Philosophiae Doctor - PhD / The continuous increase in energy consumption and decrease in fossil fuels reserves are a primary concern worldwide; especially for South Africa. Therefore, there is an urgent need for alternative energy resources that will be sustainable, and environmentally friendly in order to tackle the ecological degradation generated by the use of fossil fuels. Among many energy ‘niches’, solar energy appears to be one of the most promising and reliable for the African continent because of the constant availability of sun light. Organic conjugated polymers have been identified as suitable materials to ensure proper design and fabrication of flexible, easy to process and cost-effective solar cells. Their tendency to exhibit good semiconducting properties and their capability to absorb photons from the sunlight and convert it into electrical energy are important features that justify their use in organic photovoltaic cells. Many different polymers have been investigated as either electron donating or electron accepting materials. Among them, poly(3-hexylthiophene) is one of the best electron donor materials that have been used in organic photovoltaic cells. It is a good light absorber and its Highest Occupied Molecular Orbital (HOMO) energy level is suitable to allow electron transfer into an appropriate electron acceptor. On the other hand, the molecular ordering found in dendrimers attracted some interest in the field of photovoltaics as this feature can ensure a constant flow of charges. In this work, I hereby report for the first time, the chemical synthesis of a highly crystalline dendritic star copolymer generation 1 poly(propylene thiophenoimine)-co-poly(3-hexylthiophene) (G1PPT-co-P3HT) with high molecular weight and investigate its application as donating material in bulk heterojunction organic photovoltaics.

No description available.

In the last decade, organic photovoltaics (OPVs) have been attracting much attention for their low cost, and feasibility of mass production in large-area modules. Reported power conversion efficiencies (PCE) of organic solar cells have reached more than 10%. These promising PCEs can be realized by uncovering important principles: (1) rational molecular design, (2) matching of the material energy level, (3) favorable morphology of donor-acceptor (D/A) network, (4) higher carrier mobilities, and (5) suppression of charge recombination within the bulk heterojunction (BHJ). Though these key properties are frequently stated, the relationships between these principles remain unclear, which motivates us to fill these gaps. In the beginning, we show that changing the sequence of donor and acceptor units of the benzodithiophene-core (BDT) SM donors critically impacts molecular packing and charge transport in BHJ solar cells. Moreover, we find out that by adding small amount of the external solvent additive, the domain size of the SMFQ1 become relatively smaller, resulting in the FF enhancement of ~70% and thus pushing PCE to >6.5%. To further improve the device performance, we utilize another technique of device optimization: Solvent Vapor Annealing (SVA). Compared with solvent additive, the SVA creates a solvent-saturated environment for SMs to re-arrange and crystalize, leading to PCE of >8%, with nearly-free bimolecular recombination. When the systems are shifted from fullerene acceptors to nonfullerene acceptors, using solvent additives in indacenodithiophene-core (IDT) systems significantly reduces the domain size from >500nm to <50nm and also allows the SM donors to orderly packed, rising the PCE from <1% to 4.5%. Furthermore in a similar IDT-based system, it shows unexpectedly high VOC and low energy loss, and high PCE > 6% can be reached by employing the dimethyl disulfide (DMDS) as the SVA solvent to re-organize the morphology from excessive mixing to ordered phase-separated D/A network. Lastly, taking advantage of the distinct and complementary absorption of fullerene and nonfullerene acceptors, we show that the SM ternary system successfully realizes the high PCE of 11%, good air stability, and scalable property.

This thesis discusses two different photovoltaic systems, organic solar cells, and photonic power converters. The open-source software package Solcore was used to simulate and analyze optoelectronic properties of both systems. It is widely accepted that the transition from a fossil-fuel driven economy is necessary in the coming future. Organic solar cells are an alternative energy generation method with potential for fast energetic and economic payback periods. Bulk heterojunction organic solar cells are a common design, as they have particularly low manufacturing costs due to a simple device architecture. In this work, two bulk heterojunction blends are experimentally assessed using the acceptor molecule silicon phthalocyanine (bis(tri-n-butyl silyl oxide) silicon phthalocyanine ((3BS)2-SiPc) as a potential low-cost non-fullerene alternative to the typical acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM). These acceptors are compared within blends with the typical donor compound poly(3-hexylthiophene) (P3HT), and also poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo [1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1′,3′-di-2- thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c’]dithiophene-4,8-dione)] (PBDB-T). Device performance was assessed under standard conditions, increased angles of incidence, and reduced light intensities. Devices with the P3HT:(3BS)2-SiPc blend achieved a power conversion efficiency (PCE) of 3.6%, which outperformed P3HT:PC₆₁BM devices with a PCE of 3.0% due to a higher open-circuit voltage (VOC) of 0.76 V as opposed to 0.53 V. The PBDB-T:(3BS)2-SiPc achieved a high VOC of 1.09 V, but had a lower PCE of 3.4% in relation to the PBDB-T:PC₆₁BM device with a PCE of 6.4% and a VOC of 0.78 V. Photonic power converters are devices in optical networks that allow for optical power transmission rather than the conventional method of electrical power transmission. This provides benefits such as electrical isolation and resistance to electromagnetic interference, along with the ability to propagate along the same cable as data. These power converters are used to convert optical power to electrical power, and operate similarly to a solar cell with a narrow bandwidth. Multijunction designs are often used for increased operating voltage and efficiency. In such designs employing a vertical architecture, the bottom-most junction has the largest thickness along with the lowest efficiency due to increased recombination losses. To improve this lower efficiency, light trapping techniques can be employed to decrease the junction thickness while retaining the optical thickness. In this work, a current-matched 5- junction GaAs photonic power converter was simulated with both metallic and distributed Bragg reflectors at the rear of the device. These reflectors allowed for the thinning of the bottommost junction, which resulted in an increase in efficiency and overall power output of the power converter.

