Nanopatterning with PFPE elastomers: Materials and photovoltaic applications

Abstract

Nanoscale fabrication is the foundation for emerging nanotechnology applications. This work describes the development and investigation of a soft lithography technique that utilizes perfluoropolyether (PFPE) elastomers for fabricating nanostructures from a variety of organic and inorganic materials. Inorganic oxides such as anatase phase TiO₂ are patterned from sol-gel routes. π-conjugated polymers are patterned into a myriad of shapes and sizes on the sub 500 nm length scale from both organic-based and aqueous solutions. Patterns are constructed with and without flash layers, over large areas, with varying aspect ratios, on different substrates, and from precursor materials that are not traditionally used in soft lithography applications.

Ordered bulk heterojunction solar cells are made from nanostructured titania and P3HT. Compared to a flat reference bilayer device, the short-circuit current was doubled upon nanostructuring, while the open-circuit voltage remained the same. The nanostructured device with the Z907 interfacial modification led to a power conversion efficiency of 0.6%. Photovoltaic devices comprised solely of organic materials were also fabricated, showing an increase in PV performance for nanopatterned devices compared to flat bilayer devices.

Several PFPE-based elastomers are explored for high resolution replica molding applications. The modulus of the elastomeric molds was varied using synthetic and additive approaches. High resolution nanofabrication techniques are reviewed, and the relationship between mold material properties and pattern fidelity is presented. Composite molds were used to form flexible molds out of stiff, high modulus materials. High arial density, sub-20 nm nanostructures are replicated using composite molds. Mold stability is experimentally investigated using sub-100 nm periodicity grating structures fabricated using e-beam lithography. It was observed that as the feature spacing decreased, high modulus PFPE-tetramethacrylate (TMA) composite molds were able to effectively replicate the nanograting structures without cracking or tear-out defects that typically occur with high modulus elastomers. In addition, the amount of fluorinated residue on a surface after contacting a PFPE molds is analyzed as a function of mold material and surface polarity. Suggestions for future directions of PFPE-based molding techniques are also presented.