Energy transfer between Ir(ppy)3 molecules in neat film and concentration quenching of phosphorescence

Numerical calculations are performed for temperature dependences of phosphorescence quantum efficiency and lifetime of Ir(ppy)3 monomer in photo-excited neat film by solving the rate equations for three zero-field splitting substates of the triplet T1 state. In the analysis, we take into account spin–phonon relaxations among the three substates and excitation energy transfer to the excimer state by thermal activation process. Good agreement of the calculated quantum efficiency and lifetime with experimental results is obtained by considering both the excimer formation and triplet–triplet annihilation, e.g., (1) the efficiency increases with decreasing temperature from 300 K to 125 K and then decreases on going from 125 K to 5 K, and (2) the phosphorescence decay curve shows multi-exponential, where the lifetimes increase with decreasing temperature from 300 K. It is suggested that concentration quenching of monomer emission in the neat film is mainly due to the formation of triplet excimer with activation energy of 975 cm−1 and partly triplet–triplet annihilation and polariton pair annihilation.