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Nanostructures for Energy & Environment

Research Area

Electrodes for Dye-Sensitized Solar Cells (DSSCs)

          Dye-sensitized solar cell (DSSC) is a photoelectrochemical system incorporating wide-bandgap oxide semiconductor such as TiO2 and ZnO as photoanode, dye molecules as photosensitizer, and electrolyte. These DSSCs are attracting much attention due to transparency, cell-color tunability by using various color of dye, and low cost fabrication. We have demonstrated a three-dimensional TiO2 nanohelix array, fabricated by the oblique angle deposition method, as a multifunctional photoanode for high performance of (Q)DSSC. The enhanced performance of nanohelix array based (Q)DSSC is attributed to two important sequential improvements; first, a boost in the number of photo-generated carriers due to strong light scattering and, second, a very efficient collection of photo-generated carriers through near-single crystalline nanohelix structures.

Related articles

1. Seung Hee Lee et al., Solar Energy Materials & Solar Cells, 132, 47-55 (2015) [Link]

2. Seung Hee Lee et al., Optics Express, 22, A867-A879 (2014) [Link]

 

Electrodes for Organic Photo-Voltaics (OPVs)

          Nano-structured three-dimensional electrodes are promising approaches to enhance the performance of organic solar cells. Our ITO nano-helix arrays fabricated by OAD technique, can improve (i) optical properties (by anti-reflection coating and light scattering effect), (ii) electrical properties (by balancing carrier mobilities by crystalline ITO nano-helix) at the same time, eliminating the trade-off between light absorption and charge transport. Finally, we aim to fabricate the ideal structure for organic solar cells which have helically interdigitized bilayer structure by conformally coating organic materials on three-dimensional ITO nanostructures.

Related articles

1. Hyunah Kwon et al., Advanced Energy Materials, 4, 1301566 (2014) [Front Cover] [Link]

 

Electrodes for Photoelectrochemical Cells & Artificial Photosynthesis

          Photoelectrochemical (PEC) water splitting has attracted considerable attention because it is a highly promising and clean technique to produce carbon-free hydrogen from inexhaustible solar energy and water source. A variety of researches on photoanodes have been reported using metal-oxide semiconductors such as WO3, BiVO4, Fe2O3, TiO2 which have relatively small band gap energy and appropriate band edge position for efficient water splitting reactions. Many nanostructured photoanodes have been suggested to improve light absorption and charge transport, thus, solar-to-hydrogen conversion efficiency. A new type of photoanodes based on three-dimensional nanohelixes (NH) array was fabricated by a facile and cost-effective method called oblique angle deposition. It is found that the NH structure enhances light harvesting efficiency due to strong light scattering effect enabled by its unique helical shape. Micro-structural analyses by X-ray diffraction and high-resolution transmission electron microscopy reveal that individual NHs are highly crystallized, leading to efficient charge separation and transport.

Related articles

1. Xinjian Shi§, Il Yong Choi§ et al., Nature Communications, 5, 4775 (2014) [Link]

 

Active Components for Smart Sensors (Toward Electronic Noses)

          Nano-structured three-dimensional electrodes are promising approaches to enhance the performance of organic solar cells. Our ITO nano-helix arrays fabricated by OAD technique, can improve (i) optical properties (by anti-reflection coating and light scattering effect), (ii) electrical properties (by balancing carrier mobilities by crystalline ITO nano-helix) at the same time, eliminating the trade-off between light absorption and charge transport. Finally, we aim to fabricate the ideal structure for organic solar cells which have helically interdigitized bilayer structure by conformally coating organic materials on three-dimensional ITO nanostructures.

Related articles

1. Sunyong Hwang et al., Analyst, 138, 443-450 (2013) [Inside Front Cover] [Link]

 

Smart Photonics Components

  1. Anti-reflection coatings

 

Anti-reflection (AR) coatings are widely used in solar cells to suppress Fresnel reflection. To utilize the full spectrum of incident sunlight, minimizing the reflection over wide range of wavelength and incident angle is highly desirable characteristics for AR coatings. The conventional quarter-wavelength AR coating, however, can effectively reduce the reflection only at a specific wavelength and an angle of incidence where they are optimized. It is well known that the optical coatings with graded refractive index show broadband and omni-directional characteristics. So we are investigating the nanostructured multilayer AR coatings with tunable refractive index enabled by co-sputtering and even low-refractive index by oblique angle deposited (OAD) nanoporous thin film.

Related articles

1. Seung Jae Oh et al., Optics Express 21,  A157-A166 (2013). [Link]

2. Xing Yan et al., Advanced Functional Materials 23, 583-590 (2013). [Link]

3. Jong Kyu Kim et al., Advanced Materials 20, 801(2008). [Link]

 

  2. Distributed Bragg Reflector (DBR)

          White LEDs based on the combination of a GaN-based blue LED chip and a yellow phosphor layer suffer from a low phosphor conversion efficiency (PCE) because a significant amount of the yellow fluorescence is emitted towards the blue LED chip where the fluorescence is partially absorbed. Strongly enhanced PCE of a white LED is demonstrated by embedding a dichroic-filtering contact (DFC) multi-functioning as blue-transmitting but yellow-reflecting optical filter, as well as a low-resistance ohmic contact to p-type GaN. Electrically conductive DFCs, consisting of alternating layers of dense- and nanoporous indium-tin-oxide, are realized by the oblique-angle deposition technique that enables a tunable refractive index of a single material.

Related articles

1. Seung Jae Oh et al., Journal of Materials Chemistry C 1 (36),  5733-5740 (2013) [Link]

 

Fabrication Method

Oblique Angle Deposition (OAD)

          Oblique angle deposition is one of useful physical deposition methods. Unlike conventional evaporation methods, a substrate is tilted against to vapor flux. At the initial stage of the deposition, nucleuses are randomly formed on the substrate, and they produce self-shadowed regions where vapor flux cannot directly reach resulting in the creation of a slanted nano-rod array. Additionally, by controlling the substrate rotation as well as the vapor flux incidence angle we can fabricate 3-dimensional nanostructures such as nano-helix, vertical posts, and zig-zags. Any evaporable materials such as TiO2, ITO, SiO2, WO3, and even semiconductors and metals are available so that OAD can be applied in numerous applications (ex. Organic/Inorganic photovoltaic cells, Photoelectrochemical systems, Gas sensors etc.)