Thin-film solar photovoltaic (PV) cells are an exciting product as they could also be flexible and potentially can be developed much further, providing layering characteristics and shape change alternatives. The term thin-film is derived 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. They use much less material, the cell’s active area is usually only 1 to 10 microns thick (thick-films typically are as much as 200 to 400 microns thick). Thin-film cells are also usually amenable to large-area fabrication and are suitable for automated, continuous production, arraying, and packaging. They can also be deposited on flexible substrates.
Many thin-film devices are based on amorphous silicon alloys. Other thin-film devices are usually poly-crystalline materials. The fabrication of a thin-film solar cell involves depositing a layer of semiconductor material (such as amorphous silicon, copper indium gallium diselenide, or cadmium telluride) on a low-cost substrate, such as glass, metal, or plastic. Current deposition techniques can be broadly classified into physical vapor deposition (PVD), chemical vapor deposition (CVD), electro-chemical deposition (ECD), plasma enhanced chemical vapor deposition (PECVD) or some combination of them.
Thin-film materials can be produced in either single-junction or multi-junction configurations. Multi-junction cells often referred to as stacked junction, cascade, multicolor, or tandem cells, are more complicated and expensive than single-junction cells, and should result in higher efficiencies. This concept entails combining two or more single-junction cells (the top cell being semitransparent), so that each junction converts a different portion of the solar spectrum into electricity, thus using the light more efficiently. Although a variety of semiconductor materials have appropriate energy band gaps and absorption characteristics, today technologies being developed are primarily focused on amorphous silicon (a-Si), copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe).
There is an interesting comparison to be made between thin-films and wafer technologies, when it comes to inter-cell connections. To connect wafers together, metal ribbons are attached to the cells at discrete points. One of the main advantages of thin-film over crystalline solar cells is that the complete module can be deposited with the cell interconnections made during layers deposition (figure below). Unlike crystalline modules, thin-film modules are not plagued by problems associated with interconnecting individual cells. Individual cells of any size and number are made by scribing the sequential layers (either with a laser beam or mechanically), as they are made so that the top electrode of one cell contacts with the bottom electrode of another cell, linking them in series. While broken connections are not uncommon in wafer-based modules, it is virtually impossible for thin-film modules to open up because of inter-cell contact failure. Each individual cell is in the form of a long narrow strip, which reduces series resistance, and all cells are connected in series, with the outer metal strip being the negative terminal and the outer transparent conductor strip the positive terminal for the whole module. Since the voltages of cells in series add, the total voltage for a module can easily be adjusted by changing the scribing pattern that defines the number and size of individual cells. Width of each strip determines the current of individual cells. Thin-films exhibit contact redundancy that can be an important factor in module longevity. | 
