We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold I th . The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below I th , the slope of the upconverted PL is twice the value of the MAFA PL; (2) above I th , it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the I th shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of I th = 7.1 mW/cm 2 .

The emerging field of lead halide perovskite-sensitized triplet–triplet annihilation (TTA) in rubrene shows great promise in upconversion applications. By rapidly transferring single charge carriers instead of bound triplet states, perovskites enable a high triplet population in rubrene, yielding low I th values. In this contribution, we investigate the role of the triplet population on the upconverted emission. Interestingly, two independent rates of TTA can be observed, as well as a sharp drop in the visible emission intensity over several seconds. This effect can be attributed to the triplet-density-based diffusion length: (i) at low triplet populations slow diffusion-mediated TTA yields singlets far from the interface and (ii) higher triplet populations lead to rapid TTA close to the perovskite/rubrene interface. Because of the proximity of the strongly absorbing perovskite, the singlet states created closer to the interface undergo stronger back-transfer to the perovskite film and therefore appear to exhibit a lower photoluminescence quantum yield.

Green-to-blue photon upconversion bears great potential in photocatalytic applications. Current hybrid inorganic−organic upconversion schemes commonly utilize spherical CdSe nanocrystals, but size polydispersity could influence efficiencies in future solid-state applications. In this contribution, we introduce anisotropic CdSe nanoplatelets as triplet sensitizers.Here, quantum confinement occurs in only one direction, erasing effects stemming from size polydispersity. Additionally, their high quantum yields, giant oscillator strengths, and large absorption cross sections could prove useful in a triplet sensitization scheme. We investigate the triplet energy transfer process from the CdSe nanoplatelets to the surface-bound triplet acceptor 9-anthracenecarboxylic acid and the resulting upconversion in 9,10-diphenylanthracene. We further investigate the influence of nanoplatelet stacking and singlet back-transfer on the observed upconversion efficiency. We obtain an upconversion quantum yield of 5.4% at a power density of 11 W/cm 2 using the annihilator 9,10diphenylanthracene and a low efficiency threshold I th of 237 mW/cm 2 .

Solid-state bulk lead halide perovskite thin films have recently shown progress as triplet sensitizers in infrared-to-visible photon upconversion (UC) schemes. Common systems pair lead halide perovskites with a rubrene annihilator, doped with ∼1% dibenzotetraphenylperiflanthene (DBP), to prevent the efficiency limiting process of singlet fission. However, the interplay between rubrene and DBP has not been investigated in these spin-coated bilayer systems. Here, we investigate the UC photoluminescence intensity and the dynamics of the triplet sensitization process as a function of the DBP doping percentage of rubrene, finding that, as a whole, DBP does not significantly affect the UC intensity of the lead halide perovskite sensitized scheme. This indicates that, in the solution-processed devices reported here, rubrene disorder is sufficient to suppress unwanted singlet fission processes, removing the requirement of an annihilator/emitter system.

Triplet sensitization of rubrene by bulk lead halide perovskites has recently resulted in efficient infrared-to-visible photon upconversion via triplet–triplet annihilation. Notably, this process can occur under solar relevant fluxes, potentially paving the way toward integration with photovoltaic devices. In order to further improve the upconversion efficiency, the fundamental photophysical pathways at the perovskite/rubrene interface must be clearly understood to maximize charge extraction. Here, we utilize ultrafast transient absorption spectroscopy to elucidate the processes underlying triplet generation at the perovskite/rubrene interface. Our results point to a triplet generation mechanism based on hot carriers; thermally excited charge carriers in the perovskite cool more rapidly in the presence of rubrene, suggesting rapid extraction of these thermally excited carriers on the picosecond time scale. Subsequent triplet formation in rubrene is observed on a subnanosecond time scale.