Upconversion studies typically use a laser as the source of low-energy photons. A lower cost, lower power, and fully tunable system was used here. The Fluoromax-4 spectrofluorometer incorporates a 150-W xenon lamp and illuminates a 1 cm3 volume in a standard cuvette. The 1 cm x 1cm excitation beam was reduced to 1 mm x 1 mm, increasing the power density by 100. The cuvette was mounted upon a translation stage so that the excitation beam could skim the inside surface of the cuvette. This minimized self-absorption of the output. Finally, an optical filter was included between the excitation monochromator and the cuvette to ensure that the solution was not exposed to undesired wavelengths. The instrument was tested with a known sensitizer/emitter system and upconversion was detected. Subsequently, a new pair of compounds was studied but the [Ru(deab)3]2+/BPEA solution did not exhibit upconversion in the modified instrument.

Photon energy upconversion (UC), a process that can convert two or more photons with low energy to a single photon of higher energy, has the potential for overcoming the thermodynamic efficiency limits of sunlight-powered devices and processes. An attractive route to lowering the incident power density for UC lies in harnessing energy transfer through triplet-triplet annihilation (TTA). To maximize energy migration in multicomponent TTA-assisted UC systems, triplet exciton diffusivity of the chromophores within an inert medium is of paramount importance, especially in a solid-state matrix for practical device integration. In this thesis, low-threshold sensitized UC systems were fabricated and demonstrated by a photo-induced interfacial polymerization within a coaxial-flow microfluidic channel and in combination with nanostructured optical semiconductors. Dual-phase structured uniform UC capsules allow for the highly efficient bimolecular interactions required for TTA-based upconversion, as well as mechanical strength for integrity and stability. Through controlled interfacial photopolymerization, diffusive energy transfer-driven photoluminescence in a bi-molecular UC system was explored with concomitant tuning of the capsule properties. We believe that this core-shell structure has significance not only for enabling promising applications in photovoltaic devices and photochromic displays, but also for providing a useful platform for photocatalytic and photosensor units. Furthermore, for improving photon upconverted emission, a photonic crystal was integrated as an optical structure consisting of monodisperse inorganic colloidal nanoparticles and polymer resin. The constructively enhanced reflected light allows for the reuse of solar photons over a broad spectrum, resulting in an increase in the power conversion efficiency of a dye-sensitized solar cell as much as 15-20 %.

Metalloporphyrin aggregation is critical for triplet-triplet annihilation (TTA) to occur. In order to maximize the efficiency of TTA, to use as a mechanism of photon upconversion in dye-sensitized solar cells, it is important to understand the phenomenon of absorber aggregation. The aggregation of ZnTPP in polymer films was investigated by fluorescence anisotropy and total internal reflection fluorescence microscopy (TIRFM). Single molecule spectroscopy (SMS) and spectromicroscopy were the techniques used to study single molecules and multimolecular aggregates of ZnTPP/ZnTPPS in polymer films/adsorbed on glass substrates.<p> Fluorescence anisotropy measurements consistently showed depolarized emission from films most concentrated with ZnTPP. This observation was likely a result of energy transfer in and among porphyrin aggregates. Fluorescence intensity decays were also obtained and they illustrated a pattern of decreased fluorescence lifetime (i.e. faster decays) as the concentration of porphyrin in the film increased. These results are consistent with the formation of aggregates, and their increased presence in more concentrated films. The formation of these aggregates quenches the fluorophores fluorescence, resulting in the observed shorter fluorescence lifetimes. <p> TIRFM was performed to study the structure of these polymer films doped with ZnTPP. It was determined that these films consisted of discrete domains and thus lacked homogeneity, and the presence of aggregates was clearly visible. Time-resolved TIRFM measurements were also performed but no interesting results were collected.<p> SMS and spectromicroscopy were the final techniques employed to study porphyrin aggregation. Preliminary measurements were performed with polymer films doped with ZnTPP, and the single step decay time trajectories collected indicated that single molecules were being studied. Furthermore, emission spectra of these molecules were collected and they were similar to those obtained for a bulk measurement, but the bands were slightly shifted in comparison. These measurements were repeated with ZnTPPS adsorbed to glass substrates. Two different patterns of decay trajectories were measured: (i) single step decays corresponding to single ZnTPPS molecules and (ii) multi step/complex decays representative of multimolecular aggregates. Emission spectra were also collected for the multimolecular aggregates, and they were consistent with those of an ensemble measurement but slightly blue-shifted. Such a shift is common when studying aggregates on such a highly polar surface. Thus, these results demonstrate that ZnTPPS aggregates form even at concentrations as low as 10-8 M, and can be studied using SMS despite their weak fluorescence emission.

