The overall efficiency of organic photovoltaics cells (OPVs) is influenced by the nature of the charge injection barrier at the transparent conducting oxide (TCO) bottom contact. Modification of the transparent conducting oxide (TCO)/organic interface with an electroactive molecular monolayer will potentially create a robust ohmic contact that will influence the efficiency of hole injection into the TCO. Asymmetric zinc Phthalocyanines (ZnPc) with a flexible phosphonic acid (PA) linker have been synthesized and used to modify indium tin oxide (ITO) surfaces. The adsorption of PA functionalized asymmetric ZnPcs on an ITO/waveguide was monitored using attenuated total reflectance (ATR) spectroscopy. Polarized dependent ATR spectroscopy was used to determine the orientation of these absorbed subpopulations species on ITO modified surfaces as a function of wavelength using transverse electric (TE) or transverse magnetic (TM) polarized light. The first oxidation potential on absorbed monolayers was found by cyclic voltammetry to be resolved into two peaks indicative of two electrochemically distinct subpopulations of molecules, atributed to aggregates and monomerics forms of PA functionalized ZnPcs. Potential modulated ATR (PM-ATR) spectroelechtrochemistry was employed to measure the charge transfer rates constants (k(s, app)) at ITO modified surfaces using TE and TM polarized light. Faster charge transfer rate constants were found for molecules with a smaller tunneling distance. A k(s, app) of 3.9 x 104 s−1 represents the fastest rate measured for PA functionalized ZnPc chromophore tethered to an ITO waveguide electrode by PM-ATR. We synthesized and characterized the first examples of PA functionalized RuPcs to investigate the effect of molecular orientation on charge transfer properties at an ITO/organic interface. PA functionalized RuPcs have the ability to coordinate axial ligand to suppress aggregation, providing the flexibility of connecting the anchoring group through the axial position of the metal and allowing chemisorption of the molecule in plane with ITO. Cyclic voltammetry and ATR UV/vis spectroscopy on the modified ITO surface demonstrated a surface composition of a closed-packed monolayer of monomeric species. Measurement of the charge transfer rates constants demonstrated that RuPc anchored to ITO exhibited slow rates compared to corresponding surface bound ZnPcs. Finally, we describe the synthesis and characterization of a new PA functionalized N-pyridinyl perylenediimide (PDI)-RuPc donor-acceptor dyad capable of chemisorption to ITO surfaces as a molecular-level heterojunction system to study photo induced charge separated states. The developed ensemble was proven to be stable on ITO for further study of charge injection events from the dyad to the oxide surface.

Indium tin oxide serve a critical function in many organic devices, such as organic light emitting diodes and organic photovoltaics. To optimize the performances of these devices, it is desirable to tune the interface between the indium tin oxide and the next functional layer of these devices. A common surface modification of transparent conductive oxides is through the use of self-assembled monolayers. This methodology enables a simultaneously tuning of the properties and performance of this interface, including the surface energy, work function and durability of the transparent conductive oxide. Phosphonic acid and silane based monolayers have been extensively studied and used in devices for their ability to tune the interfacial properties of transparent conductive oxide. Herein, alcohol based monolayers are first demonstrated on transparent conductive oxide surfaces. The electrochemical and chemical stabilities of alcohol based monolayers, as well as changes in the optical properties of the Indium tin oxide as a function of their stability were evaluated in comparison to more traditional routes of surface modification, such as through the use of silanes and phosphonic acid based monolayers. The tunability of both work function and surface energy of the modified Indium tin oxide were also determined for assessing their electronic properties.

Phosphonic acids are known to bind strongly to a variety of metal oxide surfaces. Phosphonic acids were designed in order to impart specific properties to the surface of a range of metal oxides upon formation of a monolayer. A large number of novel phosphonic acids were synthesized and fully characterized. The binding of phosphonic acids to the surface of several metal oxides, such as indium tin oxide (ITO) and barium titanate, was studied in detail and determined to be a mixture of bidentate and tridentate binding modes. The modification of several key surface properties of ITO by phosphonic acid modification was also studied. The work function of ITO could be increased or decreased with respect to unmodified ITO by controlling the dipole of phosphonic acids bound to the surface. Additionally, the surface energy could be substantially lowered by attaching phosphonic acids with non-polar terminal functional groups to the ITO surface. The ability to control these surface properties resulted in organic light-emitting diodes (OLEDs) which showed superior lifetimes and stability with respect to OLEDs incorporating ITO without a phosphonic acid monolayer. In addition, the binding of phosphonic acids to a number of other oxides, such as zinc oxide and zeolites, was also studied.

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Photovoltaics manufactured using organic materials as a substitute for inorganic materials may provide for cheaper production of solar cells if their efficiencies can be made comparable to existing technologies. Photovoltaic devices are comprised of layered structures where the electrical, chemical, and physical properties at the multiple interfaces play a significant role in the operation of the completed device. This thesis attempts to establish a relationship between interfacial properties and overall device performance by investigation of both the organic/organic heterojunction interface, as well as the interface between the inorganic substrate and the first organic layer with useful insights towards enhancing the efficiency of organic solar cells. It has been proposed that residual chemical species may act as barriers to charge transfer at the interface between the transparent conductor (TCO) and the first organic layer, possibly causing a large contact resistance and leading to reduced device performance. Previous work has investigated the surface of the TCO but no baseline characterization of carbon-free surfaces has previously been given. In this work clean surfaces are investigated to develop a fundamental understanding of the intrinsic spectra such that further analyses of contaminated surfaces can be presented systematically and reproducibly to develop a chemical model of the TCO surface. The energy level offset at the organic/organic heterojunction has been proposed to relate to the maximum potential achievable for a solar cell under illumination, however, few experimental observations have been made where boththe interface characterization and device performance are presented. Photovoltaic properties are examined in this work with titanyl phthalocyanine used as a novel donor material for enhancement of spectral absorption and optimization of the open-circuit potential. Characterization of the interface between TiOPc and C60 coupled with characterization of the interface between copper phthalocyanine and C60 shows that the higher ionization potential of TiOPc does correlate to greater open circuit potentials. Examination of photovoltaic behavior using equivalent circuit modeling relates the importance of series resistance and recombination to the homogeneity of the solar cell structure.