Rubrene is a model organic semiconductor, regarded as one of the most promising in the field of organic optoelectronics. Specifically, rubrene allows, with very high efficiencies, both singlet exciton fission and triplet exciton fusion, which are the two main processes through which the efficiency of all photovoltaic devices can be improved.
However, to effectively exploit this material and, in particular, singlet fission, it is essential to be able to grow crystalline rubrene thin films and control their degree of order and crystallinity, while concurrently tackling the tendency of this material to degrade rapidly by photo-oxidation, a phenomenon that affects the rubrene molecule and amorphous rubrene films.
A study conducted by a group of researchers led by Prof. Adele Sassella, Dr. Alessandro Minotto, and Dr. Luisa Raimondo, researchers at the Department of Materials Science of the University of Milan-Bicocca, illustrates how these results can be achieved all together.
Specifically, the researchers obtained thin films of crystalline rubrene, grown on amino acid (β-alanine) substrates, with physical properties that are fully comparable to those of rubrene single crystals, including efficient singlet fission. The films exhibit a molecularly flat surface on the cm-scale, very low defect density and coherently oriented domains with >1 μm lateral dimensions. The crystallinity of these films also preserves them from photo-oxidation after exposure to air.
This study shows that the mechanism behind the result is a spontaneous amorphous-to-crystal transition that occurs at room temperature and is driven by the organic epitaxy between rubrene and substrate. Through the understanding and detailed study of the amorphous-to-crystal transition, the authors then propose an effective strategy to optimise the quality of rubrene films by acting on specific growth parameters.
The results obtained through the calculated choice of growing rubrene on substrates such as β-alanine, which promote epitaxy and can be removed for subsequent transfer to another substrate, pave the way for the integration of crystalline organic semiconductor films in optoelectronic devices.
The findings of this study are reported in the Advance Article “Tailoring the optical properties of rubrene films through epitaxy-induced amorphous-to-crystal transition” (doi: 10.1039/D4TC01618A) published in the Journal of Materials Chemistry C (Royal Society of Chemistry, Impact Factor 5.7, 2023 Journal Citation Reports (Clarivate Analytics, 2024)).
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