1 research outputs found
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