염료 감응형 태양전지의 탄소 촉매전극 제작 및 전기화학적 특성 평가

Abstract

In dye-sensitized solar cells (DSCs), many research groups have been investigated carbon materials as a catalyst to substitute Pt because of low cost, chemical inertness, high catalytic activity in the electrolyte containing iodide/triiodide redox couple, and low-temperature solution process. In order to improve the catalytic activity of carbon electrodes, the charge transfer resistance at the electrolyte/electrode interface should be minimized by adjusting the carbon particle size, electrode thickness or the connectivity between carbon particles. Although various carbon materials have been used as catalysts in DSCs, the electrochemical properties of carbon materials have not been interpreted clearly at various conditions such as carbon particle size, electrode thickness, and composite with other materials. In this work, we investigated the electrochemical property of carbon materials at various conditions such as various particle sizes, different thickness, and composite with other materials. Among various carbon materials, carbon blacks were chosen because they having low crystallinity and many edges may be more active than highly oriented carbon materials such as graphite and carbon nanotubes. The electrochemical property was mainly analyzed with electrochemical impedance spectroscopy (EIS) and fitted with equivalent circuit model which is adoptable to the porous structure. In composite materials, PEDOT:PSS and carbon blacks were blended because PEDOT:PSS can provide catalytic active sites and high conductivity. Finally, we fabricated FTO-free PEDOT:PSS/carbon black composite based counter electrode and studied the catalytic properties for triiodide reduction in the electrolyte. In chapter 1, the requirement of solar energy and the basic principle of solar cells were discussed. Fossil fuel resources are limited in supply and non-renewable. Besides, fossil fuels produce greenhouse gases, health or smog problems and other harmful environmental pollutants. In order to substitute fossil fuel resources having various disadvantages, solar energy has been attracted as a renewable energy because the solar resource’s magnitude, wide availability, versatility and benign effect on the environment and climate make it the most appealing source of energy. In chapter 2, DSCs were introduced and the basic theory of the counter electrode system was explained. DSCs have attracted much attention as a next generation photovoltaic devices due to their high photovoltaic conversion efficiency (~12 %), low cost and easy fabrication process. In DSCs, the role of the counter electrode is to transfer electrons arriving from the external circuit to the electrolyte for the triiodide ion reduction. Thus, the counter electrode should have high catalytic activity, large surface area and high conductivity to reduce the charge transfer resistance at the electrolyte/counter electrode interface. In order to investigate the electrochemical properties of the counter electrode, EIS is generally measured with the symmetric cell. Compared with Pt having thin film structure, carbon electrode having porous structure has the charge transport resistance in the bulk carbon electrode and the capacitive defect sites. Thus, our group suggested a new equivalent circuit model to interpret the electrochemical mechanism of carbon materials including the charge transport resistance part (Rtrns) and the capacitive defect sites part (Ctrap). In chapter 3, spray coated carbon black (CB) electrodes with various particle sizes and thicknesses were used as a catalytic layer for the triiodide ion reduction in DSCs. Their electrochemical properties were studied with EIS and the charge transfer mechanism was studied with a new equivalent circuit model to account for the porous nature of the carbon electrode. With spray coating, it is easy to control the electrode thickness by adjusting the amount of the spraying solution (or spray time). There is no need to add a binder to adjust the viscosity of the solution and low temperature processing is possible because there is no need to remove a binder. Also, it is applicable to the large-scale roll-to-roll process by controlling the nozzle size of the sprayer. As the particle size decreases, the catalytic activity is improved with low charge transfer resistance due to the increase of catalytic sites with large surface area. Increased thickness from 1 μm to 9 μm also improves the catalytic activity. 9 μm thick CB counter electrode (with 20 nm particle size) based DSCs shows the efficiency which is comparable to the efficiency of Pt counter electrode based DSC. In chapter 4, PEDOT:PSS and carbon black composites (P/CB composites) were used as catalysts for triiodide ion reduction in DSCs and the electrochemical properties were investigated as a function of the PEDOT:PSS ratio and the thickness. Added PEPEDOT:PSS can improve the catalytic activity because of its high conductivity and intrinsic catalytic activity. As the PEDOT:PSS ratio increases, the conductivity and the catalytic activity are improved with low charge transfer resistance due to the addition of the electron pathways and the catalytic sites from PEDOT:PSS. Increased P/CB composite electrode thickness also offers more catalytic active sites with large surface area and thus the catalytic activity for triiodide ion reduction is improved with decreasing charge transfer resistance. Thus, 4.5 μm thick 17 P/CB composite (17 wt% PEDOT:PSS) counter electrode based DSCs shows the efficiency of 7.4 % which is comparable to the efficiency of Pt counter electrode based DSC (7.7 %). In chapter 5, FTO-free PEDOT:PSS/carbon black composite (P/CB composites) counter electrodes based DSCs were fabricated and the electrochemical properties were investigated at various thicknesses. 17 P/CB composite shows high catalytic activity due to the high conductivity and lots of catalytic active sites in spite of FTO-free conditions. Increased P/CB composite electrode thickness provides more catalytic active sites with large surface area and thus the catalytic activity for triiodide ion reduction is improved with decreasing charge transfer resistance. Although there is no FTO electrode as a charge collector, P/CB composite electrode presented high electrochemical property. Thus, 12 μm thick 17 P/CB composite formed FTO-free counter electrode based DSCs shows the efficiency of 6.0 % which is the 80% of the Pt counter electrode based DSC efficiency (7.7 %).In additionally, highly conductive PEDOT:PSS film as a conductive layer was inserted, in order to improve the performance of FTO-free P/CB composite counter electrode based DSCs. As the PEDOT:PSS film thickness increases, the resistivity is decreased and thus the total internal resistance of the DSC is significantly reduced. Because PEDOT:PSS is high conductivity, lots of electrons can easily transport in the P/CB composite

PEDOT:PSS based counter electrodes and be transferred to the electrolyte with low charge transfer resistance. Thus, the performance of the 12 μm 17P/CB composite

200nm PEDOT:PSS counter electrode based DSCs is shown the efficiency of 7.1 % which is 93 % of the Pt counter electrode based DSCs’ efficiency (7.7 %). Finally, we fabricated the flexible counter electrodes with P/CB composite

PEDOT:PSS electrodes coated on the PET substrates and measured the performance at pristine/bending states. In both cases, the efficiency exhibits similar values in the range of 6.8~6.9 %. It indicates that P/CB composite

PEDOT:PSS based counter electrodes have high catalytic activity for triiodide ion reduction with good mechanical stability at pristine/bending states.