Keum Hwan Park

Enhanced light harvesting in bulk heterojunction photovoltaic devices with shape-controlled Ag nanomaterials: Ag nanoparticles versus Ag nanoplates

Enhanced power conversion efficiency (PCE(%)) with improved optical path length from two types of shape controlled silver (Ag) materials (Ag nanoplates versus Ag nanoparticles (NPs)) was studied in poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) or poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT)/[6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) bulk heterojunction (BHJ) devices. The Ag nanoplates and Ag NPs can be synthesized by simple solution polyol chemistry with well defined size and shape. A BHJ with a 0.5 wt% optimized blend ratio of Ag nanoplates shows improved cell performance and photo-current density than a BHJ with Ag NPs owing to the enhanced light absorption with the results of an excitation of localized surface plasmon and efficient light scattering by the Ag nanoplates embedded BHJ film. When the BHJ is combined with the Ag nanoplates at an optimized ratio of 0.5 wt%, the PCE (%) increases from 3.2% to 4.4% in P3HT/PC71BM, and from 5.9% to 6.6% in PCDTBT/PC71BM BHJ devices.

The size control of silver nanocrystals with different polyols and its application to low-reflection coating materials

The size of silver nanocrystals in polyol synthesis can be simply controlled by tuning the viscosity of the reaction medium such as ethylene glycol, 1,2-propanediol, 1,4-butanediol and 1,5-pentanediol. We found that a higher viscose medium (1,5-pentanediol) led to monodispersed smaller particles thanks to the slow addition of silver atoms into the nuclei. Size-controlled silver nanocrystals of 30 nm were obtained in a viscosity controlled medium of 1,5-pentanediol to synthesize a low refractive index filler by coating with silica and subsequent etching of the silver core. The coated low-reflection layer from the hollow silica nanoparticles on polyethylene terephthalate (PET) film can greatly reduce the reflection of the PET film from 10% to 2% over the entire visible region.

Capillary flow of amorphous metal for high performance electrode

Metallic glass (MG) assists electrical contact of screen-printed silver electrodes and leads to comparable electrode performance to that of electroplated electrodes. For high electrode performance, MG needs to be infiltrated into nanometer-scale cavities between Ag particles and reacts with them. Here, we show that the MG in the supercooled state can fill the gap between Ag particles within a remarkably short time due to capillary effect. The flow behavior of the MG is revealed by computational fluid dynamics and density funtional theory simulation. Also, we suggest the formation mechanism of the Ag electrodes, and demonstrate the criteria of MG for higher electrode performance. Consequently, when Al85Ni5Y8Co2 MG is added in the Ag electrodes, cell efficiency is enhanced up to 20.30% which is the highest efficiency reported so far for screen-printed interdigitated back contact solar cells. These results show the possibility for the replacement of electroplating process to screen-printing process.

Crack-induced Ag nanowire networks for transparent, stretchable, and highly sensitive strain sensors

Crack-based strain sensor systems have been known for its high sensitivity, but suffer from the small fracture strain of the thin metal films employed in the sensor which results in its negligible stretchability. Herein, we fabricated a transparent (>90% at 550 nm wavelength), stretchable (up to 100%), and sensitive (gauge factor (GF) of 30 at 100% strain) strain gauge by depositing an encapsulated crack-induced Ag nanowire (AgNW) network on a hydroxylated poly(dimethylsiloxane) (PDMS) film. Stretching the encapsulated AgNWs/PDMS resulted in the formation of a percolation network of nanowire ligaments with abundant percolation paths. The encapsulating polymer was designed to adhere strongly to both the AgNW and PDMS. The improved adhesion ensured the resistance of the crack-induced network of AgNWs varied reversibly, stably, and sensitively when stretched and released, at strains of up to 100%. The developed sensor successfully detected human motions when applied to the skin.

Enhancement of electrical conductivity of thick silver electrode using a tailored amorphous alloy

Metallic glass (MG) can be a promising additive for the screen-printed silver electrode, when the composition of MG is properly tailored. When Cu-Zr-Al MG is used as an additive, the contact resistance between the Ag electrode and the Si emitter significantly decreases down to 5.39 mΩ cm2 on 100 Ω/sq of emitter, while when Cu-Zr Mg is used, the contact resistance is 31.28 mΩ cm2. Wider supercooled-liquid region and high reactivity with silver of Cu-Zr-Al MG lead to wider contact area of the Ag electrode and higher density of Ag crystallites, resulting in such a low contact resistance.

