Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics

Abstract

Efficient charge harvesting at electrodes is critical for the effective performance of organic photovoltaics and is strongly influenced by the first molecular monolayer at the transparent conducting oxide electrode. Herein, we present a study of the relationship between molecular orientation and tunneling distance on charge-transfer rates between a tethered sub-monolayer/monolayer of phosphonic acid (PA)-functionalized Zn phthalocyanines (ZnPcs) and an indium tin oxide (ITO) surface by a combination of electrochemical techniques and waveguide tools such as attenuated total reflectance (ATR) UV–vis spectroscopy and potential-modulated ATR (PM-ATR) spectroelectrochemistry. The distance between the main chromophore and PA anchoring group was modulated by an aliphatic carbon of various lengths (n = 3, 9, 10, or 11) that resulted in an extended distance of 7–18 Å between the ZnPcs and the ITO surface. Modified ITO surfaces were composed of monomeric and aggregated subpopulations of ZnPcs with molecular orientations predominantly in-plane (36–39°) and out-of-plane (72–75°), respectively. Charge-transfer rate constants (ks,opt) were measured using PM-ATR. For a given tether length, the aggregated subpopulations exhibit higher ks,opt values compared to the monomeric subpopulations. The observed ks,opt values had an exponential dependence on the effective tunneling distance with a decay constant (β) that ranged from 0.32 to 0.47 Å–1, depending on the chromophore orientation and aggregation state. The fastest charge-transfer rate constants were found for the chromophores with the smallest tunneling distance (n = 3). A ks,opt of 3.9 × 104 s–1 represents the fastest rate constant measured by PM-ATR for a PA-functionalized ZnPc chromophore tethered to an ITO waveguide electrode.