Evidence for near-Surface
NiOOH Species in Solution-Processed
NiO<sub><i>x</i></sub> Selective Interlayer Materials: Impact
on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics
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Abstract
The characterization and implementation of solution-processed,
wide bandgap nickel oxide (NiO<sub><i>x</i></sub>) hole-selective
interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics
(OPVs) are discussed. The surface electrical properties and charge
selectivity of these thin films are strongly dependent upon the surface
chemistry, band edge energies, and midgap state concentrations, as
dictated by the ambient conditions and film pretreatments. Surface
states were correlated with standards for nickel oxide, hydroxide,
and oxyhydroxide components, as determined using monochromatic X-ray
photoelectron spectroscopy. Ultraviolet and inverse photoemission
spectroscopy measurements show changes in the surface chemistries
directly impact the valence band energies. O<sub>2</sub>-plasma treatment
of the as-deposited NiO<sub><i>x</i></sub> films was found
to introduce the dipolar surface species nickel oxyhydroxide (NiOOH),
rather than the p-dopant Ni<sub>2</sub>O<sub>3</sub>, resulting in
an increase of the electrical band gap energy for the near-surface
region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking
properties of the as-deposited and O<sub>2</sub>-plasma treated NiO<sub><i>x</i></sub> films are compared using both electron-only
and BHJ devices. O<sub>2</sub>-plasma-treated NiO<sub><i>x</i></sub> interlayers produce electron-only devices with lower leakage
current and increased turn on voltages. The differences in behavior
of the different pretreated interlayers appears to arise from differences
in local density of states that comprise the valence band of the NiO<sub><i>x</i></sub> interlayers and changes to the band gap
energy, which influence their hole-selectivity. The presence of NiOOH
states in these NiO<sub><i>x</i></sub> films and the resultant
chemical reactions at the oxide/organic interfaces in OPVs is predicted
to play a significant role in controlling OPV device efficiency and
lifetime
