HIGH EFFICIENCY THIN-FILM Si/Ge SOLAR CELLS
S.H. Zaidi, K. Khan
Gratings, Inc, US
Keywords: Si nanostructures, Ge heteroepitaxial growth, Thin-film solar cell
Abstract:Thin-film crystalline Si solar cells are important due to potentially significant cost savings in contrast with conventional substrate solar cells. These savings are achieved through reduced Si usage assuming highly (~ 20-30%) efficient thin film Si solar cells can be manufactured. A necessary condition for high efficiency solar cells in thin-film configurations is complete optical absorption of sunlight. Due to its indirect bandgap, Si has inherently weak absorption in near IR region. Even incorporation of scattering mechanisms (geometrical, diffractive, and physical) is not enough to achieve complete absorption in Si thin-films. We propose thin-film Si/Ge solar cell configuration in which the top surface comprises of thin Si film and the bottom surface thin Ge film. This heterojunction solar cell combines the best attributes of both materials since Si absorbs strongly in the visible and Ge in most of the near IR region; the absorption spectrum ranges from 300 nm to 1600 nm. This approach has not been investigated extensively in part due to technical challenges associated with growth of high-quality Ge layers on Si. Our recent work has demonstrated growth of very high quality SixGe1-x and Ge films on nanostructured Si substrates, thus, identifying a promising pathway in resolving the fundamental problem of heterojunction Si/Ge thin-film solar cells. At the conference, we plan to report on absorption measurements and modeling of Si/Ge solar cells.