2 research outputs found
Thermal Conductance across Phosphonic Acid Molecules and Interfaces: Ballistic versus Diffusive Vibrational Transport in Molecular Monolayers
The influence of planar organic linkers
on thermal boundary conductance across hybrid interfaces has focused
on the organic/inorganic interaction energy rather than on vibrational
mechanisms in the molecule. As a result, research into interfacial
transport at planar organic monolayer junctions has treated molecular
systems as thermally ballistic. We show that thermal conductance in
phosphonic acid (PA) molecules is ballistic, and the thermal boundary
conductance across metal/PA/sapphire interfaces is driven by the same
phononic processes as those across metal/sapphire interfaces without
PAs, with one exception. We find a more than 40% reduction in conductance
across henicosafluorododecylphosphonic acid (F21PA)
interfaces, independent of metal contact, despite similarities in
structure, composition, and terminal group to the variety of other
PAs studied. Our results suggest diffusive scattering of thermal vibrations
in F21PA, demonstrating a clear path toward modification of interfacial
thermal transport based on knowledge of ballistic and diffusive scattering
in single monolayer molecular interfacial films
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