Electrically Active Defects and Shunting in UMG-Silicon

The project shall be carried out at the Max Planck Institute of Microstructure Physics in Halle (Germany), as it is the recognized institute concerning the investigation of shunts in solar cells. The work is embedded within the research cluster Solar Focus, where Germanys leading research institutions have joined with the goal to increase efficiencies of solar cells by improved silicon processing and innovation. Electrically active defects can form internal shunts in solar cells which are reducing their efficiencies and might lead to hot-spots under reverse bias causing thermal damage of the entire module. Shunts are attaining increasing attention especially in block-cast Si solar cells, where upgraded metallurgical grade Silicon (UMG-Si) is used, which introduces much more shunts into the cell than the purer solar grade silicon. A significant part (a few percents) of nowadays produced Si solar cells has to be rejected due to shunts and low breakdown voltages. So far it is known that the major fraction of material induced ohmic shunts are consisting of n-conducting SiC filaments which precipitate preferably along grain boundaries leading to a shunting contact at the backside. Further it is known that some filaments are shunting and others are not. The reason for this is not known yet. The main task of this work shall be the explanation of the shunting mechanism of material-induced shunts in multi-crystalline Si solar cells. A special focus shall be given to the parameters influencing the ohmic contact of the SiC filaments at the back contact and how they can be prevented. Further, other shunting phenomena in mc-Si as well as thin film solar cells shall be explored in general. Explanations shall be given for the occurrence of soft-exponential pre- breakdowns. MPI Halle provides all necessary equipment for a successful conduction of this research e.g. Si processing, Focused Ion Beam (FIB) preparation, Transmission Electron Microscopy (TEM) with micro analysis features, Scanning Electron Microscopy (SEM) including EBIC and EDX. Together with another research partner electroluminescence measurements shall be further developed. Already existing research collaborations with industry, e.g. Q-Cells and CSG-Solar (both in Bitterfeld Thalheim close to Halle) ensures the relevance and potential impact to current technology. Suggestions to reduce or avoid shunts in solar cells can be expected, which is of utmost importance for UMG-Si and might lead to an increased use of this less cost-intensive material which might significantly reduce the costs in PV technology.