| Photo sensitizer, which harvests the sunlight to form excited state and achieves the process of electrons injection into the conduction band of nanosemiconductor TiO2, is one of the most important parts of dye-sensitized solar cell (DSC) for getting higher efficiency and longer stability. The resultant oxidized dye is regenerated by redox mediator in electrolyte. To decrease the cost of DSC, the metal-free organic dyes as the alternative to noble metal complex sensitizer have been a research topic of high priority and develop rapidly in recent years.Based on the prospective of metal-free organic dyes,23 novel donor acceptorπ-conjugated (D-π-A) metal-free organic dyes have been engineered and synthesized as sensitizers for the application in DSC. The electron-donating moieties are triphenylamine, phenothiazine and carbazole derivatives, and the electron-withdrawing moieties are cyanoacrylic acid group. Differentπ-conjugations (double bond, triple bond, thienyl, phenyl and anthracyl, etc) are introduced to the molecules and serve asπ-conjugations. 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. DSC based on the dyes and I-/I3- or Br-/Br3--containing electrolytes were constructed and the detailed relationship between dye properties and DSC performances has been investigated.The molecular structures of the dyes have important influence on dyes'photophysical and electrochemical properties. The introduction of a triple bond or an anthracene moiety intoπ-conjugations and fine-tune of planar configurations of molecular structures can enlargeπ-conjugations, red-shift absorptions and enhance light-absorbing abilities of the dyes. The Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO) levels of all the dyes match well with that of I-/I3--containing electrolyte and the conduction band of TiO2, respectively. And the HOMO levels of the dyes TC301-TC308, whose electron-donating moieties are carbazole derivatives and electron-withdrawing moieties are cyanoacrylic acid group, also match well with that of Br-/Br3--containing electrolyte.The introduction of a triple bond intoπ-conjugations can enhance electron transfer yields, incident photon-to-current conversion efficiency (IPCE) and solar energy conversion efficiencies of DSCs. And the introduction of an anthracene moiety intoπ-conjugations can enhance electron lifetimes in TiO2 films, suppress electron recombination thus increase the opencircuit-voltages (Voc) of DSC to a large extent. Through broadening absorption spectra of the dyes and fine-tuning planar configurations of molecular structures, solar energy conversion efficiencies of 7.03% was achieved for DSC sensitized by the dye TC501, whose electron-donating moieties are carbazole derivatives, electron-withdrawing moieties are cyanoacrylic acid group, andπ-conjugation is introduced an anthracene moiety.Upon using Br-/Br3- instead of I-/I3-, the opencircuit-voltages of DSC were enhanced to a large extent because of not only the enlarged energy difference between the potential of redox mediator and the Femi level of TiO2, but also the suppressed charge recombination. The driving forces to regenerate the dyes with I-/I3- and Br-/Br3- are both sufficient for the dyes whose HOMO levels are more positive than that of Br-/Br3-. And for the dyes whose HOMO levels are equal to that of Br-/Br3-, the driving forces to regenerate the dyes with I-/I3- is sufficient, but with Br-/Br3- is insufficient. Using Br-/Br3- based electrolytes, a very high opencircuit-voltage of 1.156 V and a good solar energy conversion efficiency of 3.68% were achieved by DSC sensitized by the dye TC301, and a high opencircuit-voltage of 0.939 V and a very good solar energy conversion efficiency of 5.22% were also achieved by DSC sensitized by the dye TC305. |
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