Novel applications of near-field scanning optical microscopy: Microluminescence from local junction breakdown in solar cells

The presence of local junction breakdown in silicon solar cells can be examined by electroluminescence (EL) imaging under reverse bias. This luminescence-extending from the near infrared into the visible spectrum-is primarily consistent with bremsstrahlung radiation resulting from the acceleration of both electrons and holes across the breakdown site. In this contribution, we introduce the application of near-field scanning optical microscopy (NSOM) to resolve the physics of the local junction breakdown in solar cells at the micro- and nano-scale. The superior resolution of NSOM reveals aspects of the local junction breakdown not seen before, providing a better description of the processes involved in the junction breakdown. Assuming that this luminescence is primarily bremsstrahlung, the photon energy distribution of the emission spectrum can be used as a local measurement of the local field at breakdown. Thus, not only does NSOM provide the precise location where the breakdown initiates, but the distribution of the field at breakdown is also accessible within the micron scale. Because of the practical significance of junction breakdown at low reverse bias in upgraded metallurgical (UMG) multicrystalline silicon (mc-Si) solar cells, this contribution is dedicated to the application of NSOM spectrum imaging to different breakdown mechanisms found in these solar cells.