Rubrene Nanoaggregate-Integrated CH3NH3PbI3 Bilayer Film: Role of Singlet Fission and Photon Upconversion

The potential to improve the efficiency of photovoltaic devices through singlet exciton fission (SF) has raised interest of researchers during the last decade. Organic small molecules of rubrene were fabricated into spherical nanoaggregates (NAs) of diameter similar to 250 nm by the re-precipitation method. J-type aggregation inside the NA with pivotal electronic coupling occurs in NAs. Observed strong fluorescence quenching indicates the presence of competing pathways in the NAs: hot and thermally activated SF, triplet-triplet annihilation (TTA), and energy transfer between inhomogeneously broadened NA ensembles. SF which proceeds directly from upper excited electronic states within 340 fs (vs 200 fs in crystals; 30 fs in films) results in a triplet quantum yield of 162%. With an aim to apply rubrene in photovoltaics, a rubrene NA-integrated film was prepared on top of the CH3NH3PbI3 layer. Triplet formation within 25 ps in the NA layer was well-documented. Subsequently, the triplet excitons are accumulated and further upconverted via TTA to populate the conduction band of the perovskite within 39 ps (80%) and >> 10 ns (20%). Our findings are crucial for understanding photophysical mechanisms of rubrene/perovskite interaction after light absorption by rubrene.