Nanoparticles present a myriad of physical, optical, electrical, and chemical properties that provide valuable functionality to thin-film technologies. In order to successfully exploit these aspects of nanoparticles, appropriate dispersion and stability measures must be implemented. In this dissertation, different types of nanoparticles are coated with polymer and metallic layers to enable their effectiveness in both anti-reflection coatings (ARCs) and organic photovoltaics (OPVs). Ionic self-assembled multilayers (ISAMs) fabrication of poly(allylamine hydrochloride) (PAH) and silica nanoparticles (SiO2 NPs) results in highly-transparent, porous ARCs. However, the ionic bonding and low contact area between the film constituents lack sufficient mechanical and chemical stability necessary for commercial application. Chemical stability was established in the film by the encapsulation of SiO2 NPs by a photo-crosslinkable polyelectrolyte, diazo-resin (DAR) to make modified silica nanoparticles (MSNPs). UV-irradiation induced decomposition of the diazonium group and the development of covalent bonds with polyanions. Crosslinked MSNP/poly(styrene sulfonate) (PSS) ISAMs exhibited excellent anti-reflectivity (transmittance >98%, reflectance <0.2% in the visible range) and chemical stability against dissolution in a ternary solvent. Mechanical stability was also achieved by the incorporation of two additional PAH and poly(acrylic acid) (PAA) layers to create PAH/PAA/PAH/SiO2 NP interlayer ISAM ARCs. Thermal crosslinking of PAH and PAA facilitates the formation of covalent amide bonds between the two polyelectrolytes, as confirmed by FTIR. Since PAH and PAA are both weak polyelectrolytes, adjustment of the solution pH causes significant variations in the polymer chain charge densities. At low PAA pH, the decreased chain charge densities caused large SiO2 NP encapsulation thicknesses in the film with great mechanical stability, but poor anti-reflection (≤97% transmittance). At high PAA pH, the high chain charge densities induced thin encapsulation layers, insufficient mechanical stability, but excellent anti-reflection. At trade-off between the two extremes was founded at a PAA pH of 5.2 with excellent anti-reflection (less than 99% transmittance) and sufficient mechanical stability. The normal force required for scratch initiation was increased by a factor of seven for films made from a pH of 5.2 compared to those made from a pH of 6.0. Organic photovoltaics (OPVs) are an attractive area of solar cell research due to their inexpensive nature, ease of large-scale fabrication, flexibility, and low-weight. The introduction of the bulk heterojunction greatly improved charge transport and OPV performance by the blending of the active layer electron donor and acceptor materials, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), into an interpenetrating network with high interfacial area between adjacent nanodomains. However, constrained active layer thicknesses restrict the total optical absorption and device performance. The localized surface plasmon resonance (LSPR) of plasmonic nanoparticles, such as anisotropic silver nanoplates (AgNPs), provides large local field enhancements and in coupling with the active layer, substantial optical absorption improvements can be realized. AgNPs were first integrated into the hole-transport layer (PEDOT:PSS) by ISAM deposition. Here, PEDOT:PSS was used as a negatively-charged ISAM layer. Encapsulation of the AgNPs by PAH (ENPs) provided a positive surface charge and allowed for the creation of ENP/PEDOT:PSS ISAMs. Stability against acidic etching by PEDOT:PSS was imparted to the AgNPs by coating the edges with gold (AuAgNPs). The AuAgNP ISAMs substantially improved the optical absorption, but were ineffective at increasing the device performance. The dispersion effects of functionalized polymer coatings on AgNPs were also deeply investigated. Functionalized AgNPs were dispersed in methanol and spin-coated onto the active layer. When the AgNPs possessed hydrophilic properties, such as unfunctionalized or functionalized by poly(ethylene glycol) methyl ether thiol (PEG-SH), they formed large aggregates due to unfavorable interactions with the hydrophobic P3HT:PCBM layer. However, the hydrophobic functionalization of AgNPs with thiol-terminated polystyrene (PS-SH) (PS-AgNPs) resulted in excellent dispersion, optical absorption enhancements, and device performance improvements. At a PS-AgNP concentration of 0.57 nM, the device efficiency was increased by 32% over the reference devices. / Ph. D. / Investigations are presented on the quality of distribution or dispersion of functional inorganic (composed of silicon dioxide or silver) particles that have dimensions of less than 100 nanometers, called nanoparticles. The