Relaxation processes in the triplet state T1 of organic Ir-compound Btp2Ir(acac) doped in PC and CBP fluorescent materials

We analyzed the temperature dependences of photoluminescence (PL) lifetime and intensity of phosphorescent Ir-compound Btp2Ir(acac) doped in polycarbonate and 4, 4′-N,N′–dicarbazole-biphenyl, using a model that (1) the emitting triplet state consists of three zero-field splitting substates, (2) non-radiative relaxation occurs among these substates, and (3) the energy dissipation occurs from the excited Btp2Ir(acac) to its neighboring Btp2Ir(acac). Rate equations for the populations of the three substates were solved numerically. The substates 2 and 3 were estimated to be at 7.6 and 27.8 cm−1 above the lowest-energy substate 1, respectively. The calculated temperature dependences of the PL lifetimes and intensity are in a good agreement with the observed ones.