Epitaxial GaInP/GaAs/Si Triple-Junction Solar Cell with 25.9% AM1.5g Efficiency Enabled by Transparent Metamorphic AlxGa1-xAsyP1-y Step-Graded Buffer Structures

III-V/Si multi-junction solar cells are potential successors to the silicon single-junction cell due to their efficiency potential of up to 40% in the radiative limit.[1] Herein, latest results of epitaxially integrated GaInP/GaAs/Si triple-junction cells are presented. To reduce parasitic absorption losses, which have limited the current density in the Si bottom cell in the previous devices, transparent AlxGa1-xAsyP1-y step-graded metamorphic buffers are investigated. Compared with previous GaAsyP1-y step-graded buffers, the transmittance is enhanced significantly, while no significant impact on the threading dislocation density is observed. Implemented into a new triple-junction solar cell, an increase in short-circuit current density from 10.0 to 12.2mA(cm-2) is achieved, leading to a new record conversion efficiency of 25.9% under AM1.5g conditions.