The sub-bandgap onset of rubrene-based organic light emitting diodes serves as an indicator of direct triplet exciton sensitization based on charge 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 photon upconversion, while overcoming previous limitations resulting from poor exciton diffusion in nanocrystal-based systems. In particular, bulk lead halide perovskite (LHPs) thin films have 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] and upconversion has been shown to be efficient at incident powers comparable to the solar flux.

Understanding the upconversion mechanism is crucial for the advancement of such devices. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are likely competing pathways for the optically excited charge carriers. As a result, we obtain lower intensity thresholds Ith for efficient upconversion using thicker perovskite films, as these exhibit lower trap densities. However, a trade-off is observed: 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 unexpected effects have been observed in perovskite-based upconversion devices: i) two rise times in the upconverted photoluminescence dynamics, and ii) a reversible ‘photobleach’ of the resulting 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. [3]

[1] Nienhaus et al. ACS Energy Lett. 2019, 4, 888-895
[2] Wieghold et al. Matter 2019, https://doi.org/10.1016/j.matt.2019.05.026
[3] Wieghold et al. J. Phys. Chem. Lett. 2019, https://doi.org/10.1021/acs.jpclett.9b01526