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Harnessing solar energy has become a promising clean and renewable energy source alternative to fossil fuels since the development of low-cost dye sensitized solar cells (DSSC) and organic photovoltaic solar cell devices. Their power-conversion efficiencies, below 13% and 9% respectively, still limit the economic viability of these technologies. The geometry and optical properties of photonic crystals can be used to improve the absorption and charge collection efficiencies of these devices. This thesis describes the fabrication of TiO2 DSSC and ZnO-polymer solar cell devices based on a three-dimensional photonic crystal structure. Photonic crystal polymer structures were produced by holographic lithography and thermally stabilized in order to be used as templates for atomic layer deposition (ALD) of various metal oxides. For this purpose, an ALD apparatus was built and ALD processes for the growth of TiO2, ZnO, Al2O3, ZnO:Al, and Zr3N4 were established and deposited on photonic crystal templates. After ALD, the template was removed by calcination at 500°C, at which ZnO:Al films lost their conductivity of 250 S/cm preventing their use as transparent conducting oxide (TCO) electrodes. The produced 90 nm TiO2 photonic crystal shell DSSC and TiO2 inverse replica devices based on the dye N-719 and iodine/iodide redox electrolyte provided power-conversion efficiencies of 0.9% and 0.49% respectively and their diffusion lengths were 2× and 3× longer than that of a nanocrystalline reference device respectively. ZnO-polymer devices, comprising a P3HT layer as absorber and PEDOT:PSS film as hole-transporter, were also investigated.

Recentemente tem sido observado grande avanço na área de fotovoltaicos orgânicos e híbridos. Esses dispositivos baseiam-se em diferenças de energia eletrônica nas interfaces dos componentes de uma heterojunção, onde as heterojunções dispersas obtidas via arranjos interpenetrantes com estes dois compostos, em dimensões sub-20-nm, permite coleção efetiva dos portadores de carga fotogerados, mesmo com pequeno comprimento de difusão do éxciton e baixa mobilidade dos portadores de carga. Desta maneira, o estudo e a modificação da interface entre as duas fases é essencial para melhorar a eficiência de conversão desses dispositivos. Este trabalho tem como objetivo investigar heterojunções de dióxido de titânio (TiO2)/poli(3-hexil tiofeno) (P3HT) como camadas ativas em dispositivos fotovoltaicos híbridos. Essas heterojunções são atraentes para fotovoltaicos híbridos por permitir a nanoestruturação do metalóxido antes da incorporação do polímero, levando à obtenção de elevada área interfacial e com a possibilidade de ajustar as propriedades interfaciais mantendo o caráter aceitador/doador desta heterojunção. Os filmes de TiO2 foram obtidos a partir de dispersões coloidais de nanopartículas de anatase, enquanto aqueles de P3HT foram obtidos a partir de sua solução de 1,2-diclorobenzeno. Como eletrodos foram utilizados o óxido de índio dopado com óxido de estanho (ITO) e ouro (Au). A caracterização elétrica e espectroscópica indicou que os parâmetros fotovoltaicos, bem como absorção e fotoluminescência, dependem da submissão dos dispositivos a tratamentos térmicos post-production, evidenciando que o aquecimento induz a mudanças conformacionais no polímero, alterando a interface óxido/polímero. A introdução de grupos carboxilato nas ramificações 3-hexil do P3HT permite melhor contato entre óxido/polímero, sendo favorecido por tratamento térmico. Mais ainda, a infiltração do polímero em filmes de TiO2 nanocristalino levou à obtenção de dispositivos fotovoltaicos de considerável estabilidade, apresentando atividade mesmo após 6 meses após a elaboração destes. Finalmente, neste trabalho também é proposta a utilização de um complexo a base de ftalocianina de zinco como modificador de interface do TiO2, cuja absorção estende a janela espectral do dispositivo até o vermelho e infravermelho próximo. Estudos de fenômenos de transporte, levando-se em conta a fotogeração de portadores e sua recombinação, foram realizados por medidas do espectro de fotocondução no dispositivo / Recently a fast development in organic and hybrid photovoltaic field has been observed. Such devices are fabricated by organic semiconductors within components of a heterojunction, in which bulk heterojunctions obtained via interpenetrating networks at the sub-20-nm length scale. It permits the effective collection of photogenerated charge carriers even with low exciton diffusion length and low charge carrier mobilities. Therefore, the study of material properties and the interface modification are essential to improve the power efficiency of such devices. The goal of this work is to investigate heterojunctions of titanium dioxide (TiO2) and poly(3-hexyl thiophene) (P3HT) as active layers in hybrid photovoltaic devices. Such heterojunctions are attractive since the metaloxide can be nanostructured previous the infiltration of the polymer within the nanocrystalline phase, leading on large interfacial area with the possibility to tune the interfacial properties keeping