a-Si:H solar cells: SiH(2)Cl(2) as a source gas and a-SiGe:H alloys

This thesis gives an overview of the reasons why solar cells are a necessity in this world of carbon constrained energy use. The most important factors upon which to work to improve the competitiveness of photovoltaic generated electricity with conventional, fossil fuel based generation are the cost of the solar cell and the cell conversion efficiency. The two main technical thrusts of this thesis--DCS as a source gas and a-SiGe:H alloys--attacked two aspects of these problems. a-SiGe:H is used in multijunction cells to achieve higher efficiencies and DCS can be used to increase the deposition rate of a-Si:H thus decreasing the time it takes to make a solar cell. We review the various experimental methods used to investigate the optical and electronic properties of a-Si:H thin films as well as the methods used to measure solar cell efficiency and determine the effectiveness of the various portions of a solar cell. Using DCS as a source gas helped us increase the deposition rate by factor of 5 while maintaining the film quality; however chlorinated intrinsic films had higher defect densities and lower photoconductivity than standard a-Si:H films. Use of DCS to deposit i-layers of a solar cell led us to the discovery that Cl enhances the doping efficiency of B in a-Si:H. The enhanced doping of a-SiC:H as the p-layer of a solar cell increased the cell's efficiency from 7.1% to 7.8%. A-SiGe:H alloys were investigated over a range of Tauc gaps from 1.7 eV down to 1.0 eV. The optical and electronic properties of these films were investigated as well as their incorporation in solar cells. Different bandgap graded structures were used in the i-layer of a solar cell with the conclusion that the bathtub shaped i-layer yields the highest stabilized efficiency. An attempt was made to fabricate solar cells using cathode-deposited a-SiGe:H alloys as the i-layer in collaboration with Harvard University. The cells fabricated could neither prove nor disprove the effectiveness of the cathode deposited material because of problems in breaking vacuum between deposited layers.