Nonlinear emission and recombination in conjugated polymer waveguides

Abstract

The emission dynamics in photoexcited planar conjugated polymer waveguides is investigated at high excitation densities. Using femtosecond pump/probe experiments and photoluminescence spectroscopy we investigate the interplay of nonlinear radiative and nonradiative recombination processes. Amplified spontaneous emission (ASE) leads to an ultrafast depletion of the excited state at excitation densities above 1018 cm−3 in an ladder-type poly(p-phenylene) film deposited on a glass substrate. Owing to the different waveguide structure ASE is not observed for the same polymer deposited on an indium–tin–oxide (ITO)-coated substrate. Instead, we observe nonradiative bimolecular annihilation with a coefficient γ=4.2×10−9 cm3 s−1. Our results demonstrate that even in the absence of a resonator collective stimulated emission can be much more efficient than nonradiative recombination. A mandatory prerequisite, however, is a suitable waveguide design. The use of ITO as a hole-injecting contact is problematic due to its high refractive index and its relatively high losses.

Exciton diffusion and relaxation in methyl-substituted polyparaphenylene polymer films

“…Third, the annihilation rate is time independent within experimental accuracy, at least on tens of picosecond time scale, and equals to about 1.3ϫ 10 −8 s −1 cm 3 . Values of the linear relaxation times are within the range of literature data for m-LPPP films, which are widely distributed between 8 and 300 ps 28,[33][34][35][36][37]. …”

Recombination of singlet excitons with geminate pairs of charge carriers in conjugated polymers

“…It is manifested by the increasing decay rate of both PF and PTA as well as by decreasing PF quantum yield upon increasing the intensity of pulsed photoexcitation. [1][2][3] Since singlet excitons can perform intraas well as interchain motion, the singlet-singlet annihilation ͑SSA͒ was suggested as a possible explanation of the observed nonlinear effects. 1,2 This phenomenon is well known for many molecular organic systems including molecular crystals ͑Ref.…”

Large gain for crystalline thin films of thiophene/phenylene co-oligomer by photopumping with femtosecond laser pulses

“…Furthermore, dilute solutions of organic laser dyes [22], and the derivatives of poly(p-phenylenevinylene) (PPV) and polyfluorene showing high fluorescence have σ values of 10 -20 -10 -16 cm 2 in both solutions [22][23][24], and films [25,26]. However, the laser dyes only exhibit fluorescence in dilute solutions and not in the solid state because of concentration quenching.…”