Ching-Mei Hsu

Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays

Hydrogenated amorphous Si (a-Si:H) is an important solar cell material. Here we demonstrate the fabrication of a-Si:H nanowires (NWs) and nanocones (NCs), using an easily scalable and IC-compatible process. We also investigate the optical properties of these nanostructures. These a-Si:H nanostructures display greatly enhanced absorption over a large range of wavelengths and angles of incidence, due to suppressed reflection. The enhancement effect is particularly strong for a-Si:H NC arrays, which provide nearly perfect impedance matching between a-Si:H and air through a gradual reduction of the effective refractive index. More than 90% of light is absorbed at angles of incidence up to 60 degrees for a-Si:H NC arrays, which is significantly better than NW arrays (70%) and thin films (45%). In addition, the absorption of NC arrays is 88% at the band gap edge of a-Si:H, which is much higher than NW arrays (70%) and thin films (53%). Our experimental data agree very well with simulation. The a-Si:H nanocones function as both absorber and antireflection layers, which offer a promising approach to enhance the solar cell energy conversion efficiency.

Carbon−Silicon Core−Shell Nanowires as High Capacity Electrode for Lithium Ion Batteries

We introduce a novel design of carbon-silicon core-shell nanowires for high power and long life lithium battery electrodes. Amorphous silicon was coated onto carbon nanofibers to form a core-shell structure and the resulted core-shell nanowires showed great performance as anode material. Since carbon has a much smaller capacity compared to silicon, the carbon core experiences less structural stress or damage during lithium cycling and can function as a mechanical support and an efficient electron conducting pathway. These nanowires have a high charge storage capacity of approximately 2000 mAh/g and good cycling life. They also have a high Coulmbic efficiency of 90% for the first cycle and 98-99.6% for the following cycles. A full cell composed of LiCoO(2) cathode and carbon-silicon core-shell nanowire anode is also demonstrated. Significantly, using these core-shell nanowires we have obtained high mass loading and an area capacity of approximately 4 mAh/cm(2), which is comparable to commercial battery values.

Nanodome solar cells with efficient light management and self-cleaning.

Here for the first time, we demonstrate novel nanodome solar cells, which have periodic nanoscale modulation for all layers from the bottom substrate, through the active absorber to the top transparent contact. These devices combine many nanophotonic effects to both efficiently reduce reflection and enhance absorption over a broad spectral range. Nanodome solar cells with only a 280 nm thick hydrogenated amorphous silicon (a-Si:H) layer can absorb 94% of the light with wavelengths of 400-800 nm, significantly higher than the 65% absorption of flat film devices. Because of the nearly complete absorption, a very large short-circuit current of 17.5 mA/cm(2) is achieved in our nanodome devices. Excitingly, the light management effects remain efficient over a wide range of incident angles, favorable for real environments with significant diffuse sunlight. We demonstrate nanodome devices with a power efficiency of 5.9%, which is 25% higher than the flat film control. The nanodome structure is not in principle limited to any specific material system and its fabrication is compatible with most solar manufacturing; hence it opens up exciting opportunities for a variety of photovoltaic devices to further improve performance, reduce materials usage, and relieve elemental abundance limitations. Lastly, our nanodome devices when modified with hydrophobic molecules present a nearly superhydrophobic surface and thus enable self-cleaning solar cells.

Light trapping in solar cells: can periodic beat random?

Theory predicts that periodic photonic nanostructures should outperform their random counterparts in trapping light in solar cells. However, the current certified world-record conversion efficiency for amorphous silicon thin-film solar cells, which strongly rely on light trapping, was achieved on the random pyramidal morphology of transparent zinc oxide electrodes. Based on insights from waveguide theory, we develop tailored periodic arrays of nanocavities on glass fabricated by nanosphere lithography, which enable a cell with a remarkable short-circuit current density of 17.1 mA/cm(2) and a high initial efficiency of 10.9%. A direct comparison with a cell deposited on the random pyramidal morphology of state-of-the-art zinc oxide electrodes, replicated onto glass using nanoimprint lithography, demonstrates unambiguously that periodic structures rival random textures.

Wafer-scale silicon nanopillars and nanocones by Langmuir-Blodgett assembly and etching

We have developed a method combining Langmuir–Blodgett assembly and reactive ion etching to fabricate nanopillars with uniform coverage over an entire 4 inch wafer. We demonstrated precise control over the diameter and separation between the nanopillars ranging from 60 to 600 nm. We can also change the shape of the pillars from having vertical to tapered sidewalls with sharp tips exhibiting a radius of curvature of 5 nm. This method opens up many possible opportunities in nanoimprinting, solar cells, batteries, and scanning probes.

