Nanostructured copper electrodes for organic photovoltaics

This thesis focuses on the development of high-performance copper (Cu) based transparent electrodes for application in organic photovoltaics (OPVs). Laboratory-scale OPV devices are typically supported on costly and brittle indium tin oxide (ITO) coated glass substrates which serve as the transparent electrode. There is however near universal consensus in the community that for the commercialization of OPVs a flexible, lower cost alternative to ITO is needed. To date transparent electrodes based on silver (Ag) nanowires, grids and optically thin planar films are considered the most viable alternatives. However, Ag is as costly as indium and so offers little advantage over ITO glass in this respect. Cu is an attractive alternative to Ag, with comparable electrical conductivity at < 1% of the material cost. Historically however, Cu has been overlooked due to its lower stability towards oxidation in air and its stronger absorption of visible and near infra-red light. The first results chapter of this thesis focuses on the development of a high-performance Cu grid electrode fabricated using microcontact printing, which is an unconventional method of patterning metal films that has been little explored for this application. Using a low-toxicity single component etchant and a printed mask of monolayer thickness flexible Cu grid electrodes with simultaneously > 90% transparency and < 10 Ω sq-1 sheet resistance are realised - comparable to ITO on glass. Additionally the grid line width achievable using this approach is 20× narrower than possible using conventional printing methods, which reduces or eliminates the requirement for a PEDOT:PSS conductive polymer interlayer. A remarkably effective approach to passivating Cu without electrically isolating it, first reported by Hutter et al. (Adv. Mater, 2013, 25, 284–288), is to cap the Cu with 0.8 nm of aluminium (Al) by simple thermal evaporation. In the second results chapter it is shown that a slab-like 8 nm thick Cu film capped with an 0.8 nm Al capping layer and perforated with ~6 million apertures per cm2 using photolithography exhibits a sheet resistance increase of less than 1% after 2 years in ambient air. The reason for this remarkable stability is elucidated for the first time: Namely, spontaneous segregation of the Al to grain boundaries in the polycrystalline film Cu results in ternary oxide plugs at those sites most vulnerable to oxidation. Cu does not adhere strongly to glass and plastic substrates, and so there is a need to develop ways to seed the growth of dense films of < 10 nm thickness of low roughness and concomitant improved stability. The third results chapter describes the development of a hybrid seed layer for the fabrication of robust, optically thin Cu films on glass and plastic substrates that outperforms the best alternative. The hybrid layer is based on a combination of molecular and inorganic (Al) layers. The final results chapter applies the insight from the second results chapter to develop novel bilayer electrodes of copper/tin and copper/zinc which improve with in-situ oxidation, which represents ongoing work. The four results chapters are prefaced with an introduction chapter which outlines the relevant background literature and theory and is followed by chapters summarizing the methods and techniques used in this thesis