**4. Silicon thin film**

Silicon thin-film cells are mainly deposited by chemical vapor deposition (typically plasma-enhanced, PE-CVD) from silane gas and hydrogen gas. Depending on the deposition parameters, these silicon thin films can be based on one or a combination of these materials [2–5]:


These silicon thin film materials can be characterized by their grain sizes ranging from none (amorphous) to large silicon (~100 μm) for polysilicon. The crystalline silicon thin films present dangling bonds, which result in deep defects (energy levels in band gap) as well as deformation of the conduction and valence bands. The solar cells made from thin films tend to have lower energy conversion efficiency than silicon bulk but are also less expensive to produce (**Table 1**).


#### **Table 1.**

*Grain size range depending on the type of the silicon [6–8].*
