<p>Over the last few decades, bulk-heterojunction organic photovoltaic devices comprising an intimately mixed donor-acceptor blend have gained serious attention due to their potential for being cheap, light weight, flexible and environmentally friendly. In this thesis, APFO-3/PCBM bulk-heterojunction based organic photovoltaic devices with an inverted layer sequence were investigated systematically. Doctor blade coating is a technique that is roll-to-roll compatible and cost efficient and has been used to fabricate the solar cells.</p><p>Initial studies focused on optimization of the electrodes. A thin film of the conductive polymer PEDOT:PSS was chosen to be the transparent anode. Different PEDOT:PSS films with respect to the film thickness and deposition temperature were characterized in terms of conductivity and transmission. Decent conductance and transmittance were obtained in the films deposited with wet film thickness setting of 35 μm, The cathode was fabricated from a metal bilayer comprising Al and Ti with an area about 1 cm², and the best-working cathodes contained a 70 nm thick Al layer covered by a thin Ti layer of about 10 -15 nm.</p><p>Optimized coating temperature and wet film thickness settings for the active layer and PEDOT:PSS layer were experimentally determined. The highest efficiency of the APFO-3/PCBM based inverted solar cells fabricated by doctor blading was 0.69%, which exceeded the efficiency of spin-coated inverted cells.</p><p>A higher efficiency (0.8 %) was achieved by adding a small amount of high molecular weight polystyrene to the active layer. Morphological changes after adding of the polystyrene were observed by optical microscopy and AFM. A coating temperature dependent phase separation of the APFO-3/PCBM/polystyrene blend was found.</p><p> </p>

<p>In this thesis, the microstructure of organic photovoltaic APFO-3:PC₆₁BM bulk-heterojunction blends was examined. Earlier studies have focused on the microstructure after spin coating. This thesis aims to give a better insight into microstructural degradation as the films are annealed above the glass transition temperature, T_g, and the mixture approaches thermodynamic equilibrium. Electro- and photoluminescence studies indicate that the polymer and PC₆₁BM are intermixed on a scale shorter than the exciton diffusion length of 10 nm, even when annealed above T_g. The temperature stability of APFO-3:PC₆₁BM was also investigated with respect to the molecular weight of the polymer. The photovoltaic performance of these blends was found to be stable up to temperatures approaching the glass transition temperature, especially if a high molecular-weight APFO-3 grade was used.</p><p> </p><p>The crystallization of PC₆₁BM was also investigated. Above T_g, PC₆₁BM crystallization was found to commence, albeit slowly at temperatures close to T_g. At elevated temperatures instead, micrometer sized crystals were observed to form. It was also noted that illumination while annealing APFO-3:PC₆₁BM thin films above T_g affected PC₆₁BM crystallization, the origin of which is so far unclear although chemical degradation could be largely excluded.</p>

In this thesis, the microstructure of organic photovoltaic APFO-3:PC61BM bulk-heterojunction blends was examined. Earlier studies have focused on the microstructure after spin coating. This thesis aims to give a better insight into microstructural degradation as the films are annealed above the glass transition temperature, Tg, and the mixture approaches thermodynamic equilibrium. Electro- and photoluminescence studies indicate that the polymer and PC61BM are intermixed on a scale shorter than the exciton diffusion length of 10 nm, even when annealed above Tg. The temperature stability of APFO-3:PC61BM was also investigated with respect to the molecular weight of the polymer. The photovoltaic performance of these blends was found to be stable up to temperatures approaching the glass transition temperature, especially if a high molecular-weight APFO-3 grade was used.   The crystallization of PC61BM was also investigated. Above Tg, PC61BM crystallization was found to commence, albeit slowly at temperatures close to Tg. At elevated temperatures instead, micrometer sized crystals were observed to form. It was also noted that illumination while annealing APFO-3:PC61BM thin films above Tg affected PC61BM crystallization, the origin of which is so far unclear although chemical degradation could be largely excluded.

Over the last few decades, bulk-heterojunction organic photovoltaic devices comprising an intimately mixed donor-acceptor blend have gained serious attention due to their potential for being cheap, light weight, flexible and environmentally friendly. In this thesis, APFO-3/PCBM bulk-heterojunction based organic photovoltaic devices with an inverted layer sequence were investigated systematically. Doctor blade coating is a technique that is roll-to-roll compatible and cost efficient and has been used to fabricate the solar cells. Initial studies focused on optimization of the electrodes. A thin film of the conductive polymer PEDOT:PSS was chosen to be the transparent anode. Different PEDOT:PSS films with respect to the film thickness and deposition temperature were characterized in terms of conductivity and transmission. Decent conductance and transmittance were obtained in the films deposited with wet film thickness setting of 35 μm, The cathode was fabricated from a metal bilayer comprising Al and Ti with an area about 1 cm2, and the best-working cathodes contained a 70 nm thick Al layer covered by a thin Ti layer of about 10 -15 nm. Optimized coating temperature and wet film thickness settings for the active layer and PEDOT:PSS layer were experimentally determined. The highest efficiency of the APFO-3/PCBM based inverted solar cells fabricated by doctor blading was 0.69%, which exceeded the efficiency of spin-coated inverted cells. A higher efficiency (0.8 %) was achieved by adding a small amount of high molecular weight polystyrene to the active layer. Morphological changes after adding of the polystyrene were observed by optical microscopy and AFM. A coating temperature dependent phase separation of the APFO-3/PCBM/polystyrene blend was found.

