In my study, I attempted to use the high electrical conductivity of graphite modified TiO2 nanoparticles to study the effect of graphite modification to the performance of dye-sensitized solar cell. Graphite oxide (GO) was successfully prepared by the improved Hummer¡¦s method. Graphenes that from the as-prepared GO reduced with hydrazine hydrate and sodium borohydride were characterized by Fourier transform infrared spectroscopy (FT-IR) and UV-visible spectroscopy. The performance of TiO2 based DSSC revealed a short-circuit photocurrent density of 11.24 mA/cm2, an open-circuit voltage of 0.66 V, and a fill factor of 0.48, yielding an overall conversion efficiency of 4.50%. The TiO2 / r-GO composite based DSSC showed higher efficiency than those standard DSSC, revealed a short-circuit photocurrent density of 18.48 mA/cm2, an open-circuit voltage of 0.68 V, and a fill factor of 0.51, yielding an overall conversion efficiency of 7.83%. On the other hand, we found the DSSC that treated with small amount of alcohol in making the TiO2 paste showed superior performance to that with untreated photoanode, the ratio of energy conversion efficiency being 7.11% to 4.50%.

Semiconductors absorb photos with energy greater than their band gap energy may induce electron-hole pairs. In semiconductor physics, increasing charge carrier improves the electric conductivity of semiconductor. The following methodology was taken to investigate the electric conductivity and the electron hole pairs affected performance of a dye sensitized solar cell. I applied 3 specific monochromatic light (365nm, 405nm and 437nm, respectively.) mixed with xenon light and normal xenon light separately illuminating on dye sensitized solar cells. At the assumption of the normalized photon to current conversion efficiency of solar cell illuminated by 437nm monochromatic light is 100%, the normalized photon to current conversion efficiency of the solar cell illuminated by 365nm monochromatic light was only 28%, however, that illuminated by 365nm monochromatic light mixed xenon light raised to 58%. The more intense mixed light produced more excited electrons than only 365nm monochromatic light. The holes generated by 365nm monochromatic light is easier to be captured by the electrons in the more intense mixed xenon light irradiation results in higher photon to current conversion efficiency. The output of photocurrent of the dye sensitized solar cell irradiated by 365nm ultraviolet light mixed xenon light was enhanced most significantly by 6.53% compared with that by normal xenon light irradiation.

In my research, I use sucrose to modify TiO2 nanoparticles to study the influence of sucrose modification in the performance of dye-sensitized solar cell. Two types of TiO2 are used in the experiments, one is P-25 (80% anatase, Degussa, Germany) and the other is ST-21 (100% anatase, Ishihara Sangyo, Japan). The results shows that the solar cells with TiO2 photoanode sintered in N2 has better conversion efficiency than that sintered in air. On the other hand, the TiO2 with 0.08 g/mL sucrose modification and sintered in N2 has the best conversion efficiency than the others with different sucrose concentration modifications and the highest conversion efficiency reaches 5.55 %. The performance of P-25 with 0.08 g/mL sucrose made solar cell is 10.9% higher than that without sucrose modification TiO2 photoanode made solar cell and the performance of ST-21 with 0.08 g/mL sucrose made solar cell is 5.4% higher than that without sucrose modification.

