A flash of light lasting a fraction of a millisecond could help create better materials for solar energy. Researchers used ultra-fast heating at rates of up to 10 million degrees Celsius per second to rearrange the atoms inside a semiconductor, producing a version that generated up to 50 times more electrical current from light than the same material in its ordinary form. Because the flash heats only the semiconductor coating and not the surface it sits on, the method works on transparent conducting glass, the see-through conducting surface used in touchscreens and solar cells, which conventional furnaces would ruin. The method could eventually help improve solar technologies and other advanced electronic devices.

Researchers have developed a way to use extremely short flashes of light to control the internal structure of semiconductor materials, potentially improving technologies used to convert solar energy into electricity. In the process, the team heated a coating to nearly 2,000 degrees Celsius while the glass sheet supporting it, known as the substrate, stayed below 100 degrees, a combination that conventional heating cannot achieve. Neither the glass nor its conducting coating tolerates temperatures much above 500 to 600 degrees Celsius, which is why high-temperature processing on these surfaces has been considered out of reach.

The study, published in Small Structures, was led by Shahar Artzi and Dr. Ronen Gottesman of the Hebrew University’s Institute of Chemistry and Center for Nanoscience and Nanotechnology.

Many materials can exist in more than one crystal form (known as polymorphs), even though they have the same chemical composition. These structures may have very different properties, but the more useful forms are often unstable and difficult to preserve. Diamond and graphite are the textbook example: both are pure carbon, and only the arrangement of the atoms differs. In electronic materials, the more desirable arrangement is usually the one that survives only at high temperature and reverts whenever the material is allowed to cool slowly.

Preserving it has been blocked by two obstacles. A furnace heats the coating and the surface underneath it together, so the glass and its conducting layer reach their limit long before the film on top reaches the temperature it needs. And a furnace cools slowly, giving the atoms ample time to settle back into their ordinary arrangement on the way down.

The researchers used a technique called flash photonic heating, exposing thin films of bismuth oxide to powerful pulses of white light, in effect the same principle as a photographic flash but far more intense and precisely timed. The pulses last between roughly a tenth of a millisecond and a few milliseconds, and the film absorbs the light directly, heating and cooling far faster than the glass beneath it can follow. By changing the length and intensity of the flashes, the team could choose between the two polymorphs of the material.

“The idea is to heat and cool the material so quickly that we can trap it in a crystal structure that would normally disappear,” Gottesman said. “This allows us to access useful properties that conventional heating methods cannot easily preserve.”

Strikingly, the total amount of energy delivered was not what decided the outcome. Two pulses carrying identical energy produced two different materials, depending only on how quickly that energy arrived.

Longer pulses produced the stable alpha phase, while shorter and more powerful pulses trapped the less stable beta phase at room temperature. The transformation was visible, with the material changing from pale gray to bright yellow.

The beta phase absorbed a wider range of visible light, capturing light that the stable alpha form simply lets through, and produced between 10 and 50 times more photocurrent, the electrical current generated by light,, depending on how the film was prepared. The researchers linked this improvement to more efficient movement of electrical charges through the material.

“What is especially promising is that we are not changing the material’s chemical composition,” Gottesman added. “We are controlling how its atoms are arranged, and that structural change leads to a major improvement in its response to light. The closest everyday analogy is the blacksmith’s trick of quenching steel: cool it fast enough and the structure has no time to relax back to what it would prefer to be. We are doing the same thing with light, on a millisecond timescale, and we are doing it on transparent conducting glass, the same kind of surface behind a touchscreen.”

The team was also able to switch the material repeatedly between the two phases, cycling it from one form to the other and back again. According to the researchers, a reversible switch of this kind, carried out directly on the coated conducting glass used in real devices, had not previously been demonstrated.

Although the study focused on bismuth oxide, the researchers say the method could potentially be applied to other materials used in solar energy conversion, photocatalysis and advanced electronic devices. The team is now examining whether the same approach can be extended to plastic and flexible substrates, whose tolerance for heat is lower still.