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

Transient and photomodulation spectroscopy is used in order to determine de cay times and densities of both emitting and absorbing species in the wide band-gap semiconductor poly-2,5-diheptyl-1,4-phenylene-alt-2, S-thienylene (PDHPT). The wide band gap of this material is a consequence of the large t wisting of the neighboring constituents of the polymer chain. Transient spe ctroscopy revealed a monoexponential decay of singlet excitons in PDHPT sol utions with a radiative and nonradiative decay time of 1.9 and 1.6 ns, resp ectively. For the solid film photoluminescence decays biexponentially. This distinct decay behavior in solid state is attributed to the migration of t he 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 photolumine scence quenching centers. Photomodulation spectroscopy is exploited to dete rmine steady-state density of triplet excitons along with their decay dynam ics and we found that the steady-state density of the triplets can be as hi gh as 10(16) cm(-3) in this material. Furthermore, we determined the genera tion probability of triplets to be 2 x 10(-3).