Solar energy is the foremost power source of our planet. Driving photosynthesis on our planet for 3 billion years the energy stored in the form of fossil fuels also originates from the sun. Consumption of fossil fuels to generate energy is accompanied with CO2 emission which affects the earth's climate in a serious manner. Therefore, alternative ways of converting energy have to be found. Solar cells convert sunlight directly into electricity and are therefore an important technology for future electricity generation. In this work solar cells based on the inorganic semiconductor titanium dioxide and hole-transporting dyes are investigated. These type of solar cells are categorized as hybrid solar cells and are conceptually related to both dye-sensitized solar cells and organic solar cells. Light absorption in the bulk of the hole-transporting dye layer leads to the formation of excitons that can be harvested at the organic/inorganic interface. Two design approaches were investigated: 1) utilizing a multilayer of a hole-transporting dye and 2) utilizing a hole-transporting dye as light harvesting antenna to another dye which is bound to the titanium dioxide surface.  Using a multiple dye layer in titanium dioxide/hole transporting dye devices, leads to an improved device performance as light harvested in the consecutive dye layers can contribute to the photocurrent. In devices using both an inteface-bound dye and a hole-transporting dye, excitation energy can be transferred from the hole-transporting dye to the interface dye.

In this thesis we synthesize two organic materials, one is discotic liquid crystal Acid-6, and the other is novel discotic liquid crystal polymer DLC-PAM. After demonstrating the molecular structures of Acid-6 and DLC-PAM by FT-MS, 1H-NMR and FT-IR, we use the two materials as photo-sensitized dyes for dye-sensitized solar cells(DSSCs) and manufacture two kinds of cells. We use polyacrylamide(PAM) as main chain of the novel discotic liquid crystal polymer DLC-PAM and graft the discotic liquid crystal monomer Acid-6 onto PAM by chemical synthesis. DLC-PAM belongs to side-chain liquid crystal polymer, and it can show the properties of it¡¦s discotic liquid crystal function. One of the properties is absorption of visible light. By observing the UV-Vis spectrum, we can realize the absorption band is located between 200 ~ 450 nm and confirm that it is able to be a photo-sensitized dye. Another property of discotic liquid crystal is the self-assembly ability, the moleculars can assemble into hexagonal columnar structure by themselves, and the property enable discotic liquid crystal to have better mobility. In this part, we can demonstrate DLC-PAM and Acid-6 really have hexagonal columnar structure by X-ray diffractmeter. After qualitative demonstrating and optical analysis, we use DLC-PAM and Acid-6 as photo-sensitized dyes for DSSCs and manufacture two kinds of cells successfully. The more photocurrent occur when the two DSSCs are woking. Besides, the two DSSCs have good performance on power conversion efficiency which can achieve 0.047 % for DLC-PAM and 0.364 % for Acid-6. Therefore, in this research we prove that DLC-PAM and Acid-6 are able to be photo-sensitized dyes for DSSCs and successfully demonstrate that using the two materials to manufacture DSSCs is feasible.

The photophysical properties of a series of Zn (II) porphyrins adsorbed onto a semiconductor were investigated using steady-state absorbance and emission measurements. The ability of the porphyrins to undergo triplet-triplet annihilation (TTA), a photophysical process through which photons in the red and near-infrared (NIR) regions of the optical spectrum can be converted into higher energy photons (upconversion), was explored. Aggregation capabilities were determined to verify possibility of these molecules to undergo triplet-triplet annihilation (TTA). TTA has significant potential for increasing the efficiency of dye-sensitized solar cells (DSSCs) by upconverting photons in the energy rich NIR region of the solar spectrum. A key requirement for efficient TTA is aggregation of the sensitizer dye, and in this thesis, we have examined the aggregation of porphyrins in TiO2-based sol-gel films. Solution phase absorption and emission studies were conducted using zinc (II) tetraphenylporphyrin and three of its functionalized derivatives, tetra(4-aminophenyl)porphyrin Zn(II), tetra(4-carboxyphenyl)porphyrin Zn(II), and tetra(4-sulfonatophenyl)porphyrin Zn(II), to evaluate their potential as DSSC sensitizers on TiO2 thin films. Mesoporous TiO2 thin films were synthesized, using a polymer-templating sol-gel route, and characterized with tunneling electron microscopy (TEM), atomic force microscopy (AFM), and UV-Vis absorbance measurements. Spectroscopy measurements were also carried out on porphyrin-sensitized TiO2 thin films and compared to solution-based results. A simple DSSC was constructed and used to further explore the application of zinc (II) porphyrin sensitizers in photovoltaic applications.

