Seung I. Cha

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.

Nanoporous cobalt foam and a Co/Co(OH)2 core–shell structure for electrochemical applications

Nanoporous metal foams have good electrical and thermal conductivities and potential catalytic activities because of their high surface areas. In this study, nanoporous cobalt foam was prepared by simple consolidation of a pearl-necklace-type CNT/Co3O4 nanocomposite powder. During heat treatment of the pre-compacted powder in an inert atmosphere, Co3O4 particles were reduced to cobalt metal and formed a three-dimensional, continuous nanoporous metallic structure. This nanoporous cobalt foam could be used as an excellent conducting framework because of the superior electrical conductivity of the metal. A Co/Co(OH)2 core–shell structure was prepared by coating Co(OH)2 onto the nanoporous cobalt foam and using it as the electrode for a supercapacitor. Because of the high surface area of the nanoporous cobalt metal frame, the Co/Co(OH)2 core–shell structure had a specific capacitance of 525 F g−1 at a current density of 0.5 A g−1.

A new hybrid architecture consisting of highly mesoporous CNT/carbon nanofibers from starch

Currently used activated carbon electrodes from commercial products contain mostly micropores (<2 nm), which are not easily accessible to electrolyte ions. Therefore, mesoporous carbons, with their more accessible porous infrastructure, are promising materials to maximize the capacitance in electrochemical capacitors. This paper reports a new hybrid carbon nanofiber architecture having mesopores with a narrow distribution, highly accessible surface area, low resistivity, and high stability by electrospinning of starch without using the template method for the first time. By using the natural ability of the starch lamellar structure and controlling the carbonization temperature, we successfully fabricated a new hybrid carbon architecture consisting of CNT reinforced-carbon nanofibers with a pore diameter of 4.76 nm and pore volume of 0.31 cm3 g−1. It shows a higher specific capacitance (170 F g−1) and electrical conductivity (2.1 S cm−1) than other carbon materials derived from synthetic polymers and free-standing CNT papers.

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.

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.

High Energy Conversion Efficiency with 3-D Micro-Patterned Photoanode for Enhancement Diffusivity and Modification of Photon Distribution in Dye-Sensitized Solar Cells

Dye sensitize solar cells (DSSCs) have been considered as the promising alternatives silicon based solar cell with their characteristics including high efficiency under weak illumination and insensitive power output to incident angle. Therefore, many researches have been studied to improve the energy conversion efficiency of DSSCs. However the efficiency of DSSCs are still trapped at the around 10%. In this study, micro-scale hexagonal shape patterned photoanode have proposed to modify light distribution of photon. In the patterned electrode, the appearance efficiency have been obtained from 7.1% to 7.8% considered active area and the efficiency of 12.7% have been obtained based on the photoanode area. Enhancing diffusion of electrons and modification of photon distribution utilizing the morphology of the electrode are major factors to improving the performance of patterned electrode. Also, finite element method analyses of photon distributions were conducted to estimate morphological effect that influence on the photon distribution and current density. From our proposed study, it is expecting that patterned electrode is one of the solution to overcome the stagnant efficiency and one of the optimized geometry of electrode to modify photon distribution. Process of inter-patterning in photoanode has been minimized.

Dependence of the electrocatalytic performance of platinized counter electrodes on the redox mediator employed in dye-sensitized solar cells

The electrocatalytic properties of platinized counter electrodes (Pt CEs) prepared by various coating methods were investigated with respect to the redox mediator, including the widely used iodide/tri-iodide (I−/I3−) and the more recently introduced cobalt(II/III)tris(2,2′-bipyridine) (Co(bpy)32+/3+), for application in dye-sensitized solar cells (DSCs). The coating methods controlled Pt loading and the surface morphology of the Pt CEs. For a high-performance DSC with a fill factorxa0>0.7, the charge-transfer resistance at the Pt CE/electrolyte interface should bexa0<4.5xa0Ωxa0cm2 for both redox mediators. The I−/I3−-mediated DSCs were insensitive to Pt loading as low as 0.001xa0mgxa0cm−2, while the Co(bpy)32+/3+-mediated DSCs required relatively large Pt loadings ofxa0>xa00.005xa0mgxa0cm−2. Our results indicated that care should be taken in the preparation of Pt CEs with high transparency and low loading to obtain high-performance DSCs employing cobalt–ligand redox electrolyte.

Improvement in Energy Conversion Efficiency by Modification of Photon Distribution within the Photoanode of Dye-Sensitized Solar Cells

The dye-sensitized solar cell (DSSC) is a potential alternative to the widely used Si-based solar cell, with several advantages including higher energy conversion efficiency under weak and indirect illumination conditions, and the possibility of practical application in urban life due to their exterior characteristics. However, despite these advantages, the energy conversion efficiency of DSSCs has remained low at ∼10%. To improve the efficiency of DSSCs, research has been done on modifying the materials used in DSSC component parts, such as the photoanode, electrolyte, and counter electrode. Another approach is to modify the photoanode to increase the diffusion coefficient, reduce the recombination rate, and enhance the light behavior. One of the most popular methods for improving the efficiency of DSSCs is by trapping and dispersing the incident light using a scattering layer. Use of a scattering layer has shown various and interesting results, depending on the application, but it is currently used only in a simple form and there has been no deep research on the further potential of the scattering layer. In this study, the scattering center was introduced to maximize the effect of scattering. Light distribution near the scattering center, within or on the photoanode, was investigated using finite differential time domain (FDTD) numerical methods. Based on the FDTD analysis, an optimized, dome-shaped three-dimensional modified structure of a transparent photoanode with minimized scattering centers was introduced and indicated the possibility of modifying the photon distribution in the photoanode to enhance the performance of DSSCs. In addition to using the scattering center, we have introduced the structure of the dome-shaped three-dimensional structure to utilize the light distribution within the photoanode. This novel three-dimensional transparent photoanode and scattering center design increased the energy conversion efficiency of DSSCs from 6.3 to 7.2%. These results provide a foundation for investigating the role of the scattering center via further in-depth research. This new three-dimensional photoanode design provides a means to overcome the previous limitations on DSSC performance.