Nanostructured copper electrodes for organic photovoltaics

Abstract

This thesis describes a body of original research focused on the development of a viable alternative to the indium-tin oxide (ITO) glass window electrode used in organic photovoltaic (OPV) devices, based on the use of ultra-thin Cu films. The first results chapter describes a low cost, robust Cu | Al bilayer window electrode that simultaneously functions as the low work function electron-extracting electrode and as a sink for oxygen/water molecules in OPVs. When the electrode is exposed to air, an ultra-thin oxide layer forms at its surface without any increase in surface roughness, and the sheet resistance of the electrode actually decreases. However, this electrode has the disadvantage of a lower far-field transparency than ITO glass. The second results chapter describes how the transparency of ultra-thin Cu films can be increased to a level comparable to that of ITO glass across most of the spectrum over which OPVs harvest light using an overlayer of tungsten sub-oxide (WO3-x) which is spontaneously doped with Cu, increasing both its refractive index and electrical conductivity. Unfortunately these electrodes are not air stable. The third results chapter describes how the developments described in the previous two chapters might be integrated to realise an electrode that is both air-stable and highly transparent. The final results chapter describes a very different approach to coupling light into an OPV based on a Cu electrode with a dense array of sub-optical wavelength apertures. These electrodes absorb light strongly, concentrating it as surface plasmon excitations. It is shown that this trapped light can be absorbed by the light harvesting organic semiconductor in organic photovoltaics so that electrodes with very low far-field transparency can perform as well as more transparent electrodes