This thesis presents a universal transfer printing method to introduce a thin layer of interlayer nanoparticle material in the cathode-organic layer interface in organic device. The use of reverse micelles for making nanoparticles restricts the nanoparticles to be directly synthesized on the organic active layer , therefore a transfer printing method using graphene was derived and a characterization method was needed to detect the transfer of nanoparticles in the whole device system. Raman spectroscopy was found to be the best candidate in studying these organic systems. The oxidation behavior and interaction of CVD graphene on Cu with oxygen plasma and mild annealing was monitored closely by a detailed Raman trilogy studies. Raman results also show evidence of graphene oxide successfully transferred to the target organic layer. Raman spectroscopy was further explored to understand all material in the transferred system including the micelles, type of nanoparticles and the organic layer, which then provides valuable insights to the evolution of the different phases of nanoparticle material formed by the reverse micelles technique. Raman was also used to confirm the first-reported formation of the hot-topic perovskites materials in reverse micelles. An extended Raman technique, the unconventional inverted-TERS, was used to detect a monolayer of micelles which was otherwise impossible for a normal Raman setting. The underlying mechanisms of this technique with high-resolution were also proposed. In order to understand and explore the tunability of reverse micelles on nanoparticle synthesis, a study with the pervovskite material was performed. There were evidence of precursors interacting with the pyridine group in the micelles core, which affects nanoparticle formation. The size of nanoparticles is also found to be tunable by using micelles of different block lengths and different solvents. All these findings contribute to future optimization on the nanoparticles to be transfer printed into devices interlayer and ultimately to benefit on the improvement on organic photovoltaics. / Thesis / Doctor of Philosophy (PhD)

In this thesis, the interface between the electron donor polymer and the electron acceptor fullerene in organic photovoltaic devices is studied. Starting from a bilayer system of donor and acceptor materials, the proximity of polymer and fullerene throughout the bulk of the devices is improved by inducing an interdiffusion of the two materials by heating the devices in the vicinity of the glass transition temperature of the polymer. In this manner, a concentration gradient of polymer and fullerene throughout the bulk is created. The proximity of a fullerene within 10 nm of any photoexcitation in the polymer ensures that the efficient charge separation occurs. Measurements of the absorption, photoluminescence, and photocurrent spectra as well as I-V characteristics are used to study the interdiffusion and its influence on the efficiency of the photovoltaic devices. In addition, the film morphology is studied on a microscopic level with transmission electron microscopy and with Auger spectroscopy combined with ion beam milling to create a depth profile of the polymer concentration in the film. Initial studies to induce an interdiffusion were done on poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as the electron donor polymer and the buckminsterfullerene C60 as the electron acceptor. Interdiffused devices show an order of magnitude photoluminescence quenching with concomitant increase in the photocurrents by an order of magnitude. Variation of the polymer layer thickness shows that the photocurrents increase with decreasing thickness down to 70 nm due to charge transport limitation. The choice of layer thickness in organic photovoltaic devices is critical for optimization of the efficiency. The interdiffusion process is also monitored in situ and a permanent increase in photocurrents is observed during the heat treatment. Transmission electron microscopy (TEM) studies on cross sections of the film reveal that C60 interdiffuses into the MEH-PPV bulk in the form of >10 nm clusters. This clustering of C60 is a result of its tendency to crystallize and the low miscibility of C60 in MEH-PPV, leading to strong phase separation. To improve the interdiffusion process, the donor polymer is replaced by poly(3-octylthiophene-2,5-diyl) (P3OT), which has a better miscibility with C60. Again, the photocurrents of the interdiffused devices are improved significantly. A monochromatic power conversion efficiency of 1.5 % is obtained for illumination of 3.8 mW/cm2 at 470 nm. The polymer concentration in unheated and interdiffused films is studied with Auger spectroscopy in combination with ion beam milling. The concentration profile shows a distinct interface between P3OT and C60 in unheated films and a slow rise of the P3OT concentration throughout a large cross-section of the interdiffused film. TEM studies on P3OT/C60 films show that C60 still has some tendency to form clusters. The results of this thesis demonstrate that thermally-controlled interdiffusion is a viable approach for fabrication of efficient photovoltaic devices through nanoscale control of composition and morphology. These results are also used to draw conclusions about the influence of film morphology on the photovoltaic device efficiency and to identify important issues related to materials choice for the interdiffusion process. Prospective variations in materials choice are suggested to achieve better film morphologies. / Ph. D.

