energies of 70, 65, 72, and 71 mN m−1, respectively. On the other hand, PVK, PMMA, and bare PEN substrate have relatively low surface energies of 26, 30, and 27 mN m−1, respectively, due to their few polar functional groups or bulky chemical structures. Assuming that the surface energies of MNIs are a critical parameter in the formation of ultrathin metal films, it can be expected that the trend of the percola-tion threshold of metal films follows the order PVP > PEO > PEI > PAA > PMMA > PEN > PVK. Meanwhile, since func-tional amines with high reactivity can induce chemical interac-tions with evaporated metal atoms,[43,44] so-called coordination bonds, we examined the contents of the functional group of the PEN surface coated by amine-containing polymers, such as PEI, PAA, PVP, and PVK, using X-ray photoelectron spectros-copy (XPS). We confirmed that after Ag deposition, the N1s peaks of the PEN/MNIs were shifted to higher binding ener-gies, indicating that the amines of the polymers lose their elec-trons and interact with Ag atoms (Figure 2b). This observation indicates that the concentration of the functional amines on the PEN/MNI surface determines the density of the sites that are chemically reactive with the evaporated metal atoms. By comparing the contents of the functional amines of the sam-ples, we can predict that PEN/PEI with the highest concentra-tion of functional amines would provide the greatest number of the reactive sites with Ag atoms, thereby enabling the for-mation of continuous ultrathin metal films with the lowest τp (Figure 2c).

2.2. Electrode Performance

Because Rsh is an important indicator of the continuity of ultrathin metal films, we compared the Rsh values of increas-ingly thick Ag films on different PEN/MNI substrates (Figure 3a). Based on previous results that metal evaporation rates can affect the growth and quality of metal films,[45,46] we experimentally optimized the metal evaporation rate to obtain highly transparent and conductive Ag films with the ultrathin thickness of 9 nm and chose the specific rate of 1.7 Å s−1 that provides the highest Top while maintaining a sufficiently low Rsh value (Figure S2, Supporting Information). A comparison of PVK-UME with PEO-UME proves that MNIs with high sur-face energies are not essential to obtain ultrathin metal films with low thickness; indeed, PVK has the lowest surface energy (26 mN m−1) among the MNIs. All UMEs based on amine-con-taining polymers are highly conductive at low thickness com-pared to other UMEs. These results indicate that the chemical interaction induced by the functional amines of the MNIs has a more dominant effect on the creation of ultrathin metal films than the surface energies of the MNIs. The trend of the per-colation threshold of the metal films on the amine-containing

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0.0

0.1

0.2

0.3

0.4

PVKPVPPAA

[N]/[

C] R

atio

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Ene

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/m)

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PAA PAA/Ag

PVP PVP/Ag

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ty (

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.)

PVK PVK/Ag

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b

Figure 2. a) Surface energies of the MNIs on PEN substrates. b) High-resolution XPS spectra of the N1s core level of the MNI and MNI/Ag samples. c) Nitrogen/carbon ratios of the amine-containing MNI samples.



polymers follows the order PEI > PAA > PVP > PVK, which corresponds to the contents of the functional amines on the surfaces of the PEN/MNI substrates. As the thickness is increased to 9 nm, the best-performing PEI-UME produces very continuous metal films with the lowest Rsh of 9 Ω sq−1 and highest Top of 76% at 550 nm without significant light scat-tering by metal islands, while PMMA-UME exhibits very poor morphology with the drastically high Rsh and a completely dif-ferent transmittance shape with an opaque feature of 45% at 550 nm, which is attributed to both light scattering and metal reflection (Figure 3b,c; Figure S3, Supporting Information).

Although PEI-UME has the highest Top, it has insufficiently low transmittance to meet the standard specifications for various optoelectronic applications. The electrode suffers from an inevitable optical loss arising from the reflection of incident light at the ultrathin Ag film. To reduce the metal reflection and further enhance the Top of UMEs, we introduced an antireflec-tive coating based on thin-film structures with alternating layers of contrasting refractive indices. By alternating layers of Ag film with a low refractive index and antireflective film with a high refractive index, the phase of the incident light can be shifted at their interfaces.[47] This phase shift can induce destructive interference that weakens the metal reflection (Figure 4a). We used several antireflective layers, including poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), molybdenum trioxide (MoO3), polystyrene (PS), and PVK, which have both high optical transparency and high refractive index (Figure S4,

Supporting Information). Figure 4b exhibits the dramatic changes in Top of the UMEs after coating with the antireflec-tive layers. All samples showed significant increases in Top over the visible region. In particular, UMEs with MoO3 (PAM), PS (PAS), and PVK (PAK) had a greater increase in their Top than that with PEDOT:PSS (PAP) due to the higher refractive index or better optical transparency of the materials (Figure 4b; Figure S3, Supporting Information). These results are quite con-sistent with the calculated Top using a transfer matrix method,[48] which is a useful method to calculate the Top and reflectance of stacked structures of thin films with their refractive indices and extinction coefficients (Figure 4c; Figure S4, Supporting Infor-mation). In addition, we investigated the mechanical flexibility of UMEs based on different antireflective layers. We confirmed that after bending the UMEs 100 000 times at a bending radius of 5 mm, there is almost no significant increase in resistance, thereby demonstrating that the UME possesses outstanding mechanical flexibility and durability (Figure 5).

