Tunnel-Junction-Limited Multijunction Solar Cell Performance Over Concentration

The simulation of tunnel junctions is performed by using nonlocal band-to-band and trap assisted tunneling models that are capable of reproducing the experimental current-voltage characteristics of p⁺⁺AlGaAs/ n⁺⁺AlGaAs and p⁺⁺AlGaAs/ n⁺⁺GaAs based devices. These simulated characteristics are then implemented within a lattice matched InGaP/(In)GaAs/Ge multijunction solar cell (MJSC) to assess the performance as a function of tunnel junction layer doping in the regime where the TJ limits the performance of the MJSC. At 500 suns, a 4.6% absolute drop in simulated efficiency is observed for an AlGaAs/GaAs bottom TJ corresponding to a degenerately p-doped layer of 2.5 × 10¹⁹ cm^-3 compared to a TJ with a doping of 4×10²⁰ cm^-3. A minimum p⁺⁺ doping level of 3.3 × 10 ¹⁹ cm^-3 is required in order to avoid bottom TJ limitation up to 1000 suns concentration for an n⁺⁺ doping of 2 × 10¹⁹ cm^-3 based on the calibrated models. Furthermore, the effects of the peak and valley current densities are shown to have a strong influence on the efficiency over concentration within the TJ limiting regime.