No description available.

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.

Les dispositifs optoélectroniques organiques possèdent plusieurs avantages pour les applications dans le domaine du biomédical. Le photodétecteur organique (OPD) est un type de dispositif optoélectronique qui n’est pas encore utilisé pour la détection d’activité cérébrale. L’objectif de cette thèse a été d’explorer l’utilisation des OPD, constitués de différent matériaux donneur-accepteur d’électrons, dans le domaine des neurosciences. Nous avons présenté différent types d’OPD possédant une structure minimale, une excellente sensibilité et un grand potentiel d’intégration dans les méthodes de microfabrication existantes. Les détecteurs organiques ont été utilisés pour l’enregistrement de signaux optiques intrinsèques et de signaux fluorescents reflétant l’activité du calcium dans le cerveau. De plus, un autre aspect des OPD est présenté (en combinaison avec les transistors électrochimiques organiques (OECT)) : des systèmes électroniques biomimétiques basé sur une architecture électronique neuro-inspirée. Cette thèse démontre le potentiel des OPD pour enregistrer des activités cérébrales. Elle ouvre une nouvelle perspective, grâce à leur grande sensibilité, comme capteur optique en combinaison avec des dispositifs neuronaux implantables. Ceci élargira les frontières de l’électrophysiologie optique pour explorer les mécanismes complexes du cerveau et des maladies neurodégénératives. / Organic optoelectronic devices have many promising qualities for biomedical applications. Organic photodetectors (OPD), one type of such devices, have yet to be utilized for the detection of signals in the brain, to the best of our knowledge. The goal of this thesis was to explore the use of OPDs, based on different electron-donor and -acceptor materials in neuroscience applications. Different types of minimal-structure OPDs are presented, which have an excellent sensitivity and a high potential for incorporation into existing microfabrication methods. The organic sensors were utilized for monitoring the brain’s intrinsic optical signals and fluorescent calcium dynamics. Additionally, another aspect of these devices is presented (in combination with organic electrochemical transistors (OECT)): neuroinspired electronics, electronics that mimic biology. This thesis establishes the promise of OPDs for monitoring brain activities, which would lead to their integration, as high-sensitive micron-scale optical sensors in organic neural probes. Such device would result in exploring optical biological activities in the deep brain on the cellular level and would push the frontiers of optical-electrophysiology by giving a better understanding of complex mechanisms of the brain function and neurodegenerative diseases.

Le projet de cette thèse consistait à élaborer de nouveaux matériaux donneurs d’électrons pour les cellules solaires organiques. Cette technologie photovoltaïque émergente en plein essor a d’ores et déjà atteint la limite d’efficacité lui permettant d’être industrialisée et commercialisée à grande échelle. Le faible coût de production des dispositifs photovoltaïques organiques les rendent compétitives vis-à-vis des technologies inorganiques déjà bien implantées. Mais leur plus gros avantage est surement leur légèreté et leurs propriétés mécaniques qui les rendent très souples. Elles devraient donc certainement avoir un rôle majeur à jouer dans le futur en complément des cellules solaires classiques, avec une utilisation pour des applications spécifiques. Nous avons ainsi développé des polymères en utilisant des chromophores réputés pour leurs propriétés photophysiques : les porphyrines, les BODIPY et les dicétopyrrolopyrroles. Ces différentes unités absorbent intensément la lumière, ce qui les rend adéquates pour être utilisées pour la conversion de l’énergie solaire en électricité. En concevant un design original et adapté à cette application, nous avons ainsi obtenu plusieurs nouveaux polymères prometteurs. Nous avons ensuite pu étudier leurs propriétés électrochimiques et électroniques, ainsi que leurs caractéristiques photophysiques. Pour cela nous avons utilisé de nombreux outils (caméra streak, absorption transitoire femtoseconde, etc.) afin de comprendre en détails leur propriétés d’absorption et de luminescence. Ces informations nous ont permis de pouvoir ensuite comprendre leur comportement une fois intégrés dans la couche active des dispositifs photovoltaïques. En effet, le mécanisme de fonctionnement pour la création d’un courant électrique met en jeu des transferts d’électrons ultrarapides (∼50 fs) vers un accepteur d’électron. Il est alors crucial de pouvoir comprendre et contrôler les paramètres pouvant influencer l’efficacité de ces transferts et la stabilisation des charges qui en résultent, pour pouvoir finalement mener à des rendements de conversion de l’énergie lumineuse élevés.

