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1.CIGS Solar Cell:
CISI is currently focused on the American and German markets and has two types of processes that differ by the substrates: One is the glass substrate by sputtering process, and the other is the stainless steel roll-to-roll process, where Uni-Solar based in USA is the forerunner in the stainless steel substrate applications. The type of substrate used is the rollable stainless steel substrate, and is another high-tech use of the super thin stainless steel plate.

CIS (Copper Indium Diselenide) or CIGS (Copper Indium Gallium Diselenide) are both compound semiconductors. These two materials have broad ranges of light (spectrum) absorption properties, and stable characteristics. Currently, conversion rates can reach up to 30% with assistance from light focal devices, and up to 19.5% under standard testing conditions; up to par with the most optimum conversion rates of single crystal solar cells. For large area processes, optimum conversion rates of flexible plastic substrates have reached 14.1%. With excellent stability and conversion rates, this is one of the thin film solar cells offering the most potential. Currently, CIS thin film solar cells are manufactured with the sputtering process, but with higher costs. However, starting from the end of June, Nanosolar in Silicon Valley announced research results for metallic foil as substitute for the substrate. The CIGS thin film solar cell can then be manufactured simply by printing (just like printing a news paper) and will greatly reduce production costs.

The Dye-sensitized solar cell is a fairly new technology in solar-powered batteries. The product comprises transparent conducting substrate, TiO 2 nano particle thin film, dye (light sensitized solution), electrolyte, and ITO electrodes. The main benefits of this type of solar cell is that the TiO 2 and dye materials are relatively inexpensive and may be mass produced by printing, resulting in more varied substrate materials to use.

2. Flexible Display Panel
The stainless steel thin panel is the better choice for the substrate material in rollable displays, as its water resistance, air resistance capabilities, workable temperatures, and price are better than plastic. Lehigh University successfully produced driving circuits such as the CMOS Ring Oscillator and Shift Register etc. from 0.1mm thick metallic substrate in high temperature multi crystal thin film components. This demonstrates the advantages of stainless steel substrate in high temperature processes, with greater process tolerance levels than other types of flexible substrates.

Substrate Material is the Key to Rollable Process Developments
The flexible display may be rolled up and is therefore not suitable to be manufactured by traditional glass substrate materials. Currently, researchers around the world are working towards finding other viable substrate materials for flexible displays. From the current development progress, there are three types of viable solutions: Develop super thin glass substrates, develop plastic substrate materials, or thin metallic substrates.

Importance of Super Thin Glass Substrate Component
Currently the thinnest glass available from production is 0.5mm, and panel suppliers have began attempting to polish glass plates to a thickness under 0.2mm for various applications. When glass drops to a specific thickness, the glass substrate exhibits flexibility, and has been used by some manufacturers for the super thin glass substrate in their flexible displays.

As we clearly saw from the 2006 Yokohama exhibition, suppliers have been pushing the super thin TFT LCD. The technologies behind the thinness include the super thin backlight panel and glass substrate. Products from Sharp, forexample, have an LCD thickness of 0.89mm approximately 0.3~0.4mm from the backlight panel, meaning that the glass thickness is approximated to be 0.2mm for its flexible characteristics.

If the flexible display structure is used for the LCD display medium technology, as the glass cannot bend, the force will cause transistor components to malfunction easily. As a result, when using the glass substrate, the selection of material and component will become critical.

For now, the majority of manufacturers are focusing on the plastic substrate because of its high flexibility among flexible substrates. The most commonly used plastic materials include PET, PC, PEN, and ARTON etc.

Plastic Substrate has yet to Overcome the Process Temperature and Component Forces
Even though glass substrate features advantages of high temperature tolerance and high transparency, we still require durability and resistance for flexible displays. In addition to their lightweight and compact components, plastic substrates have the upper hand and are the trend of the future. However, the greatest obstacles plastic substrates face will be temperature limitations during production, and will therefore lower the process temperatures for TFT components.

Future development of rollable displays require strong technical capability of transistor back panels, as well as high yields of the display medium panels. If plastic substrate is used, the shrinking characteristic will result in misaligned colors of the three substrate layers, and will require temperature control and component calibrations during the manufacturing process.

The thin metallic substrate has a thickness of less than 0.1mm, where the main advantages of the thin metallic substrate is its high temperature tolerance, water/air resistance, rollable process, and low costs.

The thin metallic substrate is capable of tolerating up to temperatures of 1,000J, which is vastly higher than plastic substrates, and therefore is applicable for various high temperature processes. Compared to plastic, the coefficient of thermal expansion (CTE) of thin metallic substrate is closer to glass, and is easier to align in the circuit layout.

Furthermore, as the thin metallic substrate itself does not allow water or air to pass through, which is unlike plastic materials that is easily penetrated by moisture and oxygen, it does not require any additional coating for water/air proofing. This will simplify the production procedures, coupled with the excellent extension properties of the thin metallic material makes it the suitable ingredient for rollable manufacturing processes.

For material costs, the thin metallic price is currently far lower than specialized high temperature plastic substrates at approximately 30%. When used in the mass production of flat panel displays, the protection layer that keeps out air/water is not required and therefore will further reduce the cost over plastic substrate materials. From this thought, the thin metallic substrate exhibits great potential and will continue to be the point of focus in the succession of plastic substrates.

Article excerpt: Micro-Electronics Issue 260, November 2007; Yuan-Qing Lin


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