Dye-sensitized solar cells (DSSC) are an efficient type of thin-film photovoltaic cell. Modern dye-sensitized solar cells, or Grätzel cells, are based on a concept invented in 1988 by Brian O'Regan and Michael Grätzel, but the concept dates back to the 1960s and 70s.
DSSCs are easy to manufacture with traditional roll-printing techniques, and is semi-transparent and semi-flexible, allowing a range of uses that are not applicable to rigid photovoltaic systems.
Most of the materials used are low-cost, however a handful of more costly materials are necessary, such as ruthenium and platinum. There is a significant practical challenge involved in designing the liquid electrolyte for DSSCs, which must be able to remain in the liquid phase in all kinds of weather conditions.
Even though the conversion efficiency of dye-sensitized PV cells is lower than that of some other thin-film cells, their price to performance ratio is sufficient to make them an important player in the solar market, particularly in building-integrated photovoltaic (BIPV) applications.
Advantages of DSSCs
The advantages of DSSCs are listed below:
- They are the most efficient third-generation solar technology available, absorbing more sunlight per surface area than standard silicon-based solar panels.
- DSSCs are an attractive replacement for current technologies in low density applications such as rooftop solar collectors, where the light weight and mechanical robustness of the printable cell is a key benefit.
- These may not be as attractive for large-scale deployments where high-efficiency, high-cost cells are more suitable. However, even minimal future increases in the conversion efficiency of the DSSC may make it suitable for some of these applications.
- DSSCs work even in low-light conditions such as non-direct sunlight and cloudy skies.
- They are economical, easy to manufacture and constructed from abundant and stable resource materials.
- The mechanical robustness of the DSSC leads indirectly to higher efficiencies at a range of temperatures.
- Normally, DSSCs are built with just a thin conductive plastic top layer, helping heat to be radiated away more easily and hence operate at low internal temperatures.
Development of DSSC Technology
DSSC technology has evolved over the years. The key developmental milestones are detailed below:
- In 1995, dyes used in experimental cells were sensitive only to the high frequency end of the light spectrum - blue light and UV.
- In 1999, newer versions were introduced with a higher frequency response that is efficient even at red and infra-red wavelengths. The dye used in these cells has a deep brown-black colour referred to as black dye, and has an overall efficiency of almost 90% - unfortunately, however, it tended to breakdown under high light intensities.
- Newer dyes have been introduced which have a range of specialialized properties, including 1-ethyl-3-methylimidazolium tetrocyanoborate (EMIB(CN)4), which is temperature-stable, and copper-diselenium (Cu(In,GA)Se2), which provides increased conversion efficiencies.
- Researchers are seeking to use quantum dots for converting high-energy light into multiple electrons, using solid-state electrolytes for superior temperature response and modify the doping of TiO2 to match it with the electrolyte that is being used.
- TiO2 nanoparticle-based dye-sensitized solar cells are a highly promising approach, giving efficiencies over 10%. The dye molecules are adsorbed onto the surface of sintered TiO2 nanoparticles. The incident light is harvested by the dye, and an electron is injected into TiO2 where it is transmitted through the TiO2 nanoparticles to reach an electrode.
- In 2004, researchers from the University of California at Santa Barbara described the performance and design of a zinc oxide nanowire-based dye-sensitized solar cell. The nanowires enable a direct electron conduction path between the conducting substrate and point of photogeneration and may offer enhanced electron transport compared to sintered nanoparticle films. The devices have a light harvesting efficiency below 10%, showing that present efficiencies and densities are enhanced by an order of magnitude by an increase in the nanowire surface area.
- Most of the present research on DSSCs is focused on improving spectral absorbance by making modifications in the dye, enhancing hole transport, replacement of the liquid electrolyte with conducting polymers or ionic solids and improving electron transport using alternative core-shell structures or wide band gap semiconductor materials.
Recent Advances in DSSC
Engineers from the University of Pennsylvania and Drexel University are using mathematical modelling and nanotechnology to design novel photoelectric cells that are more durable, efficient and economical.
The team is evaluating dye-sensitized solar panels to streamline the electron transfer process inside the solar panel so that it efficiently converts radiation to electricity.
Presently, dye-sensitized solar panels convert about 11 – 12% of the sunlight striking them into electricity. The researchers are trying to increase the efficiency and make it comparable with silicon-based solar panels.
The researchers anticipate that adding carbon nanotubes will enhance the overall charge collection efficiency of the solar cell considerably.
The next portion of the research is aimed at replacing the electrolyte solution, separating the electrodes in the solar cell with a more efficient polymer substance. The researchers believe that this will also enhance the efficiency of the solar cell.
Northwestern University researchers have reported an innovative device that eliminates the corrosive and leak-prone liquid electrolyte typical of DSSCs. Materials scientist Robert Chang, chemist Mercouri Kanatzidis, and two graduate students replaced the dye cells’ liquid electrolyte with a solid iodine-based semiconductor. This design actually increases performance as the caesium-tin-iodine semiconductor that acts as a replacement to the liquid electrolyte also helps the cell absorb light.
Conclusion
Dye-sensitized solar cells are a promising potential replacement for silicon-based solar cells. With advancements in nanostructured semiconductors, high-efficiency sensitizers and robust electrolytes, the performance of modern DSSCs is becoming more and more competitive. Simple processing, low-cost materials and a wide range of applications are all helping DSSCs to find a foothold in the marketplace.
Sources and Further Reading