Effects of of Energy Transfer and Spin-Dependent Processes on Photoluminescence in Rubrene/Alq3 Thin-Films

Rubrene (5,6,11,12 tetraphenylnaphthacene) molecules uniquely exhibit strong singlet fission and triplet fusion processes. Such properties have been impactful in achieving high performance organic light emitting diodes (OLED) and solar cells (OSC). Researchers have discovered that blending rubrene in Alq₃ (tris-(8-hydroxyquinolato) aluminum) achieves a benchmark photoluminescence quantum efficiency (PLQE) of 100%. However, the mechanisms contributing to its high PLQE as an acceptor in energy transfer processes have not been addressed. In this project, amorphous rubrene-doped Alq₃ films were fabricated in guest concentrations from 1 to 50 wt%, and the emission mechanisms of these donor-acceptor systems were studied with three distinct spectroscopic methods. In particular, the behavior of spin-dependent processes and relative singlet/triplet populations were studied by power-dependent magneto-photoluminescence, while the energy transfer efficiency and dynamics were understood by time-resolved photoluminescence and transient photo-induced absorption. The results of this study provide a fundamental description of the large PLQE measured in these thin-films, which can potentiate improvements in the power-conversion quantum efficiency of OSCs and have further applications in OLEDs.