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To increase the device lifetimes and efficiency of OLEDs, it is critical to understand the electrical charge behavior at different interfaces within. Professor Takayuki Miyamae and his team from Chiba University used the electronic sum-frequency generation spectroscopic method to understand the charge behavior and vibrational structure at different interfaces inside OLED devices. Credit: Ka Kit Pang from Wikimedia Commons
Scientists employ electronic sum-frequency generation spectroscopy to investigate charge transport mechanisms in organic light-emitting diodes.
High-resolution, full-color display devices, such as foldable smartphones and ultrathin televisions, rely on organic light-emitting diodes (OLEDs). Compared to other display technologies, OLEDs provide distinct advantages, including flexibility, self-illumination, lightweight construction, ultra-thin profiles, high contrast ratios, and low-voltage operation. These features have made OLEDs increasingly attractive in recent years.
An OLED consists of several layers of ultrathin organic films placed between two electrodes. Each layer serves a specific role in the device’s operation. When voltage is applied, electrical charges accumulate and light is emitted, often at the interfaces between these organic layers. While the multilayer structure allows precise control of charge accumulation, charge transport, and light generation, the very processes that enable OLED functionality can also degrade the organic layers over time. This degradation limits both the lifespan and efficiency of OLED devices.
Understanding how the electronic structure at these interfaces behaves during operation remains a significant challenge. To tackle this issue, Professor Takayuki Miyamae, together with Mr. Tatsuya Kaburagi and Dr. Kazunori Morimoto from Chiba University in Japan, employed a second-order nonlinear spectroscopic method known as sum-frequency generation (SFG). This technique enabled them to investigate the vibrational and electronic properties at the interfaces within operating OLEDs, offering new insights into their behavior under real-world conditions.
When voltage is applied to an OLED system, light is emitted via the recombination of charges at the organic interfaces. This alters the SFG output, allowing researchers to study how charge accumulates and what electronic structural changes occur at the interfaces under different operating conditions. This nondestructive, innovative spectroscopic technique to study charge behavior inside OLEDs was published online by the team in the prestigious Journal of Materials Chemistry C on March 10, 2025.
Studying Charge Behavior with ESFG Spectroscopy
In this study, three different multilayer OLEDs with different types and combinations of organic layers were used. Electronic SFG (ESFG) spectroscopy was conducted on three OLED devices to examine spectral changes induced by the charge behavior and electronic structure at the interfaces. “We examined the differences in the electric field intensities inside the OLED devices based on the applied voltage dependence of the ESFG spectra. This clarifies the role of field strength differences that affect the ease of internal charge flow and the light emission characteristics for the first time,” explains Prof. Miyamae about the team’s study.
ESFG spectral bands corresponding to each organic layer were identified by comparing the absorption spectra and layer configurations among the three OLED devices. The researchers observed changes in spectral signal intensities when applying voltages to the OLED devices, which were related to the changes in the electric field and charge behavior inside the OLEDs.
Upon voltage application, the spectral signal intensity increased at the absorption band of the hole transport material (positive charge carriers inside the OLED), and signal intensity decreased at the absorption band of the light-emitting layer. This shows that internal charge flow across the organic layers within the OLEDs is different, leading to changes in spectra.
Impact of Material Choice on Light Emission
The team also applied square-wave pulse voltages on these devices to study how electric fields formed within these devices varied with time. They found that adding BAlq (a material used for electron transport in OLEDs) changes the position where the light is emitted in OLEDs. This shift in emission affects both the color and shape of the emitted light and how efficiently the device converts electricity into light.
“ESFG technique represents a novel, highly effective, nondestructive, and non-invasive spectroscopic approach for examining the electric field generation caused by injected charges in solid-state thin-film devices,” notes Prof. Miyamae about this innovative research work.
With this technique, material scientists can now design OLEDs with improved device lifetimes, energy efficiency, and cost reductions, eventually increasing the use of ultrathin organic devices in our everyday lives. “Moreover, this research can greatly shorten and rationalize materials development research, which now performs trial-and-error processes and long periods of degradation verification to assess device efficiency and lifetime,” adds Prof. Miyamae.
Reference: “Probing charge behaviour in multilayer organic light-emitting diodes via electronic sum-frequency generation spectroscopy” by Tatsuya Kaburagi, Kazunori Morimoto and Takayuki Miyamae, 10 March 2025, Journal of Materials Chemistry C.
DOI: 10.1039/D4TC04970E
Funding: JSPS KAKENHI Grants-in-Aid for Scientific Research
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