NMS Research Analysis: Cutting Edge Solar Cell Technology: The Fast-Growing Trend in a Slow Global Economy

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Cutting Edge Solar Cell Technology: The Fast-Growing Trend in a Slow Global Economy

Ed Wijaranakula, Ph.D.
Chief Technology Officer, Infotix Systems, Inc. - January 5, 2009

The phenomenon known as the Staebler-Wronski effect causes a 10 to 20 percent degradation in cell efficiency over time when first exposed to light. The degree of degradation is dependent upon the intrinsic layer thickness of the amorphous silicon cell structure as well as the duration and the intensity of light at which the cell is exposed. After the cell has stabilized, its conversion efficiency is typically about 6 to 8 percent.

Amorphous silicon absorbs solar radiation about 40 times more efficiently than single-crystal silicon and thus the amorphous solar cells perform well in non-direct and weak light. The amorphous silicon solar modules are also more stable under intense light conditions and at high ambient temperatures, compared to silicon-based solar cells.

An amorphous silicon thin-film cell has a p-i-n structure consisting of a p-type top layer, an intrinsic or undoped (i-type) layer and an n-type bottom layer. The first manufacturing step of the p-i-n cell is to deposit a thin layer of p-type amorphous silicon onto a transparent conductive

tin-oxide film (TCO) via either a rf grow-discharge deposition or PECVD from silane (SiH4) and hydrogen. This is followed by intrinsic and n-type layer deposition. For p-type and n-type layers, diborane (B2H6) or phosphine (PH3) doping gas diluted in hydrogen gas are used, respectively. Subsequently, screen-printed or evaporated contacts are applied to the front and back of the cell.

In micromorph silicon thin film technology, an additional microcrystalline silicon layer is deposited onto the amorphous silicon layer in a tandem cell configuration. The amorphous film converts the visible part of the solar spectrum while the crystalline layer converts the energy of the red and near-infrared spectrum. The micromorph silicon thin film cells achieve about 30 percent higher efficiency and are less sensitive to light-induced degradation, compared to amorphous silicon solar cells.

CdTe Thin-Film Technology - The CdTe/CdS solar cell is a polycrystalline thin-film device based on a semiconductor heterojunction. The first manufacturing step is to deposit a thin n-type cadmium sulfide (CdS) emitter layer onto glass substrate or flexible polyamide foil coated with transparent conductive (TCO) or indium tin oxide (ITO) film. After CdTe base layer deposition, the CdTe layer stack has small grains and a high density of deep level defects which negatively impact the cell efficiency. A short recrystallization anneal at around 400 °C is applied to relieve the film stress and to facilitate grain growth in the CdTe layer stack, which in turn reduces the deep level defect density.

Prior to copper metal back contact, the CdTe is either chemically etched or receives a Te vapor treatment to create a Te-rich layer on the CdTe stack. According to Gartner, the average conversion efficiency of CdTe thin film solar modules is about 10 percent at a cost of approximately $1.14 per watt. Commercial modules of up to 11.5 percent efficiency have been reported by Fort Collins, Colorado-based AVA Solar Inc.

CIGS Thin-Film Technology - The CIGS solar cell is a polycrystalline thin-film device, based on a more complex heterojunction structure than that of the CdTe thin film cell. The CIGS solar cells can be manufactured using the conventional vacuum-based process in which a complex quaternary alloy p-type Cu(In,Ga)Se2 is either co-evaporated or sputtered onto glass substrate coated with molybdenum (Mo) that serves as metal back contact. An alternative low cost manufacturing technique, known as nanoparticle spray based deposition, uses nanoparticle colloids, or "ink", that is sprayed or printed onto heated molybdenum-coated glass to form

precursor Cu-In-Ga-Se films. The alloy films are subsequently seleinized using H2Se gas at about 450 °C to form the CIGS absorber layer.

Thin buffer layers of CdS and intrinsic ZnO are deposited onto the Cu(In,Ga)Se2 film via a chemical bath deposition method and LPCVD. The cell manufacturing process is complete after a deposition of an n-type aluminum-doped transparent conductive ZnO top layer, followed by evaporation of metal contacts. Commercial production CIGS module efficiency is about 10.5 percent. Austin, Texas-based, HelioVolt Corporation, claimed their proprietary FASST® reactive transfer printing process has produced thin film solar cells with 12.2 percent conversion efficiencies in a record-setting six minutes.

Major Players Still See Strong Growth Ahead - The top three major players are Osaka, Japan-based Sharp Corporation [JP:67530], Thalheim, Germany-based Q-Cells SE [WKN:555866] and Wuxi, China-based SunTech Power Holdings Co., Ltd. [NYSE:STP] with a total combined production capacity of over 2GW or about 40 percent of the total world capacity.

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About the Author: Dr. Ed Wijaranakula is presently the Chief Technology Officer at Infotix Systems, Inc. Prior to Infotix Systems, he has worked with Intel, Hewlett-Packard, Micron, Motorola and Texas Instruments and has held senior as well as managerial positions in semiconductor manufacturing companies. He has published over 80 technical papers and holds more than 12 U.S. and foreign patents. At the time of publication, Dr. Wijaranakula's portfolio does not hold any positions in any of the financial products mentioned in the article.