Phase Transformation of Biphasic Cu2S−CuInS2 to Monophasic CuInS2 Nanorods

We synthesized wurtzite CuInS(2) nanorods (NRs) by colloidal solution-phase growth. We discovered that the growth process starts with nucleation of Cu(2)S nanodisks, followed by epitaxial overgrowth of CuInS(2) NRs onto only one face of Cu(2)S nanodisks, resulting in biphasic Cu(2)S-CISu heterostructured NRs. The phase transformation of biphasic Cu(2)S-CuInS(2) into monophasic CuInS(2) NRs occurred with growth progression. The observed epitaxial overgrowth and phase transformation is facile for three reasons. First, the sharing of the sulfur sublattice by the hexagonal chalcocite Cu(2)S and wurtzite CuInS(2) minimizes the lattice distortion. Second, Cu(2)S is in a superionic conducting state at the growth temperature of 250 degrees C wherein the copper ions move fluidly. Third, the size of the Cu(2)S nanodisks is small, resulting in fast phase transformation. Our results provide valuable insight into the controlled solution growth of ternary chalcogenide nanoparticles and will aid in the development of solar cells using ternary I-III-VI(2) semiconductors.

Improved solid oxide fuel cell performance with nanostructured electrolytes.

Considerable attention has been focused on solid oxide fuel cells (SOFCs) due to their potential for providing clean and reliable electric power. However, the high operating temperatures of current SOFCs limit their adoption in mobile applications. To lower the SOFC operating temperature, we fabricated a corrugated thin-film electrolyte membrane by nanosphere lithography and atomic layer deposition to reduce the polarization and ohmic losses at low temperatures. The resulting micro-SOFC electrolyte membrane showed a hexagonal-pyramid array nanostructure and achieved a power density of 1.34 W/cm(2) at 500 °C. In the future, arrays of micro-SOFCs with high power density may enable a range of mobile and portable power applications.

Nanopore Patterned Pt Array Electrodes for Triple Phase Boundary Study in Low Temperature SOFC

This paper describes the fabrication and investigation of morphologically stable model electrode structures with well-defined and sharp platinum/yttria-stabilized zirconia YSZ interfaces to study geometric effects at triple phase boundaries TPBs. A nanosphere patterning technique using monodispersed silica nanoparticles, which are applied to the YSZ surface by the Langmuir‐ Blodgett method, is employed to deposit nonporous platinum electrodes containing close-packed arrays of circular openings 300‐400 nm in diameter through which the underlying YSZ surface is exposed to the gas phase. These nanostructured dense Pt array cathodes exhibited better structural integrity and thermal stability at the solid oxide fuel cell SOFC operating temperature of 450‐500°C when compared to porous sputtered Pt electrodes. More importantly, electrochemical studies on geometrically well-defined Pt/YSZ sharp interfaces demonstrated that the cathode impedance and cell performance both scale almost linearly with the aerial density of TPB length. These controlled experiments also demonstrated that when normalized with respect to TPB length, the performance of different cells with different TBP densities agree well each other, indicating that TPB length governs cell performance especially in the activation polarization regime, as expected. Cells with a higher TPB density achieved better fuel cell performance in terms of higher power density and lower electrode impedance.

Nanostructured Light Management for Advanced Photovoltaics

The ultimate success of photovoltaic technologies requires great improvement in both efficiency improvement and cost reduction. One of the most effective ways to achieve these two objectives simultaneously is to use light-trapping scheme. Here we discuss a novel solar cell structure with an efficient light-trapping design, involving both absorption enhancement and reflection reduction. The centerpiece of the design is the nanocone structure, which can be fabricated by a scalable low-temperature process. With this design, devices with a very thin active layer can achieve near-perfect absorption because of both efficient antireflection and absorption enhancement over a broadband of spectra and a wide range of angles of incidence. The device performance of this design is significantly superior to that of conventional devices. More strikingly, the design and process is in principle not limited to any specific material; hence, it opens up exciting opportunities for a variety of photovoltaic devices to further improve the performance, reduce material usage, and relieve the abundance limitation. Fundamental limit of absorption enhancement at sub-wavelength regime is also discussed. It is found that light trapping can greatly enhance absorption, even beyond the conventional limit 4n 2. Therefore, engineering nanostructures presents new opportunities for advanced photon management to significantly improve the performance of solar cell devices.