  Lead halide-based perovskite thin films have attracted great attention due to
the explosive increase in perovskite solar cell efficiencies. The same
optoelectronic properties that make perovskites ideal absorber materials in
solar cells are also beneficial in other light-harvesting applications and make
them prime candidates as triplet sensitizers in upconversion via
triplet-triplet annihilation in rubrene. In this contribution, we take
advantage of long carrier lifetimes and carrier diffusion lengths in perovskite
thin films, their high absorption cross sections throughout the visible
spectrum, as well as the strong spin-orbit coupling owing to the abundance of
heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite
thin films as the absorber layer and spin-mixer in inorganic/organic
heterojunction upconversion devices allows us to forego the additional
tunneling barrier owing from the passivating ligands required for colloidal
sensitizers. Our bilayer device exhibits an upconversion efficiency in excess
of 3% under 785 nm illumination.
