Utilizing lateral current spreading in multijunction solar cells: An alternative approach to detecting mechanical defects

Lateral current transport in forward biased triple junction solar cells is analyzed. A model based on distributed sheet resistances in combination with ideal p-n junctions is developed. Experimentally, the spatial distribution of the electroluminescence radiation is recorded and used as a tool to deduce the current densities in the individual junctions. In a specific test case, a cell which contains both area with and without externally injected current, the current spreading across the boundary between the two cell areas is reproduced accurately. Subsequently the impact of a mechanical defect on the current distributions in the individual junctions is addressed through this model. It is demonstrated that the carrier recombination at the surface of the crack acts as a current sink with a long range impact on the current densities. In addition a characteristic splitting of the current densities in top cell and middle cell is observed which implies that mechanical defects can be easily identified in a cell by their electroluminescence signature. The dependence of this phenomenon on the semiconductor properties in the basic cell structure is identified.