A Study on Dye-Modified TiO2 Electrode, Gel Polymer Electrolyte and Pt-Free Counter Electrode for Dye-Sensitized Solar Cells
Date Issued
2008
Date
2008
Author(s)
Lee, Kun-Mu
Abstract
The main purpose of this thesis is to investigate the behaviors of new approaches in electrodes (working and counter), sensitizers and gel polymer electrolytes for dye-sensitized solar cells (DSSCs) and discussing the influences on the cell performance and stability of DSSCs. n the first part of this thesis (Chapter 2 and 3), the optimization of solar energy conversion efficiency of DSSCs was investigated by the tuning of TiO2 photoelectrode’s morphology. Double-layered TiO2 photoelectrodes were designed by the coating of TiO2 suspension incorporated with low and high molecular weight poly(ethylene glycol) as a binder. Among four types of TiO2 electrodes, the P2P1 showed the highest efficiency under the conditions of identical film thickness and constant irradiation. This can be explained by the larger pore size and higher surface area of P2P1 TiO2 electrode than the other materials and these two factors assist for the facile transport of I3-/I- ion couple through the TiO2 matrix. The best efficiency (h) of 9.04% for a solar cell was obtained by introducing the light scattering particles to the TiO2 electrode measured under AM 1.5G. s for the part of low-temperature fabricated DSSC, the TiO2 film with the TTIP/TiO2 molar ratio of 0.08 has the best conduction. Meanwhile, the charge transport resistance at the TiO2/dye/electrolyte interface increased as a function of the MWCNT concentration, ranged from 0.1 to 0.5 wt%, due to a decrease in the surface area for dye adsorption. The DSSC with the TiO2 containing 0.1 wt% of MWCNT resulted in a JSC of 9.08 mA/cm2 and a cell conversion efficiency of 5.02 %. On the other hand, TiO2 film prepared by using binder-free TiO2 paste which developed by Prof. Miyasaka’s group was also used in plastic DSSC to optimal the SJW-E1 dye which synthesized by Prof. Wu’s group. The effects of TiOx buffer layer and co-adsorbents as well as long-term stability of plastic DSSCs were investigated. The TiOx buffer layer not only benefited the adhesion between TiO2 thin film and ITO/PEN substrate but also reduced the electron recombination, resulting in the improvement of the FF and conversion efficiency of cells. The optimized solar cell based on SJW-E1 showed a high efficiency of 6.31 % at 100 mW/cm2 (AM 1.5G), and SJW-E1 based solar cell showed a better stability than that of N719 based after 500 h light soakingest.n the second part of this thesis (Chapter 4), the co-sensitization of dyes for the complementary in the spectral characteristics in plastic DSSCs was investigated. Two co-sensitization systems for the plastic DSSCs, including N719/FL and black dye/FL showed enhanced photovoltaic performances compared with that of each dyendividually. The optimal conversion efficiencies of N719/FL and black dye/FL DSSCs reached 5.10 % and 3.78 %, respectively, which were higher than that of individual sensitizers. However, for the system co-sensitized with FL and Chl-e6, the cell performances only lay in between that of each dye. From the EIS analysis, the characteristic frequencies (C.F.) at TiO2/dye/electrolyte interface for N719/FL and black dye/FL are kept the same or lower than that of individual dyes. While for the FL /Chl-e6 co-sensitized DSSCs, the C.F. were higher than that based on only FL, indicating that they had shorter electron lifetime in the TiO2 electrode after co-sensitization.n the third part of this thesis (Chapter 5), two kinds of gel polymer electrolytes were developed and used in DSSCs. At the beginning, it was found that the donor number of solvent in electrolyte is the one of the key factors that effect the photovoltaic performance of DSSC. Meanwhile, the quasi-solid state DSSCs were fabricated with polyvinyidene fluoride-co-hexafluoro propylene (PVDF-HFP) in methoxy propionitrile (MPN) as gel polymer electrolyte (GPE), tetrabutylammonium iodide/iodine as redox couple, 4-TBP as additive and nano-silica as fillers. The energyonversion efficiency of the cell with 5 wt% PVDF-HFP is comparable to that one obtained in liquid electrolyte system. Solar cell containing PVDF-HFP with 0.8 M ofBAI and 0.12 M of I2 shows maximum photocurrent. Moreover, the addition of 1wt% nano-silica is found to improve the at-rest durability and the performance of theolar cell. A photocurrent of 14.04 mA/cm2, a VOC of 0.71 V and an overall conversion efficiency of 5.97 % under 100 mW/cm2 irradiation was observed for the best performance of a solar cell in this work. On the other hand, the ionic conductivities and performances of DSSCs of GPEs prepared by in situ polymerization with different cross-linkers were investigated. The poly(imidazole-co-butylmethacrylate)-based GPE containing the B4Br cross-linker showed a higher ionic conductivity, due to the formation of micro-phase separation that resulted in an increase of ion transport paths in the GPE. Moreover, a co-adsorbent, (4-pyridylthio) acetic acid, co-adsorbed with N3 dye on the TiO2 electrode not only reduced dye aggregation, but also reacted with the cross-linkers in the GPE at the TiO2/GPE interface after gelling, thus the value of JSC significantly increased from 7.72 to 10.00 mA/cm2. In addition, in order to reduce the ionic diffusion resistance within the TiO2 electrode, incorporation of monodispersed PMMA in the TiO2 paste was considered. With the optimal volume ratio of PMMA/TiO2 (v/v = 3.75), the micro-porous TiO2 electrode exhibited larger pores (ca. 350 nm) uniformly distributed after sintering, and the ionic diffusion resistance within the TiO2 film could significantly be reduced. The cell conversion efficiency increased from 3.61 to 5.81% under illumination of 100 mW/cm2, an improvement of ca. 55 %.n the fourth part of this thesis (Chapter 6), a series ofoly(3,4-alkylenedioxythiophene) counter electrodes prepared by electrochemical polymerization on the fluorine-doped tin oxide (FTO) glass substrate were incorporated in the platinum-free DSSCs. Cells fabricated with a PProDOT-Et2 counter electrode showed a higher conversion efficiency of 7.88 % compared to cells fabricated with PEDOT (3.93 %), PProDOT (7.08 %), and sputtered-Pt (7.77 %) electrodes. The FF was strongly dependent on the deposition charge capacity of the PProDOT-Et2 layer, but the aggregation of PProDOT-Et2 in higher deposition capacities (> 80 mC/cm2) resulted in decreases in JSC and the cell conversion efficiency. Incorporating the best ProDOT-Et2 film (40 mC/cm2) as the counter electrode in plastic DSSC was compared and showed similar tendency as mentioned above. The cell fabricated with a PProDOT-Et2 counter electrode showed a higher conversion efficiency of 5.20 % compared with that fabricated with sputtered-Pt (5.11%) electrodes under the illumination of 100 mW/cm2 (AM 1.5G).
Subjects
Dye-sensitized solar cells, TiO2 electrode
Sensitizer
Organic dye
Co-sensitization
Gel polymer electrolyte
Counter electrode
Conducting polymer
Electrochemical impedance spectroscopy (EIS)
Transient photovoltage/photocurrent analysis.
SDGs
Type
thesis
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