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A unique aspect of the inverted metamorphic multijunction (IMM) solar cell is the bandgap tunability of each junction, creating extremely flexible device designs. The optimal structure has subcell photocurrents that are matched for a given spectrum. However, the subcell photocurrents depend on the cell operating temperature, and therefore, the bandgaps need to be optimized for a certain range of operating conditions. In addition, imperfect material quality results in a loss of voltage and current that depends on the cell bandgap and thickness. In this case, an iterative process of multijunction design and subcell characterization is necessary to determine the optimal design. We compare two different three-junction devices to demonstrate the effect of bandgap selection and lattice-mismatched material quality on device performance at different temperatures. The triple-junction (3J)-IMM design with two lattice-mismatched junctions of perfect material quality (2MMJ) is theoretically optimal at room temperature but experimentally performs similarly to a simpler design with one mismatched junction (1MMJ) at higher temperature because of material quality tradeoffs and the temperature dependence of the designs. Significant progress in the growth, processing, and measurement has led to a 1MMJ design with (42.6 ± 2.1)% peak efficiency at 327 suns and (40.9 ± 2.0)% efficiency at 1093 suns under the direct spectrum.
Date of Publication: April 2013