Spectroscopy Investigation of Molecular Processes at Organic/Metal Oxide and Organic/Metal Interfaces in Organic Photovoltaic Devices
AdvisorPemberton, Jeanne E.
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PublisherThe University of Arizona.
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EmbargoRelease after 12-Aug-2018
AbstractThe purpose of this Dissertation is to investigate the chemistry at interfaces between organic active materials and two electrodes, namely organic metal oxide cathode and metal anode, in organic photovoltaic (OPV) devices. Poor compatibility and energy level mismatch at organic/transparent metal oxide (TCO) interfaces is a long standing challenge which limits interfacial electron transfer efficiency. Phosphonic acid modifiers on TCO surfaces are able to improve interface compatibility and energy alignment. Chapters 3 and 4 in this Dissertation investigate the fundamental formation, quality and orientation of phosphonic acid monolayers on indium-doped zinc oxide (IZO) surfaces, a model TCO. Metal electrode deposition on organic active layer materials is a common last step of OPV device fabrication. Chapters 5-8 in this Dissertation explore possible molecular processes at organic-metal interfaces when metal deposition occurs under ultra-high vacuum conditions. Choosing octylphosphonic acid (OPA), F₁₃-octylphosphonic acid (F₁₃OPA), pentafluorophenyl phosphonic acid (F₅PPA), benzyl phosphonic acid (BnPA), and pentafluorobenzyl phosphonic acid (F₅BnPA) as a representative group of modifiers, Chapter 3 describes polarization modulation-infrared reflectance-absorbance spectroscopy (PM-IRRAS) of binding and molecular orientation on IZO substrates. Considerable variability in molecular orientation and binding type is observed with changes in PA functional group. OPA exhibits partially disordered alkyl chains, but on average, the chain axis is tilted 57° from the surface normal; F13OPA tilts 26° with mostly tridentate binding; the F₅PPA ring orients 72° from the surface normal with a mixture of bidentate and tridentate binding; the BnPA ring orients 59° from normal with a mixture of bidentate and tridentate binding, and the F₅BnPA ring orients 45° from normal with a majority of bidentate with some tridenate binding. These trends are consistent with what has been observed previously for the effects of fluorination on orientation of phosphonic acid modifiers. The results from PM-IRRAS are well correlated with recent results on similar systems from near-edge x-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. Overall, these results indicate that both surface binding geometry and intermolecular interactions play important roles in dictating orientation of PA modifiers on TCO surfaces. This work also establishes PM-IRRAS as a routine method for SAM orientation determination on complex oxide substrates. In addition to orientation studies the effect of PA deposition method on the formation of close-packed, high-quality monolayers is investigated in Chapter 4 for SAMs fabricated by solution deposition, microcontact printing, and spray coating. The solution deposition isotherm for perfluorinated benzylphosphonic acid (F₅BnPA) on IZO is studied using PM-IRRAS at room temperature as a model PA/TCO system. Fast surface adsorption occurs in the first minute; however, well-oriented high-quality SAMs are reached only after ~48 h, presumably through a continual process of molecular adsorption/desorption accompanied by molecular reorientation. Two other rapid, soak-free deposition techniques, microcontact printing and spray coating, are also explored. SAM quality is compared for deposition of phenyl phosphonic acid (PPA), F₁₃-octylphosphonic acid (F₁₃OPA), and perfluorinated benzyl phosphonic acid (F₅BnPA) by solution deposition, microcontact printing and spray coating using PM-IRRAS. In contrast to microcontact printing and spray coating techniques, 48-168 h solution depositions at both room temperature and 70 °C result in contamination- and surface etch-free close-packed monolayers with good reproducibility. SAMs fabricated by microcontact printing and spray coating are much less well ordered.Oligothiophenes are building blocks of the popular organic donor materials polythiophene and P3HT. In Chapters 6 and 7, interfacial reactions of the model thiophene-based oligomers, ɑ-sexithiophene (ɑ-6T) and 2, 2’:5’, 2”-terthiophene (ɑ-3T), with vapor deposited Ag, Al, Mg and Ca are investigated using surface Raman spectroscopy under ultra-high vacuum conditions. Results indicate that Al and Ca cause reduction of ɑ-6T to tetrahydrothiophene and calcium sulfite, respectively, with Al exhibiting less reactivity than Ca. Partial electron donation from the sulfur atom lone pair electrons to vacant Ag and Mg d or p orbitals is observed, inducing formation of polaron states at the interface. Inter-ring C-C bond rotation is also induced by this electron sharing betweenɑ-6T and both Ag and Mg. This unexpected evolution of ɑ-6T interfaces with low work function metals alters the interfacial energetics through the formation of “gap” states which ultimately impact device performance. Vapor deposited Ag forms nanoparticles on the surface and induces considerable surface enhanced Raman scattering (SERS) of the ɑ-3T along with a change in molecular symmetry and formation of Ag-S bonds; no other reaction chemistry is observed. Vapor deposited Al and Ca exhibit chemical reaction withɑ-3T spectrum initiated by metal-to-3T electron sharing. For Al, the resulting product is predominantly amorphous carbon (a-C) through initial radical formation and subsequent decomposition reactions. For Ca, the spectral evidence suggests two pathways: one leading to ɑ-3T polymerization and the other resulting in thiophene ring opening, both initiated by radical formation through Ca-to-ɑ-3T electron transfer. In Chapter 8, metal penetration depth into ɑ-3T and ɑ-6T films is investigated and compared between Ag, Al, Mg and Ca using Raman and X-ray photoelectron spectroscopies. Mg exhibits the greatest penetration with no observable surface metallization on 50 ML (15 nm) OT surfaces. Ag shows moderate penetration and metallization ability with no reaction chemistry when in contact with ɑ-6T. Al and Ca exhibit the least penetration and greatest metallization abilities, possibly due to reaction chemistry occurring between Al (or Ca) and ɑ-6T. Al and Ca both penetrate up to 10-14 nm intoɑ-6T layers. The penetration process for Ca consists of two distinct phases. Ca tends to be more evenly distributed throughout the entire ɑ-6T film and reduce the native ɑ-6T until the composition of the top 5-7 nm of the ɑ-6T film becomes constant; beyond this point, further Ca deposition penetrates and completely reduces ɑ-6T into CaS throughout the entire 10-14 nm thickness. Al atoms are more concentrated within the top 5-7 nm of the film and gradually penetrate deeper into the film. These results reveal significant but varying depths of the impact of deposited metals on OT thin films during physical vapor deposition; these results further reinforce the critical role of interfacial chemistry on organic electronic device performance.
Degree ProgramGraduate College