Block copolymer based nanostructures for energy harvesting

Photovoltaic device performance depends on efficient conversion of absorbed photons to electronic charge. Any mismatch of refractive indices between air and the solar cell front surface results in reflection of incident sunlight and reduced device performance. We detail a new approach for texturing silicon surfaces over arbitrarily large areas, combining self-assembly of block copolymer thin films and plasma-based etching.  This process creates densely packed arrays of sub-wavelength size cones, whose tapered profile grades the refractive index transition between air and bulk silicon. The gradual change in refractive index greatly reduces reflection at the air/silicon interface from more than 35 percent in a flat film to less than 1 percent over a broad wavelength range from 350 nm to 1000 nm. A significant fraction of the nanotexture volume comprises a surface layer whose optical properties differ substantially from those of the bulk, providing the key to improved performance. The nanotexture reflectivity is quantitatively well-modelled after accounting for both its profile and changes in refractive index at the surface. I will also discuss about a newly develop block copolymer based nanopatterning scheme for designing 3D nanostructures.

Ref.
1. Atikur Rahman, Pawel W. Majewski, Gregory Doerk, Charles T. Black, Kevin G. Yager. Nat. Commun. (2016) (accepted).
2. Atikur Rahman, Ahsan Ashraf, Huolin Xin, Xiao Tong, Peter Sutter, Matthew D. Eisaman, and Charles T. Black. Nat. Commun. 6:5963 (2015).
3. Pawel W. Majewski, Atikur Rahman, Charles T. Black, Kevin G. Yager. Nat. Commun. 6:7448 (2015).