  The growth of heavily doped tunnel junctions in inverted metamorphic
multijunction solar cells induces a strong diffusion of Zn via a
point-defects-assisted mechanism. The redistribution of Zn can compensate the
n-type doping in the emitter of the GaInP top junction, degrading severely the
conductivity of the whole solar cell and its conversion efficiency. This work
evaluates different epitaxial growth strategies to achieve control on the Zn
profile of an inverted metamorphic triple-junction structure, including: the
reduction of the doping concentration in the tunnel junction to minimize the
injection of point defects that trigger the diffusion mechanism; the use of
different barrier layers to keep the injected point defects away from active
layers and, finally, the minimization of Zn concentration in the AlGaInP
back-surface-field layer of the GaInP subcell. This last approach enables a
high-conductivity multijunction solar cell device without redesigning the
tunnel junction as well as a high electronic quality in the GaInP subcell,
which shows a collection efficiency higher than 93% and an open-circuit-voltage
offset of 410 mV at 1 sun irradiance. The characterization of final
triple-junction devices, including quantum efficiency, electroluminescence, and
light current-density-voltage curves at different irradiances, demonstrates a
successful integration of all the subcell and tunnel junction components. This
way, final solar cells with peak efficiencies exceeding 40% at 500 suns are
demonstrated, despite using doping levels in the AlGaInP:Zn back-surface-field
of the GaInP subcell and using non-optimized antireflective coatings.
