Thin film solar cells (1)
The high cost of crystalline silicon wafers (crystalline silicon wafers make up 40-50% of the cost of a finished module) has led the industry to look at cheaper materials to make solar cells.
The selected materials are all strong light absorbers and only need to be about 1 micron thick, so materials costs are significantly reduced. The most common materials for solar cells are amorphous silicon (a-Si, still silicon, but in a different form), or the polycrystalline (multicrystalline) materials: cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).
Each of these three for solar cells is amenable to large area deposition (on to substrates of about 1 meter dimensions) and hence high volume manufacturing. The thin film semiconductor layers are deposited on to either coated glass or stainless steel sheet.
The semiconductor junctions are formed in different ways, either as a p-i-n device in amorphous silicon, or as a hetero-junction (e.g. with a thin cadmium sulphide layer) for CdTe and CIS. A transparent conducting oxide layer (such as tin oxide) forms the front electrical contact of the cell, and a metal layer forms the rear contact. 
Thin film technologies are all complex. They have taken at least twenty years, supported in some cases by major corporations in the industry, to get from the stage of promising research (about 8% efficiency at 1 cm2 scale) to the first manufacturing plants producing early product.
Amorphous silicon is the most well developed of this thin film technologies. In its simplest form, the cell structure has a single sequence of p-i-n layers. Such cells suffer from significant degradation in their power output (in the range 15-35%) when exposed to the sun.
The mechanism of degradation is called the Staebler-Wronski Effect, after its discoverers. Better stability requires the use of a thinner layers in order to increase the electric field strength across the material. However, this reduces light absorption and hence cell efficiency.
This has led the industry to develop tandem and even triple layer devices that contain p-i-n cells stacked one on top of the other. In the cell at the base of the structure, the a-Si is sometimes alloyed with germanium to reduce its band gap and further improve light absorption. All this added complexity has a downside though; the processes are more complex and process yields are likely to be lower.
