Organic photovoltaics (OPVs) have attracted a considerable research interest over the past few decades as a low-cost and nontoxic source of renewable energy. Although a power conversion efficiency (PCE) of up to 17.4% has been achieved for single junction, this is still lower than that of inorganic solar cell, so more work is needed to realize the full commercial potential of OPVs. One way to improve the performance and stability of OPVs is the inclusion of interfacial layers, which facilitate the transport of both electrons and holes to both electrodes. Zinc oxide is widely used electron transport layer (ETL) that has the advantage of good electrical properties and chemical stability. The most commonly used hole transporting material (HTL) is PEDOT:PSS, which has better electrical properties than the most of the alternatives, but its acidic and hydroscopic nature limits the durability of the OPV, so a replacement is needed.
This thesis focuses on the development of high-performance OPVs based on p-type metal oxides as potential alternatives to PEDOT:PSS. Firstly, in chapter 3, we have successfully demonstrated the thermal evaporation of controlled thickness, high optical quality NiOx thin films. It was anticipated that the films would be hole selective transport layers, however those prepared were surprisingly shown electron transport behaviour. This was attributed to the alloying between the Ni and tungsten boat during evaporation. After identifying the optimum thickness and tungsten content in evaporated films, we have demonstrated those films as efficient universal electron transport layers in a wide variety of organic photovoltaic devices. Next, chapter 4 presents the fabrication of CuOx films by solution process and their potential as HTLs in OPVs comparing with commonly used HTL, PEDOT:PSS. The characterisation of the CuOx films allowed a fully understanding of the structure and optoelectronic properties to identify an optimum annealing temperature and film’s thickness. The impact of processing solvent on device performance is further studied. Finally, chapter 5 demonstrates the preparation of CuOx via novel synthetic route at low temperature. The influence of annealing temperature on device performance compared to PEDOT:PSS is further investigated. This work shows the potential of CuOx as efficient HTLs for OPV.