Simulation, modeling and comparison of III-V tunnel junction designs for high efficiency metamorphic multi-junction solar cells

摘要

Simulations of Al_xGa_(1-x)As/GaAs (x = 0.3) and Al_xGa_(1-x)As/Al_xGa_(1-x)As (x < 0.2) tunnel junction J-V characteristics are studied for integration into a 2D metamorphic multi junction solar cell model composed of G&InP/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 (J_(sc)), open circuit voltage (V_(oc)), fill factor (FF) and efficiency (η) for both tunnel junction designs. Using Al_xGa_(1-x)As/GaAs top and bottom tunnel junctions, the metamorphic solar cell obtained values of J_(sc) = 12.3 mA/cm~2, V_(oc) = 2.56 V, FF = 0.81 and η = 25.5%, whereas the solar cell with the Al_x Ga_(1-x)As/Al_xGa_(l_x)As top and bottom tunnel junctions reported values of J_(sc) = 12.3 mA/cm~2, V_(oc) = 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 qV_(oc) 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 △V_(oc) = 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.