Olga D. Parashchuk

Acceptor-Enhanced Local Order in Conjugated Polymer Films.

Disorder in conjugated polymers is a general drawback that limits their use in organic electronics. We show that an archetypical conjugated polymer, MEH-PPV, enhances its local structural and electronic order upon addition of an electronic acceptor, trinitrofluorenone (TNF). First, acceptor addition in MEH-PPV results in a highly structured XRD pattern characteristic for semicrystalline conjugated polymers. Second, the surface roughness of the MEH-PPV films increases upon small acceptor addition, implying formation of crystalline nanodomains. Third, the low-frequency Raman features of the polymer are narrowed upon TNF addition and indicate decreased inhomogeneous broadening. Finally, the photoinduced absorption and surface photovoltage spectroscopy data show that photoexcited and dark polymer intragap electronic states assigned to deep defects disappear in the blend. We relate the enhanced order to formation of a charge-transfer complex between MEH-PPV and TNF in the electronic ground state. These findings may be of high importance to control structural properties as they demonstrate an approach to increasing the order of a conjugated polymer by using an acceptor additive.

Hyperdiffusive dynamics in conjugated polymer blends and fullerene absorbing solutions

We have observed the hyperdiffusive dynamics of light scatterers accompanied by their ballistic motion by a dynamic light scattering (DLS) technique in blends of semiconducting polymer (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV) with a low-molecular-weight organic acceptor 2,4,7-trinitrofluorenone, TNF) and in fullerene C60 solution. The DLS autocorrelation function has been found to follow the Kohlrausch-Williams-Watt function with the exponent β in the range of 0.5–2, depending on the laser power, scattering vector, optical absorption and type of sample. We show that the hyperdiffusive dynamics (β > 1) results from laser-induced convection caused by optical absorption at the DLS probing wavelength. The convection appears in the DLS data as a ballistic motion with velocities in the range of 5–60 μm s−1 depending on the laser power, type of sample and their absorption. The convection is driven by the local temperature difference, ΔT ∼ 0.1–0.2 K, as evaluated from thermal lens effect measurements. We demonstrate that by decreasing the absorbed laser power, laser-induced hyperdiffusion can be avoided in both the polymer and fullerene samples so that the inherent thermal molecular motion can be probed. Specifically, in the polymer solutions, we have found a slow relaxation mode with the characteristic spatial spectrum of the inverse relaxation time Γs α q4. This mode has been assigned to the coupled motion of entangled conjugated polymer chains.

Threshold formation of an intermolecular charge transfer complex of a semiconducting polymer

It has been found that the formation of an intermolecular charge transfer complex in the ground electronic state between the model conjugated polymer (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and the low-molecular-weight organic acceptor (2,4,7-trinitrofluorenone, TNF) occurs stepwise with an increase in the acceptor concentration in the blend as is observed in the optical absorption spectra of solutions. The threshold dependence of the absorption of the charge transfer complex is attributed to the stepwise change in the concentration of the charge transfer complexes, which is not explained by the standard model describing the optical characteristics of intermolecular charge transfer complexes. A kinematic model has been proposed to explain the threshold increase in the concentration of charge transfer complexes: at low acceptor concentrations, the charge transfer complex is formed primarily on the surface of a polymer coil, whereas as the acceptor fraction in the solution increases, TNF molecules penetrate inside the polymer coils, forming the charge transfer complex with the units of the polymer inside the coil.