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.

Due to climate change and its effects, alternative renewable energy sources are needed in the future human society. In the work of this thesis, the Dye-sensitized Solar Cell (DSC) has been investigated and characterized. DSCs are appealing as energy conversion devices, since they have high potential to provide low cost solar light to electricity conversion. The DSC is built up by a working electrode consisting of a conductive glass substrate with a dye-sensitized mesoporous TiO2 film, a counter electrode with a catalyst and, in between, the electrolyte which performs the charge transport by means of a redox mediator. The aim of this thesis was to develop and evaluate cheap and environmentally friendly materials for the DSC. An alternative polymer-based counter electrode catalyst was fabricated and evaluated, showing that the PEDOT catalyst counter electrode outperformed the platinum catalyst counter electrode. Different organic dyes were evaluated and it was found that the dye architecture affected the performance of the assembled DSCs. A partly hydrophilic organic triphenylamine dye was developed and applied in water-based electrolyte DSCs. The partly hydrophilic dye outperformed the reference hydrophobic dye. Small changes in dye architecture were evaluated for two similar dyes, both by spectroscopic and electrochemical techniques. A change in the length of the dialkoxyphenyl units on a triphenylamine dye, affected the recombination and the regeneration electron transfer kinetics in the DSC system. Finally, three water soluble cobalt redox couples were developed and applied in water-based electrolyte DSCs. An average efficiency of 5.5% (record efficiency of 5.7%) for a 100% water-based electrolyte DSC was achieved with the polymer-based catalyst counter electrode and an organic dye with short dimethoxyphenyl units, improving the wetting and the regeneration process.

We report the synthesis of ultra-high aspect ratio copper nanowires (CuNW) and fabrication of CuNW-based transparent conductive electrodes (TCE) with high optical transmittance (> 80%) and excellent sheet resistance (R-s < 30 Omega/sq). These CuNW TCEs are subsequently hybridized with aluminum-doped zinc oxide (AZO) thin-film coatings, or platinum thinfilm coatings, or nickel thin-film coatings. Our hybrid transparent electrodes can replace indium tin oxide (ITO) films in dye-sensitized solar cells (DSSCs) as either anodes or cathodes. We highlight the challenges of integrating bare CuNWs into DSSCs, and demonstrate that hybridization renders the solar cell integrations feasible. The CuNW/AZO-based DSSCs have reasonably good open-circuit voltage (V-oc = 720 mV) and short-circuit current-density (J(sc) = 0.96 mA/cm(2)), which are comparable to what is obtained with an ITO-based DSSC fabricated with a similar process. Our CuNW-Ni based DSSCs exhibit a good open-circuit voltage (V-oc = 782 mV) and a decent short-circuit current (J(sc) = 3.96 mA/cm2), with roughly 1.5% optical-to-electrical conversion efficiency.

The long-term outdoor durability of dye-sensitized solar cells (DSSCs) is still a challenging issue for the large-scale commercial application of this promising photovoltaic technique. In order to study the degradation mechanism of DSSCs, ageing tests under selected accelerating conditions were carried out. The electrolyte is a crucial component of the device. The interactions between the electrolyte and other device components were unraveled during the ageing test, and this is the focus of this thesis. The dynamics and the underlying effects of these interactions on the DSSC performance were studied. Co(bpy)32+/3+-mediated solar cells sensitized by triphenylamine-based organic dyes are systems of main interest. The changes with respect to the configuration of both labile Co(bpy)32+ and apparently inert Co(bpy)33+ redox complexes under different ageing conditions have been characterized, emphasizing the ligand exchange problem due to the addition of Lewis-base-type electrolyte additives and the unavoidable presence of oxygen. Both beneficial and adverse effects on the DSSC performance have been separately discussed in the short-term and long-term ageing tests. The stability of dye molecules adsorbed on the TiO2 surface and dissolved in the electrolyte has been studied by monitoring the spectral change of the dye, revealing the crucial effect of cation-based additives and the cation-dependent stability of the device photovoltage. The dye/TiO2 interfacial electron transfer kinetics were compared for the bithiophene-linked dyes before and after ageing in the presence of Lewis base additives; the observed change being related to the light-promoted and Lewis-base-assisted performance enhancement. The effect of electrolyte co-additives on passivating the counter electrode was also observed. The final chapter shows the effect of electrolyte composition on the electrolyte diffusion limitation from the perspectives of cation additive options, cation concentration and solvent additives respectively. Based on a comprehensive analysis, suggestions have been made regarding lithium-ion-free and polymer-in-salt strategies, and also regarding cobalt complex degradation and the crucial role of Lewis base additives. The fundamental studies contribute to the understanding of DSSC chemistry and provide a guideline towards achieving efficient and stable DSSCs. / <p>QC 20160517</p>