Thermal decomposition of silver acetate in silver paste for solar cell metallization: An effective route to reduce contact resistance

A screen printed silver/metallic glass (MG) paste formulated with Ag acetate resulted in a specific contact resistance in the range of 0.6–0.7 mΩ·cm2 on both the n- and p-type Si emitters of interdigitated back-contact solar cells. Silver nanocrystallites resulting from thermally decomposed Ag acetate prevented the Al MG frits from directly interacting with the Si emitter, thus reducing the amount of Al diffused into the Si emitters, and subsequently, the contact resistance. A photovoltaic conversion efficiency of 20.3% was achieved using this technique.

Electrically bistable Ag nanocrystal-embedded metal–organic framework microneedles

Ag nanocrystal-embedded metal–organic frameworks (MOF) were synthesized using a one-pot synthetic method by introducing melamine into a polyol process for the synthesis of Ag nanocrystals. The resulting MOF frameworks showed needle-like structures, inside which many Ag nanocrystals smaller than 10 nm were uniformly embedded, and exhibited electrical bistability reproducibly.

Synthesis of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)-capped silver nanoparticles and their application to blue polymer light-emitting diodes

Organic light-emitting diodes (OLED) and polymer light-emitting diodes (PLED) are promising candidates for future display applications due to their superior properties, but their efficiency and stability need to be improved to expand their application to large-size display panels and lightings. One of the most remarkable ways to enhance the efficiency of PLEDs is to incorporate metal nanoparticles and utilize their localized surface plasmon resonance (LSPR). We report on the improvement of blue PLEDs efficiency by the insertion of silver nanoparticles (Ag NPs) capped by poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS). Ag NPs were synthesized with PEDOT : PSS as a stabilizer and then deposited on an indium tin oxide (ITO) anode using a simple spin-coating process without any aggregation. The result of deposition was confirmed by SEM and TEM images, and by Raman spectrum. Optical properties of the PEDOT : PSS-capped Ag NPs on ITO and the interaction between Ag NPs and Lumation blueJ, a blue light-emitting polymer, were measured using a UV-Vis spectrophotometer, a photoluminescence (PL) spectrophotometer, and a time-resolved photoluminescence spectrophotometer (TRPL). As a result, the introduction of PEDOT : PSS-capped Ag NPs to the blue PLEDs was found to have been successfully conducted. The fabricated blue PLEDs with Ag NPs exhibited a 15% increase of external quantum efficiency. This was thought to originate from the localized surface plasmon coupling of the PEDOT : PSS-capped Ag NPs with Lumation BlueJ.

Enhanced performance of blue polymer light-emitting diodes by incorporation of Ag nanoparticles through the ligand-exchange process

A coupling interaction between localised surface plasmon resonance of metal nanoparticles (NPs) and emitting materials has been shown to improve the internal quantum efficiency in organic light-emitting diodes. In this research, Ag NPs were introduced into blue polymer light-emitting diodes (PLEDs), which suffer from low efficiency. Ag NPs were attached to the indium tin oxide (ITO) surface through a ligand-exchange process, which guarantees well-dispersed Ag NPs, to maximise resonant coupling and to prevent optical and electrical side effects. Mercaptoacid molecules not only function as a coupling agent between ITO and Ag NPs, but also aid the transfer of holes by increasing the work function of ITO, as confirmed by tests on hole-only devices and by ultraviolet photoelectron spectroscopy. Maximum current efficiencies of blue PLEDs improved by 25% with thioglycolic acid (TGA) only and by 48% with 0.302 nM Ag NPs attached to TGA. We confirmed from time-resolved photoluminescence spectra and normalised electroluminescence spectra that the enhancement mechanism is the resonant interaction between Ag NPs and the blue light-emitting polymer, SPB-02T.

Highly stable electrical manipulation of reflective colors in colloidal crystals of sulfate iron oxide particles in organic media

Monodisperse Fe3O4 particles are prepared by solvothermal reaction, in which negatively charged sulfate groups are formed on the surface by adding potassium sulfate during the high-pressure reaction. Then, those particles are successfully dispersed in the polar solvent propylene carbonate and manipulated electrophoretically between transparent electrodes. By applying an external electric field, they showed structural colors which can be tuned precisely over the visible range depending on the strength of the electric field. Because sulfate groups on the particle surface are chemically stable or inert, those particles are highly stable against irreversible deposition on the electrode during electrophoretic operation. Therefore, we demonstrate highly stable color switches over at least hundreds of cyclic operations, which may be useful in real electronic paper display applications.