nanoparticle surfaces were covered with polymer layers, where polymers are organic materials with repeating molecular structures. The study of these nanoparticle distribution effects were first examined in anti-reflection coatings (ARCs). ARCs induce transparency of windows or glasses through a reduction in the reflection of light. Here, the ARCs were fabricated as self-assembled thin-films (films with thicknesses ranging from 1 to 2000 nanometers). The self-assembly process here was carried out by immersing a charged substrate (microscope slide) into a solution with an oppositely-charged material. The attraction of the material to the substrate leads to thin-film growth. The process can continue by sequentially immersing the thin-film into oppositely-charged solutions for a desired number of thin-film layers. This technique is called ionic self-assembled multilayers (ISAMs). ARCs created by ISAM with charged polymers (polyelectrolytes) and silicon dioxide nanoparticles (SiO2 NPs) can lead to highly-transparent films, but unfortunately, they lack the stability and scratch-resistance necessary for commercial applications. In this dissertation, we address the lack of stability in the ISAM ARCs by adding additional polyelectrolyte layers that can develop strong, covalent bonds, while also examining nanoparticle dispersive properties. First, SiO2 NP surfaces were coated in solution with a polyelectrolyte called diazo-resin, which can form covalent bonds by UV-light exposure of the film. After tuning the concentration for the added diazo-resin, the coated SiO2 NPs were used to make ARCs ISAM films. The ARCs had excellent nanoparticle dispersion, high levels of transparency, and chemical stability. Chemically stability entails that the integrity of the film was unaffected by exposure to polar organic solvents or strong polyelectrolytes. In a second method, two additional v polyelectrolyte layers were added into the original polyelectrolyte/SiO2 NP design. Here, heating of the film to 200 oC temperatures induced strong covalent bonding between the polyelectrolytes. Variation of the solution pH dramatically changed the polyelectrolyte thickness, the nanoparticle dispersion, the scratch-resistance, and the anti-reflection. An optimum trade-off was discovered at a pH of 5.2, where the anti-reflection was excellent (amount of transmitted light over 99%), along with a substantially improved scratch-resistance. A change of pH from 6.0 (highest tested pH) to 5.2 (optimal) caused a difference in the scratch-resistance by a factor of seven. In these findings, we introduce stability enhancing properties from films composed purely of polyelectrolytes into nanoparticle-containing ISAM films. We also show that a simple adjustment of solution parameters, such as the pH value, can cause substantial differences in the film properties. Nanoparticle dispersion properties were next investigated in organic photovoltaics (OPVs) OPVs use semiconducting polymers to convert sunlight into usable electricity. They have many advantages over traditional solar cells, including their simple processing, low-cost, flexibility, and lightweight. However, OPVs are limited by their total optical absorption or the amount of light that can potentially be converted to electricity. The addition of plasmonic nanoparticles into an OPV device is a suitable way to increase optical absorption without changing the other device properties. Plasmonic nanoparticles, which are composed of noble metals (such as silver or gold), act as “light antennas” that concentrate incoming light and radiate it around the particle. In this dissertation, we investigate the dispersion and stability effects of polymer or metallic layers on silver nanoplates (AgNPs). The stability of the AgNPs was found to be greatly enhanced by coating the nanoparticle edges with a thin gold layer (AuAgNPs). AuAgNPs could then be introduced into a conductive, acidic layer of the OPVs (PEDOT:PSS) to increase the overall light absorption, which otherwise would be impossible with uncoated AgNPs. Next, the AgNPs were distributed on top of the photoactive layer or the layer that is responsible for absorbing light. Coating the AgNPs with a polystyrene polymer layer (PS-AgNPs) allowed for excellent dispersion on this layer and contrastingly, dispersion of the uncoated AgNPs was poor. An increased amount PS-AgNPs added on top of the photoactive layer progressively increased the optical absorption of the OPV devices. However, trends were quite different for the power conversion efficiency or the ratio of electricity power to sunlight power in the OPV device. The greatest PCE enhancements (27 – 32%) were found at a relatively low coverage level (using a solution concentration of 0.29 to 0.57 nM) of the PS-AgNPs on the photoactive layer.

