Â鶹ÒùÔº

Boosting the response speed of quantum LEDs via an excitation memory effect

Light-emitting diodes (LEDs) are widely used electroluminescent devices that emit light in response to an applied electric voltage. These devices are central components of various electronic and optoelectronic technologies, including displays, sensors and communication systems.

Over the past decades, some engineers have been developing alternative LEDs known as quantum LEDs (QLEDs), which utilize quantum dots (i.e., nm-size semiconducting particles) as light-emitting components instead of conventional semiconductors. Compared to traditional LEDs, these quantum dot-based devices could achieve better energy-efficiencies and operational stabilities.

Despite their potential, most QLEDs developed so far have been found to have significantly slower response speeds than typical LEDs using inorganic III-V semiconductors. In other words, they are known to take a longer time to emit light in response to an applied electrical voltage.

Researchers at Zhejiang University, University of Cambridge and other institutes recently showed that QLEDs exhibit an excitation-memory effect, which could help to improve their response speeds. Their proposed approach, outlined in a study in Nature Electronics, essentially entails leveraging the ability of the devices to emit light in response to electrical pulses, leveraging their "memory" of previous electrical inputs.

"Recent progress in the development of organic LEDs for visible light communications were the key inspiration for our study, as they showed that LEDs can serve purposes beyond just display technology," Dr. Yunzhou Deng at University of Cambridge and Prof. Yizheng Jin at Zhejiang University, two authors of the paper, told Â鶹ÒùÔº.

"Quantum-dot LEDs (QLEDs) are an emerging class of LEDs known for their high efficiency, brightness, and stability, making them promising candidates as light sources for optical communication."

The initial objective of this study by Dr. Deng, Prof. Jin and their colleagues was to better understand how QLEDs respond to pulsed electrical excitations. Yet their experiments led to unexpected findings, which they built on to design new high-speed QLEDs based on specialized microstructures.

"To conduct our study, we employed transient electroluminescence measurements, which aim to track how quickly the LED is turned on or shut down after in response to a voltage pulse input," explained Dr. Deng. "Using an oscilloscope, we monitored how the emission intensity evolved over time in response to microsecond-long electrical pulses. By testing QLEDs under different pulsed excitation conditions, we uncovered key insights into their response behavior."

The tests carried out by the researchers showed that the electroluminescent responses of QLEDs are influenced by remnants of electrical pulses that were applied to them in the past. This observed excitation-memory effect was found to be linked to energy states known as deep-level hole traps, which inhabit the amorphous polymer semiconductors in the device.

"Our most significant discovery is that QLEDs exhibit an excitation memory effect, meaning that they 'remember' previous pulsed excitations even milliseconds after being turned off," said Dr. Deng and Prof. Jin. "As a result, when driven at higher pulse frequencies, the devices respond faster. This effect enables QLEDs to operate at high modulation frequencies exceeding 100 MHz, making them strong candidates for high-speed optical communication applications."

To demonstrate the promise of their approach, the authors designed a low-capacitance micro-QLED with a -3 dB bandwidth of up to 19 MHz, which leverages the excitation-memory effect they observed. This QLED was found to exhibit an electroluminescent modulation frequency of 100MHz and data-transmission rates of up to 120 Mbps, while retaining a good energy-efficiency.

The results of this recent study could soon contribute to the further advancement of QLED technology, potentially paving the way for their deployment for a wide range of applications. Meanwhile, the researchers plan to continue investigating the effect they observed, while also working to speed up the responses of QLEDs even more.

"To further accelerate device response speed, we will need to develop new quantum dot materials with faster recombination rates," added Dr. Deng and Prof. Jin. "This will involve exploring novel compositions and core-shell nanostructures. Additionally, enhancing the excitation memory effect by modifying the organic components in the device could lead to even more interesting transient behaviors."