<p>För att undersöka skillnad i prestationsförmåga mellan celler sensiterade med växtextraktsbaserad färg, och celler sensiterademed ruteniumkomplex-baserad färg, samt huruvida presskraften påverkar en cells prestationsförmåga, tillverkades icke-slutna fotoelektrokemiska färg-sensiterade solceller med tunnfilmsfotoelektroder av pressad, nanoporös titandioxid.</p><p>Cellerna pressades med tre olika presskrafter och sensiterades med växtextraktsfärg från rödkål, rödbeta, viol och henna, samt en ruteniumkomplex-baserad färg som fick utgöra kontrollbetingelse. För varje cell uppmättes IPCE- och iV-värde och motsvarande fyllnadsgrad (fill factor) och dessa jämfördes.</p><p>Ingen signifikant skillnad kunde fastställas mellan celler pressade med olika presstryck. Bland cellerna sensiterade med växtextraktbaserad färg presterade rödbeta bäst. Cellen med högst effektivitet hade fyllnadsgraden 70%. Emellertid uppvisade de växtfärgade cellerna genomgående sämre effektivitet än de rutenium-sensiterade och fotoströmmarna var mycket låga. IPCE-värdena var allmännt låga: den bäst presterande cellen hade ett IPCE-värde på något över 0,06 i våglängdsintervallet 440-470 nm. En förklaring till detta är de övriga ämnen som förutom pigment återfinns i de växtbaserade färgerna. Dessa hindrar pigmentmättnad och förhindrar att växtfärgen når ruteniumfärgens intensitet. En annan anledning består i svårigheten att passa ihop energinivåerna i cellens elektrolyt-halvledarsystem med energinivåerna hos pigmentet i växtfärgen.</p> / <p>Non-sealed photoelectrochemical dye sensitised solar cells (DSSC) with pressed nanoporous TiO2 thin film photoelectrodes were manufactured for the purposes of finding out whether plant extractbased dye sensitised cells can perform as well as ruthenium complex-based dye sensitised cells and whether the pressing force affects the cell performance.</p><p>The cells were pressed with three different pressing forces and sensitised with plant extracts from red cabbage, beetroot, violet and henna, as well as with a ruthenium complex-based dye for comparison. The IPCE and iV values and the corresponding fill factors of the cells were evaluated and compared.</p><p>No significant difference between the cells pressed with different pressing forces could be established. Among the plant extract-based dye sensitised cells the ones sensitised with beetroot extract performed best. The cell that achieved the highest efficiency had a fill factor of 70%. Compared to the ruthenium-sensitised cells the overall performance of the plant dye sensitised cells were very poor and the produced photocurrents very low. The IPCE values were generally low: one of the best-performing cells had an IPCE value of slightly over 0.06 in the 440-470 nm wavelength ranges. One reason for this is that it is difficult to obtain a plant extract dye as intense and deep in colour as ruthenium complex-based dyes, since pigment saturation is obstructed by the presence of other chemical compounds in the plant extracts. Another is that it is a delicate and difficult matter to match the energy levels in the electrolyte-semiconductor system with the energy levels of the pigments in the plant extract dye.</p>

This thesis reports the study of metal oxide semiconductors for the application of photoelectrochemical water splitting with a particular emphasis on both anion and cation-doped zinc oxides. A study of the mechanisms of visible light absorption in both anion and cation-doped ZnO semiconductors, the potentials of metal oxide materials modified by impurities as one of the ideal photocatalysts in harvesting solar light has been explored. X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopes have been performed to establish the electronic structures of anion and cation-doped ZnO. Aluminium impurities in ZnO thin films reveal the relationship between the bandgap broadening and the so-called Burstein Moss effect. Both cadmium and sulphur dopants were incorporated in ZnO either as powders by the solid state synthesis or as thin films by spray pyrolysis technique. Cadmium and sulphur dopants demonstrate effective electronic bandgap reduction and an increasing absorption of visible light. Furthermore, the incorporation of cadmium and sulphur in ZnO were prepared as photoanodes and evaluated in a custom-built photoelectrochemical workstation for the measurement of photon energy conversion efficiencies.

