Material Control for High-Efficiency Amorphous Silicon Solar Cells

A total-area conversion efficiency of 12.0% (100cm2) has been achieved for a single-junction a-Si solar cell. The film deposition rate (Rd) plays an essential role in controlling the optical and electrical properties of “device-quality” a-Si:H for high-efficiency solar cells. The properties of conventional “device-quality” a-Si:H films deposited from 100% SiH4 are primarily determined by the balance between Ts and Rd. A lower or higher deposition rate results in a-Si:H with a narrower or wider bandgap, respectively. This enables the properties of a-Si:H to be controlled independent of Ts. The controllable range of a-Si:H properties can be widened by effectively utilizing factors such as ion bombardment. For example, a high dilution of SiH4 with H2 (H2 / SiH4 ≈ 10 or more) or an H2 plasma treatment after deposition results in a-Si:H with a very wide bandgap and low defect density, which cannot be achieved by using 100% SiH4. Controlling the properties of a-Si:H by applying vibrational / rotational energy is also investigated.