Solar jackets instead of wall sockets

Whether on your jacket, the roof of your car or the façade of a building, a new type of solar cell is supposed to generate electricity in places where this is difficult to do with conventional panels. An international collaboration involving DESY has now come a great deal closer to achieving this goal. The team of researchers was able to modify a special type of cell – the perovskite-organic tandem cell – and set a new efficiency record. Their prototype achieves an efficiency of 26.4 percent, meaning that it converts 26.4 percent of the incident light energy into electricity. The group has now presented its findings in the renowned journal Nature. 

Solar cells made of silicon are found on the roofs of many houses; they are tried and tested, having been installed millions of times. However, silicon only captures part of the sunlight, mainly in the visible spectrum. Other wavelengths, such as those in the near infrared, remain untapped. This is where the tandem principle comes in: two materials, each of which captures different wavelengths, are stacked on top of each other, so that together they utilise a broader spectrum – from ultraviolet all the way to infrared. Cells that use perovskite semiconductors in addition to silicon have recently become available and achieve above-average efficiency levels.

Their drawback is that, like conventional silicon modules, they are relatively heavy and rigid. This is why experts are studying a new tandem variant. It combines the perovskite with a thin layer of an organic semiconductor – essentially, a solar cell made of plastic. “Such perovskite-organic tandem cells bring together the best of both worlds,” explains DESY scientist Stephan Roth. “Using two materials makes them highly efficient in generating electricity, and using an organic semiconductor makes them flexible and bendable.”

New molecule for more electricity

However, previous prototypes were marred by the fact that the lower organic layer made inadequate use of the infrared radiation. Specifically, two functional components – donors, emiting electrons, and acceptors, absorbing electrons – need to be arranged in such a way that, although they are clearly separated, they are interlinked closely enough to work together as efficiently as possible. Using this as its starting point, the team, working under the leadership of the University of Singapore, set out to develop a new acceptor molecule. The aim was to improve its interaction with the donor compared with earlier prototypes – thereby helping to convert incident infrared light into electricity more efficiently.

Using this new molecule, the experts produced a one-square-centimetre prototype by means of spin coating – a widespread laboratory technique for depositing thin films. To determine whether this candidate displayed the correct molecular orientation in the organic layer, the prototype was analysed at DESY’s X-ray source PETRA III and also at a synchrotron in Taiwan. The result was that “using a special X-ray technique, we could see that the molecules were lying alongside each other instead of standing on end,” explains Roth. “And this significantly improves charge transport.”

It also became apparent that manufacturing the sample in Singapore had been extremely successful. The internal structure of the small prototype cell was highly uniform: the materials were finely distributed, without lumps or defects. With a certified efficiency of 26.4 %, the cell is at the cutting edge of current solar cell technologies – outperforming many silicon-based modules. The record has been confirmed by an independent laboratory.

Flexible and versatile

But the new tandem cell is not just efficient, it’s also light, flexible and versatile. Stephan Roth sees great potential for curved surfaces – for example on building façades, car roofs or even in textiles. A jacket with integrated perovskite-organic tandem cells could, for instance, charge a smartphone on the go, even in diffuse light or in the shade. Because, being highly sensitive to infrared, such a cell would also function under indoor lighting.

However, some challenges still need to be overcome before the cell is ready for market release. Production needs to be scaled up to manufacture larger surfaces. For the moment, spin coating only works for small samples, so experts are considering alternatives such as spray coating. Durability and environmental factors also need to be considered. Organic materials are sensitive to moisture and oxygen, making encapsulation essential. The scientists are also exploring the use of environmentally friendly solvents and developing recycling strategies for the various components of these perovskite-organic cells.

A future-oriented DESY project will help advance the technology further: the planned X-ray source PETRA IV could provide far more precise insights into the functioning of solar cells – for example through studies examining how materials age over time. “PETRA IV would allow us to observe the entire life cycle of a solar cell in real time,” says Stephan Roth enthusiastically, “from manufacturing through deployment to recycling.”

Reference