In this study, we used two types of materials as working electrode of DSSC, commercial TiO2 (P25) nanoparticle and ZnO nanotip. First part, we sintered TiO2 electrode in nitrogen and treat TiO2 electrode by UV light irradiation, in order to increase dye adsorption onto TiO2. Second part, we used three different buffer layers to grow ASD-ZnO nanotip as working electrode, sputtered-ZnO, sputtered-AZO and spin-coating TiO2. The hetrojuction is formed at the interface of ZnO nanotip/TiO2 buffer layers due to the different materials between ZnO and TiO2, which is beneficial for electron-hole separation. The morphology was measured by field emission scanning electron microscope (FE-SEM, Philip XL-40FEG). The sheet resistivity was measured by four-point probe. The crystallinity was examined by X-Ray diffraction (XRD, Simens D5000). Structural and spectral properties are characterized by ultraviolet-visible spectroscopy (UV-Vis) spectroscopy. The fourier transform infrared spectroscopy (FT-IR spectroscopy, BRUKER 66v/s) deals with the infrared region of absorption spectroscopy. And the Angilent B1500A is used for current-voltage (I-V) characterization of solar cells. In our results, we enhance the performance of TiO2 electrode DSSC, the open circuit voltage can reach to 0.56 V, the short circuit current density can reach to 16.65 mA/cm2, the conversion efficiency can reach to 4.6 % and the fill factor can reach to 49.2 %. On ZnO electrode, the open circuit voltage can reach to 0.57 V, the short circuit current density can reach to 4.43 mA/cm2, the conversion efficiency can reach to 1.21 % and the fill factor can reach to 47.9 %.

In this research, hills-like hierarchical structured TiO2 photoanodes for dye-sensitized solar cells (DSSCs) have been prepared. We expected these appropriately aggregated TiO2 clusters in photoanode layer could cause stronger light scattering and higher dye loading that increased efficiency of photovoltaics. For detailed light-harvesting study, different molecular weight of polyvinyl alcohol (PVA) polymers were used as binders for TiO2 nanoparticles (P-25 Degussa) aggregation. After preparing a series of TiO2 films with dissimilar morphology, the reflection of TiO2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices were measured and investigated. An optimized device had higher dye loading and well light harvesting at the same time that induced a 23% increase of short-circuit current Jsc in DSSCs. Moreover, we found that electrolyte could penetrate or diffuse easily in this higher porous structure. We fabricated dye-sensitized solar cells with MPN-based liquid electrolyte and gel polymer electrolyte. From this structure, the short-circuit current Jsc was increased around 16% and 19% respectively compared to conventional layers with liquid electrolyte and gel polymer electrolyte devices. The increase in highly viscous system of gel polymer electrolyte is due to easier penetration by such hills-like hierarchical structure.

In this study, we deposit titanium dioxide (TiO2) on the indium tin oxide (ITO/glass) substrate by a liquid phase deposition (LPD) method as a buffer layer and coat TiO¬2 particles on LPD-TiO2 films by spin-coating method as anode of dye-sensitize solar cell (DSSC). In order to adjust the optical absorption edge of titanium dioxide to the visible light, we co-dope fluorine and nitrogen into TiO2 by LPD method and Ammonium Fluoride (NH4F). In our experiment, the morphology and thickness was characterized by scanning electron microscopy (SEM), structure was characterized by X-ray diffraction (XRD), chemical properties was characterized by electron spectroscope chemical analysis (ESCA), structural and spectral properties were characterized by ultraviolet-visible spectroscopy (UV-Vis) spectroscopy and current-voltage (I-V) characterization of solar cells was measured by B1500A. In our results, we enhance the performance of TiO2 as a DSSC`s anode, the open circuit voltage can reach to 0.71 V, the short circuit current can reach to 5.14 mA, the conversion efficiency can reach to 1.91 % and the fill factor can reach to 52.5 %.