This thesis describes studies on nanohybrid systems consisting of single-walled carbon nanotubes (SWNTs) with monolayer coatings of semiconducting polymers. Steady-state and time-resolved optical and high-resolution microscopy experiments were used to investigate the blends. These materials show promise for use in organic photovoltaics (OPVs) owing to the high carrier mobilities and large aspect ratios of SWNTs, the controllable solubilisation of tubes with various polymers and the broad light-harvesting abilities of organic polymers. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising carbon nanotubes in OPV devices, revealing poor performances to date. The experimental methods used during the thesis are detailed in Chapter 3 and the solution processing techniques used to prepare the polymer–nanotube blend samples are described in Chapter 4. Chapter 5 describes a study on a nanotube blend with a thiophene polymer, a system previously unsuccessfully implemented into OPV devices. Ultrafast spectroscopic measurements showed that electrons can transfer on a 400 fs time scale from the polymer to nanotubes and the conditions to allow long-lived free charges to be produced were found. The study is extended in Chapter 6 to show that nanostructures consisting of a nanotube coated in one polymer can then be coated by a second polymer and that these nano-engineered structures could be implemented into OPV devices. The use of a competition binding process to isolate purely semiconducting nanotubes dispersed with any desired polymer is then described in Chapter 7. Finally, Chapter 8 introduces systems consisting of chains of porphyrin units, nature’s light-harvesting systems, bound to nanotubes and the blends were found to exhibit the required electronic alignment for use in OPVs. The work described in this thesis provides an explanation for the poor device behaviour of nanotube–polymer blends to date and, in particular, demonstrates several nanohybrid systems that show particular promise for improved OPV applications.

Organic solar cells show great potential to become a commercially available technology for renewable clean energy production due to their attractive properties. Inexpensive materials and manufacturing processes, including classical roll-to-roll fabrication, as well as the ability to produce flexible, low weight, semitransparent devices are some of the advantages organic photovoltaics provide. Addressing the most common issues in these new technologies, i.e., the low efficiencies of devices and rapid degradation of materials, could bring a realistic alternative for the photovoltaic industry. In this work, the performance of P3HT:PCBM based bulk heterojunction solar cells modified through low energy gold ion implantation in the hole transporting layer, the PEDOT:PSS, is studied. Reference solar cells without gold were also fabricated and characterized for comparison. Through field emission scanning electron microscopy (FESEM) micrographs, the formation of gold nanoparticles (AuNPs) in the PEDOT:PSS has been shown layer for the highest implantation doses used. Absorbance measurements of PEDOT:PSS films before and after gold implantation further confirmed this result. TRIDYN and SRIM simulation programs estimated shallow gold implantations of ~3 nm underneath the PEDOT:PSS films surface. Current-voltage (JxV) characteristics of reference solar cells under AM 1.5 illumination presented the uncommon S-shaped curves, an abnormal deviation from typical JxV curves. This was attributed to PEDOT:PSS degradation due to oxygen and water exposure, which reduced its work function significantly. As a result, deteriorated parallel and series resistances were obtained in reference devices, which ultimately reduced their field factors and power conversion efficiencies. This abnormal behavior was consistently eliminated with the introduction of AuNPs near the PEDOT:PSS/Active-layer interface, leading to the rectification of the illuminated JxV curves of modified solar cells and the reestablishment of cell parameters. Consequently, outstanding improvements in the field factors and power conversion efficiencies were observed in these devices. This was attributed to enhancement (and prevention from the reduction) of the PEDOT:PSS work function layer due to the presence of AuNPs, which rearranged the energy levels at the interface to a more favorable state: higher electron blocking and lower hole extraction barriers. / Células solares orgânicas têm mostrado grande potencial para se tornar uma alternativa tecnológica na produção de energia limpa e renovável. Baixo custo dos materiais e dos processos de manufatura, e a possibilidade de fabricar dispositivos com baixo peso, flexibilidade