| At the heart of the dye-sensitized solar cell (DSSC) system is a mesoporous oxide layer composed of nanometer-sized particles anchored by a monolayer of the charge transfer dye. Sunlight is harvested by a dye sensitizer, photo-excitation of the dye results in the injection of electrons into the conduction band of the oxide. The dye is regenerated by the electrolyte, such as an ionic liquid containing most frequently the iodide/triiodide couple as a redox system. Dye is one of the most important parts for DSSCs getting higher efficiency and longer lifetime.In this thesis, thirteen novel donor acceptorπ-conjugated (D-π-A) metal-free organic dyes have been engineered and synthesized as sensitizers for the application in DSSCs. The electron-donating moieties are substituted tetrahydroquinoline and the electron-withdrawing parts are cyanoacrylic acid group or cyanovinylphosphonic acid group. Differentπ-conjugation moieties (thienyl, thienylvinyl, dithieno[3, 2-b; 2′, 3′-d] thienyl and phenyl, etc) are introduced to the molecules and serve as electron spacers. The structures of the dyes have been characterized by mass spectra (MS) and proton nuclear magnetic resonance (1H NMR) technology. The photophysical and electrochemical properties of the dyes were studied. DSSCs were constructed based on these dyes and detailed relationship between dye structures and solar cell performances has been investigated. At the same time, two model compounds were synthesized for the intromolecular charge transfer (ICT) study. The findings are used to interpret the relationship between dye structures and solar cell performances.The change of dye structures has important effect on the photophysical and electrochemical properties of the sensitizers. The absorption band of a dye can be red-shifted and broadened by the introduction of a largeπ-conjugation spacer, such as more thienyl or vinyl moieties. This will enhance the light absorption ability of a dye, while the phenyl moiety has lower effect on the absorption character. The thienyl or vinyl in theπ-conjugation spacer will enhance aggregation of a dye on TiO2 surface and results in the blue-shift of the absorption band. Redox potentials of ground and excited states of the dyes match well with that of I3-/I- and TiO2 conduction band, respectively. Introduction of more thienyl or vinyl moieties will shift the ground state redox potential of a dye negatively, and results in a reduced gap between that and redox potential of I3-/I-. This might reduce the efficiency of regeneration of the oxidized dye by I-. A dye with cyanovinylphosphonic acid as anchoring group has more negative excited state redox potential than a cyanoacrylic acid group containing one, and thus has more efficient electron injection from the excited dye to the conduction band of TiO2.The tetrahydroquinoline derivates show well performance in DSSC applications. Dye bath solvents and coadsorbent for TiO2 sensitization influence the solar cell performances. As the result of changing of photophysical and elecrtrochemical properties, structural change of the dyes has important effect on the performances of DSSCs based on these dyes. For most cases, a dye containing the vinyl moiety inπ-conjugation spacer gives worse solar cell performance, such as lower short-circuit current density (Jsc), open-circuit voltage (Voc) and the overall solar-to-electrical energy conversion efficiency (η). The thienyl moiety in a dye molecule also leads to lower Voc value of the DSSC, but a higherηvalue could be obtained as the result of broadened spectral response and higher JSC. A maximumηvalue of 4.53% was achieved under simulated AM 1.5 irradiation (100 mW/cm2) with a DSSC based on C2-2 dye (Voc= 597 mV, Jsc=12.00 mA/cm2, FF=0.63). Under the same conditions, theηvalue of a DSSC based on N3 dye is 6.16%.From the experimental and theoretical investigations, it is found that the excited states of model compounds containing thienyl and vinyl moieties have obvious ICT character. The optimized lowest energy geometry is a twisted intramolecular charge transfer (TICT) structure because of the coexistence of thienyl and vinyl moieties. Geometrical twist in the excited state will increase the probability of de-excitation through vibration relaxation pathway, and thus reduce the probability of electron injection to the TiO2 conduction band and lower the solar cell performance.
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