Novel organic conjugated materials have led to new technologies in the field of flexible electronics, with applications in the area of sensors, field effect transistors, or photovoltaic devices. Several material parameters and properties come into play in these devices, including energy of the frontier molecular orbitals, thin film morphology, and charge transport. These properties can be controlled by the chemistry of organic materials, and through processing conditions. In particular, this dissertation focuses on the isoindigo unit as an electron deficient unit to tune polymer light absorption, charge separation, charge transport in the first part of this dissertation, and morphology control in organic photovoltaic (OPV) devices in a subsequent section. The first part of this dissertation introduces the synthesis and properties of isoindigo-containing polymers as n-type, p-type, or ambipolar semiconductors, and their application in all-polymer or polymer:fullerene blends OPV active layers. It is found that polymers with phenyl linkages along the backbone tend to have broader light absorption than polymers with alternating phenyl-thiophene rings; however, steric hindrance in the former leads to low charge mobilities, and poor device performance. In addition, this section highlights the importance of controlling phase separation in OPV devices by focusing on all-polymer blends, which show large phase separation, and polymer:fullerene blends, where the morphology can be controlled through processing additives generating a two-fold increase in device efficiency. Looking at poly(oligothiophene-isoindigo) polymers as model systems, emphasis is placed on photovoltage losses in these devices due to a decrease in effective energy gap between the polymers and fullerene as the oligothiophene donating strength is increased, as well as explanation of the device parameters through description of morphology as solubility is varied. The second portion of this dissertation focuses on solution properties of polymers and their correlation to thin film morphology. A first study investigates the influence of alkyl side chains on solubility, molecular packing, and phase separation in blends of poly(terthiophene-alt-isoindigo) with fullerenes. Specifically, as side chains are lengthened, solubility is increased, but with limited impact on the blends morphology. On the other hand increased backbone torsion leads to variations in energy levels, polymer packing and large phase separation in blends with fullerenes. These thermodynamic parameters are to put in perspective with the kinetic control of film formation during the coating process. This is discussed in a second study, which looks at the mechanism of thin film formation when processing additives are used. In particular, this study highlights the interactions that provide a driving force for polymer crystallite formation, depending on the mechanism followed when aliphatic and aromatic additives are used. These observations are then used to predict the morphology in spin-coated thin films.

While mixed ionic-electronic conductors (MIECs) show promise in a number of different device structures, their successful application has been inhibited by difficulties with characterization. The simultaneous influence of both ionic and electronic systems often foils attempts to quantify material parameters important for rational device design. In many cases, even general models of MIEC function can prove uncertain or controversial. This dissertation addresses the broader issue of ambiguity in MIEC characterization by exploring near-infrared absorbance as a method of gaining further insight into these systems. In combination with a traditional suite of techniques, this method enables determination of parameters not otherwise accessible. The determination of a concentration-dependant carrier mobility in an MIEC material will be demonstrated, and MIEC conduction in the unipolar regime will be broadly described as a system of electrochemically-supported charge injection. This model will be subsequently expanded to describe an unusual and previously unreported phenomenon of rectification when MIECs are interfaced with otherwise appropriate semiconducting contacts. A model labeled as extracting-electrode space-charge limited current will be described and experimentally demonstrated. Finally, the unique photovoltaic properties of an ionic heterojunction system comprising two MIECs will be examined. The results will be used to gain insight into the role of ionic asymmetry in the behavior of MIEC interfaces. This dissertation contains coauthored, previously published, and unpublished work.

In the first part of the study, power conversion efficiency enhancement of organic solar cells by addition of gold nanorods and gold nanostars into PEDOT: PSS (Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) layer was investigated. Efficiency of each sample set has been characterized by measuring current density-voltage characteristics. The best efficiencies obtained during this study are 2.88 % and 2.54 % by addition of gold nanostars and nanorods, respectively. The increase in PCEs is notable when these values are compared with the ones (1.67 %) obtained with a reference device which is prepared without adding any gold nanoparticles under the same conditions. In the second part of the study, branched gold nanoparticles were succesfully grown directly on different types of surfaces such as glass, silicon wafer, and indium-tin-oxide (ITO) coated glass with a simple solution-based method in order to utilize them for further applications.