the acceptor/donor character of the two components. Titania films were obtained from colloidal dispersions of anatase nanoparticles, while P3HT from its solution in 1,2-dichlorobenzene. The electrodes were indium-tin oxide (ITO) and gold (Au). Electrical and spectroscopic characterization shows the photovoltaic parameters, as well as absorption and photoluminescence, depends on post-production treatments, evidencing that heating induces conformational changes in the polymeric phase, changing the oxide/polymer interface. The introduction of carboxyl groups at 3-hexyl ramifications in P3HT permits better contacts between oxide and polymer by chemisorption, being favored by thermal treatment. Furthermore, the infiltration of the polymer within the nanocrystalline TiO2 leads on enhanced stability of the devices, working even six months after their preparation. Finally, in this work is also proposed the utilization of a interface modifier based on zinc phtalocyanine complex, which has extended the device absorption window up to the red/ near infrared, increasing light harvesting, and a transport study related to photocarriers generation and recombination process was carried out by photoconductivity action spectra

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 this dissertation novel work is presented showing the performance and device physics of Organic Field Effect Transistors (OFETs) and bulk heterojunction Organic Photovoltaic (OPV) devices fabricated using novel acceptor small molecules. Pentacene and N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN₂) were used as the active layer in p-channel and n-channel Organic Field Effect Transistors (OFETs), respectively, and novel pulsed voltage transient measurements were developed in order to extract transient mobilities and carrier velocities from the transistor response of the device, which were well correlated with the corresponding DC OFET characteristics. A distributed RC network was used to model the OFET’s channel and the transient and DC characteristics of the devices were successfully reproduced. Temperature dependent studies of the DC field effect mobilities and transient mobilities of these two materials were carried out and the results used to extract information on charge carrier transport in the materials at varying time scales. Open-circuit voltages of the OPV devices are correlated with the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) levels various acceptor small molecules and donor polymers comprising the active layers of the devices. / text

A new class of electrodes with nanometer-scale contact spacing can be produced at the edge of patterned metal/insulator/metal this film structures. A key challenge is to produce insulator layers with low leakage current and have pristine metal contacts for controlled molecular contacts. Atomic layer deposition of high quality Al2O3 thin films onto Au electrodes was enabled by surface modification with a self-assembled monolayer of -OH groups that react with a monolayer of trimethylaluminum gas source. Ar ion milling was then used to expose the edge of the Au/dielectric/Au structure for molecular electrode contacts. The junctions are characterized by atomic force microscope and tunnel current properties. The Au/self-assembled monolayer/Al2O3/Au tunnel junction, with a very thin oxide insulator layer (15.4 Å), is stable and has a small tunneling current density of about 0.20 ~ 0.75 A/cm2 at 0.5 V. Organometalic cluster molecules were attached to bridge the electrodes. Through tunnel current modeling, low temperature and photo current measurements, molecular current was found to be consistent with direct tunneling through the organic tethers to available states at the metal center. This novel electrode was also used to study the efficiency of organic conducting thin films where the photovoltaic efficiency can be improved when the electrode separation distance is below the exciton diffusion length. Copper (II) phthalocyanine (CuPc) was thermally evaporated between the nano-gap electrodes formed by Au/Al2O3/Au tunnel junctions. A large photocurrent enhancement over 50 times that of bulk CuPc film was observed when the electrode gap distance approached 10 nm. CuPc diffusion length is seen to be 10 nm consistent with literature reports. All devices show diode I-V properties due to a large Schottky barrier contact resistance between the small top Au electrode and the CuPc film. To add another dimension of nm-scale patterning, nanowires can be used as line-of-sight shadowmasks provided that nanowire location and diameter can be controlled. Lateral ZnO nanowires were selectively grown from the edge of a Si/Al2O3/Si multi-layer structure for potential integration into devices utilizing Si processing technology. Microstructural studies demonstrate a 2-step growth process in which the tip region, with a diameter ~ 10 nm, rapidly grew from the Al2O3 surface. Later a base growth with a diameter ~ 22 nm overgrew the existing narrow ZnO nanowire halting further tip growth. Kinetics studies showed surface diffusion on the alumina seed surface determined ZnO nanowire growth rate.

