Uncovering the Role of Spacers in Advancing Portable, Low-Voltage OLEDs

Summary

Inserting a nanometer-thin spacer in ExUC-OLEDs improves energy transfer, enhances blue light emission 77-fold, paving way for lightweight, low-voltage, and more flexible OLEDs

Exciplex upconversion-type organic light-emitting devices (ExUC-OLEDs) can emit light at less than half the voltage needed for conventional OLEDs, but their development remained limited by strict requirements for compatible donor and acceptor materials. Now, researchers from Japan have introduced a nanometer-thin spacer layer, boosting blue light output by 77-fold compared to previously incompatible materials. With a greater choice of materials, this design opens doors to energy-efficient OLEDs for a wide range of uses.

Research Background

Organic light-emitting diodes (OLEDs) have transformed display and lighting technology with their vivid colors, deep contrast, and energy efficiency. As demand grows for lighter, thinner, and more energy-saving devices—especially in wearables, foldables, and portable electronics—there’s increasing interest in OLEDs that can operate at lower voltages without compromising performance. A new type of OLEDs, known as exciplex upconversion OLEDs (ExUC-OLEDs), has opened newer avenues for making the display and lighting technology more efficient. These devices use a different mechanism to generate excitons, allowing light emission at much lower voltages.

Conventional OLEDs emit light when excitons (bound pairs of electrons and holes) relax to a lower-energy state. The formation of excitons requires a voltage equal to or higher than the bandgap of the emitter material—typically around 3 V for red and 4 V for blue light. In contrast, ExUC-OLEDs generate light via a lower-energy intermediate state called an exciplex, a loosely bound electron-hole pair formed at the interface between donor and acceptor molecules. The exciplex transfers its energy to the emitter’s triplet state, enabling triplet–triplet upconversion (TTU) to occur. In TTU, two triplets combine to produce a high-energy singlet state, which emits visible light. This mechanism allows ExUC-OLEDs to produce blue light at voltages as low as 1.47 V. However, their development has been limited by the need for carefully matched donor and acceptor materials to enable efficient energy transfer, which narrows material choices and hinders device design.

Research Results and Future Perspective

To unlock the full potential of ExUC-OLEDs, researchers from the University of Toyama, Japan, have now proposed a simple yet effective design that can bring ExUC-OLEDs closer to commercialization. By inserting a nanometer (nm)-thin “spacer” layer between the donor and acceptor materials, the team enabled previously incompatible material combinations to function together, resulting in a 77-fold increase in blue light output.

The study, led by Associate Professor Masahiro Morimoto and co-authored by Mr. Ryosuke Fukazawa and Professor Shigeki Naka, was published online in the journal ACS Applied Optical Materials on June 4, 2025.

“The OLED industry is already substantial, but ultra-low-voltage drive OLEDs are a game changer because the materials and design concepts are different from those of the past. Using appropriate spacer materials opens up a wider selection of usable materials in ExUC-OLEDs,” explains Dr. Morimoto.

The spacer increases the physical separation between donor and acceptor molecules, weakening the Coulombic attraction that stabilizes the exciplex. This, in turn, raises the exciplex energy (E_Ex), improving its overlap with the triplet energy of the emitter. As a result, energy transfer becomes more efficient, resulting in light emission, even with material combinations that did not previously work.

In their experiments, the team used a blue-emitting donor material, α, β-ADN, and tested two different acceptors: HFl-NDI and PTCDI-C₈, both with and without a bathocuproine (BCP) spacer. Without the spacer, the PTCDI-C₈ device performed poorly, achieving a blue light emission efficiency of just 0.00083% due to the weak overlap between the exciplex energy level and the triplet excited state of the emitter molecule. However, with a 3-nm-thick spacer, the efficiency of the device increased to 0.064%, marking a 77-fold improvement.

Moreover, by varying spacer thickness from 0 nm to 9 nm, the researchers found that increasing the distance weakened the Coulombic interaction at the donor-acceptor interface, raising the E_Ex from 0.06 eV to 0.09 eV. However, a 3-nm spacer provided the best tradeoff, raising the exciplex energy sufficiently while still allowing efficient exciplex formation.

The team also tested spacers made from materials with different permanent dipole moments. Though the electrical properties and exciplex energies were largely unaffected by the spacer material, blue light efficiency varied significantly. High-dipole spacers, such as BCP, resulted in a higher external quantum efficiency of 6.4 x 10^-2%, compared to just 7.8 x 10^-3% for nonpolar spacers like UGH-2.

With this straightforward, scalable design for enhancing ExUC-OLEDs, this study marks a significant turning point in OLED technology. It expands the selection of usable materials and brings us closer to achieving ultra-low-voltage, energy-efficient OLEDs suitable for displays, wearables, and lighting systems.

“ExUC-OLEDs can be driven at ultra-low voltages; expanding the freedom of material selection could lead to a wider range of device applications, paving the way for future, energy-saving light-emitting devices beyond just OLEDs,” concludes Dr. Morimoto, optimistically.

Reference

Title of original paper:

Improved Freedom of Material Selection for Exciplex Upconversion-Type Organic Light-Emitting Diodes by Controlling Energy Transfer at the Donor/Acceptor Interface

Journal:

ACS Applied Optical Materials

DOI:

https://doi.org/10.1021/acsaom.5c00014

About the University of Toyama, Japan

University of Toyama is a leading national university located in Toyama Prefecture, Japan, with campuses in Toyama City and Takaoka City. Formed in 2005 through the integration of three former national institutions, the university brings together a broad spectrum of disciplines across its 9 undergraduate schools, 6 graduate schools, and a range of specialized institutes. With more than 9,000 students, including a growing international cohort, the university is dedicated to high-quality education, cutting-edge research, and meaningful social contribution. Guided by the mission to cultivate individuals with creativity, ethical awareness, and a strong sense of purpose, the University of Toyama fosters learning that integrates the humanities, social sciences, natural sciences, and life sciences. The university emphasizes a global standard of education while remaining deeply engaged with the local community.

Website：https://www.u-toyama.ac.jp/en/

About Associate Professor Masahiro Morimoto from the University of Toyama, Japan

Dr. Masahiro Morimoto is an Associate Professor in the Academic Assembly Faculty of Engineering at the University of Toyama and a member of the Organic Optical Device Engineering Laboratory, which focuses on the design and application of organic materials for next-generation electronic and optical devices. His research centers on the structural manipulation of organic thin films and their integration into flexible, nature-friendly, and bio-based electronics that promote harmonious coexistence between people and technology. Since joining the University of Toyama, Dr. Morimoto has held several academic roles and teaches courses related to nanomaterial science and electronics.

Funding information

This research was supported in part by JSPS KAKENHI, Grant Numbers JP20KK0323, JP24K00921, a project (JPNP20004) subsidized by the New Energy and Industrial Technology Development Organization (NEDO), the Izumi Science and Technology Foundation, and the Shorai Foundation for Science and Technology.

Media contact

Yumiko Kato

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