| Technology |
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| Technology Overview |
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| ISET has developed a complete process for manufacturing low–cost thin-film Copper Indium Gallium Selenide (CIGS) Photovoltaic modules. A completed ISET module cleanly converts sunlight into electricity and costs much less than modules based on competing technologies. |
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| ISET’s process builds a stack of very thin film layers on a rigid glass or flexible foil substrate. These films form the structure of a solar cell that absorbs and converts sunlight into electricity. The primary semiconductor material used in ISET’s modules is CIGS. ISET has developed an innovative technique of printing a precursor ink onto various substrates and subsequently converting that coating into an efficient CIGS absorber film through controlled gas-solid reactions. This type of process, which first applies a precursor material before forming a CIGS film, is known as 2-stage process. |
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| ISET’s patented “Ink-Based” process represents a decisive advancement in low-cost CIGS solar cell fabrication. Historically, CIGS precursors have been deposited using expensive high-vacuum evaporation and physical vapor deposition techniques that utilize materials poorly and require costly capital investment. The limitations of these processes have hindered large-scale, low cost commercial production of thin-film CIGS solar cells. ISET’s non-vacuum printing process generates high-quality CIGS solar cells that can truly be commercialized on a global scale. |
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For a more detailed look at the primary features of ISET’s technology, click on the headers below:
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| What is CIGS? |
| CIGS is a compound semiconductor made up of the elements Copper, Indium, Gallium and Selenium. This material is a very powerful absorber of the Sun’s rays, with demonstrated conversion efficiencies approaching 20% for small-scale CIGS solar cells produced at the National Renewable Energy Laboratory (NREL). A CIGS absorber film requires only 1-2µm of material to convert sunlight into electricity, offering potential for significantly reduced material costs over conventional silicon solar cells. |
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| CIGS films are coated onto substrates that have been metallized with Molybdenum as a back contact to collect electric current. A thin buffer layer of Cadmium Sulfide (CdS) is deposited onto the absorber film, forming a junction between a p-type semiconductor (CIGS) and an n-type CdS layer. A transparent conductive window layer, typically doped zinc oxide (ZnO), forms the top contact of the device, resulting in a complete CIGS solar cell. |
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| As light passes through the ZnO layer, it is absorbed by the CIGS, creating electron-hole pairs. An electric field created at the CIGS/CdS junction draws negatively charged electrons to the ZnO layer, which generates a flow of positive charges in the opposite direction, producing an electric current. |
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| CIGS solar cells can be fabricated on both rigid and flexible substrates, adding to their desirability as the next generation photovoltaic material of choice. When properly sealed and laminated, CIGS modules can deliver stable performance while withstanding exposure to the elements as well as direct solar radiation for over 20 years in the field. |
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| Printed CIGS from Nanoparticle Inks |
| ISET has pioneered the non-vacuum ink-printing approach with the intent to produce high-quality CIGS films at a low cost. ISET’s patented “ink-based” technology begins with the synthesis of Copper-Oxide, Indium Oxide, and Gallium Oxide nanoparticles, which are subsequently suspended in water to prepare a precursor ink for printing onto glass or foil substrates. The most important advantage of this approach is the ability to precisely control the composition of the ink, resulting in the deposition of compositionally uniform precursor films. |
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| ISET’s printed precursor coatings are converted to CIGS films through sequential gas-solid reactions. Through a reduction reaction with ambient hydrogen gas, nanoparticle oxides are converted to a continuous film of alloyed Copper, Indium, and Gallium metals, producing water vapor as a byproduct. This film retains the precise ratio of each element as formulated in the ink, maintaining uniformity across the entire coated substrate. Subsequently, the metal alloy film is reacted with Hydrogen Selenide gas to complete the full conversion to a high quality CIGS absorber film. |
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| ISET has fully realized the inherent cost-lowering benefits of printing, resulting in exceptionally high materials utilization. Ink is selectively applied over a defined area, resulting in almost no Copper, Indium, and Gallium being wasted. The precursor coating is applied at standard room temperature and pressure, avoiding the need for costly high-vacuum equipment. Printing and wet coating technologies are well-established for high-volume production, which eases the transition from the pilot-scale to large-scale production. Selective printing also allows ISET to produce a range of possible product configurations, as cells can be printed on both small and large areas without using more material than is necessary for each type. |
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| CIGS Solar Cell Processing |
| ISET’s complete CIGS process is very straightforward and economical, as the substrate moves through sequential steps to build a complete stack of thin film layers forming a solar cell. Following ink coating and absorber formation, the buffer layer is applied to the CIGS layer by chemical bath deposition (CBD), forming a P-N junction. The CIGS solar cell is completed when a transparent conductive window layer is applied through low-pressure chemical vapor deposition. Cells are then isolated and monolithically integrated to form a complete module on a single substrate. |
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| Monolithically Integrated CIGS Modules |
| Photovoltaic modules are formed by connecting several solar cells in series to build voltage by adding the voltage of each cell in the string. This process is greatly simplified by constructing monolithically integrated thin-film CIGS modules, whereby the layers that make up a CIGS module are sequentially patterned so that a designed number of cells are defined on a single substrate. In this configuration, the cells are connected in series as the top contact of each cell connects to the back contact of its next neighboring cell. |
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| Monolithic integration truly takes advantage of the sequential nature of thin-film processing to produce an optimal configuration of cells to match the desired voltage and current output of a CIGS module. These configurations are variable without affecting the basic thin-film deposition process conditions, allowing ISET greater flexibility for product offerings. |
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