Jung-Yong Lee

Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes

We report a comprehensive study of transparent and conductive silver nanowire (Ag NW) electrodes, including a scalable fabrication process, morphologies, and optical, mechanical adhesion, and flexibility properties, and various routes to improve the performance. We utilized a synthesis specifically designed for long and thin wires for improved performance in terms of sheet resistance and optical transmittance. Twenty Omega/sq and approximately 80% specular transmittance, and 8 ohms/sq and 80% diffusive transmittance in the visible range are achieved, which fall in the same range as the best indium tin oxide (ITO) samples on plastic substrates for flexible electronics and solar cells. The Ag NW electrodes show optical transparencies superior to ITO for near-infrared wavelengths (2-fold higher transmission). Owing to light scattering effects, the Ag NW network has the largest difference between diffusive transmittance and specular transmittance when compared with ITO and carbon nanotube electrodes, a property which could greatly enhance solar cell performance. A mechanical study shows that Ag NW electrodes on flexible substrates show excellent robustness when subjected to bending. We also study the electrical conductance of Ag nanowires and their junctions and report a facile electrochemical method for a Au coating to reduce the wire-to-wire junction resistance for better overall film conductance. Simple mechanical pressing was also found to increase the NW film conductance due to the reduction of junction resistance. The overall properties of transparent Ag NW electrodes meet the requirements of transparent electrodes for many applications and could be an immediate ITO replacement for flexible electronics and solar cells.

Solution-processed metal nanowire mesh transparent electrodes.

Transparent conductive electrodes are important components of thin-film solar cells, light-emitting diodes, and many display technologies. Doped metal oxides are commonly used, but their optical transparency is limited for films with a low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates, are costly, and require a high-temperature step for the best performance. We demonstrate solution-processed transparent electrodes consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal-oxide thin films for the same sheet resistance. Organic solar cells deposited on these electrodes show a performance equivalent to that of devices based on a conventional metal-oxide transparent electrode.

Fully Solution-Processed Inverted Polymer Solar Cells with Laminated Nanowire Electrodes

We demonstrate organic photovoltaic cells in which every layer is deposited by solution processing on opaque metal substrates, with efficiencies similar to those obtained in conventional device structures on transparent substrates. The device architecture is enabled by solution-processed, laminated silver nanowire films serving as the top transparent anode. The cells are based on the regioregular poly(3-hexylthiophene) and C(61) butyric acid methyl ester bulk heterojunction and reach an efficiency of 2.5% under 100 mW/cm(2) of AM 1.5G illumination. The metal substrates are adequate barriers to moisture and oxygen, in contrast to transparent plastics that have previously been used, giving rise to the possibility of roll-to-roll solution-processed solar cells that are packaged by lamination to glass substrates, combining the cost advantage of roll-to-roll processing with the barrier properties of glass and metal foil.

Semitransparent Organic Photovoltaic Cells with Laminated Top Electrode

We demonstrate semitransparent small molecular weight organic photovoltaic cells using a laminated silver nanowire mesh as a transparent, conductive cathode layer. The lamination process does not damage the underlying solar cell and results in a transparent electrode with low sheet resistance and high optical transmittance without impacting photocurrent collection. The resulting semitransparent phthalocyanine/fullerene organic solar cell has a power conversion efficiency that is 57% of that of a device with a conventional metal cathode due to differences in optical absorption.

Wearable Textile Battery Rechargeable by Solar Energy

Wearable electronics represent a significant paradigm shift in consumer electronics since they eliminate the necessity for separate carriage of devices. In particular, integration of flexible electronic devices with clothes, glasses, watches, and skin will bring new opportunities beyond what can be imagined by current inflexible counterparts. Although considerable progresses have been seen for wearable electronics, lithium rechargeable batteries, the power sources of the devices, do not keep pace with such progresses due to tenuous mechanical stabilities, causing them to remain as the limiting elements in the entire technology. Herein, we revisit the key components of the battery (current collector, binder, and separator) and replace them with the materials that support robust mechanical endurance of the battery. The final full-cells in the forms of clothes and watchstraps exhibited comparable electrochemical performance to those of conventional metal foil-based cells even under severe folding-unfolding motions simulating actual wearing conditions. Furthermore, the wearable textile battery was integrated with flexible and lightweight solar cells on the battery pouch to enable convenient solar-charging capabilities.