An in-depth study of concentration gradients in thermally-interdiffused polymer – fullerene photovoltaic devices, with a focus on thickness and heat treatments, is presented in this thesis. Device performance is improved from the bilayer by the creation of a concentration gradient of the donor and acceptor materials throughout the active layer of the device. Concentration gradients are expected to improve device performance by optimizing the charge transfer, transport and collection processes. This is achieved through heat-induced interdiffusion of the two materials at temperatures above the glass transition temperature of the polymer. Investigation of the poly(3-octylthiophene) (P3OT) – C₆₀ system show a three-fold improvement in the external quantum efficiencies (EQE) as compared with bilayer devices. Auger spectroscopy, combined with argon-ion beam milling, serves to record the concentration depth profile and identify concentration gradients in the device through detection of the sulfur in the P3OT backbone. Concentration gradients are optimized to yield the best devices through a thickness variation study conducted on the P3OT – C₆₀ system for fixed thermal interdiffusion conditions at 118 °C for 5 minutes. An optimum thickness of 40 to 60 nm is obtained for the two materials that yields the ideal morphology of a concentration gradient as recorded by Auger spectroscopy. For such devices, the concentration gradient is seen to extend through the device, ending in a thin layer of pure material at each electrode. A monochromatic power conversion efficiency of 2.05% is obtained for 5.3 mW/cm²⁺ illumination at 470 nm. A brief study is also presented to optimize the concentration gradient profile through variations of the thermal parameters. The dependence of the concentration gradient on the interdiffusion time and temperature is investigated. The merits of heat treatment on the crystallinity of P3OT and the overall device performance are also discussed. It is shown in some case that devices with annealed P3OT layers show almost twice the EQE as non-annealed P3OT layer devices. Potential alternatives for C₆₀ in interdiffused devices with P3OT have been presented. [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a well-investigated acceptor for blend devices, is studied as an acceptor in concentration gradient devices. A method for spin-coating uniform bilayers of P3OT and PCBM, without solution damage to either layer, is presented. A thermal variation study of the interdiffusion conditions on this system indicated higher interdiffusion temperatures and times are preferred for P3OT – PCBM systems. For interdiffusion at 150 °C for ten minutes, EQE values approaching 35 % at 500 nm are obtained. Auger spectroscopy studies on this system yielded the same conclusions about the concentration gradient device morphology that gives optimum device output. 1:1 and 1:2 blends of P3OT – PCBM are also studied. The influence of various thermal treatments on these devices is described. The endohedral fullerene Sc₃N@C₈₀ is introduced as a new acceptor material. The endohedral fullerene consists of Sc₃N cluster enclosed in a C₈₀ cage. An order of magnitude increase is seen in device performance upon sublimation of these molecules on a P3OT layer confirming its effectiveness as an acceptor. Preliminary studies done on this system indicated the need for greater thermal treatment to produce optimum concentration gradients. An in depth study for varying temperatures and times is presented. The best device performance was seen for interdiffusion at 160 °C for 25 minutes. The endohedral fullerene devices also show a long-term deterioration and so best result are presented from a set of devices fabricated within the same time period. The study of these three donor-acceptor systems confirms that the conclusions on the thickness dependence and device performance study conducted for the P3OT – C₆₀ system extend to other acceptors. A model of EQE for varying thicknesses based on absorption in the interdiffused concentration gradient regions is also presented. This model effectively highlights the influence of P3OT layer thickness on the trends observed in the EQE. It did not, however, reproduce the experimental thickness variation results for varying C₆₀ thicknesses. Incorporation of the effects of the electric field intensity distribution is expected to correct for this. Suggestions have been given on how this might be achieved. / Ph. D.

