Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

The steady-state photoinduced absorption (PA), photoluminescence (PL), PL-detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) $N_0$ dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons (SE’s) and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE’s are quenched by trapped and free polarons and that the spin-$\frac{1}{2}$ magnetic resonance conditions reduce the total polaron population. Since the sublinear $N_0$ dependences of the positive (PL-enhancing) spin-$\frac{1}{2}$ PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE’s by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent π-conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.