Chiral Circularly Polarized Light Plays Its Hand to Brighten OLED Screens

Physics principles dictate that light of only a single handedness is permitted to pass through an OLED display — causing even the most advanced OLED display light sources to waste about half of the light they emit.

Seeking to ameliorate this issue, researchers at the Weizmann Institute of Science led by Binghai Yan discovered that, in contrast to traditional scientific theory, reversing the current flow in OLEDs switches the handedness of the circularly polarized light that it emits. The researchers used a method that has shown its efficacy for the control of the chirality of light to make the discovery, which the researchers said could boost the net polarization rate in OLEDs by orders of magnitude.

When particles spin in the same direction in which they travel, their rotation in relation to their motion, or chirality, is right-handed. When particles spin against the direction in which they are traveling, their chirality is left-handed. The transparent outer layer of an OLED display is made of a chiral material that allows only one-handed light to pass in and out of the display. For example, if right-handed light is allowed to pass through the display, photons with left-handed chirality will be blocked. This is done to neutralize the incoming ambient light, which has a mixed chirality.

Allowing light of the opposite chirality to pass through the screen would lower the screen’s contrast, making it difficult to view the screen in daylight.

Although it is necessary to block light of the opposite chirality for the display to operate in bright light, it is also inefficient. When the diodes of OLED screens emit light of a mixed chirality, only about half of the emitted light reaches the viewer, because light of only one chirality is allowed to pass through.

Chirality refers to the self-rotation of particles in relation to their motion; when particles flow, they move in space, and they also spin. When these particles spin in the same direction in which they travel, as a bullet does, their chirality is considered right-handed. When they spin against that direction, they have left-handed chirality. Researchers reversed the current flow in OLEDs, which they showed serves to switch the handedness of the circularly polarized light that it emits. The finding could be used to boost the speed of data transmission. Optical switches that worked faster than mechanical switches, for example, could be designed to flip the chirality of the photon flow by switching electric polarity. Courtesy of Weizman Institute of Science.

The researchers first found a way to create organic diodes that simultaneously emit light in opposite directions — forward and backward. This capability makes it possible for the diodes to emit photons that, for the most part, match the chirality of the transparent outer layer of the OLED display, the researchers said. The diode is outfitted with a back panel that is coated with a polymer containing an organic chiral material. The polymer is engineered to efficiently convert the chirality information it contains. The portion of the diode’s light that matches the chirality of the transparent layer passes through the layer unhindered. The remaining portion of the diode’s light bounces back and forth until it hits the polymer-coated back panel of the diode, which flips the diode’s opposite chirality to match the chirality of the transparent outer layer of the display.

As the Yan-led group investigated chirality at the Weizmann Institute, a researcher at Linköping University (Sweden), Li Wan, developed and tested a method for controlling and amplifying the chirality of light in organic devices. Wan and professor Alasdair Campbell experimentally showed that the chirality of an electron’s flow can be flipped by changing the polarity of the battery used to generate the electric current. Each time the polarity of the power supply was flipped, the chirality of the electron flow changed. The same material was used each time the experiment was repeated.

“These findings ran so counter to everything that was known in this field, other scientists had a hard time believing Wan’s results,” Yan said.

The research groups joined efforts, and they showed that Wan’s findings were an inevitable outcome of Yan’s theory. The researchers further found that the chirality of the light emitted by the electron flow could be controlled by ensuring that the photons were emitted along the same trajectory as the electron flow.

In addition to improving the efficiency of OLED displays, the study’s findings could be used to boost the speed of data transmission. For example, optical switches that worked faster than mechanical switches could be designed to flip the chirality of the photon flow by switching the electric polarity.

The work also explores the close connections between chiral materials, topological electrons, and circularly polarized light in the quantum regime.

“We’ve revealed an intriguing unity between seemingly unrelated aspects of chirality: the structural geometry of a material, the handedness of an electron flow, and, finally, the handedness of light,” Yan said.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01113-9).