Abstract
Simulations of AlxGa1-xAs/GaAs (x = 0.3) and AlxGa1-xAs/AlxGa1-xAs (x < 0.2) tunnel junction J-V characteristics are
studied for integration into a 2D metamorphic multi-junction solar cell model composed of GaInP/GaAs/InGaAs. A
comparison of the simulated solar cell J-V characteristics under AM1.5D spectrum is discussed in terms of short circuit
current density (Jsc), open circuit voltage (VOC), fill factor (FF) and efficiency (η) for both tunnel junction designs. Using
AlxGa1-xAs/GaAs top and bottom tunnel junctions, the metamorphic solar cell obtained values of Jsc = 12.3 mA/cm2, VOC
= 2.56 V, FF = 0.81 and η = 25.5%, whereas the solar cell with the AlxGa1-xAs/AlxGa1-xAs top and bottom tunnel
junctions reported values of Jsc = 12.3 mA/cm2, VOC = 2.22 V, FF = 0.81 and η = 22.1%. At open circuit voltage, energy
band diagrams show minimal curvature in the electron and hole quasi Fermi levels; furthermore, the difference between
the top sub-cell electron quasi Fermi level and the bottom sub-cell hole quasi Fermi level is shown to be equal to qVOC
for both designs. The energy band diagram of the complete structure is compared for both tunnel junction designs,
showing the difference in energy levels that correspond to the difference in measured open circuit voltage. The observed
decrease in open circuit voltage was ΔVOC = 0.34 V, which was attributed to the difference in tunnel junction material
band parameters such as bandgap, valence and conduction band offsets at heterojunctions and Fermi level degeneracies
due to doping concentration differences.
