Surface Treatment of NiO Hole Transport Layers for Organic Solar Cells

Recent advances in the power-conversion efficiency of organic photovoltaics (OPVs) has largely been realized through the development of conjugated polymer absorber materials that provide for increased overlap with the solar spectrum as well as proper energy level offset with the electron acceptor. These allow for increased photocurrent and photovoltage, thus resulting in increased performance. Such systems could further be improved through the application of contact materials that have been tuned to minimize losses in carrier and potential losses at the charge-extraction interfaces. To date, these devices continue to use contacts that have not been optimized for the specific active layer components employed. Here, we demonstrate that the electrical and contact properties of NiO can be tuned through careful control of the deposition parameters as well as through surface treatments. The effects of the NiO thin-film processing and properties are investigated for application as a hole transport layer (HTL) in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester OPV devices. Devices based on the NiO HTLs demonstrate equal performance to those employing poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) HTLs. Furthermore, the NiO HTLs enable the application of zinc-oxide-based materials as transparent electrodes.