För att undersöka skillnad i prestationsförmåga mellan celler sensiterade med växtextraktsbaserad färg, och celler sensiterademed ruteniumkomplex-baserad färg, samt huruvida presskraften påverkar en cells prestationsförmåga, tillverkades icke-slutna fotoelektrokemiska färg-sensiterade solceller med tunnfilmsfotoelektroder av pressad, nanoporös titandioxid. Cellerna pressades med tre olika presskrafter och sensiterades med växtextraktsfärg från rödkål, rödbeta, viol och henna, samt en ruteniumkomplex-baserad färg som fick utgöra kontrollbetingelse. För varje cell uppmättes IPCE- och iV-värde och motsvarande fyllnadsgrad (fill factor) och dessa jämfördes. Ingen signifikant skillnad kunde fastställas mellan celler pressade med olika presstryck. Bland cellerna sensiterade med växtextraktbaserad färg presterade rödbeta bäst. Cellen med högst effektivitet hade fyllnadsgraden 70%. Emellertid uppvisade de växtfärgade cellerna genomgående sämre effektivitet än de rutenium-sensiterade och fotoströmmarna var mycket låga. IPCE-värdena var allmännt låga: den bäst presterande cellen hade ett IPCE-värde på något över 0,06 i våglängdsintervallet 440-470 nm. En förklaring till detta är de övriga ämnen som förutom pigment återfinns i de växtbaserade färgerna. Dessa hindrar pigmentmättnad och förhindrar att växtfärgen når ruteniumfärgens intensitet. En annan anledning består i svårigheten att passa ihop energinivåerna i cellens elektrolyt-halvledarsystem med energinivåerna hos pigmentet i växtfärgen. / Non-sealed photoelectrochemical dye sensitised solar cells (DSSC) with pressed nanoporous TiO2 thin film photoelectrodes were manufactured for the purposes of finding out whether plant extractbased dye sensitised cells can perform as well as ruthenium complex-based dye sensitised cells and whether the pressing force affects the cell performance. The cells were pressed with three different pressing forces and sensitised with plant extracts from red cabbage, beetroot, violet and henna, as well as with a ruthenium complex-based dye for comparison. The IPCE and iV values and the corresponding fill factors of the cells were evaluated and compared. No significant difference between the cells pressed with different pressing forces could be established. Among the plant extract-based dye sensitised cells the ones sensitised with beetroot extract performed best. The cell that achieved the highest efficiency had a fill factor of 70%. Compared to the ruthenium-sensitised cells the overall performance of the plant dye sensitised cells were very poor and the produced photocurrents very low. The IPCE values were generally low: one of the best-performing cells had an IPCE value of slightly over 0.06 in the 440-470 nm wavelength ranges. One reason for this is that it is difficult to obtain a plant extract dye as intense and deep in colour as ruthenium complex-based dyes, since pigment saturation is obstructed by the presence of other chemical compounds in the plant extracts. Another is that it is a delicate and difficult matter to match the energy levels in the electrolyte-semiconductor system with the energy levels of the pigments in the plant extract dye.

