Demystifying Triplet-Triplet Annihilation Mechanism in the CsPbI₃-Rubrene-DBP Upconversion System

A triplet-triplet annihilation-based upconversion (TTA-UC) system, employing a multichromophore assembly, is convenient to harvest low-energy photons for light energy conversion and optoelectronic applications. The primary donor in the TTA-UC system, typically a low-bandgap semiconductor, captures the low-energy photons and transfers triplet energy to an annihilator dye molecule, which in turn generates a high-energy singlet excited state via T-T annihilation. We have now succeeded in revealing kinetic and mechanistic details of multistep energy transfer processes in the CsPbI₃-rubrene-perylene derivative (DBP) films by analyzing time-resolved emission and absorption measurements. The initial triplet energy transfer between CsPbI₃ and rubrene occurs with an efficiency of 70% and a rate constant of 9 × 10⁸ s^-1. The rubrene triplets undergo T-T annihilation via simple second-order kinetics to form an excited singlet state exhibiting a delayed emission up to 10 μs, which is significantly greater than the intrinsic lifetime of 15 ns. The emitter DBP (a perylene derivative) captures the singlet energy quite effectively and delivers the upconverted emission in sync with the delayed emission of rubrene. The quadratic dependence of DBP emission on the excitation light intensity shows the importance of the T-T annihilation process in dictating the overall upconversion process. The kinetic parameters evaluated in this study, which divulge the critical steps dictating energy transfer in a TTA-UC system, should aid in the design of new light harvesting assemblies.