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Surface Modification of Indium–Tin Oxide with Functionalized Perylene Diimides: Characterization of Orientation, Electron-Transfer Kinetics and Electronic Structure
journal contribution
posted on 2016-08-25, 00:00 authored by Yilong Zheng, Anthony
J. Giordano, R. Clayton Shallcross, Sean R. Fleming, Stephen Barlow, Neal R. Armstrong, Seth R. Marder, S. Scott SaavedraCharge-transfer
efficiency at the active layer/transparent conducting
oxide (TCO) interface is thought to be a key parameter contributing
to the overall efficiency of organic electronic devices such as organic
photovoltaics (OPVs). Modification of the TCO surface with a redox-active
surface modifier is a possible approach toward enhancing OPV efficiency
by providing an efficient charge-transfer pathway between either hole-
or electron-harvesting contacts and the organic active layer. Here
we report on the modification of indium–tin oxide (ITO) electrodes
with two perylene diimides (PDIs), coupled to phosphonic acid (PA)
binding groups through a p-phenylene bridge or a
biphenyl-4,4′-diyl bridge (PDI–phenyl–PA and
PDI–diphenyl–PA, respectively). We used two different
deposition techniques: adsorption from solution (SA) and spin coating
(SC), to create three types of monolayer films on ITO: SA PDI–phenyl–PA,
SA PDI–diphenyl–PA, and SC PDI–phenyl–PA.
These thin films, designed to act as “charge-transfer mediators”,
were used to study relationships between molecular structure, electron-transfer
(ET) kinetics, and electronic structure. Molecular orientation was
assessed using polarized attenuated total reflectance (ATR) spectroscopy;
the average tilt angle between the PDI molecular axis and the ITO
surface normal for both SA films was about 30°, while films deposited
using spin-coating were more in-plane, with an average tilt angle
of 45°. To our knowledge, these are the first reported measurements
of orientation in PDI monolayers on ITO electrodes. Electrochemical
and ultraviolet photoemission spectroscopy studies showed that all
three PDI–PA films have similar reduction potentials, electron
affinities, and ionization energies, indicating that differences in
bridge length and molecular orientation did not measurably affect
the interfacial electronic structure. ET rate constants ranging from
5 to 50 × 103 s–1 were measured
using potential-modulated ATR spectroscopy. The kinetic and thermodynamic
data, along with a photoelectrochemical comparison of electron injection
efficiency, show that PDI–PA films are capable of serving as
a charge-transfer mediator between an ITO electrode and an organic
active layer, and thus have potential for use as electron-collection
contacts in inverted OPV devices.