Simon H. Liu

Holographic solar concentrator

A solar concentrator receives sunlight for generating solar power with the concentrator including holographic optical element (HOE) separators for separating sunlight into separated bands, including HOE concentrators for concentrating the separated bands into concentrated bands, including HOE reflectors for reflecting the concentrated bands as reflected bands onto a multiple junction photovoltaic solar cell for generating the solar power with reduced aberrations of the bands for improved conversion of the solar light into the generator solar power, all of which can be constructed in an integrated structure using spacers, waveguides, and a substrate, where the HOEs use chirp Bragg gratings for reducing optical aberrations of the separated, concentrated, and reflected optical bands, with the option of multiple HOE separators for receiving sunlight from various angles of incidence.The approach selected is the fabrication of holographic optical elements which will focus to either a line or a point. A concentrating mirror is replicated in the hologram, which consists of dichromate gelatin exposed to a laser beam. The dichromate gelatin can be processed to produce a non-uniform microstructure, which gives the hologram a significant waveband width. Even so, it becomes necessary to stack at least three holograms, with each reflecting a different region of the solar spectrum, if we are to reflect most of the solar energy. To achieve high efficiency, it is necessary to obtain adjacent quasi-square waves for the efficiency—wavelength profile of each of the holograms in the stack. Profile information was obtained by the use of a monochromator coupled to a computer. An optical efficiency in excess of 50% was measured for a three-hologram stack. This represents approximately 70% of the efficiency achievable within the limited measuring range of the monochromator. A line-focus holographic concentrator model has been built for demonstration purposes.

Measurement and dynamics of the spatial distribution of an electron localized at a metal-dielectric interface.

The ability of time- and angle-resolved two-photon photoemission to estimate the size distribution of electron localization in the plane of a metal-adsorbate interface is discussed. It is shown that the width of angular distribution of the photoelectric current is inversely proportional to the electron localization size within the most common approximations in the description of image potential states. The localization of the n=1 image potential state for two monolayers of butyronitrile on Ag(111) is used as an example. For the delocalized n=1 state, the shape of the signal amplitude as a function of momentum parallel to the surface changes rapidly with time, indicating efficient intraband relaxation on a 100 fs time scale. For the localized state, little change was observed. The latter is related to the constant size distribution of electron localization, which is estimated to be a Gaussian with a 15+/-4 A full width at half maximum in the plane of the interface. A simple model was used to study the effect of a weak localization potential on the overall width of the angular distribution of the photoemitted electrons, which exhibited little sensitivity to the details of the potential. This substantiates the validity of the localization size estimate.

Radiation Effects and Annealing Studies on Amorphous Silicon Solar Cells

Results of radiation effects and annealing studies are presented for amorphous silicon solar cells from three manufacturers. Data scale well with ionizing dose in many cases for proton, x-ray, and electron irradiation. Significant long-term annealing occurs at room temperature. Results for small-area diodes are in reasonable agreement with findings for monolithic modules. Damage mechanisms in irradiated and illuminated devices are compared.

Effects of contamination on solar cell coverglass

As the power generation capability of solar cells depends strongly on the spectra of the incident light through the coverglass, there is a critical need to understand the impact of adsorbed molecular (organic) contaminants, which absorb light in the short wavelength range. The goal of this work is to calculate solar cell current loss based on experimentally determined coverglass transmission change in the presence of contaminant films. Two representative contaminants, di-octyl phthalate (DOP) and DC704, were photo-fixed on the coverglass samples, which were subsequently irradiated with protons under a simulated 15-year GEO space radiation environment. The coverglass transmission change was characterized before and after each process. The coverglass transmission data were then convolved with the solar cell spectral response to determine the coverglass darkening effects on solar cell performance. The results indicate that the solar cell current could be significantly reduced due to the combined effects of contamination and proton exposure.