Orientador: Prof. Dr. Cedric Rocha Leão / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, 2016. / Células fotoeletroquímicas (PECs) são dispositivos optoeletrônicos que convertem a energia solar em energia química através da fotoeletrólise da água. O vanadato de bismuto (BiVO4) é um semicondutor com propriedades fotocatalíticas promissoras para aplicação em PECs, apresentando uma das maiores eficiências teóricas na transformação da energia luminosa em energia química. Contudo, o BiVO4 pristino apresenta alguns fatores limitantes para sua eficiência, tais como baixa condutividade intrínseca e a curta duração das fotoexcitações. Resultados experimentais indicam que a incorporação de Mo ou W ao BiVO4 aumenta a geração de fotocorrente. Porém, esta incorporação apresenta resultados ótimos para as seguintes concentração dos dopantes: 10 at.% (percentual atômico) para o Mo e 8 at.% para o W. No presente trabalho, busca-se investigar por cálculos ab initio baseados na teoria do funcional da densidade como a variação na concentracão de Mo em matriz de BiVO4 altera as propriedades eletrônicas do semicondutor. Para tanto, a adição destes metais de transição foi abordada de dois modos: dopagem por Mo e formação de ligas quaternárias por Mo ou W. Os resultados de energia de formação de defeitos intrínsecos indicam que a síntese do BiVO4 em atmosfera pobre em oxigênio maximiza a formação de defeitos doadores rasos, otimizando a geração de fotocorrente no dispositivo. Os defeitos substitucionais de Mo em sítio de V são doadores rasos e apresentaram baixa energia de formação, contudo o aumento na concentração destes átomos promove o surgimento de níveis profundos que atuam como armadilhas de portadores de carga. As análises de densidade de estados projetada mostraram que os estados eletrônicos do Mo nas ligas quaternárias hibridizam-se sobretudo na banda de condução. Foram verificadas alterações nas massas efetivas de elétrons e buracos, bem como no gap de energia devido à adição dos elementos de liga. Potencialmente, a incorporação destes átomos pode propiciar a formação de ligas quaternárias com alteração também no alinhamento da banda de condução com o potencial de redução da água e no acoplamento elétron-fônon. / Photoelectrochemical cells (PECs) are optoelectronic devices that convert light energy into chemical energy through water splitting process. Bismuth vanadate (BiVO4) presents promissing photocatalytic properties for application in PECs. However, there are some limitant factors for the pristine BiVO4, such poor charge transport and excessive electron¿hole recombination. Previous experimental results show that the addition of Mo or W into BiVO4 increases the photocurrent generation. Nevertheless, these additions promote optimal photocurrent generation for 10 at.% (atomic percent) of Mo. and 8 at.% of W. In the present work, we propose to investigate using ab initio calculations based on density functional theory how the increment of Mo concentration into the BiVO4 can change its electronic properties. We approach this issue in two ways: doping using Mo and alloying by Mo or W. Results of thermodynamic studies to determine theoretically the conditions for nucleation and growth of BiVO4 pristine and doped suggest that the synthesis of BiVO4 in an oxygen poor atmosphere enhances the concentration of shallow donors, optimizing the photocurrent generation by the photoanode. Substitutional defects containing Mo into the V site are shallow donors that present low formation energy, however the enhancement in the alloy element concentration promotes the arising of deep levels which acts as trap for charge carriers. Analysis of projected density of states shows that the electronic states of Mo in quaternary alloys hybridize mainly in the conduction band. Our results indicate that this alloying changes the effective masses of electrons and holes, as well as the bandgap. Potentially, the alloying using Mo or W can change other properties, such as band edge alignment and electron-phonon coupling which will affect the device performance.

Increasing energy consumption and its environmental impacts make it necessary to look for alternative energy sources. Solar energy as huge energy source which is able to cover the terms sustainability is considered as a favorable alternative. Solar cells and solar fuels are two kinds of technologies, which make us able to harness solar energy and convert it to electricity and/or store it chemically. Metal oxide semiconductors (MOSs) have a major role in these devices and optimization of their properties (composition, morphology, dimensions, crystal structure) makes it possible to increase the performance of the devices. The light absorption, charge carriers mobility, the time scale between charge injection, regeneration and recombination processes are some of the properties critical to exploitation of MOSs in solar cells and solar fuel technology. In this thesis, we explore two different systems. The first one is a NiO mesoporous semiconductor photocathode sensitized with a biomimetic Fe-Fe catalyst and a coumarin C343 dye, which was tested in a solar fuel device to produce hydrogen. This system is the first solar fuel device based on a biomimetic Fe-Fe catalyst and it shows a Faradic efficiency of 50% in hydrogen production. Cobalt catalysts have higher Faradic efficiency but their performance due to hydrolysis in low pH condition is limited. The second one is a photoanode based on the nanostructured hematite/magnetite film, which was tested in a photoelectrochemical cell. This hybrid electrode improved the photoactivity of the photoelectrochemical cell for water splitting. The main mechanism for the improvement of the functional properties relies with the role of the magnetite phase, which improves the charge carrier mobility of the composite system, compared to pure hematite, which acts as good light absorber semiconductor. By optimizing the charge separation and mobility of charge carriers of MOSs, they can be a promising active material in solar cells and solar fuel devices due to their abundance, stability, non-toxicity, and low-cost. The future work will be focused on the use of nanostructured MOSs in all-oxide solar cell devices. We have already obtained some preliminary results on 1-dimensional heterojunctions, which we report in Chapter 3.3. While they are not conclusive, they give an idea about the future direction of the present research.

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