Anthradithiophene (ADT) derivatives have proven to be a front-runner in the world of small molecule semiconductors for organic electronics and optoelectronics. This is mainly due to the improved stability, easy tuning of chemical and physical properties, and impressive device performance that these molecules possess, especially in organic field effect transistors (OFET) and organic photovoltaics (OPV). The second chapter of this dissertation shows that reducing the amount of alkylsilylethynyl groups, used for functionalizing and solubilizing the ADT backbone, does alter the chemical, physical and crystallographic properties of ADTs. These changes offer the opportunity to study and observe different intermolecular interactions as well as monitoring their influence on sulfur scrambling in solid state. Additionally, from the early days ADTs and functionalized ADTs have been synthesized as isomeric mixtures. In chapter three, I demonstrate a new and simple method that can separate the syn and anti isomers of the F-TES-ADT and F-TEG-ADT chromatographically. The effects of isomeric purity on crystal packing and field effect transistor performance were studied extensively. Chapter four of this dissertation reveals a new generation of acceptor (electron poor) ADT derivatives obtained by attaching cyanide as electron withdrawing group (EWG) to the ADT chromophore. An extensive study was conducted on CN-ADT (acceptor) molecules in small molecule (F-TES-ADT) donor/ small molecule (CN-ADT) acceptor binary BHJ blends as well as P3HT/CN-ADT/PCBM ternary BHJ blends. Photophysical studies of the Donor/ acceptor blends (interface, domains, and crystal orientation) were conducted to obtain a better understanding of the film morphology and its effect on solar cell performance. Finally, the last part of the dissertation, Chapter five, focus on studying singlet fission in ADT derivatives, as well as the effect of varying the size of the alkylsilylethynyl functional group (used for solubilizing the ADT backbone) on altering the electronic couplings and how can that potentially affect the singlet fission rate in these molecules. We also tried to inspect the extent of the correlation between long-range order in crystal packing and singlet fission by monitoring singlet fission rate and efficiency for ADT derivatives with different thin film morphologies.

Τα οργανικά φωτοβολταϊκά (OPV) είναι συσκευές που παρουσιάζουν μια ελκυστική λύση για εφαρμογές ηλιακής ενέργειας λόγω του χαμηλού κόστους παραγωγής τους, της μηχανικής ευκαμψίας και τη δυνατότητα παραγωγής συσκευών μεγάλης έκτασης και μικρού βάρους. Οι πιο αποδοτικοί δέκτες ηλεκτρονίων μέχρι σήμερα στα OPVs βασίζονται σε τροποποιημένα φουλερένια. Ωστόσο, χρειάζονται περαιτέρω βελτιώσεις προκειμένου να επιτευχθεί πιο αποτελεσματική μεταφορά των διαχωρισμένων φορέων στα αντίστοιχα ηλεκτρόδια. Προσπάθειες προς αυτή την κατεύθυνση έχουν γίνει, είτε επηρεάζοντας την αναμειξιμότητα μεταξύ του δότη και δέκτη είτε με την ανάπτυξη πιο αποτελεσματικών δοτών ή δεκτών ηλεκτρονίων. Νέα υβριδικά υλικά με βάση το φουλλερένιο έχουν χρησιμοποιηθεί για να επηρεάσουν τις ημιαγώγιμες ιδιότητες των πολυμερών. Δεδομένου ότι οι πολυκινολίνες αποτελούν μια από τις πλέον υποσχόμενες κατηγορίες πολυμερών μεταφοράς φορτίου (οπής ή ηλεκτρονίου) για εφαρμογή σε διάφορες οπτοηλεκτρονικές εφαρμογές, ο συνδυασμός τους με C60 αναμένεται να παράσχει μια λύση για την ενίσχυση των οπτικών, μορφολογικών και ηλεκτρονικών τους ιδιοτήτων. Πρόσφατη έρευνα έχει δείξει ότι η τροποποίηση των πολυκινολινών ώστε να έχουν χαμηλότρες τιμές LUMO θα ενισχύσει τις ιδιότητες τους ως δέκτες ηλεκτρονίων. Στην εργασία αυτή μελετήθηκαν οι ηλεκτρονικές ιδιότητες διαφόρων υλικών που αντιστοιχούν στα διαδοχικά στάδια σύνθεση ενός νέου υβριδικού συμπολυμερικού δέκτη μέσω των φασματοσκοπιών φωτοηλεκτρονίων από ακτίνες-Χ και ακτινοβολία UV (XPS/UPS). Το τελικό υβριδικό πολυμερές που μελετήθηκε είναι η πολυ-πενταφθόροφενυλοκινολίνη η οποία υβριδίστηκε με C60 (P5FQ-C60). Το μονομερές πενταφθόροφενυλοκινολίνη (Ph5FQ), το καθαρό C60 και το υβριδικό μονομερές Ph5FQ-C60 έχουν επίσης μελετηθεί. Επίσης μελετήθηκαν τα υβριδικά συμπολυμερή P3OT-co-P5FQ και P3OT-co-(P5FQ-Ν-C60) με αναλογία 1:10 για χρήση ως δέκτες ηλεκτρονίων με καλύτερη αναμειξιμότητα με τον δότη. Για την φασματοσκοπική έρευνα τα δείγματα αποτέθηκαν σε υποστρώματα Si με χρήση spin coating από διαλύματα τολουολίου, THF ή χλωροφορμίου. Οι μετρήσεις πραγματοποιήθηκαν σε θάλαμο ανάλυσης υπερυψηλού κενού (βασική πίεση 5x10-9 mbar). Από τις μετρήσεις XPS η κορυφή F1s των μη υβριδικών μορίων εμφανίστηκε σε ενέργεια σύνδεσης (BE) 688.3 eV, μια τιμή που αντιστοιχεί σε