Excitonic photovoltaic devices, including organic, hybrid organic/inorganic, and dye-sensitized solar cells, are attractive alternatives to conventional inorganic solar cells due to their potential for low cost and low temperature solution-based processing on flexible substrates in large scale. Though encouraging, they are currently limited by the efficiency from not yet optimized structural and material parameters and poor overall knowledge regarding the fundamental details. This dissertation aims to achieve improved performance of hybrid solar cells by enhancing material property and designing new device architecture. The study begins with the addition of XD-grade single-walled carbon nanotube (XDSWNT) into poly(3-hexylthiophene) (P3HT) to improve the current density. By having a weight ratio of XDSWNT and P3HT equaled to 0.1:1, short-circuit current was quadrupled from 0.12 mA cm-2 to 0.48 mA cm-2 and solar cell efficiency was tripled from 0.023% to 0.07%, compared to devices with pure P3HT as a hole transport material. Secondly, a significant improvement in device efficiency with 250 nm long ZnO nanorod arrays as photoanodes has been achieved by filling the interstitial voids of the nanorod arrays with ZnO nanoparticles. The overall power conversion efficiency increased from 0.13% for a nanorod-only device to 0.34% for a device with combined nanoparticles and nanorod arrays. The higher device efficiency in solid-state DSSCs with hybrid nanorod/nanoparticle photoanodes is originated from both large surface area provided by nanoparticles for dye adsorption and efficient charge transport provided by the nanorod arrays to reduce the recombinations of photogenerated carriers. Followed by the novel layer-by-layer self-assembly deposition process, the hybrid photoanode study was extended to the longer ZnO nanorod arrays. The best performance, 0.64%, was achieved when the thickness of the photoanodes equaled to 1.2 ?m. Finally, the photovoltaic devices were modified by adding ZnO nanoarpticles into P3HT to increase interfacial area between ZnO and P3HT. The efficiency was enhanced from 0.18% to 0.45% when the ZnO nanorod arrays were 625 nm in length. Our successful design of the device morphology significantly contributes to the performance of solid-state hybrid solar cells.

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.

A novel ionic liquid crystal (ILC) system (C18IMCNBr) with a liquid crystal alignment used as an electrolyte for a dye-sensitized solar cell (DSSC) showed the higher short-circuit current density (Jsc) and the higher light-to-electricity conversion efficiency than the system using the non- alignment liquid crystalline ionic liquid (C18IMCNBr),due to the higher conductivity of liquid crystal alignment. The larger Jsc and efficiency value of liquid crystal alignment supported that the higher conductivity of liquid crystal alignment is attributed to the enhancement of the exchange reaction between iodide species. As a result of formation of the two-dimensional electron conductive pathways organized by the localized I3- and I- at liquid crystal alignment layers, the concentration of polyiodide species exemplified by Im- (m =5,7, ...) was higher in alignment C18IMCNBr. However, in the two-dimensional electron conductive pathways of C18IMCNBr, more collision frequencies between iodide species (I-,I3-, and Im-) could be achieved than that in the three-dimensional space of C18IMCNBr, which could lead to the promotion of the exchange reaction between iodide species, the contribution of a two-dimensional structure of the conductive pathway through the increase of collision frequency between iodide species was proposed.

Alternative energy sources such as solar energy have attracted an extensive interest in the petroleum shortage era. Among solar cells, dye-sensitized solar cell (DSSC) attracts the attention of widespread research teams because of the easy-production process, low cost, and good photon-to-electron conversion efficiency. In this study, both UV and acid solution such as HCl are used to improve the efficiency of DSSC. The UV illumination can eliminate organic contaminates on TiO2 by photocatalysis and enhance the adsorption of dye molecules. Meanwhile, the coordination mode between TiO2 and dye could be changed and lower the electron transportation. If the HCl solution is used after UV illumination, the coordination mode can be preserved. Moreover, H+ from HCl can attract the COO¡Ð anchoring group of dye by electrostatic force. It further increases the adsorption of dye and improves the DSSC efficiency. The coordination mode was measured by Fourier-transform infrared spectrometer (FTIR). The internal resistance was measured by electrical impedance spectroscopy (EIS). The chemical properties were characterized by X-ray photoelectron spectroscopy (XPS). The light absorbance was measured by ultraviolet-visible spectroscopy (UV-Vis). The morphology was observed by field emission scanning electron microscope (FE-SEM). The performance of the cells was measured by a semiconductor device analyzer. In our results, the conversion efficiency was improved from 6.29% of untreated one to 6.71 and 7.39% for UV and UV + HCl treated ones.