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Ultra-thin and smooth transparent electrode for flexible and leakage-free organic light-emitting diodes.

Ok KH, Kim J, Park SR, Kim Y, Lee CJ, Hong SJ, Kwak MG, Kim N, Han CJ, Kim JW - Sci Rep (2015)

Bottom Line: Leveraging the robust mechanical properties and flexibility of cPI, the thickness of the composite film was reduced to less than 10 μm, which is conducive to extreme flexibility.As a result, we were able to finely control the flexible OLEDs' electroluminescent properties using the enlarged conductive pathways.The fabricated flexible devices showed only slight performance reductions of <3% even after repeated foldings with a 30 μm bending radius.

View Article: PubMed Central - PubMed

Affiliation: Display Components &Materials Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Korea.

ABSTRACT
A smooth, ultra-flexible, and transparent electrode was developed from silver nanowires (AgNWs) embedded in a colorless polyimide (cPI) by utilizing an inverted film-processing method. The resulting AgNW-cPI composite electrode had a transparency of >80%, a low sheet resistance of 8 Ω/□, and ultra-smooth surfaces comparable to glass. Leveraging the robust mechanical properties and flexibility of cPI, the thickness of the composite film was reduced to less than 10 μm, which is conducive to extreme flexibility. This film exhibited mechanical durability, for both outward and inward bending tests, up to a bending radius of 30 μm, while maintaining its electrical performance under cyclic bending (bending radius: 500 μm) for 100,000 iterations. Phosphorescent, blue organic light-emitting diodes (OLEDs) were fabricated using these composites as bottom electrodes (anodes). Hole-injection was poor, because AgNWs were largely buried beneath the composite's surface. Thus, we used a simple plasma treatment to remove the thin cPI layer overlaying the nanowires without introducing other conductive materials. As a result, we were able to finely control the flexible OLEDs' electroluminescent properties using the enlarged conductive pathways. The fabricated flexible devices showed only slight performance reductions of <3% even after repeated foldings with a 30 μm bending radius.

No MeSH data available.


Related in: MedlinePlus

Fabrication of transparent and conductive films having extreme flexibility and smoothness:(a) a schematic description for the fabrication procedure, (b) a cross-sectional view of the Ag-nanowire-embedded structure processed by a FIB, (c) a fabricated AgNW-cPI composite film on printed Korea Electronics Technology Institute (KETI) emblems, (d) measured transmittance of the fabricated transparent electrode with varying AgNW density (including the transmittance of cPI).
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f1: Fabrication of transparent and conductive films having extreme flexibility and smoothness:(a) a schematic description for the fabrication procedure, (b) a cross-sectional view of the Ag-nanowire-embedded structure processed by a FIB, (c) a fabricated AgNW-cPI composite film on printed Korea Electronics Technology Institute (KETI) emblems, (d) measured transmittance of the fabricated transparent electrode with varying AgNW density (including the transmittance of cPI).

Mentions: The fabrication procedure for the AgNW-cPI composite is schematically illustrated in Fig. 1(a). First, a sacrificial layer—to be removed by a specific method, e.g., dissolution by an organic solvent or decomposition by exposure to ultra violet (UV) or laser light—is coated on a pre-cleaned glass substrate. Here, we employed poly(methyl methacrylate) (PMMA) as a sacrificial layer, which is easily dissolved by acetone for delamination of the AgNW-cPI composite film from the supporting glass. A PMMA suspension in chlorobenzene was coated on a glass sheet by a spin casting, followed by a baking at 120°C. An AgNW dispersion in isopropyl alcohol (IPA) was deposited on the surface of the PMMA to form a transparent and conductive network. After drying and patterning the AgNW networks, a cPI varnish was spin-coated onto the AgNW network and subsequently cured at 150°C for one hour. The thickness of the composite films was precisely controlled by varying the spinning velocity, and we fabricated samples with thicknesses of 7–8 μm, which is a suitable value for both extreme flexibility and reliable performance. In fact, the thickness could most likely be reduced to less than 5 μm without any loss of mechanical stability. Figure 1(b) shows a cross-sectional scanning electron microscope (SEM) image for the AgNW-cPI composite fabricated on glass. The buried AgNWs on the surface of the cPI, as well as the sacrificial PMMA layer, are clearly seen in this figure. Then, the composite films were peeled off from the glass by immersing the samples in acetone for 2 min. When the composite films were directly delaminated at ambient conditions without the introduction of a sacrificial layer, a rough surface with some AgNWs extruding from the originally buried sites was observed (see Fig. S3). It is worth noting that the roughness measured from peak to valley (Rpv) is large enough to cause a vertical short or leakage, while the root mean square (RMS) value would indicate stability. This implies that the surface state of an electrode cannot be determined based solely on a single parameter of roughness (either an Rpv or RRMS value). After drying at room temperature for 5 min, highly transparent and conductive films with line patterns were obtained, as shown in Fig. 1(c). As can be seen in the figure, the fabricated film is very clear and nearly haze-free.


