Where possible, experiments were carried out in duplicate to enable standard errors to be generated and calculated according to Equation 9. As a result the reproducibility of the experiments can be considered by the error values generated.. The number of DSSCs fabricated in each experiment will be clearly marked accompanying the results.
ππππππππππππππππ πΈπΈπππππΈπΈππ = π π ππππππππππππππ πππππππππππππππΈπΈππ
οΏ½ππππππππππππ πΈπΈππ πΈπΈπππ π πππππππππππππΈπΈπππ π
Equation 9
2.6 References
1. OβRegan, B. & GrΓ€tzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737β740 (1991).
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Chapter 3
3 Fabrication of Novel electrodes
3.1 Introduction
The fabrication of dye sensitised solar cells (DSSCs) is normally undertaken using transparent conducting oxide (TCO) glass electrodes. The advantages of TCO electrodes are their reasonable transparency and electrical conductivity. They also make excellent substrate materials from which to fabricate solar cells because they are rigid and flat. However, the use of glass substrates in DSSCs introduces limitations in the size and shape of the cells that can be fabricated. They also contribute a large proportion of the cost to produce and install a DSSC. Therefore, alternative electrode materials need to be considered if larger and less expensive cells are to be fabricated.
The TCO glass electrodes used in DSSC fabrication are typically indium doped tin oxide (ITO) or fluorine doped tin oxide (FTO) glass. These substrates are expensive to produce and have light transmission rates of 80-90 % in the visible spectrum (Chapter 1 Figure 7).1 This reduces the efficiency of TCO glass DSSCs due to energy losses before the photons of light enter the photoactive area of the cell.
Furthermore, large panels are considerably heavy due to the weight of the glass electrodes. This often results in the requirement for a stronger mounting structure for such panels, adding to installation costs.
All of these factors result in a large scale DSSC installation becoming an expensive proposition.
The sheet resistance of ITO or FTO glass electrodes is typically 10 β¦/β‘ (equivalent to a conductivity of 1000 S cm-1). This does not provide sufficient current carrying capacity to efficiently extract all the photo-generated charges from cells over large distances. Indeed, low conductivity factor results in the photoactive area of DCCSs normally having a width no greater than 1 cm. This point will be further explored in Section 3.2. To overcome this limitation, narrow cells are fabricated that can be as long as required, with two silver current collecting tracks running along each length of the cell collecting current over the narrow cell width. Often in larger devices made of multiple cells the silver tracks are connected in series to boost the voltage output.
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Figure 3.1: A DSSC module arranged into strips with silver current collector bus bars. Image from Aisin Seiki Co., Ltd and Toyota Central R&D Labs., Inc.2
Thin silver grids have also been deposited on a substrate within the photoactive area to lower the sheet resistance of the electrode and allow for wider cells to be fabricated as shown in Figure 3.1.2 However the use of silver adds cost and complexity to the cell design and will reduce the optical transparency of the device. These factors may reduce photovoltaic performance more than they reduce the internal resistance of the cell.
Since only one electrode needs to be transparent in a photovoltaic device, either the photoanode or the cathode could be a metallic film and a number of DSSCs of these types are reported as discussed in Chapter 1. The advantage of replacing the photoanode with a metal foil is not only in reducing the cost and weight of the DSSC but it is also a substrate that can tolerate the high sintering temperature required for titanium dioxide (TiO2).
However, the use of one metallic and one glass electrode does not necessarily mean individual DSSCs can be made larger than 1 cm in width. This would require the replacement of both electrodes with metal films. Since both electrodes would then have superior electrical conductivity to TCO glass, each cell would not be restricted by a poor current carrying capacity and would open the possibility to produce cells with a larger width. However, using a non-transparent electrode would require a modification to the basic DSSC design. As discussed in Chapter 1 Section 1.3, the use of dual metallic electrodes requires
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one of the electrodes to be porous. The porous electrode would then enable the use of a back contact DSSC (BCDSSC) to ensure good light absorbance of the photovoltaic cell. The fabrication of various porous metallic electrodes will be discussed in Sections 1.4 and 1.5.
The use of a BCDSSC design requires the photoanode to be thin, on the order of microns to tens of microns, to help reduce any recombination losses in the cell as discussed in Chapter 1. Thin metal electrodes, such as metallic foils, are flexible by nature. This inherent flexibility would result in a higher probability of the metallic electrodes touching each other during cell assembly. As a result of this, many of the BCDSSCs shown in this thesis have a porous polymer separator layer placed in between the dual metallic electrodes. This polymer layer is a battery separator or membrane such as Celgard 2500. The microns thick separator acts as a physical barrier to prevent short circuiting of the cell. At the same time, the porosity of the polymer layer allows for the electrolyte to diffuse though the membrane during device operation.The type of metal chosen for fabrication in BCDSSCs is considered in Section 3.2 of this chapter. Factors to be considered include the electrical conductivity of the metal, any oxides formed on the surface of the metal and its resistance to corrosion from the iodine electrolyte. There is a body of research discussed in Chapter 1 where metallic electrode surfaces can be treated chemically to improve photovoltaic performance. Section 3.3 will detail attempts to replicate and expand upon this work and apply it to the BCDSSCs fabricated in this thesis. Section 3.4 investigates the fabrication of porous electrodes to be utilised in BCDSSCs.