Moha Naeimi¹, Tim Völzer², Regina Lange¹, Stefan Lochbrunner², Ingo Barke¹, Sylvia Speller¹*

¹Surfaces and Interfaces, Institute of Physics, University of Rostock,

²Dynamics of Molecular Systems, Institute of Physics, University of Rostock

*sylvia.speller@uni-rostock.de

Upon optical excitation of organic semiconductors bound electron-hole pairs (Frenkel excitons) are generated. Understanding the dynamics and migration of excitons in organic crystals is important for devices such as organic solar cells, light emitting and photodiodes. A prospect is excitonic, where the employment of excitons as charge-neutral bosonic carrier species is envisioned. In rubrene the singlet exciton energy is just equal to twice that of the triplet, making fission and fusion of excitons dominating processes. Almost all emission in the visible originates from fused triplet excitons. We study the luminescence and electrical properties of a novel type of orthorhombic, tabular, sector-zoned microcrystal type, exhibiting growth shape. Sector-zoning is a phenomenon known from mineralogy and associated with anisotropic growth speeds. In rubrene the lattice of triangular sectors appears turned with respect to the diamond sectors. Due to the phenyl side groups the crystal properties are extremely anisotropic. The large signature of a peculiar emission, not part of the ordinary vibronic progression, is addressed. The delayed fluorescence together with life-time spectra as well as photoemission signatures are put in context with the lattice properties.

References

[1] P. Irkhin, A. Ryasnyanskiy, M. Koehler, I. Biaggio, Absorption and photoluminescence spectroscopy of rubrene single crystals, Physical Review B 86 (2012) 085143

[2] Y. Chen, B. Lee, D. Fu, V. Podzorov, The Origin of a 650 nm Photoluminescence Band in Rubrene, Advanced Materials, 23 (2011) 5370

[3] K. Takazawa J.-i. Inoue, Hexagonal single crystals of perylene with patterned heterogeneous domains, Chemical Physics Letters 843 (2024) 141232

[4] T. Ubide, S. Mollo, J.-x. Zhao, M. Nazzari, P. Scarlato, Sector-zoned clinopyroxene as a recorder of magma history, eruption triggers, and ascent rates, Geochimica et Cosmochimica Acta 251 (2019) 265

[5] M. Naeimi, K. Engster, I. Barke, S. Speller, Morphology and structural properties of thin rubrene crystallites grown on graphite, Physica Scripta (2024)

Funded by the German Research Foundation (DFG), SFB 1477 - 441234705