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Amorphous Silicon Solar Cells: structure and applications

Amorphous silicon (a-Si) is one of the major solar thin-film type with a wide range of applications.

What are Amorphous Silicon Solar Cells?

Amorphous silicon (a-Si) is the non-crystalline allotropic form of the semiconductor silicon. It has a high absorption capacity and can therefore be used in solar cells with very small layer thicknesses (usually about a factor of 100 smaller than in crystalline silicon), saving on material costs and compensating for performance deficiencies caused by its comparably low industry-maximum efficiency which is about 13%.

Despite its lower performance as compared to crystalline silicon (c-Si) solar cells, amorphous silicon solar cells can be deposited at very low temperatures and on various structures, not only on glass but also plastic. Due to their simplified and lower cost production, a-Si solar cells have mostly been used for electronic devices with very little power requirements such as watches and pocket calculators.

However, in recent years improving production techniques and higher achievements in performance efficiency have resulted in a wider range of applications of a-Si modules, including building integrated photovoltaics (BIPV) applications.

 

Chemical structure

Silicon is a tetravalent metalloid that has a fourfold coordination and is tetrahedrally bonded to neighboring silicon atoms. In c-Si this structure is regular and continues over a large range, forming a stable and consistent crystal lattice. On the contrary, a-Si does not have a consistent large range structure and its atoms are rather randomly ordered with some of its atoms having dangling bonds, thus causing anomalous electrical output behaviour.

Amorphous cell structure. Source: Encyclopedia of Earth

Amorphous cell structure. Source: Encyclopedia of Earth

This anomalous electrical output behaviour can be reduced by passivating the material by hydrogen, thus generating hydrogenated amorphous silicon (a-Si: H). Yet, in hydrogenated state, a-Si material is much more prone to electricity conversion efficiency degradation in the sunlight.

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