Ultra-thin and smooth transparent electrode for flexible and leakage-free organic light-emitting diodes.

Ok KH, Kim J, Park SR, Kim Y, Lee CJ, Hong SJ, Kwak MG, Kim N, Han CJ, Kim JW - Sci Rep (2015)

Fabrication of transparent and conductive films having extreme flexibility and smoothness:(a) a schematic description for the fabrication procedure, (b) a cross-sectional view of the Ag-nanowire-embedded structure processed by a FIB, (c) a fabricated AgNW-cPI composite film on printed Korea Electronics Technology Institute (KETI) emblems, (d) measured transmittance of the fabricated transparent electrode with varying AgNW density (including the transmittance of cPI).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4379502&req=5

f1: Fabrication of transparent and conductive films having extreme flexibility and smoothness:(a) a schematic description for the fabrication procedure, (b) a cross-sectional view of the Ag-nanowire-embedded structure processed by a FIB, (c) a fabricated AgNW-cPI composite film on printed Korea Electronics Technology Institute (KETI) emblems, (d) measured transmittance of the fabricated transparent electrode with varying AgNW density (including the transmittance of cPI).
Mentions: The fabrication procedure for the AgNW-cPI composite is schematically illustrated in Fig. 1(a). First, a sacrificial layer—to be removed by a specific method, e.g., dissolution by an organic solvent or decomposition by exposure to ultra violet (UV) or laser light—is coated on a pre-cleaned glass substrate. Here, we employed poly(methyl methacrylate) (PMMA) as a sacrificial layer, which is easily dissolved by acetone for delamination of the AgNW-cPI composite film from the supporting glass. A PMMA suspension in chlorobenzene was coated on a glass sheet by a spin casting, followed by a baking at 120°C. An AgNW dispersion in isopropyl alcohol (IPA) was deposited on the surface of the PMMA to form a transparent and conductive network. After drying and patterning the AgNW networks, a cPI varnish was spin-coated onto the AgNW network and subsequently cured at 150°C for one hour. The thickness of the composite films was precisely controlled by varying the spinning velocity, and we fabricated samples with thicknesses of 7–8 μm, which is a suitable value for both extreme flexibility and reliable performance. In fact, the thickness could most likely be reduced to less than 5 μm without any loss of mechanical stability. Figure 1(b) shows a cross-sectional scanning electron microscope (SEM) image for the AgNW-cPI composite fabricated on glass. The buried AgNWs on the surface of the cPI, as well as the sacrificial PMMA layer, are clearly seen in this figure. Then, the composite films were peeled off from the glass by immersing the samples in acetone for 2 min. When the composite films were directly delaminated at ambient conditions without the introduction of a sacrificial layer, a rough surface with some AgNWs extruding from the originally buried sites was observed (see Fig. S3). It is worth noting that the roughness measured from peak to valley (Rpv) is large enough to cause a vertical short or leakage, while the root mean square (RMS) value would indicate stability. This implies that the surface state of an electrode cannot be determined based solely on a single parameter of roughness (either an Rpv or RRMS value). After drying at room temperature for 5 min, highly transparent and conductive films with line patterns were obtained, as shown in Fig. 1(c). As can be seen in the figure, the fabricated film is very clear and nearly haze-free.

Bottom Line: Leveraging the robust mechanical properties and flexibility of cPI, the thickness of the composite film was reduced to less than 10 μm, which is conducive to extreme flexibility.As a result, we were able to finely control the flexible OLEDs' electroluminescent properties using the enlarged conductive pathways.The fabricated flexible devices showed only slight performance reductions of <3% even after repeated foldings with a 30 μm bending radius.

View Article: PubMed Central - PubMed

Affiliation: Display Components &Materials Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Korea.

ABSTRACT
A smooth, ultra-flexible, and transparent electrode was developed from silver nanowires (AgNWs) embedded in a colorless polyimide (cPI) by utilizing an inverted film-processing method. The resulting AgNW-cPI composite electrode had a transparency of >80%, a low sheet resistance of 8 Ω/□, and ultra-smooth surfaces comparable to glass. Leveraging the robust mechanical properties and flexibility of cPI, the thickness of the composite film was reduced to less than 10 μm, which is conducive to extreme flexibility. This film exhibited mechanical durability, for both outward and inward bending tests, up to a bending radius of 30 μm, while maintaining its electrical performance under cyclic bending (bending radius: 500 μm) for 100,000 iterations. Phosphorescent, blue organic light-emitting diodes (OLEDs) were fabricated using these composites as bottom electrodes (anodes). Hole-injection was poor, because AgNWs were largely buried beneath the composite's surface. Thus, we used a simple plasma treatment to remove the thin cPI layer overlaying the nanowires without introducing other conductive materials. As a result, we were able to finely control the flexible OLEDs' electroluminescent properties using the enlarged conductive pathways. The fabricated flexible devices showed only slight performance reductions of <3% even after repeated foldings with a 30 μm bending radius.

No MeSH data available.


Related in: MedlinePlus