Polycrystalline Thin Film: Solar Technology
Polycrystalline Thin Film
One scientific discovery of the computer semiconductor industry also has great potential in the photovoltaic (PV) industry: thin-film technology.
The "thin film" term comes from the method used to deposit the film, not from the thinness of the film. Thin-film cells are deposited in very thin, consecutive layers of atoms, molecules, or ions. Thin-film cells have many advantages over their "thick-film" counterparts. For instance, they use much less material. The cell's active area is usually only 1 to 10 micrometers thick, whereas thick films typically are 100 to 300 micrometers thick.
Thin-film cells can also usually be manufactured in a large-area process, which can be an automated, continuous production process. Finally, they can be deposited on flexible substrate materials.
Thin-Film Cell Structure
Unlike most single-crystal solar cells, a typical thin-film device doesn't have a metal grid for the top electrical contact. Instead, thin-film solar cell uses a thin layer of a transparent conducting oxide, such as tin oxide. These oxides are highly transparent and conduct electricity very well. A separate antireflection coating might top off the device, unless the transparent conducting oxide serves that function.
Polycrystalline thin-film solarcells are made of many tiny crystalline grains of semiconductor materials. The materials used in these polycrystalline thin-film solar cells have properties that are different from those of silicon. So, it seems to work better to create the electric field with an interface between two different semiconductor materials. This type of interface is called a heterojunction ("hetero" because it is formed from two different materials, in comparison to the "homojunction" formed by two doped layers of the same material, such as the one in silicon solar cells).
The typical polycrystalline thin film has a very thin (less than 0.1 micron) layer on top called the "window" layer. The window layer's role is to absorb light energy from only the high-energy end of the spectrum. It must be thin enough and have a wide enough bandgap (2.8 eV or more) to let all available light through the interface (heterojunction) to the absorbing layer. The absorbing layer under the window, usually doped p-type, must have a high absorptivity (ability to absorb photons) for high current and a suitable band gap to provide a good voltage. Still, it is typically just 1 to 2 microns thick.
Below is the figure of the structure of thin film cell.
Thin-Film Silicon
The term "thin-film silicon" typically refers to silicon-based photovoltaic (PV) devices other than amorphous silicon cells and single-crystalline silicon cells (where the silicon layer is thicker than 200 micrometers).
These films have high absorptivity of light and may require cell thicknesses of only a few micrometers or less. Nanocrystalline silicon and small-grained polycrystalline silicon—considered thin-film silicon—may be able to replace amorphous silicon alloys as the bottom cell in multijunction devices. As with other thin films, advantages include the savings of material, monolithic device design, use of inexpensive substrates, and manufacturing processes that are low temperature and possible over large areas.
