Novel Bithiazole And Triphenylamine-based Organic Dyes For Dye Sensitized Solar Cell

Dye sensitized solar cells (DSSCs) is a kind of solar cells which convert light to electricity by means of harvesting the solar irradiation by the sensitizer. DSSCs have attracted considerable attention in scientific research and practical applications for its lower cost, much more simple preparation and higher convertion efficiency. In order to improve the photovoltaic performance and stability of DSSCs, extensive efforts have been focused on the synthesis of new highly efficient sensitizers. In this paper, several series of sensitizers have been designed and synthesized. Their photovoltaic performances have been studied in detail. In addition, a series cyanine polymer has been synthesized and their properties have been studied.In chapter1, the basic definition on the structure and principle for dye-sensitized solar cells, nanoporous semiconductor electrode and electrolytes are introduced, and recent progress in high-efficiency sensitizers for dye-sensitized solar cells is reviewed. Then the research strategy of the dissertation is presented.In chapter2, five new metal-free organic dyes (T1-T5) containing bithiazole moieties were synthesized and used for dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties were fully characterized. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. All of these dyes performed as sensitizers for the DSSC test, and the photovoltaic performance data of these bithiazole-bridged dyes showed higher open circuit voltages (745-810mV). Among the five dyes, Tl showed the best photovoltaic performance:a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of83.8%, a short-circuit photocurrent density (Jsc) of11.78mA cm"2, an open-circuit photovoltage (Voc) of810mV, and a fill factor (ff) of0.60, corresponding to an overall conversion efficiency of5.73%under standard global AM1.5solar light condition, which reached93%with respect to that of an N719-based device fabricated under similar conditions. The result shows that the metal-free dyes based on bithiazole p-conjugation are promising candidates for improvement of the performance of DSSCs.In chapter3, a series of new push-pull organic dyes (BT1-BT6) incorporating electron-withdrawing bithiazole with thiophene. furan, benzene and cyano moiety as π-spacer have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). In comparison with the model compound T1, these dyes containing thiophene moiety between triphenylamine and bithiazole display enhanced spectral responses in the red portion of the solar spectrum. Electrochemical measurement data indicate that the HOMO and LUMO energy levels can be tuned by introducing different π-spacer between the bithiazole moiety and cyanoacrylic acid acceptor. The incorporation of two hexyl-substituted bithiazole is highly beneficial to prevent close π-π aggregation, thus favorably suppressing charge recom-bination and intermolecular interaction. The overall conversion efficiencies of DSSCs based on bithiazole dyes are in the range of3.58to7.51%, in which BT1-based DSSCs showed the best photovoltaic performance:a maximum monochromatic incident (IPCE) of87.8%, aJsc of15.69mA cm-2, an open-circuit photovoltage (Voc) of778mV, and a fill factor (ff) of0.61, corresponding to an overall conversion efficiency of7.51%under standard global AM1.5solar light conditions. Most importantly, long-term stability of the BT1-BT6-based DSSCs with ionic-liquid electrolytes under1000h light-soaking was demonstrated and BT2with furan moiety exhibited better photovoltaic performance of up to5.75%power conversion efficiency.In chapter4, two new bithiazole-based organic dyes (DTI and DT2) with indoline as electron donor, thiophene and furan as π-spacer were designed and synthesized, and the corresponding dyes (BT1and BT2) with triphenylamine as donor group for the purpose of comparison were also synthesized. Their absorption spectra, electrochemical and photovoltaic properties were fully characterized. It was found that the absorption spectra and HOMO/LUMO energy levels tuning can be conveniently accomplished by alternating the donor moiety. The organic dye based on indoline moiety showed broader absorption spectra in the solution, but narrower on TiO2film, which is not benefit for light-harvesting. The higher