Abstract
A dye-sensitised solar cell (DSSC) counter electrode (CE) plays a vital role in catalysing the conversion of triiodide ( I 3 − ) to iodide ions ( I − ), thereby ensuring the completion of the repetitive cycle of electricity generation. The platinum CE, despite being the standard counter electrode in DSSCs, has drawbacks of platinum’s rarity and high cost. Platinum is an excellent redox catalyst, and consequently, it is the most sought-after metal for catalytic conversions. The huge demand for platinum in the automotive industry for vehicular catalytic converters, the pharmaceutical industry, and in oil refining, as well as other industries, has driven its price to unprecedented levels. The prohibitive price of platinum has caused newer thin film technologies, such as the DSSC which depends on the platinum CE, to be cost-ineffective, thus meaning they cannot compete with the better-established silicon-based solar cells. These problems have stagnated the development of the DSSC, which in turn has dampened larger commercialisation prospects for this thin film technology. With this in mind, this review paper focuses on recent progress in the research and development of alternative cost-effective materials to replace Pt-based CEs. Ternary alloys are amongst the possible alternatives that have been explored, yielding varied results. Alloys, especially ternary sulphides, selenides, and oxides, are attractive as alternatives as they are cheap and are easily fabricated. Ternary alloys also have a synergistic effect produced by the coexistence of two metal ions in a crystal structure, which is believed to induce greater catalytic capability, thus making them ideal cost-effective materials to replace the Pt CE in DSSCs. This review intends to highlight the performance of ternary alloy counter electrodes through the analysis of charge transfer resistance and power conversion efficiencies. Focus is also given to the restrictions and impediments to the attainment of higher power conversion efficiency in alternative CEs. The advances in fabrication of simple ternary alloys, as well as more advanced hierarchical nanostructured counter electrodes, are discussed here in detail. Results obtained to date indicate that the efficiencies of ternary alloy counter electrodes are still below that of the platinum counter electrode, and hence more research is required to enhance their efficiencies.
Highlights
With the world relying mostly on nonrenewable energy sources, such as fossil fuels, which are rapidly depleting, the rising world population and unrelenting global economic development have necessitated the need for more energy sources
Other developments development of solid-state dye-sensitised solar cells with solid hole conductors has sought which have been undertaken have sought to achieve the fabrication of a cell with an efficiency to rid the cell of the problems associated with the liquid electrolyte [18]
When Gratzel fabricated a dye-sensitised solar cell (DSSC) with a binary cobalt sulphide counter electrode on ITO/PEN films which produced a 6.5% power conversion efficiency (PCE), it led to widespread effort aimed at finding the right sulphide compound which could be as effective as platinum whilst withstanding corrosion from the iodine electrolyte
Summary
With the world relying mostly on nonrenewable energy sources, such as fossil fuels, which are rapidly depleting, the rising world population and unrelenting global economic development have necessitated the need for more energy sources. The tedious fabrication processes and costly processing of materials make this solar technology expensive. It is not viable for cost-competitive generation of electricity [6]. The DSSC is a thin film technology that is based on mimicking the natural light harvesting process of photosynthesis in plants. It was first introduced by Gratzel and O’Regan in 1991. Possible improvements to the DSSC could be achieved through: (1) replacement of the liquid electrolyte; (2) development of a dye that can absorb light from the whole visible spectrum as well as the infrared region; (3) development of an inexpensive platinum-free counter electrode that has an efficiency surpassing that of the platinum-based DSSC [13]. The properties of different materials and reasons why they would make excellent replacements for the platinum counter electrode are summarised
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