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, Jsc. 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 Voc 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 Voc for thick superlattices with sufficiently-high dot density.