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A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form. The term is typically associated with silicon films in optical applications such as solar cells.
Silicon solar cells
Amorphous silicon (a-Si) is a popular solar cell material owing to its low cost and ease of production. Owing to disordered structure (Urbach tail), its absorption extends to the energies below the band gap resulting in a wide-range spectral response; however, it has a relatively low solar cell efficiency. The efficiency is higher in protocrystalline silicon and it has also been shown to improve stability, but not eliminate it. Protocrystalline Si also has a relatively low absorption near the band gap owing to its more ordered crystalline structure. Thus, protocrystalline and amorphous silicon can be combined in a tandem solar cell where the top layer of thin protocrystalline silicon absorbs short-wavelength light whereas the longer wavelengths are absorbed by the underlying a-Si substrate. Such two-layer cells can be produced at a relatively low temperature of 75 °C and have an efficiency of about 5%, and a four-layer cells yield an open circuit voltage above 3.1 V.