n Hee Seo

Pt-free transparent counter electrodes for dye-sensitized solar cells prepared from carbon nanotube micro-balls

Since their initial invention, dye-sensitized solar cells (DSSCs) have offered cost-effective photovoltaic systems. For their counter electrodes, DSSCs generally employ Pt nanoparticles. However, Pt is expensive, rare, and already widely in demand as catalyst in various chemical and electrochemical fields. Substitutes for Pt have been sought among carbon materials, such as activated carbon, carbon black, and carbon nanotubes. Carbon nanotubes (CNTs) are the most appealing candidates, because of their favorable electrochemical catalytic activities. Unfortunately, as with other carbon materials, CNTs cannot provide high charge exchange currents. To obtain performances comparable to Pt counter electrodes, large surface areas are required, resulting in thick electrodes. We have found that transparent Pt-free counter electrodes suitable for DSSCs can be prepared using MWCNT micro-balls deposited on transparent substrates. The deposition density (i.e., the number of CNT micro-balls per unit area) can be controlled, allowing transparency and DSSC performance to be tuned. For a counter electrode transparency of 70%, the efficiency of a DSSC using CNT micro-balls is more than 80% of one using Pt nanoparticles. The prepared CNT micro-balls can be usefully applied in other electrochemical devices, such as battery and supercapacitors.

Dye-sensitized solar cells on glass paper: TCO-free highly bendable dye-sensitized solar cells inspired by the traditional Korean door structure

Dye-sensitized solar cells (DSSCs) are considered a suitable photovoltaic system for urban applications and highly bendable DSSCs can be expanded to applications such as dispensable DSSCs for commercial advertising and small portable power sources. However, although many reports have shown flexible or highly bendable photoelectrodes using TCO-coated polymeric substrates or metal meshes, until now, few have shown highly bendable DSSCs using electrodes because the flexibility of a single electrode is not a critical issue for highly bendable DSSCs. Here, we report a new DSSC design, inspired by the traditional Korean door structure consisting of a paper-bonded wooden frame, and a process for TCO-free highly bendable DSSCs utilizing glass paper and metal mesh. In the new DSSC design, constituents such as stainless steel mesh and mesoporous TiO2 loaded with a Ru-complex dye were bonded on the glass paper, which was sputter-coated with Pt on one side and filled with electrolyte. The glass-paper-based flexible DSSCs showed 2% energy-conversion efficiency, which was maintained under bending until the radius of curvature reached 2 cm. The new glass-paper-based flexible DSSCs may have potential applications as low-cost highly bendable solar cells to overcome the limitations of conventional sandwich-type DSSCs.

Dispersant-free conducting pastes for flexible and printed nanocarbon electrodes.

The dispersant-free fabrication of highly conducting pastes based on organic solvents with nanocarbon materials such as carbon nanotubes and graphene nanoplatelets has been hindered by severe agglomeration. Here we report a straightforward method for fabricating nanocarbon suspensions with >10% weight concentrations in absence of organic dispersants. The method involves introducing supramolecular quadruple hydrogen-bonding motifs into the nanocarbon materials without sacrificing the electrical conductivity. Printed films of these materials show high electrical conductivity of ~500,000 S m(-1) by hybridization with 5 vol% silver nanowires. In addition, the printed nanocarbon electrodes provide high-performance alternatives to the platinum catalytic electrodes commonly used in dye-sensitized solar cells and electrochemical electrodes in supercapacitors. The judicious use of supramolecular interactions allows fabrication of printable, spinnable and chemically compatible conducting pastes with high-quality nanocarbon materials, useful in flexible electronics and textile electronics.

Highly Flexible Dye-sensitized Solar Cells Produced by Sewing Textile Electrodes on Cloth

Textile forms of solar cells possess special advantages over other types of solar cells, including their light weight, high flexibility, and mechanical robustness. Recent demand for wearable devices has promoted interest in the development of high-efficiency textile-based solar cells for energy suppliers. However, the weaving process occurs under high-friction, high-tension conditions that are not conducive to coated solar-cell active layers or electrodes deposited on the wire or strings. Therefore, a new approach is needed for the development of textile-based solar cells suitable for woven fabrics for wide-range application. In this report, we present a highly flexible, efficient DSSC, fabricated by sewing textile-structured electrodes onto casual fabrics such as cotton, silk, and felt, or paper, thereby forming core integrated DSSC structures with high energy-conversion efficiency (~5.8%). The fabricated textile-based DSSC devices showed high flexibility and high performance under 4-mm radius of curvature over thousands of deformation cycles. Considering the vast number of textile types, our textile-based DSSC devices offer a huge range of applications, including transparent, stretchable, wearable devices.

Insertion of Dye-Sensitized Solar Cells in Textiles using a Conventional Weaving Process

Increasing demands for wearable energy sources and highly flexible, lightweight photovoltaic devices have stimulated the development of textile-structured solar cells. However, the former approach of wire-type solar cell fabrication, followed by weaving of these devices, has had limited success, due to device failure caused by high friction forces and tension forces during the weaving process. To overcome this limitation, we present a new approach for textile solar cell fabrication, in which dye-sensitized solar cell (DSSC) electrodes are incorporated into the textile during the weaving process, using the textile warp as a spacer to maintain the DSSC structure. Porous, dye-loaded TiO2-coated holed metal ribbon and Pt nanoparticle-loaded carbon yarn were used as the photoanode and counterelectrode, respectively. The highly flexible textile-based solar cell was fabricated using a common weaving process with a loom. The inserted DSSCs in the textile demonstrated an energy conversion efficiency of 2.63% (at 1 sun, 1.5 A.M.). Our results revealed that additional performance enhancement was possible by considering other electrode materials and textile structures, as well as where and how the DSSC electrodes are inserted. In addition, we demonstrated that the inserted DSSCs could be electrically connected using a parallel configuration.