άτομα φθορίου με δεσμούς C-F. Η ίδια κορυφή μετατοπίζεται σε χαμηλότερες ενέργειες σύνδεσης σε όλα τα υβριδικά υλικά, υποδεικνύοντας την επίδραση των μορίων C60 στο ηλεκτρονιακό νέφος των φθορίων της κινολίνης. Από τα φάσματα UPS μετρήθηκε το υψηλότερο κατειλημμένο μοριακό τροχιακό (HOMO) σε σχέση με το επίπεδο Fermi καθώς και το κατώφλι υψηλών ενεργειών σύνδεσης (HBE) για κάθε υλικό. Από αυτά, υπολογίστηκε το έργο εξόδου τους, ενώ από το άθροισμα του έργου εξόδου και της ενέργειας σύνδεσης του ΗΟΜΟ υπολογίστηκαν οι Ενέργειες Ιονισμού (ΙΡ). Αυτή είναι μια χρήσιμη παράμετρος για τον χαρακτηρισμό των ημιαγώγιμων πολυμερών επειδή αντιστοιχεί στην απόσταση μεταξύ του ΗΟΜΟ και του επιπέδου κενού και σε συνδυασμό με το ενεργειακό χάσμα (Eg) μπορεί να υπολογιστεί η χαμηλότερο μη κατειλημμένο μοριακό τροχιακό (LUMO). Τα αποτελέσματα δείχνουν ότι οι ιδιότητες των ημιαγώγιμων πολυμερών ή μονομερών μπορούν αποτελεσματικά να επηρεαστούν με υβριδοποίηση με χρήση νανοδομών του άνθρακα, που σε αυτή την περίπτωση είναι το C60. / Organic photovoltaic (OPV) devices present an attractive solution for solar energy applications due to their inherently low material costs, mechanical flexibility, and the potential of scalability to large area, light weight, devices. The most efficient electron accepting materials used so far in OPVs are based on modified fullerenes. However, further improvement is needed in order to achieve more efficient transport of the separated charges to the respective electrodes. Attempts to this direction have been made either by influencing the miscibility between the donor and acceptor phases or by the development of more efficient electron donor or electron acceptor materials. New hybrid materials comprising of fullerene can been used to tune the semiconducting properties of polymers. Since polyquinolines are one of the most promising classes of electron-transporting and electron-accepting polymers for use in various optoelectronic applications their combination with C60 is expected to provide a route for the modulation of their optical, morphological as well as their electronic properties. Previous work has shown that the modification of polyquinolines towards lower LUMO values will increase their electron accepting properties. In this work the electronic properties of various materials that correspond to the sequential synthesis steps of a novel hybrid copolymeric acceptor are investigated by x-ray and UV photoelectron spectroscopies (XPS/UPS). The hybrid material under investigation is the newly synthesised poly-perfluorophenylquinoline(P5FQ-C60) hybridised with C60. The perfluorophenylquinoline monomer (P5FQ, Fig1a), C60 on its own and the hybrid P5FQ-C60 are also studied. The hybrid copolymers P3OT-co-P5FQ and P3OT-co- (P5FQ-N-C60) with a ratio of 1:10 are also studied for use as electron acceptors to confer better miscibility with the donor. For the spectroscopic investigation the samples were deposited on Si substrates by spin coating from toluene,THF or chloroform solutions. The measurements were carried out in an ultrahigh vacuum analysis chamber (base pressure 5x10-9 mbar) equipped with a hemispherical electron energy analyzer, a twin anode X-ray source for XPS and a discharge UV lamp for UPS. The XPS F1s photo-peak from the non hybrid samples appeared at binding energy (BE) 688.3 eV, a value that corresponds to fluorine atoms in C-F bonds. The same peak was shifted to lower binding energy in the case of all hybrid materials. Despite the fact that the F1s peak has a measurable signal, the C1s component corresponding to C-F bonds (BE=289.4 eV) appeared to be at noise level for all the materials under investigation. This is attributed to the fact that the photoionization cross section of C1s is about four times lower than that for F1s. From the UP spectra the Highest Occupied Molecular Orbital (HOMO) with respect to the Fermi Level and the high binding energy (HBE) cut off can be measured. From the latter the work function of the material is calculated, while the sum of the work function and the binding energy of HOMO correspond to the Ionization Potential (IP). This is a useful parameter for the characterization of semiconducting polymers because it corresponds to the distance between the HOMO and the vacuum level and in combination with band gap (Eg) values can be used for the calculation of the Electron Affinity or in other words the Lowest Unoccupied Molecular Orbital (LUMO) position. The results demonstrate that the semiconducting properties of polymeric or monomeric materials can be effectively tuned by hybridization with carbon based nanostructures, in this case C60.

