On the Thermal Degradation of Tunnel Diodes in Multijunction Solar Cells

Tunnel junctions are essential components of multijunction solar cells. These highly doped p/n junctions provide the electrical interconnect between the subcells that constitute a multijunction solar cell device. The conductivity and the peak tunneling current of tunnel diodes are known to be severely affected by thermal load. This is a general phenomenon observed in tunnel junctions despite the materials used, the dopants employed or the growth technique applied. Despite this generality, the explanations for this thermal degradation tend to be quite material/dopant specific. On the contrary, in this work we apply the amphoteric native defect model to explain this issue. In this context, the degradation can be explained as a consequence of the net loss of free carrier concentration produced by the creation of native compensating defects in the highly doped layers of the tunnel junction. Experiments carried out on n(++) GaAs agree well with the model.