Enhancement of optical absorption in thin-film organic solar cells through the excitation of plasmonic modes in metallic gratings

We theoretically investigate the enhancement of optical absorption in thin-film organic solar cells in which the top transparent electrode is partially substituted by a periodic metallic grating. We show that the grating can result in broadband optical absorption enhancement for TM-polarized light, due to the large field enhancement in the vicinity of the strips of the grating, associated with the excitation of plasmonic modes. The overall optical absorption in the organic layers can be greatly enhanced up to ∼50% for such solar cell structures.

Plasmonic Forward Scattering Effect in Organic Solar Cells: A Powerful Optical Engineering Method

In this report, plasmonic effects in organic photovoltaic cells (OPVs) are systematically analyzed using size-controlled silver nanoparticles (AgNPs, diameter: 10 ~ 100 nm), which were incorporated into the anodic buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The optical properties of AgNPs tuned by size considerably influence the performance levels of devices. The power conversion efficiency (PCE) was increased from 6.4% to 7.6% in 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 (PC70BM) based-OPVs and from 7.9% to 8.6% in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):PC70BM based-OPVs upon embedding the AgNPs. The external quantum efficiency (EQE) was significantly enhanced by the absorption enhancement due to the plasmonic scattering effect. Finally, we verified the origin of the size-dependent plasmonic forwarding scattering effect of the AgNPs by visualizing the scattering field with near-field optical microscopy (NSOM) and through analytic optical simulations.

The origin of enhanced optical absorption in solar cells with metal nanoparticles embedded in the active layer

We analyze the enhancement in optical absorption of an absorbing medium when spherical metal nanoparticles are embedded in it. Our analysis uses generalized Mie theory to calculate the absorbed optical power as a function of the distance from the metal nanoparticle. This analysis is used to evaluate the potential of enhancing optical absorption in thin-film solar cells by embedding spherical metal nanoparticles. We consider the trade-off between maximizing overall optical absorption and ensuring that a large fraction of the incident optical power is dissipated in the absorbing host medium rather than in the metal nanoparticle. We show that enhanced optical absorption results from strong scattering by the metal nanoparticle which locally enhances the optical electric fields. We also discuss the effect of a thin dielectric encapsulation of the metal nanoparticles.

Efficient Welding of Silver Nanowire Networks without Post-Processing

Silver nanowire (AgNW) random meshes have attracted considerable attention as flexible and high-performance transparent electrodes. Notably, post-treatment of the AgNW random meshes, such as thermal annealing, is usually required to guarantee comparable optical transparency and electrical conductivity to commercial indium tin oxide (ITO). Here, the integral elements of preparing a high-performance, large-area AgNW random mesh network are discussed. High-performance nanostructured transparent electrodes can be obtained without any post-treatment, thereby relieving the restrictions related to the substrate. Solvent washing and a large-area spray-coating method effectively reduce the wire-wire contact resistances, thus reducing or eliminating the requirement for post-treatment.

Au@Ag Core–Shell Nanocubes for Efficient Plasmonic Light Scattering Effect in Low Bandgap Organic Solar Cells

In this report, we propose a metal-metal core-shell nanocube (NC) as an advanced plasmonic material for highly efficient organic solar cells (OSCs). We covered an Au core with a thin Ag shell as a scattering enhancer to build Au@Ag NCs, which showed stronger scattering efficiency than Au nanoparticles (AuNPs) throughout the visible range. Highly efficient plasmonic organic solar cells were fabricated by embedding Au@Ag NCs into an anodic buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the power conversion efficiency was enhanced to 6.3% from 5.3% in 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 (PC70BM) based OSCs and 9.2% from 7.9% in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):PC70BM based OSCs. The Au@Ag NC plasmonic PCDTBT:PC70BM-based organic solar cells showed 2.2-fold higher external quantum efficiency enhancement compared to AuNPs devices at a wavelength of 450-700 nm due to the amplified plasmonic scattering effect. Finally, we proved the strongly enhanced plasmonic scattering efficiency of Au@Ag NCs embedded in organic solar cells via theoretical calculations and detailed optical measurements.