Since the discovery of the photovoltaic effect in bulk heterojunction devices¡Mthe considerable publications in PSCs have been reported¡OPSCs based on the concept of bulk heterojunction (BHJ) configuration where active layer comprises of a p-type donor (conjugated polymer) and a n-type acceptor (fullerene derivative) materials¡Mrepresents the most useful strategy to maximize the internal donor-acceptor interface area allowing for efficient charge separation¡OTo further enhance the power conversion efficiency from solar cells made of poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester (P3HT/PCBM) ¡M a new conducting polymer with optimized band energy levels are demonstrated to be one of the key properties¡OIn this study¡MI synthesized a soluble and strongly visible-light absorbing alternating conducting polymer using Suzuki coupling polymerization method¡OThe UV-Vis absorption spectra of copolymer contains an intramolecular charge transfer (ICT) transition band¡Mwhich leads to absorption extending to near-infrared region and optical band gaps is 1.55 eV¡OThe photo-electron spectroscopy in air(PESA) measurements show that the HOMO level of the polymer is ~5.0eV which is lower than P3HT¡O

For more than 20 years, scientists have studied solar cells made from organic semiconductors. Throughout this time, device structures have evolved from bilayer devices to bulk heterojunction (BHJ) devices and even though efficiencies are approaching 10%, scientists still know relatively little about the transport of charge carriers and recombination mechanisms in these materials. Novel structures, based on lateral BHJ solar cells, have proven to be versatile tools to study transport and recombination mechanisms. In addition, these structures can easily be employed by researchers and solar cell manufacturers to determine the quality and measure the improvement of their materials. For these studies, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been employed due to its wide use among researchers as well as potential for commercialization. DC photocurrent measurements as a function of device length have yielded the mobility-lifetime product and the generation rate of free carriers within these BHJ devices. In addition to these parameters, the recombination rate as a function of light intensity provides information about the mechanisms of recombination. For example, by measuring the recombination rate as a function of applied electric field and light intensity we have found that recombination is unimolecular in nature and shifts to bimolecular at increased electric field strengths. Additionally, the mobility-lifetime product, generation rate, and recombination mechanism have been studied as a function of applied electric field, illumination spectrum, illumination intensity, etc. This information has provided much insight on physics of the P3HT:PCBM material system which did not exist before these studies. / text

The investigation carried out in this project allowed for the development of eleven regioregular π-conjugated alternating copolymers and their implementation in organic solar cells. The eleven synthesized polymers, poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(4,7-dithien-2-yl-2,1,3-benzothiadiazole)] (CB), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(1,6-pyrene)] (LP), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(5,5’’’-(3,3’’’-dihexyl-2,2':5',2'':5'',2'''-quarterthiophene))] (CT), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(2,7-9H-fluoren-9-one)] (CF), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(1,6-pyrene)] (CP), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(4,7-dithien-2-yl-2,1,3-benzothiadiazole)] (LB), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(2,7-9H-fluoren-9-one)] (LF), poly[(5,5’’’-(3,3’’’-dihexyl-2,2':5',2'':5'',2'''-quarterthiophene))-alt-(2,7-9H- fluoren-9-one)] (TF), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(4,4'-dioctyl-2,2'-bithiophene)] (oTLT), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(4,4'-dioctyl-2,2'-bithiophene)] (oTCT), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(4,4'-dihexyl-2,2'-bithiophene)] (TCT), were investigated using theoretical methods that included semi-empirical geometry optimizations, density functional theory (DFT) energy calculations, and time-dependent density functional theory (TD-DFT) optical absorption predictions. The absorption predictions gave credence to our experimental results in which the absorption of the longer polymer chains underwent a redshift from the monomer absorption. With several of the prepared polymers, bulk-heterojunction photovoltaic cells were fabricated and their photovoltaic activity was investigated. Several of the fabricated cells exhibited photovoltaic efficiencies including polymer/PCBM composites with an aluminum back electrode (CF, CT, P3HT, and MEH-PPV), and also inverted cells with a silver back electrode (CT, P3HT, and MEH-PPV). Several polymers (CF, CT, TCT, LP, oTCT, oTLT, P3HT, and MEH-PPV) were used to solubilize single-walled carbon nanotubes (SWNTs). The solubility of the nanotubes occurred by the polymers’ ability to wrap the tubes, disrupt the bundles (ropes of tubes), and allow for the creation of a homogeneous mixture. Polymer:PCBM:SWNT mixtures were prepared and utilized as the active layer in BHJ solar cells. Some of the inverted cells (with a silver back electrode) that incorporated the nanotube composites (CT, oTCT, oTLT, P3HT, and MEH-PPV) displayed photovoltaic activity. These preliminary results illuminate the photovoltaic behavior of the polymer and provide evidence for their future use in polymer solar cells. / Thesis (Master, Chemistry) -- Queen's University, 2011-02-13 22:09:00.464

