Amorphous silicon (a-Si), silicon in which some atoms in the structure remain unbonded, lacks long-range order, but can be produced, in principle, even more cheaply than multicrystalline silicon. Such a lack of long-range order in the structural arrangement of the atoms is the result of what are referred to as unsatisfied, or "dangling" bonds. The lack of long-range order has a severe impact on the material properties of amorphous silicon, and the "passivation" of these dangling bonds is required before amorphous silicon can be used as solar cell material. Passivation incorporates atomic hydrogen with amorphous silicon to a level of 5-10%, saturating the dangling bonds, thereby improving the quality of the material.

Nevertheless, the material properties of amorphous silicon are significantly different from those of crystalline silicon. For example, the band gap increases from 1.1 eV in crystalline silicon to 1.7 eV in amorphous silicon and the absorption coefficient of a-Si is much higher than that of crystalline silicon. In addition, the presence of the large number of dangling bonds cause a high defect density and low diffusion lengths.

Amorphous silicon has short-range order to give it semiconductor properties. Extra bonds are terminated on hydrogen atoms. The change in average atomic spacing and presence of hydrogen gives amorphous silicon different electronic properties to crystalline silicon.

The different material properties of amorphous silicon require different design approaches for a-Si solar cells. In particular, the minority carrier diffusion lengths in the silicon-hydrogen alloys (a-Si:H) are much less than 1 µm. As many light-generated carriers as possible have to be generated in the depletion region to maintain a good collection efficiency. As a result the depletion region forms most of the active carrier collecting volume of the cell. The high absorption coefficient of a-Si, which makes the material only a few microns thick, also means that the depletion region is large by comparison with the thickness of the emitter and base.

Schematic of an a-Si:H solar cell.

Structural differences between a-Si and crystalline silicon mean that the fabrication technology of solar devices based on these two types of material is different. In a-Si and other "thin-film" technologies, very thin films of semiconductor are deposited onto glass or other low-cost substrates. Thin-film solar cells are used in many small consumer products. Calculators, watches and "non-critical" outdoor applications are some of these products. In principle, thin-films provide a very low cost means of cell production.

However, a-Si used in outdoor applications or under light sources with a high UV light content may experience degradation in efficiency caused by the splitting of the Si-H bond induced by UV light. Research into thin-film and other potentially low-cost solar cell materials offers the means of overcoming problems found in devices which use amorphous silicon. As a result this research contributes to the development of highly efficient, stable and low-cost solar cells.

Calculator Watch

Amorphous solar cells are ideal for consumer products where the power requirements are small and the cells can be easily integrated. The whole face of the watch is the solar panel and provides enough power for its operation.