Conjugated polymers hold tremendous potential as low-cost, solution processable materials for electronic applications such organic light-emitting diodes and photovoltaics. While the concerted efforts of many research groups have improved the performance of organic electronic devices to near-relevant levels for commercial exploitation over the last decade, the overall performance of organic light-emitting diode and organic photovoltaic devices still lags behind that of their traditional, inorganic counterparts. Realizing the full potential of organic electronics will require a comprehensive, molecular-level understanding of conjugated polymer photophysics. Studying pure, well-defined, and reproducible conjugated polymer materials should enable these efforts; unfortunately, conjugated polymers are typically synthesized by metal-catalyzed step-growth polycondensation reactions that do not allow for rigorous control over polymer molecular weight or molecular weight distribution (i.e., dispersity). Chain-growth syntheses of conjugated polymers would not only allow for precise control over the aforementioned polymer metrics such as molecular weight and dispersity, but could also potentially create new applications by enabling the preparation of more advanced macromolecular structures such as block copolymers and surface grafted polymers. Our efforts toward realizing these goals as well as toward exploiting chain-growth methodologies to better understand fundamental conjugated polymer photophysics and self-assembly will be presented. / text

Donor-acceptor and thiophene based π-conjugated molecules and polymers, along with fullerene derivatives, are extensively used active components in the photoactive layer of organic photovoltaic devices. In this dissertation, we make use of several computational methodologies to investigate structure-property relationships of these organic systems in their molecular forms. We begin with an overview of the field of organic photovoltaics and some of the important problems in organic solar cells that are currently being investigated. This is then followed by a brief review of the electronic-structure methods (e.g. Hartree-Fock theory, Density Functional Theory, and Time-dependent Density Functional Theory) that are employed. We then present the main results of the dissertation. Chapter 3 provides a broad overview on how changes to the donor-acceptor copolymer chemical structure impacts its intrinsic geometric, electronic, and optical properties. Chapter 4 focuses on the characterization of the lowest excited-states and optical absorption spectra in donor-acceptor copolymers. In Chapter 5, we investigate the effects of alkyl side-chain placements in the π-conjugated backbone of oligothiophenes and how that impacts their intramolecular properties as well as the oligomer:fullerene interfacial interactions. Chapter 6 presents our investigation on the role of oligomer:fullerene configuration and reorganization energy on exciton-dissociation and charge-recombination processes. Finally, a synopsis of the work and further considerations are presented in Chapter 7.

The widespread adoption of hybrid organic-inorganic solar cells has been delayed by low performance. Improving performance requires a firm understanding of how to optimize both material composition and processing parameters. In this thesis, we examine processing parameters that include solution composition, annealing temperature, and the rates of spin casting and evaporative coating. We also find that the optimal weight ratio for the active layer of a ZnO:P3HT solar cell is 40 wt. % ZnO. / text

Energetics, charge selectivity and interfacial charge transfer kinetics affect the efficiency of solar electric energy conversion and solar photochemical formation of fuels. The research described herein focuses on understanding and controlling the energetics, charge selectivity, and interfacial charge transfer processes in organic photovoltaics, as well as new generation semiconductor-semiconductor and metal-semiconductor heterostructured nanorods (NRs) as photocatalysts. Waveguide and transmission based spectroelectrochemistries, photoemission spectroscopies, and impedance spectroscopy were used to characterize the frontier orbital energies, charge selectivity and interfacial charge transfer kinetics in heterostructured NRs and organic photovoltaics. CdSe NRs tipped with Au nanoparticles and CdSe seeded CdS NRs tipped with Pt nanoparticles were used to study the effect of compositional asymmetry and catalytic sites on band edge energies of NRs. We used UV photoemission spectroscopy (UPS) and waveguide and transmission-based spectroelectrochemistry of NR monolayers/multilayers on conductive substrates to estimate valence/conduction band energies. Potential-modulated attenuated total reflectance (PM-ATR) spectroscopy was utilized to measure the apparent heterogeneous rate constants of reversible electron injection into NR films on indium tin oxide (ITO). We conclude from these measurements that metal tipping, which is designed to enhance the photocatalytic activity of semiconductor NRs, altered band edge energies and enhanced electronic coupling to conductive substrates, in ways that are predicted to influence their efficiency as photoelectrocatalysts. Monolayers of functionalized phosphonic acid ruthenium phthalocyanines (RuPcPA) tethered to ITO as a model organic photovoltaic donor/electrode interface were studied to understand the aggregation and orientation dependent charge transfer kinetics and energetics of these systems. The effect of surface roughness on the orientation of RuPcPA was theoretically modeled and compared to the experimental results. Electrochemical and spectroelectrochemical studies revealed the presence of only monomeric species on ITO. Impedance spectroscopy (IS) and PM-ATR were used to measure charge transfer rate constants. Further, frontier orbital energies of RuPcPA modified ITO were measured using UPS, and the results indicated favorable energetics for hole collection at the RuPcPA/ITO interface for OPV applications. The effect of "UV-light soaking" on the performance of organic photovoltaic devices employing metal oxide (MO) electron selective interlayers (ESL) was addressed using sputtered zinc oxide (ZnO) ESL films. This study provides a coherent methodology for differentiating between the proposed origins of the s-shaped current-voltage (J-V) responses in the literature for organic photovoltaics using MO ESLs. We use IS and UPS to demonstrate that the energetic barrier for charge extraction at the ZnO/active layer interface leads to the observed s-shape response in OPVs using ZnO ESLs. Furthermore, this study provides clear guidelines for minimizing the s-shaped J-V response and the effect of UV light on the performances of OPV devices using ZnO ESLs. We have developed solution electrochemical protocols to characterize nanometer-scale porosity and electronic properties of both solution-deposited and sputtered ZnO thin films used as interlayers for electron-harvesting contacts in inverted organic solar cells on ITO substrates. These electrochemical experiments were performed in order to evaluate the hole-blocking abilities of these ZnO ESLs as well as their effective "pinhole density," thus demonstrating a strong correlation to their OPV performances. These electrochemical experiments can be used to characterize and optimize ESLs rapidly, before OPV device fabrication.