With increasing concerns of global warming and the impending exhaustion of fossil fuels attention is being turned to renewable sources of energy. The sun supplies 3 x 1024 J per year to the earth which is around 104 times more energy than what the human race consumes. The world’s energy needs would be satisfied if a mere 0.1% of the planet’s surface was covered with solar cells(< = 10%)1, causing the conversion of solar energy (sunlight) into electricity to represent a very practical renewable source. Past research into solar energy has produced a photovoltaic device, which when coupled with highly coloured coordination compounds, enables this conversion. This device is known as a dye-sensitised solar cell (DSSC). Further research has been conducted into the properties of the dyes, and has shown that highly coloured coordination compounds are able to convert solar energy into electrical energy with the highest efficiencies. The dominant compounds in this area to date have been Grätzel’s ruthenium complexes and porphyrins. However, there exists a class of smaller compounds called dipyrrins, described most simply as “half a porphyrin”, which possess many of the attractive qualities for DSSC dyes. Although there are no examples of ruthenium-dipyrrin complexes in the literature, the combination of advantageous properties from both components represent very attractive synthetic targets with huge potential as dyes for DSSCs. The objectives of this thesis were firstly to develop a series of dipyrrin complexes which would be suitable as dyes for DSSCs; then to fully characterise the complexes and investigate the spectroscopic properties of each complex; and finally to determine the suitability of the complexes as dyes for DSSCs. These objectives were fully met, resulting in a set of generic target compounds characterised via 1H NMR, 13C NMR, mass spectrometry (ESI-MS), elemental analysis, and x-ray crystallography. From analyses of the UV-visible, fluorescence, emission, and Raman spectra; and electrochemistry results; the complexes were concluded to be suitable as dyes for DSSC’s. An additional bonus is that the syntheses for these complexes are applicable to any dipyrrin, thus aiding future studies into the use of dipyrrins as dyes for DSSC’s. This thesis summarises the findings of the above outlined research project.

