Strategies for Thinning Graded Buffer Regions in Metamorphic Solar Cells and Performance Tradeoffs

Compositionally graded buffers (CGBs) have enabled the development of inverted metamorphic multijunction solar cells that contain three or more junctions and exhibit record efficiencies. Typically, the CGB is grown thick with a constant strain grading rate, because that is the most straightforward way to minimize the threading dislocation density (TDD) in device active regions. The CGB growth represents a significant expense, in terms of materials consumption and capital cost, however, hampering the economic viability of metamorphic devices. This work explores strategies for thinning Ga1-xInxP CGBs from GaAs to InP while minimizing performance loss, as evaluated by the open-circuit voltage (VOC) of ~0.74 eV Ga
_0.47
In
_0.53
As solar cells. Using the Ga
_1-x
In
_x
P CGB as a model, we demonstrate that slower grading is necessary in certain regions of the grade where dislocation nucleation rates are higher. Grading must be more gradual in these regions to suppress TDD. Grading rates can be increased in noncritical areas where dislocation nucleation is lower, permitting a thinner buffer. We demonstrate the relationship between VOC and TDD, establishing tradeoffs between thickness and performance. Through careful design, we demonstrate a CGB with a 40% thickness reduction (2 μm) achieved with a VOC decrease of only 7.5% (25 mV).