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Polymer-fullerene blend systems have found relevance and application in a number of fields including organic photovoltaic devices. While synthesizing new materials with desirable electronic properties is essential to designing better photovoltaic devices, it is equally important to understand the complex phase morphology of these blends and its effect on device performance. Consequently, this knowledge could be used to further design new materials and device architecture for more efficient systems. In particular, this dissertation focuses primarily on the morphology in a series of amorphous as well as semi-crystalline polymer-fullerene blend systems both in bulk and thin films and its relation to device performance. Scattering based techniques have been used to determine in-plane and out-of-plane phase morphology. Morphological parameters derived from these studies have shown possible correlation between fullerene segregation and device performance values. The results of these studies have been used to synthesize a thermo-cross-linkable fullerene acceptor having slower diffusion through the polymer, allowing better control of the polymer-fullerene blend morphology. Consequent effects have been studied on device lifetime and thermal stability and have shown significant improvements

Fully conjugated coil-like polymer poly-(3-hexylthiophene) (P3HT) and aromatic heterocyclic copolymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b¡¬]- dithiophene)-alt-4,7-(2,1,3-benzothiadiazole] (PCPDTBT) were applied separately as donors mixed with derivatized carbon fullerence [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) serving as an acceptor. Single layer photovoltaic cells of ITO/ PEDOT:PSS/polymer:PC61BM/LiF/Al were fabricated to study photovoltaic effect of layer thickness, thermal annealing, composition variance, and processing solvent. At a P3HT:PC61BM weight ratio of 1:1, the thermally annealed photovoltaic cells achieved a conversion efficiency (£bp) of 4.58 % from enhanced contact between cathode and active layer. At a PCPDTBT:PC61BM weight ratio of 1:1.25, the best £bp was 2.62 %. The efficiency difference was due to PCPDTBT:PC61BM was highly phase separated preventing the formation of conductive interpenetrating network to facilitate charge transport. Its device fill factor was limited to be 38 %. Under the same spin coating speed, solutions of different PC61BM concentration would yield different spun film thickness leading to large change in conversion efficiency (£bp). At a constant active layer thickness, £bp tended to be stable indicating that £bp was affected more by the layer thickness than by PC61BM concentration. A layer of mixing P3HT: PCPDTBT: PC61BM would expand the absorption range from visible to near infrared. However, an increased PCPDTBT concentration did not help £bp. This is due to charge transport imbalance between P3HT and PCPDTBT leading to an £bp less than those of individual blends with PC61BM. Device £bp was consistently higher for using a solvent with a boiling point higher than polymer glass transition temperature (Tg).

Thesis (Ph. D.)--University of Akron, Dept. of Chemistry, 2009. / "August, 2009." Title from electronic dissertation title page (viewed 9/16/2009). Advisor, David A. Modarelli; Committee members, Matthew Espe, Michael Taschner, Yi Pang, Mukerrem Cakmak; Department Chair, Kim Calvo; Dean of the College, Chand Midha; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

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.

<P> In this thesis, several methods of material integration into organic photovoltaic devices are investigated by fabricating solution processed and vacuum coated devices. Each of these methods is aimed at examining and improving one or more of the four critical factors that determine solar cell efficiency: (1) photovoltage, (2) light absorption, (3) exciton separation, and ( 4) charge collection. To investigate and improve photovoltage, the photovoltaic properties of different M-Phthalocyanine/Fullerene (M-Pc/C60 ) blends are measured and demonstrate an improved open circuit voltage (Voc) using trivalent-metal phthalocyanine. Rubrene is also added to the tl-Pc/C60 cells and shown to systematically increase the Voc. To improve light absorption, two new device structures are developed: the parallel tandem cell and the heteromorphic cell. The parallel tandem cell is demonstrated using both all-vacuum coated materials as well as a combination of vacuum and solution processed materials. Results show definitive and significant current contribution from the near-infrared (NIR) wavelengths, and concomitant increase in photocurrent and power conversion efficiency (PCE). The heteromorphic cell demonstrates the integration of two polymorphs of the same M-Pc, yielding a broader external quantum efficiency (EQE) spectrum in the IR region and an increase in the overall PCE. To investigate exciton separation and charge collection, time of flight photoconductivity studies are performed on varying compositions of solution processed polymer/fullerene films as well as pristine and blended M-Pc:C60 films. Results verify the necessity for balanced carrier transport in blended systems, and t he importance of carrier mobility for achieving high fill factors. Finally, the stability of a relatively new polythiophene (PQT-12) in an organic solar cell is investigated , and shown to significantly increase the device lifetime as compared to the standard P3HT polymer. </p> / Thesis / Doctor of Philosophy (PhD)

Due to entropy and Van der Waals¡¦ interaction, carbon nanotubes tend to aggregate degrading their excellent opto-electronic properties and limiting their applications. Chemical derivatizations were applied to the multi-wall carbon nanotube (MWCNT) by esterificating with different lengths of aliphatic pendants (COOC4H9, COOC10H21, and COOC18H37) to decrease the MWCNT aspect ratio to facilitate its dispersion, and to observe its percolation behavior. FTIR analysis revealed the more relevant absorption peaks of C-H at 2917 cm-1, 2846 cm-1 and C=O at 1733 cm-1 from the derivatization. H1-NMR showed that the aliphatic pendant functionalized MWCNT from the signals of OCH2 at £_ = 3.64 ppm, CH2 at £_ = 1.25 ppm, and CH3 at £_ = 0.88 ppm. Raman scattering indicated that esterification caused the ID/IG absorption peak area ratio to decrease. In applications, the electric conductivity was measured on thin-films of MWCNT:Poly-(3-hexylthiophene) (P3HT) as a function of nanotube content. Accompanied with nanotube doping concentration increased, the electric conductivity parallel to film surface (£m||) could range from an undoped value 1.4¡Ñ10-6 S/cm up to 1.2¡Ñ10-2 S/cm. The conductivity percolation threshold concentration decreased as the MWCNT aspect ratio increased due to the average distance between the nanotubes becoming sufficiently small for charges to hopping through P3HT. By incorporating [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), bulk heterojunction photovoltaic (PV) cells of ITO/PEDOT:PSS/MWCNT:[PC61BM:P3HT]/LiF/Al were fabricated. By varying the ratio of MWCNT to the PC61BM:P3HT (0.8:1) mixtures, the PV cells showed the maximum power conversion efficiency (£bp) close to 4 % with MWCNT-COOC4H9 at a doping concentration of 0.01 wt. %.