In this work, two optical devices, organic photovoltaics (OPVs) and optical fibers, are introduced. Each of these devices have performance drawbacks. The major drawbacks of organic photovoltaics is their low absorption rate due to bandgap mismatch with the solar spectrum as well as poor charge carrier mobility and short exciton diffusion length. In order to overcome some of these drawbacks and increase the efficiency of OPVs, we use plasmonic gold nanoparticles (AuNPs). We report 30% increase in the efficiency of bulk-heterojunction OPV after incorporation of 50 nm AuNPs. The optical, electrical, and thermal impacts of AuNPs on the performance of PVs have been investigated experimentally and using Lumerical Solutions and COMSOL Multiphysics® simulation packages. The major contributions of AuNPs is causing near field enhancement and increasing the absorption of the structure by 65%, decreasing the extracted carrier density by quenching the excitons, changing the workfunction of the structure, as well as increasing the temperature of their surrounded medium when exited at their plasmon resonance frequency. Furthermore, one of the challenges in devices made from optical fibers such as wavelength division multiplexing systems, is self-phase modulation (SPM) which is a nonlinear phenomenon. We introduce a novel method to remove the SPM in liquid core optical fibers (LCOF) using nonlinear organic chromophores with a negative third-order susceptibility. The idea of this work is to eliminate the effective nonlinear refractive index that the optical pulses are experiencing while propagating through the LCOF. Further, a novel method is introduced to characterize the third-order optical nonlinear susceptibility of organic chromophores in LCOF system. The presented method is simple, and can be extended to the characterization of other nanoscale particles such as quantum dots and plasmonic metal nanoparticles in solutions. Finally, a convenient method is presented that enables researchers to investigate the mechanisms behind photobleaching of various materials. The photostability of materials is of great importance for their acceptance in commercial systems such as organic photovoltaics, electro-optic (EO) modulators and switches, etc. This method is based on the simultaneous detection of different signals such as second-, and third-harmonic generations as well as two-, and three-photon excitation fluorescence using multi-photon microscopy.

No description available.

