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Polycrystalline Thin Film Used in Photovoltaics
Polycrystalline thin-film cells are made of many tiny crystalline grains of semiconductor materials. The materials used in these cells have properties that are different from those of silicon.
Thin-film cells have many advantages over their thick-film counterparts. For example, 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. Also, thin-film cells can 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.
The term thin film comes from the method used to deposit the film, not from the thinness of the film itself. Thin-film cells are deposited in very thin, consecutive layers of atoms, molecules, or ions.
Several deposition techniques can be used for thin-film cells, and all are potentially less expensive than the ingot-growth techniques required for crystalline silicon. These deposition techniques can be broadly classified as physical vapor deposition, chemical vapor deposition, electrochemical deposition, and combination techniques. And like amorphous silicon, the layers can be deposited on various low-cost substrates (or superstrates) such as glass, stainless steel, and plastic in virtually any shape.
In addition, these deposition processes can be scaled up easily, which means that the same technique used to make a 2-inch-by-2-inch laboratory cell can be used to make a 2-foot-by-5-foot PV module. Thin films are unlike single-crystal silicon cells, which must be individually interconnected into a module. Thin-film devices can be made as a single unit—that is, monolithically—with layer upon layer being deposited sequentially on a substrate
Thin-Film Cell Structure
Unlike most single-crystal cells, a typical thin-film device does not have a metal grid for the top electrical contact. Instead, it uses a thin layer of a transparent conducting oxide, such as tin oxide. These oxides conduct electricity well. A separate antireflection coating might top off the device, unless the transparent conducting oxide also serves that function.
For these materials, it works better if the electric field is created with an interface between different semiconductor materials. This type of interface is called a heterojunction (hetero because it is formed from different materials, in contrast with homojunction, which is formed by two doped layers of the same material, such as 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.
Copper Indium Diselenide
Copper indium diselenide (CuInSe2 or CIS) has an extremely high absorptivity, which means that 99% of the light shining on CIS is absorbed in the first micrometer of the material. Cells made from CIS are usually heterojunction structures—structures in which the junction is formed between semiconductors with different bandgaps. The most common material for the top, or window, layer in CIS devices is cadmium sulfide (CdS), although zinc is sometimes added to improve transparency. Adding small amounts of gallium to the lower absorbing CIS layer boosts its bandgap from its normal 1.0 eV, which improves the voltage and therefore the efficiency of the device. This particular variation is commonly called a copper indium gallium diselenide or CIGS PV cell.
Cadmium telluride (CdTe) is another prominent polycrystalline thin-film material. With a nearly ideal bandgap of 1.44 eV, CdTe also has a very high absorptivity. Although CdTe is most often used in PV devices without being alloyed, it is easily alloyed with zinc, mercury, and a few other elements to vary its properties. Like CIS, films of CdTe can be manufactured using low-cost techniques.
Also like CIS, the best CdTe cells employ a heterojunction interface, with cadmium sulfide (CdS) acting as a thin window layer. Tin oxide is used as a transparent conducting oxide and antireflection coating. One problem with CdTe is that p-type CdTe films tend to be highly resistive electrically, which leads to large internal resistance losses. One solution is to allow the CdTe layer to be intrinsic (i.e., neither p-type nor n-type, but natural) and add a layer of p-type zinc telluride (ZnTe) between the CdTe and the back electrical contact. Although the n-type CdS and the p-type ZnTe are separated, they still form an electrical field that extends through the intrinsic CdTe. When it comes to making CdTe cells, a variety of methods are possible, including closed-space sublimation, electrodeposition, and chemical vapor deposition.
The term thin-film silicon typically refers to silicon-based 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.