Cost implications for PV modules II -
next generation solar*

*for part one, click here

Many in the industry believe that cost reduction processes in silicon processing will eventually make crystalline silicon cells competitive with traditional generating technologies (without the need for subsidies – see cost implications for PV modules I, renewable energy focus September/October, page 25). At the same time, however, many also consider that the creation of crystalline solar cells is an inherently costly process, and that the best way to achieve cost competitive solar is with a new generation of solar cells.

Thin film

The theory behind the thin-film technologies is that only a small amount of semiconductor material is required to exhibit the PV effect and create electricity. The traditional method of slicing solid ingots of silicon into wafers has its limitations, mostly due to breakages of thin wafers, and is essentially nearing the limits of ‘thinness’. These thin wafers – down to around 180 microns – use relatively large amounts of material which is not necessary for the PV effect, and therefore significant research has been focused on how to create thinner layers which still exhibit this photoelectric effect.

There are many different methodologies available to achieve the deposition of thin films of material, each with their advantages and disadvantages. And different developers have been working with different substrates, particle sizes and semiconductor materials.

Providing a look into all the different technologies under development is beyond the scope of this article but it is worth mentioning that one of the key challenges that developers have had to overcome is cell uniformity. Uniformity of the reactive layers in PV cells is essential, so that the cells provide a useful output when included as a module. This is because any slight differences in the PV cell structure make the cells exhibit slightly different electrical properties. These slight differences lead to back currents between cells when wired together, and this effect is greatly exaggerated when many cells are wired up in series to create a module with a useful voltage output (i.e. a crystalline silicon module may comprise up to 36, 0.5V cells to create a useful output ofaround 12V; large installation inverters for grid connections can require up to 200V-600V).

Thin film deposition technology has been around for some time, with common examples including vacuum deposition for CDs and sputtering for hard disks. However, it has been a challenge to scale these processes for larger surface areas as required for solar cells. Many developers have thus developed proprietary deposition methods, a number of which are variants on the discussed techniques:

A first example has to be First Solar, which is prominent in Cadmium Telluride (CdTe) solar cells. CdTe cells exhibit relatively high efficiencies of around 10%, and First Solar has developed a technique called High-Rate Vapour Transport Deposition (see figure 1) which allows the company to deposit rapidly the semiconductor layers onto an inflexible glass substrate (pane by pane). First Solar recently announced contracts which priced its output at US$1.87/Watt (compared to crystalline silicon solar cells at around US$2-US$3/Watt – depending on the cell efficiency), which indicate costs of around US$1/Watt.

The second most well known supplier of thin film solar cells is United Solar Ovonic, commonly known as Uni-Solar. Uni-Solar, a division of Energy Conversion Devices Inc., creates triple-junction amorphous-silicon (a-Si) cells. These cells are based upon the deposition of three reactive sets of silicon layers, which absorb different bandwidths and enable the cells to have a relatively high (for a-Si) efficiency of over 7%. Uni-Solar’s cells are created using a proprietary vapour deposition process, which deposits the layers onto a flexible metal backing (see figure 2).

Deposition onto a flexible metal backing is technically more difficult than onto a rigid glass substrate, but this methodology allows for continuous roll-to-roll processing, which can potentially reduce costs further. Uni-Solar currently sells its cells for around US$3/Watt and, based upon margin performance, Ambrian estimates costs to be around US$2.3/Watt, although this should decrease as the company ramps production from 60MW pa currently to 300MW pa by 2010.

PowerFilm, an Iowa-based company listed on AIM, produces a-Si based cells with a flexible plastic backing, which achieve a conversion efficiency of 5%-5.5%. Deposition onto a flexible plastic backing is extremely difficult, not only due to the physical difficulties in maintaining an even and level surface, but also because plastic materials tend to emit gases or vapourised liquids when held under a vacuum – called out-gassing.

At present PowerFilm is the only company (to the author’s knowledge) that creates a commercially-available thin film material on a flexible plastic substrate; it does this using a proprietary vacuum deposition process. PowerFilm currently sells into high-value applications for around US$5/Watt and makes around a 40% margin; however, costs should decrease below US$1/Watt when new production machinery comes on line, and the company begins to sell into the building integrated PV market (BIPV).

Other thin film companies of note:

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