Small Steps For Electrons – Big Steps For The Solar Cells Of The Future?

Physicists at the University of Regensburg and the University of Oxford are using an ultrafast microscope to reveal how electrons move in a new type of solar cell material. The results provide insights into how the material can be used even more efficiently for photovoltaics.

On the way to more efficient and sustainable methods of energy generation, the class of materials known as metal halide perovskites is a promising beacon of hope. Within a very short time after their discovery, new types of solar cells based on this material achieved efficiencies comparable to commercial silicon solar cells. In addition, perovskite solar cells have other decisive advantages: The manufacturing and energy costs are low compared to established silicon technology because they can be produced using inexpensive coating processes. In addition, the newcomers in photovoltaics are flexible and lightweight, which enables them to be used on a wide variety of surfaces - from portable electronics to innovative building facades.

But how does a solar cell actually work? Sunlight, which consists of individual light quanta - so-called photons - is absorbed in the solar cell. The photons give off their energy to electrons, which are then raised to higher-energy orbits where they can move more freely. These are extracted at suitable electrical contacts and thus converted into usable electrical energy. The efficiency of a solar cell depends crucially on how easily these short-lived charge carriers can move through the material to reach the contacts before they decay again. In order to further optimize solar cells in a targeted manner, it is therefore important to understand exactly how this transport takes place - which paths the electrons take and what restricts their movement.

This is exactly what researchers at the University of Regensburg led by Prof. Dr. Rupert Huber have now achieved using a new type of ultrafast microscope on tailor-made samples from Prof. Dr. Michael Johnston (Oxford University). The team was able to generate free electrons in a targeted manner and track their diffusion on ultrashort timescales. This has so far been a particular challenge with perovskite solar cells, as these are not homogeneous but consist of many small grains that are only hundreds of nanometers - a billionth of a meter - in size. At the same time, these nanocrystals are not all identical, but can exist in one of two different atomic structures at room temperature, only one of which is suitable for use in solar cells. It is therefore important to know exactly where you are on the sample and which crystalline structure is being examined. The researchers therefore used a microscope with which they can control the position of their measurement to nanometer precision and at the same time use optical methods to extract whether they are currently sitting on a crystallite with the correct atomic structure. "We make the atoms in the nanocrystallites vibrate. Depending on the arrangement of the atoms, this leaves clearly identifiable signatures in the scattered light - something like a fingerprint. This allows us to deduce exactly how the atoms are arranged in the respective crystallites," explains Martin Zizlsperger, lead author of the publication.

Once the team knew the exact shape and crystal structure of the nano rocks, they illuminated the sample with a short light pulse that - like the sun - excited electrons into mobile states. The researchers were then able to measure the subsequent movement of the charges with a second laser pulse. "To put it very simply, the charges act like a mirror. If these charges move downwards away from our measuring point, for example, then the second laser pulse is reflected later. From this tiny time offset of just a few femtoseconds - where a femtosecond corresponds to a millionth fraction of a billionth of a second - we can reconstruct the exact movement of the charges," explains co-author Svenja Nerreter.

This made it possible to observe exactly how the excited electrons move through the labyrinth of different crystallites. In particular, the researchers were able to investigate the technically particularly relevant movement into the solar cell after excitation. The results were surprising: Although the material consists of many different nanocrystals, the vertical charge transport on the nanometer length scale is unaffected by irregularities in the exact shape of the nanocrystallites - a possible reason for the success of perovskite solar cells. When the researchers also examined larger regions on the scale of several hundred micrometers, however, it also became apparent that there are differences between micrometer-sized regions made up of hundreds of small nanocrystallites, with some regions being more efficient at transporting charge than others.

These local hotspots could be of great importance for the development of new solar cells. The researchers' novel measurement method can provide direct insight into the distribution and efficiency of the individual regions and is an important step towards further improving perovskite solar cells. The results were published in the renowned journal Nature Photonics. "Our newly developed method allows us for the first time to observe the complex interplay between charge transport, crystal configuration and the shape of the crystallites directly on the nanoscale. This means that it can be used to further improve perovskite solar cells in a targeted manner," explains Prof. Huber. However, the novel measurement method is not only limited to modern solar cells, as the interplay between structure and charge transport is of central importance for a large number of modern applications. The breakthrough could also be of valuable help in the development of ultimately small and fast transistors and in explaining one of the greatest mysteries in solid-state physics - high-temperature superconductivity.