Increases response speed of quantum LEDs via excitation memory effects

Optical light emitting diodes (LEDs) are widely used in electroluminescent trace devices that emit light depending on the applied voltage. These devices are central components of a variety of electronic and optoelectronic technologies, including displays, sensors, and communication systems.

Over the past decades, some engineers have developed alternative LEDs known as Quantum LEDs (QLEDS), which use quantum dots (i.e. NM-sized semiconductor particles) as luminescent components instead of traditional semiconductors. Compared to traditional LEDs, these quantum dot-based devices can achieve better energy efficiency and operational stability.

Despite this possibility, it has been found that most QLEDS developed to date have significantly slower reaction rates than typical LEDs using inorganic III-V semiconductors. In other words, they are known to take time to emit light depending on the voltage applied.

Researchers from Zjiang University, Cambridge University and other laboratories recently showed that QLEDS exhibits excitation memory effects. The proposed approach, outlined in a study published in Nature Electronics, involves leveraging essentially the ability of devices to emit light in response to electrical pulses, and leveraging the “memory” of previous electrical inputs.

“Recent advances in the development of organic LEDs for visible light communication have been a key inspiration for our research as they demonstrated that LEDs can serve a purpose beyond just display technology,” Dr. Yunzhou Deng of Cambridge University and Professor Yizheng Jin of Zhejiang University told Phys.org.

“Quantum-Dot LEDs (QLEDS) are a new class of LEDs known for their high efficiency, brightness and stability, making them a promising candidate for optical communications.”

The first aim of this study by Professor Jin and his colleagues, Dr. Deng, was to better understand how QLED responds to pulsed electrical excitation. However, their experiments led to unexpected discoveries and were built to design new high-speed QLEDS based on special microstructures.

“To carry out our research, we employed temporary electroluminescence measurements, which aim to track how quickly the LED turns on or shuts down in response to a voltage pulse input,” explained Dr. Deng. “We used an oscilloscope to monitor how emission intensity evolves over time in response to electrical pulses of microseconds in length. By testing QLEDs under various pulse excitation conditions, we uncovered important insights into reaction behavior.”

Tests conducted by the researchers showed that the electroluminescent responses of QLEDS are affected by the debris of electrical pulses applied to them in the past. This observed excitation memory effect was found to be linked to an energy state known as deep-level hole traps that inhabit the semiconductors of amorphous polymers within the device.

“Our most important finding is that QLEDS exhibits excitation memory effects, meaning they “remember” previous pulsed excitations, even after milliseconds after they are turned off,” Dr. Den and Professor Zinn said. “As a result, the device responds faster when driven at higher pulse frequencies. This effect allows QLEDs to operate at high modulation frequencies above 100 MHz, making them a powerful candidate for high-speed optical communications applications.”

To demonstrate the promise of their approach, the authors designed a low-capacitance microcrew with -3 dB bandwidth of up to 19 MHz. The QLE was found to exhibit an electrolumin center modulation frequency of 100MHz and data movement speeds of up to 120 Mbps, while maintaining excellent energy efficiency.

The results of this recent study could quickly contribute to further advances in QLED technology and pave the way for deployment to a wide range of applications. Meanwhile, researchers continue to investigate the observed effects, while working to further speed up QLEDS responses.

“To further accelerate the response speed of devices, we need to develop new quantum dot materials with faster recombination rates,” added Dr. Deng and Professor Jin. “This includes searching for new compositions and core-shell nanostructures. Additionally, modifying organic components within the device can lead to even more interesting temporary behavior by increasing the excitation memory effect.”

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