Innovation in Thin-Film Silicon PV

NanoMarkets has just released a new report on thin-film silicon photovoltaics. See here for details.

Amorphous silicon (a-Si), a type of thin-film photovoltaic (PV) technology, is experiencing a dramatic growth curve worldwide and offers a compelling business opportunity in power generation, building- integrated solutions and consumer applications. Thin-film PV solutions are the most rapidly growing portion of the PV landscape with approximately 23 percent of the overall PV market in 2008 and a-Si represents the largest component at over 50 percent of the overall TFPV market production in 2008. Amorphous silicon is well positioned to become low-cost PV solution of choice for many applications in the eight-year time frame covered in this report. Lower cost per kWh is the main driver for the shift from crystalline silicon PV to thin-film PV, as well as the increasing acceptance of a-Si thin-film PV for new applications. Cost, product maturity, excellent reliability, and availability of product in high volume are all reasons a-Si has become the most popular of the thin-film technologies; other TFPV technologies include CIS/CIGS and organic PV, which have product maturity issues, and CdTe, which suffers from government regulatory issues at end of life disposal/recycling.

The economics of all photovoltaics involve a high upfront cost to pay for the solar panels, but free feedstock in the form of light from the sun and relatively low operating costs because of the relatively low, periodic maintenance costs compared to traditional methods of power generation. The PV technology that is able to provide the quickest path to lowering these upfront costs and deliver product in high volume is likely to become the dominant PV solution. Amorphous silicon thin-film PV is well positioned to be the PV solution that can provide both large volumes quickly and a roadmap to low cost faster than competing TFPV technologies.

Amorphous silicon solar cells were introduced initially in the late 1980s, with expectations that they would dominate the PV market and be competitive with fossil fuels by the mid-1990s. This did not come to pass as the efficiency was less than 5 percent and initial cell reliability was less than 10 years. These drawbacks coupled with the pullback in fossil fuel prices in the late 1980s, off of peaks in the early 1980s, eliminated almost all demand for a-Si PV except in low-cost/low-power applications such as solar calculators, watches, etc.

Despite a lack of large-scale commercial applications, research continued on a-Si, which resulted in a much better understanding of a-Si PV physics. This research resulted in the development of tandem a-Si/Si:Ge alloy and a-Si/µc-Si cells that had efficiencies nearing 10 percent and field reliability of over 20 years. This positioned the a-Si PV to capture market share when renewed interest in PV energy emerged in the early 2000s.

Several events occurred starting in the early 2000s that accelerated the adoption of PV in general and that of a-Si in particular. First was the spike in fossil fuel costs that increased interest in all PV solutions. With this increased interest, the PV demand exceeded supply. Because crystalline silicon dominated the market, the increased need for silicon combined with the robust demand for silicon in the semiconductor industry caused silicon prices to skyrocket and resulted in a silicon shortage. These high prices spurred companies to invest in capital to expand capacity for a-Si (<2 percent silicon consumption of c-Si) and CdTe-based thin-film PV, as well as accelerated research and development into CIGS and organic PV. Fortunately for a-Si, the renewed interest in PV solutions happened at the same time the industry was transitioning to tandem and multi-junction architectures with much more attractive overall efficiency and reliability than the single-junction designs, which were the dominant products available in the late 1990s and early 2000s.

In addition to the demand for alternative energy sources strictly due to cost of fossil fuels, the global warming/climate change movement helped drive demand for PV solutions as they have a zero carbon footprint. Government subsidies for PV solutions (especially in Germany and Spain where such subsidies can be viewed as either jump starting the PV industry or distorting the marketplace, depending on your point of view) have made it economically feasible to build large PV arrays. Amorphous silicon is very competitive for these applications and this has created demand for more capacity.

By the end of the period covered by this report, the roadmap for thin-film silicon PV cells will most likely transition from the a-Si/µc-Si cells, which are now becoming the mainstream a-Si product, to tandem-junction cells that most likely will be tandem- or triple- junction cells based on combinations of amorphous silicon, microcrystalline silicon and nanocrystalline silicon. The roadmap by the end of the reporting period will see the introduction of silicon-based quantum dots or silicon nanowire-based architectures ramping to high-volume manufacturing. This a-Si PV materials roadmap predicted in this report provides a path to 15-16 percent cell efficiency leveraging the cheap SiH4 as a feedstock, no changes to the TCO or reflector materials (although there are certainly improvements in materials processing that can improve efficiency), and most likely will use much of the equipment infrastructure of the current tandem cell factories that are currently coming on line. This reuse of capital equipment and infrastructure represents an excellent value proposition to constantly increase efficiency, aggressively driving down costs, while not being saddled with heavy capital costs to improve efficiency with the exception of those to satisfy increased capacity.