Spectroscopic Srudies of Model Organic Photovoltaic and Organic Light Emitting Diode Organic-Organic' and Metal-Organic Heterojunctions
AuthorSchalnat, Matthew Craig
AdvisorPemberton, Jeanne E.
Committee ChairPemberton, Jeanne E.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThe purpose of this Dissertation was to present fundamental approaches to expand the general knowledge of the chemistry that occurs at both the organic-organic' (O-O') and the metal-organic (M-O) interfaces in organic optoelectronic devices. In order to simplify the interactions in the initial studies presented herein, simple model molecules that represent the larger, highly conjugated molecules used in device construction were considered.UPS, reductive-desorption electrochemistry, and Raman surface spectroscopy were used to determine monolayer characteristics of thiophenol and pentafluorothiophenol on Ag. Proposed interfacial orientations and molecular spacing of the TP and F5TP were proposed. Benzene and hexafluorobenzene (F6-benzene) were then condensed and forcibly dewet onto the monolayers in an effort to understand the solid-liquid interfacial interactions. Benzene films on alkanethiol (UDT) and perfluorinated thiophenol (F5TP) were prone to rupturing, and spectroscopically appeared to be liquid-like in character, while molecular spacing of TP and adsorbed benzene on unmodified Ag template ordered benzene films. Polycrystalline films of F6-benzene forms at the interfaces of TP and unmodified substrates. F6-benzene induces a reorientation of F5TP molecules, but is subsequently unable to induce long range order. F6-benzene on UDT appears liquid-like. These studies show that fixed molecules can stimulate order or disorder at a molecular heterojunction, which may have profound effects in device efficiency.In an effort to begin to understand the complicated reaction chemistry that occurs at the organic-metal interfaces in optoelectronic devices, thin benzene films were reacted with typical device cathode metals, Ag, Mg, Al, and Ca, and studied using Raman vibrational spectroscopy. Ag and Mg form metal clusters and some adduct formation. Al undergoes an insertion reaction, forming a substituted benzene ring. Ca reacts with benzene to form a phenyl radical, which then decomposes the film into regions of ordered graphitic carbon. The results of these studies are attributed to atomic properties of the metal atoms.
Degree GrantorUniversity of Arizona
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Chemisorption in organic semiconductor systems: Investigation of organic semiconductor-organic semiconductor and organic semiconductor-metal interfaces.Schuerlein, Thomas John (The University of Arizona., 1995)The production of ordered thin films of organic monolayers is of general interest to the surface science community and of specific interest to our laboratory where the understanding of small molecule adsorption has been a long term goal. The production of ordered thin films may simplify the study of the interactions of adsorbate molecules on organic films. The production of ordered layers of aromatic hydrocarbons and dye molecules were performed under a variety of deposition conditions in an ultrahigh vacuum (UHV) environment. These films were studied with several UHV analytical techniques including low energy electron diffraction, photoelectron spectroscopies, thermal program desorption mass spectrometry and visible spectroscopy. The study of several aromatic hydrocarbons revealed that these molecules possess significant mobility on the Cu(100) surface, while adsorbing with their molecular plane parallel to the copper surface. A majority of phthalocyanine (Pc) molecules studied were observed to adsorb in a single packing structure at similar substrate temperatures for divalent metal centers and a slightly higher temperature for trivalent metal centers. Chloroaluminum phthalocyanine was determined to pack in a unique structure at 150°C and adopt the previously observed phthalocyanine structure at 175°C. It was determined that the perylene derivatives 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N, N-dimethyl-3,4,9,10-perylene-bis(carboxylimide) (DMPI) dissociatively interact with a Cu(100) surface while forming an epitaxial overlayer. These two structurally similar molecules were determined to possess different growth modes at the initial stages of growth, which was attributed to their different bulk packing structures. The electronic properties of a Pc-PTCDA heterostructure were investigated via ultraviolet photoelectron spectroscopy. The investigation correctly anticipated the diode behavior of this isotype heterostructure. This study also proved the validity of the electron affinity rule, developed for inorganic structures, for organic systems.
Characterization of organic/organic' and organic/inorganic heterojunctions and their light-absorbing and light-emitting propertiesAnderson, Michele Lynn, 1968- (The University of Arizona., 1997)Increasing the efficiency and durability of organic light-emitting diodes (OLEDs) has attracted attention recently due to their prospective wide-spread use as flat-panel displays. The performance and efficiency of OLEDs is understood to be critically dependent on the quality of the device heterojunctions, and on matching the ionization potentials (IP) and the electron affinities (EA) of the luminescent material (LM) with those of the hole (HTA) and electron (ETA) transport agents, respectively. The color and bandwidth of OLED emission color is thought to reflect the packing of the molecules in the luminescent layer. Finally, materials stability under OLED operating conditions is a significant concern. LM, HTA, and ETA thin films were grown in ultra-high vacuum using the molecular beam epitaxy technique. Thin film structure was determined in situ using reflection high energy electron diffraction (RHEED) and ex situ using UV-Vis spectroscopy. LM, HTA, and ETA occupied frontier orbitals (IP) were characterized by ultraviolet photoelectron spectroscopy (UPS), and their unoccupied frontier orbitals (EA) estimated from UV-Vis and fluorescence spectroscopies in combination with the UPS results. The stability of the molecules toward vacuum deposition was verified by compositional analysis of thin film X-ray photoelectron spectra. The stability of these materials toward redox processes was evaluated by cyclic voltammetry in nonaqueous media. Electrochemical data provide a more accurate estimation of the EA since the energetics for addition of an electron to a neutral molecule can be probed directly. The energetic barriers to charge injection into each layer of the device has been correlated to OLED turn-on voltage, indicating that these measurements may be used to screen potential combinations of materials for OLEDs. The chemical reversibility of LM voltammetry appears to limit the performance and lifetimes of solid-state OLEDs due to degradation of the organic layers. The role of oxygen as an electron trap in OLEDs has also been verified electrochemically. Finally, a more accurate determination of the offset of the occupied energy levels at the interface between two organic layers has been achieved via in situ monitoring of the UPS spectrum during heterojunction formation.
Spectroscopy Investigation of Molecular Processes at Organic/Metal Oxide and Organic/Metal Interfaces in Organic Photovoltaic DevicesSang, Lingzi (The University of Arizona., 2015)The 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.