On the Composite Counter Electrodes and Quasi-Solid-State Electrolytes for Dye-Sensitized Solar Cells
Date Issued
2012
Date
2012
Author(s)
Huang, Kuan-Chieh
Abstract
The exploration of solar energy is one of most intensive studies on the technologies of green energy in recent years. A new generation of solar cell, dye-sensitized solar cell (DSSC), has been investigated worldwide due to the advantages, including facile development of cell, flexibility, and colorful appearance. In terms of (1) improving the power-conversion efficiency (η) of cell (Chapter 3), (2) reducing the consumption of fabrication of cell (Chapter 4 and Chapter 5), and (3) enhancing the long-term durability of cell (Chapter 6 and Chapter 7), various nanomaterials have been synthesized and prepared for the counter electrodes (CEs) and the electrolytes of the DSSCs in this dissertation.
For improving the η of DSSC, a conducting glass substrate spin-coated with a composite thin film, consisting of platinum nanoparticles (PtNPs) and multi-wall carbon nanotubes (MWCNTs), is prepared for a CE of a DSSC (Chapter 3). A homemade polymer, poly(oxyethylene)-segmented imide (POEM), is used and served as stabilizer and dispersant for MWCNTs in the aqueous solution. The best electro-catalytic ability of the film for the reduction of tri-iodide (I3-) ions is obtained after the film is annealed at 390 oC. This is attributed to the complete decomposition of non-conducting POEM, to the formation of PtNP with a moderate crystalline size, and to the surface roughness of film. Thus, an η of 8.47 ± 0.21% of the DSSC with the CE based on the composite film shows much higher than that of a DSSC applying a Pt-coated CE (η = 7.41 ± 0.24%) under illumination of 100 mW cm-2.
For reducing the consumption of fabrication of DSSC, an electro-polymerized composite thin film consisting of nanographites (NGs) and polyaniline (PANI) on the conducting glass substrate is prepared for the replacement of costly Pt as the CE of a DSSC (Chapter 4). A well dispersing ability of NG/aniline particles in the depositing solution is obtained after the particles are treated by reflux-condensation; thereby improving the depositing quality of NG/PANI composite film on the substrate by electro-polymerization. The fine distribution of NGs in the film results in an enhanced conductivity of it, with reference to the conductivity of pristine PANI film. Thus, a percentage of 98.3%, corresponding to the η of a DSSC made of Pt-coated CE, for the DSSC with the NG/PANI CE is acquired. The competitive electro-catalytic ability of the film in comparison to that of film of Pt is confirmed by scanning electrochemical microscopy. In addition, hollow spherical PANI (hsPANI) particles are also deposited on the conducting glass substrate by means of reflux-condensation and electro-polymerization for the CE of a DSSC (Chapter 5). A larger active surface area (A) of thin film consisting of hsPANI particles is estimated to be 0.191 cm2, with reference to that of pristine PANI film (A = 0.126 cm2) by rotating disk electrode. The increased A is beneficial for the reduction of I3-. Thus, a percentage of 95.4%, corresponding to the η of a DSSC with a Pt-coated CE, for the DSSC made of hsPANI CE is obtained. The film consisting of hsPANI particles can serve as a potential alternative for the replacement of Pt catalyst on the CE.
For enhancing the long-term durability of DSSC, a liquid electrolyte based on organic solvent is converted into a quasi-solid-state electrolyte by the gelation using a polymer, poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP), for a DSSC (Chapter 6). The crystallinity of PVDF-HFP decreases when high thermal stable nanoparticles of aluminum nitride (AlN) are incorporated in the quasi-solid-state electrolyte. Thus, the diffusion coefficient of iodide (I-) is increased from 2.97 × 10-6 to 3.52 × 10-6 cm2 s-1. Under 1 sun illumination, the η of a DSSC with this quasi-solid-state electrolyte gives a higher value of 5.27 ± 0.23%, compared to that of a DSSC without adding AlN in its electrolyte (η = 4.75 ± 0.08%). Merely a loss of 5% in η of the DSSC with reference to its initial η is observed for the at-rest durability of the quasi-solid-state DSSC in a period of 1,000 h. In addition, a solvent-free ionic liquid (IL)-based electrolyte containing a synthesized composite of MWCNT/crown ether is prepared for a quasi-solid-state DSSC (Chapter 7). An IL, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), is used. Prior to the characterization of performance for the DSSC with this IL electrolyte, crown ether, 15-crown-5, is preferably chosen owning to its appropriate size of cavity for capturing the lithium (Li+) in the electrolyte in comparison to the capturing abilities of 12-crown-4 and 18-crown-6. Thus, the decrease in electrostatic force between Li+ and I- leads to an improvement of the exchange reaction of I- and I3- by adding the MWCNT/15-crown-5 composite in the EMIBF4 electrolyte. The transport of electrons is facilitated by MWCNTs. Consequently, the values of short-circuit current density and η of the DSSC with both MWCNT/15-crown-5 and EMIBF4 in its electrolyte exhibits increases by 71.2 and 38.8%, respectively, with reference to these values of a DSSC with a bare EMIBF4. The at-rest durability of this quasi-solid-state DSSC is found to be unfailing for a period of 1,200 h.
Subjects
Counter electrode
Dye-sensitized solar cell
Ionic liquid
Quasi-solid-state electrolyte
Rotating disk electrode
Scanning electrochemical microscopy
Type
thesis
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