Organic electroluminescent diodes with a thin crystalline layer

We report on the characteristics of organic light-emitting diodes (OLEDs) employing a rubrene crystalline thin film as the emission layer, representing an evolution of the traditional organic electroluminescent diodes. By combining an amorphous underlayer with precise annealing processes, we have succeeded in the crystallization of non-epitaxial vacuum-deposited thin films. Polarized optical microscopy revealed the formation of giant crystalline domains reaching approximately 1 mm in size within a 50-nm-thick rubrene film. Devices incorporating this crystalline film exhibited a dramatic increase in current density—up to 1,000 times higher than that of conventional amorphous devices—and achieved an exceptionally low turn-on voltage of 1.3 V at 1 cd/m 2 . Furthermore, the electroluminescence spectrum showed a sharp, single peak characteristic of crystalline structures, suggesting emission derived from a selective transition dipole moment.