Solar panels based on the photovoltaic effect have been used for more than thirty years and have traditionally been built using wafers of crystalline silicon, which requires expensive processing and results in ridged, heavy and fragile solar panels.
Crystalline silicon PV cells are still the mainstream products in the PV cell market because they have high conversion efficiencies. However, their output is increasingly being bogged down by shortage of raw material, high production cost and difficulty of processing. These factors have given rise to rapid development of second generation PV technology known as thin-film PV technology.
Thin-film solar cells are generated by coating a substrate (glass, thin flexible metal or plastic substrate) with layers of conductive and semi-conductive materials of a few micrometers in thickness. The individual layers of material are deposited by various processes.
The key materials for the thin-film solar cells are semiconductor elements such as amorphous silicon (a-Si, still silicon, but in a different form), cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).
Amorphous silicon (a-Si) was the first thin-film material to be commercialized, although, the PV cells built from amorphous silicon are invariably less efficient than crystalline PV. These PV cells have low efficiency and limited lifetime (approximately 10-15 years). Initially, a-Si was mostly used in consumer items such as calculators. Amorphous silicon is the most widely used for the creation of thin-film solar panels. It has a sun energy conversion rate as high as 9%.
Cadmium telluride (CdTe) is a highly useful material in the making of solar cells. Cadmium telluride PV (CdTe PV) is the first and only thin-film photovoltaic technology to surpass crystalline silicon PV in the marketplace in terms of lower system price for a significant portion of the PV market – large (multi-kW) systems.
CdTe PV cells structure includes a very thin layer of cadmium sulfide that allows most sunlight to pass through to the CdTe layer. These characteristics provide the potential for high-efficiency modules with low-cost manufacturing processes. CdTe cell efficiencies are over 16% in the laboratory; commercial module efficiencies are likely to be in the 9% range in the first manufacturing plants.
Copper indium gallium diselenide (CIGS) cells create more electricity from the same amount of sunlight than does other thin-film PV and therefore has a higher “conversion efficiency”. Besides that, CIGS conversion efficiency is very stable over time, meaning its performance continues unabated for many years.
CIGS cells use extremely thin layers of semiconductor material applied to a low-cost backing such as glass, flexible metallic foils, high-temperature polymers or stainless steel sheets. They are of interest for space applications and the portable electronics market because of their light weight. CIGS cells are also suitable in special architectural uses, such as photovoltaic roof shingles, windows, siding and others. CIGS thin-film solar cell recently reached 19.9 percent efficiency, setting a new world record for this type of cell.
Thin-film PV technology has attracted a lot of interest in the recent years. The main reason for this interest is that thin-film PV cells are less expensive than other PV systems. Rather than being manufactured laboriously piece by piece, thin-film can be mass-produced in cheap rolls like packaging – in any colour. Thin-film PV cells also can harvest as much energy from the sun with far less semiconductor material. They can be made with flexible substrates which allow them to be used in more locations than silicon cells, such as clothing and sails. A number of applications are being pursued using thin-film PV technologies, including roof-top applications (such as rooftop shingles, roof tiles), building-integrated photovoltaics (BIPV), the glazing for skylights or atria, and utility-scale applications.
Thin-film PV cells represent the most promising technology for providing more affordable solar cells for residential and other uses in the future. According to NanoMarkets, the thin-film photovoltaics (TFPV) market will produce 26GW by 2015, generating over $20 billion in revenues.