El processament convencional de materials d'òxid de titani (TiO2) inherentment implica un consum energètic important i ha esdevingut una limitació tecnològica per a la fabricació de dispositius emprant substrats termolàbils així com per a la preparació de materials híbrids orgànic/TiO2. Aquesta tesi doctoral tracta del desenvolupament de processos químics per a la preparació de capes i pols de TiO2 d'acord amb principis de Química Verda, posant especial èmfasi en el processament a baixa temperatura. Plantejaments simples, benignes amb el medi ambient i de baix cost són els desitjats sota aquestes directrius. A més a més, els mètodes a baixa temperatura (idealment fins a un màxim proper a 100ºC) haurien de donar lloc a materials que exhibeixin propietats similars a aquells processats a alta temperatura o tractats solvotermalment. S'ha focalitzat especial interès en la recerca d'aplicacions pràctiques dels materials produïts en camps com la conversió d'energia solar i materials actuadors. / El procesamiento convencional de materiales de óxido de titanio (TiO2) inherentemente implica un consumo energético importante y ha resultado una limitación tecnológica para la fabricación de dispositivos utilizando substratos termolábiles así como para la preparación de materiales híbridos orgánico/TiO2. Esta tesis doctoral trata el desarrollo de procesos químicos suaves para la preparación de capas y polvos de TiO2 de acuerdo con principios de Química Verde, poniendo especial énfasis en el procesamiento a baja temperatura. Planteamientos simples, benignos con el medioambiente y de bajo conste son los deseados bajo estas directrices. Además, los métodos a baja temperatura (idealmente hasta un máximo próximo a 100ºC) deberían dar lugar a materiales que exhiban propiedades similares a aquellos procesados a alta temperatura o tratados solvotermalmente. Se ha centrado especial interés en la búsqueda de aplicaciones prácticas de los materiales producidos en campos como la conversión de energía solar y materiales actuadores. / Conventional high-temperature processing of titanium dioxide (TiO2) materials inherently implies important energy consumption and has became a technological limitation for fabricating devices employing thermolabile substrates as well as preparing hybrid organic/TiO2 materials. This PhD thesis deals with the development of chemical processes for preparing TiO2 films and powders fitting Green Chemistry principles, putting special emphasis on low-temperature processing. Simple, environmentally benign and low-cost approaches are desired upon these guidelines. Moreover, low-temperature methods (ideally up to a maximum closer to 100ºC) should give rise to materials exhibiting properties similar than those processed at high temperature or solvothermally treated. Special interest has been focused on finding practical applications of the as-prepared materials in fields such as solar energy conversion and actuating materials.

In this research, gel-state and quasi-solid state dye-sensitized solar cells (DSSCs) were fabricated with polydimethylbenzimidazole(PDMBI) as the polymer electrolyte. These devices are stable under room light in air, even without encapsulation. The energy conversion efficiency of gel-state cells was drastically increased around 200% after the device worked. We propose that appropriately aggregated PDMBI in electrolyte layer could provide pathways which would facilitate the diffusion of ion through the electrolyte. Moreover, this arrangement induces it an ion exchange reaction which could lead to the promotion of the diffusion rate between iodide species. An optimized device performs well with a power conversion efficiency of 4.98% under air-mass 1.5 global (AM 1.5G) illumination. For the fabrication of quasi-solid state dye-sensitized solar cells, we immersed a few liquid electrolyte to improve electrical contact between TiO2 porous layer and PDMBI layer. The quasi-solid state cell efficiency fabricated with PDMBI as electrolyte was 2.26%. Furthermore, our device architecture is performing well because of the good band alignment among TiO2, dye, and PDMBI. In this research, we have successfully demonstrated gel-state and quasi-solid state dye-sensitized solar cells comprising PDMBI as electrolyte.

