Perovskite-sensitized upconversion bingo: Stoichiometry, composition, solvent, or temperature?

Triplet-triplet annihilation-based photon upconversion (UC) using bulk perovskite sensitizers has been previously shown to facilitate efficient UC at low fluences. However, the fabrication of the UC devices has not been fully optimized; thus, there is room for improvement. Here, we apply techniques that have been successful in enhancing the performance of perovskite solar cells in order to also improve perovskite-sensitized UC devices. In particular, we investigate the use of a post-fabrication thermal annealing step, overstoichiometric vs stoichiometric addition of PbI to the perovskite precursors, methylammonium vs formamidinium cation-rich lead halide perovskite compositions, and the use of different solvents for the annihilator molecules on the perovskite/annihilator interface. We find that excess PbI does not significantly affect the UC process, while the perovskite composition is crucial for the yield of extracted carriers across the interface. Comparing toluene and chlorobenzene, we find that the solvent used to deposit the annihilator is also a key factor in the overall device performance. Moreover, we find that thermal annealing of the whole device architecture significantly improves the UC performance by a factor of three.