Metalloporphyrin aggregation is critical for triplet-triplet annihilation (TTA) to occur. In order to maximize the efficiency of TTA, to use as a mechanism of photon upconversion in dye-sensitized solar cells, it is important to understand the phenomenon of absorber aggregation. The aggregation of ZnTPP in polymer films was investigated by fluorescence anisotropy and total internal reflection fluorescence microscopy (TIRFM). Single molecule spectroscopy (SMS) and spectromicroscopy were the techniques used to study single molecules and multimolecular aggregates of ZnTPP/ZnTPPS in polymer films/adsorbed on glass substrates.<p> Fluorescence anisotropy measurements consistently showed depolarized emission from films most concentrated with ZnTPP. This observation was likely a result of energy transfer in and among porphyrin aggregates. Fluorescence intensity decays were also obtained and they illustrated a pattern of decreased fluorescence lifetime (i.e. faster decays) as the concentration of porphyrin in the film increased. These results are consistent with the formation of aggregates, and their increased presence in more concentrated films. The formation of these aggregates quenches the fluorophores fluorescence, resulting in the observed shorter fluorescence lifetimes. <p> TIRFM was performed to study the structure of these polymer films doped with ZnTPP. It was determined that these films consisted of discrete domains and thus lacked homogeneity, and the presence of aggregates was clearly visible. Time-resolved TIRFM measurements were also performed but no interesting results were collected.<p> SMS and spectromicroscopy were the final techniques employed to study porphyrin aggregation. Preliminary measurements were performed with polymer films doped with ZnTPP, and the single step decay time trajectories collected indicated that single molecules were being studied. Furthermore, emission spectra of these molecules were collected and they were similar to those obtained for a bulk measurement, but the bands were slightly shifted in comparison. These measurements were repeated with ZnTPPS adsorbed to glass substrates. Two different patterns of decay trajectories were measured: (i) single step decays corresponding to single ZnTPPS molecules and (ii) multi step/complex decays representative of multimolecular aggregates. Emission spectra were also collected for the multimolecular aggregates, and they were consistent with those of an ensemble measurement but slightly blue-shifted. Such a shift is common when studying aggregates on such a highly polar surface. Thus, these results demonstrate that ZnTPPS aggregates form even at concentrations as low as 10-8 M, and can be studied using SMS despite their weak fluorescence emission.