Les travaux développés au cours de cette thèse comprennent le dessin, modélisation, synthèse et caractérisation de nouveaux matériaux polymérisés à base de fullèrene. Dans le cadre d'une utilisation au sein de cellules photovoltaïques, ces matériaux doivent démontrer propriétés telles que solubilité dans des solvants organiques, miscibilité avec le polymère absorbeur de lumière et stabilité morphologique supérieures à celles rencontrées pour les matériaux déjà en utilisation, tels les (bis-)PC60BM et les dérivés d'indene-C60. Six routes de synthèse différentes ont été étudiées et les propriétés électroniques (énergie de LUMO, électroaffinité, électrophylicité, énergies de réorganisation, intégrale de transfert et mobilité électronique) qui en résultent ont été déterminées par des méthodologies de modélisation numérique. Parmi ces composés modèles, la route dite ATRAP a été retenue pour être synthétisée vue qu'elle a été peu étudiée dans la littérature. Les propriétés physico-chimiques de ces composés synthétisés avec des différentes chaînes latérales ont été déterminées par plusieurs techniques et leur application dans des couches minces au sein des dispositifs photovoltaïques a été mise en place. Quand utilisés comme additifs, ces matériaux présentent un potentiel de stabilisation de la couche de P3HT/PC60BM, vu que la performance des cellules qui l'ont est très peu affectée. Le comportement de ces couches sous traitement thermique a montré un effet de non-stabilisation dont le mécanisme a été, lui aussi, étudié par diverses techniques expérimentales. Finalement, un mécanisme de dépolymérisation induit par lumière et/ou chaleur a été proposé. Selon ceci, la cassure de la liaison chimique entre le monomère et le fullèrene est responsable pour la création des défauts, dépolymérisation, cross-linkings et réarrangement irréversible de la couche mince. Cette cassure peut être soit thermiquement activée, soit par l'état triplet du monomère qui déstabilise cette même liaison. Outre que l'étude de ces composés, la stabilisation de l'interface organique-inorganique en dispositifs photovoltaïques, la stabilisation de la chaîne latérale des polymères conjugués et la relation entre affinité avec oxygène moléculaire et la géométrie de la molécule en question ont été adressées. Ces études parallèles ont été achevées par la proposition de nouveaux matériaux hybrides du type donneur-accepteur dérivés de hexabenzocoronene capables de s'empiler dans des structures similaires à des cristaux liquides discotiques. À partir des connaissances acquises au cours de ce document, deux composés ont été proposés et leurs propriétés électroniques montrent qu'il est possible de dessiner des matériaux qui peuvent être, à la fois, stables et efficaces pour être utilisés dans le domaine du photovoltaïque organique. / The work developed during this thesis include the design, modeling, synthesis and characterization of new polymeric materials based on fullerenes. In the optics of a use within photovoltaic cells, these materials have to present particular properties, among which a good solubility in organic solvents, a good miscibility with the light-absorber polymer as well as a morphological stability superior to those currently used materials, such as (bis-)PC60BM and the derivatives of indene-C60. Six different synthetic routes were studied and the electronic properties (LUMO orbital energy, electroaffinity, electrophilicity, reorganization energy, transfer integral and electron mobility) risen were determined by molecular modeling. Among these routes, the one called ''ATRAP'', not much studied in the literature yet, was finally retained. The physical-chemical properties of the so-synthesized materials, grafted with different lateral chains, were determined by different characterization techniques and their application in thin films for Organic Photovoltaic devices was performed. When used as additives, these materials display a potential of stabilizing the P3HT/PC60BM layer, and this does not influence the performance of the device. After a thermal treatment, this behavior was the opposite of the expected, though: a destabilization of the active layer was noted which mechanism was also studied by several experimental techniques. Finally, a depolymerization mechanism induced by light and/or heat was proposed. Within this process, the cleavage of the monomer-fullerene bond is responsible for creating defects, such as the depolymerization, cross-linkings or irreversible rearrangement of the thin layer. This cleavage can be either