Applications of nanoimprinted structures to organic photovoltaics

Abstract

Small-molecule organic photovoltaic cells (OPVs) have the potential to be a low-cost, flexible power conversion solution to many energy problems. These OPVs take advantage of an extremely thin active layer which enables this flexibility and reduces material volume. However, it is this thin quality that calls for improved power conversion efficiency compared to traditional silicon solar cells. Thin films suffer from reduced optical path lengths, which hinder light absorption and hence, power conversion efficiency. Many designs have been proposed to improve light absorption. A novel light-trapping substrate geometry for OPVs is presented which is based on a conformally-coated, subwavelength-textured substrate design which is intended to substantially increase optical path lengths. The subwavelength nature of these Nanocones/Nanowedges decouples the light propagation from the exciton diffusion path. This is an optimized situation for efficient charge transfer. Enhanced power absorption into the OPV active layer has been demonstrated via numerical computation methods, including COMSOL FEM and Lumerical FDTD. The challenge to fabricate a working device by using nanoimprinting to create the structures in a conductive polymer will be presented, where the nanoimprinting process is optimized to maintain good electrical properties of the patterned conductive film. We will also present an alternative approach that utilizes a conformal coating of the organic conductor PEDOT onto the pre-patterned nanostructures. Uniform and conformal PEDOT coverage over the nanoscale features was achieved using an all-dry deposition process.