Thin-film inorganic solar cells, such as CdTe, have demonstrated the most promise to date for a viable low-cost renewable energy resource. Their current performance, however, is far from the theoretical limit suffering from significant charge recombination losses due to grain boundaries and point defects. It is likely that the microscopic compositions of grain bulk and grain boundaries are significantly different and not optimal for the overall device performance. Good understanding of charge transport along and across the grain boundaries and other microscopic interfaces is lacking, preventing the development of reliable and predictive device models. The insufficient microscopic understanding hinders efficient characterization of photovoltaic materials and also holds back the development of process control techniques. We first show preliminary results for a novel technique, quantum-dot electron-beam induced current to characterize semiconductors in the near-field. We also propose the use of near-field optical scanning microscopy for high precision optical excitation and for local, high-resolution characterization. These imaging techniques are examined with the goal of synthesizing information obtained by both methods, of material phenomena at the relevant length scales, to other measurement methods. The most important nanoscale phenomena being the separation of compositional and electrical effects.

Orientador: Francisco das Chagas Marques / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-03T02:41:17Z (GMT). No. of bitstreams: 1 SilvaFilho_JoseMariaClementeDa_D.pdf: 74812249 bytes, checksum: 2c3eb02d615d6ec43fdb5efb29cc187b (MD5) Previous issue date: 2017 / Resumo: Perovskitas híbridas orgânica-inorgânica tem sido objeto de intensa investigação devido as suas atrativas propriedades ópticas e eletrônicas, por exemplo, banda de energia proibida direta, alto coeficiente de absorção e transporte ambipolar de cargas. Tais propriedades possibilitaram a aplicação desse material em células solares e em diodos emissores de luz de forma eficiente. Assim, o desenvolvimento de novas rotas de síntese que permitam produzir materiais com as características adequadas para cada aplicação é de extrema importância para o desenvolvimento dessa área de pesquisa. Portanto, neste trabalho de doutoramento apresentaremos resultados sobre a síntese e caraterização de filmes e nanocristais de perovskita obtidos a partir de novas metologias, baseadas na conversão de filmes finos de sulfeto de chumbo (PbS) e iodeto de chumbo (PbI2) depositados por rf-sputtering e em pontos quânticos de PbS. Na primeira rota de síntese, filmes finos amorfos de PbS, depositados por sputtering, foram convertidos em filmes finos de PbI2 através do processo de iodação em temperatura ambiente. Esse procedimento resultou em uma completa mudança estrutural, conforme atestado pelos resultados de difração de raios-x. A conversão desses filmes de PbI2 em CH3NH3PbI3 foi realizada por meio da imersão dos mesmos em uma solução de iodeto de metilamônio (CH3NH3I). Na segunda rota de síntese, filmes finos de PbI2 foram diretamente depositados por sputtering. A conversão desses filmes em CH3NH3PbI3 também foi realizada através do mergulho dos mesmo em uma solução de CH3NH3I. Esses dois métodos, permitiram-nos sintetizar filmes finos de CH3NH3PbI3 com boas propriedades ópticas e estruturais e também com uma completa cobertura do substrato, sem evidências de fissuras ou buracos, conforme indicado por microscopia eletrônica de varredura. Essas metodologias têm o potencial de abrir caminho para a produção em larga escala de células solares de CH3NH3PbI3 reprodutíveis e com alta eficiência. Como terceira rota de síntese, nanocristais de perovskita foram sintetizados utilizando pontos quânticos de PbS como precursores. Esse procedimento foi realizado através da iodação dos pontos quânticos de PbS, o que produziu nanofios de PbI2 com comprimento da ordem de 5 ?m e diâmetro de aproximadamente 200 nm. Os nanofios de PbI2 foram então convertidos em nanocristais de perovskita através de