| The pursuit of a reliable renewable energy source becomes extremely urgent and has attracted increasing attention in view of the rapidly increasing demands of energy, the depletion of fossil fuel, and consequent environmental deterioration. Since the remarkable progress made by O’Regan and Gratzel in1991, dye-sensitized solar cells (DSSCs) have aroused worldwide scientific research interest for their easy fabrication process and low cost compared with traditional silicon-based solar cells. As a key component of DSSCs, the sensitizer performs a critical function in light-harvesting, electron injection and thus the overall photo-energy conversion efficiency. In comparison with ruthenium-polypyridyl complex sensitizers, organic sensitizers have many advantages including easy purification procedure and unlimited raw materials. However, organic sensitizers tend to aggregation accompany with relatively narrower absorption bands, leading to comparatively lower energy conversion efficiency.In order to expand absorption bands and inhibit inter-molecule aggregation of the organic sensitizers, in this dissertation, we try to introduce cyclic urea/thiourea functional groups into triphenylamine electron donor unit and synthesize three series functionalized sensitizers, namely, bis cyclic urea functionalized sensitizers, bis cyclic thiourea functionalized sensitizers, and mono cyclic urea (thiourea) functionalized sensitizers. Their photophysic properties, electrochemistry property, photovoltaic properties, and frontier molecular orbitals were investigated, and the correlations of the structural modifications (including the cyclic urea/thiourea functional groups, the length of substituted alkyl chains, conjunction bridge, and acceptor) with photovoltaic performance (including incident photon-to-current conversion efficiency, short-circuit current density, open-circuit voltage, fill factor, and power conversion efficiency) were established as well. In addition, co-sensitization of bis cyclic thiourea functionalized sensitizer TC6D2with N719was also studied due to their complementary absorption bands.Detailed research contents were followed:(1) Twelve novel cyclic urea/thiourea functionalized triphenylamine based sensitizers were synthesized from urea via a series of classic reactions, involving condensation-cyclization, alkylation, iodination, Ullmann coupling, bromination, Suzuki cross-coupling, and Knoevenagel condensation, additionally, three triphenylamine based reference dyes, coded TC105,1P-PSS, and LO were synthesized as well. The structures of these sensitizers along with their intermediates were confirmed by1H nuclear magnetic resonance (1H NMR),13C nuclear magnetic resonance (13C NMR), infrared spectroscopy (IR), and mass spectroscopy (MS).(2) The UV-vis absorption spectra of these sensitizers were investigated. Compared with the corresponding triphenylamine based dyes, the maximum UV-Vis absorption peaks of cyclic urea/thiourea function/nalized sensitizers were red-shifted and the molar extinction coefficients were increased, which is desirable for high-performance sensitizer with a relatively higher short-circuit current density. And the UV-vis absorption spectra were deeply depended on the electronic property of conjunction bridges, for example, the spectra were blue-shifted for dyes with biphenyl linker but red-shifted for dyes containing bi-thiophene bridge. And the UV-vis absorption spectra were not affected by the length of substituted alkyl chains. However, the UV-vis absorption spectra were clearly red-shifted with evident enhanced molar extinction coefficients for dyes bearing rhodanine acid acceptor in comparison with the dyes containing2-cyanoacrylic acid acceptor.(3) Cyclic voltammetry was performed to test first oxidation potentials (Eox), corresponding to the HOMO levels of the dyes. The LUMO levels of the dyes could be calculated from Eox-E0-0, in which the E0-0was estimated from the onset wavelength of the absorption spectra. The HOMO levels of these dyes were more positive than the iodine/iodide redox potential, so the oxidized dye molecules could be effectively regenerated by the redox couple. The LUMO levels of these dyes were adequately more negative than the conduction band energy level of the TiO2electrode, which is energetically feasible for the electron injection from the excited dye into TiO2films. And the cyclic urea/thiourea functionalized sensitizers exhibited a relatively narrower E0-0energy band than the corresponding triphenylamine based dyes.(4) The frontier molecular orbitals of these sensitizers were calculated by density functional theory (DFT). Their HOMOs were delocalized from triphenylamine to the cyclic urea/thiourea functional groups, while LUMOs were mainly distributed at cyanoacrylic acid acceptor. This distributions of electrons will facilitate electron injection into the conduction band of TiO2. The energy gaps (△ELUMO-HOMO) of cyclic urea/thiourea functionalized sensitizers were narrower than that of corresponding triphenylamine dyes, which displayed the introduction of urea/thiourea functional groups would be beneficial the electron transfer from donor to acceptor. These are in line with UV-Vis results.(5) Electrochemical impedance spectroscopy was used to investigate the interfacial charge transfer process. The charge-transfer resistance at the TiO2/dye/electrolyte interface was enlarged due to the introduction of cyclic urea/thiourea functional groups, resulting a slower electron recombination and a lengthened electron lifetime, so an increased open-circuit voltage was obtained.(6) The DSSCs characteristics of sensitizers were evaluated. Encouragingly, the cyclic urea/thiourea functionalized dyes exhibited distinctly improved power conversion efficiency due to the increased short-circuit current density accompanied with the enhanced open-circuit voltage relative to the corresponding triphenylamine based dyes. The photovoltaic performance of sensitizers were enhanceed in the order of bis cyclic urea functionalized dyes<mono cyclic urea (thiourea) functionalized dyes<bis cyclic thiourea functionalized dyes. It is worth noting that dyes with longer alkyl chains displayed higher open-circuit voltage than dyes with shorter alkyl chains. Compared with the dyes containing2-cyanoacrylic acid acceptor, the photovoltaic performance of dyes bearing rhodanine acid acceptor was decreased clearly because of ineffective electron injection process. Furthermore, photovoltaic performance was directly related with conjunction linker, dyes with biphenyl linker generally displayed relatively poor photovoltaic performance. However, TC6D3bearing bis cyclic thiourea groups and bithiophene bridge exhibited the highest power conversion efficiency up to7.29%, which was comparable to N719(7.36%), in addition, TC6D3displayed a better stability than N719.(7) The co-sensitization of N719with a bis cyclic thiourea functionalized organic dye, coded TC6D2, for DSSCs was demonstrated. Due to its intensive absorption in ultraviolet region, TC6D2could compensate the loss of light harvest induced by triiodide, thereby the short-circuit photocurrent density was increased for co-sensitized (N719+TC6D2) DSSC. Moreover, the electron recombination and dye aggregation were retarded upon N719cocktail co-sensitized with TC6D2, thus the open-circuit voltage of co-sensitized device was enhanced as well. The increased short-circuit current density (17.9mA·cm-2) combined with the enhanced open-circuit voltage (698mV) ultimately resulted in an improved power conversion efficiency of7.91%for co-sensitized DSSC, which was raised by8.6%in comparison with that of N719(7.28%) sensitized alone. In addition, co-sensitized DSSC exhibited a better stability than that of N719sensitized device probably due to the depression of dye desorption. |