e semitransparência, inclusive pelo método clássico de roll-to-roll, são algumas das vantagens oferecidas pela fotovoltaica orgânica. Resolver os problemas mais comuns destes dispositivos, como a baixa eficiência na conversão de energia e a rápida degradação dos materiais, é necessário para sua disponibilização no mercado fotovoltaico atual. Neste trabalho, células solares de heterojunção volumétrica baseadas no polímero P3HT e modificadas através da implantação de íons de ouro de baixa energia na camada de PEDOT:PSS são estudadas. Dispositivos equivalentes sem modificação de ouro também foram fabricados e caracterizados como referência. Imagens obtidas através de um microscópio eletrônico de varredura por emissão de campo (FESEM Field Emission Scannig Electron Microscopy) mostraram a formação de nanopartículas de ouro (AuNPs) na camada de PEDOT:PSS para as doses de implantação mais elevadas. Medidas do espectro de absorbância dos filmes de PEDOT:PSS antes e depois da implantação de ouro confirmam este resultado. Simulações feitas com os softwares TRIDYN e SRIM estimaram o ouro implantado em u ma profundidade de ~3 nm abaixo da superfície do PEDOT:PSS. As curvas de corrente-tensão (JxV) características das células solares de referência sob iluminação AM 1.5 mostraram um comportamento de forma S, que corresponde a um desvio da forma típica das curvas JxV. Isto foi atribuído à degradação dos filmes de PEDOT:PSS devido à exposição ao oxigênio e à água, que reduz sua função trabalho significativamente. Como resultado, deterioraram-se as resistências em paralelo e em série destes dispositivos, o que em última instância, reduziu o Field Factor (FF) e a eficiência na conversão de energia. Este comportamento anormal foi eliminado de forma consistente após a introdução de AuNPs perto da interface PEDOT:PSS/Camada-Ativa. As curvas JxV das células solares modificadas sob iluminação foram retificadas e os valores dos seus parâmetros restabelecidos. Melhorias notáveis no FF e eficiência de conversão de energia foram obtidas para todas as células solares modificadas. Isto foi atribuído ao aumento da função trabalho da camada de PEDOT:PSS pela presença das AuNPs, que reorganizou os níveis de energia na interface para um estado mais favorável: com barreiras de potencial otimizadas para bloquear a extração de elétrons e favorecer a de buracos.

The understanding of molecular effects in nanoscale environments is becoming increasingly relevant for various emerging fields. These include spectroscopy for molecular identification as well as in finding molecules for energy harvesting. Theoretical quantum chemistry has been increasingly useful to address these phenomena to yield an understanding of these effects. In the first part of this dissertation, we study the chemical effect of surface-enhanced Raman scattering (SERS). We use quantum chemistry simulations to study the metal-molecule interactions present in these systems. We find that the excitations that provide a chemical enhancement contain a mixed contribution from the metal and the molecule. Moreover, using atomistic studies we propose an additional source of enhancement, where a transition metal dopant surface could provide an additional enhancement. We also develop methods to study the electrostatic effects of molecules in metallic environments. We study the importance of image-charge effects, as well as field-bias to molecules interacting with perfect conductors. The atomistic modeling and the electrostatic approximation enable us to study the effects of the metal interacting with the molecule in a complementary fashion, which provides a better understanding of the complex effects present in SERS. In the second part of this dissertation, we present the Harvard Clean Energy project, a high-throughput approach for a large-scale computational screening and design of organic photovoltaic materials. We create molecular libraries to search for candidates structures and use quantum chemistry, machine learning and cheminformatics methods to characterize these systems and find structure-property relations. The scale of this study requires an equally large computational resource. We rely on distributed volunteer computing to obtain these properties. In the third part of this dissertation we present our work related to the acceleration of electronic structure methods using graphics processing units. This hardware represents a change of paradigm with respect to the typical CPU device architectures. We accelerate the resolution-of-the-identity Moller-Plesset second-order perturbation theory algorithm using graphics cards. We also provide detailed tools to address memory and single-precision issues that these cards often present.