seu mergulho em uma solução de CH3NH3I, o que resultou em nanocristais de perovskita com comprimento da ordem de 5 ?m e largura de 400 nm / Abstract: Organic-inorganic hybrid perovskite has been subject of intense investigation due to their attractive optical and electronic properties, e.g., direct bandgap, high absorption coefficient and ambipolar charge transport. Such properties allowed the application of this material in solar cells and light emitting diodes efficiently. Thus, the development of new synthesis routes that allow the production of materials with the appropriate characteristics for each application is extremely important for the development of this area of research. Therefore, in this PhD work we¿ll present results on the synthesis and characterization of perovskite films and nanocrystals obtained from new methodologies, which are based on thin films of lead sulphide (PbS) and lead iodide (PbI2) deposited by rf-sputtering and on quantum dots of PbS. In the first synthesis route, amorphous PbS thin films deposited by sputtering were converted to PbI2 thin films by the iodination process at room temperature. This procedure resulted in a complete structural change, as attested by XRD measurements. The PbI2 films were converted into CH3NH3PbI3 by immersing them in a solution of methylammonium iodide (CH3NH3I). The second route consisted in depositing directly films of PbI2 by sputtering. The conversion into CH3NH3PbI3 also was performed by immersing the films in a CH3NH3I solution. These two methods allowed us to synthesize CH3NH3PbI3 thin films with good optical and structural properties and with complete substrate coverage, without evidence of cracks or holes, as verified by scanning electron microscopy images. Such methodologies have the potential to pave the way for the large-scale production of reproducible and high efficiency CH3NH3PbI3 solar cells. The third route was devoted to produce perovskite nanocrystals using PbS quantum dots as precursors. This approach was performed through iodination of PbS quantum dots. This produced PbI2 nanowires of about 5 ?m in length and 200 nm in diameter. The conversion in perovskite nanocrystals was accomplished through dip of the PbI2 nanowires into a solution of CH3NH3I. This procedure generated perovskite nanocrystals of about 5 ?m in length and 400 nm in width / Doutorado / Física / Doutor em Ciências / 165756/2014-4 / CNPQ

Solar cells and photodetectors rely on similar physical principles based on the interaction of light and matter. Both types of optoelectronic devices are indispensable in a wide range of technological applications, from large-scale renewable power conversion to everyday consumer items. In this thesis, the use of facile solution-processable semiconductors in solar cells and light sensors is studied with a focus on antimony sulfide (Sb₂S₃) and antimony sulfoiodide (SbSI). The improvement of the photovoltaic performance in Sb₂S₃ sensitized solar cells upon the controlled partial oxidation of the absorber layer is investigated. A reduction in charge carrier recombination is the reason for the improved efficiency, caused by the oxidation process. Further, a new chemical bath deposition method for antimony sulfide is developed. Carried out at room temperature, this technique eliminates the necessity of cooling equipment during the deposition process. The antimony sulfide from this method decreases the density of trap states compared to the conventional deposition. Power-conversion efficiencies of up to η=5.1% are achieved in antimony sulfide sensitised solar cells using the new room temperature deposition method. Finally, antimony sulfide is used as a precursor to form films of antimony sulfoiodide (SbSI) micro-crystals in a facile physical vapour process. These films are then used to fabricate photodetectors. With PMMA as an insulating spacer layer, the devices are built in a sandwich-type architecture. Optoelectronic characterisation shows that these devices have the shortest response and recovery times reported for SbSI photodetectors to date.