2.3. Flexible White Polymer LED Application

In consideration of the orthogonal solubility, charge transport properties, and energy levels of the antireflective layers, the semiconducting MoO3 layer is most useful as a hole-trans-port layer for device applications based on charge injection or extraction, such as LEDs and solar cells.[49,50] To investigate

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Figure 3. a) Variations in Rsh of the MNI/Ag samples with different Ag thicknesses. b) Top and c) SEM images of the MNI/Ag samples with 9 nm Ag thickness. The Top spectra of the MNI/Ag samples were measured by excluding those of the PEN substrates.



the impact of PAM on LED applications, polymer LEDs with white emissive layers were fabricated on UMEs. For com-parison, we also prepared reference LEDs using ITO elec-trodes. Figure 6a,b shows the device architecture consisting of PEN/PAM (or ITO/MoO3)/white emissive copolymer/Ca/Al together with the corresponding energy level diagram. All LED data, including the current density–voltage–luminance (I–V–L)

characteristics, current efficiency, and electroluminescence (EL) spectra, exhibit similar results for both PAM- and ITO-based LEDs, demonstrating that UMEs can be fully functional as an FTE for LED applications (Figure 6b,c,d). By contrast, we expect that PAS and PAK, with the highest Top, would be more appropriate for specific device applications, such as capacitive-sensing touch panels and voltage-operating smart windows.

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Figure 4. a) Schematic diagram of the destructive interference in the Ag film with an antireflective layer. b) Top spectra of UMEs with various antireflec-tive layers in the visible region. c) Simulated Top spectra of UMEs with various antireflective layers by using a transfer matrix method. The Top spectra of the UMEs were measured by excluding those of the PEN substrates.

Figure 5. Variation in resistance versus the bending radius or number of bends for the a) convex and b) concave bending of the flexible UMEs and ITO electrodes.


1606842 (6 of 8) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2017, 27, 1606842

Figure 6. a) Device architecture of the white flexible LED fabricated on the PAM electrode. b) Luminance–voltage characteristics of the LEDs together with the energy level diagram of the LED. c) Current efficiency and current density–voltage curves of the LEDs. d) EL spectra of the LEDs.

Figure 7. a) Schematic illustration of the fabrication procedure of the large-area UME. b) Photograph of the large-area UME. c) Uniformity of the Top and Rsh of the UMEs. The uniformity data are obtained by cutting the large-area UME into 24 pieces and measuring the electrode performance of the pieces.



2.4. Fabrication of Large-Area Ultrathin Metal Electrodes

The best feature of our UMEs is that we can readily fabricate large-area FTEs. Since the optimum PEI layer as an MNI is a few nanometers thick, it is essential to precisely control the thickness with high uniformity to fabricate large-area UMEs. We devised a method similar to layer-by-layer film deposition, which is widely used to control the film thickness with 1 nm resolution by using two oppositely charged polyelectrolytes.[51,52] In the first step to fabricate large-area UMEs, we used ultravi-olet-ozone (UV-O3) or plasma treatment to increase the wetta-bility of PEI on PEN by forming many hydrophilic hydroxyls on the surface of the substrate (Figure 7a; Figure S5, Supporting Information). The treated substrate was then dipped into a bath containing PEI solution, in which the PEI is partially proto-nated in aqueous solution (Figure S6, Supporting Information). During the instantaneous dipping (≈5 s), PEI can ionically self-assemble through two acid–base reactions, a proton transfer reaction and a neutralization reaction, between the amines of PEI and the hydroxyls of PEN (Figure S7, Supporting Informa-tion). Because the excess PEI, which remains without inter-acting with PEN, can cause substantial stains, we removed the unnecessary PEI by rinsing with alcohol (Figure S8, Supporting Information). The self-assembled PEI layer on PEN is insoluble in the alcohol solvent due to the electrostatic interactions between the negatively charged oxides of PEN and the positively charge amines of PEI (Figure S9, Supporting Information). After subsequent evaporation of Ag and MoO3, we success-fully fabricated large-area UMEs (21 × 29.7 cm2) with uniformly high Top (>95%) and low Rsh (<10 Ω sq−1) over the whole area (Figure 7b,c,d).

