Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.210
Publication date: 18th July 2019
The sub-bandgap onset of rubrene-based organic light emitting diodes serves as an indicator of triplet exciton sensitization by carrier injection. Hence, materials which have a proper band alignment to allow for direct charge injection into the triplet state can enable a new path in sensitizing excitonic upconversion. In particular, bulk lead halide perovskite (LHPs) thin films have recently emerged as efficient sensitizers for near-infrared-to-visible upconversion. The upconversion process is based on triplet-triplet annihilation (TTA) in the annihilator rubrene. Conservative estimates result in upconversion efficiencies upwards of 3%. [1]
Understanding the upconversion mechanism is crucial for the advancement of upconversion devices. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are competing pathways. As a result, we obtain lower intensity thresholds Ith denoting efficient upconversion using thicker perovskite films. Results indicate that the power-dependence of the upconverted emission is non-linear even above the Ith value, due to the bimolecular nature of the triplet sensitization mechanism. However, a trade-off can be observed: despite low Ith values for thicker LHP films, with increasing film thickness parasitic reabsorption of the singlets created by TTA also increases, which diminishes the visible light output of the device. [2]
Two other unusual effects have been observed in these bilayer devices: i) two rise times in the upconverted photoluminescence dynamics, and ii) a reversible photobleach of the upconverted emission. Both effects can be traced back to the existing triplet population level and the resulting population-dependent diffusion length, indicating that further optimization of the device is still needed for real-world applications. [3]
The authors gratefully acknowledge Florida State University startup funds.
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