Improved stability in ECR-deposited a-Si solar cells

We report on fabrication, properties and stability of a-Si:H and a-(Si,Ge):H solar cells made using remote low pressure ECR deposition. We have fabricated both substrate and superstrate type solar cells. We can make solar cells with high fill factors in both geometries, but the voltages are higher with substrate-type solar cells than with superstrate type cells. Special problems related to diffusion of B have to be solved in superstrate cells because the deposition is done at higher temperatures (350-375 C). Several novel p-layer grading schemes and buffer layers which allow us to fabricate these types of cells are described. The substrate cells were made with both H-ECR and He-ECR discharges. We find that while the cells prepared with He discharge have lower H concentration, and lower H content, they are less stable than cells prepared using H₂ discharges. The stability of cells was measured using ELH and xenon lamps, and compared with the stability of cells made using standard glow discharge techniques. We find that the cells prepared using H₂-ECR discharges are more stable than standard glow discharge cells with comparable fill factors, voltages and thicknesses of i layers. We also report on a new type of graded gap a-(Si,Ge):H cell, which appears to show improved stability