Interdependence of photon upconversion performance and antisolvent processing in thin-film halide...

We prepared triplet-triplet annihilation photon upconverters combining thin-film methylammonium lead iodide (MAPI) perovskite with a rubrene annihilator in a bilayer structure. Excitation of the perovskite film leads to delayed, upconverted photoluminescence emitted from the annihilator layer, with triplet excitation of the rubrene being driven by carriers excited in the perovskite layer. To better understand the connections between the semiconductor properties of the perovskite film and the upconversion efficiency, we deliberately varied the perovskite film properties by modifying two spin-coating conditions, namely, the choice of antisolvent and the antisolvent dripping time, and then studied the resulting photon upconversion performance with a standard annihilator layer. A stronger upconversion effect was exhibited when the perovskite films displayed brighter and more uniform photoluminescence. Both properties were sensitive to the antisolvent dripping time and were maximized for a dripping time of 20 s (measured relative to the end of the spin-coating program). Surprisingly, the choice of antisolvent had a significant effect on the upconversion performance, with anisole-treated films yielding on average a tenfold increase in upconversion intensity compared to the chlorobenzene-treated equivalent. This performance difference was correlated with the carrier lifetime in the perovskite film, which was 52 ns and 306 ns in the brightest chlorobenzene and anisole-treated films, respectively. Since the bulk properties of the anisole- and chlorobenzene-treated films were virtually identical, we concluded that differences in the defect density at the MAPI/rubrene interface, linked to the choice of antisolvent, must be responsible for the differing upconversion performance.