Abstract
The highest conversion efficiencies are reached by multi-junction III-V solar cells. Wafer-bonding offers new possibilities to advance beyond the industrial standard triple-junction cells and its efficiency limitations. Here we present for the first time results of two novel 4-junction device concepts based on direct wafer-bonding with a realistic efficiency potential of 50% under concentrated sunlight. In the first concept, an inverted grown GaInP/GaAs tandem cell is bonded to a metamorphic GaInAs/Ge tandem cell. Frist bonded 4-junction solar cell on Ge shows an efficiency of 34.5% under one sun AM1.5d. In the latter concept a GaSb cell will be bonded to an inverted metamorphic triple-junction solar cell. The main challenge in this approach is a conductive direct wafer bond between the two cell stacks. Here, bond characteristics between GaSb and Ga0.29In0.71P and another between GaSb and Ga0.79In0.21As are shown for the first time.
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