Inorganic materials have dominated electronic applications such as photovoltaic cells, thin film transistors (TFTs) and light emitting diodes (LEDs). However developments in the field of organic electronics over the past three decades have enabled the use of organic materials in these devices. While significant improvements have been made to improve their electronic properties there are several road blocks towards commercial application. One of the significant obstacles is the poor charge carrier mobility associated with organic semiconductors processed by well established printing methods. The goal of my research project is to improve the charge carrier mobility of solution cast films of acene semiconductors by partial halogenation and heteroatom substitution. Spin coated films of triisopropylsilylethynylated difluoropentacene exhibited higher hole mobility compared to TIPS pentacene due to contact induced nucleation of pentacene on perfluorobenzenethiol treated gold electrodes. The success of this project allowed me to further investigate the effect of degree of fluorination on the electronic properties of pentacene. A series of trialkylsilylethynylated tetrafluoro and octafluoropentacenes were synthesized and their performances in thin film transistors and solar cells were explored. Solar cells made from these materials using poly(3-hexylthiophene) as donor exhibited poor open circuit voltages (Voc) resulting in low power conversion efficiency (PCE). Better device performances were achieved using pentacenes having single halogen substituent. In order to improve the charge carrier mobility in TFTs soluble trialkylsilylethynylated hexacenes were explored. However these molecules exhibited a greater tendency to photo-dimerize in solution and solid state. Partial halogenation was used as a tool to improve the solution stability of reactive hexacene. The improved solution stability of partially halogenated hexacenes allowed me to successfully extend this approach to heptacene and nonacene. Finally a series of new trialkylsilylethynylated anthradiselenophenes were synthesized to improve molecular ordering in the solid state by increasing non-bonding Se – Se interaction. However single crystal x-ray diffraction studies revealed no such interaction between the acene chromophore resulting in poor device performance.

A long-term goal within the field of organic electronics has been to developflexible and functional devices, which can be processed and patterned withlow-cost and energy-efficient solution-based methods. This thesis presents anumber of functional paths towards the attainment of this goal via thedevelopment and demonstration of novel fabrication and patterningmethods involving the important organic-semiconductor family termedfullerenes.Fullerenes are soccer-shaped small molecules, with two often-employedexamples being the symmetric C60 molecule and its more soluble derivative[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We show that PCBM canbe photochemically transformed into a dimeric state in a bi-excited reactionprocess, and that the exposed material features a significantly reducedsolubility in common solvents as well as an effectively retained electronmobility. This attractive combination of material properties allows for adirect and resist-free lithographic patterning of electronic PCBM films downto a smallest feature size of 1 µm, using a simple and scalable two-stepprocess constituting light exposure and solution development. In a furtherdevelopment, it was shown that the two-step method was useful also in thearea-selective transformation of fullerene/conjugated-polymer blend films,as demonstrated through the realization of a functional complementary logiccircuit comprising a 5-stage ring oscillator.In another project, we have synthesized highly flexible, single-crystal C60nanorods with a solution-based self-assembly process termed liquid-liquidinterfacial precipitation. The 1-dimensional nanorods can be deposited fromtheir synthesis solution and employed as the active material in field-effecttransistor devices. Here, it was revealed that the as-fabricated nanorods canfeature an impressive electron mobility of 1.0 cm2 V-1 s-1, which is on par withthe performance of a work horse in the transistor field, viz. vacuumdeposited amorphous Si. We further demonstrated that the processability ofthe nanorods can be improved by a tuned light-exposure treatment, duringwhich the nanorod shell is polymerized while the high-mobility interior bulkis left intact. This has the desired consequence that stabile nanoroddispersions can be prepared in a wide range of solvents, and we anticipatethat functional electronic devices based on solution-processable nanorodscan be realized in a near future.

