TFPV: Standing on its Own
Conventional Solar Panels: Thin-Film vs. Crystalline Silicon
Alternative PV Products: Flexible Modules and BIPV
As the photovoltaic (PV) industry has grown over the past decade, the thin-film photovoltaics (TFPV) segment of that industry--and with it, the volumes of the materials used by it--has experienced even more rapid growth. And while the recession of 2008-2009 has certainly set things back a bit, this segment is poised to resume significant growth and even to surpass conventional crystalline silicon (c-Si) PV in volume over the next several years.
TFPV uses a completely distinct set of materials from the more traditional crystalline silicon photovoltaics. The three major TFPV technologies--amorphous silicon (a-Si) PV, cadmium telluride (CdTe) PV, and copper indium gallium diselenide (CIGS) PV--use very thin films (single microns or less) of their absorber materials instead of the bulky silicon wafers hundreds of microns thick that are used by c-Si PV. And the films are formed by thin-film deposition methods--like sputtering or CVD; sometimes electrodeposition or printing--so that all of them, even a-Si PV cells, which use silicon, use completely different starting materials as compared to c-Si PV.
And it is not just the absorber materials that are different. TFPV uses a completely different class of substrates--glass, metal foils, or polymers--as opposed to the silicon wafers that double as substrate and active material for c-Si PV. The electrodes are also very different, especially on the front face where TFPV uses transparent conductive films and c-Si PV does not. And some classes of materials--non-transparent electrodes, encapsulating films, etc.--are specialized for TFPV even though they are in some cases similar to those used for c-Si PV.
Up until the worldwide economic crash of 2008, while PV grew at a rapid pace, TFPV has grown even faster, with hyper-growth at times. After the crash and during the extended recession of 2008-2009, some pockets of the TFPV segment have fared better than c-Si PV or PV in general. But why should TFPV behave any differently from c-Si PV? After all, none of the TFPV technologies is up to the conversion efficiency of c-Si PV; while CIGS PV comes close in champion cells, its production cell efficiencies still lag. And nearly all TFPV modules produced are quite conventional in nature and very similar to the modules produced by c-Si PV firms.
For a long time the answer was easy: the cost and availability of the materials. As demand for PV modules grew rapidly--triggered by high energy prices, concern for the environment, and perhaps most importantly by policy choices in Europe, Japan, and California--the supply of silicon needed to produce them could not keep up. TFPV, long limited to research labs and small niches like solar-powered calculators, "grew up" suddenly to help satiate this raging demand. Like many other technologies, TFPV has historically suffered from the production-versus-demand paradox: high volumes depend on low price, but low price depends on high volumes. The imbalance between supply and demand for PV panels enabled TFPV to climb out of its traditional, limited role since a higher price could be supported yet still be low relative to the alternative.
But now that silicon is no longer in short supply, the cost advantage of TFPV has been reduced. And the new construction opportunities for PV installation--the most economical time to install PV--are still fewer than they had been a year and a half ago, causing demand to remain limited. This is a different environment than the one TFPV "grew up" in. But the volumes that TFPV has achieved now make it a competitive collection of technologies in this market. Even so, TFPV will increasingly benefit from taking advantage of its other advantages--besides cost--to drive growth.
The basic unit of photovoltaic cell installations is the conventional panel, and this is the predominant form of TFPV modules. In the solar panel market, TFPV competes with c-Si PV largely on the basis of cost. The end of the silicon shortage broadens the concern over costs to now include TFPV as well, which uses costly materials like indium, tellurium, and germanium, albeit in small quantities. But even so, TFPV will retain a significant cost advantage for a number of reasons. Aside from short-term price fluctuations, there is a floor to c-Si PV cell prices. Silicon production remains a costly process and prices cannot fall too much farther and remain that way. And besides the materials costs, TFPV benefits from the fact that the technologies involved are still relatively new. c-Si PV is a mature technology and continuing cost improvements will most likely be minor and incremental. But the TFPV technologies are still vibrant fields for innovation, and cost reduction is likely to occur more rapidly. In particular, the use of roll-to-roll processing, and perhaps printing along with it, can reduce costs significantly and allow TFPV to benefit from economies of scale to a greater extent than does discrete wafer-based c-Si PV.
Cost is not the only factor on which conventional PV panels compete. There is also conversion efficiency--the amount of power produced per square meter of the solar panel--and other factors such as weight and ease of installation. c-Si has the advantage on efficiency, although CIGS PV is likely to narrow this gap over the next several years, in large part due to developments in the materials. What's more, newer technologies like nanocrystalline silicon cells and multi-junction CIGS cells--all with their unique material needs--have the potential to exceed c-Si PV's efficiency in the coming years. And TFPV has the edge on weight. Because of their very thin nature and their lower susceptibility to fracture, TFPV modules are lighter and do not require as robust a surrounding structure as do c-Si PV modules. This can reduce the balance-of-system costs for TFPV modules and also allow for some innovative design and mounting strategies.
While TFPV can compete favorably with c-Si PV in the conventional module market, it has even greater advantages in other markets, some of which are completely inaccessible to c-Si PV. The emergence of building-integrated photovoltaics (BIPV) is an excellent example. Both c-Si PV and TFPV cells can be built into rigid BIPV products such as roofing tiles and glass facades. But TFPV cells generally suffer less of a performance drop under indirect lighting than do c-Si PV cells. Since tracking systems are generally not an option for BIPV installations, TFPV-based BIPV products retain more of their rated power than c-Si PV-based ones under actual use. And in the case of BIPV glass, TFPV is generally built on glass substrates and can be more aesthetic than a collection of c-Si wafers encased in glass.
But then there is a market completely inaccessible to c-Si PV--the market for flexible PV products. Many of the opportunities here are also for BIPV applications--like flexible roofing shingles and flexible laminates that are applied to conventional building materials--but there are also portable applications like soft briefcases and portable chargers. This is one of the key areas of interest to material suppliers because of the unique materials requirements. Besides the obvious need for flexible substrates, flexibility adds demands on electrodes--especially the transparent ones--and encapsulation materials.
Source: Thin-Film PV: More than Just a Cost Advantage
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