Solar-Electric Panel Technologies

Thin-Film PV

Unlike their crystalline cousins, thin-film PV modules have no crystal structure— the silicon atoms are arranged at random, much like in glass. Instead of growing and slicing crystalline materials, thin-film modules are created by depositing very thin layers of photo-reactive materials onto a substrate, such as stainless steel.

The chief advantages of thin-film PV over crystalline types are:

• Lighter weight
• Greater heat tolerance
• Better shading tolerance
• Lower cost (in some cases, and increasingly so)

Thin-film’s typically low efficiencies (8-12%) are often countered by its unique advantages. It does better than crystalline in high-temperature areas (roughly above 30°C. typical ambient temperature). Its shade tolerance is in part due to the fact of its normally non-cellular construction. While mono- or polycrystalline panels are comprised of series-wired cells, where each is dependent on its neighbor to help reach the total rated voltage of the module, thin-film types are not so limited. Partial shading of crystalline PV modules—due to a tree branch or a leaf blown onto the module surface—can result in near-zero current output, while thin-film modules’ output tends to be reduced more or less in step with the amount of surface area shaded.

The lack of brittle crystals and the use of alternative encapsulation techniques (i.e., no glass covering) allow thin-film modules to be made in various lightweight, flexible shapes. Thin-film has been used to make roll-out laminate modules for metal roofs; lightweight, foldable modules for military and portable needs; and even PV roofing shingles. It is very adaptable for use in consumer electronics with lower power requirements like watches, calculators, yard lights, and for charging computers, cell phones, MP3 players, and other equipment for on-the-go lifestyles. Building-integrated photovoltaic (BIPV) solutions, that blend into the architecture better than standard framed modules, are another developing use for PV and thin-film will play a big role there. Even clothing with integrated thin-film PV has made its market debut!

The big promise of thin-film technologies is not only lighter weight but lower production costs, which should ultimately overcome the technology's big drawback: low power density, as measured in Watts generated per square unit of surface area. Crystalline module types can have up to three or more times the power density of some thin-film types. If the available surface area for mounting PV modules is limited—as might be the case in a roof-mounted solar array—then thin-film modules may not have the power density to produce all the power you need in the space you have.

Another quirk of thin-film PV is the “settling-in period”. The materials used in their manufacture are initially less stable than mono- and polycrystalline cells. When first exposed to sunlight, their output decreases rapidly over a short period of time (typically weeks) by as much as 20% before they finally stabilize. That means some thin-film modules will produce well above their rated output during the first few weeks of full-time operation. If you're using new thin-film modules you may think you got a real bargain! But after settling in, such modules’ output will typically fall into line closer to their rated output.