Organic light-emitting diodes (OLEDs) have long represented a cornerstone in modern display and lighting technology, prized for their vibrant colors, deep contrast, and energy efficiency. As the demand for sleeker, lighter, and more energy-conscious devices intensifies, especially in the realms of wearables, foldable gadgets, and portable electronics, scientists are probing innovative ways to reduce the operational voltages of OLEDs without sacrificing performance. A breakthrough has emerged in the form of exciplex upconversion OLEDs (ExUC-OLEDs), which harness a fundamentally different mechanism to produce light at significantly lower voltages, potentially revolutionizing energy consumption in future devices.
Traditional OLEDs function by generating excitons—electron-hole pairs—within the emissive layer when an adequate voltage, generally aligning with or exceeding the bandgap of the emitting material, is applied. This bandgap usually sits near 3 volts for red light emissions and nearly 4 volts for blue, leading to relatively high power requirements, especially for devices emitting shorter-wavelength light. In contrast, ExUC-OLEDs leverage exciplexes, unique interfacial states formed at the junction between donor and acceptor molecules. These loosely bound electron-hole pairs create a lower-energy intermediate that facilitates an alternative pathway for exciton formation and transformation, culminating in visible light emission at dramatically reduced voltages, sometimes as low as 1.47 volts for blue light.
Despite their promise, ExUC-OLEDs have faced significant hurdles. Central among them has been the necessity for highly compatible donor and acceptor material combinations to ensure efficient energy transfer to the emitter’s triplet state, a critical step that triggers triplet-triplet upconversion (TTU). TTU is an advanced photophysical process wherein two triplet excitons merge to form a high-energy singlet exciton capable of light emission. This specificity in material pairing severely limits the spectrum of usable materials, constricting device optimization and hampering practical applications.
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In a significant advance, researchers at the University of Toyama, Japan, led by Associate Professor Masahiro Morimoto, have devised an innovative yet elegantly simple approach to circumvent these material constraints. Their strategy involves the insertion of a nanometer-scale “spacer” layer—merely 3 nanometers thick—between the donor and acceptor layers within the ExUC-OLED architecture. This minuscule modification unlocks unprecedented freedom in material selection, enabling previously incompatible donor-acceptor pairs to cooperate effectively and substantially amplifying the emitted blue light intensity by a factor of 77.
This groundbreaking work, documented in the journal ACS Applied Optical Materials on June 4, 2025, showcases the profound influence of nanoscale engineering on the electronic and photophysical properties of OLEDs. Dr. Morimoto explains that the nanoscale spacer subtly modifies the Coulombic interactions at the donor-acceptor interface—specifically, it weakens the electrostatic attraction that ordinarily stabilizes the exciplex state. This weakening elevates the exciplex energy level (E_Ex), thereby optimizing its spectral alignment with the triplet energy of the emitter molecule, streamlining energy transfer, and facilitating efficient light emission even with material combinations that had previously failed.
Experimental validation was conducted by constructing devices using the blue-emitting donor α,β-ADN alongside two different acceptors: HFl-NDI and PTCDI-C8. Importantly, the team compared device performances with and without the inclusion of a bathocuproine (BCP) spacer. The PTCDI-C8 device without the spacer exhibited an abysmally low external quantum efficiency (EQE) of 0.00083%, underscoring the poor exciplex-triplet state resonance. Remarkably, integrating the 3-nm BCP spacer elevated the EQE to 0.064%, a staggering 77-fold enhancement. This pronounced improvement signifies how judicious control of interfacial distance and electronic coupling can dramatically reshape energy dynamics within OLEDs.
Further investigations probed the influence of spacer thickness on device performance. By incrementally adjusting the spacer from 0 to 9 nanometers, researchers observed a systematic weakening of the Coulombic interaction at the donor-acceptor interface, which raised the exciplex energy from 0.06 electronvolts to 0.09 electronvolts. However, beyond the 3-nanometer thickness, exciplex formation became less efficient, highlighting that the spacer must delicately balance increased energy with sufficient exciton formation. This finely tuned optimization underscores the criticality of nanoscale engineering in bridging fundamental photophysics with practical device architecture.
The team also examined the role of spacer material properties, particularly focusing on permanent dipole moments. While the electrical properties and exciplex energy levels remained largely invariant across various spacers, the blue emission efficiency exhibited significant sensitivity to the spacer’s dipolar nature. High-dipole spacers such as BCP delivered superior external quantum efficiencies of 6.4 × 10⁻²%, whereas nonpolar substrates like UGH-2 yielded only 7.8 × 10⁻³%. This variation suggests that electric field modulation at the interface, stemming from the spacer’s dipolar character, plays a pivotal role in mediating energy transfer and exciton dynamics.
The impact of this pioneering research extends beyond immediate performance metrics. By radically expanding the palette of usable donor and acceptor materials, the spacer insertion method paves the way for ultralow-voltage OLEDs with enhanced tunability, efficiency, and device lifespan. This approach holds particular promise for the wearable technology sector, where minimizing power consumption without forfeiting brightness or color fidelity is paramount. Furthermore, the spacer technique offers a scalable, straightforward pathway to integrate into existing OLED manufacturing processes, accelerating commercialization prospects.
Moreover, ExUC-OLEDs present an enticing platform for next-generation lighting and display technologies with their ability to exploit triplet states—traditionally deemed less useful for light emission. Their low-voltage operation not only reduces energy footprint but also lowers thermal stress, improving device stability and longevity. Dr. Morimoto emphasizes that the newfound freedom in material choices heralds a new era in OLED design philosophy—departing from tight material constraints and embracing hybrid architectures that synergistically blend diverse organic semiconductors.
Industry stakeholders are particularly attentive to this development as the global OLED market expands rapidly into flexible displays, microdisplays for augmented reality, and environmentally sustainable lighting solutions. The spacer-based strategy deftly addresses one of the key bottlenecks limiting ExUC-OLED scalability and encourages new explorations into exotic molecular systems, promising vivid color tunability and robustness hitherto unattained.
In conclusion, this advance epitomizes the transformative power of nanoscopic interfacial engineering in optoelectronics. By interposing an ultra-thin spacer, the University of Toyama team has unlocked the potential of exciplex upconversion OLEDs to operate efficiently at ultra-low voltages, broadening the horizon for energy-saving, high-performance organic light-emitting technologies. As the research community continues to refine and expand upon this concept, we can anticipate a future where OLEDs become not only more sustainable but also more versatile and accessible across myriad applications.
Subject of Research: Not applicable
Article Title: Improved Freedom of Material Selection for Exciplex Upconversion-Type Organic Light-Emitting Diodes by Controlling Energy Transfer at the Donor/Acceptor Interface
News Publication Date: June 4, 2025
Web References:
https://doi.org/10.1021/acsaom.5c00014
References:
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: 10.1021/acsaom.5c00014
Image Credits: Reprinted (adapted) with permission from DOI: 10.1021/acsaom.5c00014. Copyright 2025 American Chemical Society.
Keywords
Organic light-emitting diodes, exciplex OLEDs, exciplex upconversion OLEDs, triplet-triplet upconversion, ultralow voltage OLEDs, energy transfer, donor-acceptor interface, spacer layer, bathocuproine, external quantum efficiency, nanomaterials, OLED efficiency, optoelectronics
Tags: advanced materials for OLEDsenergy-efficient display technologyexciplex upconversion OLEDsfoldable gadget displayslow-voltage OLED innovationsnext-generation lighting solutionsOLED exciton formationorganic light-emitting diodesportable OLED applicationsreduced power consumption in electronicsspacers in OLED technologywearable technology advancements