8 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 Ī²-NiĀ(OH)<sub>2</sub> to form thin
Ī²-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
Dopant Diffusion in Sequentially Doped Poly(3-hexylthiophene) Studied by Infrared and Photoelectron Spectroscopy
The
diffusivity of dopants in semiconducting polymers is of high
interest as it enables methods of sequential doping but also affects
device stability. In this study, we investigate the diffusion of a
bulky sequentially deposited p-dopant in polyĀ(3-hexylthiophene) (P3HT)
thin films using nondestructive <i>in situ</i> infrared
(IR) spectroscopy and photoelectron spectroscopy (PES). We probe dopant
diffusion into the polymer film at varying coverage by differentially
evaluating electron transfer in the bulk and at the surface. Thereby
it is possible to determine dopant coverages at which both electron
transfer and incorporation of dopants are saturated. By use of PES,
neutral and charged dopants can be distinguished, revealing that charged
dopants are less mobile in the diffusion process than neutral molecules.
We further compare the diffusivity in semicrystalline and fully amorphous
P3HT. We find that at high coverage semicrystalline P3HT seems to
yield a higher capacity for dopants than fully amorphous P3HT. A temperature-dependent
measurement of sequential doping shows directly that the incorporation
of dopants is thermally activated and requires temperatures close
to room temperature
Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes
We study two types of water/alcohol-soluble
aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated
(PEIE), for their suitability as electron injection layers in solution-processed
blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron
spectroscopy is used to determine the nominal thickness of the polymer
layers while ultraviolet photoelectron spectroscopy is carried out
to determine the induced work-function change of the silver cathode.
The determined work-function shifts are as high as 1.5 eV for PEI
and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal
that homogeneous PEI and PEIE layers are present at nominal thicknesses
of about 11 nm. Finally, we solution prepare blue emitting polymer-based
OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous
efficiency reaches 3 and 2.2 cd A<sup>ā1</sup>, whereas maximum
luminance values are as high as 8000 and 3000 cd m<sup>ā2</sup> for PEI and PEIE injection layers, respectively. The prepared devices
show a comparable performance to Ca/Ag OLEDs and an improved shelf
lifetime
Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide
The
development of medium-bandgap solar cell absorber materials
is of interest for the design of devices such as tandem solar cells
and building-integrated photovoltaics. The recently developed perovskite
solar cells can be suitable candidates for these applications. At
present, wide bandgap alkylammonium lead bromide perovskite absorbers
require a high-temperature sintered mesoporous TiO<sub>2</sub> photoanode
in order to function efficiently, which makes them unsuitable for
some of the above applications. Here, we present for the first time
highly efficient wide bandgap planar heterojunction solar cells based
on the structurally related formamidinium lead bromide. We show that
this material exhibits much longer diffusion lengths of the photoexcited
species than its methylammonium counterpart. This results in planar
heterojunction solar cells exhibiting power conversion efficiencies
approaching 7%. Hence, formamidinium lead bromide is a strong candidate
as a wide bandgap absorber in perovskite solar cells
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
StructureāProperty Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide
Studying
the structureāproperty relations of tailored dipolar phenyl
and biphenylphosphonic acids, we report self-assembled monolayers
with a significant decrease in the work function (WF) of indiumātin
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
Conformal and Highly Luminescent Monolayers of Alq<sub>3</sub> Prepared by Gas-Phase Molecular Layer Deposition
The gas-phase molecular layer deposition
(MLD) of conformal and highly luminescent monolayers of trisĀ(8-hydroxyquinolinato)Āaluminum
(Alq<sub>3</sub>) is reported. The controlled formation of Alq<sub>3</sub> monolayers is achieved for the first time by functionalization
of the substrate with amino groups, which serve as initial docking
sites for trimethyl aluminum (TMA) molecules binding datively to the
amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting
formation of highly luminescent Alq<sub>3</sub> monolayers is afforded.
The growth process and monolayer formation were studied and verified
by in situ quartz crystal monitoring, optical emission and absorption
spectroscopy, and X-ray photoelectron spectroscopy. The nature of
the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces
and porous structures with high surface areas, as demonstrated in
this work for silica aerogels. The concept presented here paves the
way to highly sensitive luminescent sensors and dye-sensitized metal
oxides for future applications (e.g., in photocatalysis and solar
cells)
Dipolar SAMs Reduce Charge Carrier Injection Barriers in nāChannel Organic Field Effect Transistors
In this work we examine small conjugated
molecules bearing a thiol
headgroup as self assembled monolayers (SAM). Functional groups in
the SAM-active molecule shift the work function of gold to n-channel
semiconductor regimes and improve the wettability of the surface.
We examine the effect of the presence of methylene linkers on the
orientation of the molecule within the SAM. 3,4,5-Trimethoxythiophenol
(TMP-SH) and 3,4,5-trimethoxybenzylthiol (TMP-CH<sub>2</sub>-SH) were
first subjected to computational analysis, predicting work function
shifts of ā430 and ā310 meV. Contact angle measurements
show an increase in the wetting envelope compared to that of pristine
gold. Infrared (IR) measurements show tilt angles of 22 and 63Ā°,
with the methylene-linked molecule (TMP-CH<sub>2</sub>-SH) attaining
a flatter orientation. The actual work function shift as measured
with photoemission spectroscopy (XPS/UPS) is even larger, ā600
and ā430 meV, respectively. The contact resistance between
gold electrodes and polyĀ[<i>N</i>,<i>N</i>ā²-bisĀ(2-octyldodecyl)-naphthalene-1,4:5,8-bisĀ(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5ā²-(2,2ā²-bithiophene) (Polyera Aktive
Ink, N2200) in n-type OFETs is demonstrated to decrease by 3 orders
of magnitude due to the use of TMP-SH and TMP-CH<sub>2</sub>-SH. The
effective mobility was enhanced by two orders of magnitude, significantly
decreasing the contact resistance to match the mobilities reported
for N2200 with optimized electrodes