HOMO energy levels of DTI and DT2lead to hard regeneration of oxidation state dyes. The DSSCs based on the dyes DT1-DT2showed the lower photovoltaic performance compared with dyes BT1-BT2:an overall conversion efficiency of1.93%and3.56%under standard global AM1.5solar condition, respectively, a short-circuit photocurrent density (Jsc) of7.66mA cm-2, an open-circuit photovoltage (Voc) of666mV, and a fill factor (ff) of0.70for DT2.In chapter5, four organic dyes (K1-K4) employing carbazole unit as electron donor, linear or branched alkyl as side chain, triphenylamine as electron-donating group and thiophene or furan as conjunction-bridge were designed and synthesized for nanocrystalline TiO2dye-sensitized solar cells. Their absorption spectra, electrochemical and photovoltaic properties were fully characterized. The triphenylamine can improve light-harvesting and tune HOMO/LUMO energy level. We introduced different alkyls to carbazole moiety and found that the contribution to absorption is in order of octyl>hexyl>4-methylamyl. All of these dyes performed as sensitizers for the DSSC test, and the photovoltaic performance data of these carbazole dyes showed higher open circuit voltages(Voc)(0.726-0.815V). The Voc increased but Jsc decreased based on electrolyte containing guanidinium thiocyanate. Among the five dyes, K2showed the best photovoltaic performance:a short-circuit photocurrent density (Jsc) of13.26mA cm-2, an open-circuit photovoltage (Voc) of724mV, and a fill factor (ff) of0.66, corresponding to an overall conversion efficiency of6.68%under standard global AM1.5solar light condition.In chapter6, two new organic sensitizer (C1and C2) with coumarine unit as electron donor, thiophene units as the π-conjugated bridge, and cyanoacrylic acid group as the electron acceptor was designed and synthesized. Their absorption spectra, electrochemical, and photovoltaic properties were extensively investigated. The absorption spectra indicate that molar extinction coefficient of sensitizer C1was relatively higher. It was found that the HOMO and LUMO energy levels can be tuned by the isomer donors. The pyrrole attached to coumarine also transfered electron to accepter and generate another charge transfer state, which was confirmed by excitation energies calculation. The dye C1exhibited a higher photovoltaic performance:a short-circuit photocurrent density (Jsc) of6.20mA cm-2, an open-circuit photovoltage (Voc) of600mV, and a fill factor (ff) of0.74, corresponding to an overall conversion efficiency of2.74%under standard global AM1.5solar light condition.In chapter7, a series of organic sensitizer (P1-P3) with5,10-dioctyl-5,10-dihydrophenazine as electron donor, ethenyl units as the π-conjugated bridge, and cyanoacrylic acid group as the electron acceptor was designed and synthesized. Their absorption spectra, electrochemical, and photovoltaic properties were fully investigated. The introduction of two octyl groups retarded dye π-π aggregation, which can be observed from absorption spectra both in solution and on TiO2film. It was found that the HOMO energy levels of PI and P3were0.54V and0.51V, respectively, which are too close to the energy level of I-/I3-, leading to hard regeneration of oxidation state dye. The dye P2, containing two cyanoacetic acid accepter groups, has lower HOMO energy level, since the electron density has been dispersed. Among the three dyes studied, P2showed the best photovoltaic performance:a short-circuit photocurrent density (Jsc) of5.96mA cm-2, an open-circuit photovoltage (Voc) of618mV, and a fill factor (ff) of0.60, corresponding to an overall conversion efficiency of2.20%under standard global AM1.5solar light condition.In chapter8, a series of cyanine polymers were synthesized by Sonogashira coupling. The structures of compounds were characterized by1H NMR and UV-Vis absorption. These polymers (CY1-CY4) have good film-forming properties and stability. From the GPC investigation, it was found that the molecular is up to120thousand, which demonstrate the main chain of cyanine is stable. The dyes exhibit broader absorption spectra both in solution and when forming film. The HOMO level and LUMO level is around-4.95eV and-3.25eV, respectively. These novel cyanine dyes can be used in bulk heterojunction solar cells. The photovoltaic performance measurement is in progress.