This thesis studies parameters influencing both the performance and stability of solid state sensitized solar cells (ssSSCs). ssSSCs benefit from their low materials and manufacturing processing costs, a consequence of using solution processed materials. However, solution processed materials are often structurally and electronically disordered. By characterizing fully operational ssSSCs and their charge transport properties, this thesis elucidates the factors limiting charge transport and proposes routes towards both improved photovoltaic conversion efficiency and long-term stability. Chapter 2 provides an explanation of the operation of ssSSCs, while Chapter 3 discusses the basic methods used in this thesis. Having set this background, Chapter 4 explores the interaction between atmospheric oxygen and charge doping mechanisms in the organic semiconductors used in ssSSCs. To understand the implications of the findings presented in Chapter 4, a new technique, “transient mobility spectroscopy”, was developed to understand the evolution of balanced charge transport behaviour of disordered semiconductors at different operating conditions in ssSSCs. This technique is presented in full in Chapter 5. The understanding gained in Chapters 4 and 5 suggest that alternative light absorbers with higher extinction coefficients may be beneficial to improving the performance of ssSSCs. Chapter 6 discusses the use of an organometal trihalide perovskite, as light absorber in ssSSCs. Using time resolved techniques, the charge transport and recombination mechanisms in various device architectures are explored, allowing suggestions to be made towards future improvements. Chapter 7 uses the technique presented in Chapter 5 to understand a rapid degradation mechanism of working ssSSCs. Particular focus is placed on the titanium dioxide charge-transporting layer. Building on this newfound understanding, two methods for attaining stable photovoltaic performance are provided, a great step forward for this technology.

Doctor of Philosophy / Department of Chemistry / Jun Li / Solar energy as a sustainable resource is a promising alternative to fossil fuels to solve the tremendous global energy crisis. Development of three generation of solar cells has promoted the best sunlight to electricity conversion efficiency above 40%. However, the most efficient solar cells rely on expensive nonsustainable raw materials in device fabrication. There is a trend to develop cost-effective biophotovoltaics that combines natural photosynthetic systems into artificial energy conversion devices such as dye sensitized solar cells (DSSCs). In this research, a model system employs natural extract light-harvesting complex II (LHCII) as a light-absorbing sensitizer to interface with semiconductive TiO₂ and plasmonic nanoparticles in DSSCs. The goal of this research is to understand the fundamental photon capture, energy transfer and charge separation processes of photosynthetic pigment-protein complexes along with improving biophotovoltaic performance based on this model system through tailoring engineering of TiO₂ nanostructures, attaching of the complexes, and incorporating plasmonic enhancement. The first study reports a novel approach to linking the spectroscopic properties of nanostructured LHCII with the photovoltaic performance of LHCII-sensitized solar cells (LSSCs). The aggregation allowed reorganization between individual trimers which dramatically increased the photocurrent, correlating well with the formation of charge-transfer (CT) states observed by absorption and fluorescence spectroscopy. The assembled solar cells demonstrated remarkable stability in both aqueous buffer and acetonitrile electrolytes over 30 days after LHCII being electrostatically immobilized on amine-functionalized TiO₂ surface. The motivation of the second study is to get insights into the plasmonic effects on the nature of energy/charge transfer processes at the interface of photosynthetic protein complexes and artificial photovoltaic materials. Three types of core-shell (metal@TiO₂) plasmonic nanoparticles (PNPs) were conjugated with LHCII trimers to form hybrid systems and incorporated into a DSSC platform built on a unique open three-dimensional (3D) photoanode consisting of TiO₂ nanotrees. Enhanced photon harvesting capability, more efficient energy transfer and charge separation at the LHCII/TiO₂ interface were confirmed in the LHCII-PNP hybrids, as revealed by spectroscopic and photovoltaic measurements, demonstrating that interfacing photosynthesis systems with specific artificial materials is a promising approach for high-performance biosolar cells. Furthermore, the final study reveals the mechanism of hot electron injection by employing a mesoporous core-shell (Au@TiO₂) network as a bridge material on a micro-gap electrode to conduct electricity under illumination and comparing the photoconductance to the photovolatic properties of the same material as photoanodes in DSSCs. Based on the correlation of the enhancements in photoconductance and photovoltaics, the contribution of hot electrons was deconvoluted from the plasmonic near-field effects.

Novel methods for the construction of dye-sensitized solar cells (DSSCs) were developed. A thin dense underlayer of TiO2 was applied on fluorine-doped tin oxide (FTO) glass using as a precursor Tyzor AA-105. Subsequently a mesoporous film of P-25 TiO2 was applied by spreading a suspension uniformly over the surface of the underlayer and allowing the plate to slowly dry while resting on a level surface. After sintering at 500° C slides were treated with TCPP as a sensitizing dye and assembled into DSSCs. A novel method was used to seal the cells; strips of Parafilm® were used as spacers between the electrodes and to secure the electrodes together. The cells were filled with a redox electrolyte and sealed by dipping into molten paraffin. A series of phosphonic acids and one arsonic acid were employed as coadsorbates in DSSCs. The coadsorbates were found to compete for binding sites, resulting in lower levels of dye adsorption. The resulting loss of photocurrent was not linear with the reduction of dye loading, and in some cases photocurrent and efficiency were higher for cells with lower levels of dye loading. Electrodes were treated with coadsorbates by procedures including pre-adsorption, simultaneous (sim-adsorption), and post-adsorption, using a range of concentrations and treatment times and a variety of solvents. Most cells were tested using an iodide-triiodide based electrolyte (I3I-1) but some cells were tested using electrolytes based on a Co(II)/Co(III) redox couple (CoBpy electrolytes). Phosphonic acid post-adsorbates increased the Voc of cells using CoBpy electrolytes but caused a decrease in the Voc of cells using I3I-1 electrolyte. Phosphonic acids as sim-adsorbates resulted in a significant increase in efficiency and Jsc, and they show promise as a treatment for TCPP DSSCs.

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