Analysis of SiGe/Si quantum dot superlattices grown by low-pressure chemical vapor deposition for thin solar cells

We have grown SiGe/Si quantum dot superlattices (QDSLs) via low-pressure chemical vapor deposition (LPCVD) in order to analyze their performance as thin-film solar cells. Self-assembled SiGe quantum dots were included in the base region in order to increase absorption of near-infrared (NIR) photons and to increase short-circuit current density, J_sc. Rather than physically separate the epitaxial layers from the substrate prior to testing, a unique cell design was used to ensure that the epitaxial layers dominate the photocurrent from the cells. We found that open-circuit voltage V_oc in our LPCVD-grown cells was comparable to that in earlier-reported cells grown by molecular beam epitaxy (MBE). The effect of both superlattice thickness and quantum dot density were also explored, and dislocations were found to sharply reduce V_oc for thick superlattices with sufficiently-high dot density.