Diamond's 8000th publication: The future of solar cells

As one of the most advanced scientific facilities in the world, Diamond strives to enable world-changing science every day. An important part of our mission is to aid in the publication of papers and results of the experiments done here into the public domain. Today, we are thrilled to celebrate the publication of the 8000th paper from Diamond research, which rather befittingly looks to the future at energy technologies, helping to power tomorrow's world. 

A collaboration between researchers in the UK and China recently led to the publication of the 8000th research article describing cutting edge science carried out at Diamond Light Source. Professor David Lidzey from the University of Sheffield and his collaborator Professor Tao Wang from Wuhan University of Technology published their findings in Nano Energy with implications for the future of solar cells. 

     

A brief overview

A representation of a
A representation of a "bucky ball" or fullerene molecule, commonly used as charge acceptors in solar panels.

Fullerene molecules known as “Bucky balls” have been used as charge acceptors in solar cells for a long time. Researchers used Diamond Light Source to investigate new acceptor molecules that would be cheaper to manufacture. They discovered that depending on the molecule and the way that it was blended with polymers, they were able to see a significant efficiency increase over traditional compositions. The added efficiency came from the fact that the new compositions could absorb light over a broader wavelength range. This means that if used in solar cells, they will be able to use more of the sun’s light than is possible using current materials.

The added efficiency comes from the molecules themselves as well as the way they are blended and cast. Using the GWAXS technique at Diamond, the researchers found that flat acceptor molecules were able to stack very efficiently and that the production method allowed them to self-organise on nanometre length scales allowing aggregates to form that extend the wavelengths that can be absorbed.

The research is in very early stages and is not ready to be translated into the commercial manufacture of solar cells yet. However, it opens the door for new, cheaper molecules to be used in the production of organic solar cells as well as casting and blending methods that can increase efficiency. In the future, this will hopefully lead to cheaper and more efficient solar cells.

How organic solar cells work

Almost all of the solar cells in general use are based on the material silicon. However new generations of solar cells are emerging that should be cheaper and easier to manufacture. One new type of solar cell is modeled on carbon-based semiconductors, and is called an ‘organic solar cell’. Organic solar cells generate electricity using a mixture of two different types of molecules, charge donors and charge-acceptors. When a donor molecule absorbs a photon, an excited-state electron is created. If this excited electron undergoes a transfer to a nearby ‘acceptor molecule’, a small electrical current is generated. By extracting such charges from the solar cell they can be harnessed as useful electricity. Typically, the acceptor molecule used in organic solar cells is the football shaped C60 molecule, fullerene. While it is possible to make efficient solar cells with fullerene, its purification and production can be a problem. Fullerenes often need to be functionalised to make them soluble and current manufacturing methods are low yield, adding significant costs to solar cell production. Fullerene acceptors also absorb light only weakly and tend to migrate in devices over time, forming large clumps which reduce the device efficiency. These effects limit the light-to-power conversion efficiency of state-of-the-art fullerene-based organic solar cells to around 11%. However, a new generation of ‘non-fullerene’ acceptor molecules are being used in organic devices and have shown efficiencies over 16%. Current research in the field now focuses on how to further improve efficiency, durability, and processability of new materials that could be applied in the manufacture of organic solar cells.

 

A more efficient way of absorbing light

Lidzey and Wang set about examining new, non-fullerene molecules that could be used in solar cells, as well as new ways of manufacturing them. Lidzey explains:

Non-fullerene acceptors have been shown to be cheaper to manufacture as well as increasing the efficiency of solar cells. This has reinvigorated the field of organic photovoltaics which had previously relied on fullerenes as acceptors. 

The new acceptors could one day lead to a new generation of solar cells with a broader wavelength range and higher efficiencies
The new acceptors could one day lead to a new generation of solar cells with a broader wavelength range and higher efficiencies

The non-fullerene acceptors are mixed with polymers but are not chemically bonded together, rather they self-organise and associate to different degrees depending on how the molecules are blended and cast. The research team took a selection of new potential acceptors and tried new ways of blending them.

When the team examined the materials using spectroscopy in Professor Wang’s lab, they made a surprising discovery. They expected to see spectra similar to the traditional fullerene acceptors but the spectra from their new acceptors were much broader. This was an exciting moment in the study and it gave some initial evidence that their mixture would able to capture light over a broader wavelength range leading to higher efficiencies. This could one day lead to a new generation of solar cells able of capturing energy from wavelengths of the sun’s light that are not possible today.

To try and explain what they were seeing in the spectra, and to understand the efficiency of their experimental solar cells, the team headed to Diamond Light Source. Lidzey explained:

The efficiency of solar cells is dependent on the organisation and structure of a material at length scales of a few nanometres.

The team worked round the clock in shifts measuring the structures of hundreds of samples on the GWAXS beamline, I07 at Diamond. Lidzey said, "I always felt lucky that some students were able to slip into a nocturnal work mode. I was always really impressed that they were so cheerful in the morning despite having been working all night!"

The results from the GWAXS were surprising. They found that their mixtures of non-fullerene acceptors and polymers were extremely crystalline. However, by applying the team’s new processing methods, those molecules were able to stack on top of one another in a very ordered new fashion and it was this that affected the way the acceptors were able to absorb light. The highly organised packing of the molecules in the mixture at nanometre scales was what allowed the mixtures to absorb light over different wavelengths. The result was an acceptor molecule and a method for mixing it with a donor-polymer that could one day form the basis of efficient organic solar cells.

What does the future hold for these new acceptors?

 Lidzey was eager to point out that this was fundamental research and the direct applications will come further down the line after more trials and experiments. However, even if this exact system is not used in the future of solar cells, it seems likely that something similar will be. Part of the genius of the study, despite it being fundamental research, is that it supplies information that can be used by future solar cell manufacturers. The paper shows that we don’t need to rely on fullerene as an acceptor which represent a major cost in currents solar panels. The paper also shows that the way that solar cells are made can have a big impact on the final efficiency.

For David Lidzey and his group, the future will also hold many more experiments at Diamond. He told of how when he started using Diamond it was his first experience doing “big science” and over the last 10 years, this has had a huge impact on his research.