Transition Metal Oxides and Their Use as Hole Extraction Materials in Organic Photovoltaic Devices

One of the limiting factors in the efficiency of organic photovoltaic devices utilising new generations of donor polymers is the ability to obtain Ohmic contacts between the electrodes and the organic layers. This is due to the fact these new donor polymers often have deeper energy levels meaning that at the interfaces between the organic layer and the electrode contact barriers are formed. PEDOT:PSS is the current benchmark material that is used to reduce or remove these contact barriers however even this material has too shallow an energy level and has serious issues with long term stability. One possible class of materials that might overcome these limitations and give increased device performance and lifetimes are metal oxides. However these materials are currently deposited mainly through vacuum deposition and the difficulty of incorporating vacuum based depositions into roll-to-roll fabrication setups limits the commercial use of these materials.
Using a combination of spectroscopic techniques and device results this work shows that metal oxides can be used to effectively reduce these contact barriers and achieve high performance using deep energy level donor polymers. It has been shown that Vanadium (V) Oxide can be deposited from solution at room temperature while maintaining high efficiencies. The work continues to look at spray coating, a roll-to-roll compatible deposition technique, and how it can be used to incorporate ultrathin film of solution processed Molybdenum (VI) Oxide for fabricating efficient devices. In addition I have shown that the processing of these materials is only limited by the possible introduction of metallic gap states that can occur due to high temperature processing.