Microcrystalline silicon (pc-Si) has been studied extensively for three decades and has been used for doped layers in a-Si solar cells for over 15 years. Microcrystalline silicon is a complex material consisting of conglomerates of silicon nanocrystallites embedded into amorphous silicon. It can be more easily doped than a-Si:H; but, on the other hand, it is also more sensitive to contaminants than a-Si:H. The nucleation and growth of pc-Si:H are determinant for device quality; a certain amount of amorphous material is needed for the passivation of the nanocrystallites and for the reduction of defect related absorption. During the growth of the layer, the formation of crystallites starts with a nucleation phase after an amorphous incubation phase. During continued layer deposition, clusters of crystallites grow (crystallization phase) until a saturated crystalline fraction is reached. These processes are very much dependent on the deposition conditions. In general, crystalline growth is enhanced by the presence of atomic hydrogen, which chemically interacts with the growing surface.
Much research effort has been put worldwide into the development of both fundamental knowledge and technological skills that are needed to improve thin film silicon multijunction solar cells. The research challenges are:
1. To enhance the network ordering of amorphous semiconductors (leading to protocrystalline networks), mainly for improving the stability;
2. To increase the deposition rate, in particular for microcrystalline silicon;
3. To develop thin doped layers, compatible with the new, fast deposition techniques;
4. To design light-trapping configurations, by utilizing textured surfaces and dielectric mirrors.
The first report on practical microcrystalline cells is given in 1992 by Faraji that reported a thin film silicon solar cell with a pc-Si: H: O i-layer by using VHF PECVD and the first solar cell with a pc-Si: H i-layer was fabricated with 4.6% power efficiency (Meier et al., 1994).
In 1996, Fischer reported that microcrystalline (pc) Si: H p-i-n junctions have an extended infrared response, and are entirely stable under photoexcitation with an efficiency of 7.7% (Fischer et al., 1996). The tandem arrangement of an a-Si: H solar cell with a pc-Si: H solar cell also appears promising a remaining problem of the insufficient deposition rates for the pc-Si: H layers. In 2002, Meier published a micromorph tandem cell with efficiency of 10.8% in which the bottom cell was deposited at a rate of Rd =0.5 nm/ s with the thickness of 2 pm (Meier et al.,2002). At least in this type of devices the highest efficiencies reported to date are 15.4% (tandem cell consisting of microcrystalline silicon cell and amorphous silicon cell) (Yan et al., 2010).
Further development and optimization of a-Si: H/pc-Si: H tandems will remain very important because it is expected that in the near future, its market share can be considerable. For example, in the European Roadmap for PV R&D, it is predicted that in 2020, the European market share for thin film silicon (most probably a-Si: H/pc-Si: H tandems) will be 30%. This shows the importance of thin film multibandgap cells as second-generation solar cells.