October 2011, Volume 2, No.5 International Journal of Chemical and Environmental Engineering X-ray solar cell based on carbon-metal junction Jacek A. Jaworski a,b and Eric Fleury b a H. Niewodnicznski institute of Nuclear Physics Polish Academy of Sciences ul. Radzikowskiego 152, 31-342 Krakow, Poland b Center for High Temperature Energy Materials, Korea Institute of Science and Technology, Seoul 136-791, Korea Abstract The article presents a new photosensitive metal - carbon junction sensitive to X - radiation produced by electro-deposition method. The junction was made by electrodeposition of amorphous carbon onto a metal with the work function higher than the deposited carbon creating Schottky junction. The junction was not sensitive for visible light but sensitive for X-rays. Values of the voltage measured in the open circuit mode were found to be dependent on the type of the metal used as a substrate. 1. Introduction Since the beginning of the industrial revolution, the reserves of fossil fuels like oil and gas shrink from year to year. The deposit of methane hydrates at the bottom of the ocean and under glaciers in Siberia and the arctic region of the North American continent might delay the shortage of energy for a few decades [1, 2]. However it will only be a temporary solution like other natural resources. Other alternatives to provide energy to the humanity are bio- fuels [3, 4, 5]. But their disadvantage is the inherent pollution of the atmosphere (carbon, nitrogen and sulfur oxides, dust, etc.) resulting from the production and utilization. Another problem with bio-fuels is the occupation of the cultivable grounds. We are now witnesses a shortage of food worldwide, which will be become worst as the ground occupied for cultivation of plants for bio-fuel will be extended [6]. One of solutions of this crisis is production of energy by solar cells [7 - 11]. For example, the European community is planning to build great solar power plants installation in North Africa and transport energy to Europe [12, 13]. But solar cells on the Earth have limitations: the Earth atmosphere is transparent only for particular wavelengths [14, 15, 16, 17] and the land area where the solar cells could be installed is limited. These restrictions disappear in the outer space and on the Moon. The potential of area for the installation of solar cells is gigantesque and the energy produced there in space could be transported to the Earth by microwaves [18] before been converted to electrical energy. In this case the efficiency of the solar cells would be not critical because of the huge area available on the Moon and in the outer space around Earth. Japan is already planning to build a solar power plant in a cosmic space [19]. Currently, most of the solar cells are based on Si, TiO 2 , GaAs, CdTe, CuInSe 2 and the production is relatively expensive. In this article is described a simple and cheap method of producing new type solar cells based on carbon - metal Schottky junction operating in the range of X-ray. These cells are produced by electro-deposition of carbon on various types of metallic substrates, which have been selected based on their work functions. The band gap of carbon can vary in a wide range from almost zero electron-volt in the case of graphite (semi- metal) though amorphous carbon created by chemical vapour deposition method with values around 4 eV [20] to a value up to 5.5 eV for diamond, which is located at the border between semiconductor and insulator. Consequently carbon-based solar cells can be sensitive for a wide spectrum of electromagnetic radiation depending on the crystallographic structure. This sensitivity can also be enhanced by doping with for example element such as nitrogen, oxygen or boron in case of diamond or diamond like carbon [21]. Because of its band gap, pure diamond should be photo-sensitive for radiation with a wavelength shorter than 200 nm but this is an extreme value for carbon solar cell. Solar cells based on semiconductor junctions were described by Chapin [7] and Shockey [22]. This article considers a metal - semiconductor diode junction. Rectifying properties of this kind of junction were discovered in 1874 by Karl Ferdinand Braun [23]. A short explanation of the phenomenon is the creation of Schottky barrier in the junction. This characteristic is dependent on the work function, which can be defined as the minimum quantity of energy that is required to remove an electron from the solid to a point just outside the surface of the given material. If the work function of a semiconductor is smaller than that of a conductor, this