  We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP
systems by the comparison of different structures that emulate the back-surface
field (BSF) and base layers of a GaInP subcell integrated into an inverted
multijunction solar cell structure. Through the analysis of secondary ion mass
spectroscopy (SIMS), electrochemical capacitance-voltage (ECV) and spectrally
resolved cathodoluminescence (CL) measurements, we provide experimental
evidence that 1) the Zn diffusion is enhanced by point defects injected during
the growth of the tunnel junction cathode layer; 2) the intensity of the
process is determined by the cathode doping level and it happens for different
cathode materials; 3) the mobile Zn is positively charged and 4) the diffusion
mechanism reduces the CuPt ordering in GaInP. We demonstrate that using barrier
layers the diffusion of point defects can be mitigated, so that they do not
reach Zn-doped layers, preventing its diffusion. Finally, the impact of Zn
diffusion on solar cells with different Zn-profiles is evaluated by comparing
the electrical I-V curves at different concentrations. The results rule out the
introduction of internal barriers in the BSF but illustrate how Zn diffusion
under typical growth condition can reach the emitter and dramatically affect
the series resistance, among other effects.