Wide band gap metal oxides have recently become one of the most investigated materials in surface science. Among these metal oxides especially TiO2 attracts great interest, because of its wide range applications, low cost, biocompatibility and ease of analysis by all experimental techniques. The usage of TiO2 as a component in solar cell technology is one of the most investigated applications of TiO2 . The wide band gap of TiO2 renders it inecient for isolated use in solar cells. TiO2 surface are therefore coated with a dye in order to increase eciency. This type of solar cells are called dye sensitized solar cells . The eciency of dye sensitized solar cells is directly related with the absorbed light portion of the entire solar spectrum by the dye molecule. Inspite of the early dyes, recent dye molcules, which are called wider wavelength response dye molecules, can absorb a larger portion of entire solar spectrum. Thus, the eciency of dye sensitized solar cells is increased by a considerably amount. In this thesis the electronic structure of organic rings, which are the fundamental components of the dye molecules, adsorbed on anatase (001) surface is analyzed using density functionaltheory. The main goal is to obtain a trend in the electronic structure of the system as a function of increasing ring number. Electronic structure analysis is conducted through band structure and density of states calculations. Results are presented and discussed in the framework of dye sensitized solar cells theory.

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Materials Science and Engineering." Includes bibliographical references (p. 164-172).

The dye sensitized solar cell (DSSC) is a promising low-cost technology alternative to conventional solar cell in certain applications. A DSSC is a photo-electrochemical photovoltaic device, mainly composed of a working electrode, a dye sensitized semiconductor layer, an electrolyte and a counter electrode. Sunlight excites the dye, producing electrons and holes that can be transported by the semiconductor and electrolyte to the external circuit, converting the sunlight into an electrical current. A material that could be useful for DSSCs is the nanoscale cupric oxide, which can act as a p-type semiconductor and has interesting properties such as low thermal emittance and relatively good electrical properties. The goal of this project was to synthesize and characterize CuO nanoparticles using three different methods and look into each products potential use and efficiency in DSSCs. The particles were synthesized using two different solution based chemical precipitation methods and a flame spray pyrolysis method, yielding nanostructures with different compositions, structures and sizes ranging from ~20 to 1000 nm. The nanoparticle powder synthesized by the flame spray pyrolysis route was tested as semiconductor layer in the working electrode of the DSSC. Current-voltage measurements presented low solar conversion efficiencies with a reversed current, meaning that the cupric oxide cells did not work in a desirable way. Further studies of the cupric oxide synthesis and its suitability in DSSCs are needed to increases the future possibilities for gaining well working p-type DSSCs with higher efficiencies.

