Solution Processed Hole Extraction Interfaces for Polymer Solar Cells

Organic semiconductors often make poor ohmic contacts with electrodes due to the deep energy levels forming an energetic barrier at the interfaces between the organic layer and the electrode contact. The mismatch in energy levels at the interface was overcome by inserting PEDOT:PSS material as an anode interlayer for achieving good ohmic contact and selecting single types of charge carriers at the polymer-electrode interface. Despite the significant development that was observed in the OPVs performance, the residual moisture and the acidic nature of PEDOT:PSS can cause degradation of the organic films and therefore affect long term stability. Metal oxides were later suggested as alternative interlayers to the PEDOT:PSS which exhibited high performance and long lifetimes. However, many of the metal oxide studies reported in literature used vacuum deposition methods, such as thermal evaporation and sputter deposition, which are not necessarily desirable for large-scale production. This thesis shows that it is possible to deposit Vanadium oxide (V2Ox) from solution in ambient conditions requiring no post-deposition treatment and achieving comparable efficiency to the most widely used interlayer materials. Using a combination of spectroscopic techniques and device characterisation, it is shown that solutionprocessed V2Ox can be used to replace evaporated metal oxides in optoelectronic devices which are fabricated at high temperatures. The work also goes on to show that it is possible to solution-process nickel oxide from a nickel acetylacetonate precursor and obtain a power conversion efficiency > 5%. Finally, the lifetime study of OPV devices utilising various anode interlayer materials shows that the stability of optimised V2Ox devices can be comparable with other interlayer materials.