The Study On The Electrolyte Of Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have been attracting widespread scientific and technological interest because they are clean and low cost. However, DSCs are not manufactured for practical application on large scale. The mainly scientific issues that lie behind this problem are the efficiency and stability. On one hand, though impressive energy conversion efficiency (η) of 11% has been achieved for the DSCs employed liquid-state electrolyte, this value is still substantially below the theoretical maximum, which is mainly caused by the back reaction. On the other hand, the use of liquid-state electrolyte always results in the issues of volatilization and leakage, affecting the long-term stability of the DSCs. So, suppressing the back reaction and improving the stability of the DSCs are necessary at present. (1) As far as the back reaction is concerned, the injected electrons transfer to I3- in the electrolytes mainly via the conducting glass and via the nanocrystalline TiO2. By deposition a "compact" layer of TiO2 as an insulating layer on conductive glass, the electron transfer from the conducting glass to the I-/I3- redox couple can be prevented; while, the addition of additive in the electrolyte can effectively suppress back reaction through the nanocrystalline TiO2, leading to an improved conversion efficiency. (2) As for the stability, the replacement of liquid electrolyte with quasi-and all-solid-state electrolyte can resolve the volatilization and leakage of the electrolyte, leading a considerable stability. Keep these in mind, we devoted to the following studies:1. In order to retard the back reaction occurred on the conductive glass, the compact titanium dioxide layer was deposited on the conductive glass by the hydrolysis of TiCl4 aqueous solution. The as-prepared compact layer has been characterized by XRD, SEM, UV-vis absorption spectroscopy, voltammetry, and electrochemical impedance spectroscopy (EIS). And then, the effect of the compact layer on the phptovolatic performance of the DSCs was studied. Experimental results show that the compact TiO2 layer with the thickness of about 100 nm is composed of anatase-phase nanoparticles with the particle size of 10-15 nm. Impedance measurements display that the compact layer of TiO2 can prevent the back reaction of electrons with tri-iodide ions via the conducting glass under low applied potentials, increase the open circuit voltage (Voc) and fill factor (FF), and finally improve the conversion efficiency for the DSCs from 7.5% to 8.1%.2. In order to suppress the back reaction occurred on the surface of the TiO2 film, an inexpensive pyridine-based additive allyl isonicotinate (AIN) for the electrolyte of DSCs was designed and synthesized. AIN can be quickly synthesized at room temperature without any solvent. The presence of AIN in the electrolyte enhances the Voc, FF, and the short-circuit photocurrent (Jsc), consequently improving the energy conversion efficiency (η) from 6.5% to 8.2%. The impedance experiments show that the adsorption of AIN on the surface of TiO2 can obviously suppress the back reaction of the injected electrons via the TiO2 film, increasing the lifetime of electrons in the TiO2. Simultaneously, the adsorption of AIN leads to the negative shift of the conduction band edge of the dye-sensitized TiO2 around 55 mV. The suppression of the recombination of the injected electrons and the negative shift of the conduction band edge together contribute to the higher power conversion efficiency.3. In order to solve the issues of volatilization and leakage of the liquid-state electrolyte, poly(methyl acrylate)/poly(ethylene glycol)-based quasi-solid-state electrolyte was prepared and employed to construct the DSCs. The poly(methyl acrylate)/poly(ethylene glycol) hybrid is beneficial to entrap large volume of liquid electrolyte. At 25℃, the ionic conductivity and the triiodide ionic diffusion constant of the as-prepared polymer gel electrolyte are 2.1 mS cm-1 and 2.3×10-6 cm2 s-1, respectively. The quasi-solid-state solar cell sensitized by triphenylamine-based dyes attains an overall energy conversion efficiency of 5.76% at the light intensity of 30 mW cm-2. The DSC exhibits excellent long-term stability. The conversion efficiency of it almost retained its initial value for 1000 h at the testing temperature of 60℃. The presence of poly(ethylene glycol) in the electrolyte obviously increases the conductivity and the energy conversion efficiency compared with that without poly(ethylene glycol).4. Further, all-solid-state electrolyte, alkyloxy-imidazolium iodide ionic polymer/SiO2 nanocomposite electrolyte, was prepared. By optimizing the content of I2, 1,2-dimethyl-3-propylimidazolium iodide, and SiO2 nanoparticles in the electrolyte, considerable ionic conductivity of 0.151 mS cm-1 is achieved due to the formation of the high efficient electron exchange tunnels. The electrolyte is further characterized by differential scanning calorimetry and scanning electron microscopy (SEM). The result of differential scanning calorimetry shows that the melting temperature of the electrolyte is 120℃. The SEM analysis reveals a favorable interfacial contact between the electrolyte and the TiO2. The all-solid-state solar cells with the as-prepared alkyloxy-imidazolium iodide ionic polymer/SiO2 nanocomposite electrolyte and triphenylamine-based metal-free organic dyes attains high energy conversion efficiency of 2.70% and 4.12% under the illumination intensities of 100 and 10 mW cm-2, respectively.