The development of organic photovoltaics (OPVs) has attracted a lot of attention due to their potential to create a low cost flexible solar cell platform. In general, an OPV is comprised of a number of layers of thin films that include the electrodes, active layers and barrier films. Thus, with all of the interfaces within OPV devices, the potential for failure exists in numerous locations if adhesion at the interface between layers is inherently low or if a loss of adhesion due to device aging is encountered. To date, few studies have focused on the basic properties of adhesion in organic photovoltaics and its implications on device reliability. In this dissertation, we investigated the adhesion between interfaces for a model multilayer barrier film (SiNx/PMMA) used to encapsulate OPVs. The barrier films were manufactured using plasma enhanced chemical vapor deposition (PECVD) and the interfacial fracture toughness (Gc, J/m2) between the SiNx and PMMA were quantified. The fundamentals of the adhesion at these interfaces and methods to increase the adhesion were investigated. In addition, we investigated the adhesive/cohesive behavior of inverted OPVs with different electrode materials and interface treatments. Inverted OPVs were fabricated incorporating different interface modification techniques to understand their impact on adhesion determined through the interfacial fracture toughness (Gc, J/m2). Overall, the goal of this study is to quantify the adhesion at typical interfaces used in inverted OPVs and barrier films, to understand methods that influence the adhesion, and to determine methods to improve the adhesion for the long term mechanical reliability of OPV devices.

Indium-tin oxide (ITO) is commonly used as the transparent electrode in organic photovoltaic (OPV) devices. ITO's transparent properties come at the expense of less than ideal electrode characteristics arising from insulating over-oxidized surface species. OPVs fabricated on the native ITO surface tend to exhibit poor performance with a high degree of variability from device to device. Aggressive acid etching of the ITO surface removes the majority of the insulating surface species leading to improvements in OPV efficiency with greater reproducibility and increased device to device consistency.Organic light emitting diodes (OLEDs) are planar electroluminescent light sources that naturally couple a portion of their emission into internally reflected modes within the device substrate. Although this coupling property is well known, few attempts have been made to integrate OLEDs as light sources for internal reflection elements. Furthermore, OPVs share the optical coupling properties of OLEDs and therefore can be used as integrated internal reflection detectors. Integrating both an OLED light source and an OPV detector onto the same substrate results in an internal reflection sensing platform that requires no free-space optics, has low power consumption requirements, and can be easily fabricated on substrates occupying an area less than one square inch. In this work we establish a functional prototype design, characterize the fundamental coupling properties, and demonstrate several surface sensing responses of this fully integrated optical sensing platform.The net solar power production from OPVs arises from the interactions between multiple currents through the device. The photocurrent is the only power producing current in the device and understanding the voltage dependent nature of this current is essential in OPV research. Analysis methods of conventional, inorganic photovoltaics do not adequately describe the photocurrent behavior commonly observed in OPVs. OPV analysis is therefore somewhat limited by the methods commonly employed. To improve upon the convention methods we develop a simplified method of OPV photocurrent analysis based on electrochemical methods that accurately describes the voltage dependence of the photocurrent and leads to greater insight into the key parameters involved in solar power production from OPVs.

