Since the first practical organic light-emitting diode (OLED) was developed in 1987 by Ching W. Tang and Steven A. VanSlyke at Eastman Kodak Company in the USA, OLED technology has come a long way. The original device used two thin organic layers to produce green light when electricity was applied, demonstrating that organic materials could be used to create practical light-emitting devices. Today, OLEDs are widely used in smartphone and television displays as well as lighting, with continuous improvements in efficiency and lifespan.
Now, researchers from the University of Toyama, Japan, led by Professor Masahiro Morimoto of the Academic Assembly Faculty of Engineering, along with Mr. Yuya Honda and Prof. Shigeki Naka from the same university, have advanced the technology further by developing an OLED that incorporates a crystalline rubrene thin film as its light-emitting layer. This paper was made available online on July 1, 2026, and will be published in Volume 320 of the journal Synthetic Metals on August 01, 2026.
Rubrene is an organic semiconductor that can transport electrical charges much more efficiently in its crystalline form. However, when deposited as a thin film using the conventional vacuum evaporation process used to manufacture OLEDs, it forms a disordered (amorphous) structure in which electrical charges move much less efficiently.
To overcome this limitation, the researchers first fabricated the OLED by depositing several ultrathin layers, including a 50 nm-thick rubrene layer, onto an indium tin oxide substrate. They then used a two-step heat treatment to transform the rubrene layer into a crystalline film. During the first heating step, tiny crystal seeds formed within the rubrene layer. After the remaining layers were deposited, a second heating step allowed these crystals to grow into large crystalline domains.
Using polarized optical microscopy, the researchers observed large crystalline regions measuring about 1 mm throughout the rubrene film. Similar crystal structures were found across the entire substrate, indicating that the crystals had grown uniformly. X-ray diffraction revealed that the rubrene had formed an orthorhombic crystal structure.
"We have extended the concept of Tang's organic electroluminescent diodes to rubrene crystalline thin films and fabricated OLEDs with superior characteristics. This represents the first application of non-epitaxial crystalline thin films to OLEDs," says Prof. Morimoto
Compared with OLEDs containing amorphous rubrene, the crystalline devices exhibited current densities up to 1,000 times higher and reduced the luminance turn-on voltage by 0.30 V to just 1.33 V. Moreover, the electroluminescence spectrum changed from the broad, two-peak emission characteristic of amorphous rubrene to a sharp single peak near 565 nm, a distinctive signature of the crystalline structure.
The study demonstrates that organic crystals with high charge-transport properties can be successfully integrated into practical thin-film OLEDs using standard vacuum-deposition techniques. Building on nearly four decades of OLED innovation since Tang and VanSlyke's pioneering work, the findings suggest that crystalline organic materials could provide a new path toward more efficient OLED devices.
"Just as the semiconductor industry has driven historical breakthroughs by precisely controlling the structural order of materials, we believe the OLED field is now at a similar turning point. While current commercial OLEDs remain amorphous, our findings clearly show that transitioning to 'crystalline' structures will define the next generation of OLED technology." says Prof. Morimoto.
