Turning On TTA: Aggregation-Induced Energy Landscape Modification

The development of efficient solid-state photon upconversion (UC) devices remains paramount for practical applications of the technology, and in recent years, incorporating perovskite thin films as triplet sensitizers for triplet-triplet annihilation (TTA) based UC has provided a promising solution. In the pursuit of finding an ‘ideal annihilator’ in order to maximize the apparent anti-Stokes shift, we investigate naphtho[2,3-a]pyrene (NaPy) as an annihilator in both solution-based and perovskite-sensitized TTA-UC systems. Surprisingly, we observe different emissive behaviors of NaPy in the solid state based on the excitation wavelength. Under direct excitation, a high energy transition S1’ dominates the emission spectrum, while upconversion results in predominate emission from a lower lying state S1”. We propose that this is the result of aggregation-induced lowering of the singlet excited state thus changing the fundamental energic landscape. Aggregation decreases the singlet energy below the energy level of the triplet pair state ¬1(TT), yielding energetically favorable emission from the aggregated state S1” and weak emission from the higher lying singlet state S1’.