Solar cell guide, part 2 - Thin Film (CdTe, CIGS) solar cells

Dricus De Rooij
15 september 2012

Thin Films

Thin Films are considered a potential alternative for silicon (Si) solar cells, because the costs have long been considered to be cheaper than silicon solar cells. The reason for that is for Thin Films less semi-conductor is used, bringing the costs down.

CdTe solar cells

Advantages

CdTe solar cells have a number of advantages that make this technology popular in the US.

  1. Fast manufacturing. One of the advantages is the relatively quick way of manufacturing. First Solar is known to produce one CdTe module in about 2.5 hours, which is fast.
  2. Few semi-conductor. The use of less semi-conductor used has long been seen as a major lost advantage. However in recent months the cost of silicon (Si) solar cells has come down significantly, making CdTe solar cells less competitive.
  3. Flexibility. CdTe solar cells can be flexible, and manufactured on various substrates, which allows integration in construction / roofing materials.

The highest efficiency ever reached with CdTe solar cells in a lab is 17.3%. Common manufacturing efficiency is around 11%, which is not enough to compete with other cell technologies such as silicon solar cells, as the installation costs are higher due to the larger amount of pv panels used.

 

CdTe production

 

Problems and risks

In recent years experts emphasized the durability of thin film PV modules, however recently First Solar has numerous quality problems with major power plants, which has led to over 5000 project replacements.

People now start to believe that this extremely thin semi-conductor might not be as durable as First Solar hoped. These defects also proves that accelerated aging tests performed by certification bodies such as TUV are not a 100% guarantee for long term product quality. Once again it’s obvious that the best test is placing the PV modules in the field.

Why is CdTe cell technology not leading the industry?

  1. Few CdTe manufacturers. So far there are limited players on the market that produce CdTe solar cells. And there’s one company that produces CdTe PV modules on a very large scale: First Solar. The past year First Solar has suffered from drop in pricing of silicon solar cells, and the quality problems that occurred with large projects.However the import tariffs imposed on Chinese PV manufacturers might help the company to keep a large market share in the US.
  2. Heavy weight. Still most Thin Film modules are manufactured with glass as a substrate, making the panels heavy, which is not favorable for the installation.
  3. Low efficiency. Further the lower efficiency of CdTe modules leads to a larger quantity of panels need = higher installation costs + more roof surface needed.

Rare and toxic elements

Tellurium is a rare element, which is extracted during copper mining. There’s only a limited quantity available on earth. It’s nearly as rare as gold! If we assume that CdTe will fulfill a large amount of our future electricity need, there will likely be a shortage of tellurium, because we can’t start mining more copper just because we need more Tellurium!

If that is not enough, CdTe Thin Film solar cells contain Cadmium, which is a highly toxic element. Even though it won’t cause a problem during the usage of the CdTe module (even though you could ask yourself what will happen in case of a fire).

After the panels have reached their end of life, the CdTe modules will be have to be recycled. First Solar includes a recycling fee for each PV module sold, and passes this on to an insurance company that guarantees the recycling of each module.

Of course it would be desirable to move on with alternative materials that are non-toxic, as Cadmium needs to be processed properly during recycling, and this involves risks. And in fact, there is plenty of research on abundant, non-toxic elements that can be used to make the perfect solar cell for the future.

 

CIGS Solar Cells

CIGS is a semiconductor material composed of copper, indium, gallium, and selenium.

Why is CIGS so appealing?

If you realize the huge success of First Solar (don’t look at the past year’s performance..), you know that a cell with a higher efficiency than TeCd AND a solar cell that does not contain the toxic element cadmium, would be a big success!

A lot of people believe CIGS solar cells are better than CdTe cells. The highest efficiency reached in the lab so far is 20.4%, and production cells come near the 14%. However producing CIGS solar cells has proven to be more difficult to manufacture than Silicon or CdTe solar cells, as it consists of four different elements, that of course need to be applied in the right ratios.

Advantages CIGS

  1. Heat resistance. Just like TeCd panels, CIGS modules show a better resistance to heat than silicon made solar panels.
  2. Non-toxic materials. CIGS does not contain the toxic element Cadmium.

 

CIGS by Nanosolar

 

Disadvantages CIGS

  1. Low efficiency. CIGS is not as efficient as Silicon (Si), and it will take a long time to increase efficiencies to the same level.
  2. Not competitive (yet). So far CIGS is not competitive with TeCd and Si made solar panels

There are various companies working on CIGS solar cells and when manufacturers get the process right and scale up production, the CIGS cells are, because of their higher efficiency, superior to CdTe solar cells. For now most CIGS manufacturers have limited production capacity.

Main Challenges CIGS Solar Cells

No new technology emerges without overcoming several obstacles. The main challenges for CIGS solar cells are:

  1. Encapsulation problems. CIGS solar cells need to be well protected, as moisture can badly damage the cells. Zinc oxide, formed on the transparent conductive oxide (TCO) layer on top of the CIGS cells is susceptible to damage by moisture. Researchers are looking to use alternative elements to replace the zinc.The traditional way of encapsulating the solar cells with glass is sufficient, however it makes the solar panels badly heavy. Therefore other encapsulation methods with polymers are now being developed by companies such as 3M, Dupont, Qsolar and Dow Corning.
  2. Efficiency improvement. CIGS cell manufacturers are working on rapidly increasing the efficiency of their cells, in order to compete with Si and TeCd cell technologies. Lab efficiencies of up to 20.3% have been research. But like always, it’s not possible to produce the same efficiencies on a large scale yet.
  3. Large scale production. So far CIGS manufacturers have not been able to produce CIGS solar panels on a scale comparable to the large Chinese Si solar cell manufacturers. Nearly all CIGS companies have announced delays in production, technical problems and postponed production capacity increases.Because of that, some are facing difficulties at the moment, because of the severe competition of Chinese silicon solar cell manufacturers.

Years of development and major investments have been made to develop CIGS solar cell technology and make it competitive with Si and TeCd cells. The advantages of CIGS solar cells, the higher efficiency and non-toxic materials, are clear. Now CIGS companies need to bring it to the next level, and make it cost effective..
Additional more detailed reading:

Thin film modules: Cadmium telluride (CdTe)

Thin film modules: Amorphous silicon (a-Si)

Thin-film modules