Which Thin Film Solar Technology is the Best?

The term "Thin film solar panels" refers to the fact that these types of solar panels use a much thinner level of photovoltaic material then mono-crystalline or multi-crystalline solar panels. Thin film solar cells consist of layers of active materials about 10 nm thick compared with 200- to 300-nm layers for crystalline-silicon cells.

The semiconductor junctions are formed in different ways, either as a p-i-n device in amorphous silicon (A-Si), or as a hetero-junction (e.g. with a thin cadmium sulfide layer) for CdTe and CIGS panels. A transparent conducting oxide layer (such as tin oxide) forms the front electrical contact of the cell, and a metal layer forms the rear contact.

The primary objective of manufacturers of these solar panels is to reduce the overall price per watt to make solar competitive (on a capital investment cost basis) with fuel based power generating technologies (i.e., nuclear, coal, and gas). The original goal was to break $1.00 per watt - but now that has been breached, the new goal is $0.70 / watt of peak power.

While these thin module prices are much lower in price, they also have a lower module efficiency (roughly 1/2 of monocrystalline solar panels, but roughly comparable with polycrystalline solar panels.


Three Common Types of Thin Film Panels

The high cost of producing crystalline silicon wafers (which makes up 40-50% of the cost of a finished module) has led many companies to look at less expensive materials, and more economical ways to make solar cells. The three most common photovoltaic materials used in mass production of thin film modules at this time are the following:

Amorphous Silicon

(a-Si, still silicon, but in a different form),

Amorphous Silicon (a-Si) PV modules were the first thin film PV modules to be commercially produced. What is amorphous silicon? It is the non-crystalline form of silicon. An amorphous silicon solar cell contains only about 1/300th the amount of active material in a crystalline-silicon cell. In its simplest form, the cell structure has a single sequence of p-i-n layers.

You are probably already familiar with this technology because it is widely used in pocket calculators and other small electronic devices. In the past few years solar panels made from amorphous silicon have gained popularity because of improvements in its efficiency (averages between 7 to 9 percent), and lower sales prices. Additionally, because of its higher resistance to overheating, Sanyo has introduced a has developed a hybrid solar cell by applying coatings of amorphous silicon onto a mono-crystalline solar cell.

Amorphous silicon is the most well-developed thin film technology to-date and has an interesting avenue of further development through the use of hybrid cells which incorporate some "microcrystalline" silicon in order to combine the stable high efficiencies of crystalline Si technology with the simpler and cheaper large area deposition technology of amorphous silicon.

To read more about the Amorphous Silicon Solar panels, including their advantages and disadvantages click here.


Thin Film Silicon

NEW: For more information about a new member of the thin film family - be sure to read the following two articles we've posted to this website about:

1. Thin film silicon cuts production costs to €0.50 / kWp; and

2. New Micromorph a-Si Solar Panels reach 11.9% Efficiency

Cadmium Telluride (CdTe)

First Solar, the largest producer of solar panels through June 2010, is the biggest advocate of using Cadmium Telluride.

So far Cadmium telluride is the first and only thin film photovoltaic technology to surpass crystalline silicon PV in price per watt of peak power, but this price advantage seems to be eroding as price of raw silicon has decreased and Chinese manufacturers increase their production of multi-crystalline panels.

There are some concerns about the future of Camium Telluride based panels, specifically the very limited availability of Telluride and increased concern in Europe about long-term toxic affects of Cadmium.

To learn more about the history, manufacturing and the advantages and disadvantages of using solar panels made with Cadmium Telluride, click here.

Copper Indium (Gallium) diSelenide (CIS or CIGS)

CIGS technology has achieved efficiency levels of 20% in the laboratory, much higher than Cadmium Telluride. Unfortunately the material is more difficult to work with and many companies are struggling to bring a sufficiently efficient module to the market at an attractive price.

Currently most manufacturers are producing relatively small amounts in the 1–30 megawatts per year range and CIGS technology remains an unproven, but promising technology with the promise of being able to eventually reach efficiencies approaching 20%..