The arrangement of soft materials through solution processing techniques is a topic of profound importance for next generation solar cells; the resulting morphology has a major influence on construction, performance and lifetime. This thesis investigates the connections between the soft matter physics of colloidal systems and solid state dye sensitised (SSDS) and bulk heterojunction (BHJ) solar cells. A study of aqueous titanium dioxide nanoparticulate suspensions was carried out in order to observe how suspension structure can be controlled by altering the inter-colloid potential via pH-induced electrostatic charging. Measurements were performed at volume fractions between 0.025% and 8.2% with the solution pH set to 3.1, 3.5 or 4.5 before mixing. Suspensions with a volume fraction above 4% formed self-supporting gels regardless of the set pre-mix pH. These gels displayed shear thinning behaviour with a power law exponent of 0.8, a yield stress of 11(1) Pa and rheological response consistent with an aggregated fractal network. At lower volume fractions, suspensions exhibited consolidation interpreted as the collapse of a gel of fractal clusters with a fractal dimension of 2.36. The velocity of the suspension/supernatant interface exhibited delayed sedimentation behaviour, as well as further fractal-based power law scalings with volume fraction. Lower volume fraction suspensions were explored using dynamic light scattering. Limited aggregation of ‘stable’ suspensions was observed when compared to primary aggregate radii measured from electron microscopy images. To connect suspension structure and cell manufacture, the behaviour of more concentrated suspensions was observed during the drying of thin films, a process which forms an essential part of a SSDS solar cell. Lowering the pH of the suspension after mixing from 4 to 3 resulted in an ordering of observed crack domains. An increase in film delamination was also observed. Rates of mass loss during drying followed the expected three phase process, although there was an unexpected increase in rate during the initial phase (where rate is usually constant in time). Dynamic light scattering was found to be a useful but demanding technique for studying cluster formation in titanium dioxide suspensions. A non-linear fitting technique utilising the method of moments was thoroughly explored using computer simulated datasets. The algorithm reduced the systematic error in fitted parameters for moderately polydisperse (0:2 < < 0:4) datasets as compared to the commonly applied linear algorithm. The fitting algorithm was also robust to bad initial estimates of parameters. Finally, test solar cells have been built using blends of titanium dioxide and poly-3-hexylthiophene. Device performance was reduced with blend standing time after mixing but could be improved by remixing the blend before spin coating, implicating a reversible process (e.g. aggregation of titanium dioxide or crystallisation of P3HT) in the loss of performance. Addition of a titanium dioxide hole blocking layer before spin coating reduced cell performance. Combining the above studies and these device designs provides a future platform for continuation of this work in the context of real devices.

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.

In organic photovoltaics (OPVs) the key processes relevant to device operation such as exciton dissociation and free carriers recombination occur at the donor-acceptor (D-A) interface. OPV devices require the bulk heterojunction (BHJ) architecture to function efficiently. In these BHJs, D-A interfaces are arranged in three dimensions, which makes molecular arrangements at these interfaces ill defined and hard to characterize. In addition, molecular materials used in OPVs are inherently disordered and may exhibit variable degrees of structural order in the same BHJ. Yet, D-A molecular arrangements and structure are crucial because they shape the energy landscape and photovoltaic (PV) performance in OPVs. Studies that use well-defined model systems to look in details at the interfacial molecular structure in OPVs and link it to interfacial energy landscape and device operation are critically lacking. We have used in situ photoelectron spectroscopy and ex situ x-ray scattering to study D-A interfaces in tailored bilayers and BHJs based on small molecule donors. We show preferential miscibility at the D-A interface depending on molecular conformation in zinc phthalocyanine (ZnPc)/ C60 bilayers and we derive implications for exciton dissociation. Using sexithiophene (6T), a crystalline donor, we show that the energy landscape at the D-A interface varies markedly depending on the molecular composition of the BHJ. Both the ionization energies of sexithiophene and C60 shift by over ~0.4 eV while the energy of the charge transfer state shifts by ~0.5 eV depending on composition. Such shifts create a downward energy landscape that helps interfacial excitons to overcome their binding energies. Finally, we demonstrate that when both disordered and ordered phases of D coexist at the interface, low-lying energy states form in ordered phases and significantly limit the Voc in devices. Overall our work underlines the importance of the aggregation and conformation states of molecular materials at and near the D-A interface in determining the operation and performance of OPV devices. This work shows that the role of D-A interfaces in complex BHJ devices can be unraveled through careful experimental design and by in depth characterization of planar heterojunction bilayer devices recreating model interfaces.