The present work reports the study of noncoherent photon upconversion (NCPU) via triplet-triplet annihilation (TTA) in polymer systems. This upconversion mechanism has application in photovoltaic devices through the utilization of sub-band gap photons for potentially enhanced power conversion efficiencies. First, homomolecular TTA was studied in zinc tetraphenylporphyrin (ZnTPP) in polymer matrices. Here, ZnTPP acts as both the sensitizer and upconverting emitter as TTA yields an S2 excited porphyrin. Use of poly(methyl methacrylate) (PMMA) as the host polymer demonstrates aggregation-driven upconverted fluorescence (UC) by TTA (TTA-UC). The dye-loading ratio of the precursor solution was varied, controlling the degree of pre-aggregation. Power-dependence studies of the champion film demonstrated that TTA-UC is occurring toward the strong annihilation kinetic limit. A sub-linear dependence of upconverted fluorescence on film thickness was observed in this system. The ZnTPP study was extended to polymers possessing low glass transition temperatures, representing molecular diffusion-driven TTA-UC. Upconverted fluorescence was not observed in ZnTPP in a polyurethane (PU) matrix, likely due to coordination of the PU to the axial position of the Zn2+ ion. Low intensity NCPU via homomolecular TTA was observed in ZnTPP in a poly(ethylene glycol) (PEG) matrix, but the kinetic limit was not determined due to film photodegradation. Dye-loading studies revealed that porphyrin self-quenching was evident at low dye concentrations. Likely reasons for the low upconverted fluorescence intensities realized are this self-quenching and the possibility of PEG coordination to the Zn2+ ion, though it is believed self-quenching is the dominant parasitic effect. Strategies to determine the effect and extent of polymer coordination to the Zn2+ ion are discussed. The study of polymer-based NCPU is extended to a pair of macromolecules, each containing a single ruthenium tris(bipyridine) (Ru(bpy)3) core and multiple pendant arms, which in turn, each contain eight 9,10-diphenylanthracene (DPA) moieties. A power-dependence study of NCPU in this system is reported, and TTA-UC in the weak annihilation kinetic limit was observed. Upconverted fluorescence quantum yields vary linearly with excitation power in both polymers, consistent with the observed kinetic limit. Stern-Volmer experiments have compared the quenching of Ru(bpy)3 phosphorescence (Ph) by monomeric and polymeric DPA. These data show an enhancement in quenching rate constant for the DPA polymer (pDPA). Kinetic analysis of the Ru-DPA polymers has revealed that the energy scheme realized in this system is intrachain TTET from Ru(bpy)3 core to DPA emitter followed by interchain TTA between excited DPA moieties. Low intensity upconverted fluorescence is observed in Ru-DPA containing thin films. Based on the results presented, the requirements of future photophysically-active polymers are discussed with regards to meaningful application in photovoltaics.

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.

No description available.

No description available.

No description available.

Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a photophysical process that facilitates the conversion of two low-energy photons into a single high-energy photon. A low-energy photon is absorbed by a sensitizer molecule that produces a triplet excited state which is transferred to an emitter molecule. When two emitter triplet states encounter each other, TTA can take place to produce a singlet excited state which decays by emission of a high-energy (upconverted) photon. While traditional single-threshold dye-sensitized solar cells (DSSCs) have a maximum efficiency limit of ca. 30%, it has been predicted theoretically that implementation of UC-STTA in DSSCs could increase that efficiency to more than 40%. A possible way to implement UC-STTA into DSSCs, would be to replace the standard sensi- tized nanostructured TiO2 photoanodes by upconverting ones loaded with emitter molecules. Following TTA, the excited emitter molecule would be quenched by injection of a high-energy electron into the conduction band of the TiO2. To explore the practical aspects of this strategy for a highly efficient DSSC, in this thesis UC-STTA is studied in model systems based on nanostructured ZrO2 films. These ZrO2 films are a good proxy for the TiO2 films used in DSSCs, and allow for relatively easy optimization and study of UC-STTA by allowing measurements of the upconverted photons without the complications of electron injection into the film. Herein it is experimentally proven that UC-STTA is viable on nanostructured metal oxide films under non-coherent irradiation with intensities comparable to sunlight. Two different system architectures are studied, differing in the position of the molecular components involved in the UC-STTA mechanism. Both architectures have the emitter molecules adsorbed onto the ZrO2 surface, but the sensitizers are positioned either in solution around the nanostructure, or co-adsorbed with the emitters onto the ZrO2 surface. A set of challenges in the study and optimization of the UC-STTA process is identified for each type of system. Proposals are also given for how to further improve the understanding and UC-STTA optimization of these systems toward application in DSSCs to overcome the present solar energy conversion efficiency limit.