Influence of Molecular Aggregation on Electron Transfer at the Perylene Diimide/Indium-Tin Oxide Interface

Abstract

Chemisorption of an organic monolayer to tune the surface properties of a transparent conductive oxide (TCO) electrode can improve the performance of organic electronic devices that rely on efficient charge transfer between an organic active layer and a TCO contact. Here a series of perylene diimides (PDIs) was synthesized and used to study relationships between monolayer structure/properties and electron transfer kinetics at PDI-modified indium tin oxide (ITO) electrodes. In these PDI molecules, one of the imide substituents is a benzene ring bearing a phosphonic acid (PA) and the other is a bulky aryl group that is twisted out of the plane of the PDI core. The size of the bulky aryl group and the substitution of the benzene ring bearing the PA were both varied which altered the extent of aggregation when these molecules were absorbed as monolayer films (MLs) on ITO, as revealed by both attenuated total reflectance (ATR) and total internal reflection fluorescence (TIRF) spectra. Polarized ATR measurements indicate that in these MLs, the long axis of the PDI core is tilted at an angle of 33˚-42˚ relative to the surface normal; the tilt angle increased as the degree of bulky substitution increased. Rate constants for electron transfer (k s,opt ) between these redox-active modifiers and ITO were determined by potential-modulated ATR spectroscopy. As the degree of PDI aggregation was reduced, k s,opt declined which is attributed to a reduction in the lateral electron self-exchange rate between adsorbed PDI molecules, as well as the heterogeneous conductivity of the ITO electrode surface. Photoelectrochemical measurements using a dissolved aluminum phthalocyanine as an electron donor showed that ITO modified with any of these PDIs is a more effective electron-collecting electrode than bare ITO.

Characterization of Charge-Transfer Kinetics at Organic/Electrode Interfaces Using Potential-modulated Attenuated Total Reflectance (PM-ATR) Spectroscopy

“…This finding is supported by complementary measurements, including electroactive PDI surface coverage, PDI tilt angle, and total internal reflection fluorescence spectra. 17 The ks values measured by PM-ATR show that faster ET kinetics are correlated with a higher degree of PDI aggregation (Table 3). This trend is hypothesized to be due to an enhanced rate of intermolecular electron self-exchange between surfacebound PDI • -and PDI species.…”

“…16,17 Figure 5 shows the PDI-PA modifiers used in their work; the key structural variables are the length of the bridge between the PDI and the PA, to modulate the ET tunneling distance, and the degree of substitution, to tune the extent of PDI-PDI intermolecular interactions. In their initial study, 16 different deposition techniques (solution adsorption (SA) and spin coating (SC)) were used to deposit PDI-phenyl-PA and PDI-diphenyl-PA to create three different types of PDI monolayers: SA PDI-phenyl-PA, SA PDI-diphenyl-PA and SC PDI-phenyl-PA. Polarized ATR UV-vis measurements show that the different deposition methods produce films with different orientations.…”

“…17 A series of PDI-PA molecules, (phenyl)2-PDI-PA, (terphenyl)2-PDI-PA, and (terphenyl)2-PDI-IP-PA (Fig. 5), was designed to have different degrees of aggregation when deposited as monolayers on ITO, based on differences in the structure and size of the substituents at the imide positions.…”

“…2,4 Combining potential-modulation with the ATR geometry enables the measurement of charge-transfer kinetics of monolayer to submonolayer thin films on waveguide electrodes, with selectivity to molecular subpopulations in the film provided by appropriate choice of the applied potential and the wavelength and polarization of light, as described further below. This method has been applied to study a wide variety of electrode-supported materials, including small molecule and polymeric semiconductors, [13][14][15][16][17] proteins, 18 inorganic complexes, 19 and semiconductor nanocrystals.…”

Axially Bound Ruthenium Phthalocyanine Monolayers on Indium Tin Oxide: Structure, Energetics, and Charge Transfer Properties

“…A redox-active modifier with a reduction potential that is matched to the frontier energy levels of the donor material may serve as a charge transfer “bridging” layer at the ITO/donor interface in an OPV. ,− , In addition to the electroactive moiety, a number of structural variables are available in the design of a redox-active modifier, such as the type of anchoring group, the composition and length of the linker, and the presence of peripheral solubilizing groups . These variables in turn control molecular orientation, degree of aggregation, interface dipole magnitude and orientation, electronic coupling between the redox center and the electrode, tunneling distance, and so on; all of these are predicted to mediate charge collection kinetics and efficiency at the ITO/donor interface. ,,,− …”