Mitigation of Surface Aggregation in Modified Phthalocyanines as Potential Photo Sensitizers
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Mitigation of Surface Aggregation in Modified Phthalocyanines as Potential Photo Sensitizers Rory J. Vander Valk, Patrick J. Dwyer, and Stephen P. Kelty Center for Computational Research, Department of Chemistry and Biochemistry, Seton Hall University, 400 South Orange Ave, South Orange, NJ 07079, U.S.A. ABSTRACT Important to the development of dye-sensitized solar cells is the longevity and photoconversion efficiency of the dye. To improve cost effectiveness, dyes of superior thermal and chemical stability are desirable to extend device performance. In this study, we examine a series of peripherally fluorinated Zinc-Phthalocyanines (FxZnPc). Introduction of chemically inert fluorine and isopropyl fluoroalkyl groups on the periphery of the Pc improve the dye stability and allow for tunable photo-physical properties. Additionally, introduction of the bulky isopropyl fluoroalkyl groups help mitigate molecular aggregation in thin films which is known to be detrimental to maintaining the desired photo-physical properties of the surface coating. Using molecular dynamics and first principles modeling, various substituent effects on surface adhesion and aggregation over TiO2 surfaces are characterized for both symmetric and asymmetric substitution.
INTRODUCTION In the pursuit of highly efficient photosensitive devices, many properties are desired such as long term stability, photo-conversion efficiency, and cost. Device performance can degrade significantly if these properties are not properly shielded from the environment or from material failure during operation. Device stability is particularly important in the design of organic photovoltaic devices and dye-sensitized solar cells (DSSCs), where the use of highly conjugated molecules leaves the devices susceptible to oxidation, photo-degradation and other undesirable reactions.1 The molecular degradation of organic materials can be improved by removing weak, labile bonds where available which can significantly improve thermal and chemical stability. Porphyrins and phthalocyanines have long been considered ideal as photo-receptors for DSSCs owing to their larger molar absorbtivities.2 The ability to form complexes with numerous metals allows the optical and electronic properties of the molecule to be tuned to absorb desired wavelengths or shift the electron affinity. This property makes them an ideal candidate for engineering charge generation in photo-sensitive devices, particularly as sensitizers for DSSCs. Unfortunately, peripheral hydrogen atoms on the Pc are highly reactive in the photo-excited state allowing for degradation of the parent molecule.3 To avoid this shortcoming, we examine peripherally substituted fluorine bonds. Fluorinated phthalocyanines offer the benefit of aforementioned tunable optical properties with improved stability offered by C-F bonds. It is these properties that lead us to study their efficacy as sensitizers for DSSC applications. The binding mode between the dye and the semiconductor surface is frequently facilitated though a linker moiet
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