A new technique for producing plastic solar cells, using ‘tandem’ layers to capture more energy from the sun, is set to make roll-to-roll fabrication of plastic solar cells financially viable.
Metres upon metres of solar cells printed on plastic could be one way of collecting energy in a future free from fossil fuels. Plastic solar cells are cheap to produce and the process is straightforward. They are also flexible and can be adapted to suit almost any surface.
Even so, they come with a bunch of challenges. The biggest obstacle is that plastic solar cells harvest less of the sun’s energy than conventional solar cells. Researchers at Technical University of Denmark (DTU) are trying to solve this problem, and a recent breakthrough has given them hope of achieving a level of efficiency that will make the technology financially viable.
“When it comes to making plastic solar cells, the challenge is that each material only has a relatively limited spectrum in which it can absorb light,” says DTU researcher Jens Wenzel Andreasen. “We have therefore started to make ‘tandem solar cells’, which involves positioning layers of solar cells with different properties on top of each other. One layer absorbs a specific part of the spectrum, and then the remaining light passes through it to be captured by the next layer.
“It sounds very simple, but it makes the solar cell much more complex because you have to have an extra layer between the two layers of cells to deal with the released charges that are to produce electricity. Another problem is that you have to make sure not to damage the layer you have just made when you position a new layer on top of it.”
The reason for this is that the polymers used to make plastic solar cells must be mixed with a solvent in order to be printed – and the same solvent is often used for different polymers. As a result, the first layer dissolves when the next layer is printed.
The solution: zinc oxide and conductive polymers
Now to the good news: Andreasen and his colleagues have come up with a solution. By adding a layer of zinc oxide and conductive polymers, they can control the electrical charges and ensure that the underlying layer does not dissolve when a new layer is applied.
The layer of zinc oxide is only 40 nanometres thick – about a thousand times thinner than a human hair – but in combination with the thin layers of conductive polymers, it makes an effective barrier against the solvents. That was demonstrated in a test performed on the German X-ray synchrotron DESY – a research facility housing very powerful particle accelerators for studying the structure of matter.
For their experiment, the researchers used what’s known as ‘3D ptychography’ – an X-ray technology that makes it possible to reconstruct the shape and chemical composition of the solar cells in detail. The technology provides much higher resolution than conventional X-ray technologies do, allowing the researchers to see that the ultrathin layer of zinc oxide was intact and working as intended. The result was a tandem solar cell able to convert 2.67 per cent of the energy from the sun into electric energy.
Although this efficiency is seven to eight times lower than that of conventional solar cells, the production cost of plastic solar cells is several orders of magnitude lower than for conventional solar cells. This low cost is a major advantage of plastic solar cells.
“We can now use the technology to see exactly what’s happening in the plastic solar cells, and we have found a way of printing one layer on top of another without damaging anything. This means we’re better able to control the structures in the cell, which will inevitably lead us to [achieve] higher efficiency,” Andreasen says.
– Adapted from DTU News