Business in MW-scale PV power plants is dominated by crystalline silicon and cadmium telluride. Crystalline silicon solar cells have many advantages: Com mercial mono- and polycrystalline silicon modules now achieve 14 to just over 20 percent efficiency. Owing to the relatively high efficiency of these modules, less installation area is needed per unit of output, which also means that fewer mounting frames and cables are required. New “cast-mono” wafers achieve similarly high efficiencies to monocrystalline solar cells thanks to the particularly cheap polycrystalline silicon wafers employed.
Innovative processes, such as the string-ribbon process, may reduce manufacturing costs further. A thin ribbon of silicon is pulled from a melting crucible between two wires. This technique facilitates the production of ultra-thin wafers just 135 micrometers (µm) thick and eliminates the heavy material losses that ensue from sawing conventional silicon wafers, which are 180 µm thick. Tandem cells that combine crystalline wafers with amorphous silicon coatings achieve extremely high efficiencies with the wafers being exceptionally thin. Novel cells with selective emitters and rear-side contacts are only between 100 and 120 µm thick. Bifacial cells that are also able to absorb sunlight on the rear sides of modules and thereby boast yields of between 25 and 30 percent higher than comparable standard modules have also been on the market since 2012.
As thin-film modules are significantly less efficient than crystalline silicon modules, they need to cover up to 30 percent more surface area than crystalline silicon modules to achieve the same output (cf. fig. 2). This entails increased costs for installation, support frames and cabling. However, thanks to intensive research and development the efficiency of thin-film modules is currently improving at a faster rate than that of crystalline silicon modules. The efficiency of research modules fitted with cadmium telluride cells has now increased to more than 18 percent, while that of cells made from copper-indium semiconductors has risen to more than 17 percent. If these efficiency values are also attained in commercial modules, the fact that thin-film solar modules require a larger surface area will no longer have so such a bearing.
Since it is now possible to manufacture thin-film solar modules in large numbers, the costs of doing so have plummeted. In terms of price per unit of output, thin-film modules now cost the same as crystalline modules. However, the manufacturing costs (and therefore the prices) of solar modules with crystalline silicon solar cells have recently fallen at a greater rate than those of thin-film solar modules, resulting in the current market share of thin-film technologies declining somewhat.
As a general rule, large-scale PV plants equipped with thin-film modules can produce power just as cheaply as those constructed using crystalline modules. The lower manufacturing costs may therefore compensate for the increased outlay incurred for installation.
In addition, thin-film modules perform favorably in diffuse light conditions and at high temperatures. They utilize weak light more efficiently and, compared to crystalline silicon, their output is not so badly impaired when they heat up. This makes them significantly more suitable for the hot climate of the sunbelt, semi-desert and desert regions.
Colossal factories for producing thin-film modules with silicon technology or with compound semiconductors of copper indium gallium selenide are currently under construction. Production capacities for inexpensive cadmium telluride (CdTe) modules have experienced the strongest growth. Both CIGs and CdTe technologies have made the successful leap to bankability, demonstrating their reliability in large solar farms. In spite of this, 2011 and 2012 saw the market share of thin-film technology recede, as the decline in prices for crystalline modules coupled with their high efficiencies led to a resurgence of crystalline silicon technology in large-scale plant construction. Nevertheless, the new factories being built across the world are virtually exclusively intended for the production of thin-film modules. Significantly smaller investment funds are required here than for silicon cell and module manufacturing technology, so it can be expected that a growing proportion of thin-film modules will be found in large-scale plant business over the medium term.