Efficient Modification of Metal Oxide Surfaces with
Phosphonic Acids by Spray Coating
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Abstract
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