Thin film photovoltaics (PV) is being pursued by several institutions as a lower cost alternative to crystalline wafer technologies. The use of much less materials and roll-to-roll continuous processing in thin film technologies have been touted as the pathway to low-cost PV. However, the efficiencies of production thin film Si solar cells are about one half that achieved with crystalline silicon. Hence, achievement of single-crystalline-like silicon photovoltaics on flexible, low-cost substrates can be game changing by combining high efficiency with low cost. We are developing such a technology by creation of an architecture that yields single-crystalline-like thin films even on polycrystalline or amorphous substrates. This technology has been very successfully demonstrated and being commercialized in the superconductor field and inserted in the U.S. electric power grid [1].

 

The enabler that we have employed in this work is a single-crystalline-like thin film template of MgO made by Ion Beam-Assisted Deposition (IBAD). Such IBAD films have been successfully employed as templates for epitaxial growth of cube-textured superconducting films on polycrystalline substrates with critical current densities as high as those achieved on single crystal substrates [1].

 

MgO templates made by IBAD on flexible metal substrate have been used for epitaxial growth of germanium films using intermediate oxide layers. All layers were deposited by reel-to-reel magnetron sputtering and strongly (400) textured Ge films with an in-plane texture spread of just 1° FWHM were achieved [2]. Optical properties of the germanium films are found to be comparable to that single crystal Ge and Hall mobility values over 700 cm2/Vs have been achieved. Epitaxial (400) textured silicon films have been grown by reel-to-reel magnetron sputtering on the Ge films. A continuous grading of germanium to silicon has been done to accommodate for the lattice mismatch. While excellent epitaxial growth has been achieved in Si and Ge on flexible metal substrates, the defect density of the films showed a high value of 10^8 per cm^2. Cross sectional TEM of the multilayer architecture showed concentration of threading dislocations near the semiconductor-oxide interface. Defect reduction strategies are being employed and recent progress in use of single-crystalline-like templates on low-cost, flexible substrates for high-efficiency silicon photovoltaics will be discussed in this presentation.

 

1. V. Selvamanickam et al. IEEE Trans. Appl. Supercond. 19 (2009) 3225.

2. V. Selvamanickam et al. J. Crystal Growth 311, (2009) 4553.