VC's must believe in the technology though, as they have invested $2.3 to date in a number of CIGS companies. At least one research report predicts CIGS technology will be the dominant solar technology by 2020.

To learn more facts and the advantages and disadvantages about panels that use the CIS / CIGS technology, click here.

Brief History

Thin film technologies are all complex. They have taken at least twenty years, supported in some cases by major corporations, to get from the stage of promising research (efficiency levels between 11% and 20% in the laboratory) to the first manufacturing plants producing early product.

During this period solar panels made from cadmium telluride, and copper indium diselenide (CIS-alloys) have attracted the largest investments. First Solar was helped by a major investment from the Walton Family, and more than $2.3 Billion has been invested into CIGS companies by VC companies.

Many of these technologies have demonstrated cell efficiencies at research scale above 13%, and best prototype module efficiencies above 10%. The technology that is most successful in achieving low manufacturing costs in the long run is likely to be the one that can deliver the highest stable efficiencies (probably at least 10%) with the highest process yields. So far that leader is clearly First Solar, but there are some big companies in the market and there are new developments monthly.

The big economic questions is … will thin film manufacturers be able to withstand the growing competitiveness of crystal silicon (c-SI)? While thin film module prices have come down – c-SI module prices have come down faster. This is likely to increase as the Chinese are making $17 billion in new loan guarantees to the 3 larges c-SI companies in China for more information click here for more information.

Advantages

Thin film solar panels are the new kids on the block. Learn about the pro’s and con’s of using thin film solar panels for your home, commercial building or even for large utility sized projects (such as First Solar is planning to build):

1. Versatility

Thin film can be applied to almost all types of surfaces - such as metal, plastic and even paper (in the laboratory). They’ve even been used as a type of roofing material. Specifically, they can actually be used instead of steel or shingles for roofing, creating an entire roof that generates power from sunlight. Unlike rigid panel types, they don’t stand out, blending in better with the roof itself.

2. Flexibility

While crystal silicon solar panels are rigid and therefore fragile, "thin film" materials can be deposited on flexible substrate materials. However, while it is true that thin film solar cells are flexible, their flexibility is a feature of how they're constructed and how you can install them, but not how they're going to end up being used. Like other solar panels, they typically still get installed flat and in a frame at an optimal angle facing the sun. They can conform a little bit to a curved roof surface, but they're typically installed pretty flat; and by the time they're installed, they become inflexible.

3. Good Performance in Indirect Light

Thin film solar have gained an important market share in Germany as they are quite cheap and work well on roofs that face North, or East/West … instead of South.

4. Good Performance in High Heat

One benefit of thin film solar panels that other types can’t offer is that they don’t suffer a decrease in output when temperatures go up – which can lower efficiency of silicon based modules by 10% or more in some locations.

Some thin film modules may even have a slight increase in their outputs at higher temperature levels. That’s impressive, since areas where sunlight is readily available are also usually hot. Because of this, thin film solar panels often have an actual output that’s very close to the one they’re rated for. This can make planning a solar power system much easier using this kind of panel.

5. Good Performance in Niche Markets

Thin films have long held a niche position in low power (<50W) and consumer electronics applications (e.g., calculators), and may offer particular design options for building integrated applications.

Disadvantages

As previously mentioned there are at least three types of thin-film technologies and they each have different performance. Therefore, as you read through the following list of disadvantages keep in mind that while these are general comments about thin film solar panels in general, but there are likely to be differences according to the type of technology used and even with different manufacturers:

1. Efficiency

There’s a reason why thin film solar panels haven’t replaced older types yet. They’re just not as efficient. Additionally, some thin film materials have shown degradation of performance over time and stabilized efficiencies can be 15-35% lower than initial values.

2. Higher Total Costs

With about a 7 to 10 percent conversion rate for energy drawn from the sun, they can only draw about half the wattage from sunlight that mono and polycrystalline panels do, which requires twice as much installation space for the same amount of power.

