A soft lithography route to nanopatterned photovoltaic devices

We have fabricated bulk heterojunction photovoltaic (PV) cells using a perfluoropolyether (PFPE) elastomeric stamp to control the morphology of the donor-acceptor interface within devices. Devices were fabricated using the Pattern Replication In Non-wetting Templates (PRINT) process to have nanoscale control over the bulk heterojunction device architecture. The low-surface energy, chemically resistant, variable modulus, fluoropolymer based molds used in PRINT provide a route to patterning, with nanometer resolution, general polymeric donor materials such as polythiophene and polyphenylenevinylene derivatives and 'hard' inorganic oxide structures typically used as acceptor materials in hybrid organic solar cells such as TiO₂, ZnO, and CdSe. This "top-down" approach allows for patterning over large areas and for the functionalization of the donor/acceptor interface. Specifically, nanostructured anatase titania with post-like features ranging from 30-100 nm in diameter and 30-65 nm in height was fabricated to form the ordered bulk heterojunction of a titania-poly(3-hexylthiophene) (P3HT) PV-cell. Nanostructured devices showed a two-fold improvement in both short-circuit current (J_sc) and power conversion efficiency (PCE) relative to reference bilayer cells. Additionally, we will discuss devices fabricated with other organic and inorganic materials in order to investigate the effect on cell performance of controlling the nanoscale architecture of the bulk heterojunction via patterning.