Observation of Dislocations in Graded Buffer Layers of IMM Single Junction InGaAs Solar Cells by Two-Photon Excitation Photoluminescence

Lattice-mismatched materials enable growth of IIIV multi junction solar cells with band-gap combinations that are closer to the theoretical optimum than those of conventional material combinations. In order to improve material quality and hence conversion efficiency, it is important to reduce threading dislocations by employing graded buffer layers (GBLs) between subcells with different lattice constants. We have to understand how threading dislocations are formed from misfit dislocations, which are generated at the heterointerfaces in the GBL to relax strain. In this report, misfit dislocations inside the GBL of inverted metamorphic (IMM) single-junction InGaAs solar cells are directly observed by using two-photon excitation photoluminescence. A clear depth dependence of the dislocations inside the GBL can be confirmed. The results indicate that the strain relaxation processes in solar cells with high open-circuit voltages are distinctly different from those in solar cells with low open-circuit voltages.