The goal is to develop devices which efficiently harness the power of the sun whilst being cheaper and easier to manufacture than the current silicon solar cells.
A sustainable energy supply - it’s a huge challenge but one we need to conquer in order to support the rapidly rising population, under the shadow of depleting fossil fuels. The sun, proposes a bright solution; within just 90 minutes enough sun hits the earth to provide the entire planet’s energy needs for a year. But we need better technology to efficiently harness it.
Solar photovoltaic devices are the fastest growing renewable power technology worldwide with an annual growth rate of over 50% in the last five years.1 The conventional solar cells however, which are made from silicon, are expensive to manufacture, timely to install and have shown little increase in efficiency over the last ten years, what’s more the energy cost associated with purifying and processing the silicon is vast. So many scientists are exploring other types of solar cells made from polymers.
Silicon cells are big, heavy, fragile, and expensive to produce at a large scale, but imagine a lightweight, thin film that’s easily transportable and potentially disposable. You could roll it out like a tarpaulin, clip in a couple of electrodes and in a matter of minutes you could harness the energy from the sun.
“Plastic polymer cells can be deposited by roll to roll printing techniques which means we can make a large volume more cheaply,” explains Andy Parnell, University of Sheffield. “This could be a huge advantage especially for off-grid applications as it’s much easier to install with minimal effort.”
These qualities make polymer solar cells an attractive option however, currently they are not as efficient as their silicon neighbours. This has made them a popular field of research and many scientists are looking at different processing methods to improve the efficiency and launch polymer cells into the forefront of renewable energy technology.
However, lower efficiency and a lifetime of seven to eight years might not be a barrier to adoption if the cells are easy to manufacture and cheap to replace.
Current technologies are more limited in where you can use them; in fact, this lowering of the cost might be the key for opening solar energy to new markets such as those developing countries that don’t yet have a grid and people who wouldn’t conventionally think about investing and generating their own electricity. Polymer cells could therefore lower the barrier to the adoption of solar cells.
The role of ISIS
Part of the reason polymer cells are easier to manufacture is because of their ability to self-assemble in to layers. The polymer cells are made up of two components; a polymer and a fullerene, which are mixed together on to a glass substrate coated with an electrode of indium tin oxide. The two components then self-assemble into a structure that works as a solar cell.
Neutrons can non-destructively probe the internal layered structure of these solar cells allowing the scientists to look at the composition of the layers.
“It’s like a goldilocks zone of trying to get the polymer chains to be just the right length in order to get the best devices,” explains Andy. ‘You get significant variability in these devices and this is what we are trying to explain with neutrons”.
The team have used neutron reflectometry at ISIS to look at a particular blend of a copolymer with a fullerene derivative to determine how different molecular weights of the polymers affect the efficiency of the devices.
“Neutrons help us to look at the depth profile of our thin layers where there is some variation, we saw this sort of enrichment zone at the bottom of the film which is essentially increasing with molecular weight up to a limiting point. When there is a too high molecular weight, too long a polymer chain, you see it start to fall out of solution and so you don’t get as much polymer in the final film and then the efficiency falls off quite remarkably. So the neutrons have helped us to understand why the devices weren’t as efficient and why we had this quite large variation.”
UK start-up Ossila has been involved in the fabrication, testing and analysis of these devices. James Kingsley from Ossila says, “Current photovoltaic devices are expensive and difficult to install, and over the last decade there has been little increase in efficiency, which in turn has limited their uptake. Plastic polymer solar cells could change the game – they can be manufactured much more cheaply and quickly, and are easier to install – but first we need to make them more efficient and stable. With the University of Sheffield we are using ISIS to understand how the length of polymer chain affects efficiency and therefore how we can optimise these devices to make them really competitive.”
From the neutron scattering experiments the team have shown that a relatively high molecular weight can create an enrichment layer at the interface which has given their highest efficiency yet for this polymer, PCDTBT.
“What’s interesting to me is how we could move away from fossil fuels,” says Andy. “Abundant energy from renewable sources has got to be an achievable goal, that’s what I would like to see for our future, but I think that’s essentially what a lot of academics and people who are interested in solar technology would like to answer. It might not be the answer for every country, particularly for the UK as we live in quite a northern hemisphere, but some countries have the ability to generate quite a large proportion of their energy needs from solar. We may have to replace the polymer solar cells every seven or eight years but it’s only by this up-scaling and this benchmarking what’s acceptable that we know what we need to do next to go from there.”
Research date: July 2014
For further information contact Andrew Parnell
This research has been published in Scientific Reports.
James W. Kingsley, Pier Paolo Marchisio, Hunan Yi, Ahmed Iraqi, Christy J. Kinane, Sean Langridge, Richard L. Thompson, Ashley J. Cadby, Andrew J. Pearson, David G. Lidzey, Richard A. L. Jones, Andrew J. Parnell. Molecular weight dependent vertical composition profiles of PCDTBT:PC71BM blends for organic photovoltaics. 2014 Scientific Reports 4, Article number:5286
Andrew J. Parnell. Nanotechnology and the potential for a renewable solar future. Nanotechnology Perceptions 7 (2011) 180–187
Solar Energy Perspectives. 2011. International Energy Agency. Renewable energy technologies.