Magister Scientiae - MSc / Solar energy generation arises as a result of direct conversion of sunlight into electricity a by solar cell; which is mainly made up of a semiconducting material incorporated into a system. It is emerging as one of the most reliable and cost efficient renewable energy sources. Within the solar field, organic bulk heterojunction photovoltaic cells have proved of being able to have a great impact in the future years; mainly due to the easy processability of the active layer and substrate, their cost effectiveness and above all, a good power conversion efficiency associated to the close 3-dimensional interpenetrating network that is generated from blending donor and acceptor semiconducting materials together in a bulk heterojunction active layer. In this research work, we therefore report on the study of a newly developed organic bulk heterojunction active layer based on a blend of a star-copolymer generation 1 poly(propylenethiophenoimine)-co-poly(ethylenedioxythiophene) (G1PPT-co-PEDOT) as donor material with N,N-diisopropylnaphthalene diimide (NDI) as acceptor material. Both materials were chemically synthesized. The synthesis of G1PPT-co-PEDOT started first by the functionalization of generation 1 poly(propyleneimine) tetramine, G1PPI into G1PPT by condensation reaction in the presence of 2-thiophene carboxaldehyde under Nitrogen gas followed by the copolymerization of G1PPT with ethylene dioxythiophene (EDOT) monomer in the presence of ammonium persulfate, (NH4)2S2O8 as oxidant. On the other hand, NDI was also synthesized via condensation reaction of 1,4,5,8-naphthalene tetracarboxylic dianhydride in the presence of two (2) equivalences of N,N-diisopropylamine at 110 oC overnight in DMF. Both materials were characterized using FT-IR, UV-Vis spectroscopy, Fluorescence spectroscopy, Voltammetry, HRSEM microscopy and XRD. Based on the cyclic voltammetry and UV-Vis results, we were able to calculate the HOMO, LUMO and band gap energy (Eg) values of both the donor and acceptor to be -4.03 eV, -6.287 eV and 2.25 eV for iii the donor G1PPT-co-PEDOT respectively and -4.302 eV, -7.572 eV and 3.27 eV for the acceptor respectively. From these results, the energy diagram for both donor and acceptor was drawn and it comes out that the separation between the HOMO of the donor and the LUMO of the acceptor ΔEg = 1.985 eV, the ideal value for a good donor-acceptor combination. Also the offset energy that is, the energy difference between the LUMO of the donor and the LUMO of the acceptor is 0.302 eV.

Les matériaux les plus répandus dans le domaine du photovoltaïque (PV) organique à base de polymères sont le poly(3-hexylthiophène) régiorégulier (rr-P3HT) et le [6,6]-phenyl-C61-butyric acide methyl ester (PCBM). La couche active est une hétérojonction en volume (BHJ) de nanodomaines interpénétrés accepteur d’électron (le PCBM) et donneur d’électron (le P3HT). Le contrôle de la morphologie de la couche active et de sa stabilité est un défi majeur pour le développement des cellules organiques et est au centre de ce travail de thèse.Dans un premier temps, nous avons cherché à contrôler la morphologie du mélange P3HT:PCBM via l’utilisation de compatibilisants. Deux familles de copolymères à blocs rigide et souple ont été étudiées. Elles ont en commun leur bloc rigide à base de P3HT et diffèrent par leur bloc souple. Pourla première famille, le bloc souple poly-butylacrylate est greffé de C60, ce qui lui confère une affinité chimique avec les domaines de PCBM, et la seconde famille a un bloc souple poly(4-vinylpiridine) (P4VP) non greffé en C60. De par son affinité chimique avec le C60, le PCBM se localisera préférentiellement au voisinage des domaines P4VP.Dans un deuxième temps, nous avons exploité les propriétés d’auto-assemblages des copolymères à blocs afin de les étudier en tant que matériaux photo-actifs. Nous avons cherché à appréhender l’influence de la nature du bloc flexible let de a taille des blocs souple et rigide sur l’organisation des copolymères ainsi que sur les propriétés de transport de charge et sur les performances PV. / Polymer/fullerene bulk heterojunction (BHJ) solar cells use an interpenetrating network of TT-conjugated electron-donor polymers and electron-acceptor C60 as photo-active layer. Currently, the most investigated blends are constituted of regioregular poly(3-hexylthiophene) (rr-P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The nanostructured active layer determines to great extend the overall performance of the devices.First of all, we introduced rod-coil block copolymers as compatibilizer into P3HT:PCBM blends and investigate their influence on the blend morphology and the related photovoltaic performance. Two P3HT-based block copolymer families have been studied. The first one has a C60-grafted polybutylacrylate coil block and the second one is based on poly(4-vinylpyridine) (P4VP) coil block. The film morphology and device efficiencies are investigated as a function of copolymer content, P3HT/PCBM mass ratio and coil nature.A second possible route to obtain a stable donor/acceptor nanostructure consists of using semiconductor block copolymers as photoactive layer, since these materials are able to self-assemble into donor/acceptor BHJ by microphase separation. We explored P3HT-P4VP rod-coil block copolymers blended with PCBM in view of their utilization in photovoltaic devices. The copolymer selfassembles into nano-domains rich in either P3HT or P4VP, while the strong chemical affinity of P4VP with PCBM leads to an accumulation of the latter in the P4VP domains, providing them an acceptorcharacter. It is found that organized and thermally stable thin films, that exhibit ambipolar field effect mobilities, can be achieved for a specific rod/coil ratio.

