3 research outputs found
Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide
Solution-processed
nickel oxide (sNiO) is known to be an excellent charge-selective interlayer
in optoelectronic devices. Its beneficial properties can be further
enhanced by an oxygen plasma (OP) treatment. In order to elucidate
the mechanism behind this improvement, we use infrared transmission
and X-ray photoelectron spectroscopy to probe the bulk and surface
properties of the sNiO. We find that increasing the annealing temperature
of the sNiO not only increases the structural order of the material
but also reduces the concentration of nickel hydroxide species present
in the bulk and on the surface of the film. This results in a decrease
of the work function, while an additional OP treatment raises the
work function to between 5.5 and 5.6 eV. For all annealing temperatures
investigated, the consequences of the OP treatment are identified
as reactions of both NiO and β-NiÂ(OH)<sub>2</sub> to form thin
β-NiOOH phases in the first atomic layers. Our results emphasize
the importance of understanding the correlation between the preparation
and resulting properties of sNiO layers and provides further insight
into the interpretation of interface properties of NiO
Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity
Nickel oxide (NiO)
is a widely used material for efficient hole extraction in optoelectronic
devices. However, its surface characteristics strongly depend on the
processing history and exposure to adsorbates. To achieve controllability
of the electronic and chemical properties of solution-processed nickel
oxide (sNiO), we functionalize its surface with a self-assembled monolayer
(SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared
and photoelectron spectroscopy shows the chemisorption of the molecules
with a nominal layer thickness of around one monolayer and gives an
insight into the chemical composition of the SAM. Density functional
theory calculations reveal the possible binding configurations. By
the application of the SAM, we increase the sNiO work function by
up to 0.8 eV. When incorporated in organic solar cells, the increase
in work function and improved energy level alignment to the donor
does not lead to a higher fill factor of these cells. Instead, we
observe the formation of a transport barrier, which can be reduced
by increasing the conductivity of the sNiO through doping with copper
oxide. We conclude that the widespread assumption of maximizing the
fill factor by only matching the work function of the oxide charge
extraction layer with the energy levels in the active material is
a too narrow approach. Successful implementation of interface modifiers
is only possible with a sufficiently high charge carrier concentration
in the oxide interlayer to support efficient charge transfer across
the interface
Structure–Property Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide
Studying
the structure–property relations of tailored dipolar phenyl
and biphenylphosphonic acids, we report self-assembled monolayers
with a significant decrease in the work function (WF) of indium–tin
oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied
through the different molecular lengths and the introduction of electron-withdrawing
fluorine atoms, the surface energy is kept constant through the electron-donating <i>N</i>,<i>N-</i>dimethylamine head groups. The self-assembled
monolayer formation and its modification of the electrodes are investigated
via infrared reflection absorption spectroscopy, contact angle measurements,
and photoelectron spectroscopy. The WF decrease in ITO correlates
with increasing molecular dipoles. The lowest ever recorded WF of
3.7 eV is achieved with the fluorinated biphenylphosphonic acid