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.