Increasing global energy demands and diminishing supplies of conventional fuels are forcing the world to focus more on alternative power sources that are both renewable and ecologically benign. Solar energy is clean, regularly available and can be harvested without sacrificing valuable land space. Due to the associated cost of solar cells, however a very small portion of the world’s energy needs are supplied by the sun. Solution-processable organic photovoltaics (OPVs) offer the promise of lower production costs relative to conventional (silicon) solar cell technology. Solution-processing can be performed using reel-to-reel manufacturing, with printing and coating techniques that are significantly cheaper than current processing methods for inorganic semiconductors. Although OPV efficiency values currently remain inferior to those of conventional solar cells, the rate of improvement is much higher in OPVs than in other solar cell technologies. Recently an efficiency exceeding 10% was reported for organic solar cells. An important difference between organic and conventional solar cells is the charge carrier mobility of the semiconductors, which tends to be relatively low in organic semiconductors. Recent advances in molecular design have led to polymer semiconductor materials that possess hole mobility values similar to that of amorphous silicon. The present study investigates potential improvements in OPV devices that can be achieved through the application of high hole mobility polymer semiconductor donors. Two diketopyrrolopyrrole-based polymers, PDQT and PDBFBT, were selected for the role of electron donor in OPV devices due to their high mobilities and their optimum optical and electrical properties. Optimization of the process parameters was performed using PC61BM as the acceptor. A relatively high quantity of PC61BM (3 - 4 × the weight of the donor) is required in the donor-acceptor blends of both polymers in order to balance the high hole mobility. For these donor-acceptor blends, a solvent system consisting of chloroform/ortho-dichlorobenzene (4:1 v/v) is necessary for proper solubility, and an additive, 1,8-diiodooctane, is required to achieve an acceptable morphology. The main benefit expected from the use of high mobility semiconductors is reduced charge recombination. This was studied in relation to the active layer thickness in standard and inverted OPV devices prepared using PC61BM as the acceptor. Normally the thickness of the active layer is required to be low (~100 nm) due to the poor charge transport mobility of the carriers. In this study, rather consistent power conversion efficiencies were achieved throughout a wide range of active layer thicknesses (~100 nm to ~800 nm). A comparison between standard and inverted device configurations demonstrates that the inverted configuration is more suitable for achieving thicker active layers when a high hole mobility donor is used. This is attributed to the longer hole collection path in the inverted structure, which can benefit from using a high hole mobility material. Increasing the absorption spectra of the donor-acceptor blend was studied by substituting PC71BM for PC61BM. The improved absorption leads to greater charge generation. In PDQT devices, the increase in absorption that is contributed by PC71BM appears to be of greatest benefit when active layers are not very thick. Therefore, when thick active layers (>500 nm) are required, the use of PC61BM is sufficient, in conjunction with a high mobility donor. Finally, an increase in a polymer’s crystallinity can often lead to greater mobility. This can be accomplished through various annealing techniques. The improved crystallinity of PDBFBT that occurs as a result of thermal annealing was studied in OPV applications. Although hole mobility of PDBFBT in the lateral direction improves with thermal annealing, mobility in the vertical direction decreases with increasing temperature. This suggests that the crystallinity of PDBFBT is oriented in the lateral direction as opposed to the vertical direction, thereby directing charge flow horizontal to the surface. With thermal annealing, an optimal amount of PC61BM added to PDBFBT can increase the vertical mobility to fairly high values. Nevertheless, the efficiency of standard and inverted OPV devices decreases with increased annealing temperature. This is attributed to agglomeration of PC61BM that occurs from an increase in annealing temperature. The results of this study demonstrate that thermal annealing is not beneficial for PDBFBT:PC61BM films in OPV applications due to the vertical orientation of devices. All of the studies presented in this work involve the use of high hole mobility polymer semiconductors as donor materials for OPV applications. This work will provide a deeper understanding of the properties required for the development of new semiconductor materials in OPV applications. Furthermore, this work will be very useful for the design of device structures for more feasible manufacturing of large area OPV devices via high speed roll-to-roll printing processes.

Organic photovoltaics (OPVs) that have been studied to date have poor power conversion efficiencies. This dissertation focuses on various molecular designs that could lead to both a fundamental understanding of photoinduced charge separation at a molecular level and also provide a solution to improve bulk properties of organic materials to overcome the poor efficiencies of OPV devices. The effect of molecular architectures on the efficiency of electron transfer, a primary step in OPVs functioning, is evaluated in this work. We have shown that even though dendrimer provides an interesting architecture for efficient electron transfer due to the presence of multiple peripheries around a single core, this architecture leads to trapping of charge at the dendritic core. This results in a decrease in the electron transfer efficiency in solution and also limits the possibility of charge transport to the electron in a photovoltaic device. Non-conjugated polymers containing conductive EDOT units at side chains were also designed and synthesized. The frontier energy levels of these polymers can be easily tuned by changing the conjugation lengths of side chain EDOT oligomers. Moreover, by incorporating crosslinkable units as co-side chains, the absorption bandwidth of these polymers can be manipulated as well.

