Thin-film Photovoltaic Modules
Thin-film solar photovoltaic (PV) cells are exciting solar products. 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 (more than 1m2) and are suitable for automated, continuous production, arraying, and packaging. They can also be deposited on flexible substrate materials.
One of the main advantages of thin-film over crystalline solar cells is that the full module can be deposited with the cell interconnections made during layers deposition (Figure on the right). 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. 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 voltage of individual cells.
While supply of silicon, the raw material for crystalline solar cells (the dominant PV technology with well over 90 percent market share) remains tight, in coming years thin-film PV modules presence is steadily raising. This silicon supply bottleneck, with no certain ending, is opening up an opportunity for thin films. The question is only, how fast can traditional thin-film PV technologies amorphous silicon, cadmium telluride, and copper indium diselenide meet expectations.