The impact of 1 wt% of Ir(ppy)3 on trapping sites and radiative recombination centres in PVK and PVK/PBD blend seen by thermoluminescence

• Thermoluminescence as a method to identification of traps and emission centres.
• An influence of dipole moment of Ir(ppy)3 on traps depth localised on host was confirmed.
• Contribution of the deep (∼0.4 eV) traps on Ir(ppy)3 is 60% in PVK and 20% in PVK/PBD.
• The presence of Ir(ppy)3 creates a new channel for the energy transfer.
• The charge carriers released from different type of traps generate excitons on the Ir(ppy)3.

Thermoluminescence (TL) of poly(N-vinylcarbazole) (PVK) and PVK + 40 wt% of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) blend, both doped with 1 wt% of fac-tris(2-phenylpiridine) iridium (Ir(ppy)3), were studied. TL spectra, registered in temperature range 15–320 K, reveal that trapping sites localised on the matrices and on Ir(ppy)3 molecules exist in both investigated systems. The traps localised on Ir(ppy)3 have depth about 0.4 eV and they dominate in the doped PVK. At the same concentration of the dopant molecules in the PVK/PBD matrix, the traps located on the matrix dominated. Contribution of deep traps on Ir(ppy)3 is much smaller. After doping, a shift of TL peak, associated with the release of carriers from the matrix traps, to higher temperature was observed. It indicates on the presence of slightly deeper traps as compared with those in the neat matrices. The effect is caused by the interaction of trapped carriers and randomly oriented high permanent dipoles of Ir(ppy)3. In addition, the experiments of spectrally resolved TL (SRTL) provide evidence that the presence of the dopant creates a new channel of energy transfer. Even at concentration of 1 wt% of Ir(ppy)3, these molecules act as emission centres which effectively compete with other centres of radiative recombination present in PVK and PVK/PBD systems.

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