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Bhavin Shah (852) 2800-8538 bhavin.a.shah@jpmorgan.com 244 Asia Pacific Equity Research 20 April 2009 Cadmium Telluride (CdTe) technology overview CdTe is a material with semiconductor properties (like silicon) that makes it well suited to absorb energy from sunlight. Best case CdTe solar cell efficiencies of 16.5% have been achieved in a laboratory setting. Commercial production of CdTebased solar cells, which is today almost exclusively by First Solar, currently results in solar cell conversion efficiency levels of 10.7%. This compares well with a-Si thin film that currently has a best production cell conversion performance of only 8.5%. The CdTe material acts in the same manner of absorbing energy from sunlight as the silicon does in an a-Si thin-film cell or in a more conventional wafer-based solar cell. CdTe materials science and cell manufacturing sequence Like all solar cells, a transparent conductive oxide (TCO) layer is required in order to enhance cell conversion efficiency. In a CdTd-based cell, tin oxide (SnO2), is typically used and is either pre-coated from the glass supplier or deposited as one of the first steps in the production process (Figure 146). The TCO layer is then laserscribed into discrete areas in order to form the basis of a series of individual cells on the larger glass panel or substrate. The glass is then preheated for the deposition of the next layer, which is first of two light-absorbing materials. The first absorber layer, cadmium sulfide (CdS), is deposited through a liquid chemical bath and is the “window” layer that allows light to pass through, while acting as one side of the junction that forms the solar cell. The CdS layer performs the same function as the P- or N-layer in silicon thin-film cells. Figure 145: Cross-section of CdTe solar device structure Source: NREL. The second most critical absorber material, CdTe, can be deposited through electrodeposition, close-spaced sublimation, or vapor-transport deposition. Unlike silicon thin films, the CdTe deposition occurs in either a modest level of vacuum or even at atmosphere. The liquid/vapor deposition also occurs through thermal or chemical activation and does not require electrically created plasma to enhance the deposition rate. With vapor transport, a carrier gas passes over vaporized CdTe. The saturated vapor is then introduced over the heated substrate (panel) surface (which already has the TCO and CdS films), thereby depositing the CdTe film on the substrate (Figure 146). By using these deposition methodologies, expensive vacuum pumps, vacuum tight seals, and advanced power delivery systems are not required for CdTe deposition, likely resulting in lower capital costs than for other types of thin-film manufacturers. During deposition and subsequent heat treatment, the substrates may
Bhavin Shah (852) 2800-8538 bhavin.a.shah@jpmorgan.com Asia Pacific Equity Research 20 April 2009 be heated to temperatures as high as 600 o C to achieve the correct film morphology and composition, thus limiting the kinds of substrate material that can be used (i.e., certain metals will melt at that temperature). Figure 146: Vapor transport deposition schematic Depleted carrier gas Carrier gas Source: Institute of Energy Conversion, Solar Review Nov '05. CdTe q Substrate Saturated carrier gas A zinc telluride layer is then deposited either through sputtering (PVD), electrochemical deposition, or vapor transport and forms an interfacial layer that promotes adhesion with the back contact metal while stabilizing the ohmic contact. The back contact metal (e.g., titanium, aluminum, and/or nickel) that closes the electrical circuit is then deposited, typically using physical vapor deposition (PVD). The panel then goes through another laser scribe to define individual cells. The cells are then encapsulated with ethylene-vinyl acetate (EVA) and a bottom sheet of glass is attached to protect the cell materials from the environment over the 20-year+ lifetime of the product. We expect thin-film technologies, including CdTe and a-Si, to garner an increasing share of the overall PV market as it ramps up at the expense of silicon-wafer-based solar cells. CIGS thin film technology We believe that copper indium gallium diselenide (CIGS) thin-film is a promising PV technology that has proven to yield high efficiency solar cells in laboratory settings utilizing low-cost materials and well understood but complex process methods. The National Renewable Energy Laboratory (NREL), the Department of Energy’s research arm (in the US) for development of renewable technology, recently produced CIGS cells with 19.9% efficiency, the highest among current thinfilm PV technologies. CdTe and multi-junction a-Si thin-film cells have reached 16.5% and 13% efficiency, respectively. Crystalline silicon cells can exceed 20% efficiency but use polysilicon wafers that have been in short supply in recent years and require an energy-intensive process to manufacture. However, the commercialization of CIGS has been slow due to challenges in scaling up the processing steps to achieve acceptable cell performance and yield in highvolume manufacturing. Several development stage companies are building CIGS factories, yet none to our knowledge have begun to produce and ship product in meaningful quantities. We believe the difficulty lies in the issue of maintaining film characteristics (uniformity, morphology) over a large surface area that is needed to produce a commercial solar cell. Although we do not believe the difficulties are insurmountable, we believe that thin film process improvement is a slow and timeconsuming process. In addition, process improvement for CIGS is more complex 245
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Tech Hardware Supply Chain - Gazhoo

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