S. K. Deb

Enhancement and stabilization of porous silicon photoluminescence by oxygen incorporation with a remote‐plasma treatment

We report a treatment that enhances and stabilizes the photoluminescence (PL) from porous Si films. Films prepared by anodization in a 50% HF/ethanol solution were annealed at 450 °C in vacuum, exposed to air, and then exposed to a remote‐hydrogen plasma. Infrared absorption spectroscopy revealed that the concentration of oxygen, rather than hydrogen, was increased by the processing steps, and that silicon dihydride species had been eliminated from the surface. The PL from a treated film was initially ∼30 times more intense than from the as‐etched films. The PL intensity increased with illumination time in air until a steady‐state intensity was reached.

An UV photochromic memory effect in proton-based WO3 electrochromic devices

We report an UV photochromic memory effect on a standard proton-based WO3 electrochromic device. It exhibits two memory states, associated with the colored and bleached states of the device, respectively. Such an effect can be used to enhance device performance (increasing the dynamic range), re-energize commercial electrochromic devices, and develop memory devices.

Hydrogen sensor based on metallic photonic crystal slabs

A hydrogen sensor based on a metallic photonic crystal using gold and WO3 is presented. Hydrogen exposure influences the optical properties of this device by gasochromic mechanisms with a theoretical limit in the sub-1000-ppm-range.

Accelerated durability testing of electrochromic windows

Abstract Prototype electrochromic windows made by several different US companies have been tested in our laboratory for their long-term durability. Samples were subjected to alternate coloring and bleaching voltage cycles while exposed to simulated 1-sun irradiance in a temperature-controlled environmental chamber with low relative humidity. The samples inside the chamber were tested under a matrix of different conditions. These conditions include: cycling at different temperatures (65, 85 and 107°C) under the irradiance, cycling versus no-cycling under the same irradiance and temperature, testing with different voltage waveforms and duty cycles with the same irradiance and temperature, cycling under various filtered irradiance intensities, and simple thermal exposure with no irradiance or cycling. The electro-optical characteristics of the samples were measured between 350 and 1,100 nm every 4,000 cycles for up to 20,000 cycles. Photographs of the samples were taken periodically with a digital camera to record cosmetic defects, the extent of residual coloration, and overall coloration and bleaching uniformity of the samples. Our results indicate that the most important cause of degradation is the combination of continuous cycling, elevated temperature and irradiance. The relative importance of these variables, when considered synergistically or separately, depends on the particular device materials and design.

Potential applications of porous silicon in photovoltaics

Porous Si formed on crystalline Si wafers using electrochemical etching exhibits photoluminescent and electroluminescent properties. The authors report on the results of their investigation of using porous Si as surface texturing to enhance the performance of crystalline silicon photovoltaic solar cells. Unlike conventional KOH-based texture etching, which can only be used on [100]-oriented single-crystalline Si substrates, porous Si can be etched onto Si surfaces of any crystallographic orientation and onto polycrystalline or microcrystalline Si surfaces. A porous-Si-covered single-crystal Si wafer showed an integrated reflectance of only 1.4% at a wavelength of 500 nm. The reflectance of a porous-Si-covered polycrystalline Si was found to be comparable to other much more complicated texturing methods.<<ETX>>

Photochemical solar cells based on dye-sensitization of nanocrystalline TiO/sub 2/

A new type of photovoltaic cell is described. It is a photoelectrochemical device that is based on the dye-sensitization of thin (10-20 /spl mu/m) nanocrystalline films of TiO/sub 2/ nanoparticles in contact with a nonaqueous liquid electrolyte. The cell is very simple to fabricate and, in principle, its color can be tuned through the visible spectrum, ranging from being completely transparent to black opaque by changing the absorption characteristics of the dye. The highest present efficiency of the dye-sensitized photochemical solar cell is about 11%. The cell has the potential to be a low-cost photovoltaic option. Unique applications include photovoltaic power windows and photoelectrochromic windows.

Ion-beam-hydrogenated amorphous silicon

A kaufman ion-beam source has been used to study the rehydrogenation and postdeposition hydrogenation of amorphous silicon. In the rehydrogenation study, hydrogen atoms were implanted into glow-dischargedeposited amorphous silicon materials in which the hydrogen content had been driven out by heating. In the posthydrogenation study, amorphous silicon samples with no hydrogen content detectable by infrared absorption and no photoconductivity were used as the starting material. These materials were deposited by thermal CVD, magnetron sputtering, or RF glow discharge.

Photoluminescence properties of porous silicon

A porous silicon (PS) layer can be produced on a crystalline silicon substrate by electrochemical or chemical etching in hydrofluoric acid (HF) solutions. There are many properties that make PS thin films interesting for photovoltaic applications, such as a possible direct band gap that can be adjusted between 1.5 and 1.9 eV, textured surfaces for light trapping, the potential for low cost and large‐area fabrication, and the possibility of tandem cell structures with Si. We report the fabrication of large area PS (up to 3‘ diameter) with quite uniform photoluminescence (PL) properties, and studies of the effects of post‐hydrogenation treatments on the intensity and stability of the PL from PS. We have observed that a remote‐plasma processing treatment can increase the PL emission intensity from PS prepared under certain conditions by 100 times or more. The emission band is narrower and centered more toward the blue for the remote‐plasma processed sample, and the PL emission intensity does not degrade in a...

DOE/OER-sponsored basic research in high-efficiency photovoltaics

A high-efficiency photovoltaic project involving many of the national laboratories and several universities has been initiated under the umbrella of the US Department of Energy (DOE) Center of Excellence for the Synthesis and Processing of Advanced Materials. The objectives of this project are to generate advances in fundamental scientific understanding that will impact the efficiency, cost, and reliability of thin-film photovoltaic cells. The project is focused on two areas: (1) silicon-based thin films, in which key scientific and technological problems involving amorphous and polycrystalline silicon thin films will be addressed, and (2) next-generation thin-film photovoltaics, which will be concerned with the possibilities of new advances and breakthroughs in the materials and physics of photovoltaics using nonsilicon-based materials.

Evidence for Long-range Hydrogen Motion in a-Si:H under Room-temperature Illumination Using Raman Scattering of Amorphous Tungsten Oxide Overlayer

We demonstrate that one can detect minuscule amounts of hydrogen diffusion out of a-Si:H under illumination at room temperature, by monitoring the changes in the Raman spectrum of amorphous tungsten oxide as a function of illumination. The Staebler-Wronski effect, the light-induce creation of metastable defects in hydrogenated amorphous silicon (a-Si:H), has been one of the major problems that has limited the performance of such devices as solar cells. Recently, Branz suggested the hydrogen collision model that can explain many aspects of the Staebler-Wronski effect. One of the main predictions of this model is that the photogenerated mobile hydrogen atoms can move a long distance at room temperature. However, light-induced hydrogen motion in a-Si:H has not been experimentally observed at room temperature. We utilized the high sensitivity of the Raman spectrum of electrochromic a-WO 3 to hydrogen insertion to probe the long-range motion of hydrogen at room temperature. We deposited a thin (200 nm) layer of a-WO 3 on top of a-Si:H, and under illumination, a change in the Raman spectrum was detected. By comparing the Raman signal changes with those for control experiments where hydrogen is electrochemically inserted into a-WO 3 , we can estimate semiquantitatively the amount of hydrogen that diffuses out of the a-Si:H layer.