  The same optical and electronic properties that make perovskite thin films
ideal absorber materials in photovoltaic applications are also beneficial in
photon upconversion devices. In this contribution, we investigate the
rubrene-triplet sensitization by perovskite thin films based on methylammonium
formamidinium lead triiodide (MAFA). To elucidate the role of trap states which
affect the free carrier lifetimes, we fabricate MAFA perovskite thin films with
three different thicknesses. By measuring the change in the photoluminescence
properties under different excitation fluences, we find that the prevalent
recombination mechanism shifts from monomolecular for thinner films to
bimolecular recombination for thicker MAFA films, indicating a reduction in
shallow trap-assisted recombination. The addition of rubrene shows a
passivating effect on the MAFA surface, but adds an additional quenching
pathway due to charge transfer to the triplet state of rubrene. We observe that
the threshold for efficient triplet-triplet annihilation shifts to lower
incident powers with increasing MAFA thickness, which suggests that the charge
transfer to the triplet state competes with non-radiative trap filling. Hence,
injection of free electrons and holes into the upconverting organic
semiconductor can provide a new avenue for sensitization of rubrene, and may
allow us to move away from the necessity of efficient excitonic
singlet-to-triplet converters.
