Charge carrier transport in an emissive layer of green electrophosphorescent devices

Charge carrier generation and transport in $fac$ tris (2-phenylpyridine) iridium (III) $\left(\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3\right)$ doped in $4,4^{\prime }\text{-}N,N^{\prime }$ -dicarbazole-biphenel (CBP) thin films have been studied in terms of time-of-flight and time-resolved photoluminescence spectroscopies. It is found that the excitation energy rapidly transfer from CBP to $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ , and that the charge carriers are generated on $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ sites. With increasing $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ concentration, the electron drift mobility is slightly decreased, while the hole transit signals become unobservable. The electron and hole transport properties of $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ doped CBP thin films result from the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital of $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ with respect to those of CBP.