Although panel costs (which account for around 50% of the total installed price) have been declining as a result of more efficient manufacturing and economies of scale – installation costs have remained about the same. Consequently if you need to install twice as many panels to get the same results – the overall cost advantages of lower panel prices disappear quickly.

Here are facts: The earth has limited land use. A 20 MW solar farm can be as large as 20 acres using silicon solar panels with high efficiency rated cells, even less if you use concentrated solar technologies. In the thin film world you would need between 40 and 60 acres to produce 20 MW depending on the technology. This is the basic difference between large land use for the same results. Is it worth the extra land? Since thin film has no recorded history we don't even know if it will last as long as silicon solar panels. Sometime you get what you pay for.

3. Longevity

You should be aware that there’s a possibility that they’ll hold up just as long as mono or polycrystalline panel, but this technology is new and still hasn’t been well tested. There are concerns that there may be a more rapid decrease in electrical production than other types of panel technologies.

4. Scarcity of Raw Tellurium

Tellurium, while fairly plentiful in the universe is a very rare element on earth. Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley, says the presence of tellurium could limit the total electricity such cells could produce because of its rarity. While total worldwide electricity demand will likely reach dozens of terawatts (trillions of watts) in the coming decades, thin film solar cells will likely be limited to producing about 0.3 terawatts, according to a study he published last year.

5. Toxicity Concerns

Both Cadmium Telluride (larger amounts) and CIGS (smaller amounts) use Cadmium, which is classified as one of the 6th most toxic substance. While use of Cadmium Telluride on a residential roof doesn't pose a significant risk – there is some concern about large utility sized projects, as well as the long term effects. Authorities in Europe are currently looking at the possible desirability of enacting stronger regulations regarding products containing Cadmium, which could have an impact on the sale of thin for thin-film solar panels in Europe.

Market Acceptance

The fact that 3 of the top 10 largest producers of solar panels in 2010 produce thin film solar panels, namely First Solar (#1), Sharp (#3), and Sanyo (#10), demonstrates there is a strong market demand for lower cost solar panels.

Based on production in 2009, solar panels based on thin film technologies represented 16.8% of total global production, up from 12.5% in 2008. As of 2010 some sixty companies are either producing solar panels using one of the three technologies listed above, or have announced plans to start production this year. The top ten producers were:

_ 1100.0 MW First Solar
_ 123.4 MW United Solar Ovonic
_ 94.0 MW Sharp
_ 60.0 MW Sunfilm
_ 50.0 MW Trony
_ 43.0 MW Solar Frontier
_ 42.0 MW Mitsubishi
_ 40.0 MW Kaneka
_ 40.0 MW Moser Baer
_ 30.0 MW Würth Solar
_ 30.0 MW Bosch (formerly Ersol)
_ 30.0 MW EPV
_ 30.0 MW Solyndra

Solar tracking

Almost all of the above photovoltaic module systems to-date have been non-solar tracking, because the output of modules has been too low to offset tracker capital and operating costs. But relatively inexpensive single-axis tracking systems can add 25% output per installed watt. This is climate-dependent. Tracking also produces a smoother output plateau around midday, allowing afternoon peaks to be met.

Future

The three most viable thin-film photovoltaic technologies - cadmium telluride (CdTe), copper-indium gallium (di)selenide (CIGS), and amorphous silicon (a-Si) - continue to mature and grow technologically and in market stature.

But apart from the dominance shown by CdTe leader First Solar, the rest of the TFPV manufacturers have had a fairly difficult time making significant commercial inroads as the price of mainstream crystalline-silicon modules plummeted over the past couple of years. Other factors delaying the long-predicted age of thin film include bankability challenges and difficulties in reducing production and system costs.

Yet entrants in all three thin-film categories have reason for optimism, as they push toward a competitive market position. This paper provides an overview of the current status of the thin-film PV sector and its players, offering insights into why certain companies might emerge successfully in the years ahead.

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