O objetivo deste estudo foi a síntese de Cu2ZnSnS4 (CZTS) por electrospinning, sem a etapa de sulfurização e com tratamento térmico em atmosfera ambiente. A solução precursora consistiu na dissolução dos sais dos metais de interesse: tiourea como fonte de enxofre e polivinil butiral (PVB) em etanol como polímero condutor. Esta solução foi submetida ao processo de electrospinning com uma tensão elétrica de 16 kV e uma distância entre coletor e capilar de 120 mm e fluxo de 3 mL/h. O efeito da temperatura de tratamento térmico sobre a formação do CZTS foi investigada. Para isso, o material resultante do processo de electrospinning foi tratado termicamente em diferentes temperaturas: 150°C durante 72h, 150°C durante 24h mais 300°C durante 24h, 300°C durante 48h, 400°C durante 1h, 450°C durante 1h, 500°C durante 1h e 550°C durante 1h. A taxa de aquecimento foi de 0,5°C/mim. A Influência do agente complexante dietanolamina (DEA) sobre as propriedades do material formado, quando adicionado à dolução precursora, foi também investigada. As amostras resultantes destes tratamentos térmicos foram caracterizadas através de análises térmicas (ATG), difração de raios X (DRX), espectroscopia Raman, microscopia eletrônica de varredura (MEV/EDS), microscopia eletrônica de transmissão (MET) e, por medidas ópticas (UV-Vis) e espectroscopia de reflectância difusa (ERD). Foram obtidos filmes homogêneos e compactos com uma espessura de aproximadamente 10 nm. Os resultados indicaram que foi obtida fase CZTS a 400°C, com as razões S/(Cu+Zn+Sn) = 1,1, Cu/(Zn+Sn) = 0,8 e Zn/Sn = 1,26. No entanto, a presença de algumas fases secundárias elevou o band gap para aproximadamente 2,2 eV. Além disso, foi verificado que a adição de DEA na solução precursora não aumenta a qualidade do CZTS formado por electrospinning. / The aim of this work was to study the production of CZTS by electrospinning method without sulfurization process and in-air heat treatment. The precursor solution was prepared by dissolving metal salts, tiourea as a sulfur source and PVB as conductive polymer. The resulting solution was electrospun onto a cylindrical target with an electric voltage of 16 kV at a 120 mm distance and flow rate of 3 mL/h. The effect of in-air heat treatment in the phase formation and morphology of electrospun CZTS fibers were investigated by the following conditions, 150°C for 72h, 150°C for 24h plus 300°C for 24h, 300°C for 48h, 400°C for 1h, 450°C for 1h, 500°C for 1h e 550°C for 1h, with a used a heating rate of 0.5°C.min-1 The Influence of the addition of complexing agent diethanolamine (DEA) on the properties of the final material was investigated. The samples were characterized by thermal analysis (TGA), R-X diffraction, scanning electron microscopy (SEM/EDS), transmission electron microscopy and optical measurements (UV-Vis). The results show the obtainment of a well crystallized CZTS phase with the heat treatment of 400°C with ratios S/(Cu+Zn+Sn) =1.1, Cu/(Zn+Sn) = 0.8 e Zn/Sn = 1.26. Homogeneous and compact films with the morphology of 10 nm spheres were found in this study. However, the presence of some secondary phases increases the band gap to approximately, 2.2 eV. Furthermore, it was found that the addition of DEA in the precursor solution does not increase the quality of CZTS formed by electrospinning.

O objetivo deste estudo foi a síntese de Cu2ZnSnS4 (CZTS) por electrospinning, sem a etapa de sulfurização e com tratamento térmico em atmosfera ambiente. A solução precursora consistiu na dissolução dos sais dos metais de interesse: tiourea como fonte de enxofre e polivinil butiral (PVB) em etanol como polímero condutor. Esta solução foi submetida ao processo de electrospinning com uma tensão elétrica de 16 kV e uma distância entre coletor e capilar de 120 mm e fluxo de 3 mL/h. O efeito da temperatura de tratamento térmico sobre a formação do CZTS foi investigada. Para isso, o material resultante do processo de electrospinning foi tratado termicamente em diferentes temperaturas: 150°C durante 72h, 150°C durante 24h mais 300°C durante 24h, 300°C durante 48h, 400°C durante 1h, 450°C durante 1h, 500°C durante 1h e 550°C durante 1h. A taxa de aquecimento foi de 0,5°C/mim. A Influência do agente complexante dietanolamina (DEA) sobre as propriedades do material formado, quando adicionado à dolução precursora, foi também investigada. As amostras resultantes destes tratamentos térmicos foram caracterizadas através de análises