Hey, what are these curved green flashes above my polymer semiconductor?

In every scientific discovery in the movies, a scientist observes something unexpected, scratches the side of his or her forehead and says “hmmmmm.” In just such a moment in real life, scientists from Canada observed unexpected flashes of curved green light from a red light-emitting polymer above its surface. The flashes were reminiscent of the colored arcs that auroras take above Earth’s poles, providing a clue as to their provenance.

Their resulting investigation of the new phenomenon could find applications towards understanding the failures of polymer materials and more. Their work has been published in Physical Review Letters.

Jun Gao, a professor and chair of Engineering Physics at the Engineering Physics and Astronomy Department at Queen’s University in Ontario, Canada, and graduate student Dongze Wang were investigating the performance of semiconductors called polymer light-emitting electrochemical cells, or PLECs.

An outgrowth of work on conductive polymers that won the 2000 Nobel Prize in Chemistry by Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa, PLECs are organic semiconductors with one side doped to create an n-type semiconductor having an excess of electrons, and the other side a p-type semiconductor with electron holes.

In its normal operation, an applied voltage causes electrons to migrate across the material and fill the holes, creating emissions of red light.

The PLECs the team used were in planar configurations, not a sandwich-type configuration. Wang and Gao were investigating “beautiful” (in Gao’s words) damages in PLECs called “electrical treeing,” where tree-like branches flash through the polymer, creating voids that disrupt its performance.

To amplify this treeing, they cooled a PLEC to 200 K (-73°C) and cranked up and reversed the applied voltage up to 1,000 volts, with the negative contact on the p-type side and the positive contact on the n-type side, creating a reverse bias. “Significant electrical treeing occurred, along with orange-red light from the treetops,” Gao told Phys.org.

“We believed the excitation of the light-emitting polymer MEH-PPV caused the orange-red light,” said Gao. MEH-PPV is a soluble polyphenylene vinylene derivative that is widely used as a light emitter in polymer light-emitting diodes (LEDs). They confirmed this light was due to an excitation by taking spectral measurements of the emissions.

“We were not prepared, however,” Gao recalled, “for the green light flashes, since MEH-PPV does not emit green light.”

Looking at images of the PLEC, they noticed strange, slightly arced flashes of green light just above the n-type side electrode, with no red light on the other side. Increasing the voltage between the electrodes increased the number and duration of the arcs.

These flashes could extend far beyond the edges of the electrodes, which ruled out the polymer film as an emission source, and the slight curvature of the light flashes raised the possibility of particles being bent in a magnetic field.

“[So] we found a small permanent magnetic disk in the lab and placed it near the PLEC under test,” Gao said, “where we saw significant deflection of the green light.”

They collected these images into a video that, according to Gao, “when played back, the flashing green light resembled an aurora in color and shape.” They determined the charge-to-mass ratio of these particles by measuring the radius of curvature of the arcs, a simple process often done in freshman physics lab experiments, and found it matched the e/m ratio of the electron.

Their investigation determined that the electrons are emitted into free space by a mechanism called “field emission” from the sharp tips of the electrical treetops.

Wang and Gao postulate that the green light flashes are caused by streams of electrons bombarding and exciting an unknown light-emitting vapor in-flight, possibly gases from the deteriorating polymer. The light-emitting vapor is composed of fragments of MEH-PPV released in the electrical treeing process.

The short MEH-PPV fragments emit shorter wavelengths (green has a shorter wavelength than red and orange) because the charge carriers are more confined in space, resulting in a larger energy separation between the optical transitions.

This is the first time a doped polymer has been seen as a strong emitter of free electrons, albeit at a relatively cold temperature. Gao says a polymer electron emitter could be used in a field emission display as an electron source.

Their work also demonstrates a new method to visualize the electron flight path and calculate its charge-to-mass ratio when deflected with a known magnetic field.

The spectral characteristics of the light emission provide important clues to the breakdown products of polymer films, according to Gao, and it allows the identification of unknown compounds via “electrical breakdown spectroscopy” and gives a way to measure the resistance of the polymer film to intense electrical stress.

Finally, Gao noted, “It can be a unique all-in-one device to educate young scientists about many physics concepts, such as doping, p-n junction, light generation, and charged particles.” Maybe someday you might even see one in the movies.

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