View on Moving Electrons in Solar Cells

Members of the Femtosecond Spectroscopy Unit working on time-resolved photoemission electron microscopy to visualize electron movements. (Source: OIST)

Members of the Femtosecond Spectroscopy Unit working on time-resolved photoemission electron microscopy to visualize electron movements. (Source: OIST)

Ever since J.J. Thompson’s 1897 discovery of the electron, scientists have attempted to describe the subatomic particle’s motion using a variety of different means. Electrons are far too small and fast to be seen, even with the help of a light micro­scope. This has made measuring an electron’s movement very difficult for the past century. However, new research from the Femtosecond Spectro­scopy Unit at the Okinawa Institute of Science and Techno­logy Graduate University OIST has made this process much easier.

“I wanted to see the electrons in the material. I wanted to see the electrons move, not just to explain their motion by measuring a change of light trans­mission and reflec­tion in the material,” said Keshav Dani, leader of Unit. The limiting factor to studying electron movement using previous techniques was that the instrumen­tation could either provide excellent time resolution or spatial resolution, but not both. Michael Man, a postdoctoral fellow in Dani’s Unit, combined the techniques of UV light pulses and electron micro­scopy in order to see electrons moving inside a solar cell.

If you shine light on a material, the light energy can be absorbed by the electrons and move them from a low-energy state to a higher one. If the light pulse that you shine at the material is a few femto­seconds short, it creates a very rapid change in the material. However, this change does not last long, as the material goes back to its original state on a very fast time scale. For a device to work, like in a solar cell, we have to extract energy from the material while it is still at the high energy state. Scientists want to study how materials change state and lose energy. “In reality, you cannot watch these electrons changing state on such a fast time scale. So, what you do is measure the change of reflec­tivity of the material,” Man explained. To understand how the material changes when exposed to light, researchers expose the material to a very short, but intense, pulse of light which causes the change, and then continuing to measure the change introduced by the first pulse by probing the material with subsequent much weaker light pulses at different delay times after the first pulse.

As the first discrete bundle of photon changes the material, by rapidly heating it, the reflection of the subsequent photon changes. As the material cools down, the reflection goes back to the original one. These differences tell the scientists the dynamic of the observed phenomenon. “The problem is that you do not actually directly observe the electron dynamics that causes the changes: you measure the reflection and then you try to find an expla­nation based on the inter­pretation of your data,” Dani said. “You create a model that explains the results of your experiment. But you do not actually see what is happening.”

Dani’s team found a way to visualize this pheno­menon in a semi­conductor device. “When the pulse hits the material, it takes some electrons out, and we use an electron microscope that forms an image of where the displaced electrons came from,” Man said. “If you do this many times, for many photons, you can slowly build up an image of the distri­bution of the electrons in the material. So you photo-excite the sample, you wait for a certain time, and then you probe your sample and you repeat this process again and again, keeping the delay between the first pulse of photons and the probing photons always the same.” As a final result, you get an image of the location of most of the electrons in the material at a specific time delay.

Then, the researchers change the time delay between the two pulses and they create another image of the location of the electrons. Once an image is created, the probing pulse is further delayed, creating a series of images that describes the positions of the electrons in subsequent times after the photo-exci­tation. “When you stitch all these images together, you finally have a video,” Dani said. “A video of how the electrons are moving in the material after photo excitation: you see the electrons getting excited, and then going back to their original state.”

“We have made a video of a very funda­mental process: for the first time we are not imagining what is happening inside a solar cell, we are actually seeing it. We can now describe what we see in this time-lapse video, we no longer have to interpret data and imagine what might have happened inside a material. This is a new door to under­standing the motion of electrons in semi­conductors materials.” Dani effused. This research provides a new insight into the movement of electrons that could poten­tially change the way solar cells and semi­conductor devices are built. This new insight brings the techno­logy field one step closer to building better and more efficient electronic devices. (Source: OIST)

Reference: M. K. L. Man et al.: Imaging electrons in motion across semiconductor heterojunctions, Nat. Nano., online 

Link: Femtosecond Spectroscopy Unit, Okinawa Institute of Science, Okinawa, Japan

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