abstract: This dissertation describes the work on two projects which involves measuring molecular conductance and studying their properties on the nanoscale using various Scanning Tunneling Microscopy (STM) techniques. The first molecule studied was a porphyrin-fullerene moiety known as a molecular Dyad for photovoltaic applications. This project is further divided into two section, the first one involving the characterization of the Dyad monolayers and conductance measurement in the dark. The Dyads are designed to form charge separated states on illumination. The lifetime of the charged states have been measured efficiently but the single-molecule conductance through the molecules have yet to be characterized. The second part of the project describes the set-up of a novel sample stage which enables the study of molecular conductance under illumination. This part also describes the subsequent study of the molecule under illumination and the observation of a unique charge-separated state. It also contains the verification of the presence of this charge-separated using other characterization techniques like transient absorption spectroscopy. The second project described in the dissertation was studying and comparing the predicted rectifying nature of two molecules, identical in every way except for one stereocenter. This project describes the formation of monolayers of the molecule on gold and then studying and analyzing the current-voltage characteristics of the molecules and looking for rectification. Both the molecules proved to be rectifying, one more than the other as predicted by theoretical calculations. / Dissertation/Thesis / Ph.D. Chemistry 2011

En vue de limiter la dégradation par l’humidité et l’oxygène des dispositifs photovoltaïquesorganiques flexibles, les cellules solaires sont encapsulées entre des films barrières aux gaz.Malgré l’importance des procédés d’encapsulation et leur potentiel impact sur les performancesinitiales et lors du vieillissement des dispositifs, ils sont rarement étudiés dans la littérature. Enoutre, plusieurs études de vieillissement sur le terrain ont montré que la détérioration mécaniquelimitait la durée de vie des échantillons bien avant que leur stabilité photochimique ne soit miseen cause. L’adhésion entre les différentes couches composant les cellules est donc un facteurcritique afin d’obtenir des dispositifs flexibles fiables après leur mise en oeuvre et lors de leurutilisation.Dans ce travail, deux procédés d’encapsulation ont été étudiés : la lamination à rouleauxd’un adhésif sensible à la pression et la lamination sous vide d’un thermoplastique. Afin dequantifier l’adhésion de chacune des interfaces comprises dans les échantillons, la technique decaractérisation mécanique par pelage à 180° a été adaptée et ensuite appliquée aux dispositifsflexibles. De plus, des techniques de caractérisation des dispositifs par imagerie non-destructiveont été développées : la cartographie du courant induit par faisceau laser (plus courammentconnu sous l’acronyme anglais « LBIC ») et l’imagerie de luminescence sous excitation optiqueet électrique. Grace à ces techniques, l’hypothèse d’une dégradation mécanique des dispositifsdurant le procédé d’encapsulation par lamination à rouleaux a été émise. Des solutions permettantl’amélioration des interfaces identifiées comme mécaniquement faibles ont été recherchées etensuite évaluées par rapport aux performances photovoltaïques des dispositifs de référence.Les techniques d’imagerie développées précédemment ont également été appliquées durant levieillissement en condition accélérées des cellules encapsulées. Un mécanisme a été proposé,qui permet d’expliquer la localisation spatiale de la dégradation mais également le type dedégradation, optique ou électrique, survenu à chaque étape du vieillissement. / In order to limit the flexible organic photovoltaic devices degradation induced by moisture andoxygen, the solar cells are encapsulated between two gas-barrier films. Despite the importanceof the encapsulation processes and their potential influence on the initial performances as well asduring aging of the devices, they are scarcely described in the literature. Furthermore, severalfield aging studies showed that mechanical degradation could limit the devices lifetime beforetheir photo-chemical stability became an issue. Thus, adhesion between the different layerscomposing the devices is a critical factor in order to develop flexible OPV devices reliable aftertheir manufacturing and during their use.In this work, two encapsulation protocols were studied: the roll-to-roll lamination of apressure sensitive adhesive and the vacuum lamination of a hotmelt thermoplastic. In order toquantify the adhesion strength of every interface comprised in the samples, the 180° peelingtest mechanical characterization was adapted for and then applied to the flexible devices. Inaddition, non-destructive imaging characterization techniques were developed: the laser-beaminduced-current mapping and the luminescence emission imaging under optical and electricalexcitation. Thank to these techniques, the hypothesis of a mechanical degradation occurringduring the roll-to-roll lamination process was made. Answers allowing for the improvement ofthe interfaces identified as weak were searched for and evaluated with respect to the photovoltaicperformances of the reference devices. The imaging techniques previously developed were alsoapplied along the accelerated aging of encapsulated cells. A mechanism was proposed, whichallows one to explain the localization of the degradation but also the failure type, either opticalor electrical, occurring during each aging step.