Single crystalline rubrene: The organic silicon

The success and supremacy of silicon based microelectronics is mainly due to the possibility to grow this semiconductor in large and pure single crystals and to its reaction with oxygen giving rise to an insulating layer of silicon oxide with controlled properties. The recent huge interest on organic semiconductors is aimed at developing an alternative technology having cost effectiveness together with additional prerogatives such as the use of flexible matrixes hosting the active semiconducting layer. Even if the research community on organic semiconductors is aware that the performances of organic based devices will unlikely equate those of inorganic semiconductors, some small molecule semiconductors have shown impressive properties in terms of charge carrier mobility and electroluminescence. Among these materials, the orthorhombic polymorph of rubrene has shown to possess a record hole mobility, making rubrene itself the most promising member of its class of materials. However, the processing of rubrene is associated to important issues that hinder its establishment as reference benchmark for organic semiconducting devices. The most important one is its spontaneous solidification in an amorphous state when processed as thin film, and the rapid oxidation of this phase. We solved this major issue by developing an epitaxial strategy for the growth of crystalline and oriented thin films of rubrene. We focus here on the interesting observation of the growth of a native oxide layer on the surface of these films. Previous investigations have shown that this layer grows through the migration of molecules at step-edges towards the top terraces, covering uniformly the film surface with a thickness of ca. 2 nm. This oxide layer is known to have a slight difference in the out-of-plane spacing with respect to orthorhombic rubrene and to induce changes of the surface potential. However, its structure is still unknown. Here, we succeeded in isolating a crystalline orthorhombic phase of rubrene endoperoxide by solution processing. This phase shows an almost perfect coincidence of lattice parameters with those of rubrene. Then, the oxide layer is crystalline and commensurate with the semiconductor, explaining why the oxidation of rubrene is spontaneous and brings about the formation of ordered layers. This finding opens an interesting scenario since, similarly to silicon, the insulating oxide layer of a good semiconductor can be exploited for fabricating complex microlectronic devices.