Fully conjugated heterocyclic aromatic rod-like polymer poly-p-phenylene- benzobisoxazole (PBO) and coil-like poly-(3-hexylthiophene) (P3HT) were applied as opto-electronically active layer. The two polymers mixed with nano-carbon particles, having excellent optical absorption and electric conductivity, of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) or esterified multi-wall carbon nano-tube (MWNT-COOC10H21) as well as a hole transporting layer of PEDOT:PSS. Photovoltaic (PV) cells of indium-tin-oxide (ITO)/PEDOT:PSS/nano-carbon particle:fully conjugated polymer/Al were fabricated for optical and electrical characterizations. Tri-layered structure of ITO/PEDOT:PSS/PBO/PCBM/Al produced a straight current-voltage relation showing no PV effects. Upon changing the active layer into PCBM doped P3HT layer (PCBM:P3HT), it produced good PV effects suggesting that the doped layer had a penetrating network to facilitate the PV effects. When PCBM or MWNT-COOC10H21 was doped into P3HT, the device PV effects were increased significantly with nano-carbon particle concentration. The direct-current electric conductivity parallel to the film surface (£m¡ü)was increased with the nano-carbon particle concentration. By changing the thickness of hole transporting PEDOT:PSS and of opto-electronically active layers, it was found that when the PEDOT:PSS layer was decreased from 90 nm to 32 nm, there was a slight increase of PV cell efficiency. The active layer of PCBM:P3HT with a thickness of 99 nm had the best optical absorption and charge transport leading to an increase of PV cell efficiency.

Transferring organic photovoltaics (OPV) from the laboratory into economically feasible products, requires the fabrication of modules, a series of connected single cells. During this transition, there is typically a drastic decrease in power conversion efficiency (PCE). This thesis reports on the design, fabrication, and characterization of state-of-the-art, high-performance organic photovoltaic modules with a novel geometry that composed of unit cells with alternating electrical polarities. Such configuration is realized by exclusive patterning of the interlayers and electrodes and avoids patterning of the photoactive layer. With this novel architecture, area losses of photovoltaic module can be significantly reduced compared with the conventional configurations. The processing of this new solar cell module is also compatible with large area processing techniques such as slot-die coating. This thesis reports on 4-cell and 8-cell modules, wherein the measured fill-factors (FF) and PCE of the constituent sub-cells and of the modules are almost identical. The 4-cell module, with a total area of 0.8 cm2, exhibits an open-circuit voltage (VOC) of 3.15 V, a short circuit-current density (JSC) of 2.3 mA/cm2 and a FF of 0.69, yielding a PCE of 5.01%. The 8-cell module, with a total area of 1.6 cm2, exhibits a VOC of 6.39 V, a JSC of 1.2 mA/cm2 and a FF of 0.63, yielding a PCE of 5.06%. Similar PCE values between 4-cell and 8-cell module is a demonstration of scalability of this novel geometry without compromising the efficiency.

Ongoing efforts to improve the efficiency of organic photovoltaic cells emphasize the significance of the architecture of molecular assemblies in thin films, at nanometer and micron length scales, to enhance both exciton diffusion and charge transport, in donor and acceptor layers. Controlled growth of molecules via self-assembly techniques presents new opportunities to develop nano-structured organic thin films for electronic devices. This thesis is focused on controlling the orientation of phthalocyanine molecular assemblies in thin films in order to demonstrate the impact of microscopic control of molecular order on electrical properties and organic solar cell device performance.The studies performed here provide insights into the self-assembling behavior, film morphology, nanoscale electrical conductivity, and photovoltaic properties of a disk-shaped peripherally substituted phthalocyanine (Pc) molecule possessing amide functional groups in the side chains. Amide functionality was integrated in the side chains of this phthalocyanine molecule with the purpose of increasing the intra-columnar interaction through formation of a hydrogen bonding network between molecules, and to guide columnar orientation in a preferred direction via specific surface-molecule interactions. It is realized that molecule-substrate interactions must dominate over molecule-molecule interactions to achieve control over the deposition of molecules in a preferred direction for organic solar cell applications. Microscopic imaging and spectroscopic studies confirm the formation of flat-lying, well ordered, layered phthalocyanine films as anticipated.The remarkable electrical conductivity of the flat-lying phthalocyanine molecules, as studied by Conducting tip Atomic Force Microscopy (C-AFM) provide the impetus for the formation of organic solar cells based on layers of these hydrogen bonding phthalocyanine molecules. The photocurrent from devices that are made with the ordered Pc molecules and disordered Pc molecules as the primary photoactive donor layer, and vacuum deposited C60 as the acceptor material, were evaluated. The results presented here demonstrate the feasibility of increasing the photogenerated current by controlling the molecular organization in the photo active layer.