Moving beyond nanocrystal-based upconversion systems, bulk lead halide perovskite (LHPs) thin films have recently emerged as triplet sensitizers for near-infrared-to-visible upconversion. The upconversion process investigated here is based on triplet-triplet annihilation in the organic semiconductor rubrene. Conservative estimates yield upconversion efficiencies upwards of 3%. <br/>Understanding the energy transfer mechanism is crucial for the advancement of such optoelectronic devices. In particular, the role of trap states in the sensitizer is key to further increase the device efficiency. Here, we investigate the underlying mechanism of energy transfer at the LHP/rubrene interface, and the effect of the LHP perovskite film thickness on the upconversion process. Excitation-dependent photoluminescence (PL) studies show that the prevalent recombination mechanism in the bare LHP film shifts from monomolecular to bimolecular recombination when increasing the LHP film thickness from ~25-200 nm, indicating a reduction in shallow trap-assisted recombination. In the LHP/rubrene bilayer device, we observe two opposing effects caused by interactions between the LHP and rubrene: i) passivation of surface defects by rubrene, which results in a longer native PL lifetime, ii) quenching of the PL lifetime due to energy transfer to the triplet state of rubrene. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are likely competitive quenching pathways. As a result, the threshold at which the upconversion process becomes efficient, observed as a slope change in the power-dependent upconverted PL shifts to lower values with increasing film thickness.<br/><br/>

NIR-to-visible upconversion sensitized by lead halide perovskite thin films

2019 Fall National Meeting

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