Sang Eon Han

Optical absorption enhancement in silicon nanohole arrays for solar photovoltaics.

We investigate silicon nanohole arrays as light absorbing structures for solar photovoltaics via simulation. To obtain the same ultimate efficiency as a standard 300 microm crystalline silicon wafer, we find that nanohole arrays require twelve times less silicon by mass. Moreover, our calculations show that nanohole arrays have an efficiency superior to nanorod arrays for practical thicknesses. With well-established fabrication techniques, nanohole arrays have great potential for efficient solar photovoltaics.

Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells

We examine light trapping in thin silicon nanostructures for solar cell applications. Using group theory, we design surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays closely follows the Lambertian limit for isotropic incident radiation. These effects correspond to a reduction in silicon mass by 2 orders of magnitude, pointing to the promising future of thin crystalline silicon solar cells.

Efficient Low-Temperature Thermophotovoltaic Emitters from Metallic Photonic Crystals

We examine the use of metallic photonic crystals as thermophotovoltaic emitters. We coat silica woodpile structures, created using direct laser writing, with tungsten or molybdenum. Optical reflectivity and thermal emission measurements near 650 degrees C demonstrate that the resulting structures should provide efficient emitters at relatively low temperatures. When matched to InGaAsSb photocells, our structures should generate over ten times more power than solid emitters while having an optical-to-electrical conversion efficiency above 32%. At such low temperatures, these emitters have promise not only in solar energy but also in harnessing geothermal and industrial waste heat.

Beaming thermal emission from hot metallic bull’s eyes

We theoretically examine thermal emission from metallic films with surfaces that are patterned with a series of circular concentric grooves (a bulls eye pattern). Due to thermal excitation of surface plasmons, theory predicts that a single beam of light can be emitted from these films in the normal direction that is narrow, both in terms of its spectrum and its angular divergence. Thus, we show that metallic films can generate monochromatic directional beams of light by a simple thermal process.

Direct and Quantitative Photothermal Absorption Spectroscopy of Individual Particulates

Photonic structures can exhibit significant absorption enhancement when an objects length scale is comparable to or smaller than the wavelength of light. This property has enabled photonic structures to be an integral component in many applications such as solar cells, light emitting diodes, and photothermal therapy. To characterize this enhancement at the single particulate level, conventional methods have consisted of indirect or qualitative approaches which are often limited to certain sample types. To overcome these limitations, we used a bilayer cantilever to directly and quantitatively measure the spectral absorption efficiency of a single silicon microwire in the visible wavelength range. We demonstrate an absorption enhancement on a per unit volume basis compared to a thin film, which shows good agreement with Mie theory calculations. This approach offers a quantitative approach for broadband absorption measurements on a wide range of photonic structures of different geometric and material compositions.

Heat Transfer Effect of Inert Gas on Multi-Tubular Reactor for Partial Oxidation Reaction

The heat transfer effect of an inert gas on a multi-tubular reactor for a partial oxidation reaction has been determined. The model reaction system in the study was partial oxidation of propylene to acrolein. Both theoretical modeling and experimental studies have been performed to determine the heat transfer effect of inert gas on the system. Among many inert gases, CO2 was selected and tested as a diluent gas for the partial oxidation of propylene to acrolein system instead of conventionally used N2. The productivity increase through changing the inert gas from N2 to CO2 was possible due to the heat transfer capability of CO2. In this study, by replacing the inert gas from N2 to CO2, productivity increased up to 14%.

The role of stress in the time-dependent optical response of silicon photonic band gap crystals

Amorphous silicon inverse opals with a complete photonic band gap have been studied with optical pump-probe spectroscopy. The pump-generated free carriers cause the reflection near the band gap to be lowered for the first few picoseconds due to induced absorption. After ∼5 ps, this effect disappears and an unexpected blue spectral shift is seen in the photonic band gap. The shift appears consistent with photoinduced stress caused by the thermal expansion mismatch between the silicon and its native oxide. However, simple mechanical models fail to quantify this behavior.