How do they work?
In dye-sensitised solar cells, or “DSSCs”, the light-active component of the cell is a molecular dye. When a photon “excites” the dye, it transfers an electron from the dye molecule to a network of titanium dioxide (TiO2) nanoparticles. This semiconductor can then transfer the electron to an electrode. The positive charge left behind on the dye transfers to the hole-transporter. In this cell the hole-transporter is a liquid solution containing iodine which allows the hole to migrate to the other electrode to complete the required charge separation that generates electricity.
Watch our animation which shows how these kinds of solar cells work!
|Dye-Sensitised Solar Cells Animation (3 min)
How is a dye-sensitised solar cell made in the research lab and how does it generate electricity when the sun is shining on it?
These cells were first developed in early 1990s shown to have a power-conversion efficiency of about 11 percent. The dye is required because TiO2 does not absorb visible light. When a dye is put onto the TiO2 surface, it gives the cell the ability to absorb this light. They are known as Dye-sensitised solar cells (DSSCs), or Grätzel cells after their inventor.
The way electrons move in a dye-sensitised solar cell can be compared to how marbles move in a marble run. The electrons act like the marbles in a marble run. Energy from the sun gives the electrons enough energy to get to the top of the “run”. Then the electrons flow downhill through the solar cell to the bottom where the sun’s energy can get it back to the top again. When the electrons move through the solar cell, electricity is created, just like the way the marbles move the wheels on the marble run!
One objective of scientists is to replace the liquid hole transporter in the cells with a solid-state hole transporter. This would make the cells more stable, easier to manufacture and more robust. Other scientists are also investigating cheaper, more efficient, more stable molecular dyes and quantum dots.
Materials for DSSCs
There are several essential components which make up a dye-sensitised solar cell, each of which has a very special set of properties. There is a huge amount of research being carried out on each of these materials to try to improve these properties, in the hope that the whole solar cell will become even better at turning sunlight into electricity.
Semiconductors are a material that conduct electricity (or electrons) under certain conditions. They are half-way between conductors like metals that easily conduct electricity, like those we use in electrical wiring, and insulators that do not conduct electricity, such as plastics which we use as the coating for electrical wires.
How does that happen?
Imagine that you are walking along a path and you reach an obstacle such as a body of water like a stream or a river. If there was no obstacle, the path would be a conductor as you are not stopped from moving along it. However, the water is there. If the water is part of a sea or ocean you’ve effectively reached a dead end and you can no longer walk forward. The path has become an insulator and you’ll have to turn around and find another path that takes you elsewhere.
But, if the water is a small stream, you could step over it; if it is a bigger stream you could leap over. If it is a river, perhaps there will be a bridge that you could use to continue your walk. The bigger the body of water, the more energy you may need to use to get to the other side (building a bridge uses a lot more energy than just jumping), but you still can.This is similar to a semiconductor. A semiconductor can conduct electricity if it is given a little help or energy for the electrons to get over the obstacle, which we call the band gap. This could be in the form of heat or light. The bigger the band gap, the more energy we need to provide.Semiconductors come in various types, but they all have this property in common. Semiconductors used for excitonic solar cells are usually in the form of solid oxides, polymers or molecules that are generally cheap and easy to get hold of.Semiconductors are used in solar cells because they can use the energy from sunlight to become conductors and can be built into a solar cell to give us electricity. One commonly used semiconductor is called “titanium dioxide”, or TiO2, because it is non-toxic, naturally occurring, cheap and there is plenty of it. It is even used in things like toothpaste, sun block and white paint!
Molecular dyes are molecules that absorb visible light and therefore they are coloured. These molecules are much bigger and contain many more atoms. They often contain mainly carbon, hydrogen, oxygen and nitrogen atoms, but sometimes they can have an extra metal atom in it too to help give the dyes a better colour. Common metal atoms used are platinum, copper and ruthenium.Sometimes the semiconductors used in solar cells do not absorb visible light. Most energy in sunlight is in the visible region of the electromagnetic spectrum but many semiconductors do not absorb in this region and appear white. By covering the surface of the semiconductor with a dye that does absorb in the visible region, we can capture more energy from the sun which can then be converted to electricity. Watch our video about Coloured Semiconductors for more information.An important feature of a dye molecule is for it to have atoms that can attach, or bond, to another surface. If we are dyeing some cotton, it needs to attach firmly to it so it does not come off in the wash. The same is true when we attach dyes to semiconductors.Dye molecules have been used to colour our fabrics, our hair and our food for a long time. Now, they are also used as one of the components in dye cells. They can be found naturally in fruits like raspberries to give them their colour, but can also be made, or synthesised, by scientists in a laboratory. Chemists can specifically design a molecule that will have the right colour, the right bonding groups to attach the dye to surfaces and the right properties for helping electrons to move around a solar cell. Nina will tell you more about dye molecules for solar cells in our Diffusion in Dye Cells video.
Quantum dots are nanocrystals made from a semiconductor. Nanocrystals are very, very small crystals that are between 2 and 10 nanometres in diameter, which is 0.000000002 metres, not easy to measure with a ruler. It is hard to picture how small they are but if you imagine how small a football is compared to the size of the earth, that is how small a nanocrystal is compared to the football.When crystals get this small, they can start to look or behave differently. That is why scientists are so interested in nanotechnology as their properties (behaviour) can be really useful.
At this size, some semiconductors start to absorb sunlight and emit different colours, and these are called quantum dots. What is particularly interesting about quantum dots is that by changing their size very slightly, the colour changes. So for example, a quantum dot with a diameter of 2 nanometres will be blue while a quantum dot of 6 nanometres will be red. The sizes in between will go through the colours of the rainbow on the way. Watch our Introduction to Quantum Dots and Molecule Effects on Quantum Dots videos for more information.Because quantum dots absorb light energy, they can be used in solar cells such as dye-sensitised solar cells, OPVs and hPVs . The quantum dots are the light-active component of the cell, and absorb photons from the visible region and then transfer the electrons and holes into different materials so they can move towards the electrodes.