Thin-Film Photovoltaic Proton and Electron Radiation Testing for a Meo Orbit

A radiation test plan for thin-film photovoltaic technologies focused on a MEO flight experiment is outlined. The proton and electron radiation response of thin film, amorphous Si solar cells and CuInGaSe 2 solar cells, with and without space coatings, is presented. The degradation of the photovoltaic output under penetrating and junction-damaging proton irradiation, and 0.6 MeV and 1 MeV electron irradiation, is measured and examined. The experimental data are presented and analyzed. These data will form the basis for an on-orbit prediction model as applied to a high-radiation MEO orbit

Proton Irradiation and Annealing of a -Si Thin -Film Solar Cells for Space Applications: Results at 160 keV

†, ‡ § ** , Thin -film solar cells are of interest for satellite power generation because of the potential advantages in terms of having hig her specific power (lightweight), lower specific volume (flexible), and higher end -of -life power (superior radiation resistance), as compared to the crystalline solar cells. The space radiation environment causes gradual solar cells performance degradatio n, thus limiting the lifetime of the solar array. Due to the self annealing effect, the radiation damage on thin -film solar cells can be partially reversed. This paper presents proton irradiation and annealing of amphorous silicon (a -Si) solar cell test results.

Synergistic effects of contamination and low energy space protons on solar cell current output

It is well known that solar cell coverglass materials are subject to darkening, or transmission degradation, due to interaction with protons. Our recent laboratory test results have shown that the transmission of coverglasses, once contaminated with organic molecular films, can be further degraded upon space proton irradiation (20–400 keV). The coverglass transmission loss occurs in the short wavelength region, thus multi-junction solar cells are expected to be particularly susceptible to such synergistic effects of contamination and proton irradiation when the top junction is the current limiting junction. In the previous work, AR/ITO coverglass materials, commonly used for space solar arrays, were photo-deposited with the model contaminant DC704. The contaminated coverglass samples were subsequently irradiated with a simulated 15-year geosynchronous orbit low energy proton radiation environment at 5-year increments. The progression of coverglass transmission change was characterized before and after each process. The measured coverglass transmission data were then convolved with the solar cell quantum efficiency and solar spectrum to determine the coverglass darkening effects on solar cell performance. Taking into account space proton radiation effects and the time dependent contaminant film accumulation process, our preliminary analysis indicates that, over a 15-year mission life, approximately 3.7 % solar cell current loss could be attributed to a beginning-of-life (BOL) contaminant film of 100 Å, with no additional on-orbit film growth. For a BOL film of 100 Å and additional film growth while on orbit, the end-of-life (EOL) solar cell current loss due to contamination is approximated at 5.5% for EOL 200 Å, and 7.3% for EOL 300 Å.

Space radiation environmental testing on POSS coated solar cell coverglass

Light weight, flexible, radiation hardened solar cells coatings are of interest for applications on satellite power generation owing to the potential advantages in terms of having higher specific power (lightweight), lower specific volume (flexible), and higher end-of-life power (superior radiation resistance), as compared to current state-of-the-art Ce-doped micro sheet solar cell coverglass. The space radiation environment causes gradual optical performance degradation of coverglass and coatings, thus limiting the lifetime of the solar array. The objective of this project is to assess the POSS (polyhedral oligomeric silsesquioxane) coating in simulated space proton radiation environments. Due to the unique molecular structure, POSS may be equipped with suitable optical property along with superior radiation hardness, thus better protecting the solar cells.

Thin-Film Photovoltaic Radiation Testing for Space Applications

Although thin-film photovoltaic technology on lightweight flexible substrates has lower beginning-of-life efficiency compared to traditional single crystalline solar cells, it can offer advantages in high-specific power and low-stowed volume for power generation in space. To date, radiation testing on thin-film solar cells has demonstrated superior radiation hardness compared to traditional crystalline solar cells. In addition, radiation induced damage in thin-film solar cells can be removed by annealing at temperatures readily achievable in space. Prior to deployment of this new technology for any mission, a more thorough understanding of its performance in the space environment will be required. The Aerospace Corporation has initiated a comprehensive study of thin-film solar cell performance in a simulated space radiation environment. A new testbed has been constructed to study the combined space environmental effect of proton irradiation and air mass zero light spectrum light soaking on a thin-film photovoltaic

Engineering Development Model Testing of the PowerSphere

The Aerospace Corporation, NASA Glenn Research Center, Lockheed-Martin, and ILC Dover over the past two years have been engaged in developing a Multifunctional Inflatable Structure for the PowerSphere Concept under contract with NASA (NAS3-01115). The PowerSphere concept consists of a relatively large spherical solar array, which would be deployed from a micro satellite. 1–8 The PowerSphere structure and the deployment method was patented by the Aerospace Corporation (U.S. Patent Numbers 6,284,966 B1 and 6,318,675). The work on this project has resulted in a number of technological innovations in the state of the art for integrating flexible thin-film solar cells with flex circuit harness technology and inflatable ultraviolet-light-rigidizable structures.