What are organic photovoltaics?
Whereas traditional solar cells, such as those made from silicon, use inorganic materials as their light absorbing component, organic photovoltaics (OPVs), which are a type of “excitonic solar cell”, turn light energy into electricity using organic materials to absorb sunlight.
“Organic” in this sense means that the material is based on the element carbon, and “inorganic” mostly describes non-carbon based compounds.
The organic molecules used are called “polymers”, which are long chain molecules that are used in the plastics all around us. For this reason OPVs are sometimes known as “plastic solar cells”.
“Hybrid photovoltaics” are systems which are made up of both organic and inorganic materials.
How do they work?
See our Excitonic Solar Cells page for details about how exitonic solar cells work. The light-active component in this case is the organic polymer (as opposed to the dye molecule in a dye-sensitised solar cell).
Semiconductor blend solar cells
These solar cells are a type of OPV which contain two different organic semiconductors blended together. You can imagine them looking a bit like raspberry ripple ice-cream. The two semiconductors stay separated, like the red raspberry layer and the cream-coloured ice-cream but they are mixed so that the junction between the two layers has a large surface area. This allows plenty of surface area for electrons and holes to move between the layers. One of the layers will be an electron-transport material and the other a hole-transport material. In the blend one or both of the layers is the light-active component that absorbs sunlight, like the red raspberry of our ice-cream model.
Watch our animation which shows how these cells work, and a way they can be made in the lab.
|Semiconductor Blend Solar Cells (1 min 57 s)
How is a semiconductor blend solar cell made in the research lab and how does it generate electricity when the sun is shining on it?
The two semiconductor materials can be different combinations, such as:
- Polymer-fullerene – the polymer is the light-active component and the fullerene (also known as Buckminsterfullerene or Bucky ball) is the electron-transporter.
- Polymer-quantum dot – the “quantum dot” layer is the light-active component and the polymer is the hole-transporter.
- Oxide-polymer – the oxide is generally the electron-transport material and the light-active component is the polymer hole-transport material.
- Molecule-molecule – one molecule layer is the electron-transport material and the other the hole-transporter. Either or both can absorb visible light.
Polymers are long chain molecules that are used in the plastics all around us. They are made up of repeating units that mainly contain carbon and hydrogen atoms. The properties of the plastics can be determined by the structures of the repeating units, which might contain other atoms like oxygen, nitrogen or sulphur or by the way the polymer chains interact with other polymer chains. You can imagine it like a plate of spaghetti where the strings of spaghetti might be tied up in a big knot, or just in a pile where they can easily slide over each other. This can change whether a plastic is bendy, hard, stretchy or squeezable.
You’ve probably heard of loads of uses for different plastics, such as bags, food and drink packaging, electronic equipment casing, furniture…they are everywhere! The plastics used for these kinds of things are insulators, which means that they don’t conduct electricity. There’s a special family of polymers, however, which have been designed to be able to conduct electricity. These polymers are called “conducting polymers”, and they contain conjugated double bonds (i.e. alternating single and double bonds) in their chains which are capable of transporting electrons. Scientists have also found ways to design polymers which act as semiconductors and absorb visible light, and this has paved the way for a new kind of solar cells made using organic materials.
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 and OPVs. 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.