Organic solar cells show great potential to become a commercially available technology for renewable clean energy production due to their attractive properties. Inexpensive materials and manufacturing processes, including classical roll-to-roll fabrication, as well as the ability to produce flexible, low weight, semitransparent devices are some of the advantages organic photovoltaics provide. Addressing the most common issues in these new technologies, i.e., the low efficiencies of devices and rapid degradation of materials, could bring a realistic alternative for the photovoltaic industry. In this work, the performance of P3HT:PCBM based bulk heterojunction solar cells modified through low energy gold ion implantation in the hole transporting layer, the PEDOT:PSS, is studied. Reference solar cells without gold were also fabricated and characterized for comparison. Through field emission scanning electron microscopy (FESEM) micrographs, the formation of gold nanoparticles (AuNPs) in the PEDOT:PSS has been shown layer for the highest implantation doses used. Absorbance measurements of PEDOT:PSS films before and after gold implantation further confirmed this result. TRIDYN and SRIM simulation programs estimated shallow gold implantations of ~3 nm underneath the PEDOT:PSS films surface. Current-voltage (JxV) characteristics of reference solar cells under AM 1.5 illumination presented the uncommon S-shaped curves, an abnormal deviation from typical JxV curves. This was attributed to PEDOT:PSS degradation due to oxygen and water exposure, which reduced its work function significantly. As a result, deteriorated parallel and series resistances were obtained in reference devices, which ultimately reduced their field factors and power conversion efficiencies. This abnormal behavior was consistently eliminated with the introduction of AuNPs near the PEDOT:PSS/Active-layer interface, leading to the rectification of the illuminated JxV curves of modified solar cells and the reestablishment of cell parameters. Consequently, outstanding improvements in the field factors and power conversion efficiencies were observed in these devices. This was attributed to enhancement (and prevention from the reduction) of the PEDOT:PSS work function layer due to the presence of AuNPs, which rearranged the energy levels at the interface to a more favorable state: higher electron blocking and lower hole extraction barriers. / Células solares orgânicas têm mostrado grande potencial para se tornar uma alternativa tecnológica na produção de energia limpa e renovável. Baixo custo dos materiais e dos processos de manufatura, e a possibilidade de fabricar dispositivos com baixo peso, flexibilidade e semitransparência, inclusive pelo método clássico de roll-to-roll, são algumas das vantagens oferecidas pela fotovoltaica orgânica. Resolver os problemas mais comuns destes dispositivos, como a baixa eficiência na conversão de energia e a rápida degradação dos materiais, é necessário para sua disponibilização no mercado fotovoltaico atual. Neste trabalho, células solares de heterojunção volumétrica baseadas no polímero P3HT e modificadas através da implantação de íons de ouro de baixa energia na camada de PEDOT:PSS são estudadas. Dispositivos equivalentes sem modificação de ouro também foram fabricados e caracterizados como referência. Imagens obtidas através de um microscópio eletrônico de varredura por emissão de campo (FESEM Field Emission Scannig Electron Microscopy) mostraram a formação de nanopartículas de ouro (AuNPs) na camada de PEDOT:PSS para as doses de implantação mais elevadas. Medidas do espectro de absorbância dos filmes de PEDOT:PSS antes e depois da implantação de ouro confirmam este resultado. Simulações feitas com os softwares TRIDYN e SRIM estimaram o ouro implantado em u ma profundidade de ~3 nm abaixo da superfície do PEDOT:PSS. As curvas de corrente-tensão (JxV) características das células solares de referência sob iluminação AM 1.5 mostraram um comportamento