Surface modification of indium-tin oxide with functionalized perylene diimides: Characterization of orientation, electron-transfer kinetics and electronic structure

Charge-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.