AGS-Energy
Building a new World with environmentally clean and renewable energy technologies
Solar Materials Solar cell materials are generally group IV elements. A group III material such as Boron is usually introduced into the solar material in order to have a ''p' type doping and a group V material such as Phosphorus is usually introduced to have an ''n'' type doping. Visible wavelength of light is between 400-700 nm wavelength. Here are some key concepts: - Bandgap Energy (Eg): Energy needed to allow an electron in an atom's shell to break away and flow freely in the material. - In indirect bandgap materials, a photon with enough energy is not enough to free electrons. A little bit extra energy must come from somewhere else. Direct bandgap materials show higher efficiency of converting photons to free electrons. - Here are some semiconductor materials used in solar cells: SILICON (Si): Indirect bandgap 1.1 eV energy.....................................Conversion Efficiencies~16% GERMANIUM (Ge): Indirect bandgap 0.66 eV energy CADMIUM TELLURIDE (CdTe): Direct bandgap 1.56 eV energy GALLIUM ARSENIDE (GaAs): Direct bandgap 1.42 eV energy................Conversion Efficiencies~40% CIS (Copper Indium Diselenide): Direct bandgap 2.40 eV......................Conversion Efficiencies~14% CIGS (Copper Indium Gallium Diselenide): Direct bandgap 1.5 eV..........Conversion Efficiencies~19% - If Eg is too high, the photons won't have enough energy to generate current.. - If Eg is too low, high energy photons (blue wavelength, UV) can't contribute their excess energy to the current generated. Let's examine the bulk and thin film solar materials in more detail: SILICON: Most popular, proven, promising materialbecause it is abundant, cheap and non-toxic. Bandgap is lower than ideal for solar spectrum. There are mainly three types: 1. Monocrystalline Silicon (c-Si): Tend to be more expensive and because they are cut from cylindirical ingots, do not cover a square solar cell module without waste of silicon cut from edges. Most c-Si panels have uncovered gaps at the four corners of the cells. 2. Ribbon Silicon: A type of monocrystalline silicon. Formed by drawing flat thin films from molten silicon using ribbon. They have multicrystalline structure, lower cost due to less waste, but also lower efficiency. 3. Poly or Multicrystalline Silicon (poly-Si/mc-Si): Made from cast square ingots. Less expensive than monocrystalline silicon, less efficient. GERMANIUM: Dominated early PV market for space applications until mid 1960's. High efficiencies. Bandgap too small for high efficiency cells, indirect bandgap. CADMIUM TELLURIDE: Another material being used, but toxic. Efficient light absorbing material for thin film cells. Easier to deposit & good for large scale production. COPPER INDIUM DISELENIDE: These semiconductors are especially attractive for thin film solar cell applications because of their high optical absorption coefficients and versatile optical and electrical characteristics which can be manupulated and tuned for specific needs. COPPER INDIUM GALLIUM DISELENIDE: Some believe that these materials will have limited use due to limited availability of Indium. GALLIUM ARSENIDE: High efficiency multijunction cells originally developed for sattelites & space explorations. Efficiencies of around 41% have been observed for GaAs under solar concentration & lab conditions. In MULTIJUNCTION SOLAR CELLS, each type of semiconductor will have a characteristic bandgap energy which causes it to absorb light most efficiently at a certain color. The semiconductors in the multijunction are carefully chosen to absorb nearly all of the solar spectrum. Tandem solar cells based on monolithic, series connected, GaInP, GaAs, Ge pn-junctions are seeing demand rapidly rise. LIGHT ABSORBING DYES: Even though degradation under UV & heat is an issue, popular emerging technology. Photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2 and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte. ORGANIC/POLYMER SOLAR CELLS: Efficiencies around 6.5% observed and are lower compared to inorganic materials. May be beneficial for applications where mechanical flexibility are important. SILICON THIN FILMS: Using Silane gas and Hydrogen one can deposit thin silicon films onto substrates with Chemical Vapor Deposition (CVD) technique. Thin films have the advantage of using less material. Amorphous silicon, protocrystalline silicon and nanocrystalline silicon are deposited. Amorphous silicon has higher bandgap Eg (1.7 eV) than crystalline silicon (1.1 eV) which means it absorbs the visible part of the spectrum more strongly than IR portion. Tandem cells of a-Si + c-Si are made. A number of different advanced technologies are currently under development. These advancements will eventually increase the solar efficiencies and decrease the cost of solar modules, making it more feasible to be installed all over the World. |
