Blue iridium complexes as electron trapping sites and efficient recombination centres in poly(N-vinylcarbazole) seen by spectrally resolved thermoluminescence

Abstract

Spectrally resolved thermoluminescence (SRTL) of poly(N-vinylcarbazole) (PVK) doped with blue iridium (III) complexes: bis(4,6-difluorophenyl)-pyridinato-N,C2)(picolinate) (FIrpic) or tris[2–(2,4–difluorophenyl)pyridine] (Ir(Fppy)3) is studied and compared with photoluminescence results. Monochromatic thermoluminescence (TL) curves registered in the temperature range 15–320K indicate that besides trapping sites localised on the PVK matrix, deep electron traps on the blue iridium complexes molecules play a very important role. After doping with a small amount of FIrpic, a shift of dominant TL peak to higher temperature was observed. It shows the existence of slightly deeper traps in comparison with those in the neat PVK. This is a result of interaction of trapped carriers with randomly oriented high permanent dipoles of FIrpic. The SRTL experiments for different concentration of FIrpic provide evidence that the dopant molecules act as emission centres which effectively compete with other radiative recombination centres present in PVK matrix. Even at 0.1wt% of the dopant molecules embedded in PVK, the emission originates almost exclusively from triplet excitons of the blue iridium complex. In addition, the highest intensity of the predominant maximum of TL occurs when the concentration of electron traps located on the dopant molecules is similar to the concentration of hole traps formed on the matrix. The studies confirm that host–guest system works very effectively as emission system in the case when the dopant molecules are not only efficient emitters but they act also as trapping sites.