Bioinspired Multifunctional Superhydrophobic Surfaces with Carbon-Nanotube-Based Conducting Pastes by Facile and Scalable Printing

Directly printed superhydrophobic surfaces containing conducting nanomaterials can be used for a wide range of applications in terms of nonwetting, anisotropic wetting, and electrical conductivity. Here, we demonstrated that direct-printable and flexible superhydrophobic surfaces were fabricated on flexible substrates via with an ultrafacile and scalable screen printing with carbon nanotube (CNT)-based conducting pastes. A polydimethylsiloxane (PDMS)-polyethylene glycol (PEG) copolymer was used as an additive for conducting pastes to realize the printability of the conducting paste as well as the hydrophobicity of the printed surface. The screen-printed conducting surfaces showed a high water contact angle (WCA) (>150°) and low contact angle hysteresis (WCA < 5°) at 25 wt % PDMS-PEG copolymer in the paste, and they have an electrical conductivity of over 1000 S m-1. Patterned superhydrophobic surfaces also showed sticky superhydrophobic characteristics and were used to transport water droplets. Moreover, fabricated films on metal meshes were used for an oil/water separation filter, and liquid evaporation behavior was investigated on the superhydrophobic and conductive thin-film heaters by applying direct current voltage to the film.

Spray-dried and pre-sintered TiO2 micro-balls for sinter-free processing of dye-sensitized solar cells

A sinter-free TiO2 electrode for flexible DSSCs was fabricated by utilizing spherical aggregates of pre-sintered TiO2 nanoparticles 2–5 μm in diameter (micro-balls), which are prepared by a spray-drying process. The network of interconnected TiO2 nanoparticles within the pre-sintered TiO2 micro-balls resulted in an electron diffusion path from adsorbed dye to an underlying TCO surface, while the nanoparticles themselves made a porous, high surface area TiO2 structure, hence improved energy conversion efficiency.

Efficient Low-Temperature Transparent Electrocatalytic Layers Based on Graphene Oxide Nanosheets for Dye-Sensitized Solar Cells

Electrocatalytic materials with a porous structure have been fabricated on glass substrates, via high-temperature fabrication, for application as alternatives to platinum in dye-sensitized solar cells (DSCs). Efficient, nonporous, nanometer-thick electrocatalytic layers based on graphene oxide (GO) nanosheets were prepared on plastic substrates using electrochemical control at low temperatures of ≤100 °C. Single-layer, oxygen-rich GO nanosheets prepared on indium tin oxide (ITO) substrates were electrochemically deoxygenated in acidic medium within a narrow scan range in order to obtain marginally reduced GO at minimum expense of the oxygen groups. The resulting electrochemically reduced GO (E-RGO) had a high density of residual alcohol groups with high electrocatalytic activity toward the positively charged cobalt-complex redox mediators used in DSCs. The ultrathin, alcohol-rich E-RGO layer on ITO-coated poly(ethylene terephthalate) was successfully applied as a lightweight, low-temperature counter electrode with an extremely high optical transmittance of ∼97.7% at 550 nm. A cobalt(II/III)-mediated DSC employing the highly transparent, alcohol-rich E-RGO electrode exhibited a photovoltaic power conversion efficiency of 5.07%. This is superior to that obtained with conventionally reduced GO using hydrazine (3.94%) and even similar to that obtained with platinum (5.10%). This is the first report of a highly transparent planar electrocatalytic layer based on carbonaceous materials fabricated on ITO plastics for application in DSCs.

High electrocatalytic activity of low-loaded transparent carbon nanotube assemblies for CoII/III-mediated dye-sensitized solar cells

The development of low-loaded electrocatalysts that can act as alternatives to platinum has been a long standing challenge for use in energy-related devices. Here we report that ultrashort carbon nanotube assemblies with a loading of <5 μg cm−2 exhibit notably high optoelectrochemical and photovoltaic performances, similar to those of conventional platinum, in dye-sensitized solar cells (DSCs) employing the redox mediator cobalt(II/III)tris(2,2′-bipyridine). The electrochemical activity of the densely packed open-end-rich nanotube assemblies is strongly influenced by the redox-active species used in the organic electrolyte. The extremely high transparency (∼97.5%) of the assembly allowed us to successfully fabricate CoII/III-mediated bifacial DSCs. The power conversion efficiencies for front- and rear-side irradiation were ∼4.7% and were almost insensitive to which face of the cell was irradiated.

Self-Catalytic Growth of β-Ga2O3 Nanowires Deposited by Radio-Frequency Magnetron Sputtering

We report the growth behavior of β-Ga2O3 nanowires (NWs) on sapphire (0001) substrates during radio-frequency magnetron sputtering. Upon fabrication, flat thin films grew initially, subsequent to which, NW bundles were formed on the surface of the thin film with increasing film thickness. This transition of the growth mode occurred only at temperatures greater than ~450 °C. The β-Ga2O3 NWs were grown through the self-catalytic vapor–liquid–solid mechanism with self-assembled Ga seeds. Secondary growth of NWs, which occurred from the sides of primary NWs resulting in branched NW structures, was also observed. Finally, the photoluminescence properties of as-grown and annealed β-Ga2O3 NW samples were investigated.