Este trabalho foi desenvolvido em duas etapas: i) síntese, caracterização e aplicação de pontos quânticos de CdS, CdSe e CdTe em células solares. ii) modificação da superfície dos pontos quânticos de CdSe através de troca de ligante, seguida de caracterização e aplicação em células solares. Os pontos quânticos foram sintetizados utilizando acetatos de cádmio, selênio, telúrio e enxofre como precursores e ácido oleico como agente de estabilização. Na segunda etapa o ácido oleico foi substituído por ligantes com maior afinidade eletrônica pelos sítios de Cd2+: ácido 3-mercaptopropiônico, 4-ácido-mercaptobenzóico e ácido 11-mercaptoundecanóico. As amostras foram caracterizadas por UV-Vis, fluorescência, microscopia eletrônica de transmissão, difratometria de raios-X e voltametria cíclica. Adicionalmente, testes de solubilidade, análises de TGA e de RMN foram realizadas para confirmar a troca de ligante. Através dos resultados, verificou-se que todos os pontos quânticos sintetizados são adequados para sensibilização de TiO2 em dispositivos fotovoltaicos. No entanto, os pontos quânticos de CdSe e CdTe apresentaram fatores que evidenciam maior confinamento quântico, sendo que a maior estabilidade do éxciton foi obtida para o CdSe. Através das análises de RMN foi possível verificar que não existe apenas uma confirguração espacial preferencial para a adsorção do ligante sobre a superfície deste ponto quântico enquanto que curvas de corrente versus potencial e de eficiência de conversão de fóton incidente mostraram que a eficiência do dispositivo é fracamente dependente do ligante. Contudo, a troca de ligantes favorece a solubilidade em solventes com diferentes polaridades, inclusive água, o que amplia as possibilidades de aplicação dos pontos quânticos sintetizados neste trabalho. / This work was developed in two stages: i) synthesis, characterization and application of CdS, CdSe and CdTe quantum dots to assemble solar cells, ii) surface modification, characterization and application of CdSe quantum dots to assemble solar cells. The quantum dots were synthesized by using cadmium acetate, Se, S or Te as precursors and oleic acid as stabilizing agent. In the second stage the oleic acid capping layer was replaced by other ligands with higher electron affinity to Cd2+: 3-mercaptopropionic acid, 4-mercaptobenzoic acid and 11-mercaptoundecanoic acid. The samples were characterized by UV-Vis, fluorescence, transmission electron microscopy, x-ray diffractometry and cyclic voltammetry. Additionally, solubility tests, TGA analysis and NMR were performed to evaluate the CdSe surface modification. The results showed that all quantum dots synthesized are adequate to sensitize TiO2 in photovoltaic devices. However, CdSe and CdTe quantum dots presented better quantum confinement and the exciton generated in CdSe presented the higher stability. NMR analysis provided information about the non-preferential orientation for adsorption of the ligands on the CdSe surface, meanwhile measurements of current vs. potential and incident photon current efficiency showed a weak dependence of photovoltaic device efficiency with the nature of the ligand. On the other side, the surface modification favors the solubility in solvents with different polarizabilities, including water, widening the range for applications of the quantum dots synthesized in this work.

Orientador: Paulo Roberto Bueno / Banca: Assis Vicente Benedetti / Banca: Germano Tremiliosi Filho / Banca: Neyde Yukie Murakami Iha / Banca: Sixto Giménez Juliá / Resumo: Diferentes morfologias nanoestruturadas foram estudadas por Espectroscopia de Impedância visando ampliar as suas funcionalidades em células solares. As análises realizadas em células solares sensibilizadas por corante à base de nanotubos de ZnO são reportadas. Essas células exibiram uma alta eficiência para coleta dos elétrons ao longo dos nanotubos de ~64 μm de comprimento. A Espectroscopia de Impedância Eletroquímica, a análise de potencial de circuito aberto e de conversão dos fótons incidentes em corrente elétrica foram usados para estudar o transporte e o tempo de vida das cargas elétricas durante o processo de conversão de energia. Apesar de o fotoanodo ser relativamente extenso, o tempo de extração da carga foi muito mais rápido do que o observado para nanopartículas de TiO2 tradicionais aplicados a esse tipo de célula. A coleta