Photovoltaic (PV) arrays comprise varying numbers of modules connected together. The modules fall into several categories, including monocrystalline, polycrystalline, and thin-film .
Monocrystalline (single-crystalline) modules 
These use the purest 'monocrystalline' form of silicon (single-crystalline silicon, or sc-Si) and typically comprise highly ordered blue-black polygons. They are the most efficient type of PV module in good light, with a conversion efficiency of typically 10-16% - but also the most expensive. The energy required to make them - their embodied energy - is greater than other PV types. Life expectancy is typically 25-30 years.
Typical monocrystalline PV panel
Polycrystalline ('multicrystalline' or 'semicrystalline') modules
Made from multicrystalline silicon (mc-Si), these are cheaper and their embodied energy is less than monocrystalline modules. However, their more disorded atomic structure means that their energy efficiency is generally slightly lower, at typically 12%. Life expectancy is around 20-25 years.
Thin-film modules
These all consist of an ultra-thin layer of photosensitive material deposited onto a low-cost backing, such as glass, plastic or stainless steel. The earliest types used a layer of amorphous silicon, but subsequently various different combinations of materials have been employed, with varying degrees of success. For example, thin hybrid silicon cells are a combination of amorphous and microcrystalline cells. Other thin-film technologies are based on semiconductor materials, such as cadmium telluride (CdTe) and copper-indium-gallium-diselenide (CIGS).
Thin films can be rigid or flexible, and can be produced in various colours, giving great versatility for different applications and sites. They require relatively low amounts of raw material to manufacture, are suited to automated production, and hence are relatively cheap to make. However, efficiencies are lower (e.g. 4-8%), as is life expectancy (at 15-20 years). Non-silicon-based thin-film modules, such as those using CdTe and CIGS, have better efficiencies, with some exceeding 10%.
Flexible thin-film PV array