Reviewed by Lexie CornerApr 24 2025
Researchers from Chiba University used electronic sum-frequency generation (ESFG) spectroscopy to study charge transport and electric field formation in organic light-emitting diodes (OLEDs) under operating conditions.
Researchers examine the charge behavior inside organic light-emitting diodes (OLEDs). 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. Image Credit: Ka Kit Pang from Wikimedia Commons
To enhance the efficiency and lifespan of multi-layered organic light-emitting diodes (OLEDs), it is critical to investigate charge behavior within these devices, particularly under operational conditions. However, this is a challenging task.
To examine the interface-specific charge behavior in OLEDs during light-emitting conditions, researchers from Chiba University in Japan used electronic sum-frequency generation (ESFG) spectroscopy. They demonstrated that ESFG is an effective and non-invasive technique for observing electric-field formation in OLEDs.
OLEDs are used in high-resolution, full-color displays, including foldable smartphones and ultrathin televisions. Compared to other display technologies, OLEDs offer advantages such as flexibility, self-lighting, low weight, thinness, high contrast, and low-voltage operation, making them a popular choice in recent years.
OLEDs consist of multiple layers of organic ultrathin films placed between electrodes, with each layer serving a specific purpose within the device. Charge accumulation and light emission likely occur at the interface of these organic layers when the device is in operation.
While multiple layers help control light formation, charge accumulation, and charge flow, exposure to charges and light can degrade the organic layers, reducing the lifespan and efficiency of OLEDs.
Understanding the electronic structure at the interfaces of OLEDs while they are powered and operating is difficult.
To address this challenge, Professor Takayuki Miyamae, Mr. Tatsuya Kaburagi, and Dr. Kazunori Morimoto from Chiba University studied the vibrational and electronic properties at the interfaces of OLEDs using sum-frequency generation (SFG), a second-order non-linear spectroscopic method.
OLED systems emit light through the recombination of charges at the organic interfaces when voltage is applied. By observing changes in the SFG output, the researchers could study charge accumulation and electronic structural changes at the interfaces under various operating conditions.
The team introduced a new, nondestructive spectroscopic method for examining charge behavior inside OLEDs.
Three different multilayer OLEDs, with various combinations of organic layers, were used in this study. These devices were analyzed using ESFG spectroscopy to observe spectral changes resulting from the electronic structure and charge behavior 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.
Takayuki Miyamae, Professor, Chiba University
By comparing the absorption spectra and layer configurations of three OLED devices, the ESFG spectral bands corresponding to each organic layer were identified. When voltage was applied to the OLED devices, the researchers observed variations in spectral signal intensities, which were linked to changes in the electric field and charge behavior within the OLEDs.
Upon voltage application, the spectral signal intensity increased at the absorption band of the hole transport material (responsible for positive charge carriers) and decreased at the absorption band of the light-emitting layer. This indicates that the internal charge flow across the OLED’s organic layers is altered, leading to variations in the spectra.
To study how the electric fields within these devices change over time, the team also applied square-wave pulse voltages. They found that the addition of BAlq, a material used for electron transport, shifted the position of light emission in the OLEDs. This change affected the color, shape, and efficiency of the device’s conversion of 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.
Takayuki Miyamae, Professor, Chiba University
This technique enables material scientists to create OLEDs with improved device lifetimes, higher energy efficiency, and reduced costs, potentially leading to broader use of ultrathin organic devices in everyday applications.
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.
Takayuki Miyamae, Professor, Chiba University
This research was partially funded by the JSPS KAKENHI Grants-in-Aid for Scientific Research, Japan, under grant numbers 22H02048 and 23K17811.
Journal Reference:
Kaguragi, T., et al. (2025) Probing charge behaviour in multilayer organic light-emitting diodes via electronic sum-frequency generation spectroscopy. Journal of Materials Chemistry C. doi.org/10.1039/d4tc04970e.