rápida dos elétrons é de importância prática porque permite uma alternativa ao uso do par redox convencional ( ), uma vez que a dinâmica na intercepção do elétron é mais rápida ocasionando uma significativa redução de perda na fotocorrente. Em outra frente são relatadas as análises efetuadas em células à base de TiO2 recobertas com ZrO2 e que empregam Spiro-OMeTAD como molécula condutora de buracos. Filmes mesoporosos de TiO2 recobertos com ZrO2 foram utilizados como elétrodos ativos para aplicação em células solares do estado sólido, sendo que os dispositivos foram caracterizados por Espectroscopia de Impedância Eletroquímica. A técnica de deposição de camadas atômicas de ZrO2 foi usada com o intuito de passivar os estados de aprisionamento superficiais durante o funcionamento do dispositivo. Isso foi realizado mediante o recobrimento (precisão em Angström) dos filmes mesoporosos de TiO2 com óxido de zircônio. Foi possível verificar que o recobrimento produz um aumento no desempenho geral da ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Different nanostructured anode morphologies - aiming at extending the functionality in solar cells - had been studied by Electrochemical Impedance Spectroscopy. The analyses were carried out in dye-sensitized solar cells based on ordered arrays of polycrystalline ZnO nanotubes. These cells exhibit efficient electron collection over the entire photoanode of ~64 μm length. Electrochemical Impedance Spectroscopy, open circuit photovoltage decay and incident-photon-to-current efficiency spectra are used to quantify conductivity and lifetime. Although the photoanode used is relatively long, the extraction time was faster than in traditional photoanodes based on TiO2 nanoparticles. The fast collection of electrons is of practical importance; it allows an alternative to the use of the conventional redox electrolyte ( ), because electron-interception dynamics is faster, which reduces significantly the photocurrent loss. On the other hand, solid-state dye-sensitized solar cells based on TiO2/ZrO2 core/shell nanoparticles, using a hole transport molecule named Spiro-OMeTAD, were also characterized. The atomic layer deposition technique was used with the intention of passivating the TiO2 surface with ZrO2. It was observed that the coating enhances the general performance of the photoelectrochemical cell after two ZrO2 deposition cycles. The Electrochemical Impedance Spectroscopy measurements provided evidence that the ZrO2 coating reduces recombination losses at the TiO2/Spiro-OMeTAD interface. Besides, the technique showed that the ZrO2 coating reduces the inductive effect at low frequency, this reduction being attributed to the passivation of localized surface states. Finally, results on the study of the effect of the Spiro-OMeTAD concentration (5 to 25 %) in the fabrication of solar cells, are discussed. Variations in recombination and capacitance correlate well with the ... (Complete abstract click electronic access below) / Doutor

Neste trabalho, é apresentada uma metodologia de fabricação de transistores de filmes finos orgânicos. Foram fabricadas células solares de heterojunção de poli(3- hexiltiofeno) (P3HT) e [6,6]-fenil-C61-butirato de metila (PCBM) por apresentarem máxima conversão de potência (PCE) de cerca 5 %. Partindo de rendimentos de 10-6 até atingir 1,7 %, são mostradas as dificuldades no processamento de filmes orgânicos e na caracterização destes dispositivos. Destacam-se dentre outros, a importância da geometria dos eletrodos, da preparação da solução de blendas orgânicas e dos cuidados na utilização de substratos flexíveis (e.g. polietileno tereftalato PET). A estrutura empregada é composta por vidro, óxido de índio dopado com estanho (ITO), poli(3,4- etilenodioxitiofeno) complexado com poli(ácido estireno-sulfônico) (PEDOT:PSS), P3HT:PCBM, fluoreto de lítio (LiF) e alumínio. PET coberto por In2O3/Au/Ag em substituição ao vidro-ITO é utilizado devido à busca da indústria eletrônica por materiais alternativos de baixo custo. Estrutura semelhante é empregada para caracterização da mobilidade dos portadores de carga em filmes orgânicos. Técnicas tais como Time of Flight (ToF), Charge Extraction in Linearly Increasing Voltage (CELIV), além da interpretação de curvas estacionárias de densidade de corrente por tensão (JxV) foram empregadas para estudo de derivados de