PRE-BREAKDOWN MECHANISMS IN MULTICRYSTALLINE SILICON SOLAR CELLS

ABSTRACT: Solar cells showing too large pre-breakdown currents, which might flow for unintentional reverse biasing by local shading, cannot be used since they would lead to thermal damage of the module. To be able to reduce the number of off-specification cells, the origin of the reverse current needs to be investigated. Pre-breakdown mechanisms in semiconductors exhibit characteristic temperature dependences. We present temperature-dependent lock-in thermography (LIT) measurements as well as illuminated LIT investigations under reverse bias that facilitate the identification of the type of breakdown observed at certain sites. There are both local regions with positive and with negative temperature coefficient of the breakdown current. It turns out that grown-in recombinative crystal defects lead to regions of local soft pre-breakdown sites. However, outside of these regions there are often some additional sites where a hard breakdown occurs which may even dominate at higher reverse bias. Lock-in EBIC images show that in the latter regions the cell surface, as a result of the acidic texturization etch, contains cone-shape