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.

Sharp's primary markets are residential and commercial solar systems, ranging from a 100 KW solar array on an ice rink rooftop to the 1.6 MW rooftop system at Google's headquarters in Mountain View, California. Residential and commercial rooftop solar projects are surging as homeowners and businesses are looking to find ways to reduce their carbon footprints and the amount of electricity they purchase from their local utility companies. Beginning January 1, 2009, homeowners and businesses will be eligible for a 30 percent federal tax credit of the entire solar project's cost. The $3.3 billion California Solar Initiative plan alone could boost solar projects in California up to a million rooftops by 2017.

Currently, Sharp's Katsuragi plant in the Nara Prefecture, Japan, which produces silicon-based and amorphous silicon TFT-based PV modules, has a capacity of 710 MW.  Early last year, Sharp announced a 72 billion yen investment in an advanced amorphous silicon thin-film solar cell plant in Sakai, Osaka Prefecture with initial production of 480 MW. Combined with the 160 MW capacity at the Katsuragi Plant, Sharp's global total production capacity for thin-film solar cells will reach 1 GW by April 2010.

Q-Cells' primary markets are in silicon-based solar PV systems for residential, commercial and utility-scale integrators, including Beltsville, Maryland-based SunEdison, North America's largest solar energy services provider. The Quebec, Canada-based market research firm Electronics.ca Publications, forecasts that the utility vendor electricity solar equipment markets, driven by large scale solar farm development worldwide, will expand from $10 billion in 2007 to about $79 billion by 2014.

Q-Cells gave guidance in August 2008 that their total production capacity would rise to about 1.3 GW by the end of 2009. Due to deterioration in project financing conditions and the uncertain market situation, the company is expecting weak demand into early 2009 and has revised its forecast for total production in 2009 to between 800 MW and 1 GW. The company is still expecting strong revenue growth of at least 40 percent to between €1.75 to €2.25 ($2.4 to $3.04 equivalent) billion.

Q-Cells has heavily invested in future technologies with high commercial potential in the coming years. The list of companies in which Q-Cells has minority stakes includes a 17.18 percent stake in Sandvika, Norway-based Renewable Energy Corporation (REC), the world largest single and multicrysalline solar wafers, a 33.3 percent stake in EverQ, a joint venture between Q-Cells, REC and Marlboro, MA-based Evergreen Solar Inc. [NASDAQ:ESLR], a 18.63 percent stake in Bitterfeld-Wolfen, Germany-based CSG Solar AG, a small crystalline silicon on glass PV manufacturer, and a 32 percent stake in Fremont, California-based SOLARIA, a small start-up company specializing in the design and development of a concentrating PV (CPV) system using proprietary plastic lenses that magnify the light on solar cells. Small thin-film solar companies in which the majority equity is owned by Q-Cells are Sontor, Calyxo, Solibro and FlexCell.

Suntech Power Holdings produces silicon-based PV modules at three Chinese production sites including Wuxi, Luoyang and Qinghai. In August 2008, the company entered Japan's PV market by acquiring Nagano, Japan-based MSK Corporation, one of Japan's largest thin film PV manufacturers, specializing in building-integrated photovoltaics (BIPV). MSK, which develops and markets BIPV products such as transparent, multifunctional solar panels or PV-TV, has a production capacity of approximately 100 MW. PV-TV is a building-integrated PV material that can function as a solar panel, window glazing as well as a video display screen.

During their Q3 2008 earnings conference call last October, Suntech revised its full year 2008 PV product shipment target from 550MW to 490MW, citing tight credit conditions and deferment of some customer orders. Suntech said, however, that their production capacity reached 750MW in Q3 2008 and was still on target to hit 1GW by the end of 2008.

In the conference call, Dr. Zhengrong Shi, Suntech's Chairman and CEO, stated that his company decided to hold further capacity expansion until there was more visibility in the status of the credit market. This could include a new 300 MW PV cell production facility in Yangzhou, about 250 km northwest of Shanghai, China. Last September, Suntech announced the groundbreaking of the Yangzhou production site. The Yangzhou product plant is part of Suntech's capacity expansion plans to reach 1.4GW by year end 2009 and 2 GW by year end 2010. 

"We have already received orders from the European customers for over 600 megawatts of PV products for 2009 and are pursuing a growing pipeline of additional orders. This indicates that core demand is strong though we are well aware of the challenges created by the turmoil in the financial market that may make it difficult for some orders to materialize." added Dr. Zhengrong Shi.

* Currency exchange rate € 1 = $1.3528

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