Toward high-efficiency thin film solar cells combining multi-junctions and nano-scale light management

We have achieved significant accomplishments on the developing of high-efficiency thin film multi-junction solar cell with nano-scale light management. The highlights include: (1) Developed a computational tool for the detailed balance analysis of nanophotonic structures showing that absorption in a solar cell at all angles is sufficient to calculate and understand the fundamental behavior of nanophotonic solar cells. (2) Elucidated the physics of open-circuit voltage enhancement by analyzing the angular and spectral distribution of the cell’s absorption spectrum. (3) Demonstrated a novel nano-window GaAs Solar cell that achieved 17% efficiency, and highest Voc (~1V) in nanostructure solar cell. (4) Demonstrated equaivalent apbsorption in a160 nm free-standing nano-structured GaAs thin film absorber to that of a 1μm thick planar cell. The absorption of this nanopyramid structure is as twice as much that of a planar structure with same thickness and equivalent to a planar structure with 1 μm thickness. (5) Successfully developed a new etching technique which produces the first batch of working thin film GaAs cells. (6) Successfully developed the process for generating large area free-standing thinfilm single-crystalline Si down to 2μm thickness. (7) We are developing a model system to realize super scatterers and test their light trapping properties on an SOI detector test platform, which can accurately quantify the light trapping ability of model scatters. We have demonstrated the light trapping ability of Si nanowires on the platform and are now designing metal coated nanowire superscatterers and absorbers.