The work in this thesis focuses on the process of hole extraction in OPVs both in terms of the development of new hole extracting materials and understanding the factors that determine the efficiency of hole extraction to enable the intelligent design of these electrodes. In Chapter 1 an overview of organic semiconductors and organic photovoltaic (OPV) devices is given including details of the fundamental processes that underpin the operation of OPVs. In Chapter 2 an overview of the experimental techniques used in this thesis is presented. Chapter 3 reports the use of surface oxidised Au nanoparticles (AuNPs) as an efficient hole-extracting material in molecular OPVs. This class of hole-extracting material is unusual in that it is remarkably effective even at low surface coverage and offers the advantage over conventional hole-extracting materials of solution processing from a pH neutral solution. The effectiveness of oxidised AuNPs as a hole extracting material is attributed to the very high work function of these particles and geometric electric field enhancement effects. Chapters 4 and 5 describe experiments designed to determine how nanoscale geometric electric field enhancement and electrode work function affects the efficiency of hole-extraction in OPVs, respectively. From these latter chapters generic electrode design rules are proposed. Finally, in Chapter 6 an overview of future work is given. With the exception of the final part of Chapter 5, the results of chapters 3, 4 and 5 have been published in peer reviewed scientific journals.