AVS2011 Session EN-ThP: Energy Frontiers Poster Session

Thermionic electron emission from nitrogen doped hydrogen terminated diamond film has been evaluated 18 months after it had been deposited on a 25 mm diameter molybdenum substrate by microwave plasma assisted chemical vapour deposition. A hydrogen enriched surface layer, enabling a negative electron affinity, was formed during the film deposition procedure. The initial thermionic emission current density of the aged film obtained at 500 °C and 0.2 V/micrometer electric field was stable but relatively low compared to the value measured on a freshly deposited film. The value at a base pressure of 2x10^-7 mbar was ~ 1.4x10^-9A/cm². Such a low current density is presumed to be a consequence of oxidation, surface adsorption of contaminats and hydrogen depletion from the surface layer, all processes which proceed during ambient atmosphere exposure since the manufacturing date. Reactivation of the surface was achieved by introducing atomic hydrogen, generated by the hot filament cathode of the ionization gauge. A stable current density of ~ 2x10^-6 A/cm² (an increase factor of ~1400) was achieved after exposure to hydrogen at the base pressure of 2.2x10^-4 mbar for 2 hours again at 500 °C and 0.2 V/micrometer. The atomic hydrogen exposure rate was estimated by subsequent measurement of the change in optical transmittance of the tungsten phosphate glass plates. The increase in emission with hydrogen was ascribed to two effects: 1) reactivation of the surface, and 2) molecular assisted emission.

Research at ASU is supported through the Office of Naval Research and at JSI through the Slovenian Research Agency (grant BI-US/09-12/021).

The effect of inserting a thin-film buffer layer on the obtainable efficiency in n-ZnO/p-Cu₂O heterojunction solar cells was investigated with a transparent conducting Al-doped ZnO (AZO) thin film/Cu₂O sheet structure. To improve conversion efficiency, various heterojunction solar cells were fabricated by forming an AZO/thin-film buffer layer/Cu₂O structure on the front surface of thermally oxidized Cu₂O sheets that function as the active layer as well as the substrate. It was found that achieving higher efficiency Cu₂O-based heterojunction solar cells fabricated by depositing various thin films on Cu O sheets requires that the surface of the Cu₂O sheets always be treated using a low-damage deposition technology at a low deposition temperature .

The solar cell efficiency is hardly connected with device fabrication processes, such as texturing silicon surface, which increases the surface absorption area of incident radiation on the cell and, consequently, reduces the substrate reflectance. Nowadays, solar cells based on silicon substrate are fabricated with texturing surfaces to increase the device efficiency. These surfaces are obtained by anisotropic wet etching in KOH and NaOH alkaline solutions [1]. The disadvantage of these solutions are K+ and Na+ ions from KOH and NaOH, which are mobile charges in Metal-Oxide-Silicon structures. Thus, these solutions are not compatible for CMOS technology, which is used for integration circuits and devices on silicon surface. As an advantage, using these alkaline solutions, the <111> planes of silicon substrate with (100) crystallographic orientation are exposed, resulting in groups of pyramid on silicon surface. The pyramids induce many reflections and scatterings of the incident radiation, which increase the light absorption on the surface. This effect is defined as light trapping [1]. In this work, the silicon substrate texturing is carried out by an anisotropic wet etching, with an alkaline solution based on NH₄OH,because this solution can form pyramids on the surface, does not release alkaline products/reagents on the silicon surface and is fully compatible with the CMOS technology. With this solution (concentration of 9%wt of NH₄OH and four magnetic stirring velocities) the <111> planes of silicon substrate with (100) crystallographic orientation are exposed and theses planes can form some pyramids on surface. Scanning Electron Microscopy analysis presents that these pyramids are observed all long on the surface, indicating that the wet etching was uniform on the substrate. Furthermore, the pyramid height values are between 3.5 and 7.2 µm, and the silicon etching wet rate was about of 0.7 µm/minute. The reflectance measurements show that the surfaces with and without texturing present reflectance values of about 16% and 38%, respectively. The reflectance values of about 16% with texturing are similar to the results from references [1], which have used KOH solution. This result indicated that our texturing process based on NH4OH, which is fully compatible with the CMOS technology, can be used to fabricate Si-based solar cells [2].

REFERENCES

[1]L.A. Dobrzański, A.Drygała, Surface texturing of multicrystalline silicon solar cells, JAMME,Volume 31 ISSUE1,November 2008.

[2J J.Lu, W.Liu, C.H.M. van der Werf,A.Y.Kovalgin,Y. Sun, R. E. I. Schropp and J.Schmitz, Above-CMOS a-Si and CIGS Solar Cells for Powering Autonomous Microsystems, IEDM, December 2010

Non-pulsing DC power supplies for highly arcing processes are gaining more and more interest slowly pushing out older pulsing DC power supplies as more efficient and more cost optimal. The importance of arc management in DC power supplies is taking on a new meaning as most advanced DC generators are more and more successful in sputtering highly arcing and difficult materials, providing high deposition rates at competitive investment.

It has been developed as the answer to the market’s demand to limit arc energy during an arc occurrence in sputtering. Industrial implementation has been already successfully performed with air-cooled DC power supplies that have been on the market for years. The possibility of using DC power supplies for sputtering highly-arcing materials is the result. It is the beginning of a new age in magnetron sputtering technology. Further improvement and optimization by using more efficient water cooling and faster transistors has resulted in a new generation of power supplies with state-of-the-art arc management parameters. A complementary feature of the newly-developed arc management circuitry is software that supports efficient arc handling: self-adjusting arc management settings and an internal oscilloscope.

Microbial fuel cells (MFCs) are able to capture the electrons produced during different biochemical microbial processes to generate electricity. They offer the promise of a new sustainable source of energy, produced from biodegradable and reduced compounds. Increased shortcoming of unrenewable fossil fuels causes the need for it’s less dendence. The enhance of use of renewable fuels requires the development of alternative sources of energy such as waste biomass for environmental benefits. MFCs investigations have attracted substantial interest in development of different devices for generating electricity and removing wastes. The development of processes that can apply bacteria of normal environmental microflora to produce electricity represents a highly effective method for bioenergy production as bacteria are self-replicating, and thus the catalysts for organic matter oxidation are self-sustaining.

Nowadays MFCs is considered to be applied as highly effective and self-sustaining model of wastewater treatment which contains energy in the form of biodegradable organic matter. At the same time wastewater contains high concentrations of xenobiotics, such as different heavy metal ions that have an overwhelming harmful effect towards all living organisms. These elements even in small concentration in the environment cause the intensive inhibiton of physiological and biochemical properties of most bacteria. Despite that, some genuses of bacteria possess high toxic heavy metals resistance because of highly-efficient defensive systems functioning. There are Feroplasma, Streptomyces, Thiobacillus etc.

Desulfuromonas acetoxidans are uncoloured gram-negative sulfur-reducing bacteria. These bacteria are considered to be used as the substrate for microbial-anode fuel cells with high electron recovery, which is more than 80%, to electric current by the interaction between proceses of acetate oxidation and S⁰ or Fe³⁺ - reduction. Sulfurbacteria have a great effect on the biogeochemistry of aquatic environments because of their ability to reduce and precipitate toxic heavy metals compounds. It’s a result of their particular binding with the final product of bacterial dissimilative sulfur-reduction ­– H₂S, and formation of the unsoluble metal sulfides. It also serves as self-defensive mechanism of these bacteria against toxic influence of toxic heavy metal ions.