Thermal runaway in multijunction solar cells

Certain GaInP
₂
/GaInAs/Ge triple junction cells, when exposed to temperatures of 523 K under the 6x concentrated AM0 spectrum, developed a region of local temperature increase. These cells severely degraded in electrical performance. The “hot spot” occurred at the very position of a pre-existing top/middle cell shunt. A model is developed that calculates the Joule heating in the Ge bottom cell around the shunt, taking into account the strongly decreasing resistivity of Ge with temperature. Provided the shunt is connected by a low series resistance to the remainder of the cell, the model predicts local melting under the experimental conditions. Furthermore it identifies a temperature dependent critical voltage that has to be surpassed for this thermal runaway to happen.