Crystalline rubrene film enhances OLED design

Crystalline organic semiconductors offer excellent charge transport but can be difficult to incorporate into practical OLEDs. Now, researchers from the University of Toyama have developed an OLED using a non-epitaxial crystalline rubrene thin film. Using a two-step annealing process, the researchers achieved a higher current density and a turn-on voltage of 1.33 V, thereby demonstrating that crystalline organic semiconductors can be integrated into practical OLEDs, paving the way for a ‘crystalline generation’ of displays.

The original OLED device, developed in 1987 by the Kodak Company in the USA, 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. Now, OLEDs are widely used in smartphone and television displays as well as lighting, with continuous improvements in efficiency and lifespan.

The researchers from the University of Toyama, led by Professor Masahiro Morimoto, have advanced the technology further by developing an OLED that incorporates a crystalline rubrene thin film as its light-emitting layer. Their research findings have been published in the journal Synthetic Materials.

Rubrene is an organic semiconductor that can transport electric charges 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 less efficiently. To overcome this limitation, the researchers 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 polarised 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.

Compared with OLEDs containing amorphous rubrene, the crystalline devices exhibited current densities up to 1000 times higher and reduced the luminance turn-on voltage by 0.30 V to 1.33 V. The electroluminescence spectrum also 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 integrated into practical thin-film OLEDs using standard vacuum-deposition techniques. The findings also suggest that crystalline organic materials could provide a new path towards 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,” Morimoto said.

Image credit: iStock.com/Anatoly Morozov