Kinetics of singlet and triplet excitons in a wide-band-gap copolymer

Transient and photomodulation spectroscopy is used in order to determine decay times and densities of both emitting and absorbing species in the wide band-gap semiconductor poly-2,5-diheptyl-1,4-phenylene-alt-2, 5-thienylene (PDHPT). The wide band gap of this material is a consequence of the large twisting of the neighboring constituents of the polymer chain. Transient spectroscopy revealed a monoexponential decay of singlet excitons in PDHPT solutions with a radiative and nonradiative decay time of 1.9 and 1.6 ns, respectively. For the solid film photoluminescence decays biexponentially. This distinct decay behavior in solid state is attributed to the migration of the excitons towards quenching sites favored by the three dimensionality in the bulk material, which hence reduces the photoluminescence quantum yield. Infrared studies suggest that upon photo-oxidation one introduces carbonyl groups in PDHPT, which shortens the decay times by introducing photoluminescence quenching centers. Photomodulation spectroscopy is exploited to determine steady-state density of triplet excitons along with their decay dynamics and we found that the steady-state density of the triplets can be as high as ${10}^{16}{\mathrm{cm}}^{\mathrm{-}3}$ in this material. Furthermore, we determined the generation probability of triplets to be $2\times {10}^{-3}.$