térmicas (ATG), difração de raios X (DRX), espectroscopia Raman, microscopia eletrônica de varredura (MEV/EDS), microscopia eletrônica de transmissão (MET) e, por medidas ópticas (UV-Vis) e espectroscopia de reflectância difusa (ERD). Foram obtidos filmes homogêneos e compactos com uma espessura de aproximadamente 10 nm. Os resultados indicaram que foi obtida fase CZTS a 400°C, com as razões S/(Cu+Zn+Sn) = 1,1, Cu/(Zn+Sn) = 0,8 e Zn/Sn = 1,26. No entanto, a presença de algumas fases secundárias elevou o band gap para aproximadamente 2,2 eV. Além disso, foi verificado que a adição de DEA na solução precursora não aumenta a qualidade do CZTS formado por electrospinning. / The aim of this work was to study the production of CZTS by electrospinning method without sulfurization process and in-air heat treatment. The precursor solution was prepared by dissolving metal salts, tiourea as a sulfur source and PVB as conductive polymer. The resulting solution was electrospun onto a cylindrical target with an electric voltage of 16 kV at a 120 mm distance and flow rate of 3 mL/h. The effect of in-air heat treatment in the phase formation and morphology of electrospun CZTS fibers were investigated by the following conditions, 150°C for 72h, 150°C for 24h plus 300°C for 24h, 300°C for 48h, 400°C for 1h, 450°C for 1h, 500°C for 1h e 550°C for 1h, with a used a heating rate of 0.5°C.min-1 The Influence of the addition of complexing agent diethanolamine (DEA) on the properties of the final material was investigated. The samples were characterized by thermal analysis (TGA), R-X diffraction, scanning electron microscopy (SEM/EDS), transmission electron microscopy and optical measurements (UV-Vis). The results show the obtainment of a well crystallized CZTS phase with the heat treatment of 400°C with ratios S/(Cu+Zn+Sn) =1.1, Cu/(Zn+Sn) = 0.8 e Zn/Sn = 1.26. Homogeneous and compact films with the morphology of 10 nm spheres were found in this study. However, the presence of some secondary phases increases the band gap to approximately, 2.2 eV. Furthermore, it was found that the addition of DEA in the precursor solution does not increase the quality of CZTS formed by electrospinning.

abstract: In the interest of expediting future pilot line start-ups for solar cell research, the development of Arizona State University's student-led pilot line at the Solar Power Laboratory is discussed extensively within this work. Several experiments and characterization techniques used to formulate and optimize a series of processes for fabricating diffused-junction, screen-printed silicon solar cells are expounded upon. An experiment is conducted in which the thickness of a PECVD deposited anti-reflection coating (ARC) is varied across several samples and modeled as a function of deposition time. Using this statistical model in tandem with reflectance measurements for each sample, the ARC thickness is optimized to increase light trapping in the solar cells. A response surface model (RSM) experiment is conducted in which 3 process parameters are varied on the PECVD tool for the deposition of the ARCs on several samples. A contactless photoconductance decay (PCD) tool is used to measure the dark saturation currents of these samples. A statistical analysis is performed using JMP in which optimum deposition parameters are found. A separate experiment shows an increase in the passivation quality of the a-SiNx:H ARCs deposited on the solar cells made on the line using these optimum parameters. A RSM experiment is used to optimize the printing process for a particular silver paste in a similar fashion, the results of which are confirmed by analyzing the series resistance of subsequent cells fabricated on the line. An in-depth explanation of a more advanced analysis using JMP and PCD measurements on the passivation quality of 3 aluminum back-surface fields (BSF) is given. From this experiment, a comparison of the means is conducted in order to choose the most effective BSF paste for cells fabricated on the line. An experiment is conducted in parallel which confirms the results via Voc measurements. It is shown that in a period of 11 months, the pilot line went from producing a top cell efficiency of 11.5% to 17.6%. Many of these methods used for the development of this pilot line are equally applicable to other cell structures, and can easily be applied to other solar cell pilot lines. / Dissertation/Thesis / M.S. Electrical Engineering 2014