Electron-Transfer Processes in Zinc Phthalocyanine-Phosphonic Acid Monolayers on ITO: Characterization of Orientation and Charge-Transfer Kinetics by Waveguide Spectroelectrochemistry.
Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered to indium tin oxide (ITO) as a model for the donor/transparent conducting oxide (TCO) interface in organic photovoltaics (OPVs), we demonstrate the relationship between molecular orientation and charge-transfer rates using spectroscopic, electrochemical, and spectroelectrochemical methods. Both monomeric and aggregated forms of the phthalocyanine (Pc) are observed in ZnPcPA monolayers. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the monomeric subpopulation undergoes oxidation/reduction with ks,app = 2 × 10(2) s(-1), independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent charge-transfer rates are observed. For in-plane-oriented Pc aggregates, ks,app = 2 × 10(3) s(-1), whereas for upright Pc aggregates, ks,app = 7 × 10(2) s(-1). The rates for the aggregates are comparable to those required for redox-active interlayer films at the hole-collection contact in organic solar cells.
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Hsiao-Chu
HC
Lin
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Nathan W
NW
Polaske
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Luis E
LE
Oquendo
Matthew
M
Gliboff
Kristina M
KM
Knesting
#Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Road MS69, Menlo Park, California 94025, United States.
Dennis
D
Nordlund
David S
DS
Ginger
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Erin L
EL
Ratcliff
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Brooke M
BM
Beam
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Neal R
NR
Armstrong
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
Dominic V
DV
McGrath
†Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
S Scott
SS
Saavedra
attenuated total reflectance; electron-transfer kinetics; molecular orientation; organic donor layer; organic photovoltaic; solar cell; transparent conductive oxide