The pure organic dye sensitized solar cells (DSSCs) were studied and a new organic dye of donor-π-2acceptors (D-π-2A configuration) was fabricated. This dye, denoted as B2, was investigated and applied in DSSCs. Density functional theory (DFT) was used to examine the electronic distribution of the frontier orbitals of the B2 dye. It was found that intramolecular charge transfer (ICT) between the donor moieties and acceptor moieties of the B2 dye may take place under photo irradiation. The LUMO, LUMO+1 and LUMO+2 of B2 are all distributed on the acceptor moieties and this is very helpful to enhance the intramolecular electron transfer from the donor moieties to the acceptor moieties, which will consequently promote the chance of electron injection into the semiconductor. DSSCs based on B2 demonstrated an power conversion efficiency of 3.62 %. This efficiency value is approximately half of the power conversion efficiency of DSSCs based on N719 (7.69 %) under the same conditions. Femtosecond transient absorption and nanosecond transient absorption (TA), and time-correlated single photon counting (TCSPC) technique were applied to examine the electron transfer processes occurring on the surface of B2/TiO2. B2 dye has life time of the excited states three orders in magnitude shorter than that of N719. The electron injection time from excited B2 to TiO2 is also three orders in magnitude shorter than that from excited N719 to TiO2. It was revealed that the delocalized electrons of π → π* transition for both the B2 dye and the N719 dye could be further guided into the semiconductor, while such injection processes may not happen for the localized electrons in π → π* transition of these dyes. The nanosecond transient absorption and transient emission spectroscopy of the ruthenium bipyridyl sensitizer N719 in different solvents were studied. Three kinds of ZnO nanoparticles were utilized to study the electron transfer process taking place on the interface of N719/ZnO with and without electrolyte by Time-Correlated Single Photon Counting (TCSPC) technique, TA and transient emission spectroscopy. Isopropanol was found to stabilize the singlet excited state of N719 and a related emission band centered at 460 nm was observed in nanosecond time scale. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics on the N719/ZnO interface. In the absence of electrolyte, the electron transfer process on the N719/ZnO interface is dependent upon the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, the impact of ZnO defects upon the electron transfer process is eliminated and the effective electron injection happens from the excited states of N719 to ZnO, in spite of the ZnO particle sizes. The polymer based solar cells were studied and a polymer incorporated with a pyrenylcarbazole pendant was synthesized and applied in the functionalization of multi-wall carbon nanotubes (MWCNT) by noncovalent π-π interaction. The polymer/MWCNT hybrids were isolated and examined. The strong interaction between the polymer and MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was investigated. The emission spectra demonstrated an effective quenching of emission from the polymer by the MWCNT. DFT calculations showed an electron delocalization phenomenon between the pyrene and carbazole moieties. The LUMO of the polymer is mainly located on the pyrene moiety while the LUMO+1 of the polymer is predominantly positioned on the carbazole moiety. The electronic transition of LUMO+1→LUMO results in intramolecular charge transfer (ICT) from the carbazole moieties to the pyrene moieties. Femtosecond TA determined the characteristic TA feature of the excited states, which are contributed from both the pyrene and carbazole moieties. The excited state lifetime of the polymer was calculated to be 659 ps and the photo excited electrons can inject into the MWCNT very fast on a time scale of 420 fs. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Dye-sensitized solar cells (DSSCs) are photoelectrochemical cells that offer efficient and potentially economical alternative to conventional solar electricity production technologies. DSSCs belong to the third generation of solar cells and offer several advantages over the solid-state junction solar cells. They utilize materials, such as titanium dioxide that are inexpensive and abundant relative to those used in conventional solar cells. Moreover, DSSCs can be fabricated with simple and scalable manufacturing processes. Finally, in DSSCs, photon absorption and charge-carrier transport are undertaken by different materials, namely molecular dyes and wide band gap semiconductors, respectively. Unlike conventional solar cells, no compromise is necessary between decreasing the band gap for visible light absorption and increasing the band gap to resist photocorrosion. For successful commercialization, a photovoltaic system incorporating DSSCs must operate reliably under a wide range of solar irradiance and operating temperatures. This experimental study reports the fabrication and characterization of the performance of a DSSC as a function of irradiance and operating temperature. The prototyped DSSCs had (i) nanocrystalline titanium(IV) dioxide, TiO₂, photoanode, (ii) platinum thin film cathode, and (iii) acetonitrile based liquid electrolyte. The photoanodes were sensitized with N-749 dye. The current-voltage characteristics of the DSSCs were measured at operating temperatures from 5 to 50° C and under 500, 1000, and 1500 W m⁻² irradiance. The open circuit voltage, V[subscript oc], decreased linearly with increasing temperature and had positive, logarithmic relation with irradiance. At temperatures lower than 15° C and 1500 W m⁻² irradiance, short circuit current density, J[subscript sc], was limited by the diffusion of I₃ in the electrolyte and increased with increasing temperature. At temperatures lower than 15° C and lower irradiance, J[subscript sc] increased with increasing temperature due to electron density limited recombination of electrons injected into the TiO₂ conduction band. At higher temperatures, the recombination was dominant over diffusion and J[scubscript sc] decreased with increasing temperature. Moreover, J[subscript sc] increased linearly with increasing irradiance. The DSSC photoconversion efficiency did not vary appreciably at temperatures lower than 15° C but decreased with increasing temperature. Finally, the DSSC efficiency increased with increasing irradiance. There was no indication of significant coupling effect of irradiance and temperature on DSSC efficiency. This study reports for the first time the coupling between irradiance and thermal effects on the operation of DSSCs. The results reported in this study can be used in recovering kinetic and transport properties that can be used in modeling and optimization of DSSCs. / text