Um dos grandes desafios do século XXI está na produção de energia limpa e renovável, já que a demanda mundial por energia continuará crescendo, assim como a necessidade de despoluir o planeta e de diminuir a emissão dos gases do efeito estufa. Nesse contexto, a conversão de energia solar em elétrica coloca-se como uma excelente alternativa, e com isso a dos dispositivos fotovoltaicos. A tecnologia fotovoltaica baseada no silício e em outros semicondutores orgânicos encontra-se em estágio relativamente avançado, porém o custo de produção e de manutenção a proíbe em uso de grande escala. Mais recentemente, iniciaram-se pesquisas com filmes de semicondutores orgânicos, e a rápida melhora na performance dessas células solares a coloca como promissora ao mercado fotovoltaico. Em nosso trabalho, realizamos estudos sobre a performance de dispositivos fotovoltaicos orgânicos baseados na estrutura de heterojunção, estudando a influência de vários parâmetros na performance dos dispositivos. Usamos como camada ativa para nossos dispositivos o poli(3-hexiltiofeno) (P3HT) regiorregular, que é um polímero condutor de gap eletrônico em torno de 1,8 eV misturado ao [6,6]-fenil-C61-ácido butírico-metil ester (PCBM). Essa mistura é apropriada à dissociação dos éxcitons gerados nas cadeias poliméricas pelos fótons absorvidos porque, sendo o PCBM muito eletronegativo, ele captura o elétron do éxciton antes do processo natural de recombinação. Como esse fenômeno ocorre em todo o volume da camada ativa, o dispositivo leva o nome de heterojunção de volume. A estrutura básica que usamos foi de ITO/P3HT-PCBM/Al, isto é, o ITO como eletrodo transparente e bom injetor de buracos e o alumínio como eletrodo injetor de elétrons. Outros dispositivos foram feitos adicionando uma camada transportadora de buracos entre o ITO e o polímero ativo, o Poli(3,4-etileno dióxido-tiofeno):poliestireno-sulfonado (PEDOT:PSS) e/ou cálcio (Ca) entre a camada de alumínio e o polímero. Verificamos que a performance do dispositivo fotovoltaico é bastante alterada quando mediante o contato utilizado, a espessura da camada ativa e a temperatura em que o tratamento térmico é realizado. Investigou-se também, os mecanismos de injeção, transporte e geração de portadores sob variação de temperatura, no intervalo de 90 à 330K. Foi mostrado que, mediante a variação da temperatura, a corrente de curto circuito (JSC), é governada principalmente pela mobilidade dos portadores. A eficiência dos dispositivos desenvolvidos neste trabalho é comparável aos principais valores obtidos na atualidade. Para obtenção destes resultados, foi necessária intensa pesquisa em processamento, principalmente mantendo todas as etapas de fabricação em atmosfera controlada. / One big challenge of the humanity along the 21st Century is to produce energy based on clean and renewable sources. The energy consumption certainly will increase, as well as the necessity in decreasing the emission of greenhouse gases. In this context, solar energy becomes an important alternative for the production of electric energy, in particular, that of photovoltaic devices. Photovoltaics made of silicon and of other inorganic semiconductors are already available, but due to the high cost is not an alternative to produce energy in a large scale. More recently, the organic photovoltaics, due to their quick progress, have becoming as promising technology for the solar energy market. In this work, we studied bulk heterojunction organic photovoltaics, varying several parameters and its influence on the device performance. We used regio-regular poli(3-hexylthiophene) (P3HT), that has an electronic gap close to 1.8 eV, mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). PCBM acts in order to dissociate the photogenerated exciton because, being highly electronegative, it captures the electron form the exciton before the recombination process. We used as basic structure the ITO/P3HT-PCBM/Al. ITO as transparent electrode and injector of holes, and aluminum as the electrons injector electrode. In other devices we added a thin layer of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), as hole transport layer and/or calcium (Ca) between the Al and the polymer. We verify that the device performance changes considerably with the insertion of such layers, and with the thickness of the active layer and the annealing treatment. We also investigated phenomena related to injection, generation and transport of charge carriers, in the 90-330 K temperature range. We showed that the temperature is the main factor that governs the short-circuit current (JSC). It is important to remark that our devices exhibited similar efficiency compared to that of the literature.