A thin-film solar cell , also called a thin-film photovoltaic cell, is a second generation solar cell that is made by depositing one or more thin layers, or thin film of photovoltaic material on a substrate, such as glass, plastic or metal. Thin-film solar cells are commercially used in several technologies, including cadmium telluride , copper indium gallium diselenide, and amorphous and other thin-film silicon.

Film thickness varies from a few nanometers to tens of micrometers, much thinner than thin-film’s rival technology, the conventional, first-generation crystalline silicon solar cell , that uses silicon wafers of up to 200 µm. This allows thin film cells to be flexible, resulting in lower weight, less drag and limited resistance to foot traffic

Thin-film has always been cheaper but less efficient than conventional c-Si technology. However, they significantly improved over the years, and lab cell efficiency for CdTe and CIGS reached almost 20 percent and are on par with polysilicon, the dominant material, currently used in most PV installations. Despite these facts, market-share of thin-film never reached more than 20 percent in the last two decades and has been declining in recent years to about 9 percent of worldwide photovoltaic production in 2013

Cadmium Telluride Thin Film

Cadmium telluride is the predominant thin film technology. With about 5 percent of worldwide PV production, it accounts for more than half of the thin film market. The cell’s lab efficiency has also increased significantly in recent years and is on a par with CIGS thin film and close to the efficiency of multi-crystalline silicon as of 2013.Also, CdTe has the lowest Energy payback time of all mass-produced PV technologies, and can be as short as eight months in favorable locations.[1]:31 A prominent manufacturer is the US-company First Solar based in Tempe, Arizona, that produces CdTe-panels with an efficiency of about 14 percent at a reported cost of $0.59 per watt.

Although the toxicity of cadmium may not be that much of an issue and environmental concerns completely resolved with the recycling of CdTe modules at the end of their life time, there are still uncertainties and the public opinion is skeptical towards this technology.

Copper Indium Gallium Selenide Thin Film

A copper indium gallium selenide solar cell or CIGS cell uses an absorber made of copper, indium, gallium, selenide , while gallium-free variants of the semmiconductor material are abbreviated CIS. It is one of three mainstream thin-film technologies, the other two being cadmium telluride and amorphous silicon, with a lab-efficiency above 20 percent and a share of 2 percent in the overall PV market in 2013.

In 2008, IBM and Tokyo Ohka Kogyo Co., Ltd. announced they had developed a new, non-vacuum, solution-based manufacturing process for CIGS cells and are aiming for efficiencies of 15% and beyond.As of September 2014, current conversion efficiency record for a laboratory CIGS cell stands at 21.7%.

Amorphous Silicon Thin Film

This type of thin-film cell is mostly fabricated by a technique called plasma-enhanced chemical vapor deposition. It uses a gaseous mixture of silane and hydrogen to deposit a very thin layer of only 1 micrometre of silicon on a substrate, such as glass, plastic or metal, that has already been coated with a layer of transparent conducting oxide. Other methods used to deposit amorphous silicon on a substrate include sputtering and hot wire techniques.

a-Si is attractive as a solar cell material because it’s an abundant, non-toxic material. It requires a low processing temperature and enables a scaleable production upon a flexible, low-cost substrate with little silicon material required. Due to its bandgap of 1.7 eV, amorphous silicon also absorbes a very broad range of the light spectrum, that includes infrared and even some ultraviolet and performs very well at weak light. This allows the cell to generate power in the early morning, or late afternoon and on cloudy and rainy days, contrary to crystalline silicon cells, that are significantly less efficient when exposed at diffuse and indirect daylight.

However, the efficiency of an a-Si cell suffers a significant drop of about 10 to 30 percent during the first six months of operation. This is called the Staebler-Wronski effect a typical loss in electrical output due to changes in photoconductivity and dark conductivity caused by prolonged exposure to sunlight