Nanostructured organic solar cells defined by nanoimprint lithography

Energy harvesting from sunlight via organic solar cells (OSCs) based on polymers as an electron donors and fullerenes as electron acceptors has been subject of intensive research due to the potential for low cost and large area devices with attractive market perspectives. One of the biggest challenges for OSCs is their low efficiency of power conversion, which is limited by quality of active layer morphology of donor-acceptor materials and interfaces between the components. Key reasons for this low efficiency include severe electron-hole recombination, which prevents charge pair propagation toward the electrodes and poor light absorptions due to thin polymer layer (∼100 nm). These problems can be dramatically alleviated if the charge-transfer polymers can be arranged as periodic nanostructures for active layer of ∼300 nm so that enough light absorption takes place and no phase overlap exists in the charge propagation path.;This work reports the formation of ordered bi-continuous interdigitized active layer morphology, well defined interfaces for charge pair formation and propagation without recombination toward the electrodes. Such nanostructure arrays of poly(3-hexylthiophene) (P3HT) with well defined interfaces have been fabricated using nanoimprint lithography (NIL). The molds required for NIL are fabricated using innovative low cost and non-lithographic technique which is scalable to commercial use. Simultaneous control of nanostructured and 3-D chain alignment in P3HT nanostructures (nanowires and nanopillars) defined by NIL is revealed using out-of-plane and in-plane grazing incident X-ray diffraction measurements and enhancement in anisotropic charge carrier mobility favorable to solar cells and field effect transistors (FETs) is measured making FETs. Separate acceptor deposition is required for nanostructured solar cells which is challenging due to the limitation of solvent compatibility and self shadowing effect for thermal deposition. For this purpose, orthogonal solvent is investigated for spin processing, oblique angle deposition is used for thermal deposition and low glass transition temperature acceptor materials are researched for transfer imprinting process. The fabricated solar cells using the developed recipe, show improved performances as compared to bilayer devices.