Optimization Of TiO₂Photoanodes And Photovoltaic Performance Of Sensitizer Based On Diketopyrrolopyrrole And Bithiazole

As a new generation of solar cells, the dye-sensitized solar cells (DSSCs) have attracted widespread concern in the world due to its relatively low cost of preparation. The photoanodes and sensitizers play an important role as the key part of capturing sun ligh and transporting electron in DSSCs. In this paper, modification and optimization of the photoanodes were studied. And a series of D-π-A-π-D-and D-A-π-A type sensitizers with diketopyrrolopyrrole and bithiazole core were synthesized and their photovoltaic performances have been investigated in detail.In chapter1, the definition of the structure and principle for the dye-sensitized solar cells, electrolytes and counter electrode are introduced. Recent researches of nanoporous semiconductor electrode and new dye-sensitizers have been reviewed. Then the research strategy of the dissertation is presented on that basis.In chapter2, in the process of titanium sulfate as the precursor, the glucose was added to control the growth of particles to obtain a one-dimensional structure of nano-rod-shaped titanium oxide which was about150nm lengths and31.60nm diameters. The one-dimensional rod-like structure of titanium dioxide has higher electron transmission efficiency and improves the rate of electron transmission to the external circuit. Electrons from the excited dye were injected into the conduction band of titanium dioxide can reduce the probability of electron recombination. Finally, the cell performaned an improving open circuit voltage of848mV and overall conversion efficiency reached8.57%. Ethyl cellulose was added in the process of Titanium (IV) Isopropoxide as the titanium precursor to control the size of titanium dioxide nano-particle size. A smaller particle size of titanium dioxide was obtained (Particle size decreased from36.72nm to15.75nm) successfully. The short-circuit current for DSSCs based on N719was increased from14.61to18.05mA/cm2, and an overall conversion efficiency was up to9.59%.In chapter3, two new metal-free organic dyes (DPP-Ⅰ and DPP-Ⅱ) with diketopyrrolopyrrole (DPP) core were designed and synthesized to enrich the application of DPP unit in DSSCs, in which triphenylamine or N,N-bis(4-methoxyphenyl)benzenamine moieties was used as the electron donor, DPP units as theπ-conjugated bridge, and carboxylic acid group as the electron acceptor. Photophysical and electrochemical properties of two dyes were investigated by UV-vis spectrometry and cyclic voltammetry. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. The DSSC based on dye DPP-I showed better photovoltaic performance:a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of80.6%corresponding to an overall conversion efficiency of2.68%. Although the power conversion efficiencies are not so high, this work explores new donor-π-accepter-π-donor models and the effects of molecular design on optical propertiesIn chapter4, the quantum dot CuInS2layer was deposited on TiO2film using successive ionic layer absorption and reaction (SILAR) method, then two bithiazole-bridged dyes (BT-C1and BT-C2) were sensitized on the CuInS2films to form dye/CuInS2/TiO2photoanodes for DSSCs. It was found that the quantum dots CuInS2as an energy barrier layer can improve the solar cell open-circuit voltage (Voc) effectively. Moreover, the CuInS2barrier layer can also increase short-circuit photocurrent (Jsc) compared to the large decrease in Jsc with ZnO as energy barrier layer. The electrochemical impedance spectroscopy (EIS) measurement showed that the CuInS2formed a barrier layer to suppress the recombination from injection electron to the electrolyte and improve Voc. Finally, the Voc increased about22and27mV for CuInS2coating TiO2-based BT-C1and BT-C2-sensitized cells, the overall conversion efficiencies also reached to7.20%and6.74%, respectively.In chapter5, three metal-free bithiazole organic dyes (BT-T1～T3) based on D-A-π-A building blocks were designed and synthesized for dye-sensitized solar cells (DSSCs) to study the influence of different electron donors on photovoltaic properties, in which the electron donors of BT-T1～T3were carbazole, triphenylamine and indoline moieties, respectively. The UV/Vis absorption spectra showed that BT-T3containing indoline as electron-donor displayed red-shift absorption compared to the other two dyes and the most broadened spectrum was an onset close to700nm.n And the IPCE spectra of BT-T3were also broadened and kept higher IPCE value during580-650nm. Electrochemical measurement data indicated that the HOMO and LUMO energy levels could be tuned through introducing different electron-donor in the dye molecular. It was found that the overall conversion efficiency of indoline donor based dye BT-T3showed the highest efficiency of7.86%under AM1.5irradiation (100mW/cm2). The electron lifetime calculated from electrochemical impedance spectroscopy (EIS) measurements demonstrated the reduced charge recombination and the higher open-circuit voltageIn chapter6, thiophene as the π-conjugated bridge chain could expand the absorption spectrum and improve the molar extinction coefficient of the dye when it was introduced into the dye sensitizers. Thiophene was introduced into the dyes molecular (BT-T2with triphenylamine as donor, BT-T3with indoline as donor) between donor and bithiazole to afford TBT-T2and TBT-T3. For the dyes with triphenylamine, the introduction of thiophene afforded a red-shift maximum absorption and threshold wavelength and improved molar extinction coefficient. The dye-sensitized cell efficiency increased from7.12%to7.51%. While the experimental and the theoretical density functional calculation result indicated that the introduction of thiophene in the indoline dye leaded to a blue shift absorption spectra. The intramolecular charge transmission was blocked. The decreased rate of charge separation efficiency caused the short-circuit current of TBT-T3sensitized solar cell reduced significantly, from14.77mA/cm2to4.52mA/cm2. And the efficiency also reduced from7.86%to1.93%.