Dye-sensitized Solar Cells:Study of Pt-free Counter Electrodes and Iodide-free Electrolytes
Date Issued
2015
Date
2015
Author(s)
Li, Chun-Ting
Abstract
This dissertation aimed to systematically develop Pt-free counter electrodes (CEs) and to design a novel iodide-free electrolyte for the dye-sensitized solar cells (DSSCs) with low-costs and highly cell efficiencies (η’s). This dissertation is divided into two parts: (1) Pt-free CEs (Chapter 3~Chapter 7) and (2) iodide-free electrolyte (Chapter 8). In the case of Pt-free CEs, we aim to reduce the costs of the DSSCs using various electro-catalysts for completely replacing the expensive Pt via the simple, non-vacuum, and low-cost fabrication processes. Accordingly, three types of Pt-free composite films were studied using a standard iodide electrolyte. (I) Transition metallic compound-type CEs, including the composite film of TiS2/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (TiS2/PEDOT:PSS) in Chapter 3, the composite films of Si3N4/PEDOT:PSS, SiO2/PEDOT:PSS, SiS2/PEDOT:PSS, SiSe2/PEDOT:PSS in Chapter 4, and the composite films of Zn3N2/PEDOT:PSS, ZnO/PEDOT:PSS, ZnS/PEDOT:PSS, ZnSe/PEDOT:PSS in Chapter 5, were indivitually investigated. In a composite film, the transition metallic compound nanoparticles were separately used to provide attractive electro-catalytic abilities and large active areas for I3- reduction; among those NPs, TiS2, Si3N4, SiS2, SiSe2, Zn3N2 and ZnSe were applied in DSSCs for the first time. Among them, the cell with Zn3N2/PEDOT:PSS CE reached a higher η than that of Pt-based cell. (II) Carbonaceous-type CEs, i.e., the composite films of nano-porous carbon black nanoparticles/ sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]-2,5-diyl) (CB NPs/s-PT), were investigated in Chapter 6 to provide large surface area, fast eletrolyte penetratoin, and rapid reaction rate for I3– reduction. When a composite film contains 5 wt% CB NPs, the pertinent DSSC reached best η of 9.02%, which is even higher than that of Pt-based DSSC. The s-PT is introduced as a novel thiophene-based water-soluable conducting polymer for CE in DSSC for the very first time. Under weak sunlight, the cell with CB NPs/s-PT composite CE still maintains good performance, indicating its good compatibility at both outdoor or indoor electronics. (III) Conducting polymer type CEs, including poly(3,4-ethylenedioxythiophene) (PEDOT) and six different ionic-liquid-doped PEDOT films were systematically investigated in Chapter 7. Six different ionic liquids containing three imidazolium cations with different alkyl chains (–C2H5, –C6H13, –C10H21) and four anions (BF4−, PF6−, SO3CF3−, TFSI−) were used as the chemical dopants to increase the surface area of PEDOT films and to ehance the conjugation of the PEDOT films, respectively. Among them, the cell with HMIPF6-doped PEDOT and HMITFSI-doped PEDOT CEs reached higher η’s than that of Pt-based DSSC. In brief, this dissertation explores four Pt-free composite films of Zn3N2/PEDOT:PSS, HMIPF6-doped PEDOT, HMITFSI-doped PEDOT, and CB NPs/s-PT are a promising substitutions of Pt due to their outstanding properties, i.e., good electro-catalytic ability for I3– reduction, low-cost, simple preparation process, and easy for large-scale production. In the case of iodide-free electrolyte, we designed a novel dual-channel ionic liquid compound (Chapter 8), 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-yl)amino]ethyl}-1H-imidazol-3 -ium selenocyanate (ITSeCN), to further improve the cell effiency of DSSCs. ITSeCN is designed to contain dual redox channels of imidazolium-functionalized TEMPO (cationic redox mediator) and selenocyanate (anionic mediator). Thereby, the ITSeCN shows the favorable redox natures, which gave more positive standard potential, larger diffusivity, and better kinetic heterogeneous rate constant than those of iodide. To further investigate a suitable electro-catalytic material for triggering the redox of ITSeCN mediator, several materials were used: (1) Pt (metal type), (2) PEDOT (conducting polymer type), (3) CoSe (transition metallic compound type), and (4) carbon black (carbonaceous type) films were chosen to represent four types of electro-catalytic materials in DSSCs. Finally, the DSSCs with PEDOT and CoSe CEs achieved better performance than that of the Pt-based DSSC. Therefore, it can be infered that the transition metallic compound type CEs would be more suitable for our new synthesized-ITSeCN mediator than the others.
Subjects
Counter electrodes
dye-sensitized solar cells
electro-catalyst
electrolyte
iodide-free
Pt-free
Type
thesis