Understanding the fundamental chemistry of conjugated polymers and fullerenes has been the subject of intense research for the last three decades, with the last ten years seeing increased research toward the application of these materials into functional organic electronic devices such as organic photovoltaic devices (OPVs). This field has seen significant advances is cell efficiency in just the last few years (to >10%), in large part due to the development of new donor and acceptor materials, the fine tuning of fabrication parameters to control material nanostructure, as well as the introduction of new interfacial materials such as ionically functionalized conjugated polymers, also known as conjugated polyelectrolytes (CPEs). This dissertation aims to further understand the fundamental chemistry associated with charge injection processes in CPEs and ionically functionalized fullerenes. The role of ionic functionality on electrochemical, chemical, and interfacial charge injection processes is explored. The results presented demonstrate the use of ionic functionality to control the spatial doping profile of a bilayer structure of anionically and cationically functionalized CPEs to fabricate a p-n junction (Chapter II). The role of ionic functionality on chemical charge injection processes is explored via the reaction of polyacetylene and polythiophene based CPEs with molecular oxygen (Chapters III and IV). The results show the dramatic effect of ionic functionality, as well as the specific role of the counterion, on the photooxidative stability of CPEs. The control of reaction pathway via counterion charge density is also explored (Chapter IV) and shows a continuum of reaction pathways based on the charge density of the counter cation. Finally, the role of ionic functionality on interfacial charge injection processes in a functional OPV is explored using a cationically functionalized fullerene derivative (Chapters V and VI). Cell performance increases due to an increase in open-circuit voltage and substantial reduction in series resistance resulting from the high conductivity of the interfacial fullerene layer. The chemical origin of this high conductivity is explored in Chapter VI and shown to likely be the result of chemical reactions occurring between the counter anion and the fullerene core. This dissertation contains coauthored, previously published and unpublished work.

Organic photovoltaics has emerged as a promising technology for electricity generation. The essential component in an organic solar cell is the bulk heterojunction absorber layer, typically a blend of an electron donor and an electron acceptor. Efforts have been made to design new materials such as donor polymers and novel acceptors to improve the power conversion efficiencies. New fullerene free acceptors providing low cost synthesis routes and tenability of their optoelectronic and electrochemical properties have been designed. Despite the efforts, still not much is known about the photopysical processes in these blends that limit the performance. In this respect, time-resolved spectroscopy such as transient absorption and time-resolved photoluminescence, can provide in-depth insight into the various (photo) physical processes in bulk heterojunction solar cell. In this thesis, PCE10 was used as donor and paired with different non fullerene acceptors. In the first part of this thesis the impact of the core structure (cyclopenta-[2, 1-b:3, 4-b’]dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, abbreviated as CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells is reported. The IDTT-based acceptor achieves power conversion efficiencies of 8.4%, higher than the CDT-based acceptor (5.6%), due to concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10: IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage. In the second part of this thesis, I report the impact of acceptor side chains on the photo-physical processes of BHJ solar cells using three different IDT-based acceptors, namely O-IDTBR, EH-IDTBR and O-IDTBCN blended with PCE10. Power conversion efficiencies as high as 10 % were achieved. The transient absorption spectroscopy experiments provide insight into sub-picosecond exciton dissociation and charge generation which is followed by nanosecond triplet state formation in PCE10:O-DTBR and PCE10:EH-IDTBR blends, while in O-IDTBCN triplets are not observed. Time delayed collection field experiments (TDCF) were performed to address the charge carrier generation and examine its dependence on the electric field.

The study of the new materials potentially usable for organic photovoltaic and electronics are getting very important from the point of ecological and financial view. Organic electronic devices are getting more and more popular and it is only up to us to search for the new ones that are able to improve their physical properties. The aim of this thesis is to search for materials like have been mentioned above which have very good semiconducting properties. Solutions of pure materials and its mixtures with different concentrations of fullerene have been investigated by ultra-violet spectroscopy, classical fluorescence and time resolved spectrometry. Mainly, were studied the influence of the central atom and side substituents for the optical and electronical properties of our materials of interest. With adding fullerene was observed quenching phenomena of the fluorescence, because all these new materials show usually high photoluminescence. Based on the given results, the most suitable materials had been chosen to provide trial of making organic solar cell, and therefore investigated by the mean of electric measurements (direct current).

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