poli(para-fenileno vinileno) (PPV). Foram obtidas mobilidades de 10-810-6 cm2/Vs para modelos de corrente limitada por carga espacial (SCLC) com armadilhas rasas e profundas. Mobilidades de efeito de campo caracterizadas em TFTs bottom gate bottom contact com porta comum são pelo menos duas ordens de grandeza superiores às obtidas através das técnicas anteriormente citadas. Foram utilizados diversos substratos (e.g. silício monocristalino e vidro-ITO), dielétricos (e.g. oxinitreto de silício (SiOxNy) por PECVD e SiO2 térmico), tratamentos de superfície (e.g. vapor de hexametildissilazana HMDS), semicondutores (derivados de PPV, P3HT, pentaceno) e eletrodos (e.g. camada de aderência de titânio). Definiu-se assim uma metodologia de seleção de novos semicondutores orgânicos para a indústria eletrônica. / In this work, it is presented a methodology for organic thin-film transistor (OTFT) fabrication. Poly(3-hexylthyophene) (P3HT):[6,6]-phenyl-C61-butyric acidmethyl ester (PCBM) bulk heterojunction solar cells were studied for their maximum power conversion efficiency (PCE) around 5 %. Efficiencies evolution in time from 10-6 to 1.7 % show the difficulties involved in organic thin-film processing and device characterization. It is of remarkable importance the electrodes geometry, the organic blend solution preparation and the extra-care while processing on flexible substrates (e.g. polyethylene terephthalate PET). Devices are composed of indium tin oxide covered glass, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS), P3HT:PCBM active layer, lithium fluoride (LiF) and aluminum. PET covered with In2O3/Au/Ag substituting glass-ITO was employed due to the electronic industry research for low cost alternative materials. Similar structure is used for charge carrier mobility characterization. Techniques such as Time of Flight (ToF), Charge Extraction in Linearly Increasing Voltage (CELIV), and charge transport modeling of current density vs. voltage (JxV) stationary curves were applied on semiconductors like poly(para-phenylene vinylene) (PPV) derivatives. Mobilities around 10-810-6 cm2/Vs for space charge limited current (SCLC) with shallow and deep traps were obtained. Field effect mobilities characterized in bottom gate bottom contact TFTs with common gate are at least two orders higher than previous values. During this work, it was tested different substrates (e.g. monocrystalline silicon and glass-ITO), insulators (e.g. PECVD silicon oxynitride and thermal SiO2), surface treatments (e.g. hexamethyldisilazane vapor), semiconductors (PPV derivatives, P3HT, pentacene) and electrodes (e.g. titanium adhesion layer). It was defined that way a methodology of new semiconducting material selection for the electronic industry.

No description available.

The buffer layer is a crucial component in thin film solar cells. Defects at the interface between absorber and buffer layer lead to high recombination rate and the band structure at the interface highly affects the performance of the solar cell. In this thesis a method to synthesize thin films containing cadmium, zinc and sulfur, CdZnS, by chemical bath deposition has been developed and evaluated. A higher current from the device is expected when replacing the common buffer layer cadmium sulfide, CdS, with the more transparent CdZnS. It is also possible that the alternative buffer provides a more favorable energy band alignment at the interface with the absorber Copper-Zinc-Tin-Sulfide (CZTS). The deposition process was developed by studying depositions on glass. Increasing [Zn2+]/[Cd2+] initially led to films with higher band gap (Eg). By varying deposition time the time before colloidal growth became dominant was observed. Addition of triethanolamine showed that triethanolamine binds stronger to zinc ions than to cadmium ions. Two recipes that led to Eg=2.63 eV were evaluated as buffer layer in Copper-Indium-Gallium-Selenide (CIGSe) and CZTS solar cells. The short circuit current of the devices increased in general with the CdZnS buffers compared to CdS. The best CZTS cell with a CdZnS buffer layer had 7.7 % efficiency compared to the 7.5 % reference. For future research it is recommended that the effect of thickness variation and deposition temperature is evaluated and that additional material characterization is performed in order to further understand and develop the deposition method.

