Investigation of optical absorption in thin-film Si/Ge solar cells

Thin-film, high efficiency crystalline Si solar cells promise 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. However, due to its indirect bandgap, Si has inherently weak absorption in near IR region so thicker films are required to accommodate longer path lengths. Even incorporation of scattering mechanisms (geometrical, diffractive, and physical) is not enough to achieve complete absorption in Si thin-films. We propose thin-film crystalline Si/Ge solar cell configuration in which the top surface comprises of thin Si film and the bottom surface of thin Ge film. This heterojunction solar cell combines the best attributes of both materials with strong Si absorption in the visible followed by absorption in Ge through most of the near IR region. The absorption spectrum ranges from 300 nm to 1600 nm in this solar cell with only two thin film layers. This approach has not been investigated extensively in past due to technical challenges associated with growth of high-quality Ge layers on Si. We have demonstrated growth of very high quality SixGe1-x and Ge films on nanostructured Si substrates, thus, identifying a promising pathway in addressing the fundamental problem of pseudomorphic growth of Ge on Si, a necessary condition for high efficiency heterojunction Si/Ge thin-film solar cells. Preliminary optical measurements on Si/Ge films exhibit strong absorption dependence on growth quality.