A stability criterion for tunnel diode interconnect junctions in cascade solar cells

The degradation due to high-temperature annealing of the peak current of p(+)-n(+) tunnel junctions in GaAs is studied. Such junctions are intended for use as intercell ohmic contacts in cascade solar cells. The current degradation is a consequence of the broadening of the tunnel junction space charge caused by dopant diffusion. It is shown that the peak current is correlated with the dopant concentration profile of the linearly graded junction. To avoid serious degradation of the peak tunnel current, the product of the larger diffusion coefficient of donor or acceptor and the time at high temperature must be less than about 1 x 10 to the -12th sq cm, and the impurity diffusion length shorter than about 10 nm. This criterion is also applicable to materials other than GaAs.