O trabalho apresenta o desenvolvimento de células solares fotoeletroquímicas baseadas na sensibilização de n-TiO2 por corantes sintéticos e naturais. Abrange as etapas envolvidas na sua obtenção, desde a preparação dos compostos sensibilizadores, filmes semicondutores e meio eletrolítico, até a montagem e medidas de eficiência, com a otimização dos diversos componentes do sistema, num arranjo do tipo sanduíche. O fotoanodo consiste num filme de TiO2 nanocristalino sensibilizado pelo corante num substrato condutor. O contraeletrodo é composto de um substrato condutor recoberto com camada transparente de platina. Entre os eletrodos é disposta a camada de eletrólito, geralmente solução de l2/Lil em acetonitrila. A espessura do filme de semicondutor é controlada com os métodos empregados na deposição de sua dispersão. A transparência final depende ainda do método de preparação das dispersões. Utilizando as espécies cis-[(dcbH2)2RuLL\')2+, dcbH2 = 4,4\'-(CO2H)2-2,2\'bipiridina e L/L\' = cianopiridina, quinolina e H2O, como sensibilizador, obteve-se conversão eficiente de luz visível em eletricidade com valores de IPCE de até 50 % entre 400 e 550 nm. Os processos de transferência e recombinação de elétron na interface sensibilizador/semicondutor foram investigados por experimentos com técnicas resolvidas no tempo empregando sensibilizadores ancorados aos filmes transparentes de TiO2. O trabalho envolveu ainda incorporações técnicas para aumentar a eficiência da célula, como filamentos metálicos protegidos e a combinação de unidades independentes numa associação modular, que são objetos de patentes depositadas. / This work presents the development on photoelectrochemical solar cells based on sensitization of n-TiO2 by synthetic and natural dyes. It comprises several steps involved in the fabrication and assembling of the cells in a sandwich-type design. Preparation of sensitizers, semiconductor films and electrolyte media, as well as efficiency measurements and optimization of several components are presented. The photoanode consists of a dye-sensitized nanocrystalline n-TiO2 film onto a conductive substrate. The counterelectrode is a conductive substrate covered with a thin, transparent platinum layer. The electrolyte layer, usually l2/Lil in acetonitrile, is placed between the electrodes. The deposition process of the semiconductor dispersion onto the substrates regulates the resulting thickness of the film. The final transparency is also controlled by the procedure employed in the preparation of the semiconductor dispersion. The species cis-[(dcbH2)2RuLL\']2+, dcbH2 = 4,4\'-(CO2H)2-2,2\'-bipyridine and L/L\' = 4-cyanopyridine, quinoline and H2O, were employed as sensitizers resulting in IPCE (Incident Monochromatic Photon to Current Conversion Efficiency) values as high as 50 % from 400 to 550 nm. Electron transfer and recombination processes across the semiconductor/sensitizer interface were investigated with time-resolved experiments employing transparent films of n-TiO2 sensitized by the anchored species. Other approaches aiming improvements in efficiencies and the preparation of photoelectrochemical solar cells for different applications or through environmentally friendly processes are discussed. The use of protected metallic filaments and the modular arrangement of single cells are issues of patents.

A range of titanium dioxide (titania) samples provided by Huntsman Pigments and Additives were investigated for their suitability for use in various optoelectronic devices, specifically dye-sensitized solar cells (DSSCs). Five of the titania samples are 1-20 micrometre size spherical 3D porous beads made up of titania nanoparticles and a further six samples are porous titania nanoparticle clusters of no specific shape, all of which possess high surface areas from 85 to 276 m2g-1. The samples were compared to commercially available nanocrystalline TiO2 powders and paste. All of the samples were initially assessed for suitability in DSSC devices by investigating various properties such as crystal phase, particle size, band gap, morphology and N719 dye adsorption, both as a powder sample and as a sintered film, employing techniques such as powder x-ray diffraction, UV/Vis spectroscopy and scanning electron microscopy. Different methods of formulating the samples into pastes for application to a substrate were attempted and electrochemical properties of a selection of films were also compared. The more promising titania samples were formulated into dye-sensitized solar cells and cell efficiencies calculated. DSSC devices were also fabricated with low temperature (125oC) sintering of the titania layer to assess the suitability of the samples for use in devices with flexible substrates. Initial devices incorporating the Huntsman TiO2 samples provided low efficiencies (< 0.1%). The samples were then modified with pre-sintering treatment prior to paste formulation to optimize crystallinity, particle size, porosity and surface area. The modified titania bead samples showed great promise in low temperature sintered devices, providing device efficiencies of 2.8%, more than double that of those incorporating the standard P25 TiO2 (1.3%). After sample modification a superior solar cell performance (3.2%) was also observed in 510oC sintered devices when compared to the standard P25 TiO2 devices (2.9%), with higher photocurrent and open circuit voltage than devices fabricated from commercially optimized TiO2 paste. Devices were also fabricated using pre-sensitized titania in an attempt to reduce device manufacturing time. The modified samples again showed good performance, providing working devices with efficiencies comparable to the equivalent pre-sensitized P25 devices.