The triplet excited-state $(T_1\to T_n)$ absorption cross section of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) was calculated from quasi-steady-state photoinduced absorption measurements of films of the polymer blended with iridium (III) [(bis 4,6-difluorophenyl)-pyridinato-$N,\mathrm{C}{2}^{\prime }$] picolinate (FIrpic). Steady-state photoluminescence spectra and photoinduced absorption results showed that quenching of the FIrpic phosphorescence originated from triplet energy transfer from FIrpic to F8BT. By comparing the strength of the photoinduced absorption with the known rate of triplet generation in the F8BT, the triplet excited-state absorption cross section of F8BT was found to be $3.1\times {10}^{-16}{\mathrm{cm}}^2\pm 20\%$. This value is relatively small compared with that for poly($p$-phenylenevinylene)s, but is consistent with previous quantum-chemical calculations. Accurate knowledge of the triplet excited-state absorption cross section is valuable for quantifying the populations of nonemissive triplet excitons in polymer-based devices.

• Received 22 May 2007

DOI:https://doi.org/10.1103/PhysRevB.76.245201