3. Conclusion

Our work demonstrates that the chemical interaction between the functional amines of polymeric MNIs and evaporated metal atoms has a dominant impact on forming continuous ultrathin metal films compared to the surface energies of the polymeric MNIs. Moreover, we found that amine-containing polymeric MNIs ionically self-assemble on plastic substrates through chemical interaction with the hydroxyls of the substrates and play a key role in fabricating large-area UMEs (the size of an A4 sheet) while maintaining highly uniform electrode perfor-mance (Top > 95% and Rsh < 10 Ω sq−1) over the entire area. Our findings offer a strategic MNI design rule to further improve the electrode performance of UMEs and an upscaling method to transition from small-area laboratory-scale FTEs to large-area industrial-scale FTEs.

4. Experimental Section

Material Preparation: PEI (Aldrich, branched type, 50 wt% in H2O), PAA (Sigma-Aldrich, 20 wt% in H2O), PVP (Sigma), and PEO (Aldrich) were dissolved in DI water to 0.3, 0.5, 0.5, and 0.3 wt%, respectively. PVK (Aldrich) and PMMA (Aldrich) were dissolved in chlorobenzene to 1 wt% and chloroform to 0.5 wt%, respectively. PS solution (2 wt%) was prepared by adding PS beads (Aldrich) to toluene. White emissive

copolymer (SPW-111, Merck, Mw ≈ 328 000 g mol−1) was dissolved in toluene to obtain a 1 wt% solution.

Electrode Fabrication and Characterization: The precleaned PEN substrates were treated with UV-O3 treatment for hydrophilic nucleation inducers (PEI, PAA, PVP, PEO). All nucleation inducers were spin-coated onto the PEN at 5000 rpm for 20 s and dried at 100 °C for 20 min. Ag films were thermally evaporated at 1.7 Å s−1 under vacuum pressure (<10−6 Torr). The PEDOT:PSS (H. C. Starck, VPAI4083), PS, and PVK films were formed on the Ag films by spin-coating at 3000, 5000, and 4000 rpm for 40 s. MoO3 was deposited by thermal evaporation at 0.7 Å s−1 under high vacuum pressure (<10−6 Torr). To fabricate the A4-size PEI/Ag/MoO3 electrodes, UV-O3-treated PEN substrates were dipped in PEI solution for 5 s and dried at 80 °C in an oven for 10 min. Ag and MoO3 were thermally evaporated under vacuum pressure (<10−6 Torr) with a customized thermal evaporator in sequence. The surface energy values of the nucleation inducers were calculated by measuring the water contact angle using a Phoenix 300 Touch. The sheet resistance data were measured by four-point probe method with a Keithley 2400 Source Meter unit and a HP 34401A multimeter. The Top of the films in the visible range were acquired using a Perkin-Elmer Lambda 750 UV/vis/NIR spectrophotometer. The SEM images were obtained using a Quanta 200 FEG microscope operated at 10 kV. The XPS analysis was performed using a VG Multilab 2000 (Thermo Electron Corporation) at a base pressure of 1 × 10−9 Torr. The mechanical flexibility of the PAM film was measured using a JIRBT-620 (Junil Tech, Korea). The film thicknesses were determined using a Surfcorder ET-3000 profilometer (Kosaka Laboratory Ltd.).

Device Fabrication and Measurement: The LEDs were fabricated with PAM/SPW-111/Ca/Al or ITO/MoO3/SPW-111/Ca/Al structures. Electroluminescent SPW-111 layers (70 nm) were formed by spin-coating at 2500 rpm for 40 s in an N2-filled glove box and were annealed at 80 °C for 10 min on a hotplate. Finally, the Ca (20 nm) and Al (100 nm) for the cathode were deposited by thermal evaporation under high vacuum conditions (<10−6 Torr). The area of the cathode was 4.64 mm2. The I–V–L characteristics of the LEDs were determined using a PR650 spectrophotometer with a Keithley 2400 Source Measure unit.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsS.J. and S.J. contributed equally to this work. The authors thank the Heeger Center for Advanced Materials (HCAM) and the Research Institute of Solar and Sustainable Energies (RISE) at the Gwangju Institute of Science and Technology (GIST) of Korea for their assistance with the fabrication and characterization. This work was primarily supported by the R&D program of MSIP/COMPA (2016K000222). This research was also supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (NRF-2014R1A2A1A09006137 and NRF-2015K1A3A1A16002247) and the Technology Development Program to Solve Climate Changes of the NRF funded by the Ministry of Science, Information and Communication Technology and Future Planning (NRF-2015M1A2A2057510). K.L. also acknowledges the support provided by the “GIST Research Institute (GRI)” Project through a grant provided by GIST in 2017 and the GIST-ICL International Collaboration R&D Centre.

Conflict of InterestThe authors declare no conflict of interest.



Keywordsflexible and transparent electrodes, nonconjugated polyelectrolytes, nucleation inducers, polymer light-emitting diodes, ultrathin metal electrodes

Received: December 28, 2016Revised: March 3, 2017

Published online: April 4, 2017

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