4 research outputs found
PM-IRRAS Determination of Molecular Orientation of Phosphonic Acid Self-Assembled Monolayers on Indium Zinc Oxide
Self-assembled
monolayers (SAMs) of phosphonic acids (PAs) on transparent
conductive oxide (TCO) surfaces can facilitate improvement in TCO/organic
semiconductor interface properties. When ordered PA SAMs are formed
on oxide substrates, interface dipole and electronic structure are
affected by the functional group properties, orientation, and binding
modes of the modifiers. Choosing octylphosphonic acid (OPA), F<sub>13</sub>-octylphosphonic acid (F<sub>13</sub>OPA), pentafluorophenyl
phosphonic acid (F<sub>5</sub>PPA), benzyl phosphonic acid (BnPA),
and pentafluorobenzyl phosphonic acid (F<sub>5</sub>BnPA) as a representative
group of modifiers, we report polarization modulation-infrared reflection–absorption
spectroscopy (PM-IRRAS) of binding and molecular orientation on indium-doped
zinc oxide (IZO) substrates. Considerable variability in molecular
orientation and binding type is observed with changes in PA functional
group. OPA exhibits partially disordered alkyl chains but on average
the chain axis is tilted ∼57° from the surface normal.
F<sub>13</sub>OPA tilts 26° with mostly tridentate binding. The
F<sub>5</sub>PPA ring is tilted 23° from the surface normal with
a mixture of bidentate and tridentate binding; the BnPA ring tilts
31° from normal with a mixture of bidentate and tridentate binding,
and the F<sub>5</sub>BnPA ring tilts 58° from normal with a majority
of bidentate with some tridenate binding. These trends are consistent
with what has been observed previously for the effects of fluorination
on orientation of phosphonic acid modifiers. These results from PM-IRRAS
are correlated with recent results on similar systems from near-edge
X-ray absorption fine structure (NEXAFS) and density functional theory
(DFT) calculations. Overall, these results indicate that both surface
binding geometry and intermolecular interactions play important roles
in dictating the orientation of PA modifiers on TCO surfaces. This
work also establishes PM-IRRAS as a routine method for SAM orientation
determination on complex oxide substrates
Efficient Modification of Metal Oxide Surfaces with Phosphonic Acids by Spray Coating
We report a rapid method of depositing
phosphonic acid molecular
groups onto conductive metal oxide surfaces. Solutions of pentafluorobenzyl
phosphonic acid (PFBPA) were deposited on indium tin oxide, indium
zinc oxide, nickel oxide, and zinc oxide by spray coating substrates
heated to temperatures between 25 and 150 °C using a 60 s exposure
time. Comparisons of coverage and changes in work function were made
to the more conventional dip-coating method utilizing a 1 h exposure
time. The data show that the work function shifts and surface coverage
by the phosphonic acid were similar to or greater than those obtained
by the dip-coating method. When the deposition temperature was increased,
the magnitude of the surface coverage and work function shift was
also found to increase. The rapid exposure of the spray coating was
found to result in less etching of zinc-containing oxides than the
dip-coating method. Bulk heterojunction solar cells made of polyhexylthiophene
(P3HT) and bis-indene-C<sub>60</sub> (ICBA) were tested with PFBPA
dip and spray-modified ITO substrates as well as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)
(PEDOT:PSS)-modified ITO. The spray-modified ITO solar cells showed
a similar open circuit voltage (V<sub>OC</sub>) and fill factor (FF)
and a less than 5% lower short circuit current density (<i>J</i><sub>SC</sub>) and power conversion efficiency (PCE) than the dip-
and PEDOT:PSS-modified ITO. These results demonstrate a potential
path to a scalable method to deposit phosphonic acid surface modifiers
on metal oxides while overcoming the limitations of other techniques
that require long exposure and post-processing times
Integer Charge Transfer and Hybridization at an Organic Semiconductor/Conductive Oxide Interface
We
investigate the prototypical hybrid interface formed between
PTCDA and conductive <i>n</i>-doped ZnO films by means of
complementary optical and electronic spectroscopic techniques. We
demonstrate that shallow donors in the vicinity of the ZnO surface
cause an <i>integer</i> charge transfer to PTCDA, which
is clearly restricted to the first monolayer. By means of DFT calculations,
we show that the experimental signatures of the anionic PTCDA species
can be understood in terms of strong hybridization with localized
states (the shallow donors) in the substrate and charge back-donation,
resulting in an effectively integer charge transfer across the interface.
Charge transfer is thus not merely a question of locating the Fermi
level above the PTCDA electron-transport level but requires rather
an atomistic understanding of the interfacial interactions. The study
reveals that defect sites and dopants can have a significant influence
on the specifics of interfacial coupling and thus on carrier injection
or extraction
Orientation of Phenylphosphonic Acid Self-Assembled Monolayers on a Transparent Conductive Oxide: A Combined NEXAFS, PM-IRRAS, and DFT Study
Self-assembled monolayers (SAMs) of dipolar phosphonic
acids can
tailor the interface between organic semiconductors and transparent
conductive oxides. When used in optoelectronic devices such as organic
light emitting diodes and solar cells, these SAMs can increase current
density and photovoltaic performance. The molecular ordering and conformation
adopted by the SAMs determine properties such as work function and
wettability at these critical interfaces. We combine angle-dependent
near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and
polarization modulation infrared reflection absorption spectroscopy
(PM-IRRAS) to determine the molecular orientations of a model phenylphosphonic
acid on indium zinc oxide, and correlate the resulting values with
density functional theory (DFT). We find that the SAMs are surprisingly
well-oriented, with the phenyl ring adopting a well-defined tilt angle
of 12–16° from the surface normal. We find quantitative
agreement between the two experimental techniques and density functional
theory calculations. These results not only provide a detailed picture
of the molecular structure of a technologically important class of
SAMs, but also resolve a long-standing ambiguity regarding the vibrational-mode
assignments for phosphonic acids on oxide surfaces, thus improving
the utility of PM-IRRAS for future studies