thermally activated or induced by the triplet state of the monomer, which also destabilizes this bond. Beyond that, this work was also interested i) to the stabilization of the organic-inorganic interface within photovoltaic devices, ii) to the stabilization of the lateral chain of conjugated polymers, as well as iii) the relation between the geometry of a carbon-based molecule and its reactivity to molecular oxygen. These studies, performed in parallel, drove to the proposition of new donor-acceptor hybrid materials based on hexabenzocoronene, which are capable of stacking over itself to form supramolecular structures similar to discotic liquid crystals. From the conclusions of this document, two products were proposed, which electronic properties reveal that it is possible to design new materials that may be stable and efficient at the same time for application in organic photovoltaics.

To this day, fossil fuels still make up over 80% of the earth’s energy production. Many sources of renewable energy are available, but photovoltaics is the only source with the capacity proven to meet the increasing world energy needs. Third generation devices such as dye-sensitized and organic solar cells have gained much interest due to their cost effectiveness and flexibility but have yet to become commercially viable. Here methods have been studied to improve these devices with the use of Gold nanowire arrays. These additions provide plasmonic and light scattering enhancements in dye-sensitized solar cells. Different TiO2 deposition methods have been studied to protect the gold from the redox couple in the electrolyte. Several novel methods have been undertaken to incorporate gold nanowire arrays in organic solar cells with some success. Structural characterization shows the proposed architecture is achieved, but working devices met suffered from low success rate.

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.

Since the carbon nanotubes were first discovered by Iijima in 1991, CNTs have been the focus of intense research by many groups. Nearly 7000 papers and 700 theses on carbon nanotubes can be found from the eminent journals such as Nature and Science in the last decade. Since carbon nanotubes show impressive mechanical, physical and electronic properties such as high stiffness, high strength, low density, and excellent thermal conductivity, suggesting its role in light-weight high strength material application. A great quantity of important research has evidently been done in this field. The purpose of this thesis research is to investigate the feasibility of MWCNTs for the application of polymer organic photovoltaics, and to study the formed MWCNTs-P3HT polymer nanocomposites properties, which are optical absorption, fluorescence emission, and morphology, as well as the formed photovoltaic device performance. This fundamental research would significantly contribute to the advanced technology development for how to improve the efficiency of the polymer organic photovoltaics.

Solution deposition of thin film photovoltaic materials leads to large variations in the morphological and chemical compositions of the film. In order to improve device functionality, it is important to understand how morphology and chemical composition affects charge generation, separation, and collection. This PhD work will first study bulk methods in order to characterize materials in solution and films. The results are then correlated with microscopy studies examining morphology. Other methods used in this PhD work will directly couple spectra and microscopy. Microscopic regions of such films and devices can be illuminated using scanning confocal microscopy or near-field scanning optical microscopy (NSOM), which allows for one to directly probe regions of the film at or below the optical diffraction limit. By scanning the sample over a fixed laser spot we can simultaneously create image maps of the topographical, electrical and optical properties. This technique, known as laser beam induced current (LBIC) allows one to directly probe a local area of a device with 100-300nm resolution. Along with bulk device efficiency studies, near field and confocal data of inorganic and organic materials are investigated. These include devices fabricated with a blend of P3HT (poly[3-hexylthiophene]) and perylene diimide derivatives, and Cu(InxGa1-x)Se2 [CIGS] nanoparticle devices. Finally, we use a new device architecture, a lateral organic photovoltaic (LOPV) in order to spatially resolve transport in functional organic devices. / text