Origin of efficient light emission from a phosphorescent polymer/organometallic guest-host system

Time-resolved photoinduced absorption measurements were performed at 77 K and room temperature on thin films of tris[9,9-dihexyl-2-(phenyl-$4^{\prime }$-(-pyridin-$2^{″}$-yl)fluorene]iridium(III) $\left[\mathrm{Ir}\right(\mathrm{DPPF}{)}_3]$ doped into a blend of poly$(N$-vinylcarbazole) (PVK) with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). We observe that in the PVK-PBD host blend, charge trapping (CT) plays an important role in the excited-state dynamics, in addition to exciplex formation and intensity-dependent decay of primary excitations. We develop a physical model which includes all interactions and which is in excellent agreement with the data. We find that 35% of the initial photoexcitation channels into CT states and that exciplexes are formed at a rate of $1/10{\mathrm{ps}}^{-1}.$ For the $\mathrm{Ir}(\mathrm{DPPF}{)}_3$ doped host composite, we write the rate equations for all population densities (which include the above excited-state species) and include energy-transfer rates from the host to the guest molecules. In both 0.2% and 2% $\mathrm{Ir}(\mathrm{DPPF}{)}_3:(\mathrm{PVK}-\mathrm{PBD})$ blends, Förster energy-transfer rates drop to half their low-temperature values at room temperature. We attribute this difference to a limited availability of guest molecules ready for energy transfer following charge trapping and insufficient spectral overlap due to shifts in the highest occupied and lowest unoccupied molecular orbital levels of the guest upon hole trapping. We conclude that the overall host-guest energy transfer is almost complete at room temperature in the 2% phosphorescent blend, with a large contribution (35%) from CT states which exhibit emission at all probe wavelengths.