Morphology-Dependent Defect States in Rubrene Crystalline Films Grown by Hot-Wall Epitaxy
DOI:
https://doi.org/10.15407/ujpe71.4.359Keywords:
rubrene, hot-wall epitaxy, nanostructures, defect state, film morphology, energetic disorder, thermally stimulated luminescenceAbstract
Rubrene nanostructures grown on mica substrates by Hot-Wall Epitaxy were investigated using photoluminescence (PL) and thermally stimulated luminescence (TSL) spectroscopy. Different film morphologies, characterized by optical and electron microscopy, were obtained by varying the deposition conditions. Temperature-dependent PL spectra are found to depend substantially on the morphology of the rubrene layers. Films grown with short deposition times exhibit PL spectra dominated by intrinsic exciton emission, with only weak quenching with increasing temperature. In contrast, films grown at longer deposition times and higher substrate temperatures display additional emissive species, and their PL intensity shows a strong temperature quenching. The main results of this study are as follows: (i) A specific structure-related defect state is created in rubrene films grown at extended deposition times and elevated substrate temperatures, acting as a distinct hole trap with the depth Et = 0.23 eV, as measured by TSL. This defect gives rise to a broad, low-energy emission band that dominates the room-temperature PL spectra; (ii) deliberate oxidation of rubrene crystals demonstrates that the hole trap observed in TSL is not oxygen-related but originates from an intrinsic crystalline defect; (iii) TSL reveals a continuous distribution of shallow localized states, reminiscent of band-tail states, arising from intrinsic energetic disorder in the material. These shallow states are responsible for the very low temperature TSL features near 30 K and for the red shift of the PL spectra with increasing delay time observed in time-resolved PL measurements.
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