Two-photon triplet-triplet annihilation upconversion for photovoltaics

Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic cells. Molecular systems provide an environment in which long lived triplet states can be exploited to achieve high upconversion efficiencies under solar illumination. We report on the investigation of bi-molecular triplet-triplet annihilation upconversion (TTA-UC) in a Palladium (II) tetrakisquinoxalino porphyrin (PQ₄Pd)/rubrene solution. These molecules were studied in solution using UV/VIS spectroscopy to determine their stability in air over a period of weeks. Transient absorption spectroscopy (TAS) was used to directly measure the lifetime of triplet states within these mixtures and hence determine the photoinduced kinetics of the system. The lifetime of porphyrin triplets was reduced from 92.4 μs in pristine PQ₄Pd to 2.4 μs in the presence of rubrene. From this change, the rate constant associated with triplet energy transfer (k_TET) was calculated as 3.38 × 10⁸M^-1s^-1. Additionally, a reduction in the absorption of 530 nm light (the ground state rubrene absorption peak) was observed, while the mixture was pumped at the absorption peak of the porphyrin (670 nm). This change became apparent nearly 6 μs after the laser pulse, showing energy transfer from the porphyrin to the rubrene, and allowing further insight into the kinetics of the mechanism.