Development of new redox couple and poly(ethylene oxide)-based polymer electrolyte for dye-sensitized solar cells

Abstract

Since the pioneer work (~7% reported) in 1991 by O’Regan and Grätzel, dye-sensitized solar cells (DSCs) have shown promise as a cost competitive photovoltaics. Currently, record efficiency of 13% is reported in the DSCs, but still falling short of their theoretically achievable efficiency of ~20%. Obstructive factors for the maximum efficiency could be categorized: 1) loss-in-potential for charge transfer; 2) incomplete light harvest and conversion in a non-optimal wavelength range; and 3) resistive losses for charge transport. This dissertation comprises 6 Chapters. Chapter 1 is Introduction, where fundamental and basics for photovoltaics are briefly described. In addition, concepts on cyclic voltammetry and electrochemical impedance spectroscopy are also introduced. Chapters 2-4 are on new redox electrolytes and perspectives on polymer electrolyte for DSCs. In Chapter 2, it is attempted to attenuate the potential-loss of ~ 0.3 eV for dye regeneration with the I-/I3- redox couple. First, the thiolate/disulfide organic redox couples are investigated as an alternative for the I-/I3- redox couple. The new organic redox couple shows improvement of photovoltaic performances through its negative redox potential and higher ionic conductivity. In addition, the carbon black counter electrode is introduced to increase the fill factor which is limited due to adsorption of thiolate on Pt surface. Moreover, the carbon black counter electrode shows much better stability in thiolate/disulfide-based DSCs, compared to the Pt counter electrode. Second, with respect to the classical I-/I3- and the well-known Co(bpy)II/III redox mediator mixtures the new cobalt complex redox couple considerably improves the short-circuit photo current density, JSC in DSCs both with cationic and neutral RuII based sensitizers in the presence of 4-tert-butylpyridine and lithium cations. In Chapter 3, the origin of the differences in the performance parameters between liquid electrolyte and solid polymer electrolyte based on poly(ethylene oxide) for DSCs has been investigated. This work has been motivated because DSCs employing solid polymer electrolytes showed surprisingly a higher energy conversion efficiency than those with liquid electrolytes when the TiO2 layer thickness is thin (~ 4 μm). We have found that the higher efficiency is mostly attributable to the faster dye-regeneration rate with solid polymer electrolyte case than liquid cells, even though the ionic conductivity through solid polymer electrolyte is almost 100 ~ 1,000 times lower than that through liquid electrolyte. One critical reason to have faster dye regeneration rate in the solid polymer electrolyte is found to be the increased driving force for dye regeneration associated with the lowering of the HOMO energy level of dye upon contact with the solid polymer electrolyte. In addition, limitations associated with poor polymer electrolyte penetration and ionic diffusion have been analyzed together with other effects such as the dye regeneration rate, the conduction band edge shift and the electron recombination kinetics occurring in the solid polymer electrolyte. These understandings may contribute to the further increase in the energy conversion efficiency of DSCs employing solid polymer electrolyte. Through the understandings from Chapter 3, a simple and effective avenue to increase the energy conversion efficiency is proposed and demonstrated in Chapter 4. The simple and effective way to increase performances of DSCs based on solid-state polymer electrolyte is that increasing the ion flux by the reduction of the thickness of the solid polymer electrolyte layer, the mass transport distance of I-/I3- redox couples. In addition, a thin alumina layer is incorporated to prevent from short-circuiting two electrodes and to have additional light scattering effects. Solid state DSCs employing polymer electrolyte and bifunctional insulating layer show the power conversion efficiency of 8.9 % at 1 sun conditions. In conclusion, fusion of new redox couples and solid-state polymer electrolyte has been demonstrated to improve for highly efficient and stable DSC devices. The maximum theoretical efficiency of ~30% could be realized by optimizing the energetic and kinetic properties of each component, developed so far, in the near future.