Frenzy in perovskite

When it comes to new methods of energy generation, metal halide perovskites are promising beacons of hope. Within a very short time of their discovery, these novel solar cells achieved efficiencies comparable to commercial silicon solar cells. In addition, perovskite solar cells have further advantages: The manufacturing and energy costs are low compared to established silicon technology, as they are produced using cost-effective coating processes. The newcomers in photovoltaics are also flexible and lightweight, which enables them to be used on a wide variety of surfaces - from portable electronics to innovative building façades.

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

Researchers at the University of Regensburg led by Prof. Dr. Rupert Huber have now succeeded in doing this with a new type of ultrafast microscope. The team generated free electrons and tracked their diffusion on ultrashort time scales. This was previously a challenge with perovskite solar cells, as they 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 identical, but can occur at room temperature in one of two different atomic structures, 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 investigated. The researchers therefore used a microscope with which they can control the position of their measurement with nanometer precision and at the same time use optical methods to extract whether they are 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 clear signatures in the scattered light. This allows us to deduce exactly how the atoms are arranged in the respective crystallites,” explains Martin Zizlsperger, first author of the study.

Once the team knew the shape and crystal structure of the nanorocks, they illuminated the sample with a short light pulse, which, 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. "Put simply, the charges act like a mirror. If the charges now 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, the team was able to reconstruct the exact movement of the charges. A femtosecond is one millionth of a billionth of a second, 10^-15 s.

This made it possible to observe how the excited electrons move through the labyrinth of different crystallites. In particular, the researchers also investigated the technically relevant movement after excitation into the solar cell. 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 examined larger regions on the scale of several hundred micrometers, they found that some regions are more efficient in charge transport than others.

The local hotspots could be of great importance for the development of new solar cells. The measurement method provides insight into the distribution and efficiency of the individual regions and is an important step towards the further improvement of perovskite solar cells. "Our method allows us to observe the complex interplay between charge transport, crystal configuration and the shape of the crystallites on the nanoscale. It can therefore be used to further improve perovskite solar cells in a targeted manner,” explains Prof. Huber. The new measuring method is not just limited to modern solar cells. The interplay between structure and charge transport is important for a large number of modern applications. For example, the breakthrough could also help in the development of the ultimate small and fast transistors and in explaining one of the greatest mysteries of solid-state physics, high-temperature superconductivity.

Origina lpublication:

Martin Zizlsperger et al.

In situ nanoscopy of single-grain nanomorphology and ultrafast carrier dynamics in metal halide perovskites

Nature Photonics (2024).