Comprehensive Research on Stability of Amorphous Silicon and Alloy Materials and Devices

In this report, we describe the work done to improve the material and device properties of a-Si:H and a-(Si,Ge):H alloys prepared using electron cyclotron resonance (ECR) plasma deposition and to understand the growth chemistry. Major results were obtained in the following areas: 1) Influence of plasma chemistry on properties and stability of a-Si:H single-junction solar cells; 2) Fabrication ofgood-quality tandem-junction cells. We made tandem-junction a-Si/a-Si cells with excellent voltages and fill factors using the H-ECR process; 3) Growth of high-quality a-(Si,Ge):H films using the ECR deposition process; 4) Fabrication of single-junction devices in a-(Si,Ge):H for diagnosing the material; and 5) Graded-gap cells in a-(Si,Ge):H. Good devices were produced using a graded-gapI-layer. In summary, the most important finding from our research has been that plasma chemistry plays a very important role in determining the properties of the materials, particularly the properties of the a-(Si,Ge):H alloy system. Even in a-Si:H, plasma chemistry plays a role in determining stability. This result suggests that by deliberately changing the chemistry of deposition, one may beable to further improve the a-(Si,Ge):H materials system and make its properties comparable to the properties of a-Si:H. The ECR reactor has proved to be a very useful chemical tool, with excellent control over growth chemistry.