de forma S, que corresponde a um desvio da forma típica das curvas JxV. Isto foi atribuído à degradação dos filmes de PEDOT:PSS devido à exposição ao oxigênio e à água, que reduz sua função trabalho significativamente. Como resultado, deterioraram-se as resistências em paralelo e em série destes dispositivos, o que em última instância, reduziu o Field Factor (FF) e a eficiência na conversão de energia. Este comportamento anormal foi eliminado de forma consistente após a introdução de AuNPs perto da interface PEDOT:PSS/Camada-Ativa. As curvas JxV das células solares modificadas sob iluminação foram retificadas e os valores dos seus parâmetros restabelecidos. Melhorias notáveis no FF e eficiência de conversão de energia foram obtidas para todas as células solares modificadas. Isto foi atribuído ao aumento da função trabalho da camada de PEDOT:PSS pela presença das AuNPs, que reorganizou os níveis de energia na interface para um estado mais favorável: com barreiras de potencial otimizadas para bloquear a extração de elétrons e favorecer a de buracos.

Les recherches sur le photovoltaïque organique depuis une dizaine d’année ont permis une constante progression de l’efficacité des cellules solaires. Plusieurs groupes de recherche ont amorcé une réflexion sur les procédés de synthèse qui permettraient de limiter les déchets. Ainsi le couplage par arylation directe qui évite l’utilisation de dérivés organométalliques a émergé comme alternative au couplage organométallique classique. Une première partie est consacrée à l’étude méthodologique du couplage par arylation directe de la triphénylamine avec des dérivés du thiophène-2- carboxaldéhyde substitués en C3 et C4 par des groupements électro-accepteurs et électro-donneurs. Ce motif de base permet l’accès à trois séries de petites molécules par simple condensation. Des cellules photovoltaïques atteignant des rendements de photo-conversion de 3% en cellules bicouches, ont été obtenues. Dans une deuxième partie, la polymérisation par arylation directe sur des monomères du thiophène substitué par des groupements nitriles comme accepteur et des groupements alcoxy ou thioalkyl comme donneur conduit en une seule étape à une nouvelle famille de polymères conjugués. Ces polymères ont été utilisés comme matériaux donneurs et accepteur dans des cellules. Enfin en dernière partie, un travail exploratoire sur les propriétés d’émission de dérivés du benzofurane a été entrepris. Des réactions de condensation de Knoevenagel ont été développées pour obtenir toute une série de dérivés contenant l’unité cyano-vinylbenzofurane. La plupart des composés présente une forte émission à l’état solide. / Research on organic photovoltaics over a decade allowed a steadily increasing in the efficiency of solar cells. Several research groups have begun a reflection on the synthesis processes to limit wastes. Thus the direct arylation coupling which avoids the use of organometallic compounds, has emerged as an example of green reaction. The first part is devoted to the methodological study of direct arylation of bromotriphenylamine with thiophene-2-carboxaldehyde derivatives substituted in C3 and C4 by electron-withdrawing groups and electron-donor groups. This basic pattern allows access to three small molecules by condensation. Photovoltaic cells reaching photoconversion efficiencies of 3% were obtained. In the second part, the polymerization by direct arylation of sustituted thiophene monomers by nitrile groups as the acceptor and by alkoxy or thioalkyl as the donor led in one step to a novel family of conjugated polymers having donor and acceptor groups alternately. Solar cells were made with the polymers as donors or as acceptors materials. Finally, in the last part, exploratory work on the emission properties of benzofuran derivatives have been made. Knoevenagel condensations have been developed for a range of compounds containing the cyano-vinylbenzofuran unit. Most compounds have high emission in the solid state.